鄂尔多斯盆地奥陶系马家沟组五段膏盐下白云岩储层形成的主控因素

近期勘探实践表明，鄂尔多斯盆地东部奥陶系马家沟组五段膏盐下白云岩具有良好的天然气勘探开发潜力。为此，综合岩心观察、薄片鉴定、物性分析及地球化学特征等资料，对其岩石学、储集空间和孔渗特征等开展了系统研究。结果表明，鄂尔多斯盆地奥陶系膏盐下储集层为孔隙型白云岩储层，纵向上分布在马五6亚段、马五7亚段和马五9亚段。马五6亚段储集层岩性主要为颗粒白云岩和斑状粉晶白云岩，马五7亚段和马五9亚段储集层岩性主要为颗粒白云岩和粉-细晶白云岩。三个储层段储集空间均以溶蚀孔洞为主，次为残余粒间孔、晶间溶孔、晶间孔和微裂缝，平均孔隙度分别为5.84%、6.81%和7.03%。沉积古地貌、海平面变化和准同生期大气淡水溶蚀作用等因素共同控制膏盐下白云岩储集层的发育与分布。研究认为位于盐岩边界线外侧的中央古隆起带和榆林-横山隆起带地势相对较高，颗粒滩沉积厚度大，在准同生期和浅埋藏期易于发生大气淡水溶蚀作用，其内部赋存的孔隙不易被膏盐岩充填，是盆地内膏盐下有利白云岩储集层的发育区。

Main factors controlling formation of dolomite reservoir underlying gypsum-salt layer in the 5th member of Ordovician Majiagou Formation,Ordos Basin

The dolomite underlying gypsum-salt layer in the 5^th member of Ordovician Majiagou Formation in the eastern Ordos Basin proves to be of great gas potential by latest exploration practice.Thus we conducted systematic study on its petrological features,reservoir space,porosity and permeability by core observation,thin section identification,physical property analysis and geochemical analysis.The results show that the reservoirs underlying the gypsum-salt layer in the Ordovician are of porous dolomite type in the Ordos Basin,and they occur vertically in O₁m⁶₅,O₁m⁷₅ and O₁m⁹₅.The litho- logies of the O₁m⁶₅ reservoirs are mainly granular dolomite and porphyritic dolomite with very fine-crystalline,while the lithologies of O⁷₁m₅ and O₁m⁹₅ reservoirs are dominated by granular dolomite and dolomite with very fine-to fine-crystalline.The reservoir spaces for the three reservoir intervals are primarily dissolved pores or cavities,with residual intergranular pores,intercrystalline dissolved pores,intercrystalline pores and micro cracks ranked second.The average porosity for each is 5.84%,6.81% and 7.03%.Sedimentary paleo-geomorphology,sea level eustacy and penecontemporaneous meteoric water dissolution collectively control the development and distribution of the dolomite reservoir underlying gypsum-salt layer.It is concluded that the central paleo-uplift belt and Yulin-Hengshan uplift belt,located outside the boundary of the gypsum-salt layer,are relatively higher in topography,and the deposits of grain shoal facies are relatively thicker,so they are prone to undergo meteoric water dissolution during penecontemporaneous and shallow burial stages.Its internal pores are not easily subject to filling of gypsum salt.All in all,they are the favorable areas for the growth of dolomite reservoirs underlying the gypsum-salt layers inside the basin.