Depositional and diagenetic heterogeneities in sequence stratigraphic framework of a mixed carbonate-siliciclastic reservoir: A case study from Oligocene–Miocene Asmari Formation in the Persian Gulf

Abstract

Mixed siliciclastic–carbonate reservoirs, such as the Asmari Formation in the Zagros Basin, present significant heterogeneities that influence hydrocarbon exploration and production. This study examines the depositional and diagenetic heterogeneities of the Asmari Formation in the northwest Persian Gulf through a multi-methodological approach, including petrographic analysis, petrophysical measurements, core CT-scanning, X-ray diffraction, and sieve analysis. The primary goal is to identify the origins, scales, and predictability of these heterogeneities within a sequence stratigraphic framework to enhance reservoir characterization. Facies analysis reveals depositional environments ranging from sabkha to shoreface, peritidal, lagoonal, and shoal settings, with carbonate sedimentation interrupted by clastic influxes associated with sea-level fluctuations. Diagenetic processes, including dolomitization, dissolution, cementation, and fracturing, were identified through petrographic analysis, highlighting their effects on reservoir quality. To quantify these heterogeneities and delineate reservoir flow units, methods such as Winland classification, hydraulic flow unit (HFU) analysis, and Lorenz zonation were applied. Results show that depositional heterogeneities, predominantly driven by sea-level changes, range from small to large scales and can be predicted within a sequence stratigraphic context. Diagenetic modifications substantially affect porosity and permeability, with dolomitization improving reservoir potential, while compaction and cementation tend to reduce it. Fracturing, particularly in the lower Asmari carbonates, enhances permeability. The study concludes that accurately recognizing and modeling reservoir heterogeneities in three dimensions can significantly improve exploration and development strategies in the Persian Gulf. A better understanding of the spatial distribution of high-quality reservoir zones and impermeable baffle layers will facilitate more precise well placement, optimize hydrocarbon recovery, and mitigate exploration risks.