Lithofacies, Geochemical Trends, and Reservoir Potental, Thirteen Finger Limestone, Hugoton Embayment, Kansas

Abstract

The Amoco Rebecca K. Bounds core from Greeley County, KS was described and sampled for various analyses including conodont biostratigraphy, total organic carbon (TOC), x-ray diffraction and x-ray fluorescence to determine reservoir depositional setting, reservoir potential and establish the chronostratigraphic and lithostratigraphic boundaries of the Atokan Stage in western Kansas. The base of the Thirteen Finger Limestone (Atokan/Morrowan boundary) is placed at a prominent exposure surface at 5017.5 feet. Immediately above this surface is a thin coal followed by marine shale and limestone. The top of the Thirteen Finger Limestone (Atokan/Desmoinesian boundary) is chosen as the exposure surface at 4939.7 feet. The 83 feet of Atokan strata contains limestones (70%) separated by fossiliferous dark gray shale (10%) or black shales (20%) with few macroinvertebrates. Seven prominent flooding surfaces and two hardgrounds help define Atokan high-frequency cycles that consist of dark shale (initial flooding) that transitions upward to shallower-water limestone. The dark gray-black uranium-rich shales are higher TOC (up to 9.7% in noncarbonaceous shale), have higher transition metal content and few normal marine invertebrates. The conodonts recovered from the Thirteen Finger Limestone are upper Atokan, but the size of core prevented sampling of the entire interval and confident determination of chronostratigraphy. Atokan limestones are dominantly dense wackestones and packstones with average matrix porosity and permeability values of 0.8% and 0.1 md, respectively. However, these limestones contain numerous open- and healed-vertical fractures that may provide the pore network necessary to produce oil and gas.