Petroleum migration and accumulation in a shale oil system of the Upper Cretaceous Qingshankou Formation in the Songliao Basin, northeastern China

Petroleum migration and accumulation associated with ostracod-bearing layers in shale oil systems were investigated using a large suite of high-resolution geochemical and mineralogical data combined with petrology description and pore characterization of shale core samples from the first member of the Upper Cretaceous Qingshankou Formation in the Qijia-Gulong sag of the Songliao Basin, northeastern China. The first member of the Qingshankou shale, deposited in a moderately deep lake setting, is dominated mainly by massive mud rock and laminated shale with numerous ostracod-enriched interbedded layers. The thin carbonate-rich ostracod-bearing layers dominate the total organic carbon (TOC)–lean facies having TOC values of <2 wt. %. Petroleum yields (volatile free hydrocarbons in programmed pyrolysis [S1]) and solvent extract yields share similar vertical variation trends with TOC content, indicating that TOC content exerts an important control on retained hydrocarbon. However, there exist some upward increasing trends of TOC-normalized petroleum yields (S1) and solvent extract yields around the organic-lean interbedded carbonate-rich layers dominated by ostracods. This phenomenon, combined with bitumen and fluorescing oil within fractures and ostracod-associated pores, abnormal Rock-Eval pyrolysis parameters, compositional differences, and molecular size differences, indicate the presence of migrated hydrocarbon in organic-lean interbedded ostracod-bearing layers. The oil production of ostracod-related organic-lean shale reservoirs is much more effective due to the high brittleness of organic-lean lithofacies, improved oil quality, and low content of adsorbed oil. Three stages were proposed in a conceptual model for petroleum migration and accumulation associated with ostracod-bearing layers in the Qingshankou shale oil system.