The Application of a Coalbed Methane...Model in Defining Coalbed Methane ExplorationExamples from the San Juan, Sand Wash, and

This report applies a basin-scale coalbed methane producibility model in defining fairways and sweet spots in three coal basins in the United States. The Bureau of Economic Geology, The University of Texas at Austin, has developed a basin-scale coalbed 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 and on reconnaissance studies of several other producing and prospective coal basins in the United States. In this report we discuss the application of the producibility model in defining coalbed methane exploration fairways and sweet spots in coal basins in the United States. The producibility model also has direct application to worldwide coalbed methane exploration and development. The model indicates that tectonic and structural setting, depositional systems and coal distribution, coal rank, gas content, permeability, and hydrodynamics are controls critical to coalbed methane producibility. Simply knowing a basin's geologic and hydrologic characteristics will not lead to a conclusion about coalbed methane producibility, however, 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 ground water through coals of high rank and gas content orthogonally toward no-flow boundaries (regional structural hingelines, fault systems, facies changes, permeability contrasts, and/or discharge areas); (4) generation of secondary biogenic gases; and (5) conventional and hydrodynamic trapping along those boundaries to provide additional gas beyond that generated during coalification. Understanding the dynamic interaction among these key geologic and hydrologic controls is the basis for delineation of exploration fairways in frontier basins and for targeting "sweet spots" in basins with established production.