鄂尔多斯盆地奥陶系盐下天然气来源及勘探潜力

Sources and exploration potential of Ordovician subsalt natural gas in Ordos Basin

随着新区、新层系勘探力度持续加大,鄂尔多斯盆地奥陶系盐下马五6亚段—马四段天然气勘探取得重大突破,盐下天然气的来源及勘探潜力问题备受关注。应用地质、地球化学方法,在明确天然气地球化学特征、成因及来源的基础上,对其源岩的沉积环境、有机地球化学特征、空间发育规模及资源潜力开展综合研究。结果表明:①奥陶系盐下天然气以“高温裂解干气”为主,其中,低(不含)硫天然气的δ^(13)C_(1)值偏低,平均值为−39.6‰,δ^(13)C_(2)值为−35.6‰~−25.8‰,变化幅度大,高硫天然气的δ^(13)C_(1)、δ^(13)C_(2)值均偏高,反映TSR作用阶段不同,天然气组分及碳同位素组成受影响程度不同;②盐下天然气属于自生自储“油型气”,供气源岩为奥陶系盐下海相烃源岩;③奥陶系盐下发育黑色泥质岩、暗色泥质云岩(云质泥岩)和薄层藻泥晶灰岩3种烃源岩类型,除泥晶灰岩外,主要形成于高盐缺氧的局限海潟湖沉积环境,沉积水体分层显著,属还原环境;该套源岩有机质丰度较高,TOC平均值为0.45%,生烃母质以菌、藻类为主,有机质演化达到高-过成熟阶段,总生气量约为43.8×10^(12) m^(3),可为远离上古生界气源的盐下有利储集体供烃。

With the continuous increase of the exploration efforts in new zones and new strata,a major break⁃through has been made in the natural gas exploration of O1m56 to O1m4 in Ordos Basin.Thus,the origin and explo⁃ration potential of subsalt natural gas have attracted much attention and need to be solved urgently.On the basis of certain geochemical characteristics,genetic types and sources of natural gas,a comprehensive study on the sedimentary environment,organic geochemical characteristics and spatial distribution scale of the source rocks is conducted in this paper by using geological and geochemical methods.The study shows that:(1)The Ordovician subsalt natural gas is mainly“pyrolysis dry gas”;theδ^(13)C_(1) of the Ordovician subsalt low sulfur(non sulfur)nat⁃ural gas is lighter,with an average value of−39.6‰;theδ^(13)C_(3) ranges more largely from−35.6‰to−25.8‰;theδ^(13)C_(1) andδ^(13)C_(3) of high-sulfur natural gas are both heavier,which reveals that different Thermochemical Sul⁃fate Reduction(TSR)reaction stages have different degrees of influence on natural gas components and carbon isotope composition.(2)Subsalt natural gas belongs to“oil-type gas”of self-generation and self-accumulation,whose source rocks are mainly Ordovician subsalt marine source rock.(3)Three types of marine source rocks such as black argillaceous rock,dark argillaceous dolomite(dolomitic mudstone),and dark micrite(bioclastic)limestone,are developed in Ordovician subsalt.In addition to micrite limestone,it was mainly formed in a con⁃fined lagoon sedimentary environment with high salinity and anoxia.Sedimentary water was stratified significant⁃ly and the environment was reduced.The organic matter abundance of the source rocks is relatively high,with an average TOC of 0.45%.The hydrocarbon generating parent materials are mainly composed of bacteria and al⁃gae,and the organic matter evolution reaches high-over maturity stage.The total gas generation amount of the marine source rocks in Ordovician subsalt is about 43.8×10^(12) m^(3),which can provide hydrocarbon and accumu⁃late for the subsalt favorable reservoir facies far away from the Upper Paleozoic gas sources.