The sedimentary filling differences of subsags and their impacts on oil shale and coal accumulation in a rift lacustrine sag: A case study from the Lower Cretaceous in the Zhangqiang sag, southern Songliao Basin, Northeast China

Abstract

In order to characterize the sedimentary filling differences of two rift lacustrine subsags and their control factors and to construct the depositional model for coal and oil shale, the Lower Cretaceous Jiufotang and Shahai formations in the Qijiazi and Zhanggutai subsags, Zhangqiang sag, Songliao Basin, one of the most petroliferous basins in Northeast China have been analyzed. Seven facies were identified in the Lower Cretaceous strata of two subsags and were grouped into three facies associations, namely alluvial fan, fan delta and lacustrine. During the deposition of Jiufotang Formation, the Qijiazi subsag is mainly characterized by alluvial fan deposits, while the Zhanggutai subsag is dominated by fan delta and littoral-sublittoral lacustrine deposits. During the deposition of Shahai Formation, the Qijiazi subsag is characterized by fan delta, littoral-sublittoral lacustrine, and deep lacustrine deposits, whereas the Zhanggutai subsag is filled with fan delta and littoral-sublittoral lacustrine deposits. Coal and oil shale deposits are only preserved in the Shahai Formation of the Qijiazi subsag. Total organic carbon content, organic matter type, vitrinite reflectance, thermal alteration index and total reduced sulfur content also show that there are differences in organic matter preservation and redox conditions between two subsags. In the Jiufotang Formation, the total organic carbon contents are relatively low, the kerogen type is dominated by type III and the source rocks entered a mature phase in the Zhanggutai subsag, and the total reduced sulfur contents show a suboxic to oxidized environment in the Zhanggutai subsag and an oxidized environment in the Qijiazi subsag. In the Shahai Formation, the total organic carbon contents are relatively high, the kerogen type is dominated by type II and the source rocks entered an immature to mature phase in the Zhanggutai subsag, whereas the kerogen type is dominated by type III and the source rocks display an overall mature phase in the Qijiazi subsag. The total reduced sulfur contents of the Shahai Formation show the reduced environment in two subsags. Tectonics, sediment supply and redox conditions control the temporal and spatial differences in sedimentary filling and distribution of organic-rich rocks in two subsags. A comparison of rift lacustrine Qijiazi subsag and Fushun Basin shows that the rift basins are both characterized by deepening-upward to shallowing-upward successions controlled by tectonic subsidence and climate changes. The relatively high subsidence along with warmer climate resulted in the enhanced input of lake plankton and the decrease in higher plant input and triggered the transition from coal to oil shale formation.