ABSTRACT

The recent interest in coalbed methane as an energy source in Rocky Mountain sedimentary basins is leading to increased exploration in the Alberta Basin. One of the critical factors for the economic production of coalbed methane is permeability, for which little information has been gathered in Alberta. However, coal permeability depends, among other factors, upon the magnitude of the smallest principal stress, for which much relevant data do exist. This paper describes attempts to use available subsurface information to map stress magnitudes at horizons that are relevant to buried coal seams of Cretaceous age, to use the stress information to infer permeability, and to identify areas with production potential.

Reliable in situ stress magnitudes have been assembled from micro-fracture and mini-fracture records. Leak-off pressures and fracture breakdown pressures have been recovered from, respectively, drilling and completion records. All data sources were screened for reliability and the qualifying leak-off pressures and fracture breakdown pressures were then adjusted so as to be quantitatively compatible with the micro-fracture and mini-fracture and mini-fracture stress magnitudes. Vertical stress magnitudes were calculated to ensure that the former dataset represented the smallest principal stresses in the basin.

Inferred stress magnitudes are presented separately for Upper Cretaceous-Tertiary rocks and for Lower Cretaceous rocks. Horizontal stress orientations are also mapped for Upper Cretaceous rocks and for Lower Cretaceous rocks. In both sequences the stresses are oriented so as to favour permeability along the major fracture network in the coal seams.

Maps of inferred stress gradients and magnitudes were prepared and examples are included in this paper. Data from other basins were cited to demonstrate how such maps might be used to high-grade areas for potential coalbed methane production. However, at the present time, there are insufficient data to validate the stress mapping reported here, or to quantify stress/permeability relationships in the Alberta Basin. Therefore this study must be considered as a reconnaissance endeavour. The stress mapping also has potential application to the production of oil and gas.

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