We present a robust method to estimate the properties of kerogen necessary for petrophysical interpretations of organic shales based on a single estimate of thermal maturity. This "thermal maturity-adjusted log interpretation" replaces unknown or default kerogen properties with automated and optimized values for the organic shale of interest. The method is analogous to well-known industry workflows for "matrix-adjusted logging" in conventional reservoir rocks in which default matrix properties are replaced with values determined specifically for the formation of interest.
The method is founded on the study of more than 50 kerogens with thermal maturity ranging from immature to dry gas (vitrinite reflectance range from 0.5% to 4% Ro) and representing eight major oil- and gas-producing sedimentary basins from three continents, and ∼ 300 million years of depositional age. The determined kerogen properties include measured chemical (C, H, N, S, O) composition and skeletal (grain) density, as well as calculated nuclear properties such as apparent log density, hydrogen index, thermal and epithermal neutron porosities, macroscopic thermal-neutron capture cross section, macroscopic fast-neutron elastic scattering cross section, and photoelectric factor. For kerogens relevant to the petroleum industry (i.e., type II kerogen with thermal maturity ranging from early-oil window to dry-gas window), it is demonstrated that petrophysical properties are controlled mainly by thermal maturity, with no observable differences between sedimentary basins/formations. As a result, "universal curves" are established relating kerogen properties to thermal maturity, and the curves apply equally well in all the prolific basins studied here.
These universal curves enable robust and accurate estimation of kerogen properties in organic shale from knowledge or measurement only of thermal maturity. The benefit of thermal maturity-adjusted log interpretation of organic shales globally is shown by sensitivity analyses and field examples, enabling more accurate and confident interpretation of porosity, saturation, and hydrocarbon-in-place.
The petrophysical interpretation of downhole logs requires accurate knowledge of matrix properties, commonly referred to as "matrix adjustments". In organic-rich mudrocks (also termed unconventional petroleum source rocks or shales), the presence of abundant kerogen (solid, insoluble organic matter) has a disproportionate impact on matrix properties because kerogen is compositionally distinct from all inorganic minerals that comprise the remainder of the solid matrix. As a consequence, matrix properties can be highly sensitive to kerogen properties. Moreover, the responses of many downhole logs to kerogen are similar to the responses to fluid (Passey et al., 1990). As a result, the relevant petrophysical properties of kerogen must be accurately known to separate the tool response to kerogen (in the matrix volume) from fluid responses (in the pore volume). Just as small changes in fluid content (porosity) and fluid properties (oil vs. gas) yield large changes in log responses, small changes in kerogen content (kerogen has properties similar to fluids) and kerogen properties (kerogen properties vary by amount similar to difference in oil vs. gas properties) yield large changes in log responses. Unfortunately, relevant petrophysical properties of kerogen are poorly known in general, nearly always unknown in the formation of interest, and otherwise impractical or impossible to measure.