准噶尔盆地玛湖凹陷北部—乌夏断裂带天然气成因与来源

Origin and source of natural gas from Wuxia Fault Belt in the northern Mahu Sag,Junggar Basin

针对准噶尔盆地目前勘探程度相对较高的玛湖凹陷北部—乌夏断裂带天然气特征变化较为复杂的实际,通过天然气赋存特征、天然气与相关烃源岩有机地球化学特征的详细分析,对天然气的成因与来源进行了系统的研究。研究发现,该区已有的天然气主要为原油溶解气,少量气层气主要分布在乌夏断裂带的中生界,为原油溶解气溢出而形成的浅层气。天然气以烃类化合物为主要组分,二氧化碳与氮气等非烃含量相对较低。天然气干燥系数介于0.66～0.99之间,主要为湿气,少数干气主要分布在较浅的部位,是运移分异作用较强、保存条件较好的结果。天然气成因类型主要为腐泥型和偏腐泥型的过渡型,未见腐殖型天然气。气源分析表明天然气主要来自风城组湖相腐泥型—混合型母质的烃源岩,乌尔禾组腐殖型母质烃源岩有少量贡献,佳木河组腐殖型母质烃源岩对玛湖凹陷北部斜坡区断裂发育部位的局部天然气有贡献。

There is higher exploration degree and complicated natural gas composition in Wuxia Fault Belt and the west slope region of Mahu Sag in the Junngar Basin at present.Based on the analyses of characteristics of occurrence,and organic geochemistry of natural gases and source rocks,the authors systematically investigated the origin and source of natural gases.The natural gas which has been discovered substantially belongs to dissolved gas in crude oil,while a minor amount of gas-layer gas mainly distributes in the Mesozoic of Wuxia Fault Belt,belonging to shallow gas-layer gas resulted from ex-solution of gas which was previously dissolved in crude oil.Natural gas was dominated by alkane gas with a relatively low amount of carbon dioxide,nitrogen and other non-hydrocarbon compounds.The gas dryness coefficient ranges between0.66 and 0.99,and mainly belongs to wet gas,while minor dry gas distributes in shallower part as a consequence of stronger migration differentiation.The natural gas is mainly of sapropelic and mixed origin which is approaching to sapropelic.Gas of humic origin is rarely found.The analytical results of gas-source indicate that the gas is mainly derived from Fengcheng Formation lacustrine source rock with sapropelic-mixed precursors.The contribution of the Wu＇erhe Formation source rock with humic precursors is small,and the Jiamuhe Formation source rock with humic precursors has contribution to local gas accumulation in the slope region with developed faults.