Kerogen Volume and Maturity

Summary

TOC is a weight fraction of carbon, but porosity and inversion work in volumes. This page converts Total organic carbon to Kerogen volume, and relates the thermal maturity measured as Vitrinite reflectance to the Level of organic maturity used by the Passey method. Use it whenever TOC feeds a volumetric calculation.

Inputs and outputs

Equations

The kerogen weight fraction is the TOC divided by the carbon fraction of kerogen:

The kerogen volume follows from the densities of kerogen and the remaining matrix:

Maturity can be converted from vitrinite reflectance to LOM with a rational fit:

Single-value calculator

Behavior

Kerogen volume is close to proportional to TOC over the usual range. Kerogen is about half as dense as the matrix, so the volume fraction is about twice the weight fraction: 5 wt% TOC at a carbon fraction of 0.8 is about 12.5% of the rock by volume. A higher carbon fraction means less kerogen is needed to supply the same carbon, so the volume is smaller.

Parameter guidance

Kerogen carbon fraction is the weight fraction of carbon in kerogen, typically 0.6 to 0.9 and rising with maturity. Kerogen density is typically 1.1 to 1.4 g/cm³ and also rises with maturity. Matrix density is the grain density of the non-kerogen solids. Ideally all three come from core or from a regional study. Use one set of values across a project, so that the kerogen volume agrees with the density-based TOC calculation. Vitrinite reflectance comes from vitrinite reflectance measured on core or cuttings.

Worked example

A TOC of 5 wt% with a carbon fraction of 0.8, a kerogen density of 1.26 g/cm³ and a matrix density of 2.7 g/cm³, followed by the vitrinite reflectance to LOM fit:

toc, f_c, rho_k, rho_ma = 5.0, 0.8, 1.26, 2.7
w_k = toc / (100 * f_c)
a = w_k / rho_k
v_k = a / (a + (1 - w_k) / rho_ma)
print(f"Wk = {toc:g} / (100 x {f_c:g}) = {w_k:.4f}")
print(f"Vk = {v_k:.4f}  ({100 * v_k:.1f} vol%)")
print()
def lom(ro):
    num = -8.3180734 + 53.367852 * ro - 45.319273 * ro**2 + 13.422786 * ro**3
    den = 1 + 1.0895416 * ro - 1.4275785 * ro**2 + 0.54231042 * ro**3
    return num / den
print(f"{'Ro (%)':>7} {'LOM':>6}")
for ro in (0.5, 0.7, 0.9, 1.1, 1.3, 1.6, 2.0):
    print(f"{ro:7.1f} {lom(ro):6.2f}")

Output

Wk = 5 / (100 x 0.8) = 0.0625
Vk = 0.1250  (12.5 vol%)

 Ro (%)    LOM
    0.5   6.94
    0.7   9.16
    0.9  10.49
    1.1  11.25
    1.3  11.68
    1.6  12.24
    2.0  13.57

Assumptions and limitations

• The kerogen and the rest of the rock are two components with constant densities. In practice kerogen density changes with maturity and organic type.
• TOC is expressed as a weight percent of the dry rock, and the carbon fraction applies to the kerogen present.
• TOC is only organic carbon from kerogen. Bitumen and retained hydrocarbons carry carbon that is not in kerogen.
• The rational fit from reflectance to LOM is only valid for the range of reflectance it was fitted over.

QC checks

• Kerogen volume is zero where TOC is zero, and is about twice the TOC weight fraction in the usual range.
• The kerogen volume agrees with the one used in the porosity and inversion calculations. Mismatched densities between steps are a common error.
• LOM is between about 6 and 14 across the oil and gas windows. Values far outside this range suggest a units or conversion error.
• LOM increases monotonically with reflectance over the range used.

Going Deeper

There are several ways to convert weight to volume. If the bulk density is known, the volume fraction of kerogen is the weight fraction multiplied by the ratio of bulk density to kerogen density, which does not need the matrix density, but needs a bulk density that is itself affected by the kerogen. The mixture form used here avoids that circularity. Different vendors use different factors in their conversions, so a TOC to kerogen volume conversion is one of the first things to check when comparing results between software. LOM is a scale developed to relate maturity to the response of resistivity and porosity logs, and is only loosely tied to reflectance.

References

1. Hood, A., Gutjahr, C.C.M. and Heacock, R.L., 1975. Organic metamorphism and the generation of petroleum. AAPG Bulletin, 59(6), 986–996.

Python reference implementation

Related methods

• TOC from Passey ΔlogR
• TOC from Schmoker
• TOC from Vernik
• TOC from Faust
• Combining TOC Estimates