THE ROLE OF SALT IN THE PRESERVATION AND EXPLORATION PROSPECT OF SHALE GAS IN SICHUAN BASIN, WESTERN CHINA

Sichuan basin is located in western China, bounded by the Longmen Shan fold thrust belt to the west, North China Block and Qinling-Dabie Orogen to the north, and Jiangnan Orogen to the southeast. The basin consists of a Proterozoic basement of the Yangtze Craton covered by Sinian to Middle Triassic shallow-marine rocks and Upper Triassic to Paleogene nonmarine sedimentary rocks. It is an important petroliferous basin, and recently the most promising shale gas exploration target basin in China. At present, a large number of evidences of shale gas have been found in the Sichuan Basin, and quite a few wells show high production. However the "sweet spots" of the shale gas in the Sichuan basin is still a mystery to be solved. Since the Basin experienced complex tectonic movements during the evolution from Craton basin to foreland basin, it is widely suspected that impact of tectonic deformation is critical to solove the shale gas "sweet spots" mystery of Sichuan basin. In this research, based on the drilling data, seismic data interpretation, physical and numerical modeling, we analyzed the role of salt layers in the kinematics and mechanism of structural deformation in the basin, and hence the impact on the preservation and exploration prospect of shale gas. Results show that the halite layer of Lower Cambrian play an important controlling role in tectonic deformation, resulting in different structural styles in Sichuan Basin. The halite layer of Lower Cambrian is primarily distributed in the eastern Sichuan basin, which works as a weak detachment. As result, a wide region of thin-skinned fold-thrust belt formed in the eastern Sichuan basin, bounded in the east by the Qiyueshan fault and in the west by Huayinshan. In the western and northern Sichuan basin, due to the absence of Lower Cambrian salt layer, narrow region of basement involved thrust belt formed. Salt layer brought in the variation of the kinematics and mechanism of structural deformation in the Sichuan basin, and as a result, significantly influenced the buried depth, temperature, and fracture of the primary target shale layer, the Longmaxi Formation. Combined the distribution of salt layers, depth and thickness of shale, and organic geochemistry characteristics, we suggest that southeast region is the most favorite shale gas exploration region of Sichuan Basin.