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

The Upper Triassic Xujiahe Fm. in Sichuan basin is typical terrestrial coal-series clastics. It is composed by three sets of pervasively coal-series source rocks and three sets of large fluvial delta sand-bodies, superimposed vertically as sandwich-structure plays.

The dark muddy source rocks in Xujiahe Fm. are mainly type-II₂ kerogen, but coal series are type-III. The source maturity is generally poor, with R_o of 0.8~1.4%. Since the hydrocarbon (HC) generation peak during the Cretaceous period, 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 HC inclusions formed at different geologic time, little liquid HC 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⁺ HCs, 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⁺ HCs, while that of non-HC (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‰x-45‰ and δ¹³C₂ of -24.8‰x-28.1‰, characterized as coal-type gas. The δ¹³C_CO2 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, 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-HC CO₂. The study results have some indicative maker significance for gas-generating evolution and characteristics identification of transportation and accumulation.