6 research outputs found
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Prospectus for a Design Well in the Blessing Area Matagorda County, Texas
In recent years, the Bureau of Economic Geology has conducted regional subsurface studies of the Wilcox Group and Frio Formation of Texas as part of the U. S. Department of Energy's assessment of deep geopressured geothermal resources along the Gulf Coast. These studies resulted in two reports (Bebout and others, 1978; 1979) that describe several areas in Texas where temperatures are greater than 300°F and where the geology and reservoir conditions are suitable for resource testing by a design well.
Throughout the Texas Coastal Plain, the 300°F isotherm generally occurs at depths ranging from 12,000 to 16,000 ft. The overlying geopressured sediments represent a substantial portion of the sedimentary column that contains significant quantities of entrained methane, making them a significant portion of the resource base (Gregory and others, 1980). The lower temperatures and pressures of these shallow geopressured sediments result in lower methane solubility, but drilling costs would be substantially lower, and perhaps reservoir quality would be better in comparison to the deep geopressured intervals. Although the latter condition has not been substantiated, several areas that are geologically favorable for testing shallow geopressured aquifers with temperatures less than 300°F were identified in a recently completed study (Weise and others, 1980) funded by the Gas Research Institute.
The Blessing Prospect (fig. 1), one of the shallow prospects in Matagorda County, Texas, is presented in this prospectus as a candidate for the DOE design well program. The prospectus focuses on the geological and engineering aspects of the test site. Although legal and environmental considerations are mentioned, they have not been studied in detail and additional work would necessarily follow if the prospect is approved for drilling and testing. Likewise, a drilling program and an economic analysis would be necessary before final approval of a design well.Bureau of Economic Geolog
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Wave-dominated deltaic systems of the Upper Cretaceous San Miguel Formation, Maverick Basin, south Texas
textSandstone units of the Upper Cretaceous San Miguel Formation in South Texas are wave-dominated deltaic sequences deposited during a major marine transgression. San Miguel sediments were deposited in the Maverick Basin within the Rio Grande Embayment. Cross sections and sandstone maps reveal that during deposition of the San Miguel Formation, the Maverick Basin consisted of two subbasins. A western subbasin received sediments from the northwest; the eastern subbasin received sediments from the north. Net-sandstone patterns show that the thickest parts of the sandstone bodies are generally strike oriented; where not eroded, updip sand-feeder systems are indicated by dip-aligned components. The San Miguel deltas vary considerably in morphology and make up a spectrum of wave-dominated delta types. Modern analogs of these San Miguel deltas include the Rhone, Nile, Sao Francisco, Brazos, Danube, Kelantan, and Grijalva deltas. Final sandstone geometries depended on three primary factors: (1) rate of sediment input, (2) wave energy, and (3) rate of sea-level change. Delta morphology was determined by all three factors, but the degree of reworking of deltaic sediments after delta abandonment was determined by wave energy and rate of transgression. The most common vertical sequences in the San Miguel coarsen upward from silt and clay to fine sand. Burrows are the dominant structures. The few primary structures are of small scale; large-scale cross beds are observed only in outcrop. Strandplain or barrier-island facies sequences, which prevail in most wave-dominated deltaic deposits, are incomplete in the San Miguel. In most places, only the lower shoreface is preserved. The upper parts of the sequences, which normally bear large-scale primary structures, were lost by marine reworking during subsequent transgressions. Intense burrowing destroyed any primary structures at the tops of the truncated sequences. Most of the San Miguel sandstones are arkoses. Cements include sparry and poikilotopic calcite, quartz overgrowths, feldspar overgrowths, illite rims, and kaolinite. The primary destroyers of porosity are the two types of calcite cement, which tend to completely cement the coarsest, best sorted, and originally most porous zones of the San Miguel vertical sequences. Zones of secondary porosity resulted from leaching of shell material, calcite cement, and feldspars. Laterally, the zones of either high secondary porosity or calcite cementation are unpredictable.Geological Science
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Padre Island National Seashore : a guide to the geology, natural environments, and history of a Texas barrier island
Tx Doc no. : Z, UA220.7, G941, no.17UT Librarie
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Geopressured Geothermal Prospects and Test Well Sites Wilcox Group and Frio Formation, Texas Gulf Coast
The four geopressured geothermal prospects and test-well sites described in this report (fig. 1 and table 1) are believed to represent the most favorable locations for testing the resource along the Texas Gulf Coast. The regional and site-specific studies used in selecting the Frio prospects have been published by the Bureau of Economic Geology (Report of Investigations 91); the supporting studies for the Wilcox prospects will be published by mid-1979. Detailed stratigraphic and structural cross sections and net-sandstone and structure maps have been prepared for the fairways in which these prospects are located. It is anticipated that other prospects will be identified in these and other fairways during the course of further study and will be reported on later.
It should be emphasized that these prospects and sites have been chosen on the basis of geology alone and that equally important environmental and legal (surface and mineral rights) aspects have not been considered. The total-resource values shown in the last two columns of Table 1 are very general estimates and are intended only to project an impression as to the magnitude of the resource in these local areas.
A significant departure has been made in the manner in which the top of geopressure is picked in this report. In previous Bureau of Economic Geology reports, the top of geopressure is defined as the point at which the pressure gradient exceeds 0.7 psi/ft; the pressure gradient was determined primarily by plotting drilling-mud weights, a method recognized as highly tentative. For this report, the top of geopressure is picked at the depth at which the plot of the shale resistivity from the electrical log departs from the normal compaction curve. This method is believed to be considerably more reliable.Bureau of Economic Geolog
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Geologic Studies of Geopressured and Hydropressured Zones in Texas: Test-Well Site Selection
The primary objective achieved in this project was to identify sites for test wells capable of long-term production of methane-bearing water from the shallow geopressured and deep hydropressured zones. The process of test-well site selection involved several steps, each contributing to the basic knowledge of shallow geopressured and deep hydropressured aquifers.
First, zones within the geopressured and deep hydropressured section of the Texas Gulf Coast Tertiary were defined based on pressure gradients and temperatures. Next, high-sandstone corridors, corresponding to the trends of the Wilcox Group and Frio Formation, were identified for each of these zones. Five fairways, or areas of greatest net-sandstone thickness, were located within the corridors. Areas most prospective for testing entrained methane resources in the shallow geopressured and deep hydropressured zones were identified in each fairway. Finally, test sites were selected in four of the prospect areas: the Blessing Prospect in Matagorda County, the Nueces Bay and Corpus Channel Prospects in San Patricio and Nueces Counties, and the Sarita Prospect in Kleberg County.
Knowledge gained from these geologic studies and subsequent testing will be significant in (1) evaluating the technical and economic feasibility of producing solution gas from the shallow geopressured and deep hydropressured zones, and (2) comparing these zones with deeper, hotter geopressured zones (studied previously in DOE-funded projects) as sources of entrained methane.Bureau of Economic Geolog