Shale Oil Enrichment Mechanism of the Paleogene Xingouzui Formation, Jianghan Basin, China

Abstract

Organic-rich lacustrine shales are widely developed in China, and they have long been simply regarded as homogeneous source rocks, which restricts the understanding of intrasource oil accumulation. At present, the study of the LXF (Lower Member of the Xingouzui Formation) in the Jianghan Basin as an unconventional oil reservoir is still in its infancy, and the hydrocarbon accumulation mechanism is still unclear. Geochemical and mineralogical studies were carried out on a suite of samples from the 100-m-thick sequence, i.e., LXF II Oil Bed, by using XRD, SEM, MICP, and Rock-Eval pyrolysis. The results show that the II Oil Bed is rich in carbonate and poor in clay, so it shows a good fracturing tendency. The high degree of heterogeneity in mineral composition leads to frequent interbedding of different lithofacies. In the II Oil Bed, intercrystalline pores, interparticle pores, and intraparticle pores are developed, and micro-fractures are often observed. However, the main pore types, pore size distribution, and connectivity are quite different among lithofacies, and the carbonate-rich lithofacies have better reservoir capacity. The OM (organic matter) abundance of the II Oil Bed varies greatly and generally ranges from fair to very good. Coupled with its early-mature to mature Type II OM, it is considered to have the characteristics required for oil generation. Comprehensive analysis shows that the II Oil Bed has good shale oil exploration prospects, and the enrichment of shale oil in the sequence is the result of multiple factors matching. Firstly, high organic matter abundance is the material basis for shale oil enrichment. Secondly, thermal maturity is a prerequisite, and the difference in burial depth leads to the differential enrichment of shale oil in different areas. Thirdly, pores and micro-fractures developed in shale not only provide space for hydrocarbon storage, but also form a flow-path network. Finally, multi-scale intrasource migrations are key processes ranging from the scale of lithofacies to the intervals, which further results in the differential shale oil enrichment in different lithofacies and intervals. Considering the hydrocarbon generation capacity and reservoir quality, the prospective depth for shale oil exploration in the study area is >1350 m. The findings of this study can help in the better-understanding of the shale oil enrichment mechanism, and the optimization of future exploration strategies.

Organic matter enrichment mechanism in saline lacustrine basins: A review

“…et al, 2021). These findings highlight the significant impact of BSR on the consumption of sedimentary organic matter, which aligns with the relatively high sulphate content and low abundance of organic matter observed in the Xinguozui Formation shale in the Jianghan Basin (Li et al, 2022;Li Qiqi et al, 2021).…”

“…Hence, higher deposition rate in the late stage of saline lacustrine evolution lead to a dilution effect on organic matter, which is unfavourable for its enrichment. Thus, understanding aligns with the relatively high content of salt minerals and low abundance of organic matter observed in the shales of the Xingouzui Formation in the Jianghan Basin (Li et al, 2022;Li Qiqi et al, 2021).…”

“…In the shales of Xingouzui Formation, glauberite frequently occurs as interbeds with organic‐rich shales, exhibiting high sulphate content and direct contact with organic matter. These conditions are conducive to TSR, which contributes to the consumption of organic matter and aligns with the relatively low abundance of organic matter observed in the Xingouzui Formation shale (Li et al, 2022; Li Qiqi et al, 2021). In addition, significant pyrite mineralization is present in the shales of Xingouzui Formation, with pyrite particle sizes averaging around 5 microns, some of which are associated with organic matter (Figure 8c).…”