Research On Sedimentary Facies And Reservoir Characteristics Of Carboniferous Formation In Yunhezhai Gasfield

There are many NNE steep structural belts in east area of Sichuan,Yunhezhai is a buried gaseous structure among them,which is a fault- anticline complex gasfield with excellent trap conditions.In order to study the sedimentary facies and reservoir characteristics of Carboniferous carbonate reservoir in depth,the essay is based on previous research achievements,take the carbonate sedimentology, logging and reservoir geology as a theories guidance,adopt a combination research method of micro-macro,and make full use of various datum containing core,well logging,physical property,production performance, thin section identification and analysis,and so on.In Late Carboniferous period,Yunnan event happened,a tectonic uplift movement, which made a denudationon on the top level of Huanglong group in Carboniferous , and resulted in a missing of 3rd member in Yunhezhai structure.Through the study of stratigraphic classification and correlation,it shows that Carboniferous distributes continuous and its thickness is stable in the horizontal.Quoting"carbonate sedimentary model of surface of the lake fluctuate in the salted lagoon",which is proposed by predecessors for the eastern Sichuan especially,combining with regional sedimentary,taking the traditional petrological character , paleontologic evidence and electrofacies analysis to identify the sedimentary facies and microfacies.The study area develop two main depositional systems of restricted platform and evaporitic platform,which are composed by 5 types of microfacies,the evaporitic lagoon subfacies developed in the previous facies is most important and widespread,and the latter facies only occur in the bottom of Huanglong group.The character of the plane distribution of sedimentary facies in different periods indicate that abundant microfacies types and shallow is developed in 2nd member of Huanglong group.Besides sedimentary facies,characteristics of the reservoir is another important part of this paper.The results show that Carboniferous is fractured–porous type reservoir,crystallite dolomite and dolomite breccia are the main rock types in this area,both intercrystal pore and emposieu are well-developed.Generally speaking,the reservoir has poor properties and strong heterogeneity, specifically expressed in a low - medium porosity, low permeability and low -medium water saturation.There are a lot of fracture found in the drilling core,taking advantage of log response,the fracture is described in detail,and parameters about it are quantitative calculated.Fracture has a great effect on improving permeability,while little to change the reservoir capacity.Mesh fracture develop in the south of Yunhezhai structure,and high angle fracture in the north.Ⅰ,Ⅱ,Ⅲtype reservoir are found in Huanglong group,especially in the 2nd member of it.Ⅰ+Ⅱtype reservoir distribute amongⅢtype reservoir thinly laminated and odd,which are considered the best reservoir of Huanglong group.Speaking from permeability,low and extremely low permeability type reservoir are widespread,except the southern area,which show a medium-high permeability.Through a detailed analysis,it suggest that reservoir is integrated controlled by sedimentary facies, diagenesis and tectonic action,Favorable reservoir always developed in such an environment : shallow microfacies ,dolomitization,dissolution,recrystallization diagenesis and fractures with half filled or not filled developed,which are generated by tectonic movemen(tespecially Himalayan orogeny),all of the above conditions are very benefit to form favorable reservoir.It is Comprehensive considered that the favorable reservoir of Yunhezhai structure is distributed mainly in the area between well Yunhe 1 and well Yunhe2,and the area around well Yunhe 11.However,there is still great exploration potential in the northern part,and the region in the vicinity of well Yunhe 3 is preferred to be the next prospect area.