Outcrop Analogs for the Eastern Paradox Basin Shale Gas Play from the Hermosa Cliffs, Southwestern Colorado
Thick wedges of calcareous shale seen at Hermosa Mountain, north of Durango, Colorado, are possibly slope deposits of the Chimney Rock, Gothic or Hovenweap shales. These packages are now correlated into the framework of clastic and carbonate shelf deposits previously described to the north.
The Hermosa Cliffs north of Durango, Colorado expose a 15km long, dip-oblique section of the Pennsylvanian (Desmoinesian) Hermosa Group that includes 19 unconformity bounded sequences (Gianniny and Miskell-Gerhardt, 2007). These sequences are characterized by late lowstand to transgressive fluvio-deltaic clastics capped by transgressive to highstand shallow-water carbonates. Basin-restricted evaporites and enclosing black shales seen at Hermosa Mtn. correlate 9km/5.5mi. updip to 15m of calcareous dissolution breccias, laminites and shale which onlap the sequence 11 basal unconformity.
Age control based on conodonts and fusulinids bracket the section from top Akah (sequence 11) to lower Honaker Trail (sequence 16) (Gianniny, Miskell-Gerhardt and Ritter, 2008). Therefore, a large portion of the producing section in the Paradox Basin either has an equivalent shelf facies exhibited here or is represented by exposure / bypass surfaces.
New physical correlations now tie the shelf framework above sequence 11 at Goulding Creek to downdip deposits at Hermosa Mtn., which are interpreted to be primarily upper slope. Over this distance three trends are observed. Firstly, highstand carbonates thin, change facies from grain/packstones to wackestones, and terminate by downlap. Secondly, lowstand clastic deltas thin (by downlap) and become much finer-grained and less arkosic. Thirdly, three shaley intervals develop between downlapping deltaics and thicken basinward into the subsurface. The shales are black, fissile, calcareous and sparsely fossiliferous. While many shaley intervals updip are associated with pro-delta settings and contain visible, plant-derived organic matter, the downdip shales appear to be more marine. (Overmaturity of shales in the Hermosa Cliffs due to the proximity of the San Juan volcanic field limit the value of TOC and pyrolysis analyses.) As correlated, these shales lie above the upper Akah marker and below the lower Honaker Trail, suggesting a possible tie to the Chimney Rock, Gothic and/or Hovenweap shales. Initial attempts to extract conodonts from these shale intervals to refine correlations were unsuccessful.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009