--> Abstract: Lithology and Characteristics of the Upper Cretaceous Baxter Shale, Vermillion Basin, Northwest Colorado, by Longman, Mark W., Richard Newhart, and Scott Goodwin; #90071 (2007)

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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