Characteristics And Distribution Of Carboniferous Carbonate Buried-hill Reservoirs In Weixi’nan Sag

Weixi’nan Sag is an important exploration and development block in Beibuwan Basin.The exploration and development of buried-hill reservoirs in the study area are generally low.The carbonate reservoirs are widely distributed and have great potential for oil and gas exploration.It is urgent to study this kind of buried-hill reservoirs.In this paper,the Carboniferous carbonate buried-hill reservoirs in Weixi’nan Sag is taken as the research object.Based on the core observation,microscopic identification,cathodoluminescence test,inclusion temperature measurement,drilling and logging,imaging logging（FMI）and other geological data of Weixi’nan Sag,through the study of reservoir petrology,sedimentary facies,reservoir space types,diagenesis,palaeogeomorphology and tectonic fracture,the distribution law and development mode of buried-hill reservoirs are proposed according to the control factors of buried-hill reservoirs.The Carboniferous buried-hill strata drilled in the study area are Menggongao Formation,Shidengzi Formation,Shishui Formation,Zimenqiao Formation and Huanglong Formation from bottom to top.The lithology of the study area mainly includes limestone,dolomite,and other lithology.Limestones include granular limestone,micritic limestone,grain limestone and dolo-limestone;dolomite include crystalline dolomite,granular dolomite and limy dolomite;and other lithology are mainly karst breccia,conglomerate and miscellaneous sandstone.The sedimentary environment of the study area include two subfacies（Restricted Platform and Open Platform）and seven microfacies types（Dolomite Flat,Limestone Flat,Dolomitized Limestone Flat,Lagoon,Tidal Channel,Inter-beach Sea and Intra-platform Beach）.Three reservoir space types in the study area are pores,fractures and caves,and the secondary pores formed by dissolution are the most developed reservoir space.Physical property statistics show that the average porosity of carbonate buried-hill reservoirs in the study area is 0.69%,and the average permeability is 0.08×10^-3μm².The physical properties of Huanglong Formation in the study area are the best,with an average porosity of 0.65%and an average permeability of 0.29×10^-3μm².There are four combination reservoirs types in the study area,namely,cave-fracture composite reservoir,cave/fracture reservoir,micro-fracture reservoir and tight reservoir,and the most favorable reservoir is the cave-fracture composite reservoir.The diagenesis in the study area are complex and diverse,mainly including cementation（filling）,dissolution,compaction-pressure dissolution,metasomatism（dolomitization,silicification and pyritization）,structural fracture and recrystallization.Among them,dissolution and structural fracture are the most important constructive diagenesis.The reservoirs in Weixi’nan Sag are controlled by sedimentary facies,karstification,palaeogeomorphology and structural faults.On the whole,the genesis of carbonate reservoirs in the study area is mainly controlled by karstification and tectonic action,represented by the development of reservoirs in No.1 and No.2structural belts.By studying the vertical and horizontal distribution of buried-hill reservoirs in the study area,considering the characteristics of buried-hill reservoirs and main controlling factors of reservoirs in study area,two reservoirs development models of karst buried-hill and fault block buried-hill are proposed.