Palaeoenvironmental and tectonic controls on organic matter enrichment in the Middle Jurassic Dameigou Formation (Qaidam Basin, North China)

Abstract

Lacustrine organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. These fine-grained sediments have been proven to have good hydrocarbon generation potential; therefore, understanding the sedimentary palaeoenvironment of these shales and organic matter enrichment mechanism is essential to increase prospects for shale oil and gas resources. The mineralogy, elemental geochemistry, and organic geochemical data combined with lithology of ZK23-1 well from the Yuka Depression, Qaidam Basin have been comprehensively analyzed to determine the hydrocarbon potential, palaeoenvironmental changes, and tectonic setting of the lacustrine organic-rich shales in the seventh member of Middle Jurassic Dameigou Formation (J 2 d 7 ). The profile of the ZK23-1 well is composed of three stages (Stages I to III) based on variations in bulk geochemical parameters and lithological observations. Mineral compositions of studied samples are dominated by clay minerals and quartz, with small amount of pyrite, calcite, dolomite, and plagioclase. Interestingly, all samples contain siderite, which is closely related to palaeoenvironment changes. Total organic carbon (TOC) content and hydrogen index (HI) vary significantly in the various lithologies, suggesting that oil shale holds the best hydrocarbon potential, followed by shales and mudstones. A warm-humid palaeoclimate prevailed in the Qaidam Basin during the Middle Jurassic; however, significant palaeoclimatic fluctuations occurred from Stages I to III in the J 2 d 7 , especially in Stage II. The climate varies from hot-arid to warm-humid in Stage II, likely resulting in changes in palaeowater column, chemical weathering intensity, as well as palaeoproductivity. The results of this study suggest that the Yuka Depression in the Middle Jurassic could be located at an active continental margin. Warm and humid conditions, as well as the tectonically quiescent setting with stable subsidence rate, are considered as two primary drivers for the enrichment and preservation of OM in the sediments of the J 2 d 7 . Ultimately, a comprehensive depositional model is proposed to interpret the evolution of deposition of the J 2 d 7 , providing new insights into the deposition of lacustrine organic-rich sediments linked to palaeoenvironment conditions and tectonic setting. • Three stages have been identified from ZK23-1 in the J 2 d 7 . • An obviously environmental fluctuation occurred from stage I to III of the J 2 d 7 . • Depositional models have been proposed to interpret the enrichment of OM. • The warm and humid condition and tectonically quiescent setting are popular. • Palaeoclimate and tectonic setting are primary drivers for OM enrichment.