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Reservoir Quality in Salt-Encased Microbial-Dominated Carbonates From the Late Neoproterozoic Ara Group (South Oman Salt Basin)


The intra-salt carbonates of the Ediacaran-Early Cambrian Ara Group in the South Oman Salt Basin represent a self-sourcing petroleum system, which has been successfully explored in recent years. Depositional facies and carbonate/evaporite platform architecture are well understood, but original reservoir properties have been modified by diagenesis. Therefore, some of the carbonate reservoirs failed to produce at acceptable rates which triggered this study. It investigates in detail the interaction of petrography, facies and diagenetic control on reservoir properties in space and time. A number of methods were used to quantify the extent of primary porosity reduction by diagenesis. Near-surface diagenesis is dominated by early marine diagenesis and reflux-related processes leading to porosity inversion in initial highly porous facies and a patchy distribution of early cements (dolomite, anhydrite and halite). This strong diagenetic overprint of primary and early diagenetic porosity by reflux related cements leads to a reduction of stratigraphic and facies control on porosity. Calcite was identified as a burial-related cement phase that leads to an almost complete loss of intercrystalline porosity and permeability in dolomites. Microporosity forms an important component of the pore system, contributing often more than half of the complete pore space. The preservation of early diagenetic, very fine crystalline dolomite in a closed diagenetic system contributed to the relatively high microporosity values. Confocal laser scanning microscopy revealed the close association between bitumen and microporosity indicating an inhibition of recrystallization by an early influx of hydrocarbons. Reservoir bitumen was identified as important pore occluding phase and time marker of the deep-burial realm. Pressure and temperature reduction during a ‘deflation’ event at the end of salt tectonic times seems to be the most likely process leading to formation of reservoir bitumen, whereas further phases of bitumen formation, related to uplift-events, cannot be excluded. Maps confirm the heterogeneous distribution of key diagenetic phases on a field-scale. The diagenetic history within the South Oman Salt Basin is to a large extent determined by the successive change in fluid chemistry typical for many evaporite basins. The sequence of diagenetic events established in this study can be diagnostic for other carbonate-evaporite associations worldwide.