鄂尔多斯盆地北部地区马家沟组岩溶储层的差异性和发育模式

鄂尔多斯盆地北部地区奥陶系马家沟组普遍发育岩溶型储层，但其岩溶发育模式明显不同于经典的传统岩溶模式。通过矿物学、岩石学和地球化学分析，共识别出3类具有成因联系且受含膏地层影响的储层，并建立了研究区马家沟组含蒸发盐矿物层系的岩溶模式。结果显示，含膏白云岩-膏模孔型储层形成于萨布哈白云岩化作用，富含蒸发盐矿物，沉积环境为含膏云坪，储集空间为石膏溶解形成的膏溶铸模孔；白云岩-溶孔-晶间（溶）孔型储层形成于回流渗透白云岩化作用，流体来自于上覆的含膏层系，主要发育于浅滩相白云岩地层中，溶孔和晶间（溶）孔为主要储集空间；灰岩-缝洞型储层主要发育在（含云）灰岩地层之中，原始沉积环境为灰坪，储集空间为裂缝和溶洞溶孔-晶间（溶）孔型储层，最初始裂缝的形成与下伏含膏岩层的溶解和变形有关。在空间分布上，鄂北中东部3类储层均有发育，西部以溶孔-晶间（溶）孔白云岩储层为主，北部则主要发育灰岩裂缝型储层。在不同古地貌单元，储层岩石也表现出不同岩石学和地球化学特征，大致分为渗流区和汇聚区。在较高的部位多表现为大气降水渗流区的特征，受大气降水影响较少，储层保存较好，充填有少量的白云石，杂质元素（Al，K，Sr等）含量较低；而较低的部位则表现出汇聚区的特征，受大气降水影响较大，储层破坏严重，大量的方解石充填，杂质元素含量较高，δ¹⁸O值偏负。

Differences and development patterns of karst reservoirs in Majiagou Formation,northern Ordos Basin

Karst reservoirs are widely developed in the Majiagou Formation, northern Ordos Basin, though they are markedly different from the classic karst model.An integrated analysis of petrology and geochemistry functions to identify 3 types of karst reservoirs related in genesis and affected by gypsum-bearing strata, and a karst model of strata bearing evaporite minerals in the Majiagou Formation is established.The results show that the gypsum-bearing dolomite reservoirs with gypsum mold pores are formed under sabkha-associated dolomitization, featuring richness in evaporate minerals, sedimentary setting of gypsum-bearing dolomite flat, and reservoir space of dissolved gypsum mold pores.Dolomite reservoirs with dissolved-intercrystalline (dissolved) pores are generated under seepage-reflux dolomitization, with the fluid coming from overlying strata bearing gypsum, and mainly occur in dolomite of shallow shoal facies with reservoir space dominated by dissolved pores and intercrystalline (dissolved) pores.Fractured-vuggy limestone reservoirs are mainly developed in (dolomitic) limestone formations, which originally are deposited in limestone flat, with cracks and dissolved-intercrystalline (dissolved) pores as dominant reservoir space, and the development of primary cracks within is associated with the dissolution and deformation of underlying gypsum-bearing rocks.In terms of spatial distribution in the study area, reservoirs of the 3 types are all discovered in the central and eastern parts of northern Ordos Basin; while dolomite reservoirs with dissolved-intercrystalline (dissolved) pores are popular in the western part, and the fractured-vuggy limestone reservoirs dominate the northern part.The reservoirs of the same type also exhibit different petrological and geochemical characteristics in different paleo-geomorphic units, which could be in turn subdivided into seepage zones of the high and catchment zones of the low.The former rarely impacted by atmospheric precipitation, is characterized by well-preserved reservoirs filled with a small amount of dolomite and of low impurity level (Al, K, and Sr, etc.).The latter under the significant impact of atmospheric precipitation, features seriously-damaged reservoirs filled with a large amount of calcite, and of high impurity level and generally negative δ¹⁸O.