SEDIMENTOLOGIC EVALUATION OF THE BAXTER SHALE

The Baxter Shale is one of a myriad of Rocky Mountain mudstone systems that have been the interest of potential shale hydrocarbon development. This projects aims to integrate sedimentological, geochemical, and geomechanical data of different organic-rich Rocky Mountain shales (including cores from the Baxter, Mowry, Ismay, Phosphoria, Sussex, Carlisle, and Thermopolis Formations) in order to quantitatively establish the interdependence of these parameters in specific facies intervals and between different formations. Preliminary results from the Baxter shale—a Coniacian to mid-Campanian shale deposited in the Cretaceous Western Interior Seaway and located near the Rocksprings Uplift area of Wyoming—indicate the presence of five sedimentary facies: 1) dark, fine siltstone with a bioturbation index (BI) of 0 - 1; 2) dark to gray fine siltstone with a BI of 2; 3) gray fine siltstone with a BI of 2 - 3; 4) fine siltstone-dominated heterolith with a BI of 2-3; and 5) very fine sandstone-dominated heterolith with a BI of 3 - 4. This core has also been analyzed stratigraphically using X-Ray Fluorescence (XRF) and the resulting elemental compositions have been characterized as a function of sedimentary facies. We interpret the Baxter core to represent prodeltaic deposition, as evidenced by the strongly heterolithic character, unidirectional flow features such as current ripples, and overall low bioturbation index. Further support for this depositional environment is based on the interpretation of a low diversity, diminutive ichnofossil assemblage composed predominantly of planolites and little to no skeletal debris. This suggests a stressed, likely brackish environment with episodic riverine sediment and freshwater input. The facies scheme established in this core will be used to guide further geomechanical and geochemical testing in order to define a quantitative predictor of natural and hydraulic fracture morphology. Specifically, this project seeks to define sedimentary and/or geochemical properties that enhance fracture complexity in order to more sustainably develop hydrocarbon resources.