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

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 8 tight 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 differentiation 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."