Characterization of lithofacies in shale oil reservoirs of a lacustrine basin in eastern China: Implications for oil accumulation.

The present research study was aimed to identifies the effect of the lithofacies (assemblages) on lacustrine shale oil accumulation, the shale oil layers of the Eocene Shahejie Formation in the Bohai Bay Basin, China, were characterized. The petrological analysis, included core, thin section and scanning electron microscopy observations and X-ray diffraction, indicated that the shale oil layers of the Shahejie Formation are dominated by carbonates, which are characterized by uniform or laminar distributions. The laminar carbonate components were formed through sedimentogenesis or diagenesis. Based on the genesis patterns and the petrology, five major lithofacies (assemblages) were identified in the formation: LF (lithofacies) 1 + 5 is characterized by thin lenticular laminae of crystalline carbonates with black shale; LF2+3 depicts laminae of micritic carbonates with massive lime mudstone; LF3 is a thick layer of massive lime mudstone; LF4 is massive claystone; and LF5 is black shale. The relationship between the lithofacies and the favorability for hosting shale oil reservoirs was investigated using microdrill sampling of the total organic content, reservoir quality analysis, and shale oil-bearing property. In LF1+5, the crystalline carbonates contain abundant interparticle pores, which can act as reservoir spaces, whereas the shale layers have a high potential for hydrocarbon generation. In addition, the hydrocarbon reserves can enter the crystalline carbonates of the assemblage after migrating a short distance. Consequently, LF1+5 is the most favorable assemblage for shale oil accumulation. In addition, LF4 and LF5 have high hydrocarbon generation potentials, and thus a relatively smaller amount of hydrocarbons could accumulate in these assemblages through clay mineral adsorption. Thus, LF4 and LF5 are also valuable sites for shale oil accumulation. • The lacustrine shale oil accumulation could be classified into two categories, "crystalline carbonate reservoir (CCR)" and "in-situ generation and in-situ reservoir (GR)" model. • In CCR model, the crystalline carbonate provides reservoir space and the adjacent shale generate hydrocarbon. • In GR model, the hydrocarbon in-situ is generated and accumulated in shale and mudstone. [ABSTRACT FROM AUTHOR]