Abstract

Hydrocarbon production from Late Carboniferous (Pennsylvanian) phylloid algal mound reservoirs in the Paradox Basin of the southwestern United States has exceeded 400 MMBO over the last 40 years and estimates suggest that a similar amount of recoverable reserves are still in-place within the basin. The exploration and development history of the basin indicates that the cycles containing prolific algal mound reservoirs are characterized by extreme lateral variability resulting in close step-out dry holes, or conversely, reservoirs encountered with virgin pressures. An outcrop-based study combining detailed facies analysis and cyclostratigraphy from equivalent facies on the exposed Paradox shelf has provided insight into the three-dimensional architecture and distribution of both the algal mound reservoirs and their associated facies, as well as the timing of mound growth and porosity development relative to sea level change.

Lateral variability within the shelfal cycles is observed in both the distribution and thickness of facies. This variability in reservoir and associated facies occurs within tens or a few hundreds of meters, as well as on a kilometer scale, and is a function of antecedent topography, facies types, exposure, unfilled accommodation space, and the amplitude and frequency of relative sea level change. Landward and seaward shifting of facies belts across the shelf by tens of kilometers, for example, occurs in response to fourth order (100-400 k.y.) relative sea level changes. An awareness of facies shifts at this scale can help to direct and refine exploration efforts. In addition, rapid lateral variability in the thickness of algal mound reservoirs and associated facies observed in 4th and 5th order (<100 k.y.) depositional cycles may influence both exploration and production scale efforts in the basin.