Differential mechanisms of organic matter accumulation of source rocks in the Lower Cambrian Qiongzhusi Formation and implications for gas exploration fields in Sichuan Basin

Abstract

The marine facies highly mature natural gas system with the Lower Cambrian Qiongzhusi Formation as the source rock in the Sichuan Basin is a global model in this field. To deepen the geological understanding of high quality source rocks and the natural gas exploration areas in the Sichuan Basin, taking two hot exploration blocks in the central and northeastern parts of Sichuan Basin as examples, the differential mechanisms of organic matter accumulation in source rocks of the Qiongzhusi Formation were studied, and some gas exploration areas were identified by the means of geochemical methods from three perspectives, including paleoproductivity, redox conditions and input of terristral organic matters. The TOC of source rocks within deep-water shelf in the intracratonic rift sampled from the Gaoshi 17 and Zhongjiang 2 wells in the central Sichuan were higher than that of source rocks within the shallow-water shelf sampled from the Xiaoyangba outcrop, northeastern Sichuan. The organic matter accumulations of both sample sets were mainly controlled by redox condition, but in comparison, the source rocks in the central Sichuan Basin were formed in relative anoxic environment, which was more conducive for the preservation of organic matter than the suboxic environment in northeastern Sichuan Basin. In addition, the dilution effection by terrestrial organic matter input was particularly significant during the formation of source rocks in the northeastern Sichuan Basin, leading to poorer source rock quality. The exploration of natural gas system in the Qiongzhusi Formation still needs to be carried out closely around the intracratonic rift and its surrounding margin in the future, and the deep-water shelf area around the basin also deserves attention. Both conventional and unconventional natural gas resources should be considered.

Keywords

• natural gas
• redox conditions
• mechanism of organic matter accumulation
• intracratonic rift
• deep water shelf
• qiongzhusi formation
• lower cambrian
• sichuan basin