Lithology and Characteristics of the Upper Cretaceous Baxter Shale, Vermillion Basin, Northwest Colorado
Longman, Mark W.1, Richard Newhart2, and Scott Goodwin1
1Questar Exploration and Production Co, Denver, CO
2Questar
Exploration and Production Co, Salt Lake City
The Coniacean to Campanian Baxter Shale, a stratigraphic
equivalent of the Niobrara, Mancos, and Hilliard formations, was
deposited in hundreds of feet of water in the Western Interior Seaway
about 90 to 85 million years ago. It consists of about 2500 ft of
dominantly siliceous, illitic, and calcareous shales that contain
regionally correlative coarsening-upward sequences of quartz- and
carbonate-rich siltstones several tens of feet thick. Planktonic
foraminifers and chalk-rich copepod fecal pellets are locally common
in the shales between the siltstones.
Bioturbation ranges from non-existent to minor in most of the
Baxter, suggesting anoxic bottom conditions. The siltstones range in
thickness from a single layer of grains to ~3 cm. The thinnest planar
siltstone beds represent dominantly hypopycnal flow whereas thicker
beds contain ripple laminae and were deposited by bottom currents.
Total organic carbon content ranges from 0.5 to 2% in the shales and
from 0.25 to 0.75% in the siltstones. Measured porosities in both
shales and siltstones range from 3 to 6% with matrix permeabilities of
100 to 1500 nanodarcies.
In the Vermillion Basin at depths >10,000 ft, the Baxter Shale
has vitrinite reflectance values approaching 2.0% and is in the gas
generation window. The 2500 ft of shale between the Blair and
Frontier sandstones is overpressured with gradients ranging from 0.6
to 0.8 psi/ft. Fracture stimulation of the gas-bearing Baxter with up to
10 stages averaging about 300 ft thick allows vertical wells to be
completed with IPs of 1 to 3 MMCFGPD, but one recent well was
completed for >9 MMCFGPD. Production is commonly commingled
with that from the underlying Frontier and Dakota sandstones. Baxter
production comes mainly from silt-rich intervals and naturally
fractured shales after hydraulic fracture stimulation, although most of
the natural fractures are filled with calcite.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah