Geochemical Tracers of Dolomitizing Fluids: A Tool for Predicting Diagenetically Controlled Porosity on a Reservoir Scale

Mole-per-mole replacement of calcite by dolomite yields an approximately 12% increase in porosity because dolomite is denser than calcite. However, data from the Pliocene-Pleistocene Seroe Domi Formation of Bonaire, Netherlands Antilles, demonstrate the dolomitization of these rocks resulted in porosity reduction. Rocks proximal to the source of dolomitizing fluids exhibit a greater amount of porosity occlusion than more distal rocks, indicating that dolomitization is a porosity-destroying process and that the degree of destruction can be calibrated to the flow path of dolomitizing fludis. Porosity observed in Bonaire dolomite varies over a distance of hundreds of meters along fluid-flow paths. In three examples of dolomites in which the pathways of dolomitizing fluids can be interpreted from the spatial geometry of dolomite compositions, the distances over which these changes occur are pertinent to interpreting diagenetically controlled reservoir heterogeneity in oil and gas fields. These include the Bonaire Dolomite, the Lower Ordovician Ranger Peak Formation of west Texas, and the Permian Clear Fork Formation of west Texas. Tracing dolomitizing fluid pathways may predict diagenetically controlled porosity trends in ancient rocks. Because porosity trends are associated with significant changes in petrophysical characteristics and fluid storage capacity at a between-well scale, this interpretation scheme maymore » aid mapping of flow units in hydrocarbon reservoirs.« less