Abstract: The Upper Cretaceous Khasib Formation carbonate reservoir of X Oilfield in the Middle East is a shallow-sea shelf deposit, it has the characteristics of multiple biological types and rock types, high porosity and low permeability reservoirs, and complex pore structure. Based on the Folk limestone classification scheme, this paper highlights the biological characteristics, and comprehensively compares the pore structure characteristics and control factors of different types of reservoirs on the basis of rock classification. Studies have shown that the reservoir rock types include four types of micrite bioclastic sand-clastic limestone, micrite algal clastic limestone, micrite bioclastic limestone and micritic planktonic foraminifera limestone; the type of pore structure can be divided into three types: high porosity and medium permeability medium throat type, high porosity and low permeability medium fine throat type, high porosity and low permeability fine throat type; the main factors affecting the pore structure are physical properties and structural components. Among different lithologies, the difference in biological species and content affects the development of primary pores and the strength of later diagenesis, resulting in differences in pore structure. Among the same lithology, the content of plasters controls the quality of the pore structure.

Key words: carbonate reservoir, high porosity and low permeability, pore structure, controlling factor, bioclastic limestone