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

New discovery and favorable areas of natural gas exploration in the 4^(th) Member of Ordovician Majiagou Formation by Well Mitan 1 in the eastern Ordos Basin

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。

Recently,a new discovery of natural gas exploration was achieved in Well Mitan 1 in the 4^(th) Member of Ordovician Majiagou Formation(hereinafter referred to as"Ma 4 Member")in the Ordos Basin,indicating that Ma 4 Member in the eastern Ordos Basin has a good exploration prospect of natural gas.However,the natural gas accumulation conditions of Ma 4 Member in this area are rarely researched,which restricts the next deployment of natural gas exploration in this area.Therefore,based on new drilling results of Well Mitan 1 in the Ma 4 Member,combined with the data of field outcrop,seismic,logging,core,micro thin section,reservoir physical property analysis and geochemical analysis,this paper systematically studies the natural gas accumulation conditions,establishes a natural gas accumulation model of Ma 4 Member,and predicts the favorable areas of natural gas exploration.And the following research results are obtained.First,the natural gas reservoir of Ma 4 Member in the eastern Ordos Basin is an oil-type gas reservoir of self generation and self preservation,whose source rocks are distributed in the marine formation of Ma 3-Ma 1 Member in the Taolimiao and Mizhi depressions.Second,reservoirs of intra-platform beach facies and intra-platform mound facies are developed in Ma 4 Member along the Hengshan uplift and the uplift belt of the Mizhi Depression.The lithology of the intra-platform beach facies reservoirs is doloarenite(dolorudite)and powder–fine crystalline dolomite and the reservoir space is mainly intergranular pores,intercrystalline(dissolution)pores and microfractures,with an average porosity of 4.95%.The lithology of the intra-platform mound facies reservoirs is mainly microbial dolomite,porphyritic power crystalline dolomite,porphyritic limy powder crystalline dolomite and porphyritic dolomitic limestone,and the reservoir space is acted by intercrystalline(dissolution)pores and microfractures,with an average porosity of 2.47%.Third,faults and microfractures are the migration channels of Ma 3-Ma 1 Member marine gas source to Ma 4 Member.Fourth,lithologic and structural-lithologic traps are developed in Ma 4 Member,and there are many sets of limestone and gypsum-salt caprocks in it and its overlying formation,which provide favorable sites and barrier conditions for the accumulation of natural gas.In conclusion,Ma 4 Member in the eastern Ordos Basin has good conditions for natural gas accumulation and is the key replacement field of natural gas exploration in the future.The intra-platform beach in the Yulin-Jingbian area and the intra-platform mound in the Shenmu-Mizhi area are the favorable areas of natural gas exploration.