Glossary
Every parameter, variable and curve used on this site, with one definition, symbol and unit each.
- Alias dictionary method
-
A table that maps each known mnemonic of a curve family to one canonical name, usually with the expected units and a priority order for choosing among duplicates.
- Anisotropy ratio \(R_v/R_h\) variable
-
Ratio of vertical to horizontal resistivity. It is 1 in an isotropic rock and above 1 in laminated rock.
- Apparent resistivity \(R_a\) variable
-
Resistivity read by an induction tool in a deviated well or dipping beds. It lies between the horizontal and the vertical resistivity.
- Apparent water resistivity \(R_{wa}\) variable
-
Water resistivity back-calculated from a log reading by assuming the rock is 100% water-saturated. It equals Rw in a clean wet zone and exceeds it in hydrocarbon-bearing rock.
- Archie water saturation \(S_{w,A}\) variable
-
Water saturation from the clean-rock Archie equation, which is used as a reference against the shaly-sand models.
- Axial stress at the wall \(\sigma_{z}\) variable
-
Total stress along the axis of a vertical well at the wall, the vertical stress corrected for the horizontal stress contrast.
- Azimuth around the wellbore \(\theta\) variable
-
Angle measured around the wall of a vertical well from the direction of the maximum horizontal stress.
- Bad hole concept
-
Borehole condition (washout, breakout, rugose wall, mudcake) that damages the contact between a pad tool and the formation and so the log reading.
- Bad-hole flag \(F_{bh}\) variable
-
Indicator with value 1 where the borehole condition is judged to have damaged the log measurement and 0 elsewhere.
- Biot coefficient \(\alpha\) parameter
-
Fraction of the pore pressure that acts as an effective stress reduction in a porous elastic rock. It is 1 for unconsolidated or very soft rock and falls toward the porosity for stiff, tight rock.
- Biot coefficient from moduli \(\alpha_{M}\) variable
-
Biot coefficient computed as one minus the ratio of drained bulk modulus to grain bulk modulus.
- Biot coefficient from porosity \(\alpha_{K}\) variable
-
Biot coefficient estimated from total porosity with the Krief empirical frame-modulus relation.
- Bit size \(d_{\mathrm{bit}}\) parameter
-
Nominal diameter of the drill bit for the interval, which is the borehole diameter in gauge hole.
- Bound volume irreducible \(BVI\) variable
-
Part of the NMR porosity with a T2 shorter than the cutoff: the pore fluid that is held by capillary and clay-bound forces.
- Bound-water resistivity \(R_{wb}\) parameter
-
Resistivity of the water bound to clay surfaces in the dual-water model, at formation temperature. It is usually lower than the resistivity of the free formation water.
- Bound-water saturation \(S_{w,b}\) variable
-
Volume of clay-bound water as a fraction of the total pore volume. It is the clay volume times the total porosity of the clay, divided by total porosity.
- Bowers mudline velocity \(V_0\) parameter
-
Velocity of the virgin curve at zero effective stress, about 5000 ft/s in unconsolidated, water-saturated sediment.
- Bowers parameter A \(A\) parameter
-
Coefficient in the virgin velocity-effective stress curve V = V0 + A sigma^B, with stress in psi and velocity in ft/s. Calibrated for each basin.
- Bowers parameter B \(B\) parameter
-
Exponent in the virgin velocity-effective stress curve. Calibrated for each basin.
- Bowers unloading parameter \(U\) parameter
-
Parameter of the unloading curve. Larger values mean a stronger hysteresis between the loading and unloading curves: velocity is retained to lower stress.
- Breakdown mud weight \(MW^{bd}\) variable
-
Breakdown pressure expressed as an equivalent mud weight.
- Breakdown pressure \(P_{w}^{bd}\) variable
-
Wellbore pressure at which the hoop stress at the wall first reaches the tensile strength and a fracture starts.
- Brittleness index (brittle minerals) \(BI_{W}\) variable
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Sum of quartz and dolomite volumes divided by the sum of quartz, dolomite, calcite, clay and organic volumes.
- Brittleness index (modulus-based) \(BI_{R}\) variable
-
Mean of the normalised Young's modulus and the normalised Poisson's ratio. It is one of many brittleness definitions and has no absolute physical meaning.
- Brittleness index (quartz fraction) \(BI_{J}\) variable
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Quartz volume divided by the sum of quartz, calcite and clay volumes.
- Brooks-Corey pore size distribution index \(\lambda\) parameter
-
Exponent that describes the spread of pore sizes. A large value means a narrow, uniform distribution and a flat capillary pressure plateau; a small value means a wide distribution.
- Bubble point pressure \(p_b\) variable
-
Pressure at which the first gas comes out of solution as pressure falls at constant temperature. Above it the oil is undersaturated.
- Bulk density \(\rho_b\) curve
-
Formation bulk density measured by the density tool.
- Bulk density (SI) \(\rho_{b,\mathrm{SI}}\) variable
-
Bulk density expressed in kg/m³, which is 1000 times the value in g/cm³. Some files and LWD tools deliver density this way.
- Bulk volume water \(\mathrm{BVW}\) variable
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Volume of water per unit bulk volume of rock. It is the product of porosity and water saturation on the same basis.
- Bulk volume water at irreducible saturation \(BVW_{irr}\) parameter
-
Effective porosity times irreducible water saturation. It is the Buckles number and is close to constant within one rock type at irreducible conditions.
- Calcite density endmember \(\rho_c\) parameter
-
Density response of pure calcite used as an inversion endmember.
- Calcite U endmember \(U_c\) parameter
-
Volumetric photoelectric cross section of pure calcite.
- Calcite volume \(V_{cal}\) variable
-
Volume fraction of calcite in the rock, from mineral inversion or core.
- Calcite volume (inversion) \(V_c\) variable
-
Volume of calcite per unit bulk volume from the three-component inversion example.
- Calcite volume standard deviation \(\sigma_{V_c}\) variable
-
Standard deviation of the inverted calcite volume caused by the log errors.
- Caliper \(d_c\) curve
-
Measured borehole diameter.
- Caliper (millimetres) \(d_{c,\mathrm{mm}}\) variable
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Borehole diameter in millimetres. It equals the value in inches multiplied by 25.4.
- Caliper excess \(\Delta d\) variable
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Measured caliper minus bit size. Positive values mean the hole is larger than the bit.
- Caliper excess limit \(\Delta d_{\max}\) parameter
-
Largest caliper excess accepted before a sample is flagged as bad hole.
- Caliper rugosity \(\mathcal{R}_c\) variable
-
Local roughness of the borehole wall, measured from the caliper either as a running standard deviation or as the deviation from a smooth trend.
- Caliper rugosity limit \(\mathcal{R}_{c,\max}\) parameter
-
Largest caliper rugosity accepted before a sample is flagged as bad hole.
- Caliper trend \(d_{\mathrm{trend}}\) variable
-
A smooth reference value for the caliper, such as a running median over several tens of feet, against which local deviations are measured.
- Cation exchange capacity \(\mathrm{CEC}\) parameter
-
Number of exchangeable cations per unit mass of dry rock or clay, measured on core.
- Cation exchange capacity per unit pore volume \(Q_v\) parameter
-
Cation exchange capacity of the rock per unit of total pore volume. It measures how much excess clay conductivity the rock has in the Waxman-Smits model.
- Cementation exponent \(m\) parameter
-
Exponent relating formation factor to porosity in Archie's equation.
- Class imbalance concept
-
A large difference in the number of samples per facies. A classifier can reach a high overall accuracy while missing a rare facies altogether.
- Clay bulk density \(\rho_{b,clay}\) parameter
-
Bulk density read in 100% clay on the crossplot.
- Clay gamma ray \(\mathrm{GR}_{clay}\) parameter
-
Gamma ray value of a 100% clay (shale) interval. Picked once per zone and shared by all gamma-ray-based clay volume methods.
- Clay neutron porosity \(\phi_{N,clay}\) parameter
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Neutron porosity read in 100% clay, which is high because of hydrogen in bound water and hydroxyl groups.
- Clay porosity \(\phi_{cl}\) parameter
-
Bound-water volume per unit volume of clay, that is the porosity of the wet clay.
- Clay resistivity \(R_{cl}\) parameter
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Resistivity of the adjacent clay or shale used as the clay endmember in a shaly-sand equation, read from a thick shale on the deep resistivity.
- Clay slowness \(\Delta t_{clay}\) parameter
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Compressional slowness read in 100% clay.
- Clay SP \(\mathrm{SP}_{clay}\) parameter
-
SP value of the shale baseline, which is the reference for the clean-interval deflection.
- Clay volume \(V_{cl}\) variable
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Volume fraction of clay minerals per unit bulk volume of rock.
- Clay volume cutoff \(V_{cl,cut}\) parameter
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Maximum clay volume for rock to count as reservoir. Used to flag gross reservoir.
- Clay-bound water volume \(V_{cbw}\) variable
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Volume of water bound to clay per unit bulk volume of rock.
- Clean gamma ray \(\mathrm{GR}_{clean}\) parameter
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Gamma ray value of a clean (clay-free) reservoir interval. Picked once per zone and shared by all gamma-ray-based clay volume methods.
- Clean neutron porosity \(\phi_{N,clean}\) parameter
-
Neutron porosity read in clean, clay-free rock, on the same lithology scale as the log.
- Clean SP \(\mathrm{SP}_{clean}\) parameter
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SP value in a thick, clean, water-bearing reservoir interval, read relative to the shale baseline.
- Clean-sand porosity \(\phi_{sa}\) parameter
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Total porosity of the clean sand end member, the sand with no shale in it. It is the clean-sand vertex of the Thomas-Stieber diagram.
- Closure concept
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The constraint that the component volumes add to one.
- Coefficient of internal friction \(\mu\) variable
-
Tangent of the internal friction angle, the slope of the Mohr-Coulomb envelope. A value near 0.6 is common for frictional sliding on existing faults.
- Cohesion \(S_{0}\) variable
-
Shear strength of the rock at zero normal stress on the Mohr-Coulomb envelope, computed here from UCS and the friction angle.
- Collapse mud weight \(MW^{sh}\) variable
-
Shear-failure wellbore pressure expressed as an equivalent mud weight.
- Compaction constant \(C\) parameter
-
Empirical multiplier in the compaction factor. A value of 1 leaves the factor equal to the shale slowness divided by 100.
- Compaction factor \(C_p\) variable
-
Divisor applied to the time-average sonic porosity in uncompacted formations. It is never less than 1.
- Compaction length \(L_{c}\) parameter
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Depth scale over which the bulk density approaches its deep value in the exponential density trend.
- Compressional slowness \(\Delta t\) curve
-
Interval transit time of the compressional wave measured by the sonic tool.
- Compressional slowness (SI) \(\Delta t_{\mathrm{SI}}\) variable
-
Compressional slowness expressed in µs/m. It equals the value in µs/ft divided by 0.3048.
- Compressional velocity \(V_{p}\) variable
-
Speed of the compressional wave, 304.8 divided by the compressional slowness in µs/ft.
- Compressional velocity in ft/s \(V\) variable
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Compressional velocity, 10^6 divided by the slowness in µs/ft. Bowers' relations are written in ft/s.
- Compressional velocity in km/s \(V_p\) variable
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Compressional velocity in kilometres per second, 304.8 divided by the slowness in µs/ft. Most published Vp-Vs regressions use these units.
- Condition number \(\kappa\) concept
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Ratio of the largest to the smallest singular value of the weighted inversion matrix. A large value means small log errors produce large volume errors.
- Confusion matrix concept
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Table of true classes against predicted classes. The diagonal counts correct samples, and the off-diagonal cells show which classes are mistaken for which.
- Conventional water saturation \(S_{w,conv}\) variable
-
Water saturation from the bulk porosity and the unseparated horizontal resistivity, with no laminated-shale correction. It is shown for comparison.
- Corey exponent for oil \(n_o\) parameter
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Exponent of the normalized mobile oil saturation in the power-law oil relative permeability.
- Corey exponent for water \(n_w\) parameter
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Exponent of the normalized mobile water saturation in the power-law water relative permeability.
- Cumulative flow capacity fraction \(FC\) variable
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Cumulative permeability-thickness divided by its total, accumulated from the highest permeability downward.
- Cumulative storage capacity fraction \(SC\) variable
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Cumulative porosity-thickness divided by its total, accumulated from the highest permeability downward.
- Curve family concept
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A group of curves that measure the same physical quantity, such as bulk density, deep resistivity or compressional slowness, whatever their mnemonics, tools or processing versions.
- Curve mnemonic concept
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The short name a curve carries in a log file. It identifies the measurement only by convention, and the convention varies between vendors, tool generations and processing houses.
- Curve splice method
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The merging of curves from several runs or passes into one continuous curve, after depth-matching them and deciding what to do with the overlap.
- Curve value before normalization \(x_{\mathrm{in}}\) variable
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The value of the curve being normalized, in its own units, before any shift or scale is applied.
- Data leakage concept
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Information from the test samples reaching the model during training, which makes a validation score look better than the model's real performance.
- Deep asymptotic bulk density \(\rho_{\infty}\) parameter
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Bulk density the compaction trend approaches at great depth.
- Deep-compaction slowness \(\Delta t_{\infty}\) parameter
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Slowness that the sonic normal compaction trendline approaches at great depth, the fully compacted shale value.
- Density at first clean point \(\rho_{b,A}\) parameter
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Bulk density of the first of two points that define the clean-rock line on a crossplot, usually the matrix end of the line.
- Density at second clean point \(\rho_{b,B}\) parameter
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Bulk density of the second point on the clean-rock line, at a higher porosity than the first.
- Density baseline \(\rho_{b,base}\) parameter
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Bulk density read in the same organic-lean shale used for the resistivity baseline.
- Density correction \(\Delta\rho\) curve
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Correction that the density tool applies for mudcake and standoff. A large correction means poor pad contact and an unreliable density.
- Density log uncertainty \(\sigma_{\rho}\) parameter
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Standard deviation of the error assigned to the bulk density in an inversion.
- Density porosity \(\phi_D\) variable
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Porosity from the bulk density log, assuming a known matrix density and fluid density.
- Density residual \(r_{\rho}\) variable
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Measured bulk density minus the density predicted by the inversion model.
- Depth of top of density log \(z_t\) parameter
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True vertical depth where the density log starts. Overburden above it has to be extrapolated.
- Depth reference concept
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The zero point of a depth scale, such as kelly bushing, drill floor, ground level or mean sea level. Curves and tops measured from different references differ by a constant.
- Depth reference elevation \(z_{KB}\) parameter
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Elevation of the depth reference (kelly bushing or rotary table) above mean sea level, used to convert true vertical depth to subsea depth.
- Depth shift parameter
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A constant (or slowly varying) correction added to the depths of one run or curve so that its features line up with a reference curve.
- Depth-block cross-validation method
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Cross-validation in which each fold is a contiguous depth interval, so that neighbouring samples of the same bed are never split between training and test data.
- Disequilibrium compaction concept
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Overpressure caused by rapid burial: pore fluid cannot escape fast enough, so it carries part of the overburden and the rock stays less compacted than its depth implies. Effective stress stays on the loading curve.
- Dispersed shale concept
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Shale (clay) that fills the pore space of the sand, so that porosity falls as shale volume rises.
- Dispersed shale volume \(V_{disp}\) variable
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Volume of dispersed shale, as a fraction of bulk volume.
- Dolomite fraction \(X_{dol}\) parameter
-
Volume fraction of dolomite used to weight the lithology lines.
- Dolomite volume \(V_{dol}\) variable
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Volume fraction of dolomite in the rock, from mineral inversion or core.
- Dolomite-line shear velocity \(V_{s,dol}\) variable
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Shear velocity from the dolomite regression at the measured Vp.
- Drainage area \(A\) parameter
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Map area over which a per-unit-area volume is multiplied to give a total. It is a mapping or drainage-area input, not a log result.
- Drained bulk modulus \(K_{dry}\) variable
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Bulk modulus of the dry rock frame, at constant pore pressure.
- Dynamic bulk modulus \(K\) variable
-
Resistance to uniform compression computed from density, compressional and shear velocity at sonic frequencies.
- Dynamic Poisson's ratio \(\nu_{d}\) variable
-
Ratio of lateral contraction to axial extension in uniaxial loading, computed from the velocity ratio.
- Dynamic shear modulus \(G\) variable
-
Resistance to shear deformation computed from density and shear velocity at sonic frequencies, G = rho Vs squared.
- Dynamic Young's modulus \(E_{d}\) variable
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Ratio of axial stress to axial strain in uniaxial loading, computed from the sonic and density logs. It is higher than the static value measured on core.
- Eaton exponent for resistivity \(x_r\) parameter
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Exponent on the ratio of observed to normal resistivity in the Eaton resistivity equation. 1.2 is the usual starting value; it is calibrated for each basin.
- Eaton exponent for sonic \(x_s\) parameter
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Exponent on the ratio of normal to observed slowness in the Eaton sonic equation. 3 is the usual starting value; it is calibrated for each basin.
- Effective hoop stress at the wall \(\sigma_{\theta}'\) variable
-
Hoop stress at the wall minus the pore pressure.
- Effective porosity \(\phi_e\) variable
-
Pore volume per unit bulk volume excluding clay-bound water.
- Effective water saturation \(S_{w,e}\) variable
-
Water volume that excludes clay-bound water, as a fraction of the effective pore volume.
- End-point mobility ratio \(M\) variable
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Mobility of the displacing water at its endpoint divided by the mobility of the displaced oil at its endpoint. Above 1 the displacement is unfavourable.
- Endmember concept
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The log response of a pure component, a mineral or a fluid, used as a column of the inversion matrix.
- Environmental correction chart concept
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A vendor chart or algorithm that removes the effect of hole size, mud, temperature, pressure, standoff and salinity from a log reading. Neutron logs need a chart that matches the exact tool.
- Equivalent counterion conductance \(B\) parameter
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Equivalent conductance of the clay counterions in the Waxman-Smits model. It depends on temperature and on water salinity.
- Equivalent depth \(z_e\) variable
-
Depth in normally compacted shale at which the log property equals its observed value, and so where the effective stress equals the effective stress at the observed depth.
- Equivalent mud filtrate resistivity \(R_{mfe}\) variable
-
Mud filtrate resistivity converted to the value that gives the same electrochemical potential in the SP equation.
- Equivalent mud weight \(\mathrm{EMW}\) variable
-
A pressure expressed as the density of drilling fluid that would exert it at the depth: pressure divided by 0.052 times the true vertical depth in feet.
- Equivalent mud weight \(\rho_{eq}\) variable
-
Pressure at a depth expressed as the density of a mud column that would balance it, using 0.052 psi per foot for each pound per gallon.
- Equivalent mud weight from resistivity variable
-
Resistivity pore pressure expressed as an equivalent mud weight.
- Equivalent mud weight from sonic variable
-
Sonic pore pressure expressed as an equivalent mud weight.
- Equivalent water resistivity \(R_{we}\) variable
-
Formation water resistivity converted to the value that gives the same electrochemical potential in the SP equation. Rw is read from it with a chart or fit.
- Excess pressure \(\Delta p_{ex}\) parameter
-
Pore pressure above the hydrostatic value at the same depth. Zero for a normally pressured interval.
- Facies code variable
-
Integer label given to each depth sample, standing for one lithofacies or one cluster. A code means nothing until it is tied to a named facies.
- Faust multiplier \(F_m\) parameter
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Scaling factor that converts the sonic porosity separation to TOC. It is fitted to core TOC.
- Faust sonic \(\Delta t_F\) variable
-
Compressional slowness predicted from resistivity and depth with Faust's empirical velocity-resistivity relation.
- Flow zone indicator \(FZI\) variable
-
Reservoir quality index divided by normalized porosity. Points of one hydraulic flow unit share a similar value.
- Fluid density contrast \(\Delta\rho\) variable
-
Brine density minus hydrocarbon density at reservoir conditions. Multiplied by the freshwater pressure gradient of 0.433 psi/ft it gives the buoyancy gradient of the column.
- Fluid pressure gradient \(G_p\) parameter
-
Pressure increase per foot of true vertical depth in a static water column. About 0.433 psi/ft for fresh water and 0.465 psi/ft for a typical formation brine.
- Fluid slowness \(\Delta t_w\) parameter
-
Compressional slowness of the pore fluid, usually water.
- Fluid U endmember \(U_f\) parameter
-
Volumetric photoelectric cross section of the pore fluid. It is small and depends on salinity.
- Flushed-zone resistivity \(R_{xo}\) curve
-
Resistivity of the zone flushed by mud filtrate near the borehole.
- Flushed-zone water saturation \(S_{xo}\) variable
-
Water saturation of the zone next to the borehole that mud filtrate has flushed. It is 1 minus the sum of the residual and any bypassed hydrocarbon saturations.
- Formation conductivity \(\sigma\) variable
-
Formation conductivity as delivered by induction tools, in millisiemens per metre (the same number as mmho/m). It is 1000 divided by the resistivity in ohm·m.
- Formation factor \(F\) variable
-
Ratio of the resistivity of a rock fully saturated with water to the resistivity of that water.
- Formation temperature \(T_f\) variable
-
Temperature of the formation at the depth of interest, from bottom-hole temperature corrected to equilibrium or from a geothermal gradient.
- Formation water resistivity \(R_w\) parameter
-
Resistivity of the formation water at formation temperature.
- Fractional flow of water \(f_w\) variable
-
Fraction of the total flowing volume that is water, at a given water saturation, neglecting gravity and capillary pressure.
- Free fluid index \(FFI\) variable
-
Part of the NMR porosity with a T2 longer than the cutoff: the movable fluid. The sum of BVI and FFI is the NMR porosity.
- Free water level \(z_{FWL}\) variable
-
Depth at which the capillary pressure is zero and the water and hydrocarbon pressures are equal. Below it the rock is water-saturated; above it the hydrocarbon column builds capillary pressure.
- Friction angle from porosity (sandstone) \(\varphi_{WP}\) variable
-
Internal friction angle estimated from porosity with a linear relation published for sandstones.
- Friction angle from Vp (shale) \(\varphi_{L}\) variable
-
Internal friction angle estimated from the compressional velocity with a relation published for shales.
- Friction factor \(q\) variable
-
Ratio of the largest to smallest effective principal stress at Mohr-Coulomb failure, (1 + sin phi) / (1 - sin phi).
- Gamma ray \(\mathrm{GR}\) curve
-
Natural gamma radiation measured by the logging tool, mainly from potassium, uranium and thorium in clays and other minerals.
- Gamma ray index \(I_{GR}\) variable
-
Linear scaling of the gamma ray between the clean and clay values, clamped to the interval 0 to 1.
- Gas compressibility factor \(Z\) variable
-
Ratio of the real gas volume to the ideal gas volume at the same pressure and temperature.
- Gas formation volume factor \(B_g\) variable
-
Volume of gas at reservoir pressure and temperature that expands to one standard cubic foot at surface conditions.
- Gas gravity \(\gamma_g\) variable
-
Gas density relative to air at the same conditions. In the oil correlations it is the gravity of the solution gas, referred to a separator pressure of 100 psig.
- Gas in place per acre \(G_A\) variable
-
Surface-condition gas originally in place under one acre of the evaluated interval, from net thickness, porosity, water saturation and gas formation volume factor.
- Geothermal gradient \(G_T\) parameter
-
Temperature increase per 100 ft of true vertical depth. Typical sedimentary basins are 1 to 2 °F per 100 ft, with wide local variation.
- Grain bulk modulus \(K_{g}\) parameter
-
Bulk modulus of the solid mineral grains, about 37 GPa for quartz and 70 GPa for calcite.
- Gross reservoir flag \(F_{g}\) variable
-
One where the rock passes the lithology test (clay volume at or below its cutoff) and is a candidate reservoir, otherwise zero.
- Hampel filter method
-
A despiking filter that replaces a sample with the local median only when it is further from that median than a chosen number of scaled median absolute deviations.
- Height above free water level \(h\) variable
-
Vertical distance above the free water level at which the irreducible water saturation is evaluated. For the Swirr result it is normally the maximum column height of the reservoir.
- Height above free water level \(h\) variable
-
Vertical distance of a depth above the free water level. It sets the capillary pressure that the hydrocarbon column exerts on the pores.
- Height of constant bulk volume water \(h_{B}\) variable
-
Height above the free water level at which the height-dependent bulk volume water has fallen to the Buckles number. Above it the Buckles model treats the rock as being at irreducible saturation.
- Height of the entry pressure \(h_e\) variable
-
Height above the free water level at which the buoyancy pressure of the hydrocarbon column equals the entry pressure. Below it the rock stays at 100% water saturation, so it is the height of the oil-water contact above the free water level.
- Height-BVW coefficient a \(a_h\) parameter
-
Multiplier of the power law that relates bulk volume water to height above the free water level. Fitted to data; no universal value.
- Height-BVW exponent b \(b_h\) parameter
-
Exponent of the power law that relates bulk volume water to height above the free water level. Negative, because water volume falls with height.
- Hoop stress at the wall \(\sigma_{\theta}\) variable
-
Total circumferential stress at the wellbore wall at a given azimuth, from the Kirsch solution.
- Horizontal resistivity \(R_h\) variable
-
Resistivity measured with current flowing along the bedding. In a laminated sequence the shale laminae pull it down toward the shale resistivity.
- Horizontal shale resistivity \(R_{sh,h}\) parameter
-
Resistivity of the pure shale for current flowing along the bedding.
- Horizontal stress ratio \(R_{H}\) parameter
-
Ratio of effective maximum to effective minimum horizontal stress, (SH minus Pp) divided by (Sh minus Pp). It is 1 when the horizontal stresses are equal.
- Hydraulic flow unit concept
-
Interval of rock with similar pore geometry, and so a similar flow zone indicator, capillary pressure behaviour and porosity-permeability relation.
- Hydrocarbon pore thickness \(\mathrm{HCPV}\) variable
-
Sum over the net intervals of thickness times porosity times hydrocarbon saturation. It is the hydrocarbon pore volume per unit area expressed as a length.
- Hydrostatic gradient \(G_h\) parameter
-
Pressure increase per unit depth in a column of formation water. About 0.433 psi/ft for fresh water and up to about 0.465 psi/ft for saline brine.
- Hydrostatic pressure \(P_h\) variable
-
Pressure of a continuous column of water from the surface to the depth: the water gradient times the true vertical depth.
- Hydrostatic pressure in ppg variable
-
Hydrostatic pressure as an equivalent mud weight.
- Inorganic shale density \(\rho_{b,s}\) parameter
-
Bulk density of the rock with no kerogen, used as the matrix end of the density mixing relation for kerogen.
- Interfacial tension times cosine of contact angle \(\sigma\cos\theta\) parameter
-
Product of the interfacial tension and the cosine of the contact angle for the reservoir fluid pair. It converts capillary pressure to pore throat size and scales the J-function. About 26 dyne/cm for oil and water with 30 dyne/cm and 30 degrees.
- Internal friction angle \(\varphi\) variable
-
Angle of the Mohr-Coulomb failure envelope, from triaxial tests or a log correlation. Its tangent is the coefficient of internal friction.
- Interparticle porosity \(\phi_{ip}\) variable
-
Porosity between grains or crystals of a carbonate, excluding separate vugs. Fluid flow and capillary behaviour are controlled by it.
- Inversion average error \(E_{avg}\) variable
-
Total error divided by the number of active curves. A value near 1 means the misfit is at the level of the assigned uncertainties.
- Inversion RMS error \(E_{rms}\) variable
-
Root-mean-square of the residuals of the active curves, each divided by its uncertainty.
- Inversion total error \(E_{tot}\) variable
-
Sum of the absolute residuals of the active curves, each divided by its uncertainty.
- Irreducible water saturation \(S_{wirr}\) variable
-
Water saturation that remains in the pore space and cannot be displaced by hydrocarbon at the pressure reached.
- J-Sw coefficient a \(a_J\) parameter
-
Multiplier of the power law J = a (Sw,n)^b. It is the value of J at a normalized water saturation of 1, so it is an entry-pressure scale. Fitted to core capillary pressure.
- J-Sw exponent b \(b_J\) parameter
-
Exponent of the power law J = a (Sw,n)^b. It is negative, and equals minus the inverse of the Brooks-Corey pore size distribution index. Fitted to core capillary pressure.
- k-means clustering method
-
Unsupervised method that splits samples into k groups by repeatedly assigning each sample to the nearest group centre and moving each centre to the mean of its members.
- Kerogen as density porosity \(\phi_{D,k}\) parameter
-
The density porosity that pure kerogen would read, computed from the matrix, kerogen and fluid densities.
- Kerogen carbon fraction \(f_C\) parameter
-
Weight fraction of carbon in kerogen. It converts between kerogen weight fraction and TOC.
- Kerogen density \(\rho_k\) parameter
-
Bulk density of kerogen. It rises with maturity.
- Kerogen neutron response \(\phi_{N,k}\) parameter
-
Neutron porosity that pure kerogen would read, on the same lithology scale as the neutron log.
- Kerogen volume \(V_k\) variable
-
Volume of kerogen per unit bulk volume of rock.
- Kerogen weight fraction \(W_k\) variable
-
Weight of kerogen per unit weight of rock.
- Kerogen-corrected density porosity \(\phi_D^{c}\) variable
-
Density porosity after the contribution of kerogen has been removed.
- Kerogen-corrected neutron-density porosity \(\phi_{ND}^{c}\) variable
-
Average neutron-density porosity after the contribution of kerogen has been removed.
- Key well concept
-
The well whose curves are taken as correct. Other wells are normalized so that their reference-interval distributions match it.
- Laboratory capillary pressure \(P_{c,lab}\) variable
-
Capillary pressure measured in the laboratory on a core plug, with the lab fluid pair (for example air and brine, or mercury and vacuum).
- Laboratory contact angle \(\theta_{lab}\) parameter
-
Contact angle, measured through the wetting phase, for the laboratory fluid pair. Taken as 0 degrees for air and brine and about 140 degrees for mercury and air.
- Laboratory interfacial tension \(\sigma_{lab}\) parameter
-
Interfacial tension of the fluid pair used in the capillary pressure measurement. About 72 dyne/cm for air and brine and about 480 dyne/cm for mercury and air.
- Laminated shale concept
-
Shale present as thin continuous layers between sand layers, each layer much thinner than the vertical resolution of the logs. The log reads the average of the sand and shale layers.
- Laminated shale volume \(V_{lam}\) variable
-
Volume of shale present as laminae, as a fraction of bulk volume.
- Level of organic maturity \(\mathrm{LOM}\) parameter
-
Scale of thermal maturity of organic matter. Higher values mean more mature. It sets how much resistivity separation a given amount of organic carbon produces in Passey's method.
- Leverett J-function \(J\) variable
-
Dimensionless capillary pressure, scaled by interfacial tension, contact angle, permeability and porosity so that the curves of different rock samples collapse towards one curve.
- Limestone fraction \(X_{ls}\) parameter
-
Volume fraction of limestone used to weight the lithology lines.
- Limestone-line shear velocity \(V_{s,ls}\) variable
-
Shear velocity from the limestone regression at the measured Vp.
- Line selector parameter
-
Selects the Castagna line: 0 mudrock line, 1 limestone, 2 dolomite.
- Linear conversion intercept \(b_{E}\) parameter
-
Intercept, in GPa, of a linear relation between static and dynamic Young's modulus.
- Linear conversion slope \(a_{E}\) parameter
-
Slope of a linear relation between static and dynamic Young's modulus. It is specific to the rock type, the modulus range and the data set it was fitted to.
- Linear-space normalized resistivity \(R_{t,\mathrm{lin}}\) variable
-
Deep resistivity after a two-point linear map applied directly to the resistivity, shown for comparison with the log-space map.
- Lithology scale concept
-
The matrix (usually limestone, sandstone or dolomite) assumed when a porosity log is converted from its raw response. A porosity value is only meaningful together with its scale.
- Log permeability \(\log_{10}k\) variable
-
Base-10 logarithm of permeability in millidarcies. Permeability is close to lognormal, so it is plotted and regressed on this scale.
- Log-space normalized resistivity \(R_{t,\log}\) variable
-
Deep resistivity after a two-point linear map applied to log10 of the resistivity.
- Log-space shifted resistivity \(R_{t,\mathrm{ls}}\) variable
-
Deep resistivity multiplied by one factor that moves the middle pick onto its reference value. It is a shift of log10 resistivity.
- Log-space slope (exponent) \(s_{R}\) variable
-
Slope of the two-point map in log10 space. It is the power to which resistivity is raised in the equivalent power law; 1 means a pure multiplicative factor.
- Lower mud weight limit \(MW_{\min}\) variable
-
Larger of the pore pressure and the shear-failure (collapse) mud weight.
- Lucia cementation exponent \(m_L\) variable
-
Cementation exponent that rises with the separate-vug fraction, from about 1.76 for interparticle porosity only.
- Lucia class number \(\mathrm{class}\) variable
-
Rock fabric class, 1 to 3, from the rock fabric number: 1 for 1.5 or below, 2 above 1.5 up to 2.5, and 3 above 2.5.
- Lucia rock fabric class concept
-
One of three petrophysical classes of carbonate interparticle pore space. Class 1 is grain-dominated and coarse, class 2 is intermediate and class 3 is mud-dominated and fine.
- Machine-learning infill method
-
Filling intervals of a curve with values predicted by a machine-learning model trained on other curves. The values are predictions, not measurements.
- Margin above the lower limit \(\Delta MW_{\mathrm{low}}\) variable
-
Mud weight minus the lower limit. Negative values mean the planned mud weight is below the limit.
- Margin below the upper limit \(\Delta MW_{\mathrm{up}}\) variable
-
Upper limit minus the mud weight. Negative values mean the planned mud weight is above the limit.
- Matrix density \(\rho_{ma}\) parameter
-
Grain density of the non-kerogen solids in the rock.
- Matrix slowness \(\Delta t_{ma}\) parameter
-
Compressional slowness of the rock matrix with no pore space.
- Maximum bridged gap \(h_{gap}\) parameter
-
Thickest non-net interval between two net intervals that is bridged, so that the two count as one bed. Zero switches bridging off.
- Maximum effective stress \(\sigma'_{max}\) variable
-
Effective stress at the onset of unloading, the stress on the virgin curve that corresponds to the velocity at onset.
- Maximum hoop stress \(\sigma_{\theta,\max}\) variable
-
Hoop stress at the wall at the azimuth of the minimum horizontal stress, 3 SH minus Sh minus Pw, where breakouts form.
- Maximum horizontal stress \(S_{H}\) variable
-
Largest total principal stress in the horizontal plane. It is the least well constrained of the three stresses.
- Mean density above the log \(\bar\rho_0\) parameter
-
Average bulk density assumed for the interval between the surface (or seafloor) and the top of the density log, from an extrapolation or a regional trend.
- Mean density in the logged interval \(\bar\rho_L\) parameter
-
Average bulk density from the top of the density log to the depth of interest.
- Mean overburden density \(\bar{\rho}\) variable
-
Vertical stress divided by depth and by the density-to-pressure factor, the depth-averaged bulk density above the point.
- Mean overburden gradient \(G_{ob}\) parameter
-
Overburden stress divided by depth. About 1 psi/ft is a common rule of thumb; offshore and in young sediment it is lower.
- Mean TOC \(\overline{\mathrm{TOC}}\) variable
-
Arithmetic mean of the TOC estimates.
- Median filter method
-
A running filter that replaces each sample with the median of the samples in a window around it. It removes narrow spikes and keeps sharp bed boundaries.
- Median TOC \(\mathrm{TOC}_{med}\) variable
-
Median of the TOC estimates.
- Mineral inversion method
-
Simultaneous solution of a set of component volumes from several logs, by least squares with a closure constraint.
- Minimum bed thickness \(h_{min}\) parameter
-
Thinnest continuous interval that is kept as net. Thinner flagged intervals are removed, because they are below what a well test or a completion can use.
- Minimum GR for source rock \(\mathrm{GR}_{min}\) parameter
-
Gamma ray below which the interval is treated as non-source rock and TOC is set to zero.
- Minimum hoop stress \(\sigma_{\theta,\min}\) variable
-
Hoop stress at the wall at the azimuth of the maximum horizontal stress, 3 Sh minus SH minus Pw, where tensile fractures start.
- Minimum horizontal stress \(S_{h}\) variable
-
Smallest total principal stress in the horizontal plane. It sets the pressure at which a fracture closes and opens, and is the upper bound of the mud weight window.
- Minimum horizontal stress (equivalent mud weight) \(S_{h}^{\mathrm{ppg}}\) variable
-
Minimum horizontal stress expressed as an equivalent mud weight, a fracture gradient estimate.
- Minimum TOC \(\mathrm{TOC}_{min}\) variable
-
Lowest of the TOC estimates.
- Mobile hydrocarbon saturation \(S_{hc,m}\) variable
-
Hydrocarbon saturation above the residual: 1 minus water saturation minus residual oil saturation, not less than zero.
- Movable hydrocarbon index \(S_w/S_{xo}\) variable
-
Ratio of true-zone to flushed-zone water saturation. A value well below 1 shows that invasion has moved hydrocarbons.
- Mud filtrate resistivity \(R_{mf}\) parameter
-
Resistivity of the drilling-mud filtrate at formation temperature. It sets the water resistivity in the flushed zone.
- Mud weight \(MW\) parameter
-
Density of the drilling fluid, the pressure at the wellbore wall divided by 0.052 times true vertical depth.
- Mud weight window width \(\Delta MW\) variable
-
Upper limit minus lower limit. A zero or negative width means no mud weight satisfies both limits.
- Mudline resistivity \(R_0\) parameter
-
Resistivity of the trendline at zero depth.
- Mudline slowness \(\Delta t_{ml}\) parameter
-
Slowness of the trendline at zero depth (the seafloor or surface), where the sediment is almost uncompacted.
- Mudrock-line shear velocity variable
-
Shear velocity from the Castagna mudrock line, shown for comparison.
- Near-surface bulk density \(\rho_{0}\) parameter
-
Bulk density of the sediment at the top of the density trend, used to extrapolate density above the logged interval.
- Net pay flag \(F_{p}\) variable
-
One where the rock is net reservoir and also passes the water saturation cutoff, otherwise zero.
- Net pay thickness \(h_n\) variable
-
Thickness of the interval that passes the net pay flags, in true vertical thickness. It is the sum of the sample steps of the flagged samples.
- Net reservoir flag \(F_{r}\) variable
-
One where the rock is gross reservoir and also passes the porosity and permeability cutoffs, otherwise zero.
- Net thickness, zone 1 parameter
-
Net pay thickness of the first of two example intervals.
- Net thickness, zone 2 parameter
-
Net pay thickness of the second of two example intervals.
- Net-to-gross ratio \(NTG\) variable
-
Net thickness divided by gross thickness for a chosen interval. It exists at each level: net reservoir to gross, and net pay to gross.
- Neutron baseline \(\phi_{N,base}\) parameter
-
Neutron porosity read in the same organic-lean shale used for the resistivity baseline.
- Neutron log uncertainty \(\sigma_{\phi N}\) parameter
-
Standard deviation of the error assigned to the neutron porosity in an inversion.
- Neutron porosity \(\phi_N\) curve
-
Apparent porosity from the neutron tool, responding mainly to hydrogen index. Reported on a stated lithology scale.
- Neutron porosity (percent) \(\phi_{N,\mathrm{pu}}\) variable
-
Neutron porosity expressed in porosity units (percent), 100 times the volume fraction.
- Neutron porosity at first clean point \(\phi_{N,A}\) parameter
-
Neutron porosity of the first of two points that define the clean-rock line on a crossplot.
- Neutron porosity at second clean point \(\phi_{N,B}\) parameter
-
Neutron porosity of the second point on the clean-rock line, at a higher porosity than the first.
- Neutron residual \(r_{\phi N}\) variable
-
Measured neutron porosity minus the neutron porosity predicted by the inversion model.
- Neutron tool family concept
-
A group of neutron tools with the same design: for example epithermal sidewall tools, compensated thermal tools and LWD thermal tools. Each family, and each model within it, has its own environmental response.
- Neutron-density porosity (average) \(\phi_{ND}\) variable
-
Arithmetic mean of the neutron porosity and the density porosity.
- Neutron-density porosity (RMS) \(\phi_{ND,rms}\) variable
-
Root-mean-square of the neutron porosity and the density porosity. It is weighted toward the larger of the two and so is less reduced by gas.
- Neutron-density separation \(\Delta\phi_{ND}\) variable
-
Neutron porosity minus density porosity, both on the same matrix scale, here the limestone scale. It is positive in shale and negative in clean sandstone, and near zero in limestone.
- Normal compaction resistivity \(R_n\) variable
-
Deep resistivity expected in normally compacted shale at the depth, from the normal compaction trendline.
- Normal compaction slowness \(\Delta t_n\) variable
-
Compressional slowness expected in normally compacted shale at the depth, from the normal compaction trendline.
- Normal compaction trendline concept
-
The expected trend of a log property (sonic slowness or resistivity) with depth in clean shale that is compacting normally, with the pore pressure hydrostatic. Departures from it are read as abnormal pore pressure.
- Normalised Poisson's ratio \(B_{\nu}\) variable
-
Poisson's ratio rescaled linearly to 0 to 1, with 1 at the lowest Poisson's ratio, the second term of the Rickman brittleness index.
- Normalised Young's modulus \(B_{E}\) variable
-
Young's modulus rescaled linearly to 0 to 1 between its lower and upper bound, the first term of the Rickman brittleness index.
- Normalization reference interval concept
-
A thick, laterally consistent zone, usually a shale or other uniform rock, in which the curve distributions of different wells are compared.
- Normalization slope \(s_{\mathrm{n}}\) variable
-
Multiplier applied to the distance from the low pick by a two-point linear normalization. A value of 1 means a pure shift.
- Normalized curve value \(x_{\mathrm{norm}}\) variable
-
The curve value after a normalization map has been applied. It is in the same units as the input.
- Normalized mobile water saturation \(S_e\) variable
-
Water saturation rescaled between irreducible water saturation and one minus residual oil saturation: 0 where water stops flowing and 1 where oil stops flowing.
- Normalized porosity \(\phi_z\) variable
-
Pore volume over grain volume, the porosity divided by one minus the porosity. Used with the reservoir quality index to form the flow zone indicator.
- Normalized water saturation \(S_{w,n}\) variable
-
Water saturation rescaled between irreducible water saturation and 1, so that it is 0 at irreducible saturation and 1 at full water saturation.
- Null propagation concept
-
Rule that a result is set to null when a required input is null, instead of being computed from a filled or default value.
- Null sentinel value concept
-
A reserved number, such as -999.25, written in place of a missing or invalid sample. It is a flag, not data, and must be converted to a true missing value before any arithmetic.
- Oil formation volume factor \(B_o\) variable
-
Volume of reservoir oil, with its dissolved gas, needed to produce one stock-tank barrel of oil at surface conditions.
- Oil gravity \(\mathrm{API}\) variable
-
Stock-tank oil density expressed on the API scale, 141.5 divided by the specific gravity minus 131.5. Higher values are lighter oil.
- Oil in place per acre \(N_A\) variable
-
Stock-tank oil originally in place under one acre of the evaluated interval, from net thickness, porosity, water saturation and oil formation volume factor.
- Oil relative permeability \(k_{ro}\) variable
-
Ratio of the effective permeability to oil at the given saturation to the base permeability.
- Oil relative permeability endpoint \(k_{ro}^0\) parameter
-
Relative permeability to oil at irreducible water saturation, relative to the base permeability. Equal to 1 when the base permeability is the effective permeability to oil at Swirr.
- Oil viscosity \(\mu_o\) parameter
-
Dynamic viscosity of the oil at reservoir conditions.
- Oil-water contact \(z_{OWC}\) variable
-
Depth of the highest 100% water saturation seen in a well. It lies above the free water level by the height that the entry (threshold) pressure supports.
- Original gas in place \(G\) variable
-
Surface-condition gas originally in place over a stated area, from the petrophysical per-unit-area volume multiplied by an area.
- Original oil in place \(N\) variable
-
Stock-tank oil originally in place over a stated area. In this section it is the petrophysical per-unit-area volume multiplied by an area, not a reserves estimate.
- Overburden stress \(S_v\) variable
-
Total vertical stress at a depth: the weight of the rock and fluid above it, from integrating bulk density with depth.
- Overburden stress in ppg variable
-
Overburden stress as an equivalent mud weight.
- Parallel conductor model method
-
Model in which a laminated sand-shale sequence conducts like sand and shale layers in parallel for current flowing along the layers, and in series for current flowing across them.
- Percent-range fraction \(f_{\mathrm{n}}\) variable
-
Position of the curve value between the low and high picks, 0 at the low pick and 1 at the high pick. It is not limited to 0 to 1.
- Percentile level \(q\) parameter
-
The percentage of reference-interval samples that lie at or below a pick, between 0 and 100.
- Permeability \(k\) variable
-
Ability of the rock to transmit fluid, as measured or estimated from logs.
- Permeability cutoff \(k_{cut}\) parameter
-
Minimum permeability for rock to count as net reservoir. A zero value switches the permeability test off.
- Permeability model multiplier P \(P\) parameter
-
Empirical multiplier of a log-based permeability equation. Its value, and sometimes its units, depend on the equation, the fluid and the rock type, so it is calibrated to core.
- Permeability model porosity exponent Q \(Q\) parameter
-
Empirical exponent on porosity in a log-based permeability equation. Fitted to core for each rock type.
- Permeability model saturation exponent R \(R\) parameter
-
Empirical exponent on the water saturation term in a log-based permeability equation. Fitted to core for each rock type.
- Permeability, arithmetic mean \(\bar{k}\) variable
-
Arithmetic mean permeability of a lognormal population about the trend. It is larger than the geometric mean, and is the value that applies to flow in parallel layers.
- Permeability, high estimate \(k_{hi}\) variable
-
Permeability 1.28 standard deviations above the regression trend, the high end of the 10th to 90th percentile range of the scatter.
- Permeability, low estimate \(k_{lo}\) variable
-
Permeability 1.28 standard deviations below the regression trend, the low end of the 10th to 90th percentile range of the scatter.
- Permeability, zone 1 parameter
-
Average permeability over the net thickness of the first example interval.
- Permeability, zone 2 parameter
-
Average permeability over the net thickness of the second example interval.
- Permeability-thickness \(kh\) variable
-
Sum over the net intervals of thickness times permeability, the layer-parallel flow capacity of the interval.
- Photoelectric factor \(P_e\) curve
-
Photoelectric absorption index from the density tool. It depends mainly on lithology and is very sensitive to barite mud and to poor pad contact.
- Photoelectric factor \(\mathrm{PE}\) curve
-
Photoelectric absorption index of the formation from the density tool. It depends mainly on lithology.
- Poisson's ratio (dynamic) \(\nu\) variable
-
Dynamic Poisson's ratio from the velocity ratio: (r^2 - 2) / (2 (r^2 - 1)) with r = Vp/Vs.
- Poisson's ratio (static) \(\nu\) variable
-
Poisson's ratio that governs the stress model: static where core is available, and often taken equal to the dynamic value where it is not.
- Poisson's ratio lower bound \(\nu_{\min}\) parameter
-
Poisson's ratio at which the normalised Poisson term of the Rickman brittleness index is one (most brittle). The published value is 0.15.
- Poisson's ratio upper bound \(\nu_{\max}\) parameter
-
Poisson's ratio at which the normalised Poisson term of the Rickman brittleness index is zero (most ductile). The published value is 0.40.
- Pore fluid density \(\rho_f\) parameter
-
Bulk density of the fluid in the part of the formation the density tool reads, normally the mud filtrate in the flushed zone.
- Pore pressure \(P_p\) variable
-
Pressure of the fluid in the pores of the rock, from log-based methods such as Eaton, Bowers and equivalent depth.
- Pore pressure (equivalent mud weight) \(P_{p}^{\mathrm{ppg}}\) variable
-
Pore pressure expressed as an equivalent mud weight, pressure in psi divided by 0.052 times true vertical depth in feet. Normal hydrostatic pressure is about 8.5 to 9 ppg.
- Pore pressure (stress analysis) \(P_{p}\) variable
-
Pressure of the fluid in the pore space at the depth analysed, taken from a pore-pressure model or measurements.
- Pore pressure from resistivity variable
-
Pore pressure computed from the resistivity log.
- Pore pressure from sonic variable
-
Pore pressure computed from the sonic log.
- Pore volume per acre \(PV_A\) variable
-
Reservoir-condition pore volume under one acre, which is the porosity-thickness times the barrels in an acre-foot.
- Pore-volume-weighted water saturation \(\bar{S}_w\) variable
-
One minus the ratio of hydrocarbon pore thickness to porosity-thickness. It weights each interval by its pore volume, not by its thickness.
- Porosity (inversion) \(\phi_{inv}\) variable
-
Fluid volume per unit bulk volume from the three-component inversion example.
- Porosity cutoff \(\phi_{cut}\) parameter
-
Minimum effective porosity for rock to count as net reservoir.
- Porosity standard deviation (inversion) \(\sigma_{\phi}\) variable
-
Standard deviation of the inverted porosity caused by the log errors.
- Porosity, zone 1 parameter
-
Average effective porosity over the net thickness of the first example interval.
- Porosity, zone 2 parameter
-
Average effective porosity over the net thickness of the second example interval.
- Porosity-permeability intercept \(a_\phi\) parameter
-
Intercept of the semi-log regression of log10 permeability on porosity in percent, the value of log10 k at zero porosity.
- Porosity-permeability scatter \(\sigma_k\) parameter
-
Standard deviation of log10 permeability about the regression line. A measure of how well porosity predicts permeability.
- Porosity-permeability slope \(b_\phi\) parameter
-
Slope of the semi-log regression of log10 permeability on porosity, per percent porosity.
- Porosity-thickness \(\Phi H\) variable
-
Sum over the net intervals of thickness times porosity. It is the pore volume per unit area expressed as a length.
- Power conversion coefficient \(c_{E}\) parameter
-
Multiplier of the power-law relation between static and dynamic Young's modulus, with moduli in GPa.
- Power conversion exponent \(p_{E}\) parameter
-
Exponent of the power-law relation between static and dynamic Young's modulus.
- Pressure at the datum \(p_0\) parameter
-
Absolute pressure at zero depth, normally atmospheric pressure.
- Pseudo-critical pressure \(p_{pc}\) variable
-
Mole-fraction-weighted critical pressure of a gas mixture, here estimated from the gas gravity with a correlation.
- Pseudo-critical temperature \(T_{pc}\) variable
-
Mole-fraction-weighted critical temperature of a gas mixture, here estimated from the gas gravity with a correlation. Used in Rankine.
- Pseudo-reduced pressure \(p_{pr}\) variable
-
Absolute pressure divided by the pseudo-critical pressure.
- Pseudo-reduced temperature \(T_{pr}\) variable
-
Absolute temperature divided by the pseudo-critical temperature.
- Pure-shale bulk density \(\rho_{b,sh}\) parameter
-
Bulk density read in pure shale. It is used for the neutron-density shale volume and for removing the shale contribution from the density log.
- Pure-shale neutron porosity \(\phi_{N,sh}\) parameter
-
Neutron porosity read in pure shale, on the same scale as the neutron porosity of the sand.
- Quartz density endmember \(\rho_q\) parameter
-
Density response of pure quartz used as an inversion endmember.
- Quartz U endmember \(U_q\) parameter
-
Volumetric photoelectric cross section of pure quartz.
- Quartz volume \(V_{qz}\) variable
-
Volume fraction of quartz in the rock, from mineral inversion or core.
- Quartz volume (inversion) \(V_q\) variable
-
Volume of quartz per unit bulk volume from the three-component inversion example.
- Quartz volume standard deviation \(\sigma_{V_q}\) variable
-
Standard deviation of the inverted quartz volume caused by the log errors.
- Radioactive feldspar concept
-
Potassium-bearing feldspar that raises the gamma ray without adding clay, which biases gamma-ray-based clay volume high.
- Raymer-Hunt-Gardner exponent \(x_{RHG}\) parameter
-
Exponent in the empirical Raymer-Hunt-Gardner form that relates the matrix-to-measured slowness ratio to porosity.
- Reduced density (Hall-Yarborough) \(Y\) variable
-
Dimensionless reduced density that the Hall-Yarborough equation of state is solved for. It is not a mole fraction.
- Reference high value \(r_{\mathrm{hi}}\) parameter
-
The value the high pick is mapped to. It is the same percentile of the key well, or a fixed target chosen by the analyst.
- Reference low value \(r_{\mathrm{lo}}\) parameter
-
The value the low pick is mapped to. It is the same percentile of the key well, or a fixed target chosen by the analyst.
- Reference middle value \(r_{\mathrm{mid}}\) parameter
-
The value the middle pick is mapped to: the key-well median, or a fixed target value.
- Regression band, high \(\hat{\rho}_{b,\mathrm{hi}}\) variable
-
Approximate upper edge of a 95 percent band around the prediction, using the training residual only.
- Regression band, low \(\hat{\rho}_{b,\mathrm{lo}}\) variable
-
Approximate lower edge of a 95 percent band around the prediction, using the training residual only.
- Regression coefficient, gamma ray \(\beta_1\) parameter
-
Change in the repaired density per unit of gamma ray in a two-predictor regression.
- Regression coefficient, neutron \(\beta_2\) parameter
-
Change in the repaired density per unit of neutron porosity in a two-predictor regression.
- Regression intercept \(\beta_0\) parameter
-
Constant term of a multi-linear regression used to repair a curve.
- Regression residual standard deviation \(\sigma_{\mathrm{fit}}\) parameter
-
Root-mean-square residual of the regression over its training samples, in the units of the repaired curve.
- Regression-predicted density \(\hat{\rho}_b\) variable
-
Bulk density predicted from other curves by the fitted regression. It is a prediction, not a measurement.
- Relative dip \(\theta\) parameter
-
Angle between the tool axis and the normal to the bedding. It is 0 for a vertical well in flat beds and 90 degrees for a well drilled along the bedding.
- Reservoir capillary pressure \(P_{c,res}\) variable
-
Capillary pressure between the hydrocarbon and water phases at reservoir conditions. It equals the buoyancy pressure of the hydrocarbon column above the free water level.
- Reservoir contact angle \(\theta_{res}\) parameter
-
Contact angle, measured through the water, between the reservoir hydrocarbon and brine on the rock surface. A water-wet rock has a small angle.
- Reservoir interfacial tension \(\sigma_{res}\) parameter
-
Interfacial tension between the reservoir hydrocarbon and brine at reservoir conditions. Typical textbook values are about 30 dyne/cm for oil and water and about 50 dyne/cm for gas and water.
- Reservoir pressure \(p\) variable
-
Pressure of the fluid in the pores at reservoir depth, in absolute units. It may be hydrostatic, overpressured, or a measured value.
- Reservoir quality index \(RQI\) variable
-
Square root of permeability over porosity, scaled to micrometres. It tracks the effective pore throat size and is used to separate flow units.
- Residual oil saturation \(S_{or}\) variable
-
Oil saturation left in the rock when displacement has reduced the oil to the point where it no longer flows. Equal to one minus the flushed-zone water saturation when that is used as the source.
- Resistivity anisotropy concept
-
The difference between the resistivity measured along the bedding (horizontal) and across it (vertical), caused by thin laminations.
- Resistivity baseline \(R_{t,base}\) parameter
-
Resistivity read in a thick, organic-lean (non-source) shale where the resistivity and porosity curves are in equilibrium.
- Resistivity compaction rate \(c_r\) parameter
-
Rate at which the resistivity trendline increases with depth, per thousand feet.
- Resistivity index \(I_R\) variable
-
Ratio of the true resistivity of a partly water-saturated rock to the resistivity of the same rock fully saturated with water.
- Reuss (harmonic) mixed shear velocity \(V_{s,R}\) variable
-
Volume-weighted harmonic mean of the lithology-line shear velocities.
- Rock fabric number \(RFN\) variable
-
Lucia's continuous index of carbonate rock fabric, from about 0.5 (grain-dominated, coarse) to 4 (fine mud-dominated). It selects the porosity-saturation-height relation.
- Sampling step parameter
-
The spacing between successive samples of a log. Common values are 0.5 ft, 0.25 ft, 0.1 ft, 0.1524 m and 0.1 m.
- Sand resistivity \(R_{sa}\) variable
-
Resistivity of the sand layers alone, with the laminated shale removed from the horizontal resistivity.
- Sand water saturation \(S_{w,sa}\) variable
-
Water saturation of the sand layers, computed from the sand porosity and the sand resistivity.
- Sand-only density porosity \(\phi_{D,sa}\) variable
-
Density porosity of the sand fraction, after the laminated shale has been removed from the density porosity.
- Sand-only neutron porosity \(\phi_{N,sa}\) variable
-
Neutron porosity of the sand fraction, after the laminated shale has been removed from the neutron porosity.
- Sand-only neutron-density porosity \(\phi_{ND,sa}\) variable
-
Root-mean-square combination of the sand-only density and neutron porosities.
- Sandstone fraction \(X_{ss}\) parameter
-
Volume fraction of sandstone (quartz-rich clastic) used to weight the Greenberg-Castagna lines. The four fractions are normalized to sum to 1.
- Sandstone-line shear velocity \(V_{s,ss}\) variable
-
Shear velocity from the sandstone regression at the measured Vp.
- Saturation exponent \(n\) parameter
-
Exponent relating resistivity index to water saturation in Archie's equation.
- Scaled value \(x_{\mathrm{scale}}\) variable
-
The curve value after a pure multiplicative scale that moves the middle pick onto its reference value.
- Schmoker coefficient A \(A\) parameter
-
Coefficient multiplying the inverse bulk density in Schmoker's linear relation for TOC. It is fitted to core TOC for a formation.
- Schmoker coefficient B \(B\) parameter
-
Constant subtracted in Schmoker's linear relation for TOC. It is fitted to core TOC for a formation.
- Separate-vug fraction \(\phi_{sv}/\phi_t\) variable
-
Separate-vug porosity as a fraction of total porosity.
- Separate-vug porosity \(\phi_{sv}\) variable
-
Porosity of vugs that are connected only through the interparticle pore network, for example moulds and intragrain pores. It adds storage but little flow and no capillary signal.
- Shale fraction \(X_{sh}\) parameter
-
Volume fraction of shale used to weight the lithology lines.
- Shale slowness (compaction) \(\Delta t_{sh}\) parameter
-
Compressional slowness of adjacent shale, used as an indicator of how uncompacted the formation is.
- Shale total porosity \(\phi_{sh}\) parameter
-
Total porosity of 100% clay or shale, which is the volume of clay-bound water in a unit volume of clay. It is the porosity term in the dual-water bound-water saturation.
- Shale volume \(V_{sh}\) variable
-
Volume fraction of shale per unit bulk volume, where shale is the clay plus the silt and bound water it carries.
- Shale volume from gamma ray (linear) \(V_{sh,GR}\) variable
-
Shale volume from the linear gamma ray index between a clean-sand and a shale reading, limited to 0 to 1.
- Shale volume from neutron-density separation \(V_{sh,ND}\) variable
-
Shale volume from the separation of the density porosity and the neutron porosity, scaled by the separation in pure shale.
- Shale volume, average of methods \(V_{sh,avg}\) variable
-
The mean of the ratio, gamma ray and neutron-density shale volumes.
- Shale volume, minimum of methods \(V_{sh,min}\) variable
-
The smallest of the ratio, gamma ray and neutron-density shale volumes.
- Shale-line shear velocity \(V_{s,sh}\) variable
-
Shear velocity from the shale regression at the measured Vp.
- Shaly-sand cementation exponent \(m^{*}\) parameter
-
Cementation exponent m* measured on the same shaly core samples used for Qv in the Waxman-Smits model. It is slightly above the clean-rock m.
- Shear slowness \(\Delta t_{s}\) curve
-
Transit time of the shear wave per unit distance, the reciprocal of the shear velocity. It comes from a dipole sonic log or from a shear-log model.
- Shear slowness \(\Delta t_s\) variable
-
Shear slowness, 304.8 divided by the shear velocity in km/s. Here it is a modeled value for wells without a measured shear log.
- Shear velocity \(V_{s}\) variable
-
Speed of the shear wave, 304.8 divided by the shear slowness in µs/ft.
- Shear velocity \(V_s\) variable
-
Shear velocity in kilometres per second, from a regression on the compressional velocity or measured by a dipole sonic.
- Shear-failure mud pressure \(P_{w}^{sh}\) variable
-
Lowest wellbore pressure at which the wall of a vertical well stays intact in shear. Below it the rock fails in compression and breakouts form.
- Shifted value \(x_{\mathrm{shift}}\) variable
-
The curve value after a pure additive shift that moves the middle pick onto its reference value.
- SHmax, stress-strain model \(S_{H}^{\varepsilon}\) variable
-
Maximum horizontal stress from the poroelastic model with imposed tectonic strains.
- SHmax, strike-slip frictional limit \(S_{H}^{ss}\) variable
-
Upper bound of the maximum horizontal stress set by frictional equilibrium in a strike-slip stress state.
- Shmin, Eaton uniaxial strain \(S_{h}^{E}\) variable
-
Minimum horizontal stress from the uniaxial strain relation with Poisson's ratio and effective stress.
- Shmin, Hubbert-Willis \(S_{h}^{HW}\) variable
-
Minimum horizontal stress from a constant fraction of one third of the effective vertical stress added to the pore pressure.
- Shmin, normal-faulting frictional limit \(S_{h}^{nf}\) variable
-
Lower bound of the minimum horizontal stress set by frictional equilibrium in a normal-faulting stress state.
- Shmin, poroelastic (Eaton-Biot) \(S_{h}^{B}\) variable
-
Minimum horizontal stress from the uniaxial strain relation with a Biot coefficient in the effective stress.
- Shmin, stress-strain model \(S_{h}^{\varepsilon}\) variable
-
Minimum horizontal stress from the poroelastic model with imposed tectonic strains.
- Silhouette width \(s\) concept
-
Measure of how well a sample fits its own cluster compared with the nearest other cluster, from -1 to 1. Its mean over all samples is used to compare numbers of clusters.
- Simandoux water saturation \(S_{w,Sim}\) variable
-
Water saturation from the original Simandoux equation, which is used as a reference against the modified form.
- Slowness at first clean point \(\Delta t_A\) parameter
-
Compressional slowness of the first of two points that define the clean-rock line on a crossplot, usually the matrix end.
- Slowness at second clean point \(\Delta t_B\) parameter
-
Compressional slowness of the second point on the clean-rock line, at a higher porosity than the first.
- Slowness on the loading curve variable
-
Loading-curve velocity expressed as slowness.
- Slowness on the unloading curve variable
-
Unloading-curve velocity expressed as slowness.
- Solution gas-oil ratio \(R_s\) variable
-
Standard cubic feet of gas dissolved in one stock-tank barrel of oil at the stated pressure and temperature.
- Solution gas-oil ratio at bubble point \(R_{sb}\) parameter
-
Solution gas-oil ratio of a saturated oil at its bubble point. It is the total gas dissolved in the oil and is normally the flash gas-oil ratio of a PVT report.
- Sonic baseline \(\Delta t_{base}\) parameter
-
Compressional slowness read in the same organic-lean shale used for the resistivity baseline.
- Sonic compaction rate \(c_s\) parameter
-
Rate at which the sonic trendline approaches its deep value, per thousand feet of depth.
- Sonic log uncertainty \(\sigma_{\Delta t}\) parameter
-
Standard deviation of the error assigned to the compressional slowness in an inversion.
- Sonic porosity \(\phi_s\) variable
-
Porosity from the sonic log by the time-average relation between slowness, matrix slowness and fluid slowness.
- Sonic porosity (Raymer-Hunt-Gardner) \(\phi_{RHG}\) variable
-
Porosity from the compressional slowness by the Raymer-Hunt-Gardner transform.
- Sonic porosity with compaction factor \(\phi_{S,c}\) variable
-
Wyllie time-average sonic porosity divided by the compaction factor.
- Sonic residual \(r_{\Delta t}\) variable
-
Measured compressional slowness minus the slowness predicted by the inversion model.
- Source high pick \(p_{\mathrm{hi}}\) parameter
-
A high percentile of the curve in the reference interval of the well being normalized, for example the 95th percentile.
- Source low pick \(p_{\mathrm{lo}}\) parameter
-
A low percentile of the curve in the reference interval of the well being normalized, for example the 5th percentile.
- Source middle pick \(p_{\mathrm{mid}}\) parameter
-
A middle percentile (usually the 50th) of the curve in the reference interval of the well being normalized.
- Spontaneous potential \(\mathrm{SP}\) curve
-
Naturally occurring potential difference measured between a borehole electrode and a surface reference, driven mainly by the salinity contrast between mud filtrate and formation water.
- Static spontaneous potential \(\mathrm{SSP}\) variable
-
Maximum SP deflection between a thick clean sand and an adjacent shale, corrected for bed thickness and invasion.
- Static to dynamic ratio \(E_{s}/E_{d}\) variable
-
Ratio of the linear-conversion static modulus to the dynamic modulus. It is below 1 for most rocks, and closer to 1 for stiff, low-porosity rock.
- Static Young's modulus \(E_{s}\) variable
-
Young's modulus measured in slow quasi-static loading of core, or a log-derived value calibrated to it. It is the modulus that governs deformation under drilling and fracturing stress changes.
- Static Young's modulus (linear conversion) \(E_{s,\mathrm{lin}}\) variable
-
Static Young's modulus estimated from the dynamic value with a linear relation, floored at zero.
- Static Young's modulus (power conversion) \(E_{s,\mathrm{pow}}\) variable
-
Static Young's modulus estimated from the dynamic value with a power-law relation.
- Stieber relation method
-
Nonlinear transform of the gamma ray index that reduces the clay volume estimated in lower-clay intervals, after Stieber.
- Stress regime index \(R_{s}\) variable
-
Index of the Anderson stress regime from the ordering of the three stresses: 1 normal, 2 strike-slip, 3 reverse.
- Structural shale concept
-
Shale present as grains or clasts that take the place of sand grains in the framework. The intergranular pore space of the sand is kept and the shale grains add their own porosity.
- Structural shale volume \(V_{str}\) variable
-
Volume of structural shale, as a fraction of bulk volume.
- Surface temperature \(T_0\) parameter
-
Mean ground or sea-bed temperature at the top of the temperature profile.
- Swirr-permeability coefficient a \(a_k\) parameter
-
Multiplier of the power law between irreducible water saturation and permeability, fitted to core capillary pressure or relative permeability data.
- Swirr-permeability exponent b \(b_k\) parameter
-
Exponent of the power law between irreducible water saturation and permeability. Negative, because Swirr falls as permeability rises.
- Swirr-RQI coefficient a \(a_{q}\) parameter
-
Multiplier of the power law between irreducible water saturation and reservoir quality index, fitted to core.
- Swirr-RQI exponent b \(b_{q}\) parameter
-
Exponent of the power law between irreducible water saturation and reservoir quality index. Negative, because Swirr falls as quality rises.
- T2 cutoff \(T_{2,cut}\) parameter
-
Relaxation time that separates the bound-fluid part of an NMR T2 distribution from the free-fluid part. Set by core NMR on saturated and centrifuged plugs.
- T2 distribution width \(\sigma_{T2}\) parameter
-
Standard deviation of ln(T2) for a single-mode lognormal T2 distribution. Used here to model the distribution in the calculator.
- T2 logarithmic mean \(T_{2,lm}\) variable
-
Geometric mean of the T2 distribution, the central relaxation time of a single-mode distribution.
- Tectonic strain, maximum horizontal \(\varepsilon_{H}\) parameter
-
Imposed tectonic strain along the maximum horizontal stress direction in the stress-strain model, in microstrain. It is calibrated, not measured.
- Tectonic strain, minimum horizontal \(\varepsilon_{h}\) parameter
-
Imposed tectonic strain along the minimum horizontal stress direction in the stress-strain model, in microstrain. It is calibrated, not measured.
- Tensile strength \(T_{0}\) parameter
-
Tensile stress at which the rock fails in tension. It is small, often a tenth of UCS or less, and is lower still for rock with natural fractures.
- Terzaghi effective stress concept
-
The principle that the vertical stress carried by the rock framework is the total vertical stress minus the pore pressure. Rock properties such as velocity and resistivity respond to this effective stress, which is why they can be used to estimate pore pressure.
- Thickness-weighted permeability \(\bar{k}\) variable
-
Permeability-thickness divided by total net thickness: the arithmetic average appropriate for flow parallel to the layers.
- Thickness-weighted porosity \(\bar{\phi}\) variable
-
Porosity-thickness divided by total net thickness.
- Thomas-Stieber model method
-
A framework that relates total porosity to shale volume for sand with laminated, structural or dispersed shale, and so identifies how the shale is distributed.
- Three-point normalized value \(x_{\mathrm{pw}}\) variable
-
The curve value after a piecewise linear map through the low, middle and high picks.
- Threshold (entry) pressure \(P_{c,e}\) parameter
-
Capillary pressure that the hydrocarbon must exceed to enter the largest connected pore throats and start displacing water. It sets the gap between the free water level and the oil-water contact.
- TOC estimate 1 \(\mathrm{TOC}_1\) variable
-
First of several independent TOC estimates.
- TOC estimate 2 \(\mathrm{TOC}_2\) variable
-
Second of several independent TOC estimates.
- TOC estimate 3 \(\mathrm{TOC}_3\) variable
-
Third of several independent TOC estimates.
- TOC scaling factor parameter
-
Multiplier applied to a computed TOC curve to match core TOC.
- TOC shift parameter
-
Constant added to a computed TOC curve to match core TOC.
- Tortuosity factor \(a\) parameter
-
Empirical constant in the Archie relations that scales the formation factor. It is 1 in the simple form and about 0.6 to 0.9 in published sandstone fits.
- Total net thickness \(H\) variable
-
Sum of the net thicknesses of the intervals included in the sum.
- Total organic carbon \(\mathrm{TOC}\) variable
-
Mass fraction of organic carbon in the rock.
- Total porosity \(\phi_t\) variable
-
Total pore volume per unit bulk volume, including clay-bound water.
- Total porosity, dispersed shale \(\phi_{t,disp}\) variable
-
Total porosity of a sand whose pores are partly filled with shale. It falls as shale volume rises, until the pores are full.
- Total porosity, laminated shale \(\phi_{t,lam}\) variable
-
Total porosity of a sand with laminated shale: the straight line from clean sand to pure shale.
- Total porosity, structural shale \(\phi_{t,str}\) variable
-
Total porosity of a sand in which shale grains replace sand grains and the sand pore space is kept.
- Total water saturation \(S_{w,t}\) variable
-
Water volume, including clay-bound water, as a fraction of the total pore volume.
- Triangulation weight \(w_c\) variable
-
Barycentric weight of a log sample toward the structural or dispersed vertex of the Thomas-Stieber triangle. It is the fraction of the sample that behaves like that end member, from 0 to 1.
- True formation resistivity \(R_t\) curve
-
Resistivity of the uninvaded formation, taken from the deepest-reading resistivity curve after any needed corrections.
- True vertical depth \(z\) variable
-
Vertical depth below the reference elevation.
- True vertical depth subsea \(z_{SS}\) variable
-
True vertical depth measured from mean sea level, positive downward. It equals true vertical depth below the depth reference minus the elevation of that reference above sea level.
- U uncertainty \(\sigma_U\) parameter
-
Standard deviation of the error assigned to the volumetric photoelectric cross section in an inversion.
- UCS from porosity (clean sandstone) \(\mathrm{UCS}_{V}\) variable
-
UCS estimated with a porosity relation derived for clean, well-consolidated sandstones of porosity below about 0.3.
- UCS from sonic (sandstone, exponential) \(\mathrm{UCS}_{McN}\) variable
-
UCS estimated with an exponential relation of compressional slowness derived for Australian coal-measure sandstones.
- UCS from sonic (shale, power law) \(\mathrm{UCS}_{H}\) variable
-
UCS estimated with a power-law relation of compressional slowness derived for North Sea Tertiary shales.
- UCS from static modulus (sandstone) \(\mathrm{UCS}_{B}\) variable
-
UCS estimated with a linear relation of static Young's modulus for sandstones.
- Unconfined compressive strength \(\mathrm{UCS}\) variable
-
Axial stress at which a cylindrical sample fails with no confining stress. Log estimates are empirical and must be calibrated to core tests.
- Undersaturated oil compressibility \(c_o\) variable
-
Fractional volume change of an undersaturated oil per unit pressure change, at constant temperature.
- Unloading concept
-
A decrease in vertical effective stress after the rock has reached a higher maximum, caused by fluid expansion such as gas generation or clay diagenesis, or by lateral transfer of pressure. Velocity and density do not retrace the loading trend.
- Unloading switch parameter
-
0 uses the virgin (loading) curve everywhere. 1 uses the unloading curve where the velocity is below the velocity at onset.
- Upper mud weight limit \(MW_{\max}\) variable
-
Smaller of the minimum horizontal stress and the breakdown mud weight.
- Variable cementation exponent \(m_{v}\) variable
-
Cementation exponent that changes with porosity, in the Shell sandstone form. It is higher in tight rock and flattens toward 1.9 in porous clean sand.
- Velocity at onset of unloading \(V_{max}\) parameter
-
Velocity on the virgin curve at the point where the observed velocity leaves the loading trend, which marks the maximum effective stress reached.
- Velocity on the loading curve \(V_{load}\) variable
-
Velocity predicted by the virgin curve at a given effective stress.
- Velocity on the unloading curve \(V_{unl}\) variable
-
Velocity predicted by the unloading curve at a given effective stress, for rock that has previously been at a higher stress.
- Vertical effective stress \(\sigma'\) variable
-
Vertical stress carried by the rock framework: the overburden stress minus the pore pressure (Terzaghi).
- Vertical resistivity \(R_v\) variable
-
Resistivity measured with current flowing across the bedding. In a laminated sequence it is higher than the horizontal resistivity.
- Vertical shale resistivity \(R_{sh,v}\) parameter
-
Resistivity of the pure shale for current flowing across the bedding. It is equal to or higher than the horizontal shale resistivity.
- Vertical stress \(S_{v}\) variable
-
Total stress from the weight of the overlying rock and water, the integral of bulk density times gravity over depth.
- Vertical stress (equivalent mud weight) \(S_{v}^{\mathrm{ppg}}\) variable
-
Vertical stress expressed as an equivalent mud weight, the stress in psi divided by 0.052 times true vertical depth in feet.
- Vitrinite reflectance \(R_o\) parameter
-
Reflectance of vitrinite particles under oil immersion, a standard measure of thermal maturity measured on core or cuttings.
- Voigt (arithmetic) mixed shear velocity \(V_{s,V}\) variable
-
Volume-weighted arithmetic mean of the lithology-line shear velocities.
- Volumetric photoelectric cross section \(U\) curve
-
Photoelectric factor times bulk density. Unlike PE it mixes linearly with volume fractions.
- Vp/Vs ratio \(V_{p}/V_{s}\) variable
-
Ratio of compressional to shear velocity. It equals the ratio of shear to compressional slowness and is about 1.5 in clean carbonate and 1.6 to 2.2 in shale and sandstone.
- Vp/Vs ratio \(V_p/V_s\) variable
-
Ratio of compressional to shear velocity. A quality check on a modeled shear log.
- Water depth \(h_{w}\) parameter
-
Depth of the sea floor below sea level for an offshore well, zero on land.
- Water relative permeability \(k_{rw}\) variable
-
Ratio of the effective permeability to water at the given saturation to the base permeability.
- Water relative permeability endpoint \(k_{rw}^0\) parameter
-
Relative permeability to water at residual oil saturation, relative to the base permeability. Usually well below 1 in water-wet rock.
- Water resistivity at 77 °F \(R_{w,77}\) variable
-
Water resistivity at the 77 °F reference temperature, from salinity.
- Water salinity \(C_{w}\) parameter
-
Dissolved salt content of formation or filtrate water, expressed as sodium chloride equivalent.
- Water saturation \(S_w\) variable
-
Fraction of the pore volume filled with water.
- Water saturation cutoff \(S_{w,cut}\) parameter
-
Maximum water saturation for net reservoir to count as net pay.
- Water saturation from height-BVW power law \(S_{w,h}\) variable
-
Water saturation at a height above the free water level, from a power law of bulk volume water on height divided by total porosity.
- Water saturation, zone 1 parameter
-
Average water saturation over the net thickness of the first example interval.
- Water saturation, zone 2 parameter
-
Average water saturation over the net thickness of the second example interval.
- Water viscosity \(\mu_w\) parameter
-
Dynamic viscosity of the formation water at reservoir conditions.
- Water-saturated resistivity \(R_0\) variable
-
Resistivity the rock would have if its pore space were fully filled with formation water. It is the water-line value used in the Pickett plot.
- Wellbore (mud) pressure \(P_{w}\) parameter
-
Pressure the drilling fluid exerts on the wellbore wall at the depth analysed, the mud weight times 0.052 times true vertical depth.
- Winland R35 \(R_{35}\) variable
-
Pore throat radius at 35% mercury saturation, estimated from porosity and permeability with the Winland regression. Used to classify pore throat size and rock quality.
- Within published range variable
-
1 if the compressional velocity is inside the range for which the regression was published, 0 otherwise.
- Young's modulus lower bound \(E_{\min}\) parameter
-
Modulus at which the normalised Young's modulus term of the Rickman brittleness index is zero. The published value is 1 million psi.
- Young's modulus upper bound \(E_{\max}\) parameter
-
Modulus at which the normalised Young's modulus term of the Rickman brittleness index is one. The published value is 8 million psi.
- ΔlogR separation \(\Delta\!\log R\) variable
-
Separation between the resistivity curve and a scaled porosity curve, in logarithmic resistivity units, relative to a non-source baseline. It is the quantity that Passey's method converts to TOC.