Study On The Reservoir Characteristics Of The Second Member Of Xujiahe Formation In The Central Sichuan Area

Natural gas is a type of clean energy that is in line with the energy development strategy and environmental protection policy of China.China has clearly proposed that natural gas should be cultivated as one of the main energy sources in modern clean energy system.The Sichuan Basin is an important natural gas production base in China,in which,several large gas fields,such as Zhongba and Pingluoba Gas field have been discovered successively.In recent years,one hundred billion-scaled Hechuan and Anyue Gas field are discovered in central Sichuan area and their main gas producing zone is Xu2 Member,showing Xujiahe Formation has great exploration and development potential in central Sichuan area.The reservoirs of Xujiahe Formation are typical low-porosity and low-permeability tight reservoirs.The development and distribution regularity of high-quality reservoirs are closely related with reservoir genesis.The correct understanding of reservoir characteristics and the controlling factors of reservoir development is the foundation for the exploration and development of natural gas.In this paper,focused on the reservoirs of the second member of Upper Triassic Xujiahe Formation in central Sichuan area,the controlling factors of reservoir development were summarized and deeply studied and the following understandings were obtained:1?The Xu2 Member of Xujiahe Formation in central Sichuan area was 75~200m thick,showing the paleogeomorphologic characteristics of high in the northwest and low in the southeast.It could be divided into four sand groups from bottom to top based on its lithological association and well logging characteristics,including Xu2~1,Xu2~2,Xu2~3 and Xu2~4 sand group.Xu2~2,Xu2~3 and Xu2~4 sand group are evenly distributed in the study area,but Xu2~1 sand group is limitedly distributed in the northwestern part,with a thickness of 0~80m.2?Based on facies markers,combined with sand/formation ratio,it was considered that the delta front subfacies of delta sedimentary system developed in the Xu2 Member of Xujiahe Formation in central Sichuan area,which can be further subdivide into underwater distributary channel,mouth bar and sheet sand microfacies.3?The reservoir rocks of Xu2 Member were dominated by feldspar lithic sandstone and lithic feldspar sandstone.The reservoirs were low-porosity and low-permeability tight reservoirs,with an average porosity of 7.34%and average permeability of 0.35mD.The reservoir petrophysical properties of southern area were better than that of northern area.The reservoir petrophysical properties of Xu2~1 and Xu2~2 sand group were better than that of Xu2~3 and Xu2~4 sand group.The reservoir space was dominated by residual primary intergranular pore,intergranular dissolved pore and intragranular dissolved pore,with minor micro-pore in matrix,dissolved pore and micro-fracture at granule margin and mould pore.In plane,residual primary pore took dominance in Hechuan and Yingshan district and intergranular dissolved pore took dominance in An'yue and Penglai district.The throat was dominated by narrow necks and flaky narrow throat.The pore throat structure was the best in Hechuan district.The reservoirs were dominated by fracture-pore typed,followed by pore typed.4?Based on previous results,it was determined that the lower porosity limit of the reservoirs of Xu2 Member in central Sichuan area was 6%and the lower porosity limit of high-quality reservoirs was 8%.Taking the upper submember of Xu2Member(Xu2~3 and Xu2~4 sand group)as example,the distribution of high-quality reservoirs was analyzed.High-quality reservoirs were mainly distributed in Penglai-An'yue-Hechuan district,which were the thickest in Well Hechuan1,up to more than20m.5?The reservoir petrophysical properties of Xu2 Member were the best in the underwater distributary channel microfacies.Dissolution,chlorite cementation and tectonic disruption were constructive diagenesis.Compaction and calcareous cementation were destructive diagenesis.Based on the division results of diagenetic facies,siliceous kaolinite cementation and dissolution facies was favorable reservoir diagenetic facies,where pores were the most developed.The existence of fractures greatly improved reservoir permeability.As a result,fracture-pore system was formed,which was of great significance for reservoir modification.