关键词: 美索不达米亚盆地, Mishrif油藏, 生物碎屑灰岩, 沉积-成岩相, 储层结构, 注采关系

Abstract: In Mishrif reservoir of A oilfield, Mesopotamian Basin, the hugely thick bioclastic limestone reservoirs are classified into six depositional-diagenetic facies, i.e., extremely intense dissolution high-energy facies, intense dissolution high-energy facies, intense dissolution medium-high energy facies, moderate dissolution medium-energy facies, weak dissolution medium-low energy facies, and intense cementation facies. Within the third-order sequence, four types of reservoir architecture (Ⅰ-Ⅳ) are identified based on the scale, shape, orientation, and superimposition relationships of various depositional-diagenetic facies. In Architecture I, the thicknesses of different depositional-diagenetic facies vary. The extremely intense dissolution high-energy facies and intense dissolution high-energy facies are relatively thin, with limited lateral extension. These facies appear patchy or local continuous distribution pattern on the plane, and are vertically distributed as interlayers within the moderate dissolution medium-energy facies. In Architecture Ⅱ, the depositional-diagenetic facies show significant variations in thickness. The extremely intense dissolution high-energy facies and intense dissolution high-energy facies are distributed in a planar network-like pattern, and exhibit a vertical segmented distribution within the weak dissolution medium-low energy facies. Spatially, different depositional-diagenetic facies are superimposed in a maze-like configuration. Architecture Ⅲ is subdivided into Type Ⅲ_A and Ⅲ_B. Type Ⅲ_A is characterized by thin, extremely intense dissolution high-energy facies, which are stably distributed in a sheet-like pattern across almost the entire area. Type Ⅲ_B features a greater thickness of geological facies, with a progradational pattern extending from north to south. Spatially, it exhibits a thickness gradient featuring "larger at the top than at the bottom, larger in the north than in the south". The top of Architecture Ⅳ consists of two thin intense dissolution medium-high energy facies interbedded with moderate dissolution medium-energy facies, which are stably distributed laterally, forming a mille-feuille-like structure. The lower part is composed of thick moderate dissolution medium-energy facies and thick intense cementation facies, exhibiting a massive structure. A vertical well pattern is employed in reservoirs with Architecture I, Ⅱ, and Ⅲ_A. Production is high during depletion development, but the stable production period for architecture Ⅱ is relatively short. After waterflooding, all architectures respond quickly; however, water cut increases rapidly after water breakthrough in oil wells. Architecture Ⅲ_B utilizes a horizontal well pattern, achieving high initial production and a long stable production period. The produced water is derived from the overlying vertically drilled injection wells. Only horizontal water injection wells are deployed in Architecture Ⅳ. The study concludes that the extremely intense dissolution high-energy facies and intense dissolution high-energy facies primarily control the initial production and stable production periods, and the extremely intense dissolution high-energy facies are the main factor for rapidly increased water cut. Given that Architecture Ⅰ, Ⅱ, and Ⅲ_A currently share a common vertical well pattern, it is recommended to phase in the re-perforation of the wells of Type Ⅲ_A to align with Architecture Ⅰ and Ⅱ. Additionally, complete plugging of the water injection wells in the lower sections of Architecture Ⅲ_A and Ⅱ is essential to prevent injected water in the upper sections of Architecture Ⅰ and Ⅱ from migrating into the lower section of Architecture Ⅲ_B. Meanwhile, reducing the water injection volumes in Architecture Ⅰ and Ⅱ can effectively mitigate the increase in water cut.

Key words: Mesopotamian Basin, Mishrif reservoir, bioclastic limestone, depositional-diagenetic facies, reservoir architecture, injection-production relationship