Porosity Distribution in the Stuart City Trend, Lower Cretaceous, South Texas
D. G. Bebout, R. A. Schatzinger, R. G. Loucks, 1979. "Porosity Distribution in the Stuart City Trend, Lower Cretaceous, South Texas", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
Download citation file:
The Lower Cretaceous Stuart City Trend is a complex of biogenic reefs, banks, tidal bars, channel fills, and islands that accumulated on a broad carbonate shelf encircling the Gulf of Mexico. A variety of carbonate facies were deposited in environments with a wide range of energy levels along this shelf-margin complex. Only four of these facies, however, have greater than 5 percent porosity and 5 millidarcys permeability—the algae-encrusted miliolid-coral-caprinid pack-stone, mollusk grainstone, rudist grain-stone, and coral-stromatoporoid boundstone. Rudist grainstone is potentially the most consistent in terms of porosity and permeability, thickness, and lateral extent.
Intraparticle, interparticle, and fracture porosity are present in the thick limestone section along the Stuart City shelf margin. Intraparticle porosity, in places reaching 20 percent, is common although permeability in facies with intraparticle porosity is low. Facies with interparticle porosity of greater than 5 percent have good permeability of up to 10 millidarcys. Permeability in any facies may be enhanced by the presence of thin fractures which were noted to be common in several cores.
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.