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Salt Quality and Net Salt

Salado net salt and percent salt. Direct measurements of cumulative salt-bed thickness from wells with caliper logs in useful log suites are posted. Percent salt is calculated on the interval from top upper Tansill (marker 15) to the top of salt. Other thickness values are based on regional percent-salt average and the top salt to top Tansill interval thickness where it could be determined.

Log quality was sufficient to directly measure the amount of Salado salt in 55 logs in the study area (appendix 3 PDF file), generally because caliper-log response made it possible to reproducibly separate anhydrite from clean salt. Uncertainties remain in distinguishing mudstone-halite mixtures from mudstone in wells where the borehole has been enlarged in both lithologies. Additional measurement uncertainty is introduced by imprecise bed-thickness estimates in typical finely interbedded lithologies. Comparison of measurements from adjacent logs suggests that error of about 5 to 10 percent in measuring cumulative salt thickness is expected. In addition to measured salt thickness, thinned intervals of high gamma-ray-log response were interpreted as beds from which halite has been dissolved and used for defining the limits of salt. From measured salt thickness, the percent of salt from the salt-bearing intervals was calculated, and results ranged from 53 to 84 percent. The salt-bearing interval selected for this calculation was a minimum, from top salt to top upper Tansill clastic. This removes the insoluble material in the Alibates and above-salt insoluble residue and variable amounts of anhydrite and siliciclastic beds in the Tansill from the calculation. Typical values of percent salt were contoured, with the lowest percent salt (<70) over the Central Basin Platform and the highest percent salt (>75) toward the north and east updip edges of the Midland Basin. Inspection of the North–South and East–West cross sections suggests that thicker and more abundant anhydrite beds are the reason for increased impurities on the Central Basin Platform; in updip areas, decreased anhydrite bed abundance and thickness is partly but not wholly offset by increased abundance of siliciclastic beds.

The generalized percent salt in the salt-bearing interval was then used to estimate the salt thickness in logs from which salt beds could not be directly measured. The thickness from top salt to top Tansill siliciclastics was multiplied by the decimal percent salt mapped for the area and the estimated salt thickness calculated. In some logs top salt or top Tansill was difficult to pick and no value was posted. Resistivity logs are particularly useful in defining this interval because the salt section has low permeability and, therefore, has high resistivity, in contrast to the conductive saline-water-bearing insoluble residue and Tansill siliciclastics. The Alibates-Salado-Tansill isopach was used to guide the contouring of the net salt, and a large contour interval was used because of the measurement uncertainties.

The net salt map, like the Alibates-Salado-Tansill isopach, shows thick salt in the Midland Basin center. Even though the percent salt decreases slightly over the Central Basin Platform, the net salt continues to increase because the Salado thickness increases toward the Delaware Basin. In the Delaware Basin, the base of salt stratigraphically equivalent to the Salado Formation of the Midland Basin was approximated using the top of MB134, as the base of the Salado shows a moderate thickness increase. Salado thickness in the Delaware Basin is the result of increased accommodation in a dominantly shallow-water environment in a subsiding basin.

Toward the east margin of the Midland Basin, the net salt decreases fairly abruptly between 200 and 0 ft of salt, and this is where the depositional trend toward decreased interval thickness is overprinted by cross-cutting near-surface salt dissolution. A large zero-salt area is mapped over the south end of the Central Basin Platform and a small area is mapped over the Howard-Glasscock High. The depression over the Capitan Reef contains thin salt where it was intersected by wells, so in this area salt has not been completely remove.

Outcrop Geology: Generalized Geologic Map
Depth to Salt: Alibates – Surface Isopach
Above Salt Structure: Structure on Top of the Alibates
Thickness of Salt-Bearing Interval: Ochoan Isopach
Below Salt Structure: Structure on Top of the Yates
Geologic Processes in Salt