Pore System Evolution in a Cretaceous Carbonate Beach Sequence
Clyde H. Moore, Jr., James M. Smitherman, Stephen H. Allen, 1979. "Pore System Evolution in a Cretaceous Carbonate Beach Sequence", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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The carbonate beach sequence can be divided into a backshore (dolomite), an upper foreshore (mollusc lime grainstone), a lower foreshore (mollusc lime packstone) and an offshore zone (pellet lime packstone). This sequence represents a prograding beach-supratidal complex. The early diagenetic history can be divided into two phases. (1) The beach-backshore accretion phase is characterized by intertidal and submarine cementation with aragonite cements under marine phreatic and vadose conditions. A meteoric phreatic lens within the beach was the site of concurrent grain solution and precipitation of calcite cements. (2) The post-backshore accretion phase is characterized by dolomitization of the supratidal, and wholesale silicification of the upper foreshore by reflux waters from the prograding supratidal. Late post-burial solution took place after the sequence was uplifted, probably during Balcones faulting (Miocene-Pleistocene).
The beach proper is characterized by secondary mollusc moldic porosity. The highest porosity-permeability values (39% and 460 md.) are found in the base of the upper foreshore and top of the lower foreshore zones. The backshore dolomites have intercrystalline porosity averaging 33% with a permeability of 18 md. The offshore pellet packstones have intercrystalline porosity averaging 29% and 36 md. The concentration of secondary porosity and permeability in the middle of the beach is a function of early cementation and silicification of the upper foreshore that created a permeability barrier at the top of the sequence while the normal depositional fabrics of the offshore zone had the same effect at the base of the sequence, effectively restricting the major flow of a later groundwater system to the middle of the beach. Total rock history, therefore, including both deposition and diagenesis, is the key to effective pore system analysis.
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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.