Geologic and Hydrologic Characterization of Coalbed Methane Reservoirs, Fruitland Formation, San Juan Basin, Colorado and New Mexico

AYERS, W. B., JR., W. A. AMBROSE, W. R. KAISER, S. E. LAUBACH, A. R. SCOTT, and J. S. YEH, Bureau of Economic Geology, University of Texas at Austin, Austin, TX; C. M. TREMAIN, Colorado Geological Survey, Denver, CO; and N. H. WHITEHEAD III, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM

Geologic and hydrologic studies clarify coalbed methane reservoir characteristics of the Fruitland Formation. In geologic studies, data from geophysical logs were used to map coal (reservoir) occurrence and structural features; fractures in coal seams were described at outcrops and in cores. In hydrologic studies, Fruitland bottom-home pressures calculated from well head shut-in pressures were used to map hydraulic head and pressure regime and to estimate vertical and lateral pressure gradients. Chemical facies of produced waters were defined by Piper and Stiff ionic-ratio diagrams.

The thickest coal seams trend northwestward and occur in the north-central part of the basin; these seams are the most permeable units in the Fruitland Formation. In the southwest part of the basin, coal seams trend either northwestward or northeastward. Coal trends are predictable and are controlled by depositional setting. Fracture trends in coal seams in the southern two-thirds of the basin are north-northeast, whereas fracture trends in the northern third of the basin are northwest. The Fruitland Formation behaves locally as a heterogeneous aquifer, but regionally it behaves as a homogeneous aquifer that is overpressured in the northern part of the basin and underpressured in much of the rest of the basin. Overpressuring is attributed to artesian conditions. Large vertical and/or ateral pressure gradients indicate reservoir heterogeneity. Low-chloride, high-alkalinity waters coincide with overpressuring, and saline NaCl-type waters coincide with underpressuring.

Geologic studies delineated coalbed methane reservoirs (coal seams), whereas hydrologic studies indicated reservoir conditions such as permeability and pressure regime. By integrating geologic and hydrologic findings, we divided the San Juan basin into regions having similar reservoir characteristics and defined areas with optimal conditions for coalbed methane production. The approaches used in this study are transferable to other coal basins.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)