Formation and evolution of coal– and oil–bearing source rocks in the Lower Cretaceous Shahai Formation in the Fuxin Basin (NE China): Evidence from organic and inorganic geochemical analyses

Two source rocks were deposited in the Lower Cretaceous Shahai Formation (K1sh) in the Fuxin Basin (NE China): the third Member (K1sh3) coal–bearing and the fourth Member (K1sh4) oil–bearing source rock. However, the differences between the two Fuxin Basin source rocks remain poorly constrained. This study systematically reveals the formation and evolution of these source rocks through sedimentological and geochemical analyses. During the K1sh3 deposition, the intermittently warm–humid climate promoted organic matter (OM) input from terrestrial plants. Meanwhile, the restricted swamp with suboxic water conditions enhanced OM preservation and reduced OM dilution. Thus, warm–humid climate, suboxic water conditions of swamp controlled the formation of excellent coal–bearing source rock with highest total organic carbon (TOC) content (avg. 28.68 wt%) and Type III kerogen (perhydrous). The post–mature maturity (avg. 484 °C Tmax) indicates that the K1sh3 coal–bearing source rocks have reached the wet to dry gas generation stage.During the K1sh4 deposition, rapid basin subsidence and a humid–prone climate resulted in lake expansion, enhancing the dysoxic bottom water conditions. Moreover, the rapid basin subsidence triggered frequent gravity flows. The turbidity flows delivered terrestrial OM fragments and nutrients, promoting the lacustrine productivity and producing OM dominated by aquatic organisms with some terrestrial plants. However, hyperconcentrated density flows disturbed the dysoxic bottom water conditions and strongly diluted the OM. Hence, the humid–prone climate, dysoxic bottom water conditions and gravity flows have controlled the formation of the K1sh4 oil–bearing source rock with high TOC contents (avg. 2.73 wt%). The oil–bearing source rocks show good hydrocarbon potential (avg. 6.17 mg HC/g rock S2) and Type II to III kerogen (perhydrous). The mature oil–bearing source rocks (avg. 443 °C Tmax) in the lower K1sh4 were at the oil generation peak and may be the chief source rock in the Fuxin Basin. During the Early Cretaceous, similar rift basins were developed in Northeast China, so this study provides a typical sample for source rock prediction in other adjoining rift basins.