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Origin and Development of a Stratiform Dolomite in Barremian Oil Field Carbonates, Offshore Abu Dhabi, United Arab Emirates

Abstract

A stratiform dolomite layer is developed in the Barremian shallow-water carbonates from offshore Abu Dhabi, United Arab Emirates. The average thickness is only 1.1 m, but the porosity-permeability values are significantly variable within the layer. We discuss the origin and development of the stratiform dolomite based on an integrated geochemical, petrographic and petrophysical evaluation using 81 cores recovered from the entire field. Carbon isotope values (δ13C values) of the dolomite are relatively high (>5‰) and the strontium isotope ratios (87Sr/86Sr) mostly fall in the range of Barremian seawater. These geochemical signatures and the thin stratiform geometry suggest an early dolomitization phase below the seafloor driven by the diffusion of Mg2+ from the overlying mature seawater on the shallow platform top. Lateral changes in the dolomite content and permeability at the inter-well scale seem to be controlled by the permeability of the precursor facies. Dolomitization progressed further in grainier precursor (packstone) with higher permeability than muddier precursor (wackestone to mudstone), which led to higher dolomite content and greater permeability improvement in the grainier precursor. Oxygen isotope values (δ18O values), trace element (Sr, Na, Fe and Mn) concentrations and existence of oil inclusions in the dolomites indicate a later burial dolomitization phase during and after oil emplacement. Late burial dolomitization yielded significant pore-filling dolomite cementation that seems to have preferentially impacted the higher permeability early dolomite, to the point of significantly deteriorating reservoir quality in some instances. The degree of cementation and the Na, Fe and Mn contents of dolomite generally increase toward the flank of the field in response to the delay in oil charge and the higher water saturation compared to the crest. Subsequently, dolomite-to-dolomite recrystallization is a dominant phase under current burial temperatures (>100 degrees Celsius), which have reset the δ18O values and prevented the isotopically lighter Sr of formation water being incorporated into the dolomite. This recrystallization does not have significant impact on the petrophysical properties in the oil leg. Variance in reservoir quality of the stratiform dolomite is mainly dependent on the intensity and duration of dolomitization which is controlled by the interplay of depositional facies, structural position and oil charge.