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Geology of Salt

Salt is deposited as horizontal beds but, in some settings, deformation forms salt diapirs (Jackson, 1997). In Texas, the two main types of salt available to host caverns are piercement domes of the Gulf Coast and East Texas Basin and bedded salt in the Permian Basin of the Texas Panhandle. In this study, I describe the characteristics of bedded salt in the Midland Basin, one of the sub-basins of the Permian Basin (Index map of the Permian Basin, Texas-New Mexico, 99k).

The characteristics of bedded salt and domal salt are quite different. Typical Texas domal salt in the East Texas and Gulf Coast basins is derived from the Jurassic Louann Salt and is relatively pure and homogeneous. The lateral extent of domes is limited, however, and therefore the dome margins delimit the area useful for cavern development. Domal salt that has flowed upward to the surface has been dissolved where it is in contact with fresh water. Concentration of the impurities in salt produces cap rock at the top and, in some locations, sides of the domes. Cap rock may have low permeability and armor the dome against dissolution or, it may be permeable (Kreitler and Dutton, 1983). Structurally introduced anisotropy such as internal-boundary shear zones, foliation, bedding, mineralogy, moisture content, and grain-size variation may be features of concern in solution mining (Seni and others, 1995).

Bedded salt of the Permian Basin is much less pure than Texas dome salt. Permian salt is interbedded with limestone, dolomite, anhydrite, polyhalite (Na2MgK2(SO4) 4 · H2O), and fine-grained siliciclastic red beds (mudstone, siltstone, and sandstone). The distribution of these low-solubility impurities is one of the limitations of engineering solution-mined caverns, and characterizing impurities is one major focus of our study. Salt beds are typically continuous over large areas, so that experience with solution mining in one property may be a good indicator of what to expect at a nearby site. However, salt beds thin, pinch out, or change facies laterally into other rock types; in this study I document the various types of lateral changes in bedded salt. Permian salt, like domal salt, has been dissolved where it has been in contact with fresh water. In the Permian Basin, concentration of impurities does not form a cap rock but, rather, forms a heterogeneous and mechanically weak insoluble residue. In this paper, I describe the geometries and criteria for identifying salt thinning as a result of dissolution.

Purpose, Scope, and Methods of Our Study