Exploration progress and geochemical features of lacustrine shale oils in China

With the consumption of conventional petroleum resources, the exploration and development (E&P) of unconventional hydrocarbons in recent years have supplemented the shortage of petroleum resources to some extent. Shale gas has been successfully exploited from marine strata in southern China; however, E&P of lacustrine shale oil resources with large resource potentials have not reached expectations. This paper reviews the progress of exploration for lacustrine shale oil in China and discusses the geochemical features of shale and shale oil from a key shale oil exploration area in the Jiyang Depression to accelerate lacustrine shale oil exploration and to provide an improved understanding of prospective shale oil locations. For the first time, shale oil exploration, shale geochemistry, and geological features for nine major lacustrine sedimentary basins in China are summarized explicitly, and a detailed comparison analysis with Bakken, Eagle Ford, and Permian shales is conducted. The geochemical data (more than 600 analytical data points) and shale oil geochemical characteristics from four key shale oil exploration wells in the Jiyang Depression were analyzed in detail and were also used to analyze the prospect of shale oil exploration in this area. Finally, the research status of shale oil sweet spot evaluation is summarized, and key research directions needing further work are presented. In China, many wells have accidently produced certain amounts of shale oil from mudstone/shale fractures. The exploration and geochemical characteristics of lacustrine shales indicate not only that shale oil is a rich resource but also that it shows good exploration breakthrough potential, such as intersalt shale oil with an OSI (oil saturation index) generally higher than 300 and located in the Jianghan Basin. Samples from four key wells reveal geochemical features of lacustrine shale in ES3 (the third member of the Paleogene Shahejie Formation) and ES4 (the fourth member of the Paleogene Shahejie Formation) that demonstrate a high richness of organic matter in the ES3 and ES4 shales from the Jiyang Depression. Kerogen types are mainly I-II1, in a low maturitymature stage, and OSI values are higher than 100; shale oil flows are also being produced from shale fractures. In general, the progress of exploration for lacustrine shale oil in China has been unsatisfactory in recent years. We believe that shale oil mobility and shale fracability are vital factors for shale oil exploitation, which should be strengthened and further studied.