Red algal evidence for a marine regression during closure of the Paleo-Asian Ocean in the Junggar Basin and its linkage to hydrocarbon generation

Abstract

The closure of the Paleo-Asian Ocean was a significant geological event in northern Pangea during the Carboniferous-Permian. It had a significant effect on climate, biota, and environmental conditions of the Late Paleozoic Ice Age, and resulted in the development of vast energy resources. This paper reports on the first discovery of marine red algal fossils in the Junggar Basin and its linkage to hydrocarbon generation. Red algae occur mainly in the Fengcheng Formation (ca. 300 Ma) and provide direct fossil evidence for closure of the Paleo-Asian Ocean. The red algal fossils contain well-preserved reproductive organs, such as cystocarps and carpospores. High concentrations of C₂₇ steranes (C₂₇ regular steranes/sum of C₂₇–C₂₉ regular steranes×100=14.30%–21.30%) and the marine biomarker 24-n-propylcholesterane (C₃₀ diasterane [βα20S]/sum of C₂₇–C₃₀ diasteranes [βα20S]×100=1.15%–1.85%) were detected in the red-algae-bearing hydrocarbon source rocks. Thermal experiments that simulate hydrocarbon generation show that the oil generation potential of the red-algae-bearing source rocks is 363.71 mg g⁻¹ total organic C. This result, combined with oil-source rock correlations, indicates these rocks contributed to the formation of oil and gas resources, particularly in the marginal areas of the depression. The lake basin inherited the water and biological conditions of the Paleo-Asian Ocean during a marine regression, which was an important factor in the development of these high-quality hydrocarbon source rocks in an alkaline saline lake. The results advance our understanding of the evolution of the Paleo-Asian Ocean, interactions between the ocean and a lake during the deposition of terrestrial hydrocarbon source rocks, and whether red algae can effectively generate hydrocarbons.