Outcrop to Subsurface Stratigraphy of
the Pennsylvanian Hermosa Group
Southern Paradox Basin, U.S.A.
Dr. Alan L. Brown
Landmark Graphics Corporation

Pennsylvanian (Desmoinesian) sedimentary rocks within the Paradox Basin Four Corners area of the western United States afford a unique opportunity to study the development of sedimentary successions in a complex marine to nonmarine depositional setting. The close association of thick intervals of nonmarine fan-delta facies adjacent to and in time equivalent position to marine carbonate-evaporite facies suggests complex relationships between the factors affecting deposition. Development of an effective scheme to differentiate the depositional signatures from within these sedimentary successions is the primary goal of this study. To achieve this goal, two objectives were pursued. The first was to calibrate the diverse range of rock-types in the Hermosa Group to insitu wellbore measurements. To facilitate this process, a neural network evaluation procedure coupled with standard petrophysical evaluation techniques were employed to aid in facies succession prediction and lateral facies correlation. This process proved to be as accurate as standard wireline analysis procedures and was able to account for variations not as detectable in conventional scheme. The second objective was to correlate the stratigraphy of the Hermosa Group from outcrops of the Animas Valley to the subsurface along the southern Paradox Basin. The key to understanding the depositional sequences within the Middle Pennsylvanian section is to determine spatial and temporal relationships between the evaporites and black-shale deposits associated with carbonate algal mound buildups and juxtaposed terrigenious clastic fan-delta depositional facies. Once the relationships of these facies successions are delineated, then a three dimensional architectural framework can be manipulated to examine all possible lateral facies successions. By utilizing these analyses, several members of the Paradox Formation were shown to be laterally equivalent and physically continuous with parts of the previously designated undifferentiated Honaker Trail Formation of the San Juan Dome region.

The study required a rigorous integration process utilizing a digital workstation environment combining large and more diverse datasets than previously utilized for improved correlation control. Techniques for evaluation of facies successions involved core (42), subsurface wells (4000+), and measured sections (12+) were employed.