Impact Of Gypsolith On The Preservation And Exploration Prospect Of Shale Gas In Sichuan Basin

Shale gas is one of the unconventional gas resources which are under exploration. The shale itself serves as source and reservoir in shale gas systems. Shale gas storage is usually free gas, adsorbed gas, or dissolved gas. The depth and thickness of shale, tectonic movement and overlying strata all have great impact on the accumulation of shale gas.Sichuan basin is located in western upper Yangtze region. It is a polycyclic superimposed basin which undergone six stages evolution. Because of the plate movement since Yanshan movement period, Sichuan basin now is bounded by Longmenshan fold thrust belt, Qinling-Dabie Orogen and Jiangnan Orogen. Two sets of rich-organic marine sediment rocks are deposited in the basin, which are high in TOC, high maturity and deep buried, The halite layer of Lower Cambrian and Triassic plays an important controlling role in tectonic deformation, resulting in different structural styles in Sichuan Basin.Compared with the geological characteristics of different shale gas basins in USA and lower Yangtze region, it is suggested that Qiongzhusi Formation of Lower Cambrian and Longmaxi Formation of Lower Silurian have good hydrocarbon potential of shale gas. Since Sichuan basin experienced complex tectonic movement during its evolution, it is suspected that impact of tectonism on deformation of basin is the key factor to the preservation and exploration of shale gas. According to analysis of structural deformation feature of Sichuan basin, it is considered that the decollement fold-thrust area in eastern Sichuan basin where tectonic deformation is weak is the favorable area of shale gas exploration.Based on analysis of former researches, drilling data and seismic data, this paper interpreted the impact of gypsolith on the preservation and exploration prospect of shale gas in Sichuan Basin. Results show that the halite layer of Lower Cambrian and Triassic results in different structural styles in Sichuan Basin. The halite layer of Lower Cambrian distributed between Qiyueshan fault and Huayinshan fault, as a detachment, leads to the formation of the thin-skinned fold-thrust structure in the eastern Sichuan basin. The formation under the wide-distributed gypsolith tends to be overpressure because of the halite layer. Gypsolith in Sichuan Basin is one of the important factors which control the shale gas exploration of Longmaxi Formation. Based on the distribution of salt layers, depth and thickness of shale, and organic geochemistry characteristics, we suggest that Zigong-Yibin, Chongqing-Yongchuan, and Fuling are the favorite shale gas exploration region of Sichuan Basin. Weiyuan area is the potential region of shale gas exploration.