Discovery of Wanyuan-Dazhou Intracratonic Rift and its significance for gas exploration in Sichuan Basin, SW China

Abstract: New understandings of the geology of Sichuan Basin were achieved in the progress of natural gas exploration in the Sinian-Cambrian strata in Sichuan Basin. An NE trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic data interpretation, outcrop data and regional tectonic background analysis, we studied the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift. The following findings were obtained: (1) The seismic section indicates that a steep-slope belt indicating platform margin facies was developed during the deposition of the first and second Member (Z2dn1-Z2dn2) of the Sinian Dengying Formation. The rift generally strikes NE. (2) The thicknesses of the first and second Member of the Dengying Formation are thicker than the third and fourth Member (Z2dn3-Z2dn4) at the periphery of the rift and vice versa inside the rift. (3) The rift formed during the deposition of Z2dn1-Z2dn2 in the Sinian. Filling and subsidence occurred during the deposition of Z2dn3-Z2dn4 in the Sinian. The shrinkage of the rift happened during the deposition of the Lower Cambrian Maidiping Formation – Qiongzhusi Formation. Regional extension and uplift in the Nanhua controlled rift formation. The discovery of the Wanyuan-Dazhou rift changed the traditional understanding of the tectonic and sedimentary framework of Sichuan Basin in the Sinian and Early Cambrian. The periphery of the rift has significant potential for natural gas exploration due to its superior natural gas accumulation conditions in the Sinian. Key words: Sichuan Basin, Northeastern Sichuan, Wanyuan-Dazhou, Sinian-Early Cambrian, intracratonic rift, geological feature