Impact and conditions of calcite cementation on reservoir quality evolution of Upper Cretaceous limestones, Onshore Abu Dhabi, United Arab Emirates

This petrographic, stable isotopic, fluid inclusion and petrophysical study constrains the conditions of calcite cementation and its impact on reservoir quality of limestones of the Upper Cretaceous Simsima Formation across the anticline of an oilfield, onshore Abu Dhabi, UAE. The various crystal morphologies and occurrence habits of calcite cements and paragenetic relationshipsamong them and with other diagenetic minerals suggest various origins and formation timing. Scalenohedral and bladed calcite rims around allochems, which are restricted to packstones and grainstones, have precipitated as early cements from marine pore waters. Syntaxial calcite overgrowths around echinoid fragments engulf dolomite cement. Micro-thermometric measurement of fluid inclusions in one sample containing syntaxial calcite overgrowths revealed Th as lower as 64 ºC and salinity of nearly 20.5 wt% NaCl equivalent corroborate an origin of fluids from a highly evaporative setting. The precipitation of coarse equant blocky calcite is common in limestones which display ample evidence of dissolution of peloids, which may have acted as source for this cement. An additional cement source is expected to by the incipient stylolitisation of the limestones. The presence of this late equant calcite, which post-dates and replaces moldic pore filling dolomite, both in the oil and water zones may pre-date the oil emplacement. However, in the oil zone these moldic pores have, in many cases, escaped cementation by calcite and were instead filled with oil indicating that oil emplacement retarded or even stopped diagenesis. The higher δ18OVPDB values of some of the micrite/microspar rich samples than the values expected for marine calcite precipitated at around 25o C were indicate that the waters were enriched in 18O due to evaporation or that precipitation occurred at low temperatures. Conversely, the coarse equant calcite cements have moderate depletion in δ18OVPDB (-6.2 to -3.2‰). Combining these relatively moderately negative oxygen isotopic values with high homogenisation temperatures (76-95 °C) and high fluids salinity 16-19 wt% NaCl) suggest precipitation insemi-closed diagenetic systems from geochemically evolved formation waters. The higher homogenisation temperatures than maximum burial temperatures reached suggest that the role of flux of hot basinal brines on diagenesis of the limestones should not be ignored. The formation of secondary pores (e.g. vugs) might indicate that these brines were charged with organic acids.