The Role of Genetic Stratigraphy in Reservoir
Characterization
of Mixed Siliciclastic/Carbonate Valley-Fill Reservoirs:
Example from the Pennsylvanian Wapanucka Limestone and Spiro Sandstone, Arkoma
Basin, Southeastern Oklahoma
HORN, BRIAN, W.
The Wapanucka Limestone and Spiro sandstone comprise a (3rd order) genetic
sequence bounded by deposits and surfaces that record marine deepening. This
final stage of passive-margin deposition contains two smaller scale (4th order)
mixed carbonate-siliciclastic cycles that record progradation (highstand system
tract), valley incisement (lowstand systems tract), valley filling and
transgression (transgressive systems tract). Reservoir
development within these
smaller scale cycles show systematic lateral and vertical variation and
compartmentalization that can be attributed to their stratigraphic position
within the larger stratigraphic cycle. Core analysis and regional mapping
demonstrates the presence of a 4th order regional erosion surface (sequence
boundary) which incised 10-60 feet into the underlying Wapanucka shelf carbonate
cycles. Progradation of the seaward-stepping clastic sub-Spiro shoreface (HST),
created laterally continuous and interconnected reservoirs that are locally
truncated by younger "Foster" paleovalleys with 60-140 feet of relief,
juxtaposing fluvial-estuarine sandstones and Wapanucka shelf
carbonates
. In each
of the systems tracts, these 4th order cycles show systematic variation in
reservoir
quality, thickness and compartmentalization that may be related to
their position within the larger genetic sequence. Shelf carbonate facies are
primarily a non-
reservoir
unit, whereas the regressive clastic shoreface cycles
are productive in the northern part of the basin. While both valley fills
contain
reservoir
quality sandstones, the older Wapanucka valley-fill sandstones
form isolated
reservoir
units in the northern part of the basin. Younger
"Foster" valley-fill reservoirs, found across the entire basin, are thicker,
contain little carbonate skeletal material and have larger areas of drainage.
Comparison of these two valley-fills show differences in
reservoir
quality,
architecture and compartmentalization that can be understood from the context of
the stratigraphic position within a stratigraphic hierarchy.