Defining Coalbed Methane Exploration Fairways in East-Central Texas

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1999

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The Bureau of Economic Geology at The University of Texas at Austin has developed a basin-scale coal bed methane producibility and exploration model based on a decade of Gas Research Institute-supported research performed in the San Juan, Sand Wash (Greater Green River), and Piceance Basins, as well as reconnaissance studies of several other producing and prospective coal basins in the United States. As part of a cooperative agreement between the Bureau of Economic Geology (BEG) and the U.S. Geological Survey (USGS), BEG is to provide a preliminary assessment of the coal bed methane potential of the east-central Texas Gulf Coast coal basins based on previously published literature and data.

The objective of this report is to discuss the application of the producibility model in defining coalbed methane exploration fairways in an east-central Texas coal basin. The producibility model indicates that tectonic/structural setting, depositional systems and coal distribution, coal rank, gas content, permeability, and hydrodynamics are critical controls to coalbed methane producibility. However, simply knowing a basin's geologic and hydrologic characteristics will not lead to a conclusion about coalbed methane producibility because it is the interplay among geologic and hydrologic controls on production and their spatial relation that governs producibility.

High producibility requires that the geologic and hydrologic controls be synergistically combined. That synergism is evident in a comparison of the prolific producing San Juan Basin and marginally producing Sand Wash and Piceance Basins, where high productivity is governed by (1) thick, laterally continuous coals of high thermal maturity; (2) adequate permeability; (3) basinward flow of groundwater through coals of high rank and gas content orthogonally toward no-flow boundaries (regional structural hinge lines, fault systems, facies changes, permeability contrasts, and/or discharge areas); (4) generation of secondary biogenic gases; and (5) conventional trapping along those boundaries to provide additional gas beyond that generated during coalification.

Understanding the dynamic interaction among these key geologic and hydrologic controls will be critical for delineation of exploration fairways in east-central Texas frontier basins and for targeting "sweet spots" along the Gulf Coast.

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