Research On Diagenetic Evolution Of Tight Reservoirs And Gas Accumulation Controlling Factors Of Upper Paleozoic In Kangning Area, Eastern Ordos Basin

Recently,Kangning block in Shaanxi Province of Ordos basin is the greatest natural gas exploration and development.Multi-gas reservoirs develop well in Upper Paleozoic tight sandstone with characteristics of abundant gas reservoirs,high gas productivity of single well and great potential prospecting for development.However,there are still some unknown recognitions on diagenetic stage classification,porosity evolution,gas reservoir type and controlling factors of gas accumulation.Applying the theory of reservoir geology and advanced experiments analysis,with the studies of pore structure,diagenetic evolution,and combining fluid inclusions,gas filling time,reveals the tight sandstone reservoir type and mechanism.Gas reservoirs are majorly from sandstone flat,braided channel and distributary channel facies with sandstone thickness larger than 10 m.Moreover,gas reservoirs develop in sandstone including fine,medium and coarse particle sizes.The larger particle size enhance the reservoir physics.Upper Paleozoic sandstones are comprised of feldspar lithic sandstone and lithic feldspar sandstone with medium sorting.The roundness of the sands is mainly sub-angular.The contact interfaces between the sands are mainly the line.The secondary porosity make great contribute to reservoirs.Micro fractures can connect the isolated pores,which improve reservoir permeability.The reservoirs are featured with strong heterogeneity,complex pore structure,various pore type and high ratio of effective porosity.Microscopic petrography and SEM results indicate that multi-diagenesis develop in tight reservoirs,which experienced complex diagenesis evolution and up to stage A and B of middle diagenesis.Compaction and cementation are the major reasons to lower the reservoir porosity.Clay mineral cements and siliceous cements exists in the early stage of diagenesis and carbonate cements in the late stage of diagenesis.Dissolution has improved the reservoir physics of Shihezi formation and Taiyuan formation,which contributes to the high porosity and high permeability.On the basis of porosity evolution and the inclusion temperature analysis,the tight period of Upper Paleozoic sandstone ranges from late Triassic to early Jurassic,which is early than gas accumulation period from late Jurassic to early Cretaceous.The gas filling form is continuous and tight sandstone reservoirs belong to “pre exiting” gas reservoir.Different gas accumulation combinations are controlled by different controlling factors.Firstly,hydrocarbon generation rate in source rocks has strong effects on far source accumulation and near source accumulation;Secondly,sands distribution and reservoir physics both control far source combination,near source combination and inside source combination;The last factor is fault development,which evidently control the far source combination.Fault research reveals that the probability of gas accumulation is large when is adjacent to fault.