鄂尔多斯盆地东部米探1井奥陶系马四段天然气勘探新发现及勘探方向

近期,鄂尔多斯盆地米探1井在奥陶系马家沟组四段(以下简称马四段)取得了天然气勘探新发现,展示了该盆地东部马四段具有良好的天然气勘探前景,然而,该区马四段天然气成藏条件研究十分薄弱,制约了该区下一步天然气的勘探部署。为此,基于米探1井马四段钻探新成果,综合野外露头、地震、测井、岩心、微观薄片、储层物性分析和地球化学分析等资料,系统研究了天然气成藏条件,建立了马四段天然气成藏模式,并预测了天然气勘探有利区带。研究结果表明:①鄂尔多斯盆地东部马四段天然气藏为自生自储油型气藏,烃源岩分布于桃利庙坳陷和米脂坳陷中的马三段—马一段海相地层;②该区马四段沿横山隆起、米脂坳陷中凸起带发育台内滩、台内丘相储层,其中台内滩相储层岩性为砂(砾)屑白云岩和粉—细晶白云岩,储集空间主要为粒间孔、晶间(溶)孔和微裂缝,平均孔隙度为4.95%,台内丘相储层岩性主要为微生物白云岩、斑状粉晶白云岩、斑状灰质粉晶白云岩和斑状白云质石灰岩,储集空间类型有晶间(溶)孔和微裂缝,平均孔隙度为2.47%;③断裂和微裂缝是马三段—马一段海相气源向马四段运移的通道;④马四段发育岩性圈闭和构造—岩性圈闭,其内部及上部地层中发育多套石灰岩和石膏盐岩盖层,为天然气聚集提供了有利的场所和遮挡条件。结论认为,鄂尔多斯盆地东部马四段天然气成藏条件良好,是未来天然气勘探的重点接替领域,其中榆林—靖边地区的台内滩和神木—米脂地区的台内丘为天然气勘探有利方向。

New understandings on gas accumulation and major exploration breakthroughs in subsalt Ma 4 Member of Ordovician Majiagou Formation,Ordos Basin,NW China

Geological conditions and main controlling factors of gas accumulation in subsalt Ma 4 Member of Ordovician Majiagou Formation are examined based on large amounts of drilling,logging and seismic data.The new understandings on the control of paleo-uplift over facies,reservoirs and accumulations are reached:(1)During the sedimentary period of Majiagou Formation,the central paleo-uplift divided the North China Sea in central-eastern of the basin from the Qinqi Sea at southwest margin of the basin,and controlled the deposition of the thick hummocky grain beach facies dolomite on platform margin of Ma 4 Member.Under the influence of the evolution of the central paleo-uplift,the frame of two uplifts alternate with two sags was formed in the central-eastern part of the basin,dolomite of inner-platform beach facies developed in the underwater low-uplift zones,and marl developed in the low-lying areas between uplifts.(2)From the central paleo-uplift to the east margin of the basin,the dolomite in the Ma 4 Member gradually becomes thinner and turns into limestone.The lateral sealing of the limestone sedimentary facies transition zone gives rise to a large dolomite lithological trap.(3)During the late Caledonian,the basin was uplifted as a whole,and the central paleo-uplift was exposed and denuded to various degrees;high-quality Upper Paleozoic Carboniferous-Permian coal measures source rocks deposited on the paleo-uplift in an area of 60000 km^(2),providing large-scale hydrocarbon for the dolomite lithological traps in the underlying Ma 4 Member.(4)During the Indosinian-Yanshanian stage,the basin tilted westwards,and central paleo-uplift depressed into an efficient hydrocarbon supply window.The gas from the Upper Paleozoic source rock migrated through the high porosity and permeability dolomite in the central paleo-uplift to and accumulated in the updip high part;meanwhile,the subsalt marine source rock supplied gas through the Caledonian faults and micro-fractures as a significant supplementary.Under the guidance of the above new understandings,two favorable exploration areas in the Ma 4 Member in the central-eastern basin were sorted out.Two risk exploration wells were deployed,both revealed thick gas-bearing layer in Ma 4 Member,and one of them tapped high production gas flow.The study has brought a historic breakthrough in the gas exploration of subsalt Ma 4 Member of Ordovician,and opened up a new frontier of gas exploration in the Ordos Basin.

鄂尔多斯盆地奥陶系盐下马四段天然气成藏新认识及勘探重大突破

基于大量钻井、测井和地震资料分析,对鄂尔多斯盆地奥陶系盐下马家沟组四段天然气地质条件与成藏主控因素开展研究,提出古隆起控相、控储、控藏新认识:①马家沟组沉积期,中央古隆起分隔盆地中东部华北海与西南缘秦祁海两大沉积体系,在马四段台缘带发育巨厚丘状颗粒滩相白云岩,同时控制盆地中东部形成“两隆两凹”古地理格局,其中水下低隆带发育台内滩相白云岩,隆间低洼区发育泥灰岩。②由中央古隆起至盆地东缘,马四段白云岩逐渐减薄并相变为灰岩,灰岩致密带侧向封挡形成大面积白云岩岩性圈闭。③加里东末期中央古隆起遭受不同程度剥蚀,面积达6×10^(4) km^(2),上古生界石炭系—二叠系煤系优质烃源岩大面积披覆沉积,成为下伏奥陶系盐下白云岩岩性圈闭规模供烃主体。④印支期—燕山期盆地西倾掀斜,中央古隆起下拗转变为高效供烃窗,上古生界煤系烃源岩通过中央古隆起高孔渗白云岩体向上倾高部位侧向供烃,盐下海相烃源岩作为重要的气源补充,通过加里东期断裂、微裂缝输导供烃。以新认识为指导,转变勘探思路,综合评价优选盆地中东部马四段两大有利勘探区,部署实施两口风险探井均钻遇较厚马四段(含)气层,其中1口井获高产工业气流。该研究推动了奥陶系盐下马四段天然气勘探的历史性突破,开辟了鄂尔多斯盆地天然气勘探的重要新领域。