Abstract: Test Site for Geopressured Geothermal Well--Frio Sandstone, Texas Gulf Coast
D. G. Bebout, R. G. Loucks, A. R. Gregory
A geopressured geothermal reservoir should have a sand volume of 3 cu mi (4.8 cu km)--50 sq mi (130 sq km) in area and 300 ft (90 m) thick--subsurface fluid temperature of 300°F (150°C), and permeability of at least 20 md. Data available suggest that the water will be saturated with methane. Regional and detailed studies of the Frio Formation along the Texas Gulf Coast have been completed by the Bureau of Economic Geology to develop such a prospect.
In Hidalgo, Kenedy, Nueces, Matagorda, Brazoria, and Galveston Counties, several large fairways have been identified which have sufficient sand thickness and fluid temperatures to be prospective. However, optimum permeability in sand bodies deeper than 12,000 ft (3,600 m) is present in the quartz-rich sands of the Brazoria-Galveston County Fairway. For this reason, more detailed work has centered on this area and the Austin Bayou prospect has been identified.
The Austin Bayou prospect has maximum dimensions of 10 by 25 mi (16 by 40 km) but, because of faulting, individual reservoirs will be less than 60 sq mi (90 sq km) in area. The 10 to 30 ft (3 to 10 m) thick sands cumulate to 1,200 ft (360 m) and permeabilities of 100 md are common. Subsurface fluid temperatures are as high as 320°F (160°C).
The reservoir sand in the Austin Bayou prospect is in the Anomalina bilateralis zone of the lower Frio where an early sand depocenter prograded unusually far gulfward and was deposited on the downdip side of large growth faults. The anomalous gulfward progradation of this lower sand relative to later Frio depocenters caused it to be buried deeper and high subsurface fluid temperatures resulted.
The Austin Bayou prospect meets all of the geologic requirements for a geopressured geothermal reservoir. Barring environmental and legal problems, it is anticipated that a test well will be drilled in this area to evaluate further this potential alternate source of energy.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC