The depositional models for the Stuart City Trend and the Zelten Field are similar. Both comprise high-energy facies which were deposited on the edge of a broad platform far from the ancient shoreline. The platform edge carbonates were deposited as grainstone/packstone bars and shoals and boundstone/framestone banks or reefs. The energy level was probably higher at the Stuart City setting as shown by the greater abundance of mud-free grainstones there and the predominance of packstones at Zelten. The presence of carbonate mud between the grains at Zelten played a major role in inhibiting deposition of submarine and shallow subsurface porosity-destroying cements. Wackestone facies predominate landward of the platform-edge facies in both areas. Seaward, the slope into the basin is yery gentle and gradational in both models.
The vertical sequence of Stuart City and Zelten is progradational or shoaling upward. Widespread leaching throughout the Zelten carbonates is evidence of a significant period of subaerial exposure after deposition of the Zelten platform facies; along the Stuart City Trend local development of vadose cements and hardgrounds and sparse occurrence of palmwood are evidence of limited subaerial exposure of scattered islands after deposition of the shallow-water carbonates. Significant subaerial leaching is lacking along the Stuart City Trend and a gradation contact with the overlying deeper-water facies is more common. Both are capped by slope or offshore facies deposited in slightly deeper water.
The Zelten reservoir rocks are predominantly packstones and grainstones with little evidence of submarine diagenesis (isopachous cement or micrite rims). Subaerial vadose
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.