Print this pageMississippian "Reef" Reservoirs: Seep/Vent Related or Not?
AHR, WAYNE M., and ROBERT J. STANTON
Lower Mississippian ("Waulsortian") mounds, like those in the Williston Basin, occur on deeper portions of carbonate ramps. Mounds may nucleate on paleobathymetric highs, but they are not usually associated with faults, fractures, or diapirs. These mounds consist of clotted micrite and submarine cementstone, especially radiaxial calcite cements, all of which have "normal" carbon isotopic signatures. Cm-scale vugs ("stromatactis cavities" that may contribute significantly to reservoir porosity) are typical and contain radiaxial calcite linings (commonly on fenestrate bryozoan sheets), geopetal peloids, cavity-dwelling microfossils, and polymuds. Mound fossils exhibit high species richness and moderate dominance. Mound fossil assemblages are similar to those of the coeval level- bottom strata, but are better preserved and more diverse because taphonomy and habitat diversity on the mounds are different than on the level bottom. Waulsortian faunas are not chemoautotrophic. Seep mounds on the other hand, exhibit a distinctive chemoautotrophic biota (mainly molluscan), the mounds are associated with structures or vents rather than specific depositional settings, seep mounds have extremely light carbon isotopic signatures, and radiaxial fibrous calcites and stromatactis vugs are apparently absent. Seep mound faunas exhibit low species richness and high dominance, they are autochthonous with little indication of disturbance, shells may be articulated, and shell abrasion is not evident, although shell dissolution may be extreme because of the high CO[2] or H[2]S content in interstitial waters. Waulsortian-type mounds are associated with deep-ramp environments, with certain facies tracts in a sequence stratigraphic framework, and will exhibit a characteristic porosity system with distinctive radiaxaial calcite cements and vugs. Seep mounds are associated with fractures, diapirs, vents, and methanogenic zones rather than dip position or specific facies tracts on platforms, they contain a restricted biota, different cement characteristics, and a different type of porosity.