鄂尔多斯盆地奥陶系马五段低孔低渗岩溶白云岩储层特征及孔隙成因

Characteristics and porosity development of the karst dolostone reservoirs in the fifth member of the Ordovician Majiagou Formation in the Daniudigas field,northern Ordos Basin,northern Shaanxi

关键词:  鄂尔多斯盆地  马家沟组五段  岩溶白云岩  晶间孔隙  白云石化

Key words:  Ordos Basin  fifth member of the Majiagou Formation  karst dolostone  intercrystal pore  dolomitization

摘要:

针对鄂尔多斯盆地大牛地气田马五段低孔低渗岩溶白云岩储层发育特征,展开对致密岩溶白云岩储层孔隙成因进行研究。通过岩心、薄片观察,结合扫描电镜、CT扫描分析,明确了白云岩储层岩石学类型、孔隙类型及成因。结合测井及地球化学分析,认为表生期孔隙主要为膏溶孔、溶孔及地开石晶间孔隙;白云石含量及结构控制了白云岩晶间孔隙;残留有机质保护了藻云岩残余晶间孔隙。通过岩溶白云岩孔隙成因分析,最后认为:(1)白云石化作用及成因模式是造成岩溶白云岩储层低孔低渗的主要原因,准同生期-浅埋藏期形成的白云石晶粒普遍较为细小,晶粒结构以半自形-它形为主,晶间孔隙为微孔,造成孔隙喉道较窄;泥微晶白云岩与粉-细晶白云岩(储集岩)分布在纵向上具有不均匀性,造成白云岩整体上孔渗较低;(2)岩溶作用增加了白云岩储层的非均质性,裂缝、溶洞及较大扩溶孔被泥质、方解石充填,很大程度上降低了岩溶白云岩储层的孔渗性能,正因为膏质泥微晶白云岩和粉-细晶白云岩为低孔低渗特征,使得地表水携带的泥质不易充填孔隙。

Abstract:

The present paper gives a detailed description of the characteristics and porosity development of lowporosity and low-permeability karst dolostone reservoirs in the fifth member of the Ordovician Majiagou Formation in the Daniudi gas field,Ordos Basin. According to core observation,thin section examination and scanning electron microscopy analysis,the emphasis here is laid on dolostone types,porosity types and genesis of the karst dolostone reservoirs. The dolostone types include gypseous dolostones,microcrystalline dolostones,fine-grained dolostones and algal-laminated dolostones. The porosity types contain solution openings developed during the epidiagenetic periods, intercrystal pores generated by dolomitization, and residual intercrystal pores of algal-laminated dolostones. The solution openings and microfissures of gypsum,solution openings and microfissures,intercrystal pores of dickite and intercrystal pores of dolostones are interpreted to be valid reservoir spaces of the dolostone reservoirs. The development of intercrystal pores in the dolostones tend to be governed by the contents and structures of dolomites,and the residual intergranular pores of algal dolostones are protected by residual organic matter in the pore spaces of the algal dolostones. The dolomitization may be responsible for the low porosity and low permeability of the karst dolostone reservoirs. The dolomites generated during the penecontemporaneous-shallow burial stages generally have small,subhedral to anhedral dolomite crystals,microporosity and narrow pore throat radius. Therefore,both the microcrystalline dolostones and fine-grained dolostones have low porosity and low permeability in the fifth member of the Middle Ordovician Majiagou Formation. The karstification may give rise to the heterogeneity of the dolostone reservoirs. The filling of cracks,caves and larger dissolved pores by argillaceous calcitemay also reduce the porosity and permeability of the karst dolostone reservoirs. The gypseous microcrystalline and fine-grained dolostones with low porosity and low permeability are not easily filled by the mud carried by the surface water,which,in turn,may result in the decrease of the porosity and permeability of the karst dolostone reservoirs in the study area.