ABSTRACT

A regional study of the Tuscaloosa Formation in Louisiana, undertaken to assess geopressured-geothermal potential, revealed lobate, downdip extensions of the hydropressured zone in lower Tuscaloosa massive sandstone facies below the regional top of geopressure. Normal pressure zones within geopressured section were identified by drilling mud weights less than 13 pounds per gallon on electric logs of massive lower Tuscaloosa sandstone, and cross sections demonstrated updip continuity of these zones with the regional hydropressured zone. These hydropressure "tongues" are permitted by the anomalously high permeabilities reported from the deep Tuscaloosa trend which have been attributed to both primary and secondary porosity. The hydropressure tongues correspond with lobes of thick net sandstone, principally in Pointe Coupee, East Feliciana, East Baton Rouge, and Livingston parishes in the central Tuscaloosa trend. Limited control suggests at least one hydropressure tonge in the Chandeleur Sound area to the east.

Dimensions of hydropressure tongues range up to 17 miles (27 kilometers) parallel to strike and 11 miles (17 kilometers) oblique to strike. In many places, tongues are terminated downdip by faults, which, by acting as pressure "seals," prevent the tongues from extending to the downdip edge of the massive sandstone in the expanded sections of the downthrown blocks. The areal extent of geopressured Tuscaloosa sandstone is controlled updip by these fault zones, and downdip by pinch-out of the sandstone units basinward. Local hydropressure tongues diminish the geopressured-geothermal potential of the Tuscaloosa trend, but show no discernible relation to gas-productive areas.