Oil content evaluation of lacustrine organic-rich shale with strong heterogeneity: A case study of the Middle Permian Lucaogou Formation in Jimusaer Sag, Junggar Basin, NW China

Abstract

Large shale oil resources were recently been found in lacustrine organic-rich shale with strong heterogeneity, however, few studies have been conducted to examine their oil contents, resulting in considerable risks in lacustrine shale oil exploration. The Middle Permian Lucaogou Formation (P2l) shale in Jimusaer Sag is a typical lacustrine deposit with strong heterogeneity, and its shale oil resource shows great potential. Integrated geochemical characterization of 265 core samples were conducted and results show that the P2l shale developed in an anoxic lacustrine with stratified salty water and the organic matter in the upper sub-member shale of the P2l is more oil-prone than that in the lower sub-member shale of the P2l. The hydrocarbon generation potential of the P2l shale decreases with the kerogen types changing from Type I to Type III, however, the residual hydrocarbon contents of the P2l shale increases from Type I to II1 kerogen, and then decreases from Type II1 to III kerogen, this is mainly due to differentiated hydrocarbon expulsion efficiencies among different kerogen types. Based on S1 and TOC values and the S1/TOC ratios, considering the oil enrichment degree, this study classified the shale oil resources in the P2l shale into four categories: enriched, moderately enriched, less efficient, and invalid resources. The enriched and moderately enriched resources are mainly shales with Type II1 kerogen, followed by Type II2 kerogen, and the middle interval of the Lower P2l shale is the most favourable target for further shale oil exploration. The improved evaluation criteria are applicable for evaluating shale oil plays with strong heterogeneity qualitatively and quantitatively in terrestrial lacustrine basins in other parts of the world.