Discovery of the Large Early Platform Margin of Upper Sinian Dengying Formation in Central and Northeast Sichuan Basin and Its Implications for Hydrocarbon Exploration

This study focuses on the distribution of high-quality carbonate reservoirs and new exploration areas in the Dengying Formation (Sinian) on the eastern side of Mianyang-Changning intracratonic sag. Using drilling, logging and seismic data in the Gaoshiti-Moxi, we identify the large early platform margin of Dengying Formation and characterize its internal configuration, based on the theories including the transgression-regression cycle analysis, the layer flattening, the seismic line drawing, the impression method paleogeomorphology, and quality reservoir prediction. It is found that, on the SE-NW seismic profile in the east of Mianyang-Changning intracratonic sag, there are several progradation reflections identified for the first time within the third sequence (SQ3) of Dengying Formation, which prograded from southeast to northwest with a S-shaped, mound or short axis in cross section. At the top of these progradation reflections, there are an obvious slope break point and a series of onlap points of SQ4 transgressive system tract. The thickness of SQ3 from the slope break point to the northwest is sharply thinned and wedge-shaped. The results show that there is a large early platform margin belt about 270 km long in the carbonate platform in the east of Mianyang-Changning extensional trough, which starts from the Gaoshiti, extends northeastward to the middle of Longgang area through Moxi, Longnvsi, Nanchong and Peng'an areas. The discovery of this large platform marginal zone will provide a new area for the oil and gas exploration of the Dengying Formation in the study area, and it will no longer be limited to the vicinity of Mianyang-Changning intracratonic sag.