THE RELATIONSHIP BETWEEN PETROLEUM EMPLACEMENT AND CARBONATE RESERVOIR QUALITY - EXAMPLES FROM ABU-DHABI AND THE AMU DARYA BASIN

The relative importance of petroleum emplacement in inhibiting diagene tic processes and preserving porosity and permeability in Lower Cretac eous, Thamama Group (Kharaib Formation) carbonate reservoirs of Abu Dh abi, UAE, and in Callovian-Kimmeridgian carbonate reservoirs of the Am u Darya Basin in Uzbekistan and Turkmenistan, has been evaluated by co mbining geologic, petrophysical and geochemical data. When petroleum e mplacement is synchronous with and prior to significant burial cementa tion in carbonates, primary petroleum inclusions are trapped in the ce ments. The process appears to be characrerised by steep intra-field po rosity-depth trends within a more gradual regional decline in porosity with depth. This has profound implications for the prediction of poro sity in carbonate reservoirs. Reservoir quality is better in grainston es and packstones compared to adjacent wackestones and lime mudstones in the Kharaib Formation because of preserved macroporosity (intergran ular, vuggy, mouldic); the pore system in the finer units is dominated by micropores. These features indicate a primary textural control on porosity and permeability. Within the grainstones and packstones, macr oporosity is variably filled by late equant sparry calcite cements. Po rosity and permeability variations in grainstones and packstones at a reservoir scale are therefore controlled by the variation in amount of equant sparry calcite cement. This in turn depends on the timing of t he precipitation of this cement relative to petroleum emplacement, as shown by fluid inclusion data. Where petroleum emplacement has occurre d relatively early, at migration foci, prior to significant burial cem entation by equant sparry calcite, reservoir duality is preserved. Whe re it has occurred after significant burial cementation, reservoir qua lity has been destroyed. In the Amu Darya sequences, primary macroporo sity is commonly preserved down to depths of 11,000 ft (3.5 km) with d ifferences in the porosity and permeability characteristics of grainst ones being controlled by variations in the amount of early, probably f reshwater, cement and the extent of associated dissolution. Small volu mes of burial cements do occur, but they do not contain petroleum incl usions. Consequently, there is no firm evidence that petroleum emplace ment has inhibited diagenesis in this area. This part of the study has shown that it is not always possible to obtain conclusive evidence fr om the diagenesis to pin down the processes responsible for the preser vation of reservoir quality and that petroleum filling may not always be the primary cause. The relationships documented here show that the 'race for space' between diagenetic waters and petroleum is a major co ntrol on reservoir quality in the Thamama Group carbonate reservoirs, but is not so important for the Jurassic carbonates in the Amu Darya b asin. (C) 1998 Elsevier Science Ltd. All rights reserved.