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Strike-Parallel Facies Heterogeneity and Base-Level Change in the Campanian Fruitland Formation Coal System, North Eastern San Juan Basin

Gianniny, Gary L.1; Skyles, Emilee 1; Stetson-Lee, Tor 1; Carroll, Christopher J.2; Houck, Karen 2
1 Geosciences, Fort Lewis College, Durango, CO, CO.
2 Colorado Geological Survey, Denver, CO.

Understanding the scale of lithologic heterogeneity in ancient shoreline/fluvial coal systems is critical to predicting potential aquifer dynamics, coalbed methane resources, and the degree of reservoir compartmentalization. In many regressive sequences, basal coals are expected to be more continuous along strike, while higher coals are expected to be thinner and less continuous. Similarly, siliciclastic abundance is expected to follow the inverse pattern, with sandstone continuity increasing up section. Exceptions to these generalities seen in the Campanian Fruitland Formation/Pictured Cliffs Sandstone system suggest a more complex pattern that may be driven by coupled relative sealevel-baselevel rises, during the longer term retreat of the Cretaceous interior seaway.

Here we present new data from 22 measured stratigraphic sections across a 32 km strike-parallel transect in the Fruitland Formation, as defined by the presence of coal and carbonaceous clastic deposits, of the northeastern San Juan basin. With thichnesses of 45 to 60 meters in this area, the formation contains up to 8 distinct coal beds interbedded with fluvial sandstones, and distributary lake-margin deltas containing bivalves. Interestingly, the thickest (4-6m), and most laterally continuous coal (12 km) occurs at 30-35 meters above the basal contact of the nonmarine facies on top of the marine Pictured Cliffs Sandstone. Similarly, stratigraphically high, thick coals occur in the eastern portion of the region. These thick, high coals suggests increased baselevel due to a minor rise in relative sealevel. In addition, 2 basal parasequences include lithologies consistent with lower shoreface marine sandstones of the Pictured Cliffs Sandstone. Together these features may represent 3 parasequences within this system. This interpretation is consistent with previous work to the west and south that has identified eastward stepping parasequences in the along this basin margin. Because our work is done along the northeastern margin of the outcrop belt, and because this shoreline was retreating to the northeast, these sandstones and the stacking patterns of this fluvial coal system provide a unique signal of the now eroded marine system that lay to the northeast of this area. Recognition of these patterns provide an improved predictive framework for facies-controlled permeability and porosity heterogeneity in aquifers and coals in this in this coalbed methane producing region.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009