Analysis of a destroyed oil reservoir in the Marine Jurassic of the southern Qiangtang basin, Qinghai-Tibet Plateau

摘要

The Qiangtang basin located in the north of Qinghai-Tibet Plateau is the most developed region of Triassic to Jurassic marine sediments in China. It contains favorable assemblages of source-reservoir-cap rocks. Among of more than 100 oil and gas seepages discovered in the basin, the most typical is the destroyed oil reservoir in Long'eni area of the southern depression. The destroyed reservoir, which is discovered in the Middle Jurassic Buqu Formation, is an anticline cut through by faults. Its exposed area is about 10 km2. The main reservoir rock is dolomites with a cumulative thickness up to 400.9 m. Its maximum porosity and permeability are 15.5% and 283 X 10~(-3) μm~2 respectively. The analysis of oil components shows that it is mainly light oil and characterized by source from marine shale of the Middle Jurassic Xiali Formation. According to the analysis of tectonic evolution and maturation of source, the periods of peak HC generation and reservoir formation in the basin are simultaneous and in the Late Yanshanian. Therefore, the discovery of the destroyed reservoir indicates that the processes of generation, migration and accumulation of oil and gas have happened indeed, and that a good prospectivity of petroleum exploration is especially in the large area where the Triassic to the Middle Jurassic strata are in the subsurface of the basin.