In a five-county area of South Texas, geopressured-geothermal reservoirs in the upper Wilcox Group are colocated with heavy-oil reservoirs in the overlying Jackson Group. In 1990, research at the Bureau of Economic Geology concentrated on evaluating the potential of using geopressured-geothermal water for hot-water flooding of heavy-oil reservoirs. Favorable geothermal reservoirs are defined by thick deltaic sandstones and growth-fault-bounded compartments. Potential geothermal reservoirs are present at a depth of 11,000 ft (3,350 m) to 15,000 ft (4,570 m) and contain water at temperatures of 350°F (177°C) to 383°F (195°C) in Fandango field, Zapata County. One potential geothermal reservoir sandstone in the upper Wilcox (R sandstone) is composed of a continuous sand body 100 ft (30 m) to greater than 200 ft (>61 m) thick. Fault blocks average 2 to 4 mi^2 (5.2 to 10.4 km^2) in area.
Both heavy-oil (average API=19) and light-oil (average API=26) reservoirs in South Texas are present in sandstones of the Jackson Group Mirando trend. The updip pinch-out of strike-oriented sheet sandstones in the Jackson Group largely controls the distribution of Mirando-trend heavy-oil reservoirs. The lateral continuity of heavy-oil reservoirs minimizes reservoir compartmentalization, which could disrupt injected-fluid flow paths.
Geologic and engineering research that still needs to be conducted includes (1) studies of the chemical compatibility between injected geothermal fluids and clay matrix of heavy-oil reservoirs, (2) detailed field studies of geometry and size of geothermal reservoirs, (3) detailed field studies of geometry and size of heavy-oil reservoirs, and (4) studies of changes in the temperature and chemistry of geothermal fluids when injected into heavy-oil reservoirs.Bureau of Economic Geolog
