Geochemical screening of source rocks and reservoirs: The importance of using the proper analytical program

Abstract

Geochemical screening is routinely integrated into larger exploration (and sometimes development) programs that also include assessments of the geological setting, petrophysics, mechanical properties of the rock, etc. The Rock-Eval analytical equipment and its classical Basic/Bulk-Rock method have been developed mainly to characterize potential source-rock intervals in petroleum systems. However, with the increasing interest in unconventional plays, it has been recently demonstrated that the use of modified pyrolysis-temperature regimes improves the quantification of hydrocarbons still present in oil-impregnated samples. In spite of their availability, the use of such modified pyrolysis-temperature regimes still remains scarce among users of pyrolysis data (e.g., exploration geologists and geochemists, reservoir engineers, petrophysicists, and other geoscientists).Several cases were selected to portray how different analytical programs are necessary to obtain less biased and more accurate answers to critical questions during prospect and play evaluations and appraisals. Samples originating from conventional and unconventional plays in the Greater Permian Basin of West Texas (Wolfcamp & Spraberry formations), the DJ Basin in Colorado (Niobrara Formation), the Williston Basin (Lower Bakken Shale), and source-rock reservoirs in the Middle East were analysed each using three known different pyrolysis methods, namely the Institut Franҫais du Pétrole's “Basic/Bulk-Rock”, “Reservoir”, and “Shale Play”. The Shale Play and Reservoir pyrolysis methods yield oil-in-place estimates 20–42% higher than those yielded by the Basic/Bulk-Rock method on the same sample (e.g., for the Niobrara Formation – 87bbl oil/ac-ft Bulk method, 118bbl oil/ac-ft Reservoir method, 119bbl/ac-ft Shale method; for the Lower Bakken – 194bbl oil/ac-ft Bulk method, 246bbl oil/ac-ft Shale method). In addition, a mature, source-rock interval believed to contain gas-prone organic matter (Type III) based solely on TOC and pyrolysis data, was re-interpreted as composed mainly of amorphous oil-prone kerogen, following a multi-component study (which included transmitted and reflected-light organic petrography). These results present unequivocal evidence that underestimating the importance of selecting the proper analytical program can change interpretations dramatically.