Tectonic paleogeography of Late Sinian and its significances for petroleum exploration in the middle-upper Yangtze region, South China

The paleotectonic pattern, lithofacies paleogeographic features, sedimentary evolution and its controlling effects on hydrocarbon accumulation assemblages during the depositional period of the Sinian Dengying Formation in middle-upper Yangtze region were investigated based on outcrops, drilling, log and seismic data. The study shows that, (1) Affected by the breakup of the Rodinia supercontinent, the middle and upper Yangtze areas were in extensional tectonic environment during the depositional period of Dengying Formation. The carbonate platform was structurally differentiated. Intra-platform depressions controlled by syndepositional faults developed, forming a tectonic-paleogeographic pattern of “three platforms with two depressions”. (2) During the depositional period of the first and second members of the Dengying Formation, rimmed platforms and intra-platform fault depressions developed in upper Yangtze area and isolated platform developed in middle Yangtze area, and there was the Xuanhan-Kaijiang ancient land block in eastern Sichuan. The depositional period of the third member of the Dengying Formation is the transformation period of tectonic-paleogeographic pattern, when a set of shallow water shelf sediment rich in mud was deposited due to transgression on the background of the eroded terrain formed in Episode I of Tongwan Movement. The sediment of the fourth member of the Dengying Formation inherited the paleogeographic pattern of the first and second members of the Dengying Formation in general, but the Deyang-Anyue intra-platform fault depression further expanded, and the middle Yangtze platform evolved into two separated platforms. (3) Tectonic-sedimentary differentiation and evolution of carbonate platform in the Sinian gave rise to two types of accumulation assemblages with wide distribution and great exploration potential, which are platform margin and intra-platform.