Sedimentary differentiation triggered by the Toarcian Oceanic Anoxic Event and formation of lacustrine shale oil reservoirs: Organic matter accumulation and pore system evolution of the Early Jurassic sedimentary succession, Sichuan Basin, China

The Da'anzhai Member of the Lower Jurassic Ziliujing Formation of the Sichuan Basin is one of the most important targets for lacustrine shale oil exploration in China. Organic matter (OM) content is a critical factor when considering the exploration and production potential of lacustrine shale reservoirs. However, the complexity of the lacustrine paleoenvironment necessitates a fuller understanding of OM accumulation mechanisms. The Early Jurassic (similar to 183 Ma) Toarcian Oceanic Anoxic Event (T-OAE) occurred concurrently with the deposition of the Da'anzhai Member. However, the control of sedimentary differentiation triggered by the T-OAE on the formation of lacustrine shale oil reservoirs remains a matter of debate. In this study, we used total organic carbon (TOC) analysis, X-ray diffraction (XRD), major and trace element analyses, field emission scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MIP) measurements, and nitrogen adsorption (NA) analysis to investigate the Da'anzhai Member in the Langzhong region of the Sichuan Basin. Our results demonstrate that the deposits of the Da'anzhai Member accumulated in a warm and humid oxygenated environment under freshwater to brackish salinities. OM accumulation in the Da'anzhai Member was largely controlled by redox and salinity conditions. The pore systems of the analyzed samples of the Da'anzhai Member were diverse but dominated by clay mineral-associated pores. The T-OAE appears to have modified the redox conditions and salinity of the lacustrine environment, in which sediment of the middle part of the Da'anzhai Member accumulated by influencing paleoclimate and hydrological conditions, resulting in the enrichment of OM in contemporaneous sediment. Furthermore, the onset of T-OAE affected the productivity of the water column, inducing changes in the kerogen type that favored the formation of organic pores. In summary, T-OAE influenced the exploration and production quality of lacustrine shale by engendering OM enrichment and organic pore formation. Our study provides new insights into the effects and influence of OAEs on the accumulation in lacustrine shale oil reservoirs.