岩石学组成及其成岩演化过程对致密砂岩储集质量与产能的影响:以鄂尔多斯盆地上古生界盒8天然气储层为例

Impact of Petrologic Components and Their Diagenetic Evolution on Tight Sandstone Reservoir Quality and Gas Yield:A Case Study from He 8 Gas-Bearing Reservoir of Upper Paleozoic in Northern Ordos Basin

在岩心观察基础上,通过常规、铸体与荧光薄片镜下观察与定量统计,图像粒度与图像孔喉、毛细管压力、孔隙度与渗透率、流体包裹体均一温度测定,结合LA-ICP-MS碎屑锆石微区U-Pb定年等物源示踪成果,研究了鄂尔多斯盆地北部上古生界盒8段致密砂岩的类型与岩石学组成及其成岩演化过程对各砂岩孔隙结构、储集性能与产能的影响.结果显示:砂岩类型、岩石学组分及其原始物性特征主要受源区母岩性质的控制,其经历的成岩演化过程存在一定差异,后者决定了各砂岩孔隙发育特征、储集性能的优劣及产能的大小.石英砂岩与岩屑石英砂岩经历了复杂的多期成岩作用过程,前者的粒间溶孔和高岭石晶间微孔与微溶孔发育,其储集性能及含气性最好;后者较发育的绿泥石薄膜保存了较多的原生粒间孔,加之粘土矿物与凝灰质填隙物中大量晶间微孔与微溶孔较发育,其储集性能与含气性次之.高塑性岩屑砂岩与钙质胶结砂岩经历的成岩演化过程相对较简单,前者经历早成岩阶段强烈压实作用后大部分已成为低孔低渗、部分成为致密储层;碳酸盐胶结物是导致钙质胶结砂岩低孔、低渗-致密储层的主要胶结物,此类砂岩经历中成岩阶段A期之后,基本成为致密的非储层.研究成果对于深入理解致密砂岩成岩-烃类充注演化序列及致密化过程与机理,寻找天然气储层"甜点"均具有重要的理论意义与实际意义.

Sandstone types,detrital components and cements,pore types and reservoir quality characteristics,differential diagenetic evolution processes resulted from sandstone types and their petrologic components,impact of the diagengetic evolution on pore structures,reservoir quality and gas yield of the He 8tight gas-bearing sandstone reservoir,Middle Permian,Upper Paleozoic from the northern Ordos basin are studied,based upon observation of cores,identification and quantitative study of conventional,fluorescent and casting thin sections by vacuum impregnated with epoxy resin under microscope,image measurement of sandstone grain size,pores and pore throats,measurement of porosity and permeability,capillary pressure and homogenization temperature of fluid inclusions,combined with LA-ICP-MS detrital zircon in situ U-Pb dating provenance tracing.The results show that sandstone types and their petrologic components,primary reservoir capacity are controlled by provenance and parent rocks.Different sandstone types experienced differential diagenetic evolution processes and in turn resulted in differenti-ation in pores and pore throats development,thus reservoir quality and gas yield.Quartzarenite and sublitharenite have experienced relatively more complex multi-diagenetic processes,with inter-crystal micro-pores and micro-dissolution pores developed in kaolinite cements and inter-granular dissolution pores,turning quartzarenite into an excellent gas-bearing reservoir.A great quantity chlorite thin film formation on detrital grains have resulted in significant primary inter-granular pores preserved,together with a large number of inter-crystal pores in clay minerals and in interstitial tuff materials,turning sublitharenite into the second best gas-bearing reservoir.Diagenetic evolution of litharenite with high content of plastic fragments and calcareous cemented sandstone is relatively simple.Litharenite with high content of plastic fragments has experienced intensive compaction in early diagenetic phase,leading to rapid reduction of pores,of which the majority became reservoir with low porosity and low permeability and few turned into tight reservoir at that time;however,part of the primary pores in part litharenite are preserved,which could have formed gas-bearing reservoir if pore network formed by later dissolution event occurred in sandstones.Carbon cements are the major cement which led the calcareous cemented sandstone to be reservoir with low porosity and permeability to tight reservoirs,thus non-reservoir for natural gas.The results are of significance both in theory and practice,for the understanding of diagenetic-hydrocarbon filling and densification process and mechanism,and for tracing＂sweet spots＂in natural gas-bearing reservoir as well.