Shallow-Seated Dissolution of Bedded Evaporites in Northern Delaware Basin: ABSTRACT

Studies of boreholes penetrating the Dewey Lake, Rustler, and uppermost Salado Formations in the northern Delaware basin (southeastern New Mexico) have investigated subsurface of bedded evaporites in the vicinity of Nash Draw, a depression 5 to 10 mi (8 to 16 km) wide and about 250 ft (75 m) deep. The thickness of the section between the top of the Salado Formation and the base of marker bed 103 ranges from an intact 210 ft (64 m, east of Nash Draw) to a residual 45 ft (14 m in the Draw), where gypsification of Rustler anhydrite and removal of Rustler halite are virtually complete. The uppermost Permian halite has been previously described as a zone (the brine aquifer). Within 130 ft (40 m) below this zone are halite-filled fractures, cubic-shape cavities, and gypsum after anhydrite. Above are remnant islets of halite and anhydrite, gypsum replacing anhydrite and polyhalite, and breccia. The mineralogy and stratigraphy suggest that the shallow-seated dissolution front is a series of fingers moving laterally along bedding planes, rather than a single surface migrating downward. The sequence of alterations appears to be: (1) fracture of brittle rock, (2) of halite adjacent to the fracture rock, (3) gypsification of interbedded polyhalite and then anhydrite, and (4) of gypsum. Waters of higher salinity and lower flow rate in the brine aquifer east of Nash Draw show an oxygen isotope enrichment with respect to meteoric waters, indicating that the low fluid-to-rock ratio there has thus far recluded significant alteration of rock by water. End_of_Article - Last_Page 764------------