Reservoir Characteristics Of The Upper Deng 4 Member Of The Sinian Dengying Formation In The Gaoshiti Area

With major breakthrough made in the exploration of the fourth Member in the Sinian Dengying Formation in Gaoshiti area in Sichuan Basin in recent years.the Gaoshiti area has been widely concerned by domestic and foreign scholars.The reservoirs of the fourth Member in Dengying Formation of the study area are mainly developed in the upper part of the fourth Member,which have the characteristics of complex lithology and strong heterogeneity.At present,there is little understanding of the reservoir development,distribution patterns and the main controlling factors of the upper part of the fourth Member,which limits the further efficient development.Therefore,based on the existing research results,this paper does systemic research to characterize the reservoirs in the upper part of the fourth Member of the Dengying Formation in Gaoshiti area with regional geological data,physical property analysis and logging data.Then this paper studied the reservoir main controlling factors and reservoir distribution characteristics,which provides geological basis for the development of gas reservoirs of the upper part of the fourth Member in Dengying Formation in the study area.Based on drilling cores and thin sections,we found that the reservoir rocks of the upper part of the fourth Member in Dengying Formation include algal-sand-clastic dolomite,frame dolomite,clotted dolomite and stromatolite dolomite and so on.Among them,the algal-sand-clastic dolomite and the clotted dolomite are the main reservoir rocks.Reservoir storage space include various types of pores,caves and fractures,mainly dominated by the intergranular dissolved pores,inter-frame dissolved pores,and pore-shaped caves.Reservoir physical property studies show that the values of the reservoir porosity and permeability of the upper part of the fourth Member are both low,but they are partially affected by fractures and caves,which forms a high-porosity section.Based on the combination relationship of different types of the reservoir storage space,the reservoirs of the upper part of the fourth Member in this area can be divided into three types:pore type,pore-vuggy type and fracture-vuggy type.Factors affecting the formation and evolution of the reservoirs of the upper part of the fourth Member are also discussed from sedimentation,diagenesis and tectonism.it is believed that the algal mounds and grain beach deposits are the material basis of reservoir development.The most favorable sedimentary microfacies are clotted mounds microfacies and algal-sand-clastic microfacies.There are many diagenesis types in the upper part of the fourth Member,epigenic karstification,simultaneous dissolution and burial dissolution are beneficial for reservoir formation,and the silicification is the destructive diagenesis,hydrothermal action is both constructive and destructive.Effective fractures which formed by tectonic movement can improve reservoir permeability.This study establishes the reservoir development and evolution model of the upper part of the fourth Member in Gaoshiti area.It is believed that the reservoir formation and evolution process can be divided in to five stages:Deposition period-Syngenetic period-Shallow burial stage:most of the primary pores were damaged by cementation,causing porosity below 4%.Epigenic karstification stage:the epigenic karstification produced a large number of dissolved pores,which led to the increase of reservoir porosity to more than 10%,and the porosity decreased to 5%due to the filling of some holes later in this phase.Shallow burial stage:the hydrothermal activity both expanded and filled the reservoir space formed in the early stage,and the porosity still maintained at about 5%,Buried diagenetic stage:on the one hand,there were both structural-hydrothermal activity to increase porosity and acidic fluids to dissolve early holes during the organic matter hydrocarbon generation period,on the other hand,there were also burial minerals filling the reservoir space,and the porosity reduced to 4%at this stage.Deep burial-fold stage:the burial dissolution contributed little to the reservoirs,which kept the total porosity of the reservoir of the upper part of the fourth Member at about 4%.The reservoirs are mainly of "algal mounds and grain beach deposits+epigenic karstification" composite genetic type,the reservoir development and distribution patterns of the upper part of the fourth Member has the following characteristics:Vertically,the reservoirs are mainly developed in the upper middle part of the upper part of the fourth Member.Laterally,the reservoirs are worser in the restricted platform zone than that in the platform margin zone.Regionally,The reservoirs are distributed in a north-south direction,the reservoirs of the north-south banded area of the west side in the study area are thicker than the reservoirs of the east,and fractures and caves are more developed,so the north-south banded area is the most advantageous area of reservoir development.