The tight oil of lacustrine carbonate-rich rocks in the Eocene Shulu Sag: Implications for lithofacies and reservoir characteristics

摘要

Tight oil has been discovered in calcilutite and carbonate conglomerate reservoirs in the lower part of the third member of the Eocene Shahejie Formation of the Shulu Sag, Bohai Bay Basin, Northern China. Understanding the nature of lithofacies is essential to evaluating the quality of tight reservoirs. This study analyzed the sedimentary characteristics, geochemistry, and reservoir characteristics of each lithofacies based on thin-section and fieldemission-scanning electron microscope observations combined with X-ray diffraction, geochemical parameter, and physical property analysis. Eight lithofacies were identified: (1) varve-like laminated calcilutite, (2) non-regular laminated calcilutite, (3) interlaminated calcisiltite-calcilutite, (4) massive calcilutite, (5) massive calcisiltite-calcarenite, (6) grain-supported carbonate nconglomerate, (7) matrix-supported carbonate conglomerate, and (8) mixed-source carbonate conglomerate. The varve-like laminated calcilutites have intrabasinal organic matter with a low Pr/Ph ratio (0.50-0.63) and high gammacerane/C-31 homohopane (0.13-0.29), indicating that they are normal sediments and have good hydrocarbon generation potential. The other lithofacies are event sediments containing a certain amount of terrestrial organic matter. Laminated, organic-rich lithofacies develop organic matter pores and other storage spaces modified by organic acid. Their reservoir quality is better than that of massive or organic matter-lean lithofacies. The conglomerates containing less matrices have better reservoir quality than conglomerates with high matrix contents. The hydrocarbon generation ability and reservoir quality of lithofacies as well as the fracture density are the key factors that control the formation of tight oil reservoirs in the Shulu Sag.