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Mystery in the Mushwad: The Origin of Gas in the Big Canoe Creek Field Saint Clair County, Alabama, USA


The Big Canoe Creek Field produced gas from the Middle Cambrian Conasauga Formation. The production established in 2005 was hailed as an exciting new play in the oldest and most structurally complex formation in North America. By 2010 the field was abandoned having produced only 187 MMcf of dry gas from 13 wells out of a predicted recovery of 1 bcf. What happened to the other 800 MMcf? Is the issue with the rocks, the size of the resource or the thermal maturity?

The Conasauga is a weak rock unit that contains the basal Appalachian Thrust detachment. It has been tectonically thickened as a result of this deformation into what is identified as the “Mushwad”, producing the multiple cycles of fracturing and cementation. Subsequent erosion has brought the Conasauga to the surface in the area of the field.

Conasauga cuttings are low in organic richness. However, thin zones were identified in core samples containing marginal-to-good organic richness. The Conasauga rocks and gases have equivalent thermal maturity within the dry-gas generation window, consistent with local generation of the hydrocarbons. Yield calculations suggest that thin moderately rich intervals were sufficient to charge the field. The kerogen porosity would be high; but total porosity would be minimal due to the low organic richness. Production from individual wells had relatively high initial rates, followed by an exponential decline to low values. The low predicted porosity is consistent with the low residual production after the fracture gas is recovered.