Paradox Basin source rock, southeastern Utah : organic geochemical characterization of Gothic and Chimney Rock units, Ismay and Desert Creek zones, within a sequence stratigraphic framework
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The Chimney Rock and Gothic units of the Pennsylvanian Paradox Formation have long been considered source rocks for the rich hydrocarbon fields of southeastern Utah. Fundamental questions about these units include: source and nature of the organic material, source rock character, and position of the source rocks in the existing sequence stratigraphic framework. The Chimney Rock and Gothic, historically referred to as shales, are composed of calcareous mudstone, dolomudstone, and calcareous sandstone. High total organic carbon (TOC) values are more closely linked to sequence stratigraphic position than lithology. In the Gothic, TOC values decrease upwards. Terrestrial maceral content increases upwards in both the Gothic and the Chimney Rock as determined through point-count and qualitative observation. Pyrolysis indicates that greater than anticipated terrestrial influence is present and is consistent for all wells. No distinct difference in geochemical character exists between the two units. Sequence stratigraphic boundaries appear to be as good as, or better, than traditional lithostratigraphic boundaries for determining high TOC occurrence and source rock location. Within repetitive major sea level transgressions the organic matter that fed the basin evolved from a marine-dominated signature to a terrestrial-dominated signature.