Characteristics and genesis of interlayers in thick bioclastic limestone reservoirs: a case study of Cretaceous Mishrif Formation of the M oilfield in the Middle East

Thick bioclastic limestone reservoirs are developed in the Cretaceous Mishrif Formation of the M oilfield in the Middle East, and there are some fators constraining the reservoir development, such as interlayer uneven liquid production and low producing reserves degree. To deepen the understanding of the internal heterogeneity of carbonate reservoirs, based on core observation and description, casting thin sections, physical property analysis, mercury injection experiment and logging data, a study is performed on the type, origin and distribution of interlayers in Mishrif Formation. The results show that the interlayers in Mishrif Formation can be divided into 3 categories (sedimentary type, diagenetic type and coal-forming type)and 5 sub-categories (lagoon sedimentary type, deep-water sedimentary type, weathering cementation type, lagging cementation type and lagoon-exposed type). The lagoon sedimentary type and deep-water sedimentary type were developed in low-energy lagoons and open shallow seas, respectively, with high shale content and low primary permeability; they are classified as lithologic interlayers. The weathering cementation type were developed in alternate environments with arid and humid climates. The sea level fell significantly, and the strata were exposed for a long time. Under the weathering and leaching, a breccia zone was formed. Dense cementation occurs to saturated fluids in fractures, forming permeability barrier. The lagging cementation type were developed in high-energy shoals. The upper part of the shoal underwent leaching and dissolution, and the bottom was cemented to form a compact zone. The interlayers are small in thickness, and are not easy to be found. The lagoon-exposed type was developed in a warm and humid climate; the lagoon was exposed for a long time and peatification occurred. Through fine rock-electricity calibration, this paper establishes logging identification standards and discrimination methods for different types of interlayers, and clarifies the spatial distribution of interlayers in thick reservoirs in Mishrif Formation. The lagging cementation type and lagoon sedimentary type were developed in Mishrif Formation, while the deep-water sedimentary type was only developed in the MB2.2 layer, the weathering cementation type was only developed on the top of Mishrif Formation, and the lagoon-exposed type was only developed on the top of the MB2.1 layer. The development type and distribution characteristics of interlayers are controlled by such factors as sedimentary environment migration, eustatic cycles, exposure time of sediments and climate conditions. © 2021, Editorial Office of ACTA PETROLEI SINICA. All right reserved.