鄂尔多斯盆地寒武系-奥陶系天然气成藏模式及有利勘探区

Natural gas accumulation models and favorable exploration areas of the Cambrian-Ordovician in the Ordos Basin

鄂尔多斯盆地下古生界寒武系—奥陶系海相地层经过30多年的勘探开发,已累计提交了超万亿立方米的天然气三级规模储量,并取得了一系列重大理论成果。面对中国逐年上升的油气需求,维持产量快速增长与勘探接替领域后备资源不足的矛盾日益突出,迫切需要寻找规模储量来夯实资源基础。为了寻找新的潜在天然气勘探领域,在已取得的天然气勘探成果基础上,通过地震、野外剖面、钻井岩心、微观薄片及化验测试等资料分析,重点对鄂尔多斯盆地寒武系—奥陶系烃源岩条件、储集层特征及天然气成藏模式开展了系统研究,并预测了有利勘探区。研究结果表明:①盆地寒武系具有“新生古储、侧向供烃”和“下生上储、断裂输导供烃”天然气成藏模式,上古生界煤系烃源岩和下寒武统烃源岩是寒武系气藏的供烃源岩,发育规模的裂缝—孔隙(洞)型白云岩储层有利于天然气聚集;②盆地东部奥陶系盐下地层具有“自生自储、断裂输导供烃”天然气成藏模式,其内部发育的泥质烃源岩和分散可溶有机质2类海相烃源岩具备供烃能力,沿古隆起分布的白云岩储层可为天然气富集提供规模储集空间;③盆地西部具有海相页岩“源内自生自储”与白云岩“上生下储、断裂侧接供烃”2种天然气成藏模式,克里摩里组和乌拉力克组泥(页)岩具备规模供烃能力,沿隆起发育的白云岩储层和沿坳陷发育的页岩储层具备规模储集能力。结论认为,盆地南部吴起—环县与正宁—黄陵地区寒武系、盆地东部榆林—靖边地区奥陶系马二段—马三段和盆地西部陶乐地区与马家滩地区奥陶系白云岩是下一步寻找油气规模发现的战略领域,该研究成果对于指导鄂尔多斯盆地寒武系—奥陶系油气勘探选区及有利目标评价具有重要意义。

After over 30 years of exploration and development,the Cambrian-Ordovician marine strata of Lower Paleozoic in the Ordos Basin have submitted over one trillion cubic meters of 3P reserves in total,and a series of significant theoretical achievements have been obtained.With increasing oil and gas demands in China,the contradiction between sustaining rapid production growth and insufficient exploration reserve replacement is intensified,so it is necessary to search for substantial reserves to solidify resource foundations.To identify new potential natural gas exploration areas,this paper systematically studies the source rock conditions,reservoir characteristics,and natural gas accumulation models of the Cambrian-Ordovician in the Ordos Basin and predicts the favorable exploration areas by analyzing seismic,field profiles,drilling cores,microscopic thin sections and test data,on the basis of existing natural gas exploration achievements.The following research results are obtained.First,the Cambrian in the Ordos Basin exhibits the natural gas accumulation models of"new source rock,old reservoir,lateral hydrocarbon supply"and"lower source rock,upper reservoir,fault-conducted hydrocarbon supply",with the Upper Paleozoic coal-measure source rocks and the Lower Cambrian source rocks serving as the hydrocarbon source rocks of Cambrian gas reservoirs and the extensive fractured-porous(vuggy)dolomite reservoirs facilitating natural gas accumulation.Second,the Ordovician sub-salt layers in the east of the Ordos Basin exhibits a natural gas accumulation model of"self-source rock,self-reservoir,fault-conducted hydrocarbon supply",with the internal argillaceous source rocks and the dispersed soluble organic matters capable of hydrocarbon supply and the dolomite reservoirs along paleo-uplifts providing substantial reservoir space for natural gas enrichment.Third,there are two natural gas accumulation models in the west of the Ordos Basin,i.e.the"self-source rock and self-reservoir within the source rock"of marine shale,and the"upper-source rock,lower-reservoir,fault lateral hydrocarbon supply"of carbonate rock,with the Kelimoli Formation and the Wulalike Formation mudstone(shale)capable of substantial hydrocarbon supply and the dolomite reservoirs along uplifts and the shale reservoirs along depressions offering significant reservoir capacities.In conclusion,the Cambrian in Wuqi-Huanxian and Zhengning-Huangling areas of southern Ordos Basin,the 2nd and 3rd Members of Ordovician Majiagou Formation in Yulin-Jingbian area of eastern Ordos Basin,and the Ordovician dolomite in Taole and Majiatan areas of western Ordos Basin are strategic domains for future substantial petroleum discoveries.The research results are of great guidance to the selection and favorable target evaluation of Cambrian-Ordovician oil and gas exploration areas in the Ordos Basin.