Elemental chemostratigraphy and reservoir properties of the Mowry Shale in the Bighorn and Powder River basins, Wyoming, USA

Abstract

The Mowry Shale is an organic-rich, siliceous shale that deposited in the shallow, epeiric Mowry Sea during the early stages of the Greenhorn Transgression. The proximal to intermediate Mowry Shale (Bighorn Basin) consists primarily of siltstone, sandstones, and mudstones. Storm processes likely had strong influence on the deposition and oxygenation of the Mowry Shale in the Bighorn Basin. Based on elemental geochemistry analysis, it is unlikely that the Mowry Shale in the Bighorn Basin was anoxic for extended lengths of time and never euxinic. The distal Mowry Shale in the Powder River Basin consists primarily of silt laminae and mudstones. Elemental geochemistry, coupled with stratigraphic analysis, indicates that even the most distal Mowry Shale was prone to reoxygenation and benthic activity for brief periods. The Mowry Shale likely fluctuated rapidly between times of suboxic and anoxia, with rare times of euxinia. Molybdenum and uranium enrichment crossplots indicate a time of varying levels of oxygenation and basin restriction. Molybdenum-TOC crossplots suggest that the Mowry Sea was a weakly, possibly moderately restricted basin. Understanding the diagenesis of the silica, organic matter, and clays is key to unlocking the Mowry Shale as an unconventional tight oil play. Data suggests that the water rich fluid saturations seen in the Mowry Shale are indicative of competition for pore space with water from dehydrated opal and smectite, as well as hydrocarbons from maturing kerogen. Additionally, the excess silica in the Mowry Shale is possibly preventing cation exchange during illite-smectite diagenesis, resulting in highly expandable layers, even at depth. It is also possible that the addition of silicon cations during this process is even further saturating the pores with silica, reducing porosity significantly. The hot, deep Powder River Basin offers opportunities for Mowry Shale development. Areas with Muddy Sandstone bottom-hole temperatures greater than 220 °F are suggested to be prospective in the Powder River Basin. Favorable areas for exploration are located in areas with average resistivity values greater than 8 ohm-m. This thesis provides a base level for understanding the stratigraphic framework and geochemistry and the compilation of a large dataset from multiple sources provides a stepping stone for further research.