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Abstract: Waulsortian Mounds as Reservoir Rocks

AHR, WAYNE M., Texas A&M University, College Station, TX

Classical Waulsortian mounds are associated with Osagian time, but Waulsortian depositional attributes are not limited to that period. Early Mississippian, Waulsortian mounds in North America, the UK, and Belgium typically occur in deep water, subphotic environments that characterize the distal portions of carbonate ramps. Waulsortian mounds are typically associated with transgressive events, or TST's. Mounds may nucleate on paleostructural features or depositional irregularities, but mounds are not genetically-linked with faults, fractures, or diapirs. They are not genetically associated with methane seeps or submarine vents. Mounds in oil-producing zones at the Dickinson Area, Williston Basin and mounds that crop out at Swimming Woman Canyon, Montana exhibit normal carbon isotopic signatures as compared to "light" carbon isotopic values for known seep mounds. In general, all Waulsortian mounds are composed mainly of submarine cements, especially radiaxial calcite, and clotted lime mud, as compared to discrete micrograins of detrital micrite that may include current-aligned allochems. Cm scale vugs ("stromatactis cavities" that may contribute significantly to total reservoir porosity) are characteristic. They contain radiaxial calcite linings (commonly on fenestrate bryozoan sheets), geopetal peloids, cavity-dwelling microfossils that are absent from coeval, level-bottom beds, and internally-resedimented polymuds. Mound fossils are not chemoautotrophic, they are usually disarticulated, they are not oriented by currents, and they 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. Because Waulsortian mounds form in deeper marine environments, they may be buried without exposure to meteoric water. Instead, they commonly exhibit diagenetic attributes that reflect only marine and burial environments. Because Waulsortian and other Mississippian mounds are composed mainly of cementstone and mudstone with discrete stromatactoid vugs plus-or-minus porous, detrital beds as "pockets" in mounds, reservoir connectivity commonly depends on post-depositional porosity enhancement such as fracturing and diagenetic dissolution.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas