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以柴达木盆地东坪—牛东地区天然气组分及碳同位素数据为基础,分析了天然气地球化学特征及成因和来源。研究区烃类气体以甲烷为主,重烃(C~(2+))含量低,介于0.5%~6.5%之间。东坪地区天然气干燥系数>97%,为干气;牛东地区天然气干燥系数介于90.3%~98.1%之间,具有干气、湿气共存的特点。天然气δ13 C1值介于-17.58‰~-36.4‰之间,δ~(13)C_2平均>-29‰,其中东坪3区块烷烃碳同位素倒转,呈现无机成因气的特征,结合CO2含量与δ~(13)C_(CO_2)值的关系,以及研究区发育高成熟侏罗系煤系源岩的地质背景,综合研究认为天然气为有机成因的煤型气。通过天然气成熟度和流体包裹体研究,结合阿尔金山前东段的成藏模式,认为牛东地区天然气来源于昆特依凹陷下侏罗统煤系源岩,存在2期充注:第一期以油为主;第二期以高成熟天然气为主。东坪地区天然气来源于坪东—一里坪凹陷下侏罗统煤系源岩,存在4期高成熟和过成熟天然气的充注,并导致了烷烃碳同位素的倒转。
Based on the natural gas composition and carbon isotope data of Dongping-Niu Dong area in the Qaidam Basin, the geochemical characteristics of natural gas and their origin and origin are analyzed. Hydrocarbon gas in the study area is mainly methane, with low content of heavy hydrocarbon (C ~ (2+)), ranging from 0.5% to 6.5%. Natural gas drying coefficient of Dongping is> 97%, which is dry gas. The drying coefficient of natural gas in Niudong area ranges from 90.3% to 98.1%, with the coexistence of dry gas and moisture. The δ13C1 value of natural gas ranges from -17.58 ‰ to -36.4 ‰, and the δ13C_2 average> -29 ‰, of which the carbon isotopes of alkanes in Dongping 3 block are reversed, showing the characteristics of inorganic gas. Combined with the correlation between CO2 content and δ ~ (13) C_ (CO_2), and the geological background of Jurassic coal-bearing source rocks in the study area. Comprehensive studies suggest that natural gas is organic-type coal gas. Based on the study of gas maturity and fluid inclusions, combined with the accumulation model in the front section of the Altun Mountains, it is considered that the natural gas in the Cathay East region originated from the Lower Jurassic coal source rock in the Quaternary depression. There are two stages of filling: the first stage Oil-based; the second phase of high-maturity natural gas-based. Natural gas from Dongping area originated from the Lower Jurassic coal-measure source rocks in the Pingdong-Yiliping depression. Four high-mature and over-mature natural gas were charged, which led to the inversion of the carbon isotopes of alkanes.