鄂尔多斯盆地奥陶系盐下马家沟组四段天然气勘探突破及意义

Breakthrough and significance of natural gas exploration in the fourth member of Majiagou Formation of subsalt Ordovician in Ordos Basin

鄂尔多斯盆地奥陶系马家沟组发育碳酸盐岩—膏盐岩共生体系沉积,马家沟组四段(马四段)是马家沟组沉积厚度最大的一套海侵碳酸盐岩沉积层,长期以来是天然气勘探关注的重点层位。基于地球物理、钻井岩心及有机地球化学等资料,重新认识了鄂尔多斯盆地奥陶系盐下古构造、储层分布、烃源岩和圈闭等成藏条件,并取得以下成果认识:(1)盐下马四段天然气为油型气,主要来自奥陶系盐下海相烃源岩,该套烃源岩生烃母质除常规干酪根外,还发育分散有机质、有机酸盐,生烃物质丰厚,具备规模生烃潜力;(2)奥陶纪鄂尔多斯盆地中东部坳陷存在乌审旗—靖边古隆起和东部盐下低隆两大次级构造单元,分别控制了盐下马四段台内滩、台内丘白云岩储层的发育,储集空间主要为白云岩晶间孔;(3)盆地中东部盐下马四段台内滩、台内丘白云岩上覆厚层膏盐岩封盖,上倾方向致密石灰岩侧向遮挡,海相烃源岩供烃,形成了大面积分布的自生自储式岩性气藏。在新的地质理论认识的指导下,针对鄂尔多斯盆地奥陶系盐下马四段部署的风险探井MT1井钻遇气层43.4m,采用水力加砂压裂新工艺,试气获35.24×10^(4)m^(3)/d的高产工业气流,实现了盆地战略接替领域的重大突破。

The carbonate-evaporite paragenetic deposits are developed in the Ordovician Majiagou Formation in Ordos Basin.The fourth member of Majiagou Formation(Ma 4 member)is composed of a set of transgressive carbonate rocks with the largest thickness in Majiagou Formation,which has always been the target layer for natural gas exploration.Based on geophysical,drilling,core samples and organic geochemical data,the hydrocarbon accumulation conditions such as paleo structure,reservoir distribution,source rocks and trap are restudied,and the following achievements are obtained:(1)Natural gas in Ma 4 member is oil-type gas,generated by marine source rock of the subsalt Ordovician.In addition to conventional kerogen,the source rock has a large amount of parent materials for hydrocarbon generation,such as the dispersed organic matter and organic acid salt,showing great potential of hydrocarbon generation;(2)In the Ordovician,two secondary structural units,Wushenqi-Jingbian paleo uplift and the eastern subsalt low uplift,were developed in central-eastern depression in Ordos Basin,which controlled the development of dolomite reservoir in the intra-platform shoal and intra-platform mound of Ma 4 member,with the main reservoir space of intergranular pores;(3)The wide spread self-generation and self-storage type lithologic gas reservoir is formed in the central-eastern Ordos Basin by marine source rocks of subsalt Ordovician,reservoir of the intra-platform shoal and intra-platform mound dolomite of Ma 4 member,and cap rocks of the overlying thick evaporites and the lateral barrier provided by tight limestone in the updip direction.Guided by the new geological understanding,the risk exploration Well MT1 was drilled targeting at Ma 4 member,in which a gas layer of 43.4 m was penetrated,and high gas flow of 35.24×10^(4)m^(3)/d was tested by applying new hydraulic sand fracturing technology,achieving a major breakthrough in strategic replacement field in the basin.