Analysis of the Potential for Carbon Sequestration in the Subsurface Tuscaloosa Group, Southwestern Alabama

Abstract

In this project, the sedimentology and stratigraphy of the Upper Cretaceous Tuscaloosa Group of the subsurface of an area in southwestern Alabama has been analyzed for carbon sequestration purposes. While collecting the samples for this study, it was noted that the sandstones that make up part of the Tuscaloosa Group were found to contain large amount of glauconites. Glauconitic sands, sometimes referred to as greensand formations, are associated with continental shelf sediments typically deposited during Cambrian, Cretaceous, and Cenozoic . Previous studies suggest that the Tuscaloosa’s sands, which include some glauconite intervals, may be adequate reservoirs for geologic CO2 storage. The Tuscaloosa Group has an overall subsurface thickness of 183 m -369 m (600 ft – 1213 ft) in southwestern Alabama, and an approximate porosity of 25%. An essential part of this study was to observe and explore how the injected CO2 fluid interacts with porous rocks. Samples from the drill core Julian F. McGowin No #1 were collected in order to observe lithological changes, sedimentary structures and vertical changes in grain size and mineralogy, as well as measuring thickness of porous units. Petrographic thin sections, as well as core observations, show evidence of glauconite throughout most of the core. XRD results show the mineralogical bulk composition of mostly quartz, berlinite, micas, K-feldspar, glauconite, chlorite, and illite. A design for a geochemical experiment was formulated using brine data collected by the Geological Survey of Alabama. Tuscaloosa brine is mostly composed of Na-Ca-Cl, with high concentrations of Mg. This part of the experiment took place inside a rocking autoclave apparatus at the University of Wyoming, and the software Geochemist's Workbench was used to create a geochemical model. An activity diagram was created to observe stability fields of Fe minerals in the Tuscaloosa Group. The model shows evidence of siderite precipitation under reducing conditions. Given its wide geographic distribution, substantial thickness, substantial porosity, permeability, the Tuscaloosa Group could potentially serve as a near-limitless reservoir for storing CO2. In support of this notion, geochemical results are positive in that they showed release of Fe2+, which suggests the onset of carbonate mineral precipitation.