柴达木盆地东坪—牛东地区天然气地球化学特征及来源探讨

Geochemical characteristics and origin of natural gas in DongpingNiudong areas,Qaidam Basin,China

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。

The geochemical characteristics,origin and source of natural gas in Dongping-Niudong areas are researched based on the information of components and stable carbon isotopes of natural gas.The main component of hydrocarbon gas is methane,and C2＋possesses relatively low content,ranging from 0.5% to 6.5%.The dryness coefficient of Dongping area is more than 0.97,belonging to dry gas.The dryness coefficient of Niudong area ranged from 0.90 to 0.98,having the characteristics of dry gas coexisting with wet gas.δ^13_1varies from-17.58‰ to-36.4‰,and δ^13_2 is greater than-29‰.The gas of Dongping 3 block have the characteristics of abiogenic gas,such as reversed carbon isotope series.Combined with the relationship between the component of CO2and δ^13_（CO_2） and the background of development of hydrocarbon-generating sag,it suggests that it is coal-derived gas.The ROand fluid inclusion studies show that the gas in Niudong area is mainly derived from Early Jurassic coal-measure source rocks in Kunteyi Sag,and there are two periods of hydrocarbon fillings.The first filling is mainly oil,and the second is mainly high mature gas;gas in Dongping area is mainly derived from Early Jurassic coal-measure source rocks of Pingdong and Yiliping Sags,and there are four periods of high-over mature gas fillings.The phenomenon of reversed carbon isotope series in Dongping area resulted from the multiple gas charging.