Mechanisms for the Accumulation of Organic Matter in Sediments of the Middle Permian around Bogda Mountain, Southern Junggar Basin, NW China

Abstract

The organic-rich shales and mudstones of the Middle Permian are the most important unconventional petroleum targets in the southern Junggar Basin of northwestern China. Although numerous studies have been vigorously conducted on paleoenvironment reconstructions, the organic matter enrichment mechanism is still controversial due to the utilization of problematic geochemical proxies established in early studies. In this study, major and trace elemental compositions, molecular markers, stable carbon isotopic compositions, and organic petrology were used to study multiple factors affecting the accumulation of organic matter in sediments. According to the results, a relatively hydrostatic, brackish, anoxic environment is proposed in the northern periphery of the Bogda Mountain, where the original structure of organic matter could be preserved with non-correlations between the productivity proxies (P and Ba) and TOC. In the western Bogda area, organic-rich sediments were deposited under suboxic conditions, with frequent fluctuation occurring between the top and bottom water columns. The accumulation of organic matter was the result of productivity blooms and rapid sedimentation. This improved study of the enrichment of organic matter in the Middle Permian around the Bogda Mountain can contribute to improved evaluations of the petroleum potential and distinguishing the characteristics of different organic matter enrichment models.