Abstract
Natural gas in the tight sandstone reservoirs in Member two of the Upper Triassic Xujiahe Formation in the Western Sichuan Depression of the Sichuan Basin has complex geochemical characteristics, and the origin and source of the tight gas are revealed based on the geochemical analysis and comparison in this study. The tight gas has the dryness coefficient of 0.950–0.994, which is positively correlated with the CH4 content. The gaseous alkanes display positive carbon isotopic series, with the δ13C1 and δ13C2 values ranging from −35.6‰ to −30.3‰ and from −29.1‰ to −21.0‰, respectively, and the δD1 values range from −176‰ to −155‰. Genetic identification based on the carbon and hydrogen isotopic compositions indicates that a small amount of tight gas is typically coal-derived gas, whereas most of the tight-gas samples have experienced mixing by oil-associated gas. Geochemical comparisons suggest that the tight gas displays distinct differences with the typical oil-associated gas in Member four of the Middle Triassic Leikoupo Formation in the Chuanxi gas field and the typical coal-derived gas in Member five of Xujiahe Formation in the Xinchang gas field. It is also apparently different from the typical coal-derived gas in Member two of Xujiahe Formation in both Zhongba and Qiongxi gas fields. Among the tight gas in Member two of the Xujiahe Formation from the Western Sichuan Depression, the coal-derived gas is generated mainly by the humic mudstone in the Upper Triassic Ma’antang and Xiaotangzi formations, with assistance of the humic mudstone in Member two of the Xujiahe Formation, whereas the oil-associated gas is derived from the sapropelic limestone in the Ma’antang Formation.
Highlights
Tight-gas reservoirs refer to the tight sandstone fields or traps which accumulate commercial natural gas, and they can be divided into continuous-type and trap-type (Dai et al, 2012a)
The T3x2 gas from Western Sichuan Depression (WSD) is mainly composed of CH4, and the CH4 content ranges from 93.03% to 98.14% with an average of 96.49%, whereas the heavy alkane (C2–C4) content ranges from 0.59% to 4.92% with an average of 1.72% (Table 1)
The T3x2 gas from WSD is dry with the dryness coefficient (C1/C1-4) ranging from 0.950 to 0.994 (Table 1), and it displays a positive correlation between the CH4 content and C1/C1-4 ratio (Figure 3A), suggesting the effect of thermal maturity
Summary
Tight-gas reservoirs refer to the tight sandstone fields or traps which accumulate commercial natural gas, and they can be divided into continuous-type and trap-type (Dai et al, 2012a). The Sichuan Basin is one of the important onshore petroliferous basins in China, and the exploration fields include terrigenous and marine strata (Dai et al, 2009; Dai et al, 2012b; Liu et al, 2012; Liu et al, 2013; Liu et al, 2014; Liu et al, 2016), with the former consisting of tight sandstone reservoirs in both the Upper Triassic Xujiahe Formation (T3x) and Jurassic strata. The Zhongba gas field in Member two of the Xujiahe Formation (T3x2), located in the western Sichuan Basin, is the first tight-gas field discovered in China (Dai et al, 2014). Several giant tight-gas fields (e.g., Xinchang, Guang’an, Hechuan, and Anyue) with proven reserves exceeding 10 × 109 m3 have been revealed during the gas exploration in the Xujiahe
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