Abstract
A common problem in dolomite reservoirs is the heterogeneous distribution of porosity-reducing diagenetic phases. The intrasalt carbonates of the Ediacaran-Early Cambrian Ara Group in the South Oman Salt Basin represent a self-sourcing petroleum system. Depositional facies and carbonate/evaporite platform architecture are well understood, but original reservoir properties have been modified by diagenesis. Some of the carbonate reservoirs failed to produce hydrocarbons at acceptable rates, which triggered this study. The extent of primary porosity reduction by diagenetic phases was quantified using point counting. To visualize the distribution of diagenetic phases on a field scale, we constructed 2D interpolation diagenesis maps to identify patterns in cementation. The relative timing of diagenetic events was constrained based on thin-section observations and stable isotope analyses. Near-surface diagenesis is dominated by reflux-related processes, leading to porosity inversion in initial highly porous facies and a patchy distribution of early cements. 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. Bitumen is an important pore-occluding phase and time marker of the deep-burial realm. The stratigraphic position of the dolomite reservoirs embedded at the base of a salt diapir had a strong impact on its diagenetic development. The salt isolated the dolomites from external fluids, leading to a closed system diagenesis and the buildup of near lithostatic fluid pressures. In combination, these processes decreased the impact of further burial diagenetic processes. The study highlights that cement distribution in salt-encased carbonate reservoirs is mainly related to early diagenetic processes but can be very heterogeneous on a field scale. Further work is needed to implement these heterogeneities in an integrated numerical reservoir model.
Highlights
Large rock inclusions totally encased in salt occur in many evaporite basins worldwide [1, 2]
Changes in fluid chemistry from normal marine seawater to halite-saturated, Mg-rich brines during near-surface diagenesis determined to a large extent the porosity development within the reservoir
Precipitation of dolomite cement reduces the concentration of Ca2+ in the pore water and might have led to dissolution of sulfate phases close to the entry points of refluxing brines, an observation that is consistent with reactive transport models of reflux dolomitization
Summary
Large rock inclusions totally encased in salt (so-called rafts, floaters, or stringers) occur in many evaporite basins worldwide [1, 2]. The diagenetic characteristics of salt-encased reservoirs have been studied in the Late Permian (Zechstein) of the northern Netherlands [12], from a Jurassic clastic reservoir from the Shabwa Basin in Yemen [13] and in the Late Neoproterozoic to Early Cambrian Salt Basins of the Interior Oman [14] and the South Oman Salt Basin [3, 5, 15,16,17] The latter provide diagenetic histories based on samples from different stratigraphic levels and burial depth.
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