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Methods for Site Evaluation

Areas where changes in salt-thickness, salt-quality, or salt-dissolution trends are noted or potentially exist.

Geologic data can be applied to engineering needs, risk reduction, and assessing the future stability of the salt during site evaluation for solution-mined caverns. Geologic data include salt-bed thickness, salt quality, the type of salt dissolution, and the distribution of associated non-salt beds that may be of interest as horizons in which to set seals or as potential permeable beds to be avoided.

Regional trends and facies relationships are the basic tools to assess salt-bed thickness and quality. Facies models of the Permian depositional environment (Fracasso and Hovorka, 1986; Hovorka, 1994) suggest that salt beds have high continuity over the region. Mapping high-frequency cycles over the Midland Basin study area supports this model and provides confidence that experiences with salt quality in one part of the Midland Basin are likely to be reproduced in other areas. Measurement of individual salt-bed and interbedded non-salt units shows horizontal continuity of strata over wide areas and relatively minor variation in maximum salt-bed thickness and impurity content. Average net salt and percent salt show gradual regional variations from >75 percent salt in updip areas, where net-salt thickness is <400 ft, to <70 percent salt in areas where net salt is >600 ft. Throughout the study area, salt is interbedded with non-salt. Mudstone interbeds more than a few feet thick occur at intervals of 10 to 30 ft. Anhydrite beds 2 to 30 ft thick occur regularly through the salt at spacing of 50 to 150 ft. Some of the thickest and most pure salt beds are found near the top of the Salado Formation along the Midland Basin axis. These units, however, show the most complex facies relationships of any unit examined in the study. The complexity observed at a regional scale suggests that there may be variation over short distances in the character and thickness of the upper salt units. If these beds are a significant component of the engineering design for the cavern, I suggest that site-specific data be acquired to address the heterogeneity of these units.

The observations made in this study support the validity of the common practice of assessing a solution-mined site based on examining logs of wells in the area. The exception to this rule is areas where complex facies variations are expected. In this study, most of the areas where complex facies variations are expected generally overlap areas where there is risk of salt dissolution described in the following paragraphs and shown in this figure. The east and north margins of the Midland Basin are areas of depositional salt thinning. Across the Central Basin Platform, facies changes to more abundant and thicker anhydrite beds are observed, and the effect of these relatively high-strength, low-solubility units on salt-cavern design should be assessed. The area of most abrupt lateral changes corresponds approximately to the structural platform edge.

Salt dissolution may create risk factors to be assessed in salt-cavern design for three reasons. (1) Dissolution can cause the salt to thin over a short distance laterally into water-bearing, mechanically weak insoluble residue. The geometry of the salt-dissolution edge may be complex and difficult to map because of hidden hydrologic controls and the potential of feedback mechanisms to focus dissolution where previous dissolution has created fractures and breccia. (2) Drilling and other invasive activities have the potential to create fractures and conduits that might focus future dissolution around the facility. Therefore, in an area of active dissolution, a thick, preserved salt section might have a risk of developing engineering problems. (3) In an area of salt dissolution, there is increased risk that some beds within the salt, particularly carbonates and sandstones, may have had halite cement dissolved and, as a result, allow leakage from the caverns. Overlying beds such as the Alibates, that are commonly used for setting casing and seals, may also be of variable quality in areas of salt dissolution because of fracture permeability and hydration of anhydrite to gypsum.

The reality of these risk factors has not been tested in this study. I show the areas of interpreted salt dissolution and recommend that the potential risks associated with past or ongoing salt dissolution be assessed for sites developed near those areas. Other factors that might create potential for dissolution are also shown. High elevation contrast may create hydrologic gradients and favor active dissolution. Areas of focused structural deformation having the potential to create fractures are also mapped, although they have no correspondence to thin salt at the regional scale mapped. Large saline lakes and Pleistocene lake deposits are also shown because of the unassessed potential risk that salt dissolution may have played a role in basin formation.