Oil-gas formation, preservation and tectonic evolution of the southern Qiangtang Basin
Yang Yao1,2, Liu Yan2, Wang Xianfeng3, Yuan Tingyuan2,4
1. Earthquake Administration of Sichuan Province, Chengdu 610041;
2. State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037;
3. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059;
4. China University of Geosciences(Beijing), Beijing 100083
The Qiangtang Basin is the largest marine basin of Chinese mainland. Tectonic evolution of the basin and oil-gas prospect has been disputed for a long time. Based on recently completed 1︰ 50000 geological survey, this article discusses the tectonic evolution and oil-gas prospects of southern Qiangtang Basin again. There is a nearly EW-trending mountain in the center of the basin, which divided it into southern and northern Qiangtang blocks. The latest geological survey and research show that there was an ancient ocean between the southern and northern Qiangtang blocks in the Late Cambrian-Early Ordovician. Late Triassic to Early Jurassic Nadi Kangri volcanic rocks, which its upper part is rhyolite, was found at the southern margin of the northern Qiangtang block, indicating that the southern Qiangtang block northward subducted beneath the northern Qiangtang in the Late Triassic, while led to the formation of organic-rich foreland basin at the northern margin of the southern Qiangtang block. Jurassic flysch which represent a mature basin is exposed in south of southern Qiangtang Basin, implying that the southern region of South Qiangtang has the characteristics of passive continental margin. Following the continuous expansion of Bangong-Nujiang tethyan ocean, Jurassic to Early Cretaceous shallow marine strata, which is characterized by organic-rich reef-limestone, was widely deposited in southern and northern Qiangtang blocks. Late Cretaceous to Paleocene terrigenous clastic rock is exposed in the area vary greatly inside the basin, which unconformably overlie the early marine sedimentary rocks, implying that the southern Qiangtang block transformed from marine basin situated on passive continental margin into a continental sedimentary basin from Late Cretaceous to Paleocene times. In Cenozoic, during the ongoing collision between India and Asian continents, the southern Qiangtang Basin was southward juxtaposed over the Lhasa terrane. Several large-scale thrust faults were found in southern Qiangtang Basin, which led to much Upper Triassic, Jurassic and Cretaceous organic-rich marine reef-limestone buried deeply in the basin again. Structural section across southern Qiangtang Basin shows that structural deformation is extremely weak between these large thrust faults, broad and gentle synclines are widely developed in this basin and Cenozoic continental strata is distributed in the syncline core, so we speculate that the organic-rich marine strata is still exist under the Cenozoic continental strata. Therefore, it indicates huge amounts of oil-gas potential resources in the southern Qiangtang Basin. We should look for oil-gas resources in the footwall of thrust faults and broad, gentle syncline core region.