Reservoir Characteristics and Main Controlling Factors in Mixed Sedimentary Background: A Case Study from Asmari Formation of N Oilfield, Middle East

Abstract

Based on cores analysis, sidewall cores, thin section, scanning electron microscopy (SEM), mercury injection data, as well as porosity and permeability analysis of core, reservoir characteristics and controlling factors of Asmari Formation developed in mixed sedimentary environment have been discussed. The result indicate that the rock type of reservoir is complex, including sandstone, dolomite, sandy dolomite and dolomitic sandstone etc. The sandstone is dominated by subarkose and sublitharenite, the pores are mainly intergranular pores, and the sandstone reservoirs belong to medium ~ high porosity and permeability reservoirs. The dolomite is dominated by micritic dolomite and micrite/calcsparite grain dolomite, the pores are mainly secondary dissolution pores, the physical property of dolomite reservoirs belong to low ~ middle porosity and low permeability reservoirs. The pores of mixed sedimentary reservoirs, such as sandy dolomite and dolomitic sandstone, includes dissolution pores and intergranular pores. Sedimentary, penecontemporaneous karst, diagenesis and tectonism control the development of mixed sedimentary reservoir in Asmari formation. The difference of sedimentary facies controlled the reservoir types and distribution. Sandstone reservoirs mainly developed in the delta facies, while dolomite reservoirs often developed in the in-platform shoal of the restricted and semi-restricted platform. Mixed sedimentary reservoirs, such as sandy dolomite and dolomitic sandstone, usually developed in the transitional position of delta facies and platform facies in the longitudinal and horizontal direction. The penecontemporaneous karst is the key factor for the formation and optimization of carbonate reservoir. During the deposition process, the Asmari formation was frequently exposed and accepted the atmospheric fresh water leaching transformation, which transformed the carbonate reservoir in study area. The development of various diagenesis are the important controlling factors for different reservoir quality. Dolomitization is the basis of formation of dolomite reservoir, and dissolution has constructive effects on the development of dolomite reservoir, whereas the higher the content of anhydrite cement, the worse the physical property and the lower development degree of the dolomite reservoir. The fractures and micro-fractures generated by post-burial tectonism enhance reservoir heterogeneity and improve reservoir physical properties.KeywordsMixed sedimentary backgroundAsmari formaitonReservoir characteristicsControlling factorN OilfieldMiddle East