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Many Middle Eastern reservoirs are hosted in carbonate rocks. These rocks are chemically highly reactive systems. One of the most common physico-chemical processes affecting sediments after deposition (diagenesis) that alter limestone is dolomitization. The latter process, i.e. the transformation of calcite (CaCO3) into dolomite (CaMg(CO3)2), can be associated with a porosity gain if a mole-per-mole replacement is achieved and thus if overdolomitization (plugging gained porosity by dolomite cement) is avoided. Dolomitization occurs over a range of environmental conditions, i.e. low to high temperature environments and saline to hypersaline fluids. Despite field evidence of dolomitization at low temperature, abiotic dolomite precipitation was only achieved in the lab for temperatures of 60ÂșC or higher due to kinetic inhibition of dolomite formation. A key factor for dolomitization is fluid flow, i.e. input of a significant amount of dissolved Mg needed to form dolomite. Fluid flow depends clearly on permeability. However, the reactive surface is another key factor in transforming limestone into dolomite. In this respect, the aim of the current study is to determine the respective importance and impact of reactive surface versus permeability on the dolomitization process based on lab core flooding experiments. The experiments use two types of Jurassic ooidal limestones, 1) a porous ooidal grainstone in which porosity between ooids is well preserved and 2) an ooidal grainstone to packstone with lime mud present between ooids. Small core plugs of these limestones are flooded with saline solutions and mineralogical and morphological changes in the cores are captured by computed tomography (CT) imaging at regular time intervals. In addition, petrography of stained thin sections and mineralogical X-ray diffraction is used to determine semi-quantitative estimates in bulk rock and to validate mineralogical variation in the CT scans. The results of this study jointly funded by Qatar Petroleum, Shell and the Qatar Science & Technology Park, i.e. understanding the relative role of reactive surface versus permeability in the dolomitization process, will significantly help in the prediction of dolomite reservoirs.

Publication date: 
Thursday, April 10, 2014