High Resolution Sedimentological and Trace Metal Analysis of Late Paleozoic and Cretaceous Mudstones: Preliminary Results From the Baxter Formation, Rock Spring Uplift, WY

Abstract

Cretaceous and late Paleozoic mudstone systems, both in the Rocky Mountain region and mid-continent, are a source of great interest due to their potential for shale hydrocarbon development. However, despite of the economic interest, paleodepositional conditions of heterogeneous mudstone systems remains poorly understood. This projects aims to integrate high resolution sedimentological, geochemical, paleoecological, and geomechanical data of different organic-rich shales in order to infer mudstone depositional environments, develop sequence stratigraphic interpretations, and gain insight into the geomechanical properties variations as a function of specific facies, depositional environments, and different sequence stratigraphic systems tracts. Cores targeted in this study include Baxter, Mancos, Mowry, Ismay, Phosphoria, Sussex, Carlisle, Thermopolis, and Cline (Upper and Lower) 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. 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. Furthermore, trace element concentrations obtained from Handheld X-Ray Fluorescence (HHXRF) corroborate the above interpretations. Finally, stratigraphic trace metal trends demarcate potential flooding surfaces within an overall aggradational to progradational systems tract (APPS). Further relationships between trace metal concentrations, total organic carbon, facies association, depositional environment, stratigraphy, and geomechanical behavior are explored using preliminary data from additional cores of other Cretaceous and late Paleozoic formations.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016