Geochemical truths in large data sets. I: Geochemical screening data

Organic geochemistry, especially when undertaken on a well by well or outcrop by outcrop basis, arguably lacks statistically significant numbers of analyses to establish reliable trends. Can a reliable vitrinite reflectance vs. depth trend be established from (say) 12 analysed samples? When the number of analysed samples are in the hundreds or even thousands, much clearer general trends are established and anomalous measurements more readily recognised. Interpretation of anomalies is itself highly instructive, but forms another paper. In this paper, a global data set of some 5000 source rock samples is assembled from the open literature and released data (e.g. open file reports) and the results discussed to produce practical oil-industry interpretations from basic screening data. A complex conclusion can be extracted from simple data sets and are supported by mass balance and kinetic theory. It is concluded that interpretation always falls back on a comparison of new data with pre-existing knowledge in the form of tables and plots. Total organic carbon (TOC) values are generally log-normally distributed so that using mean ± standard deviation values will under-estimate the upside potential when fed into risked (probabilistic) hydrocarbon charge prediction models. Kerogen type is initially best-determined using plots of Rock-Eval S2 vs. TOC rather than the hydrogen index (HI = S2/TOC) vs. oxygen index (OI = S3/TOC) or “pseudo-van Krevelen” plot. The intercept of the S2 vs. TOC plot predicts the average dead carbon (inertinite) content of the source rock set, with the slope corresponding to the hydrogen index of the labile component (HIL). Maturity trends of the hydrogen index (vs. Tmax or %Ro) show reversals at high maturities as supported by the kinetics of the thermal breakdown of the three standard kerogen types. At elevated maturities, Type III kerogens have higher hydrogen indices than Types I and II kerogens, a feature that must be taken into account when correcting mature kerogens to their original HI values (HIo). The reduction in hydrogen index is a measure of generation with the corresponding increase in production index (PI = S1/[S1 + S2]) being effectively a measure of hydrocarbon retention. A cross-plot of HI vs. PI can be interpreted in terms of expulsion efficiencies (expelled = generated minus retained hydrocarbon). Measured data support a pore saturation and/or generation pressure-driven process for expulsion and indicate a limiting pore saturation equivalent to a production index of 0.6 for rich (> 2%TOC) source rocks.