Evaluation of the compositions of lacustrine shale oil in China’s typical basins and its implications

Shale oil composition serves as both a basis for revealing the shale oil enrichment mechanism and an essential parameter used to explore the interactions among oil, water, and rocks in the pores. We investigate the shale oil reservoir of pure shale type in the 1^st member in the Qingshankou Formation in the Gulong Sag, Songliao Basin; the shale oil reservoir of transitional type in the Chunshang interval of the upper sub-member of the 4^th member of the Shahejie Formation in the Dongying Sag, Jiyang Depression, Bohai Bay Basin; and the shale oil reservoir of pure shale type in the 3^rd sub-member of the 7^th member of the Yanchang Formation, Ordos Basin. Shale samples taken bypressure-retained coring and conventional coring, as well as oil produced from the three shale intervals and the products of autoclave-based thermal simulation experiment, are subjected to composition analysis. The composition of shale oil of diverse types and with varying maturity is characterized through chromatography to determine the total petroleum hydrocarbons (TPH) and pyrolysis-gas chromatography (PY-GC). The methods for deriving shale oil compositions are comprehensively summarized and compared in terms of result, and the factors affecting the composition after evaporative loss are discussed. The assessment scheme is proposed at last. Consequently, we identify the compositional differences for the produced oil, thermally desorbed hydrocarbons, shale extracts, and products from thermal simulation experiment, as well as clarify the limitations of the above-mentioned evaluation methods. Additionally, the phenomenon that shale intervals with high total organic carbon (TOC) content tend to be of high oil content is illustrated, as revealed in previous studies. However, these intervals of high oil content do not necessarily reflect a high ratio of mobile to total oil volume. Shale maturity directly determines the composition of shale oil, while the abundance of organic matter and pore structures exert certain effects on the composition of residual hydrocarbons in shales. As indicated by the results of this study, it is necessary to consider hydrocarbon evaporativeloss in evaluating oil content in shales and exploring fluid occurrence state and shale oil enrichment mechanism, especially for shales of medium to high maturity. The composition evaluation of shale oil at varying maturity can provide new insights for revealing the fluid occurrence characteristics in shale nanopores.