Excellent source rocks discovered in the Cryogenian interglacial deposits in South China: Geology, geochemistry, and hydrocarbon potential

Abstract

The Cryogenian (720–635 Ma) is characterized by the well-known Sturtian and Marinoan glaciations which plays a key role in understanding remarkable climate and sea level changes, and oxygenation events that occurred during the Neoproterozoic Era. However, less attention has been paid to the interglacial deposits. Recently, we paid great attentions to the excellent hydrocarbon source rocks (TOC > 2%) in the Cryogenian interglacial Datangpo Formation (663 ± 4 Ma–654 ± 3.8 Ma) in South China, which mainly consists of black shales with thin manganese interbeds, making the interglacial interval a new focus. To elucidate their characteristics and depositional environments, petrological and geochemical studies were conducted on fresh rock samples collected from six sections (Gucheng, Sanlian, Xiaoxi, Gaodongyuan, Xiaochayuan, Yanglizhang) in manganese mining areas at southeastern South China. Sedimentary reconstructions suggest a transition from restricted to open ocean environment during the deposition of the Datangpo Formation. And, this formation records variations from warm and humid to dry and cold climate. Black shales were deposited in its lowest member during the warm period and showed greatest thicknesses more than 20 m at depocenters. Microscopic observations of the black shale samples exhibited abundant benthic algae with great diversities, indicating a gradual recovery of the ecosystem. Biomarker analyses further reveal that these algae combined with bacterial microorganisms prosperous in low-salinity reducing environments were the dominant organic contributors to the source rocks, which were inferred by low pristane (Pr) to phytane (Ph) ratio (Pr/Ph), low gammacerane, high tricyclic terpene and high C27 sterane contents. Trace element records combined with isotopic data suggest that the deposition of source rocks in Datangpo Formation were related to a burst in surface-water productivity due to warm climate and better preservations resulted from anoxic waters within a stratified water column. These findings indicate the Cryogenian source rocks may provide a new potential field for future explorations of shale gas and deep oil and gas.