Abstract:

The Upper Triassic Xu-2-Xu-3 Member in the western Yuanba of Northeast Sichuan is an important oil and gas reservoir. In this study, we used drilling cores, logging data, and 3D seismic data to study the sedimentary facies, sedimentary characteristics, depositional models, and their effects on the reservoir development of the Upper Triassic Xu-2-Xu-3 Members, as to guide the deployment of oil and gas exploration in the region. The results show that Xu-2-Xu-3 Members are mainly braided river delta facies and lacustrine facies. Four subfacies have been recognized, as delta plain, delta, front delta, and coastal shallow lake, which can be further divided into 9 microphases. Two depositional models were developed during the depositional period of Xu-2-Xu-3 Member. Xu-2 Member shows a model of gentle slope wide channel braided river delta deposition, and mainly develops wide river channels with overlapping sand bodies. Xu-3 Member shows a model of steep slope narrow channel braided river delta deposition, and mainly develops narrow river channels, and the sand bodies are distributed as strips along the river channel. Xu-2 Member mainly develops two favorable reservoir rock types, medium feldspar sandstone and medium-grained quartz sandstone. Xu-3 Member also develops two favorable reservoir rock types, medium-coarse calcareous sandstone and fine sandy conglomerate. The high-quality reservoirs of Xu-2 Member are mainly distributed in the sand bodies of the underwater distributary channel of the delta front. The high-quality reservoirs of Xu-3 Member are mainly distributed in the sand bodies of the braided channel-delta front of the underwater distributary channel transition zone in the delta plain. The physical properties and pore structure of feldspar sandstone and quartz sandstone are significantly better developed than those of the calcareous sandstone and sandy conglomerate.

Key words: braided river delta, sedimentary model, high-quality reservoir, Xujiahe Formation, Northeastern Sichuan Basin