Assessment of Mesozoic tight-oil and tight-gas resources in the Sichuan Basin of China, 2018

In 2018, the U.S. Geological Survey (USGS) quantitatively assessed the potential for unconventional (continuous) oil and gas resources in Mesozoic nonmarine clastic rocks in the Sichuan Basin of China (fig. 1) that include tight gas in the Upper Triassic Xujiahe Formation and tight oil in Lower Jurassic lacustrine strata of the Lianggaoshan Formation and the Da’anzhai Member of the Ziliujing Formation. Previous USGS oil and gas assessments in this basin include a 2015 shale-gas assessment in three Paleozoic stratigraphic intervals in the Sichuan Basin (Potter and others, 2015; Potter, 2018) and a 2012 assessment of conventional oil and gas resources in six major Chinese basins (Charpentier and others, 2012). Chinese national oil companies currently produce tight gas from the Xujiahe (Zhao, Bian, and others, 2013) and tight oil from the Da’anzhai and Lianggaoshan (Chen and others, 2015; Yang and others, 2016) in the central part of the Sichuan Basin. The Xujiahe is a thick fluvial unit that includes three widely distributed coaly gas-prone (Type III) source intervals, each generally 50–150 meters (m) thick and containing numerous coal beds that are a few meters thick (Zou, Tao, and others, 2009; Zhu and others, 2012). These source intervals are alternately stacked with three low-permeability sand reservoir intervals (up to 40 m thick with individual sand reservoirs 3–8 m thick) (Zou, Tao, and others, 2009; Zhao, Bian, and others, 2013; Zou, Gong, and others, 2013). The Da’anzhai and Lianggaoshan contain lacustrine black shales that are rich in Type I and II (oil-prone) organic matter (Li and others, 2014) and are interbedded with tight reservoir units that include a shelly limestone (Da’anzhai) and a sandstone (Lianggaoshan) (Yang and others, 2016). The Da’anzhai is 0–60 m thick, and the Lianggaoshan is 0–100 m thick (Yang and others, 2016).