THE GEOCHEMICAL STUDY ON THE UPPER TRIASSIC XUJIAHE GASES IN FLUID INCLUSIONS AND IN TIGHT SAND GAS FIELDS OF THE MIDDLE SICHUAN BASIN

The Sichuan basin has stable structures with areas up to 18×10⁴km², a prolific gas-pay-zones and gas-fields basin. The Upper Triassic sand-mudstone-dominated Xujiahe formation of Sichuan basin is a typical terrestrial coal-series clastics deposited on the Middle Triassic Leikoupo erosion surface. Our research is based on the comparative geochemical analysis of gas samples from inclusions and tight sand gas Fields, integrated with geologic conditions, intend to clarify geochemical characteristics of Sichuan Xujiahe gas formation and evolution.

The Xujiahe dark muddy source rocks are mainly type-Ⅱ₂～Ш kerogen. The source maturity is of 0.8～1.4%. The Xujiahe reservoir rocks have large amount of gaseous hydrocarbon inclusions, little liquid hydrocarbon inclusions, indicating coal series type generates mainly gas but less oil. 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/C_1-5) 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 tight sand gas Fields, but slightly heavier totally, with the δ¹³C₁ of -36‰～-45‰ and δ¹³C₂ of -24.8‰～-28.1‰, characterized as coal-type gas. The δ¹³C_CO2 of tight sand 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, a relatively closed system, was derived from source rocks, and abiogenic CO₂ was mixed less, therefore, characterizing as heavier carbon isotopic composition for alkane gas and lighter for that of CO₂.

The gases captured in fluid inclusions reflect the primitive state that source rocks generated gas during the ancient time, 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. The study results have some indicative maker significance for gas-generating evolution and characteristics identification of migration and accumulation.