The Fate of Primary Porosity During and After Burial-Diagenetic Constraints on the Depositional Model Strategy
1979. "The Fate of Primary Porosity During and After Burial-Diagenetic Constraints on the Depositional Model Strategy", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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As mentioned earlier the utilization of the depositional model as an exploration tool necessitates the preservation of at least some of that porosity during and after burial. This however, is not a valid working strategy for carbonate rock sequences. The preservation of effective primary porosity under deep burial conditions is an exception rather than the rule, since most carbonate reservoirs seem to be characterized by secondary leached porosity, or at best, intercrystalline porosity associated with later dolomitization — both situations controlled by chemical diagenetic processes rather than physical depositional processes. Indeed, under those circumstances where primary porosity is preserved, the actual detailed distribution of primary pore space may be controlled to a great extent by presence or absence of pore fill cement, rather than a change of facies associated with the physical processes present at the site of deposition.
The relative solubility of carbonate minerals as a group, particularly at the time of formation (aragonite and magnesian calcite), makes carbonate sediments and rock sequences particularly susceptible to chemical diagenetic changes through time, particularly when it is realized that pore fluid composition changes and evolves during the burial process. This susceptibility necessitates an expansion of the basic exploration predictive strategy when dealing with carbonate rock sequences to include potential porosity controlling diagenetic pathways after burial.
The following two papers illustrate the point forcefully. The first paper by Moore, et al., describes a Lower Cretaceous beach sequence that contains all the sedimentary structures normally associated with quartzose clastic beaches, including both texture
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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.