鄂尔多斯盆地海相碳酸盐岩主要储层类型及其形成机制

对于鄂尔多斯盆地海相碳酸盐岩储层宏观分布规律和有利区带的认识目前尚不十分清楚，难以满足区带评价和目标优选的需求。为此，利用野外露头、钻井和地球物理测井资料并结合实验分析数据，深入探讨了该盆地海相碳酸盐岩储层的特征、形成机制和分布规律，并指出了下一步天然气勘探的方向。研究结果表明：①鄂尔多斯盆地海相碳酸盐岩主要发育岩溶型和颗粒滩型两类白云岩储层，前者发育于蒸发台地的含膏云岩和膏质云岩坪微相，主要分布在下奥陶统马家沟组马五段上组合及马一段和马三段，后者主要发育于台地边缘颗粒滩和台内颗粒滩微相，分布在中寒武统张夏组、上寒武统三山子组、马四段和马五段中组合；②颗粒滩型储层岩性为鲕粒云岩、砂屑云岩、晶粒云岩及微生物云岩，储集空间以残余粒间（溶）孔、微生物格架（溶）孔、晶间（溶）孔和溶洞为主，少量裂缝，孔隙度介于2.00%～18.03%，平均孔隙度为6.16%，机械沉积和微生物造丘是原生孔隙形成的重要机制，准同生溶蚀和裸露期风化壳岩溶作用是溶孔溶洞形成的主控因素，早期白云石化和封闭体系有利于孔隙的保持；③岩溶型储层岩性为（含）膏模孔细粉晶云岩和粉晶云岩，储集空间为膏模孔、溶洞及微裂缝，孔隙度介于2.00%～16.36%，平均孔隙度为5.98%，同生期层间岩溶和裸露期风化壳岩溶是膏模孔形成的主要机制，而膏模孔的保存受矿物充填和封闭体系两大因素的控制。结论认为，有利储层主要分布在该盆地鄂托克前旗—定边—上韩、桃利庙—吴起和榆林—志丹一带，其中桃利庙—吴起和榆林—志丹一带是深层碳酸盐岩气藏勘探最有利的区带。

Main types and formation mechanisms of marine carbonate reservoirs in the Ordos Basin

The macroscopic distribution laws and favorable distribution zones of marine carbonate reservoirs in the Ordos Basin have not been understood clearly, so it is difficult to meet the needs of zone evaluation and target selection. To this end, this paper discusses deeply the characteristics, formation mechanisms and distribution laws of marine carbonate reservoirs in the Ordos Basin based on outcrop, drilling and geophysical data, combined with experimental analysis data, and then accordingly points out the direction of the next natural gas exploration. And the following research results were obtained. First, two types of dolomite reservoirs are mainly developed in the marine carbonate rocks of the Ordos Basin, i.e., karst dolomite reservoirs and grain beach dolomite reservoirs. Karst dolomite reservoirs are developed in the gypsum-bearing dolomite and gypseous dolomite flat microfacies of evaporative platform, and it is mainly distributed in the upper assemblage of the fifth Member of Lower Ordovician Majiagou Formation (O1M5), the O1M1 and the O1M3. Grain beach dolomite reservoirs are mainly developed in the marginal platform grain beach microfacies and the intra-platformgrain beach microfacies, and it is distributed in the Zhangxia Formation of Middle Cambrian, the Sanshanzi Formation of Upper Cambrian, the O1M4 and the middle assemblage of O1M5. Second, the lithology of grain beach reservoirs is oolitic dolomite, doloarenite, crystalline dolomite and microbial dolomite. Its reservoir space is mainly acted by residual intergranular (dissolved) pores, microbial framework (dissolved) pores, intercrystalline(dissolved) pores and karst caves with a small number of fractures, and the porosity ranges from 2.00% to 18.03%, averaging 6.16%. Mechanical sedimentation and microbial mounding are the important formation mechanisms of primary pores, penecontemporaneous dissolution and weathering crust karstification in the exposure period are the main factors controlling the formation of dissolved pores and karst caves, and the early dolomitization and sealing system are favorable for the preservation of pores. Third, the lithology of karst reservoirs is fine-silt crystalline dolomite containing anhydrite moldic pores and silt crystalline dolomite. Its reservoir space is mainly acted by anhydrite moldic pores, karst caves and micro-fractures, and the porosity ranges from 2.00% to 16.36%, averaging 5.98%. Inter-layer karstification in the syngenetic period and weathering crust karstification in the exposure period are the main formation mechanisms of anhydrite moldic pores, and the mineral filling and sealing system are two major factors controlling the preservation of anhydrite moldic pores. In conclusion, the favorable reservoirs are mainly distributed in the areas of Eketuoqianqi–Dingbian–Shanghan, Taolimiao–Wuqi and Yulin–Zhidan, and Taolimiao–Wuqi and Yulin–Zhidan areas are the most favorable zones for deep carbonate gas exploration.