ABSTRACT

Contoured hydraulic head cross sections constructed from well logs in the central Tuscaloosa gas trend of Louisiana provide information about fluid flow near the hydropressure-geopressure interface. Extensive head inversions correspond to long, dip-oriented convolutions of the interface. The inversions occur where permeable, hydropressured massive sandstone facies in the lower Tuscaloosa underlie geopressured Eagle Ford Shale updip of fault zones which preserve geopressures downdip. Thus, regional fluid pressure regimes in the Tuscaloosa and Eagle Ford are predominantly structurally controlled, with some lithofacies control updip.

Hydraulic head trends indicate an overall pattern of regional upward flow from depth, with highest hydraulic gradients corresponding to the top of the Austin Chalk in most places. This pattern is complicated by inversions, which are nearly horizontal and tend to show very high hydraulic gradients, and by a sharp, upward-protruding head peak which becomes nearly vertical along a trend above the Lower Cretaceous limestone shelf edge. This peak represents the escape of highly pressurized fluids from depth along a preferred path which may be fault controlled.

Highest hydraulic gradients occur locally and regionally where fluids flow from geopressured shale toward permeable, hydropressured sandstone. Salinities are also reported to show a regional increase toward hydropressured, sandy, updip facies of the Tuscaloosa. Thus, reverse osmotic effects are probably unimportant. The head peak above the Lower Cretaceous shelf edge, although characterized by relatively low hydraulic gradients, could be a locus of membrane filtration at the top of Tuscaloosa sandstone if bypassing of reverse osmosis by fluid escape along faults has been minimal. The role of permeable Tuscaloosa sandstone as a sink for fluids near the top of geopressure may have localized gas there, along the producing trend.