Carbonates Sedimentary And Reservoir Characterization Of The Lower-Middle Ordovician In The Shunnan-Gucheng Area,Tarim Basin

Based on cores,thin sections,well logging,seismic and analytical test data,we have analyzed the sequence stratigraphy,sedimentary facies,and diagenetic modifications of the Lower-Middle Ordovician Yingshan and Yijianfang Formations in Shunnan-Gucheng area and revealed the main controlling factors of high-energy shoal reservoir.This provides a valuable basis for the prediction of favorable zones of high-energy carbonate shoal reservoirs.Based on the identification of sequence boundaries,we have divided the Lower-Middle Ordovician in the Shunnan-Gucheng area into three third-order sequence interfaces(T₇⁶?T₇⁵and T₇⁴).and two fourth-order sequence interfaces(T₇^6-1?T₇^5-1).Therefore,the Lower-Middle Ordovician comprise three third-order sequences,and the Yingshan Formation comprises four fourth-order sequences,Finally,we have established the sequence stratigraphic framework of this study target.The thicknesses of the Yingshan and Yijianfang Formations are generally stable in the south of Shunnan area and tends to decrease towards Gulong and Gucheng area,while they become significantly large in the Well 4 of the Gucheng area near the platform margin.The sedimentary facies of the lower member of the Yingshan Formation consist of restricted and open platforms,platform margin,and slope from west to east in the Shunnan-Gucheng area.In the restricted platform,the grain shoals distribute on a small scale and are dominated by the low-energy shoal.In the open platform and platform margin with a high hydrodynamic condition,the carbonate shoals distribute on a large scale.With the continuous rise of sea level,the sedimentary facies of the upper member of Yingshan and Yijianfang Formations consist of open platform,platform margin,and slope from west to east.In addition,the overall hydrodynamic force is strong and the high-energy shoals distribute more widely.The platform margin migrates westward with the rise of sea level and becomes wide due to the existence of platform margin shoal.These target intervals underwent complex diagenesis with multiple-stage superimposed.Silicification,dissolution,and fracturing are three main constructive diagenesis processes for shoal reservoirs,while the occurrence of cementation is in general destructive to carbonate reservoirs.Pervasive silicification contributes to the formation of a large number of pores and cavities in the study area,as well as the occlusion of tectonic fractures and dissolution fractures.However,a large amount of pore space retained between pure and large quartz crystals provides the possibility of hydrocarbon accumulation.The intercrystallite pores in the silt-crystal dolomite developed in the lower part of the Yingshan Formation provide pore space for later hydrocarbon accumulation.The shoal reservoirs in this study are controlled by multiple factors,including high-energy sedimentary facies,tectonic activities,and diagenetic alterations.(1)The higher content of grains in high-energy shoals than that in low-energy shoals are easier to be affected by dissolution in the early diagenetic stage and favorable for the high-quality reservoirs.(2)fractures are formed along the NE-trending strike-slip faults due to the tectonic movements.(3)dolomitization contributes to the formation of effective dissolution intercrystalline pores.Vertically,the favorable zones of high-energy shoal reservoirs become wide from the lower part of the Yingshan Formation to the upper part of this formation to the Yijianfang Formation.Laterally,the favorable zones of high-energy shoal reservoirs distribute in the areas of Wells Shunnan 1,Shunnan 6,Shunnan 7,Shunnan Peng1,Gulong 1,Gulong 2,Gulong 3,and the whole platform margin belt of the Gucheng area.Furthermore,the platform margin belt has largely favorable zones of carbonate reservoirs.