Influence of reservoir oil charging on diagenesis and reservoir quality: An example from Dunlin Reservoir Sandstones, East Shetland Basin, North Sea, UK

The sandstone facies from two reservoir blocks (extensional fault walls) of the Dunlin Field have been studied to evaluate the impact of the reservoir charge history on the diagenesis and reservoir quality of these sandstones. The study has identified seven main reservoir sandstone facies (D1-D7) from the reservoir crestal block (oil leg) to the flank block (water leg). These sandstone facies exhibited similar diagenetic patterns, controlled by their depositional parameters, hence having the same porosity and permeability values in both reservoir blocks (legs) until hydrocarbon charging in the Late Cretaceous-Pliocene. The burial and thermal model indicates that these reservoirs were charged at a temperature of 60-75? during the 80-50 Ma and 95-100? during the 10 Ma to Present, and significantly controlled the mesodiagenetic output, notably illite, and quartz authigenesis. The reservoir oil leg recorded a higher amount of recovered bitumen (ca. 95%) than the water leg (ca. 5%), indicating that hydrocarbon charging of the sandstone reservoir was progressive rather than instantaneous, first filling the water leg (palaeo-oil leg). Subsequent leak-off depleted this reservoir block and remigrated to fill the reservoir leg (palaeo-water). The fluctuating oil charging and leakage between these two reservoir fault blocks modulated diagenetic alteration of these reservoir sandstones; hence is the cause of the minor disparity in porosity values between these reservoir legs contrary to the wide variation between conventional reservoir oil- and water-legs distinguished by hydrocarbon emplacement with no such complex history. This study, therefore, demonstrates the importance of evaluating the depositional and diagenetic controls on reservoir quality and charging of hydrocarbon-bearing sandstones for optimum oil production and recovery in clastic depositional settings.