Paper No. 176-9
Presentation Time: 10:20 AM
WIDESPREAD DOLOMITIZATION BY EXTREMELY EVAPORATED SEAWATER ASSOCIATED WITH LATE PERMIAN HALITE DEPOSITION, PERMIAN BASIN, USA (Invited Presentation)
Geochemistry of existing formation waters in the Permian Basin indicate that waters associated with late Permian (Salado) halite deposition displaced most formation waters in the Permian Basin. High concentrations of dissolved magnesium result when seawater is evaporated to halite precipitation. The existing Permian Basin formation waters contain high calcium and low magnesium concentrations which is opposite of what is produced by evaporation of seawater indicating that the evaporated seawater responsible for halite deposition subsequently dolomitized large quantities of limestone. Salado halite is approximately 460 m thick in the Permian Basin, and that would have required the evaporation of more than 30,000 m of seawater across the entire basin. This amount of evaporated seawater could dolomitize an average of 260 m of limestone across the entire basin. Those waters would have descended and could have caused dolomitization in a number of areas including: (1) along fractures in the Capitan reef, forereef and slope as well as fractures and caverns in the Capitan backreef, (2) limestones deeper in the Paleozoic section, and (3) additional zones on pre-existing dolomite throughout the Permian section. Seawater evaporated through halite precipitation contains little calcium, but has substantial dissolved sulfate. As a result, calcium liberated during dolomitization could combine with the dissolved sulfate, and precipitate anhydrite which is common in the subsurface Capitan as well as many other intervals throughout the Permian Basin. Other possible diagenetic processes associated with these highly evaporitic waters are the formation of magnesite in the Tansill (observed in the Gulf PDB-04 well), and hydrothermal dolomitization when these descending waters were heated and then ascended.