Sedimentary Characteristics And Reservoir Evaluation Of The Ordovician Carbonate Shoal In Tazhong And Shuntuoguole Area,Tarim Basin

The main study object of this research is the Ordovician carbonate shoal in the Tazhong and Shuntuoguole area ofTarim Basin,in order to recognize geological background and sedimentary characteristics of shoals,build their sedimentary models,analyze the macroscopic control conditions,reservoir characteristics and diagenesis during the development of carbonate shoal reservoir,and evaluate the favorable area.According to the theory systems of Sedimentology,Sequence Stratigraphy,Tectonics,and Carbonate Reservoir,through the description of geological background,the analysis of core,outcrop,the 2d/3d seismic data,logging/drilling,the microscopic thin section,and other research methods,the research on the Ordovician carbonate shoal in Tazhong and Shuntuoguole area has been conducted.Based on the former study,the definition and types of carbonate shoal is clarified,the microfacies and sedimentary environment of shoals has been recognized.Three types of shoals,as intraclast shoal,bioclastic shoal,oolitic shoal,and algal shoal,were developed during Ordovician in Tazhong and Shuntuoguole area,most of shoals developed on the platform margin and micro geomorphic uplift in the carbonate platform.In the period of Yingshan Formation most of shoals are intraclast shoal during Early-Middle Ordovician;Shoals which developed in the period of Lianglitage Formation during late Ordovician mainly are bioclastic shoal,intraclast shoal and oolitic shoal.Through the interpretation of seismic profile,logs and logs contrast,the longitudinal stack types and plane distribution characteristics have been summarized.The sedimentary models of shoals in Yingshan Formation and Lianglitage Formation are also built.In the sedimentary period of Yingshan Formation,shoals developed at the south and north margin of platform had different characteristics.The longitudinal stack types of shoals would change with the euatatic sea level changes,would be also different in different system tract.Various spaces,diagenesis and diagenetic environment of shoal reservoirs has been analyzed with the using of thin section,core and geochemical data.The reservoir space of shoal reservoirs mainly are secondary porosity(such as mold pore and intragranular solution pores,intergranular dissolved pore,intergranular dissolved pore),cave and fracture.The reservoir thickness of single layer of Ordovician shoals is between 8 ~ 25 m,shoal reservoirs were often stacked.Shoal reservoirs of Yingshan Formation generally is intraclast shoal which interbedded with dolomitic flat in some area;and shoal reservoirs of Lianglitage Formation mainly are bioclastic shoal and intraclast shoal.With the rare earth elements,carbon and oxygen isotope analysis,it is concluded that dolomite of shoals in Yingshan Formation is burial dolomite,and fluid environment is sea water environment.The development of shoal reservoirs in Tazhong and Shuntuoguole area was controlled by many factors,such as(1)structural control factors,include the unconformity surface,fracture systems and tectonic palaeohigh;(2)sediment control factors,include sequence interface and sedimentary microfacies;(3)palaeoclimate.The horizontal permeability of shoal reservoirs in Yingshan and Lianglitage Formation is better than vertical permeability.The reservoir porosity of oolitic shoal is best,followed by sand shoal and bioclastic shoal,and the porosity of algal shoal is smallest.There are three favorable main reasons for shoals to develop good reservoirs.(1)the particles of shoals are vulnerable to dissolution in the early stage of diagenesis.(2)Carbonate shoals could form more cracks by cataclasis.(3)Carbonate shoals could develop more intergranular pore during dolomization.High quality shoal reservoirs of Yingshan Formation are usually distributed in fault zone and dolomitic flat in restricted platform,also developed in area of thick intraclast shoals.High quality shoal reservoirs of Lianglitage Formation were mainly distributed in karst zone,platform margin,and fault zone,some quality shoal reservoirs also occurred at thick strata area in open platform.