Depositional system and favorable reservoir distribution of Xujiahe Formation in Sichuan Basin

By comprehensive analysis on the depositional system and on the main controlling factors of reservoirs in the Upper Triassic Xujiahe Formation in Sichuan Basin, the formation conditions of high-quality reservoirs under low-porosity and low-permeability background were studied. Except the transitional facies of Xu-1 member, other members of Xujiahe Formation are of terrestrial deposit, with alluvial fan, fluvial, fan-delta, fluvial-delta, and lacustrine deposit systems developed. Both the facies belt distribution and paleogeographic evolution are closely related to the regional tectonic movement. The original sedimentary facies belt (sandbar microfacies of main stem of braided river) and later denudation co-control the distribution of favorable reservoirs, and the quartz overgrowth and cementation is one of the controlling factors of the tight sandstones. The favorable reservoirs are mainly developed in the high-energy channel of delta plain, with medium-coarse sandstone as the best lithology. Combined with the analysis data of effective thickness and physical property, the favorable reservoirs in the Xu-2, 4, 6 members were evaluated and predicted comprehensively. The results indicate that the center and south of Sichuan are the most favorable reservoir distribution areas.