Sedimentary facies and palaeogeography of the Middle Jurassic Buqu Formation in the Longeni-Esima ancient oil reservoirs zone,Qiangtang Basin,northern Xizang

The Longeni-Esima ancient oil reservoirs zone is the largest ancient oil reservoirs zone in the Qiangtang Basin, northern Xizang, where the oil-bearing formations generally occur in the dolostone strata from the Middle Jurassic Buqu Formation. In the Longeni-Esima ancient oil reservoirs zone, the oil-bearing dolostones horizons in the Middle Jurassic Buqu Formation consist mainly of the tidal-flat and subtidal shoal grainstones, algal-laminated dolostones and grainy dolostones. The grainstones and algal-laminated dolostones constitute the cyclic deposits in which the sedimentary structures such as algal-laminated structure and birdeye structure are developed, and the lagoon dark grey micritic limestone, argillaceous limestone and shell limestone are sandwiched as well. In the comparative analysis on the basis of the principle of dominant facies, the authors in this paper have reconstructed the sedimentary facies and palaeogeographic map showing the distribution of uplifted erosional areas, tidal flat facies, the lagoon facies and subtidal shoal facies, and the spatial distribution of source rocks and reservoir rocks and oil-gas reservoirs, and delineated the potential areas of residual oil and gas reservoir distribution in the study area. It is very important for the evaluation of the prospects of oil and gas resources, and the future oil and gas exploration and development in the study area.