Control of sedimentary and diagenetic processes on the reservoir heterogeneity: A case study of the cretaceous carbonate reservoir in an onshore oilfield, Abu Dhabi

To address the issue of pronounced heterogeneity in carbonate reservoirs, this study utilizes core and well log data to identify the microfacies types of the carbonate rocks in XY oilfield, while the reservoir characteristics and heterogeneity genesis were analyzed. The microfacies types and their combinations suggest depositional environments ranging from restricted to open platforms, with a shoaling-upward evolutionary sequence from middle ramp to inner ramp, shoal, and lagoon. These microfacies dictate the reservoir quality: MF3 and MF4 are characterized by ooid grainstone reservoirs where intergranular pores predominate; MF5, MF6, and MF7 feature Lithocodium-Bacinella limestone with reservoir spaces encompassing algal framework pores, intragranular dissolved pores, mold pores, and intergranular dissolved pores. The fluctuating relative sea level resulted in an orderly vertical superposition of the various sedimentary facies belts, with variations in microfacies and diagenetic activities both between different belts and within the same belt being the fundamental causes of reservoir heterogeneity. Meteoric water significantly contributes to pores through selective dissolution of aragonitic components such as Bacinella irregularis, while multi-stage calcite cement during diagenetic evolution markedly damaged the petrophysical properties. Based on these findings, diagenetic facies are categorized, and a genetic classification of reservoir types is established; conventional well logs indicate that GR, RHOB, and NPHI are more sensitive parameters for identifying reservoir types, leading to the development of a logging identification chart for reservoir types. The formation and evolution of the high-permeability layers are influenced by both sedimentation and diagenesis, with the original high-energy shoal environment serving as the foundation; during diagenetic evolution, dissolution increases porosity, and early hydrocarbon charging prevents cementation, thus preserving the pores. The high-permeability layers exhibit lateral stability, predominantly high-energy shoal facies, with occasional laterally discontinuous layers attributed to Bacinella beach facies.