Abstract

The Pennsylvanian age Northeast Shelf of the Palo Duro Basin is a relatively unexplored area with potential for Upper Pennsylvanian production from a number of facies and traps. Along this 75 mile long trend, which is similar to the outer shelf margin of the present-day Florida Keys, are five fields of Lower Cisco age which have produced over 3 MMBO total.

Production is from three facies which commonly occur on the shelf. They are: 1) ooid grainstone; 2) skeletal packstone/grainstone; and 3) mound bafflestones. Although these facies are prevalent on the shelf margin, mound bafflestones and grainstone shoals are also found scattered on the inner shelf. A major sea level drop of at least 30 meters (third order) exposed the Lower Cisco shelf margin to subaerial diagenesis and erosion. The subaerial exposure served to enhance reservoir porosity and permeability, while erosion entrenched the channels between and behind shelf margin complexes enhancing closures.

The porous mound bafflestones appear to have been more susceptible to late stage dolomitization and can be distinguished, on logs, from lime-rich porous grainstones, which were often encased in tight lime-rich porous grainstones to wackestones and therefore not available to dolomitizing fluids. By cross plotting porosity logs, three facies can be identified: 1) porous lime-rich grainstones; 2) dolomitized bafflestones; and 3) tight lime mudstone/wackestones.

By determining the percentage of each facies, areas with good quantities of potential reservoir rocks can be identified. Modern seismic can then be used to detect presence of potential traps.