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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received December 29, 1997;
revised manuscript received December 9, 1998; final acceptance January
10, 1999.
2Department of Geology and Geophysics,
University of Calgary, Calgary, Alberta, Canada T2N 1N4; e-mail: [email protected]
ABSTRACT
In the Medicine River field, oil is trapped in
discrete pools close to the unconformity edge. Reservoir facies comprise
three lithologies. Medium-crystalline dolostone with intercrystalline and
vuggy porosity forms the reservoir in an elongate dolostone body that has
replaced grainstone. Fine-crystalline dolostone is facies selective within
a lime-mudstone unit in which the common reservoir rock is bioturbated
dolomudstone with intercrystalline porosity. Grainstones have secondary
porosity formed though leaching of microcrystalline calcite allochems and
intergranular cements.
Reservoir quality is assessed from consideration
of orthogonal permeability values (Kmax = maximum horizontal
permeability, K90 =horizontal permeability at 90°
to the maximum, and Kv = vertical permeability) obtained from
conventional whole-core analyses. Sedimentary lamination primarily affects
the Kv, and the presence of fine-grained lamination dictates
that Kv < K90 in laminated rocks. An empirical
relationship between the three orthogonal permeability values records the
presence of fractures is where Kmax > 3K90 and Kv
> K90. From this relationship, the fracture density in
all three lithologies appears to be similar. Low and variable porosity
and permeability values in grainstones indicate that the fracture system
is poorly connected to the matrix.
Despite leaching and fracturing at the unconformity
edge, reservoir distribution closely follows patterns of facies that were
susceptible to dolomitization. Future exploration potential lies in fine-crystalline
dolostone bodies that may form stratigraphic traps downdip from the unconformity
edge.
The Pekisko Formation in western Canada is a
third-order sequence comprised of an open-marine grainstone through restricted
carbonate mudstone succession. Truncation of the Pekisko along an unconformity
edge prior to the Jurassic and several periods of incision from Early Jurassic-Early
Cretaceous have formed an intricately sculptured subcrop belt.
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