Key factors controlling oil contents in different lithofacies shales from the Funing Formation, Subei Basin: Evidence from scanning electron microscopy

Abstract

Lacustrine shales with complex mineral compositions and rock formations are associated with different lithofacies, and shale oil contents vary significantly depending on the lithofacies. This study examines lacustrine shales from the Funing Formation, Gaoyou Sag, Subei Basin using scanning electron microscopy, total organic carbon (TOC), Rock-Eval pyrolysis, X-ray diffraction, thin section, and nuclear magnetic resonance T1–T2 to determine the key factors controlling oil content in different shale lithofacies. Shale lithofacies were determined based on the mineral composition, rock structure, and organic matter content. The micro-distributions of shale oil in different lithofacies were disclosed. The results show that the studied shales in Gaoyou Sag are primarily divided into massive felsic-rich, stratified felsic-rich, laminated felsic-rich, laminated felsic, laminated calcareous-rich, and laminated calcareous shales. Massive felsic-rich shales have the largest TOC, S1, and free oil contents associated with organic-rich shales. Stratified felsic-rich shales have the larger TOC, S1, and free oil contents, followed by the laminated felsic-rich and felsic shales. However, laminated calcareous-rich shales are characterized by larger TOC but lower S1 and free oil contents, whereas laminated calcareous shales have the lowest organic matter and oil contents. Shale oil content is mainly affected by the organic matter content and the development of reservoir space. Massive felsic-rich shales are characterized by almost all pores saturated with oil. The oil contents in stratified felsic-rich, laminated felsic-rich, laminated felsic, and laminated calcareous-rich shales are primarily affected by the development of reservoir spaces, followed by organic matter abundance. However, laminated calcareous shales are the opposite. Interparticle pores at the edge of granules are the most developed pores, whereas interparticle pores associated with quartz are generally saturated with oil, followed by those related to calcite or dolomite. Intraparticle pores in clay minerals are usually free of oil. Moreover, shale oil primarily occurs in large interparticle pores. The organic-rich massive felsic-rich and organic-bearing laminated felsic-rich shales are favorable for shale oil enrichment, and organic-bearing laminated felsic-rich shales with larger pores and interlayer micro-fractures may be the optimal lithofacies for shale oil. This study provides insight into the micro-distribution of shale oil in different lithofacies, which will guide shale oil development in the Gaoyou Sag, Subei Basin.