Alternative Hypothesis for Development of Vugular Porosity in Mission Canyon Formation

Mark R. Luther

Several writers have discussed the development of vugular porosity in carbonates of the Mission Canyon Formation along the eastern margin of the Williston basin. Suggestions have included dissolution of carbonates by fresh water in the mesogenetic and/or telogenetic zones, or dissolution by meteoric waters in a vadose setting. An alternate model in which dissolution occurred in an eogenetic, subaqueous, marine setting is proposed here.

Apparently, the most pervasive vugular or enlarged-fenestral porosity is located near the contact between limestone and penecontemporaneously deposited gypsum bodies. During deposition, gypsum (now anhydrite) precipitation occurred in less oxygenated water than that from which the limestone formed. Microbial processes within a less oxygenated, gypsum-precipitating area can reduce sulfate ions to sulfides. This process, in the study area, produced sulfide-rich brines that flowed seaward into the more oxygenated, limestone-forming areas. The introduction of sulfides into a more oxygenated environment may produce an acid corrosive to limestone by the reaction HS^- + 20₂ ^rarr SO_4^2- + H⁺. Dissolution of limestone by this acid probably ook place near the sediment-water interface and to a shallow depth in the underlying sediments. Vugs formed by this process would be morphologically similar to those formed by vadose processes.

Implications of this model to hydrocarbon exploration include the possibility of extensive dissolution porosity occurring near depositional-topographic lows and near limestone-anhydrite contacts. The occurrence of stacked porosity zones can also be explained by this model.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988