Paleoenvironment evolution of the lacustrine organic-rich shales in the second member of Kongdian Formation of Cangdong Sag, Bohai Bay Basin, China: Implications for organic matter accumulation

Abstract The second member of Kongdian Formation (Ek2) of Cangdong Sag represents an excellent hydrocarbon source rocks interval target for crude oil and shale oil production in the Bohai Bay Basin. To reconstruct the palaeoenvironmental conditions and reveal the mechanisms of organic matter enrichment of the Ek2 shales, a series of analyses (total organic carbon (TOC), Rock-Eval pyrolysis, major and trace element analyses, and X-ray diffraction (XRD)) were performed on samples collected from the borehole of Cangdong Sag. The Ek2 consists of fine-grained rocks deposited in a lacustrine environment, and it can be further subdivided into four members (Ek24 to Ek21). The results show that the Ek2 contains a high abundance of thermally mature Type I and II kerogen, and most of the samples show good to excellent generative potential. The mineralogical compositions, dominated by quartz, feldspar, and dolomite, with subordinated calcite, clay minerals, and analcime. Specific major and trace elements were employed to analyze the changes in paleoclimate conditions, lake water chemistry, and the detrital influx of Ek2. The vertical changes of paleoenvironmental indicators suggest that during the sedimentary period of Ek2, the climate changed from semi-humid to arid, with the input of terrigenous detrital gradually decreased, and the salinity of the water increased. The relationship among the primary productivity, redox conditions, detrital influx indicators and TOC content suggests that primary productivity was the most critical factor in controlling organic enrichment, and the input of terrigenous debris promotes the increase in productivity. The high primary productivity, together with anoxic bottom water conditions, resulted in the greatest enrichment of organic matter in the lower part of Ek2 shale (Ek23). The upper part (Ek21) is mainly composed of moderate-TOC calcareous shale, which is deposited under arid paleoclimate, with low detrital input, moderate productivity, and favorable preservation conditions.