Evolution Of Mesozoic Sedimentary Basin In Qiangtang And Its Significance In Petroleum Geology

The Qiangtang basin is located in the east part of the Tethys realm and the central-northern Tibetan Plateau, and bounded by Jinshajiang suture zone to the north and Bangong-Nujiang suture zone to the south, respectively. The Qiangtang basin is a large composite basin which experienced Paleozoic-Cenozoic multi-cycle evolution. The Mesozoic Qiangtang basin bagan to form in the closure of the Paleo-Tethyan ocean, and existed with the Mid-Tethyan ocean. So the Mesozoic Qiangtang basin has great implications for the Mid-Tethyan ocean. The Mesozoic Qiangtang basin is the largest Mesozoic marine basin which has bright oil and gas prospect, and is a key exploration area. Allow for the geological significance and the application value, we selected the Mesozoic Qiangtang basin as the research object.The sedimentology, the petrology and the isotopic geochemistry are the main research methods. Based on the correlation and analysis of the Mesozoic strata in the Qiangtang, divided the filling sequences, reconstructed the lithofacies paleogeography. At the end, based on the paleomagnetic results and the achievements of the predecessors, the basin evolution was discussed.It is suggested that, the evolution of the Mesozoic Qiangtang basin can be divided into two periods, and the paleowethering crust marked the basin changing. The geochemistry data of the Zangxiahe Formation indicates that the Carnian basin of the Qiangtang is a foreland basin, and the main provenance is the syncollision granites.The geochemistry results of the Nadigangri Formation in Qiangtang basin show that the basin is a rift basin after the paleowethering crust. The rifting started at about220Ma, and persisted at205Ma in the central-nothern Qiangtang. The rifting of the southern Qiangtang may start in the early Norian and ended in the early Jurassic.The Qiangtang basin gradually changed into depressive basin from the early Jurassic to the late Jurassic. The zircon fission track data indicated the basin shrinkage occurred in between147and128Ma and the basin closure maybe started in87-86Ma.The research results of sedimentology show that the Mesozoic Qiangtang basin experienced six filling sequences:the foreland filling before the rifting, the synrifting alluvial-deluvial filling, the littoral facies filling in the early depression, two transgressive and one regressive and the mesa-tidal flat carboniferous sediment in the main depression stage, coarse clastic rocks and evaporation filling in the shrinkage and closure stage.The early coarse clastic rocks are the reservoir, the carbonate rocks and the mudstones in the middle stage are the main hydrocarbon source rocks, the evaporation rocks and the oil shale in the late stage are good seal rocks, so the Mesozoic filling rocks in the Qiangtang basin formed a super source-reservoir-seal combination.