--> --> The Geochemical Characteristics of Tight Sandstone Gas on the Upper Triassic Xujiahe Formation of Middle Sichuan Basin in China

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The Geochemical Characteristics of Tight Sandstone Gas on the Upper Triassic Xujiahe Formation of Middle Sichuan Basin in China

Abstract

There is a prolific gas-pay-zones and gas-fields in Sichuan basin with areas up to 18×104km2. The exploration on the study area indicated that its reserves is over 0.6×1012m3. The research is to clarify geochemical characteristics of Sichuan Xujiahe gas formation and evolution, based on the comparative geochemical analysis of gas samples from inclusions and gas fields, integrated with geologic conditions. The Sichuan Xujiahe dark muddy source rocks are mainly type-II2 and type-III kerogen, but coal series are type-III. The source maturity is generally poor, with Ro of 0.8–1.4%. They are presently at post-mature to early high-mature stage, but the gas to oil ratio within the present gas reservoirs is high and low condensate oil content (average of 3.8–91.9g/m3). The Xujiahe reservoir rocks have large amount of gaseous hydrocarbon inclusions formed at different geologic time, little liquid hydrocarbon inclusions, indicating coal series type generates mainly gas but less oil (Dai et al., 1997; Dai et al., 2012). The Xujiahe natural gases are dominated by methane, with higher concentration of heavier C2+ hydrocarbons, belonging to kerogen-degraded gas, and most gas dryness ratios (C1/C1–5) less than 0.95, main wet gas. The content of methane in the inclusions is low, rather lower for those of C2+ hydrocarbons, while that of non-hydrocarbons (CO2) is higher. The gas composition in gas fields does not include H2S; the gas δ13C1 ranges from −45.5‰ to −36.5‰ and δ13C2 from −30‰ to −25‰, and those two values in fluid inclusions are similar to those of gas fields, but slightly heavier totally, with the δ13C1 of −36‰–−45‰ and δ13C2 of −24.8‰–−28.1‰, characterized as coal-type gas (Dai et al., 2012). The δ13CCO2 of gas fields ranges from −15.6‰ to −5.6‰, and that of inclusions is totally lighter, from −16.6‰ to −9‰, as organic origin gas. The CO2 captured in the inclusions in a relatively closed system, characterizing as heavier carbon isotopic composition for alkane gas and lighter for that of CO2, was derived from source rocks, and abiogenic CO2 was mixed less. The gases captured in fluid inclusions reflect the primitive state that source rocks generated gas during the ancient time (Tao et al., 2003a; Tao et al., 2003b; Tao et al., 2003c), and rather weak isotopic filtration for gases in a closed system, thus it is characterized as heavier carbon isotopes for alkane gas and lighter for non-hydrocarbon CO2.