Reservoir characteristics and influencing factors of Lower Permian Shan-2 Member,northern Ordos Basin:A case study of Wellblock A in Dongsheng gas field
In order to identify the reservoir characteristics and influencing factors of the second member of Lower Permian Shanxi Formation(referred to as Shan-2 Member)in Wellblock A of Dongsheng gas field,northern Ordos Basin,the petrological and physi-cal characteristics,reservoir spaces and pore structure properties are qualitatively and quantitatively analyzed by means of grain size analysis,thin section identification,mercury injection test,and physical property testing.The main factors affecting reservoir physical properties are discussed from the aspects of sedimentation and diagenesis.The results are obtained as follows.(ⅰ)The Shan-2 Member in Wellblock A belongs to the deposition of braid river delta plain.The reservoir rocks are composed of medium-to coarse-grained lithic quartz sandstones and quartz sandstones,with the interstitial materials of argillaceous hybrid matrices and cements(carbonate cements,clay mineral cements,and siliceous cements).(ⅱ)The reservoir features ultra-low porosity and permeability,with the pore types comprising mainly intergranular and intragranular dissolved pores,as well as a few of residual intergranular pores and microf-ractures.The pores and the throats are dominated by micropores and small-medium throats,respectively,resulting in a pore-throat combination of micropores and small-medium throats.(ⅲ)The reservoir physical properties are jointly controlled by sedimentation and diagenesis.There presents a significant positive correlation between the reservoir physical properties and rock grain size.The mid-channel bar located in the distributary channel is the most favorable sedimentary microfacies.(ⅳ)Destructive diagenesis(i.e.mechanical compaction and cementation)is the major factor reducing the porosity of reservoir,while constructive diagenesis(i.e.dis-solution and fracture)significantly improves the porosity and permeability of sandstone.
Ordos BasinDongsheng gas fieldEarly PermianShanxi FormationReservoir characteristicsSedimentationDiagenesis
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