Area Balance in Extensional Structures: Comparison Between the Black Warrior and Gulf Coast Basins

PASHIN, JACK C., Geological Survey of Alabama, Tuscaloosa, AL; and GROSHONG, JR., RICHARD H., University of Alabama, Tuscaloosa, AL

Area balancing techniques can be used to construct and validate structural cross sections, quantify requisite strain, and predict the distribution of fractures. The first direct application of area balance to petroleum exploration and field development was in Pennsylvanian coalbed methane reservoirs of the Black Warrior basin. Area-depth plots indicate that extensional structures are detached within the coal-bearing section. In a full graben, requisite strain increases downward toward the basal detachment. Analysis of caliper logs suggests that strain is expressed as fractures and small-scale faults that affect well performance.
Results from the Black Warrior basin are transferable to similar geologic settings, but salt withdrawal and synsedimentary growth present challenges for applying area balancing techniques to extensional structures of the Gulf Coast basin. Ongoing research in Cretaceous fractured chalk reservoirs of Gilbertown Field in southwest Alabama is identifying the critical factors that need to be accounted for in salt-tectonic settings. Theoretical models indicate that salt withdrawal causes a downward shift of hanging-wall regional, resulting in prediction of a basal detachment that is too shallow. Synsedimentary growth results in a non-linear area-depth relationship and prediction of a basal detachment that is too deep.
In Gilbertown Field, hanging-wall regional is slightly offset in Jurassic strata, growth strata are well-developed in the Cretaceous section, and a known detachment is at the base of the Jurassic salt. Area-depth plots predict a detachment below the salt, indicating that offset regional is a less critical factor than synsedimentary growth. A new equation of area balance that accounts for synsedimentary growth has been developed, and efforts are now focused on developing a new measure of requisite strain that can be used for fracture prediction in chalk reservoirs of the Gulf Coast basin.

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