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Pennsylvanian Source Rocks in the Anadarko Basin: An Example From the Missourian Series Hogshooter Formation in Mills Ranch Field (TX & OK)


The Anadarko Basin is one of the most petroliferous basins in the world, and has a poly-basin history that can be subdivided into Ordovician, Devonian-Mississippian, and Pennsylvanian petroleum systems. Several previous studies within the Anadarko Basin highlight the potential of Pennsylvanian mudstones/shales as viable hydrocarbon sources. However, unlike source rocks in the Ordovician and Devonian-Mississippian petroleum systems, where clear geochemically validated examples of sourcing contributions in the basin exist, those of the Pennsylvanian system lack any clear geochemical ties. This study highlights the hydrocarbon-source relationship within Mills Ranch field (Hogshooter Formation: Missourian Series) along the Texas and Oklahoma border by clearly demonstrating the genetic relationship between produced oils/condensates and Missourian shales. Bitumen extracts from two mudstone-shale cores in the Hogshooter portion of the field were used to link the produced fluids to Missourian source beds. To validate key geochemical correlation characteristics, low maturity extracts from other representative source facies within the Anadarko basin were also used for comparison. Additionally, data from Woodford shale produced oils were used as a Devonian source control. Based on gas chromatography, coupled gas chromatography/mass spectrometry, and stable carbon isotope-ratio-mass-spectrometry analyses, light oils and condensates produced from Mills Ranch field are genetically related to local Missourian mudstones/shales. Key geochemical attributes that set the Pennsylvanian oils apart from the Ordovician and Devonian-Mississippian systems are Mango K1 and K2 ratios, C27/C29 14α and 14β-sterane ratios > 1, abundant diahopane relative to 17α-hopane and high saturate/aromatic fraction δ13C canonical variables. At least three organic-facies are identified within the Missourian section from integration of sedimentology, petrography and geochemistry. However, there is just one primary type-II organic-facies that is geochemically tied to the petroleum liquids. Models for the distribution of the primary type-II liquids-generating organic-facies have led to an improved assessment of the Mills Ranch field Missourian section, and a better understanding of the economic potential of Pennsylvanian shales in the Anadarko Basin.