Chemostratigraphy Of The Austin Chalk And Upper Eagle Ford Shale

Abstract

A single drill core from La Salle, TX was analyzed for its chemical composition and percent concentration of both major and trace elements in order to understand depositional environments and local tectonic activity of the Balcones Fault Zone. The drill core contained the entirety of the Austin Chalk and the upper portion of the Upper Eagle Ford Shale. Samples were taken from 47 boxes at 3 inch intervals which accumulated in a total of 1,680 samples. Each sample was analyzed using a handheld x-ray fluorescence instrument which provided quantitative analysis of the following elements: Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Mo, Ni, P, S, Si, Ti, and V. Furthermore, x-ray diffraction analysis was conducted at every 7 feet of the Austin Chalk and every 4 feet of the Upper Eagle Ford Shale. Calcium values ranged from 5% to 40% with the former values arising from the Upper Eagle Ford Shale and the latter values coming from the Austin Chalk. Sulfur percent composition ranged from 0%-20% and had similar trends as Magnesium which had values from 0.04% to 4.8%. The Austin Chalk contained great values of pyrite, illite, quartz, calcite, and dolomite. Trace element concentrations suggest that the depositional environment had periods of anoxic or euxinic events. Mineralogical and major elemental geochemistry suggests a carbonate rich Austin Chalk and calcareous Eagle Ford Shale.