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

Organic geochemistry, especially when undertaken on a well by well or outcr op by outcrop basis, arguably lacks statistically significant numbers of an alyses to establish reliable trends. Can a reliable vitrinite reflectance v s. depth trend be established from (say) 12 analysed samples? When the numb er of analysed samples are in the hundreds or even thousands, much clearer general trends are established and anomalous measurements more readily reco gnised. Interpretation of anomalies is itself highly instructive, but forms another paper. In this paper, a global data set of some 5000 source rock s amples is assembled from the open literature and released data (e.g. open f ile reports) and the results discussed to produce practical oil-industry in terpretations from basic screening data. A complex con elusion can be extra cted from simple data sets and are supported by mass balance and kinetic th eory. 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. T otal organic carbon (TOC) values are generally log-normally distributed so that using mean +/- standard deviation values will under-estimate the upsid e potential when fed into risked (probabilistic) hydrocarbon charge predict ion models. Kerogen type is initially best-determined using plots of Rock-E val S-2 vs. TOC rather than the hydrogen index (HI = S-2/TOC) vs. oxygen in dex (OI = S-3/TOC) or "pseudo-van Krevelen" plot. The intercept of the St v s. TOC plot predicts the average dead carbon (inertinite) content of the so urce rock set, with the slope corresponding to the hydrogen index of the la bile component (HIL). Maturity trends of the hydrogen index (vs. T-max or % R-o) show reversals at high maturities as supported by the kinetics of the thermal breakdown of the three standard kerogen types. At elevated maturiti es, Type III kerogens have higher hydrogen indices than Types I and II kero gens, a feature that must be taken into account when correcting mature kero gens to their original HI values (HIo). The reduction in hydrogen index is a measure of generation with the corresponding increase in production index (PI = S-1/[S-1 + S-2]) being effectively a measure of hydrocarbon retentio n. Across-plot of HI vs. PI can be interpreted in terms of expulsion effici encies (expelled = generated minus retained hydrocarbon). Measured data sup port a pore saturation and/or generation pressure-driven process for expuls ion and indicate a limiting pore saturation equivalent to a production inde x of 0.6 for rich (>2%TOC) source rocks. (C) 1998 Elsevier Science Ltd. All rights reserved.