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Purpose, Scope, and Methods
of Our Study


The purpose of this report is to present data specific to bedded salt that will be of interest to both industrial operators and to government regulators in the context of salt-cavern development. This information is intended to be both a regional description of bedded salt in the Midland Basin and a template for useful and geologically based description of salt in other basins worldwide. In particular, the objectives are to: (1) create and compile maps and cross sections documenting the regional extent, thickness, geometry, and quality of salt resources suitable for cavern development in the Midland Basin of Texas; and (2) identify some of the geologic factors and outline the methods for assessing variables that make specific sites more or less suitable for cavern development. To meet the second objective, I present conceptual models and interpretations that support and explain the descriptive data.

Some potential applications from this data set are to: (1) provide basic descriptive information such as stratigraphic nomenclature and log characteristics for describing existing or newly developed facilities; (2) match areas where storage or disposal facilities are needed with areas of salt of optimal characteristics in terms of thickness, depth, purity, and stability; (3) to provide context for comparing the history and performance of one solution-mined cavern with another; and (4) provide criteria useful for detailed site characterization of existing or newly developed facilities.

The data presented here builds upon a previous study (Hovorka, 1997) of gross salt thickness in the Midland Basin. The maps presented in this report supersede the reconnaissance results of that study. High-quality well location, increased well density, improved log interpretation, and integration with previous salt dissolution and hydrologic studies are the principal areas of improvement upon the previous study.

Methods

Map and cross-section compilation through the bedded salt section in the Midland Basin included a 31-county area (well and cross-section locations). Basic materials used in this study are 558 photocopied wireline logs from the Bureau of Economic Geology historic log library (appendix 1, downloadable PDF file). This data set was selected because (1) older logs more commonly include curves from the salt section, compared with modern log suites, that focus more on the subsalt-producing intervals, (2) it includes many wildcat wells and wells from productive fields and, therefore, provides regional coverage, and (3) it is available at no cost. Previous experience suggested that the most useful logs for West Texas bedded-salt mapping are gamma-ray, caliper, sonic combinations. If these log types were not available in the log files, neutron or resistivity logs were used. SP logs are of minimal use in salt. The log data base assembled is not exhaustive; thousands more logs through the salt interval are commercially available but were not incorporated because of the regional scope of the study. Denser well data were collected in areas where reconnaissance investigation (Hovorka, 1997) showed complex geometry.

We purchased API numbers from Petroleum Information/Dwights and georeferenced latitude/longitude locations from Tobin Data Graphics to improve well-spotting accuracy and to register the data on a 1:24,000-scale georeferenced U.S. Geological Survey (USGS) county base using ArcInfo1 Geographic Information System (GIS). The 90 wells for which the API number search was unsuccessful were located on a blueprint survey base (Midland Map Company, 1995) using survey information from the log header. The datum elevations (kelly bushing or equivalent) were extracted from the log header or from a 1:250,000-scale USGS topographic map. Well location and elevation data were checked by comparing the elevation of the top of the Yates to a published regional structure map (Geomap, 1986), and logs with erroneous header data were corrected or discarded. Stratigraphic units were marked on log photocopies and the datum and unit tops were entered into a spreadsheet (appendix 2, downloadable PDF file) and used to calculate unit thickness and structural elevation. These data were plotted on maps using ArcView GIS. Hand contouring was used to optimize interpretation of the regional data, using the published Yates structure map (Geomap, 1986), USGS 1:250,000-scale topographic maps, and surface geology (Barnes, 1992), in coordination with conceptual models to guide interpolation.

To supplement interpretation of this data, I have drawn on previous published and unpublished investigations elsewhere in the Permian Basin (index map of the Permian Basin, 99k). Salt cores collected by the U.S. Department of Energy (DOE) investigations of bedded salt in the Palo Duro Basin (Hovorka, 1994), cores collected by U.S. Army Corps of Engineers in an area of salt dissolution in the Hollis Basin (Hovorka and Granger, 1988), and the Gulf Research PDB-03 core from Loving County, Texas (Hovorka, 1989; 1990) are outside the Midland Basin study area but provide background information used to interpret the log response and geometric relationships seen in the Midland Basin. These cores are stored at the University of Texas Bureau of Economic Geology Core Research Center. Descriptions of salt geometry in the Delaware Basin used for this study include Adams (1944), Bachman (1984), Anderson and others (1972), and Snider (1966).

Areas in the Midland Basin were selected for case studies to document salt characteristics and hydrologic processes that are thought to affect the suitability of salt for hosting caverns, and detailed cross sections were prepared across these areas. We used a literature search to find information documenting the hydrologic setting and to identify areas of brine discharge.


Previous Work: Geologic Setting of the Bedded Salt in the Permian Basin