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Chemostratigraphy and Magnetic Susceptibility Characterization of Rock Cuttings from a Well With Oil-Bearing Intervals: Upper Pennsylvanian Series, North Central Kansas


Samples of rock cuttings from an oil well drilled into the Upper Pennsylvanian Series in north central Kansas were analyzed to investigate metal concentrations and magnetic susceptibility variations. The rock samples were collected from a well drilled through reservoir intervals that produced heavy degraded oil as well as reservoir intervals that did not produce oil. Analyses were made from representative powdered samples of the rock cuttings obtained every 10 feet between 3150 feet and 3900 feet below the surface. The objective was to (1) characterize the chemostratigraphic variations in metal composition of the sampled interval (2) compare and correlate the metal composition to petrophysical properties analyzed from well logs and (3) compare magnetic susceptibility and Fe content from the oil-bearing and non-oil bearing intervals. The goal was to describe the chemostratigraphic succession of the study interval and to determine whether the degraded oil intervals correlate to elevated magnetic susceptibility and high Fe content. The rock cuttings were finely crushed and the powdered samples analyzed for high frequency magnetic susceptibility values using a laboratory Bartington MS2 susceptibility meter. Portions of the powdered samples were acid digested and analyzed for major and trace metals using an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The major metals detected during analysis were Al, Ca, Fe, K, Mg, Na, and others occurred as trace amounts. Results show that Ca is by far the most abundant element analyzed, representing up to 35% by weight in some samples. Na has the lowest composition with less than 1.0% weight in all samples. High Fe content correlate closely with elevated magnetic susceptibility values in the intervals that produced heavy degraded oil. High natural gamma ray values slightly correlate with high potassium content. Elevated magnetic susceptibility correlating with high Fe content in the oil bearing interval suggest that some iron minerals in the rocks may have formed during the metabolism of hydrocarbons by microbes, thus contributing to the heavy degraded oil in these intervals. Magnetic susceptibility associated with oil degradation has potential as a well site tool for the qualitative assessment of reservoir oil.