Sedimentary characteristics of Middle Jurassic Sewa and Buqu Formations from Qiangzi Well-2, southern Qiangtang basin, Tibet

Abstract：Based on core logging and sample analysis of Qiangzi Well-2 petroleum shallow drilling hole in southern Qiangtang basin, the authors hold that the southern Qiangtang basin was in the evolution stage of passive margins in Middle Jurassic Bajocian and Aalenian period, and formed an upward coarsening and shoaling delta facies prograding sequence of Sewa Formation. On the cumulative curve in the grain size arithmetic probability graph, characteristics of saltation mass and suspension are obvious. The C-M diagram of grain sizes is characterized by typical braided river delta deposition. Pollen analysis also indicates a dry and hot climate of tropical and subtropical zones. In Middle Jurassic Bajocian period, the basin developed a comparatively stable stage of depression basin and formed carbonate platform shoaling sedimentary sequence of Buqu Formation, comprising mainly abundant oolitic, rudaceous and bioclastic limestone. The entire southern Qiangtang basin developed a regular transgressive-regressive cyclic succession from Aalenian period to Bajocian period in Middle Jurassic, forming in upward succession delta subfacies, delta frontal subfacies, platform-frontal slope subfacies, open platform subfacies and limited platform subfacies.