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