准噶尔盆地沙湾凹陷周缘中、浅层天然气地球化学特征及成因

Geochemical characteristics and genesis of mid-to-shallow natural gas on the periphery of Shawan Sag， Junggar Basin

李二庭 靳军 王剑 马万云 陈世加 刘翠敏 王海静

Erting Li Jun Jin Jian Wang Wanyun Ma Shijia Chen Cuimin Liu Haijing Wang

沙湾凹陷周缘天然气混源现象普遍，前期缺少对地区的整体研究，制约了研究区天然气成藏研究。为此，系统开展了天然气地球化学特征分析，结合烃源岩热模拟技术，明确研究区中、浅层天然气的成因。研究显示，沙湾凹陷周缘中、浅层天然气以甲烷为主，干燥系数分布在0.73～1.00，δC值分布在-56.0 ‰～-31.5 ‰，反映研究区成熟与高-过成熟天然气共存；δC值分布在-30.4 ‰～-22.8 ‰，反映研究区煤型气、油型气和混合型气均有分布。结合烃源岩热解气碳同位素特征，认为研究区天然气具有4种成因类型：Ⅰ类天然气来源于佳木河组烃源岩，主要分布在红车断裂带中段白垩系，具有极重的δC值，大于-25.5 ‰，C轻烃中甲基环己烷含量大于50 %；Ⅱ类天然气分布少，主要为原油降解次生生物气，具有异常偏负δC值和极高的干燥系数；Ⅲ类天然气来源于下乌尔禾组烃源岩，主要分布在小拐地区及红车断裂带南段侏罗系，δC值分布在-27.9 ‰～-26.4 ‰，具有混合型烃源岩特征；Ⅳ类天然气为下乌尔禾组烃源岩与风城组烃源岩混源，主要分布在红车断裂带南段、北段及金龙地区，以下乌尔禾组来源为主的天然气δC值大于-29 ‰，以风城组来源为主的天然气δC值小于-29 ‰。

Natural gas on the periphery of Shawan Sag， is generally derived from mixed sources. A lack of holistic study on this area in the early stage has restricted the understanding of natural gas accumulation therein. In this regard， a systematic analysis of geochemical characteristics of the natural gas is carried out by means of source-rock thermal simulation， by which the genetic types of mid-to-shallow gas on the periphery of the Shawan Sag are identified. The research results show that the natural gas in the study area is dominated by methane， with a drying coefficient ranging from 0.73 to 1.00， and a δC value from -56.0 ‰ to -31.5 ‰， indicating coexistence of mature and high-to-over-mature natural gas therein. A δC value varying between -30.4 ‰ and -22.8 ‰ reflects a mixed distribution of coaliferous， petroliferous and mixed gas. In addition， the analysis of carbon isotopic characteristics of source-rock pyrolysis gas， shows that the natural gas in the study area is of 4 types in genesis： Type Ⅰ natural gas， featuring a very high δC value greater than -25.5 ‰ and a methylcyclohexane content greater than 50 % in lighter hydrocarbon C7， is derived from source rocks of the Jiamuhe Formation， and mainly distributed in the Cretaceous in the middle of Hongche fault belt； Type Ⅱ natural gas of sparse distribution is mainly secondary biogas from crude oil degradation， with an abnormally negative δC value and an extremely high drying coefficient； Type Ⅲ natural gas is originated from the Xiawuerhe Formation source rocks and mainly distributed in the Jurassic Formation in Xiaoguai area and southern Hongche fault belt， with a δC value varying from -27.9 ‰ to -26.4 ‰， a characteristic of mixed source rocks； and Type Ⅳ natural gas is mainly derived from source rocks of both Xiawuerhe Formation and Fengcheng Formation， and mainly distributed in the southern Hongche fault belt and northern Jinlong area. Among others， the natural gas mainly derived from the Xiawuerhe Formation features a δC value greater than -29 ‰， and that from the Fengcheng Formation features a δC value less than -29 ‰.

10.11743/ogg20220114