Abstract

Carbonate-mound reservoirs within the Pennsylvanian (Desmoinesian) Paradox Formation are major producers of oil and gas in the Paradox Basin of Utah, Colorado, and Arizona. Outcrops of the Paradox Formation along the San Juan River of southeastern Utah provide small-scale analogues of reservoir heterogeneity, flow barriers and baffles, depositional facies, and geometry. These characteristics can be used in reservoir-simulation models for secondary/tertiary recovery of oil from small fields in the basin.

Morphologically, algal buildups within the Ismay zone of the Paradox Formation consist of large, northwest-trending algal banks separated by interbank troughs or channels. Smaller secondary algal mounds and intermounds define the upper surfaces of the algal banks. Cyclic sedimentation is recorded by four dominant facies recognized in a single, shoaling-upward sequence: (1) substrate carbonate, (2) phylloid algal, (3) intermound, and (4) skeletal capping. An outcrop locality in the Wild Horse Canyon area displaying these and additional facies was selected for detailed study.

Along the walls of Wild Horse Canyon, depositional facies characteristic of a typical algal bank transition laterally from a shallow shelf phylloid-algal mound, to bank-margin reef wall, to an off-mound detrital wedge and fan bounded at the top by a flooding surface. The phylloid-algal facies generally exhibits a textural succession composed, from base to top, of rudstone and cementstone, packstone, bafflestone, and skeletal grainstone. Algal plates, brachiopods, bryozoans, and rugose corals are commonly found in the mound complex. The reef wall, deposited in a higher-energy setting, is composed of rudstone, lumpstone, and cementstone textures. The detrital wedge/fan consists of transported algal-plate material and skeletal debris in interbedded grainstone, wackestone, and mudstone textures with open-marine fossils. Within the mound complex is an intermound trough, interpreted to be a tidal channel, through which material was transported to the detrital fan.

Similar algal buildups constitute the primary reservoir facies in most small oil and gas fields in the basin to the east. Understanding facies relationships observed at outcrop enhances prediction capabilities when these same facies and relationships are observed in the subsurface. For example, the presence of reef-wall facies in subsurface core might serve as a proximity indicator for a prospective drilling target. Similarly, extent of porosity and permeability development and distribution in buildup-associated facies (intermound and interbank troughs, reef wall, detrital wedges, and fans) would control flow paths and flow barriers, thereby impacting sweep efficiency in secondary/tertiary recovery programs. The nature of stratigraphic boundaries, vertical facies successions, lateral facies relationships, and facies characteristics within the outcropping buildup complex provide the keys to understanding distribution and behavior of algal-buildup reservoirs in the subsurface.