Characterization of Bedded Salt for Storage Caverns - A Case Study from the Midland Basin, Texas

The geometry of Permian bedded salt in the Midland Basin is a product of the interaction between depositional facies and postdepositional modification by salt dissolution. Interpretation of high-frequency cycle patterns in cross section and map view inferred from wireline logs documents the salt geometry. Geologically based interpretation of depositional and dissolution processes provides a powerful tool for mapping the geometry of salt to assess the suitability of sites for development of solution-mined storage caverns. In addition, this process-based description of salt geometry complements existing data about the evolution of one of the best-known sedimentary basins in the world and can serve as a genetic model to assist in interpreting other salt basins. Solution-mined caverns in salt in the Midland Basin Salado Formation are low-cost, large-volume storage facilities used for chemical feedstock. Caverns are also created when salt is dissolved to produce sodium chloride brine for drilling mud and other applications. Recently, solution-mined caverns have been used for disposal of oil-field wastes. This log-based regional analysis of salt character provides basic descriptive information on the geometry of salt needed to site and regulate the development, use, and decommissioning of these facilities in the context of an exploration of facies relationships and implications for depositional history in this part of the Permian Basin. Three regional trends are recognized in the Permian Salado Formation of the Midland Basin: (1) The thickest (700 ft of net salt) and most extensive salt units in the lower part of the Salado Formation show a strong and consistent regional thickening toward the southwest across the Central Basin Platform toward the Delaware Basin, reflecting accommodation resulting from subsidence in the Delaware Basin depocenter. Typical salt beds (10 to 30 ft thick) are interbedded with mudstone and mud-salt beds (3 to 10 ft thick). Anhydrite beds (2 to 30 ft thick) separate the salt-mudstone units into 5 to 10 intervals (50 to 350 ft thick). (2) Toward the upper part of the Salado Formation, salt beds are variable in thickness and discontinuous in lateral extent, and they pinch out toward the Midland Basin margins. This geometry is interpreted as the result of complex depositional patterns that developed toward the end of basin filling as well as salt dissolution beneath the sequence boundaries preceding and following Alibates deposition. (3) Salt beds have been thinned as a result of ongoing postdepositional dissolution toward the east structural margin of the Midland Basin where salt is near the surface. Focused salt dissolution is noted in four areas: (1) over the structural high at the south end of the Central Basin Platform, (2) on the south side of the Howard-Glasscock high, (3) from both the top and the bottom of the Salado salt over the Capitan Reef rimming the Delaware Basin, and (4) over the southernCentral Basin Platform structural high in the modern Pecos valley. This fourth relationship is attributed to localization of the drainage in depressions caused by enhanced salt dissolution where salt is at shallow depth.