Cyclic organic matter accumulation in a plateau salinized lacustrine basin: Major controls and hydrocarbon significance

Abstract

Exploring the sedimentary process and organic matter enrichment mechanism of plateau saline lake basin holds unique theoretical research and practical production significance for the paleoenvironment restoration and hydrocarbon exploration in the plateau. This study aims to analyze the high-frequency sedimentary evolution and organic matter enrichment mechanism of the mixed deposits from the Eocene Lower Ganchaigou Formation (LGCG) in the Qaidam Basin on the Tibet Plateau, by means of petrology, mineralogy, and organic-inorganic geochemistry. Lithological characteristics indicated the high-frequency sedimentary sequences (<10m) of the LGCG which are identified by a single cycle with four lithofacies from bottom to top: massive sandstones, laminated silty shales, laminated limestones, and bedded dolomites. Bulk organic geochemistry data showed the periodic variations of organic matter contents in the high-frequency sedimentary cycles. Organic matter predominantly occurs in dark gray laminated limestones, followed by the laminated silty shales, with the lowest contents in the bedded dolomites and massive sandstones. Inorganic geochemical proxies implied the cyclical changes of paleowater properties, paleoproductivity and sedimentation rate in the vertical profile that conformed to the high-frequency sedimentary sequences. Correlation analysis revealed that the enrichment of organic matter in the plateau saline lake is mainly related to high paleoproductivity and low sedimentation rate (>5 cm/ka), and anoxic and low-salinity water conditions also promote the preservation of organic matter. Distinct from typical saline lake basins, the LGCG in the Qaidam Basin, a plateau saline lake, is characterized by lower organic matter abundance and higher hydrocarbon generation efficiency, due to reduced biological productivity and terrigenous input.