CHARACTERIZING COUPLED TECTONIC-HYDROLOGIC PROCESSES IN COAL BEDS OF THE POWDER RIVER BASIN, SOUTHEASTERN MONTANA, FROM GEOPHYSICAL WELL LOGS AND AQUIFER TESTS

As part of a multidisciplinary investigation designed to assess the potential for coalbed methane development and its implications in terms of water resources for the Powder River Basin of southeastern Montana, six wells were drilled through Fort Union Formation coal beds along a 32-km transect within the Tongue River drainage basin. Geophysical logs were obtained in all of these wells and an aquifer test was conducted at one site across a 4.6-m thick coal unit to quantify coalbed hydraulic properties and evaluate transmissivity anisotropy. Natural gamma and electrical resistivity logs were particularly effective in distinguishing individual coal beds and complementary televiewer logs, both acoustic and optical, provided magnetically oriented images of the borehole walls. Inspection of these images and comparison with fractured coal cores infer a master cleat orientation of approximately N30°E, in close alignment with the general northeast trend of the basin and the nearby Tongue River. Further analysis of these televiewer data from all 6 wells and across all lithologies reveals 152 planar features that generally fall within two primary fracture sets, one striking east-west and the other north-south. Finally, results of the aquifer test determined from the concurrent responses in three observation wells yield an effective coal transmissivity of approximately 14 m2/d. The drawdown ellipse generated during this test indicates that this transmissivity has a transverse anisotropy ratio of 2.4 with its major axis aligned east-west. In addition to these direct field measurements, the local tectonic stress field in this physiographic province as inferred from a nearby 1984 earthquake was examined. Focal plane solutions characterize this magnitude 4.9 event as an oblique-normal faulting mechanism with dominant east-west compression and north-south extension, stress directions coincident with one set of fracture planes and the axes of the drawdown ellipse but oblique to apparent cleat orientation. Consequently, the preliminary integration and analysis of these geophysical and hydrologic field measurements indicate that transmissivity anisotropy in this coal bed is predominantly controlled by the contemporary stress field rather than by cleat structure.