Lithologies and Petrophysical Characteristics of the Upper Cretaceous Baxter Shale Gas Reservoir, Canyon Creek Field, Sweetwater County, Wyoming

Abstract

Abstract The Baxter Shale, stratigraphically equivalent to the Cody, Steele, and Mancos shales of Wyoming, was deposited in the Western Interior Seaway about 90 to 85 million years ago. In Canyon Creek Field, it consists of about 2700 ft of dominantly carbonate-rich, siliceous and illitic marine siltstones and shales. Siltstone beds range in thickness from a single layer of grains to ~3 cm and are separated by shalier beds in coarsening-upward packages that average about 200 ft thick in the upper part of the Baxter and are correlative for miles. Bioturbation ranges from non-existent to minor in most beds. The thinnest planar siltstone beds represent dominantly hypopycnal flow or eolian dust whereas the thicker beds contain ripple laminae and were deposited by bottom currents including hyperpycnal flows. In Canyon Creek Field, the total organic carbon content ranges from 1.5 to 2.3 wt% in the thermally mature (>1.4% Ro) shales and from 0.25 to 0.75 wt% in the siltstones. Measured porosities in both the shales and siltstones are typically 4.5 to 7.5% with low matrix permeabilities ranging from about 80 to 700 nanodarcies. Lying between the sandstones of the Frontier and Blair, the Baxter Shale appears to be gas-charged throughout its thickness in Canyon Creek Field. The entire Baxter interval is highly overpressured (0.72 to 0.88 psi/ft) and exhibits fair to excellent shows of gas during drilling against mud weights of 13 to 17 pounds per gallon. Multi-stage hydraulic fracture stimulation of this interval along with the underlying Dakota and Frontier sandstones allows vertical wells to be completed with IPs of 1 to 5 MMCFGPD. The best production generally comes from the silt-rich intervals as determined with production logs, but the extremely thin-bedded character of the siltstones and shale complicates petrophysical interpretation and quantification of gas in place. Natural fractures in the Baxter are mostly cemented with calcite and quartz, but localized open fractures may enhance production. Despite the significant overpressure and encouraging shows of gas, Baxter drilling results in four horizontal wells located in and around Canyon Creek Field during 2007 and 2008 have been disappointing with cumulative production of less than 0.5 BCF/well over about 8 years. This is even less than several of the vertical wells in the field. Optimization of horizontal wellbore orientation, changes in drilling strategy and mud weights, along with improved fracture stimulation techniques are being evaluated with the goal of turning the Baxter resource in Canyon Creek Field into an economic play that may potentially extend across several townships. There is no doubt that the Baxter contains significant amounts of gas but a better strategy for production must be developed to tap this large resource.