Study On Reservoir Of The Sinian Dengying Formation In Central Sichuan,China

In Sichuan basin,due to the large burial depth,complex gas reservoir forming conditions of proterozoic carbonate rocks,technical means and other reasons,the natural gas exploration of(Sinian)Dengying Formation had no significant progress in the past 20 years.Until 2011,PetroChina successfully gained 137.8 × 104m3/d natural gas flow in Dengying Formation of GS1 well,and high yielded natural gas in subsequent drilling wells.It indicated that there is a good exploration prospect in Dengying Formation in central Sichuan.The existing exploration and research shows that the oil and gas production is closely related to reservoir properties.Therefore,it has important significance to study the reservoir of Dengying Formation in central Sichuan and provide important theoretical basis for today's oil and gas exploration and development.Under the guidance of structural geology,sedimentology,sedimentary petrology,geochemistry,karstology,reservoir geology,logging geology,seismic exploration and other disciplines,making full use of regional geological data,drilling,mud logging,well logging,physical property analysis and other information of Gaoshiti-Moxi area,this paper systematically analyzes stratigraphic characteristics,reservoir characteristics(petrology characteristics,physical characteristics,reservoir space types and their distribution characteristics)as well as deposition,diagenesis of Dengying Formation and tectonic which affected its reservoir forming.At the same time,the logging response characteristics of different rock types,sedimentary microfacies and reservoir space are systematically studied.The main conclusions are as follows:(1)In Dengying Formation,the bluegrey mudstone(Z2dn3)stably distributed in central Sichuan with obvious electrical characteristics,and is typical marker bed.At the same time,according to other strata s lithology characteristics and electrical characteristics,the Dengying Formation is divided into four sections from bottom to top:Z2dn1,Z2dn2,Z2dn3 and Z2dn4.The reservoir mainly developed in the middle and upper part of Z2dn4 and Z2dn2.The rock types forming Dengying Fonnation are dolomite(dolarenite,dolorudite,algal dolomite and crystalline dolomite),breccia(karst breccia,pseudo karst breccia and weathered residual breccia),mud stone(bluegray mudstone and darkgray mudstone shale)and chert,but the main rock types forming reservoir space are palmitic dolomite,stromatolite dolomite,micritic dolomite,fine grain dolomite and dolomitic breccia rock.In Dengying Fonnation,the reservoir space is mainly composed of secondary pores,holes and seams which are formed by diagenesis.The pore cave,gravel hole,"residual grape lace" hole,dissolved holes along the direction of laminar algae have an advantage over other reservoir space.The physical data analyzing shows that the reservoir is low in porosity and middle in permeability.Meanwhile,based on the combination mode of pore,holes and seams,the reservoir types could be divided into fracture pores(or pores)reservoir,fracture holes(or holes)reservoir and cave reservoir.(2)Based on the core and thin section data and combined with regional geological data and previous research results,it is thought that the study area is mainly in shallow water platform environment in Dengying period.According to landform,hydrodynamic condition and water depth,etc,the sedimentary facies in Dengying period could be further divided into two kinds of sedimentary facies(shallow water platform and Semi deep water platform),two kinds of sedimentary sub facies(beach and platform)and five kinds of sedimentary micro facies(particle beach,particle adhesion beach,dolomite platform,algal platform and mud platform).According to the relationship between each phase,the corresponding depositional model was established.In different sedimentary micro facies,the rock types and reservoir space are different,and the logging response characteristics also have great differences.On the basis of these study,the logging response models of different sedimentary micro facies,different rocks and reservoir space were established;but the statistically studying result shows that the algal platform mainly controlled distribution of effective reservoir,a large number of dissolved pores,karst caves and dissolution seams that distribute along algal layer developed in algal platfonn.According to its formation process,the corresponding evolution model is established.The reservoir space was based on the algal platform and formed by supergene karstification.Therefore,it is thought that the algal platform was the base for large-scale dissolved pores,karst caves and dissolution seams which distribute along algal layer development.(3)In Dengying Formation,the diagenetic environment multiply overlaped,there were many diagenesis types.The compaction,cementation,filling and silication were destructive diagenesis,which hinders reservoir space forming.Recrystallization and karstification were constructive diagenesis.Recrystallization improves effective porosity and permeability and was the basis of forming pore reservoir.Hypergene karstification was the key to forming reservoir.In the vertical direction,the karst zone could be divided into three belts:vertical vadose karst belt,the horizontal subsurface karst belt and deep slow flowing karst belt.The horizontal subsurface karst belt was the main development belt of reservoir space;Buried karstification promotes the forming of high-quality reservoir.(4)In Dengying Formation,a large number of cracks in the study area have been formed in the multi stage tectonic movement.These cracks suffered varying degrees of damage under the action of cementation and filling.Through the study of the filling degree and effective width of the crack,the research shows that the cracks had a limited contribution to reservoir space but improve the reservoir permeability.(5)The reservoir space of Dengying Formation in study area was interactively formed by sedimentation,diagenesis and tectonism in long geologic history,its formation and evolution process is successively divided into five stages:Deposition period-Syngeneticperiod-the first shallow burial stage(majority of primary pores formed and disappeared in this stage),hypergene dissolution stage(various karst caves formed in this stage),the second shallow burial stage(a small amount of intergranular pores and intergranular dissolution pores formed in this stage),The middle-deep diagenesis stage(a large number of intergranular pores reservoir,intergranular dissolution pores reservoir,porous cave and frac-tured porous reservoir was formed),and the fold stage(effective cracks was formed in this stage).