Peter A. Scholle, 1979. "Porosity Prediction in Shallow Versus Deep Water Limestones- Primary Porosity Preservation Under Burial Conditions", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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Preservation of primary porosity is an important factor in many oil and gas fields producing from carbonate rocks. Initial porosities in the range of 30–40% (in grainstones) and 60–75% (in mudstones) are common in carbonate sediments and under a number of circumstances much of this primary porosity can be preserved. However, many factors such as early cementation, compaction, stylolitization and subsurface cementation tend to reduce primary porosity. Thus, we must understand and be able to predict the effects of a variety of diagenetic processes before we can effectively explain the distribution of preserved primary porosity.
Prediction of preserved primary porosity in carbonate rocks is at best a difficult undertaking. Most carbonate hydrocarbon reservoirs drilled have been in shallow-water limestones. In such rocks, depositional patterns are complex and variable, and diagenetic alteration of original sediment textures and compositions may be extreme. Few, if any, general rules can be used to predict the porosity of such carbonate rocks in the absence of drilling data. Even where a significant number of wells have been drilled, useful porosity trends that enable accurate porosity prognostication over even relatively short distances may be difficult to discern.
Deep-water limestones, on the other hand, generally have much more predictable porosity. They have more uniform depositional facies, more predictable primary chemical compositions, and simpler diagenetic alteration patterns than their shallow-water equivalents. Alteration of deep-water limestones is controlled mainly by their maximum burial depth. Average rates for such diagenetic porosity loss can be determined as a function of burial.
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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.