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

摘要： 将美索不达米亚盆地A油田Mishrif油藏巨厚生物碎屑灰岩储层划分为6种沉积-成岩相:极强溶蚀高能相、强溶蚀高能相、强溶蚀中高能相、中等溶蚀中能相、弱溶蚀中低能相、强胶结相。在三级层序内,基于不同沉积-成岩相的规模、形态、方向及叠置关系识别出4类储层结构(Ⅰ—Ⅳ)。Ⅰ型结构中不同沉积-成岩相厚度不一,极强溶蚀高能相和强溶蚀高能相厚度较薄,横向延伸规模小,平面呈斑状或局部连片,垂向上呈夹层状分布于中等溶蚀中能相。Ⅱ型结构中沉积-成岩相厚度跨度大,极强溶蚀高能相和强溶蚀高能相平面上呈网络状,垂向上呈切割状分布于弱溶蚀中低能相,空间上不同沉积-成岩相呈迷宫状叠置。Ⅲ型结构细分为Ⅲ_A型和Ⅲ_B型:Ⅲ_A型为薄层极强溶蚀高能相,近全区呈席状稳定分布;Ⅲ_B型厚度较大,自北向南呈进积状,空间上呈"上优下差、北优南差"的平缓渐变。Ⅳ型结构顶部为两套薄层强溶蚀中高能相夹中等溶蚀中能相,横向稳定分布,呈千层饼状;下部厚层中等溶蚀中能相和厚层强胶结相,呈块状。Ⅰ型、Ⅱ型和Ⅲ_A型结构的储层采用直井井网开发,衰竭式开采期产量高,但Ⅱ型结构稳产期较短,注水开发后均快速受效,但油井见水后含水率均快速上升;Ⅲ_B型结构储层采用水平井井网,初期产量高,稳产期长,产出水来源于上部直井注入水下灌,Ⅳ型结构储层仅部署水平注水井。研究认为,极强溶蚀高能相和强溶蚀高能相控制开发初期产量和稳产时间,而极强溶蚀高能相是造成含水快速上升的主要因素。由于Ⅰ型、Ⅱ型和Ⅲ_A型结构储层目前共用一套直井井网,建议将Ⅲ_A型的井分阶段上返至储层结构Ⅰ型和Ⅱ型,并全力封堵在Ⅲ_A型和Ⅱ型下部的注水井,防止上部Ⅰ型和Ⅱ型的注入水进入下部Ⅲ_B型中,同时,降低Ⅰ型和Ⅱ型的注水量,可有效控制含水率上升。

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