--> --> ABSTRACT: Barrier-Strandplain Deposits of the Pictured Cliffs Sandstone and Coal Occurrence in the Fruitland Formation, Northern San Juan Basin, Colorado and New Mexico, by William A. Ambrose, Walter B. Ayers, Jr.; #91002 (1990).

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ABSTRACT: Barrier-Strandplain Deposits of the Pictured Cliffs Sandstone and Coal Occurrence in the Fruitland Formation, Northern San Juan Basin, Colorado and New Mexico

William A. Ambrose, Walter B. Ayers, Jr.

Major transgressive/regressive episodes during the Late Cretaceous in the northern San Juan basin resulted in deposition of three upper Pictured Cliffs tongues that controlled deposition of the thickest coal deposits in the Fruitland Formation. In a study funded by the Gas Research Institute, structure, isopach, and net-sandstone maps of the Pictured Cliffs tongues and Fruitland Formation in the San Juan basin were made with data from approximately 2500 well logs. The objective was to clarify geologic controls on the occurrence and trend of the coal seams, which are important coalbed methane reservoirs.

Upper Pictured Cliffs tongues typically consist of fine-grained, crossbedded, burrowed sandstone grading upward into medium-grained, planar-bedded sandstone that was deposited in a wave-dominated shoreline setting. Outcrop and subsurface studies show that each tongue is an amalgamated barrier/strandplain unit 20 to 150 ft thick and trends northwestward, parallel to depositional strike. Episodes of shoreline migration were controlled by eustacy and sporadic differential subsidence across a structural hingeline; the lowermost tongue pinches out landward (southwestward) against this hingeline. Successive upper Pictured Cliffs tongues display an imbricate relation and are offset basinward because of net progradation of the shoreline.

During periods of relative sea-level rise, progradation of the Pictured Cliffs shoreline was temporarily arrested, which stabilized the Fruitland coastal plain and allowed thick peats to accumulate. The thickest and most extensive Fruitland coal deposits occur in three northwest-trending belts that formed landward of Pictured Cliffs tongues. Within these deposits individual coal seams are commonly thicker than 20 ft, netcoal thickness is greater than 70 ft, and in-place coalbed methane commonly exceeds 25 bcf/mi2.

Lower Fruitland coal seams form extensive coalbed methane reservoirs, commonly overriding upper Pictured Cliffs tongues. Compaction-induced folds and fractures where coal seams are draped over these barrier/strandplain sandstones may enhance coal-seam permeability and coalbed-methane productivity. These areas are potential sites for coalbed methane wells.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990