Characterization of organic-rich shales for petroleum exploration & exploitation: A review-Part 2: Geochemistry, thermal maturity, isotopes and biomarkers

Abstract

As shale exploitation is still in its infancy outside North America much research effort is being channelled into various aspects of geochemical characterization of shales to identify the most prospective basins, formations and map their petroleum generation capabilities across local, regional and basin-wide scales. The measurement of total organic carbon, distinguishing and categorizing the kerogen types in terms oil-prone versus gas-prone, and using vitrinite reflectance and Rock-Eval data to estimate thermal maturity are standard practice in the industry and applied to samples from most wellbores drilled. It is the trends of stable isotopes ratios, particularly those of carbon, the wetness ratio (C₁/Σ(C₂+C₃)), and certain chemical biomarkers that have proved to be most informative about the status of shales as a petroleum system. These data make it possible to identify production “sweet-spots”, discriminate oil-, gas-liquid- and gas-prone shales from kerogen compositions and thermal maturities. Rollovers and reversals of ethane and propane carbon isotope ratios are particularly indicative of high thermal maturity exposure of an organic-rich shale. Comparisons of hopane, strerane and terpane biomarkers with vitrinite reflectance (Ro) measurements of thermal maturity highlight discrepancies suggesting that Ro is not always a reliable indicator of thermal maturity. Major and trace element inorganic geochemistry data and ratios provides useful information regarding provenance, paleoenvironments, and stratigraphic-layer discrimination. This review considers the data measurement, analysis and interpretation of techniques associated with kerogen typing, thermal maturity, stable and non-stable isotopic ratios for rocks and gases derived from them, production sweet-spot identification, geochemical biomarkers and inorganic chemical indicators. It also highlights uncertainties and discrepancies observed in their practical application, and the numerous outstanding questions associated with them.

Keywords

kerogen type / shale organic lithofacies / shale thermal maturity / shale isotopes / shale biomarkers / shale trace elements

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