ABSTRACT: Geologic Controls on Occurrence and Producibility of Coal-Bed Methane Resources in the Upper Cretaceous Fruitland Formation, San Juan Basin, Colorado and New Mexico

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

A recent, marked increase has occurred in the number of coal-bed methane wells drilled in the Fruitland Formation; we estimate that more than 1500 such wells will exist by January 1990, a substantially higher number than in 1985 when there were less than 100 wells. In a study funded by the Gas Research Institute, we used data from approximately 2500 well logs to map depositional systems, and data from 1730 well logs to evaluate the occurrence, resource, and producibility of coal-bed methane in the Fruitland Formation.

Fruitland coal occurs in as many as 16 seams. The thickest and most continuous seams have a combined thickness greater than 50 ft and occur in belts 5 to 10 mi wide in Fruitland coastal-plain deposits that intertongue basinward (northeastward) with northwest-trending, retrogradational upper Pictured Cliffs barrier/strand-plain deposits. Northeast-trending belts of Fruitland coal seams formed in a floodplain setting. Coal seams in these dip-elongate belts, which are 2 to 8 mi wide, have a combined thickness of less than 50 ft.

We conclude that the Fruitland Formation contains 44-49 tcf of methane in 245 billion short tons of coal between the depths of 400 and 4000 ft. More than half of the Fruitland coal-bed methane occurs in the northwest quarter of the basin, in the northwest-trending belts where net-coal thickness exceeds 50 ft. In this area, in-place gas commonly exceeds 25 bcf/mi² and locally exceeds 35 bcf/mi².

Targets for fracture-enhanced permeability are areas where brittle Fruitland coal seams have been folded, such as the Hogback monocline, Ignacio anticline, and several minor tectonic folds that have less than 100 ft of structural relief. Additional targets for fracture-enhanced permeability may exist, due to differential compaction, in areas where coal seams override and drape Pictured Cliffs shoreline sandstones or Fruitland channel-fill sandstones; structural relief on these compaction-induced folds may be as great as 100 ft.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990