Dolomite Reservoir Rocks: Processes, Controls, Porosity Development
Graham R. Davies, 1979. "Dolomite Reservoir Rocks: Processes, Controls, Porosity Development", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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Dolomitization of pre-existing limestones commonly enhances permeability and improves porosity. Dolomitization may have a “homogenizing” effect on a carbonate reservoir, so that vertical and lateral permeability within the reservoir is improved. Because of its low ductility relative to limestone (and sandstone), the reservoir characteristics of dolomite often are enhanced by fracturing. Secondary dolomitization may create porosity and permeability by post-burial diagenesis closely preceding or perhaps coincident with onset of hydrocarbon migration. For all of these reasons, dolomite rocks far outweigh limestones as the major carbonate reservoir rocks in North America.
Dolomite may be formed by penecontemporeous replacement of unconsolidated carbonate sediment (“primary” dolomites, often characterized by peritidal rocks), but more frequently by replacement of pre-existing limestones (“secondary” dolomite). Dolomitization is controlled by permeability, composition and particle size of the host, and by physico-chemical parameters including temperature, pressure?, and ionic concentration and composition of the pore fluid. Models for dolomitization include evaporative reflux, capillary concentration and evaporative pumping, fresh water-brine mixing, connate water expelled by shale compaction, cannibilization of Mg-calcite sediments, and others. Major dolomite units may be the product of variations on the mixed-water and connate water models. However, most dolomitization models ignore or overlook the potential impact of temperature (particularly post-burial geothermal) and the role of organic reactions (on alkalinity, etc.) that may favor dolomitization by post-burial diagenesis.
Patterns of dolomitization are controlled by microscale and macroscale factors, in turn controlled by permeability. Replacement dolomite commonly shows strong fabric selectivity. Typically, fine matrix carbonate is replaced first, followed either by later clast replacement or moldic/vuggy solution porosity enhancement. Conversely, early cementation such as pervasive submarine cementation or early spar cementation of a grainstone, may destroy permeability in a limestone and prevent or inhibit later dolomitization. Fracture development under differential loading or tectonic stress also characterizes low-ductility dolomite.
On a larger scale, patterns of dolomitization are controlled by:
Paleogeographic setting of originally permeable limestone units relative to a source of dolomitizing pore fluids (eg. porous shelf edge reef between hypersaline lagoon and shale basin; tidal flat deposits).
Exposure at unconformities.
Paleogeographic distribution relative to tectonic highs or regions of subareal exposure (sites for fresh water recharge).
Timing of eustatic or tectonic fluctuations controlling movement and evolution of ground water and connate water.
Relationship to paleo-groundwater conduits, or water tables.
Figures & Tables
Geology of Carbonate Porosity
In clastic situations, primary porositv is a direct function of texture and fabric, including size, sorting and shape (Fig. 1). Grain size, sorting, fabric, as well as sedimentary structures are related directly to sedimentary processes acting at the time of deposition (Fig. 1). Each depositional environment is characterized by a distinct suite of processes distributed across the active sediment water interface in a pattern unique for that environment (Fig.2). This suite of processes gives rise to a group of products, including sediment texture, fabric, and structures distributed across the active sediment water interface in a pattern unique for each depositional environment (Figs. 1 and 2). In a prograding or regressive situation, when sedimentation is taking place at the active sediment-water interface, a vertical sequence of sediments is formed which reflects, in an orderly fashion, from deepest at the base, to shallowest at the top, the progressive changes in texture, fabric and sedimentary structures resulting from the progressive changes in processes found along this interface from shallow to deep water (Fig. 3). Each sedimentary environment then, can be characterized by a unique vertical sequence of sediment textures, fabrics and sedimentary structures. It is this unique suite of characteristics that is commonly used for the identification of depositional environments in ancient rock sequences, and most importantly, is used to predict the presence and detailed distribution of the most porous (best sorted, coarsest) potential reservoir facies (Fig. 3).
In a regional setting, the recognition of distinct sedimentary environments and knowledge of logical lateral relationships is the keystone for prediction of the lateral extension or even presence of potential reservoir facies.