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Geochemical Comparison of Oils From Upper Pennsylvanian Kansas Reservoirs, NW Kansas to Woodford Shale Source Rocks: A Case for Long Distance Migration

Abstract

Upper Pennsylvanian oils in western Kansas have been produced since the early 1900’s, but the primary source rock for these oils has not been definitively determined. In this study, oils from these reservoirs, as well as potential source rocks from the region and from the Anadarko Basin in Oklahoma, have been collected and characterized using various geochemical techniques to understand the characteristics of the oils and potential sources. Techniques used to characterize oils and source rock extracts included gas chromatography and gas chromatography-mass spectrometry. A number of biomarker families were investigated including steranes, terpanes, and arylisoprenoids. Diamondoids, C7 compounds and various aromatic compounds, while not true biomarkers, have also been studied extensively. Collectively, all the families of compounds provide a comprehensive picture on the potential sources of these oils as well as providing information on factors that may have impacted the composition of the oils.

Previous and extensive studies of Woodford oils and source rocks in the Anadarko Basin yield a clearly defined set of biomarker characteristics. In particular Woodford oils and source rock extracts have a unique distribution of arylisoprenoids derived from green sulfur bacteria that thrived under the euxinic conditions of deposition prevalent during the formation of the Woodford. In addition to the other families of biomarkers, these arylisoprenoids provide a powerful oil/source rock correlation tool. The geochemical characteristics of the oils found in the Upper Pennsylvanian Lansing-Kansas City (LKC) formation are consistent throughout western Kansas and very similar to those found in the oils from the Anadarko Basin in Oklahoma. It has been proposed previously that the LKC oils originated from the Late Devonian Woodford (Chattanooga) Shale in the deep Anadarko Basin and underwent long distance migration to the reservoirs in NW Kansas. The LKC oils show strong similarities to both Woodford sourced oils and rock extracts in the Anadarko Basin supporting the concept of long distance migration. As a part of this study, an investigation into the organic rich shales within the LKC in NW Kansas is being undertaken as these shales may contribute to the overall oil composition. Determining the origin of these oils is important to help negate the risk of overestimating hydrocarbon accumulations in these reservoirs and to explain the variability of the accumulations.