Depositional Models for Pennsylvanian Reservoirs of Southeastern New Mexico: Thinking a Little Outside the Box

Louis J. Mazzullo
President Mazzullo Energy Corp. Golden, Colorado

Subtle reservoir traps are common features of the late Pennsylvanian Cisco-Canyon series and earlier Pennsylvanian Atoka and Morrow Formations of southeastern New Mexico. These features have been largely overlooked by exploration for more structurally-expressed traps because they are the product of a complex series of depositional, diagenetic, and structural events, and do not always express themselves obviously through subsurface mapping or on 3D seismic. Despite their subtle nature, however, some of these features could hold per-well reserves that are comparable to larger, more conventional traps. On the Northwest Shelf, the Cisco-Canyon (and lower Wolfcamp) series is built up as discrete time units that were deposited in response to often radical changes in sea level. Deposition of shelf-edge algal reefs here was also influenced by the shallow slope of the bathymetric ramp on which they were deposited, which resulted in continuum of depositional units that are spread out over a lateral distance of over 20 miles. These algal reef fairways are found in parallel trends and appear to be developed as discontinuous and reworked units that vary in size from the smaller, vertically segregated mounds of the Bough series in the Tatum Basin, to large, stacked sequences that built in response to contemporaneous structural events, such as Indian Basin and Dagger Draw. Other, smaller algal reef fairways have been discovered, but many remain underdeveloped or undiscovered because of their subtle expression through conventional mapping techniques.

The Morrow and Atoka Formations in southeastern New Mexico have important plays for natural gas reserves for many years, and continue to be some of the more prolific gas producers in the region. Reservoirs there are found in a variety of stratigraphic and structural traps in sandstones that were deposited in fluvial, deltaic, and marine environments. These formations offer other types of more poorly-understood reservoirs that arose from peculiar early and postdepositional diagenetic and structural events that have been poorly recognized by conventional mapping. These modifications to the section create problems in correlating pay sands and tracing reservoir trends, even in areas where it is not readily apparent that such factors were operative.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009