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Case Study 1: Permian Facies Controls on the North Margin of the Midland Basin

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Detail of north–south cross-stratigraphic section in the deep part of the northern Midland Basin showing salt-character changes controlled by Permian facies change and base of sequence dissolution. Cross-section location is shown on study area index map.

Thinning is observed in the salt-bearing interval near the north edge of the Midland Basin. The structure on the Yates shows that the current structural margin of the Midland Basin is defined by the Matador Arch and Roosevelt positive. The following change in salt thickness and quality are noted along a dip section on this Permian structure. Between Terry well 16 and Hockley well 8, the salt section below the top of salt and above the Tansill siliciclastics thins from 650 to 320 ft. Most of this thinning occurs gradually, with each individual bed decreasing in thickness by about one half. For example, anhydrite bed 20 decreases from about 8 to about 2 ft thick, and the overlying halite decreases from 150 to 100 ft thick. Anhydrite bed 30 and several thin polyhalite beds pinch out or decrease to a thickness that does not produce a recognizable signature on logs. Above anhydrite bed 50 the thickness changes follow a different pattern. The upper 150 ft of the salt section in Terry County, containing four anhydrite beds and two mudstone intervals, thins to 40 ft of mudstone with one recognizable anhydrite bed at the north edge of Hockley County. The halite beds pinch out sequentially into mudstone to the north, so that the top of the halite climbs up the stratigraphic section toward the south. Anhydrite beds extend further to the north than the halite, but they also pinch out. The two Alibates anhydrite beds can be traced across the area with little change in thickness.

Thickness changes below bed 50 are interpreted as the result of depositional effects related to slower Permian subsidence, and, therefore, creation of less accommodation toward the depositional basin margin. Decreased accommodation did not result in formation of more mudstone-halite, indicating that variation in the depositional environment was subtle. In fact, salt quality in the area of less accommodation may be superior for some salt-cavern designs because anhydrite beds are thinner and less abundant in the area of thinner salt section.

Thickness changes observed above bed 50 could be interpreted several ways: (1) as the result of salt nondeposition, (2) as base-of-cycle dissolution, or (3) as regional dissolution. Current depth of salt >1,900 ft below surface suggests that modern dissolution is not a likely process. Observed map distribution of the salt beds corresponds closely to Midland Basin structure. I tentatively propose that the observed thickness changes correspond to a change in depositional style during the final stages of Salado deposition in which salt deposition was focused in the topographically low areas in the basin center. Evidence to support this is the unusually clean profile (low gamma-ray-log profile) of these upper salt beds, which suggests a change to rapid episodic salt deposition in isolated depocenters. Additional fabric and geochemical evidence is needed to support this interpretation. Any thin salt beds deposited toward the basin margin could then have been removed by base-of-cycle dissolution, or by dissolution under burial conditions prior to Alibates deposition, at the end of the Permian, or during the Mesozoic.

Case Study 2: Post-Permian Dissolution at a Structural Positive on the Eastern Basin Margin