Enrichment and high productivity mechanisms and accumulation patterns of shale oil in lacustrine shales with low total organic carbon content in the Subei Basin

Shales in the 2 nd member of the Funing Formation (also referred to as the Fu 2 Member) in the Subei Basin are characterized by low organic matter abundance, complex lithological composition, and small fault blocks. These geological characteristics pose considerable challenges for shale oil exploration, including uncertainties regarding resource potential, enrichment mechanisms, and exploration targets. To address these issues, we systematically investigate the mechanisms behind the hydrocarbon generation, reservoir formation, and hydrocarbon accumulation in shales with low total organic carbon (TOC) content in the Fu 2 Member. Accordingly, the potential and favorable targets for shale oil exploration of the shales are determined. The results indicate that shales in the Fu 2 Member consist primarily of five lithofacies types, in which lamellar clayey-felsic and felsic-carbonate mixed shales occurring predominantly in the 1 st and 2 nd sub-members are identified as favorable lithofacies. The shales contain types Ⅰ and Ⅱ kerogen, as well as a range of favorable hydrocarbon-generating parent materials, including the Leiosphaeridia, Botryococcus, dinoflagellates, and suberinite, exhibiting early-stage hydrocarbon generation, high conversion rates, low hydrocarbon expulsion efficiency, and significant free oil enrichment. Shale reservoirs in the Fu 2 Member are classified as fractured-porous reservoirs. In these reservoirs, pores are dominated by intercrystalline pores within clay minerals, intergranular pores within felsic minerals, and dissolution pores. Reservoir fractures consist primarily of bedding-parallel fractures, structural fractures, and diagenetic fractures, contributing to 17% ~ 21% of the reservoir porosity. Influenced by diagenesis and tectonic processes, shale oil experienced three stages of micro-migration within the source interval, resulting in the shale oil accumulation characteristics of micro-migration and self-sealing. Four shale oil accumulation patterns are identified in the Fu 2 Member: stable shale oil within deep low-lying zones, as well as shale oil hosted in highly-steep faulted terraces, complex fault blocks, and shallowly-buried monoclines. The enrichment and high productivity of shale oil are primarily governed by sedimentary diagenesis, preservation conditions, and fracability. A total of 12 exploration and appraisal wells, along with three test well groups, were deployed in the Qintong Sag. All wells yielded high productivity, with proven shale oil reserves exceeding 40 million tons having been reported. These results reveal that shales with low TOC content in the Subei Basin hold considerable potential for shale oil exploration and exploitation.