Crude oil and formation history of hydrocarbon in the Shenglihe area, northern Qiangtang basin

These is a favorable assemblage of source, reservoir and cap in the Jurassic marine strata in the Shenglihe area, northern Qiangtang depression. The Middle-Lower Jurassic dark-gray mudstone and shale are the major source rocks, the Middle Jurassic dolomite and fractured limestone of the Buqu Formation form the major reservoir, and the overlying cap is the Middle Jurassic mudstone and gypsum of the Xiali Formation. Drilling as deep as 500 m reveals 2 layers of oil-saturated dolomite at depth 0~6 m and 15~17 m, and 4 layers of fractured limestone filled with crude oil at depth of 24~40 m, 65~97 m, 100~109 m and 155~157 m, respectively. Dark brown crude oil exists in drilled core of the fractured limestone, and liquid oil seeps slowly from fractures to surface of the core at depth 88~90 m. Hydrocarbon generation in the Jurassic marine source rock in the Shenglihe area is closely related to thrust caused by India-Eurasia collision. Vitrinite reflectance Ro of the Middle-Lower Jurassic source rock increased to ~0.52% at ~138 Ma in the Early Cretaceous, and the source rock retained its immature-low maturity in the Middle-Late Cretaceous. The collisional orogenesis converted the Shenglihe area to foreland basin of the Xiangyang Lake thrust (XLT) in the Early Cenozoic, where formation of thick redbeds led to rapid growth in buried depth of the Jurassic marine source rock in the Early Paleogene, and Ro of the source rock increased to 0.80%~0.83% at ~51 Ma. Hydrocarbon generation in Paleocene-Eocene formed liquid oil in the fractured limestone and dolomite of the Buqu Formation, which might be key targets for future oil exploration in the Qiangtang basin. © 2022 Geological Society of China. All rights reserved.