RIESE, W. C.¹, S. G. FRANKS², UDO FEHN³, and JEAN E. MORAN^4
1Vastar Resources, Inc., Houston, TX
²ARCO Exploration and Production Technology, Plano, TX
³University of Rochester, Rochester, NY
⁴Lawrence Livermore National Lab, Livermore, CA

Abstract: Fruitland Formation Waters, San Juan Basin, Colorado, and Their Relevance to Understanding Fruitland Coalbed Methane Reservoirs

Published descriptions of reservoir dynamics for coalbed methane reservoirs of the Cretaceous Fruitland Formation of the San Juan Basin, New Mexico and Colorado, USA, hypothesize the importance of present day groundwater recharge to maintain a state of hydrodynamic overpressure and to assure that methane generated in the coals will remain adsorbed. Without recharge, hydrodynamic pressures would eventually fall, and methane would desorb and be lost. Recharge is also thought to be responsible for sustaining convergent, upward flow of water on which other hydrodynamic elements of the trap are partially dependent. Our work suggests that the pervasive permeability (cleat) system necessary to sustain this type of hydraulic conductivity does not exist in the Fruitland Formation. Nor do we find evidence for the upward (cross-formational) flow that this model requires.

Waters from wells in the Colorado portion of the basin show higher TDS and chloride concentrations closer to outcrop than should be expected if the proposed models are correct. Iodine (¹²⁹I) dating of waters indicates minimum ages of 4 mybp (Pleistocene) to 45 mybp (Eocene). Spatial distributions of these ages and water chemistry suggest significant barriers to down-dip water flow from the outcrop and appear to correlate with patterns in well performance. These data, interpreted in the context of available production data, indicate that at least some portions of the basin contain waters relatively unaffected by outcrop recharge.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas