准噶尔盆地盆1井西凹陷石炭系—二叠系天然气特征及成藏主控因素

Characteristics and main controlling factors of natural gas of Carboniferous-Permian in western well Pen-1 sag,Junggar Basin

综合利用地震、钻井、测井、岩心、薄片、有机地球化学等资料,对准噶尔盆地盆1井西凹陷石炭系—二叠系天然气的地球化学特征、成因、来源、成藏主控因素以及勘探潜力进行了探讨。研究结果表明:①盆1井西凹陷深层石炭系—二叠系天然气中甲烷的体积分数为70.93%~96.55%,平均值为81.78%,总体为湿气。碳同位素分布相对较广,甲烷碳同位素值为−41.62‰~−30.42‰,平均值为-34.75‰,天然气大多处于成熟—高成熟演化阶段;乙烷碳同位素值为−31.69‰~−24.16‰,平均值为−27.50‰,可见天然气成因以煤型气为主,其次为混合成因气。C7轻烃化合物中甲基环己烷和正庚烷的相对含量均较高,甲基环己烷相对质量分数为32.14%~58.37%,平均值为38.84%,正庚烷相对质量分数为25.37%~56.56%,平均值为47.79%。②研究区天然气成因类型及来源复杂,主要来自于二叠系下乌尔禾组烃源岩的煤型气及其与风城组油型气的混合气,来自风城组烃源岩的油型气较少。③研究区二叠系风城组和下乌尔禾组烃源岩生气潜力大,多种岩性储层发育,断裂和不整合面渗透性砂体为深层天然气的运移成藏提供了输导体系,且石炭系的古凸起和二叠系的岩性圈闭形成互补之势,对天然气在斜坡区的聚集起到了良好的侧向遮挡作用。④研究区鼻凸构造带石炭系火山岩和洼槽二叠系风城组碎屑岩2类规模较大的勘探领域是下一步盆1井西凹陷深层天然气勘探的潜力区。

Based on the seismic,drilling,logging,core,thin section and organic geochemical data,the geochemical characteristics,genesis,sources,main controlling factors of gas reservoir accumulation and exploration potential of natural gas of Carboniferous-Permian in western well Pen-1 sag of Junggar Basin were studied.The result shows that:(1)The volume fraction of methane in deep Carboniferous-Permian natural gas in western well Pen-1 sag is 70.93%-96.55%,with an average of 81.78%,which is generally wet gas.The carbon isotope distribution is relatively wide.The carbon isotope value of methane ranges from -41.62‰to -30.42‰,with an average value of -34.75‰,and most of them are in mature to high mature stage.The carbon isotope value of ethane ranges from -31.69‰to-24.16‰,with an average value of -27.50‰,which means that coal type gas is the main origin of natural gas,followed by mixed origin gas.The relative contents of methylcyclohexane and n-heptane in C7 light hydrocarbon compounds are large,the relative mass fraction of methylcyclohexane is 32.14%-58.37%,with an average of 38.84%,and the relative mass fraction of n-heptane is 25.37%-56.56%,with an average of 47.79%.(2)The natural gas in the study area has complex origin and sources,the coal-type gas from the source rocks of lower Urho Formation and its mixture with oil-type gas of Fengcheng Formation are absolutely dominant,while the oil-type gas from the source rocks of Fengcheng Formation is less.(3)The source rocks of Permian Fengcheng Formation and lower Urho Formation in the study area have great gas generation potential,and multiple lithologic reservoirs are developed.Faults and unconformity permeable sand bodies constitute a transport system for deep gas migration and accumulation.Carboniferous paleo convex and Permian lithologic trap are complementary,which plays a good role of lateral shielding for the accumulation of natural gas in the slope area.(4)Carboniferous volcanic rocks in nose-bulge and clastic rocks of Permian Fengcheng Formation in sags in the study area are potential favorable areas for deep natural gas exploration in western well Pen-1 sag.