Characteristics of the Upper Cretaceous Baxter Shale in the Vermillion Basin, Northwestern Colorado

Kaiser, Kimberley¹, Erika Davis¹, Richard Newhart², Mark Longman³, and Randy Koepsell^4
1Questar Exploration and Production, Denver, CO
²Questar Exploration and Production Co, Salt Lake City
³Questar, Denver, CO
⁴Schlumberger, Greenwood Village, CO

     The overpressured Coniacian to Campanian-aged Baxter Shale overlies the Frontier Sandstone and is about 2500 ft thick. This darkgray shale forms the reservoir in a developing shale gas play in the Vermillion Basin of northwest Colorado and southwest Wyoming. Cores were cut in the Baxter Shale in the Hiawatha Deep #5 well at depths of 12,343 to 12,390 ft and 12,951 to 13,005 ft. The cores reveal lithologies ranging from relatively pure, laminated shales to thin (<1 inch), planar to rippled, interbedded very fine sandstones, siltstones, and shales interpreted as hyperpycnal deposits. The laminated shales contain 0.5 to 2% total organic carbon. Burrowing is rare or absent suggesting that most deposition occurred under anoxic conditions on a relatively deep sea floor.
     The shale-rich and siltstone-rich intervals both contain quartz (28 to 45%), calcite (6 to 28%), dolomite (4 to 18%), plagioclase (4 to 13%), authigenic pyrite (0.5 to 3.5%), and traces of other minerals. Clay content ranges from 12.8 to 37%, dominated by illite (5-23%) and chlorite (6 to 12%).
     Core plugs from shales and siltstones have from 3.6 to 6% measured porosity and crushed-rock pressure-decay permeabilities of 52 to 115 nanodarcies. With such low permeability, natural fractures may play an important role in production. The natural fractures present in the core are of four types: 1) single-stage calcite-filled vertical; 2) single-stage oblique calcite-filled; 3) oblique partially open; and 4) multi-stage (or reactivated) oblique calcite-filled. Healed natural fractures reveal a distinctive “electrical halo” response on the image log that can be tied to healed fractures seen in the core.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah