The formation of the Paleocene lacustrine organic-rich shale in the Subei Basin, East China associated with the early late Paleocene event and marine incursions

Abstract

The second member of the Funing Formation (E1f2) is a Paleocene lacustrine organic-rich shale and has been identified as a primary source rock for both conventional and unconventional oil and gas in East China. It is correlated with the Early Late Paleocene Event (ELPE), a significant temperature inflection point in the Paleocene. However, the impact of the ELPE on the lacustrine organic-rich sedimentation is still not well understood. This study pinpoints the ELPE in the lacustrine E1f2 shale of the Subei Basin, identifies the resultant marine incursions by biomarkers, correlates the ELPE, marine incursions, and the depositional process, and clarifies organic matter accumulation mechanisms in the E1f2 shale. The interval preceding the ELPE has relatively low organic matter content, having been deposited in an arid paleoclimate with saline water conditions and low productivity. Eukaryotes predominantly contributed to the organic matter accumulation. The ELPE is identified in the E1f2 shale by negative carbonate carbon and oxygen isotope excursions. Subsequently, increasing temperatures and rising global sea levels induced marine incursions into the Subei Basin, supported by the appearance of 24-n-propylcholestane and the C30/C27-30 steranes ratio. The interval following the ELPE shows relatively high organic matter content and was deposited in an arid to semihumid-semiarid climate and a saline to brackish waterbody. Marine incursions likely introduced nutrients and enhanced lacustrine productivity, as indicated by the consistency between signals of marine invasion and bioavailable nutrient elements. Different origins contribute to organic matter accumulation at this stage, with prokaryotes being more dominant than eukaryotes. The E1f2 shale provides valuable insights into lacustrine organic-rich sedimentary processes and the responses of lacustrine systems to global events.