Abstract: Area Balance and Strain in Coalbed Methane Reservoirs of the Black Warrior Basin

Jack C. Pashin, Richard H. Groshong Jr.

Investigation of coalbed methane reservoirs in the Black Warrior basin of Alabama has established a correspondence between productivity and structural position, but the reasons for this correspondence remain uncertain. In Cedar Cove field, for example, exceptionally productive wells are concentrated in a rollover anticline, whereas in Oak Grove field, exceptionally productive wells are aligned along a synclinal axis. This suggests that factors controlling gas production are a derivative of the structural geometry, and not the geometry by itself.

Natural fractures and a low state of in-situ stress facilitate depressurization of coalbed reservoirs by dewatering, and hence, desorption and production of coalbed gas. Our hypothesis is that the abundance and openness of natural fractures in the Black Warrior basin are a direct expression of the layer-parallel strain dictated by map-scale structural geometry. Area balancing techniques can be used to quantify requisite strain, which is the homogeneous layer-parallel strain required for local area balance, and can also be used to constrain and verify structural cross sections.

Application of area balancing techniques to extensional structures in the Black Warrior basin indicates that coalbed gas is produced from thin-skinned structures detached within the coal-bearing Pottsville Formation. Within reservoir intervals, requisite strain values are as high as 10 percent and increase downward toward the basal detachment. Mapping structure and production indicates that some productivity sweet spots correlate with enhanced bed curvature. Whereas requisite strain is the homogeneous strain calculated for discrete bed segments, curvature affects the distribution of strain within those segments. Recognizing this, our research is now focused on integrating area balancing techniques with curvature analysis to explain production patterns in coalbed methane reservoirs.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas