Coal Bed Stratigraphy and Surface Mapping of the Fruitland Formation in Archuleta County, Colorado
Archuleta County, Colorado, is a new frontier for coalbed methane exploration in the northern San Juan Basin (SJB) and detailed coal stratigraphy is important to exploration efforts. A new geologic map compiled along 23 miles of late Campanian Fruitland Formation coal outcrop depicts detailed clastic bed stratigraphy. The 1:16,000-scale map shows designated formation contacts between the Pictured Cliffs Sandstone, the Fruitland Formation, and the overlying Kirtland Shale, units that were previously combined as one unit. The map depicts correlated coal beds, gas seeps, distressed vegetation, coal cleat, clinker, and regional fractures.
The surface coal beds were correlated using an integrated approach of coal bed mapping, measured sections, aerial photography, and ERDAS GIS imagery. This information was then correlated with known subsurface correlations from coalbed methane wells. Fluvial sandstones intercalated with siltstone, shale, and coal beds were correlated to determine the lateral extent of the coal bed facies. Detailed correlations of these beds show a complex relationship between brackish-water sediments intertonguing between the Pictured Cliffs Sandstone and the basal Fruitland coal beds. The en-echelon geometry of time-transgressive stratigraphy between the Pictured Cliffs and the basal Fruitland coals show how these two units interfinger near Cabezon and Archuleta Canyons. Fresh-water Fruitland coal beds up to four feet thick in Cabezon Canyon have been observed that are stratigraphically over 50 ft below the top of the Pictured Cliffs Sandstone locally. The southwest-northwest orientation of the outcrop on this side of the SJB is also parallel to the Cretaceous shoreline interpretation, showing very long sheet sand beds in the Pictured Cliffs Sandstone.
New development of coalbed methane resources in this area may show productive coal gas from this area. Coal cleat measurements along this outcrop are nearly parallel to the exposure indicating low permeability at the outcrop. Gas seeps usually develop along low topographic areas on the Fruitland outcrop and the Piedra River with nearby tributary creeks will be monitored closely. Colorado Oil and Gas Conservation Commission (COGCC) recently drilled six monitoring wells at three locations down dip of the Fruitland coal outcrop to monitor water levels and formation pressures in this part of the SJB as new coalbed methane drilling develops in Archuleta County.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009