Characteristics Of Carbonate Shoal Reservoir Of Ordovician Yingshan Formation In The Northern Slope Of The Central Tarim Basin

With the increasing demand for oil and gas and continuous improvement of exploration technology,carbonate shoal reservoirs have become a very important target for petroleum exploration.Currently,carbonate shoal reservoirs from the Lower-Middle Ordovician Yingshan Formation(O_1-2y)have in general been studied in less detail in the northern slope of the Tazhong area in Tarim Basin,although this study area has abundant oil and gas resources with huge exploration potential reservoirs.Therefore,it is of significance for studying the carbonate shoal reservoirs and providing a valuable geological basis for further exploration and deployment.The main contributions investigate the sedimentary characteristics of carbonate shoal reservoirs and summarize their development patterns in the study area under the guidance of the theories of sedimentology and reservoir geology,based on analysis of cores,outcrops,thin sections,scanning electron microscopy,cathodoluminescence,mercury injection capillary pressure(MICP),logs,and seismic data.Observations of thin sections show that there are four main types of carbonate shoals in the study area,including intraclastic shoals,algal shoals,bioclastic shoals and oolitic shoals.Shoal bodies have more obviously identificational characrteristics with the lower values being GR?30 API,and the conventional logging curves corresponding to the occurance of shoal intervals show box shape,funnel shape,bell shape and tooth shape.In addition,shoal bodies in the outcrop sections are mainly distributed in the relatively stable positions among the highstand system tract,where dissolution pores are always present at the top of shoal bodies.Furthermore,the geometry of shoal bodis can be determined by the locations of onlap-points around the shoal bodies on seismic profiles.The pore space of carbonate shoal reservoirs show three types:pores,holes and microfractures.The petrophysical properties of shoal reservoirs are generally poor,with extremely low porosity and permeability.Additionally,the throat types mainly include pore-reduced throat,necked throat,flaky throat and tubular throat.The pore structure of shoal reservoirs can be divided into four types according to testing parameters obtained from MICP.Porosity,permeability,maximum mercury saturation,maximum pore throat radius,average pore throat radius and median radius are gradually decreasing in the pore structures from type I to type IV,however expulsion pressure and median pressure show an increasing trend.The characteristic differences between these four reservoir types are mainly controlled by the variations of depositional environments.After calibration based on cores and imaging logs,carbonate shoal reservoirs of the O_1-2y Formation can be divided into fracture type,hole(pore)-microfracture type,and dissolution hole(pore)type.The carbonate shoal reservoirs have strong vertically superimposed and poorly laterally connected characteristics.The shoal reservoirs of the Yingshan Formation in the study area are controlled by multiple factors,including sedimentary facies,diagenesis,and tectonic movements.Shoal core microfacies and dolomitic tidal flat subfacies in the carbonate shoal subfacies are favorable depositional settings for potential reservoirs.Additionally,dissolution,dolomitization and silicification are three main constructive diagenesis processes for shoal reservoirs,while the occurace of cementation and compaction and pressure solution are in general destructive to carbonate reservoirs.The microfractures accompanied by tectonic movements make the dissolution holes or pores effective connected.Finally,after a fully consideration of sedimentary facies,tectonism,porosity,permeability,and residual thickness of carbonate strata,a comprehensive evaluation for the O_1-2y Formation shoal reservoirs has been established,and the favorable development zones of shoal reservoirs have also been predicted in the northern slope of the Tazhong area.