Shiyou shiyan dizhi (Mar 2023)
Reason for abnormally high drying coefficient of natural gas in Cainan area, Junggar Basin
Abstract
In order to clarify the reason for generally high drying coefficient of natural gas in Cainan area of the Junggar Basin and find out the law of natural gas migration and accumulation, the analysis of natural gas components and carbon isotopes, rock mineral composition in reservoir, bulk carbon and oxygen isotope of calcite, and laboratory hydrocarbon oxidation simulation experiments were carried out. The Jurassic natural gas in Cainan area is dominated by methane, with drying coefficient of generally greater than 0.95, and δ13C1 value of basically greater than -32‰. Among C7 light hydrocarbons, methylcyclohexane is dominant, with methylcyclohexane index of greater than 50%, indicating that natural gas in the study area came from high-over mature Carboniferous source rocks. Judging from natural gas migration identification index of ln(C1/C2) with δ13C1-δ13C2, from well block Cai-47 to well block Cai-31, and then to well block Cai-003, ln(C1/C2) values gradually increased, but δ13C1-δ13C2 values did not show a trend of decreasing or increasing, indicating that migration or maturity is not the main controlling factor for the changes of natural gas composition and carbon isotope in the study area. Hydrocarbon thermal oxidation simulation experiments showed that alcohols in oil and gas were oxidized by MnO2 to generate methane and carbon dioxide at 125℃, and methane could only be oxidized to generate CO2 when the temperature reached 200℃, thus changing the composition of oil and gas and increasing methane content in natural gas. Using backscattered electron probe technology, it was found that there are two types of calcite in Jurassic dry gas interval in the study area. One type of calcite has a high Mn content, which can be as high as 3%. It appears bright orange and orange under cathodoluminescence. In addition, the bulk carbon isotope of calcite is negatively biased, which is 5‰-10‰ more negative than that of normal calcite. The higher content of Mn in calcite, the more negative of bulk carbon isotope, confirming that there is widespread weak oxidation of hydrocarbons in the Jurassic dry gas interval in Cainan area. Comprehensive analysis suggested that the reason for abnormally high drying coefficient of Jurassic natural gas in Cainan area is that the humic source rocks have undergone high-over mature evolution to generate natural gas that accumulated in Jurassic reservoirs rich in oxidizing minerals. Then the oxidation of hydrocarbons caused methane content in natural gas to increase further, resulting in a generally high drying coefficient of natural gas.
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