Exploration for coalbed methane in remote or frontier regions is often hampered by the absence of an adequate data base, thus inhibiting detailed evaluation of coalbed methane exploration potential. The key hydrogeologic factors affecting producibility include depositional setting and coal distribution, tectonic and structural setting, hydrodynamics, permeability, coal rank and gas generation, and gas content. Although much of this information may be difficult to obtain in areas where limited or no exploration has taken place, generally sufficient information is available to perform at least a preliminary assessment of the coalbed methane potential.

High coalbed methane producibility commonly occurs in areas of upward-flow potential associated with permeability barriers (no-flow boundaries). Outcrop studies combined with examination of existing geologic maps and precipitation patterns therefore provide valuable information on coal resources (coal thickness and orientation), ground-water flow and permeability barriers (cleat orientation, faulting, coal pinchout), and gas generation potential (surface coal rank and coal quality). These data are then combined to delineate areas of potentially higher gas content and, therefore, higher coalbed methane potential.

The depositional setting and structural style in a basin may be evaluated from outcrop studies and remote-sensing techniques. Ideally this type of evaluation will determine the depositional setting of the coal beds (and, thus, coal quality and hydrocarbon-generation potential), the shoreline-progradation direction, and the possible location of thicker coal beds in the subsurface. Outcrop studies additionally provide measurements of net coal thicknesses and cleat orientation. Existing geologic maps and remote sensing techniques may also delineate fault locations, orientations, and structural style, all of which may indicate potential permeability barriers. Significant information regarding ground-water-flow direction is obtained from geologic, topographic, and annual precipitation maps, as well as cleat-orientation measurements.

Gas content, one of the more important controls on coalbed methane producibility, is impossible to determine accurately without direct measurements on coal samples. Areas of potentially higher gas content can be inferred, however, from the coalification history (coal rank), hydrocarbon or ground-water-flow direction, and the location of coal pinchout and faulting (permeability barriers). Vitrinite reflectance (VR) data collected from outcrop may indicate areas of higher coal rank and thermogenic gas generation. Shale or coal VR data from existing wells are used to construct VR profiles, which determine the depth to the threshold of thermogenic gas generation and, therefore, potentially higher gas content. Areas containing abnormally high gas contents associated with secondary biogenic gas generation may be predicted on the basis of an integration of ground-water-flow direction and location of permeability barriers.

Understanding the synergistic interplay among key hydrogeologic factors is critical to evaluating areas of potentially higher coalbed methane in remote areas. Integrating outcrop studies, existing geologic maps, precipitation patterns, vitrinite reflectance data, and existing well data (however limited) can nevertheless provide information sufficient to target areas of higher gas content and, therefore, potentially higher coalbed methane production.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah