Geochemical characteristics and formation mechanism of organic-rich source rocks of mixed sedimentary strata in continental saline lacustrine basin: A case study of Permian Pingdiquan formation in the Shishugou Sag, Junggar Basin, Northwest China

Abstract

The Permian Pingdiquan Formation (P₂p) in Shishugou Sag, Junggar Basin, Northwest China, is a typical example of mixed sedimentary strata formed in a saline lacustrine environment, characterized by complex lithological assemblages and extensive distribution of organic-rich source rocks. To further investigate the differences in oil-source contribution and hydrocarbon generation characteristics of source rocks with different lithologies, studying the difference of their geochemical characteristics and the formation mechanism of organic-rich source rocks is the top priority. Based on a comprehensive analysis of lithologic types, geochemical characteristics, and hydrocarbon generation potential of source rocks using organic petrology, molecular geochemistry and element geochemistry, this paper reveals the formation background and organic matter enrichment mechanism of the P₂p organic-rich source rocks. The results show that the mixed sedimentary rocks of the P₂p were deposited under relatively arid and saline lacustrine conditions. Paleoclimatic conditions tended to be more arid and hydrothermal activity became more frequent during the sedimentation process from mudstone, dolomitic mudstone to argillaceous dolomite. Consequently, the water column salinity elevated, leading to an increased proportion of salt-tolerant green algae and slight salt-tolerant cyanobacteria compared to salt-intolerant algae. Frequent changes in paleoclimate and hydrothermal activity control the interbedding deposition of mudstone, dolomitic mudstone and argillaceous dolomite, and the resulting salinity fluctuations contribute to variations in the proportion of salt-tolerant and intolerant algae in source rocks with different lithologies. Furthermore, the source rocks within the mixed sedimentary strata (mudstone, dolomitic mudstone and argillaceous dolomite) all exhibit substantial hydrocarbon generation potential, and there exists heterogeneity among source rocks of the same lithology. The key factor influencing the development of these organic-rich source rocks is the enhancement of palaeoproductivity driven by volcanic activity. Additionally, the water stratification caused by saline water promotes anoxic water columns, also contributing to organic matter enrichment in the source rocks.