Abstract

The Dry Branch Section is a mixed siliciclastic/carbonate sedimentary sequence containing carbonate rocks that experienced incomplete diagenesis due to early introduction of oil from the Oil Creek Formation (Ordovician). The section is part of the Deese Group (Desmoinesian) exposed in the western portion of the Mill Creek Graben, Arbuckle Mountains, Oklahoma. The sequence was deposited in shallow marine shelf setting within a tectonically active basin; deposition was strongly influenced by adjacent terrigenous coastal depositional environments. The section was synclinally deformed by wrench faults to the north which were active soon before and after deposition. Two of these faults cut both the Dry Branch Section and the underlying Oil Creek Formation, a known petroleum reservoir and the source of fault-related oil seeps throughout the central Arbuckle region. These faults acted as avenues for reservoir fluids to enter section as a result of remigration driven by orogenic stress. Carbonate rocks which show evidence of diagenesis influenced by reservoir fluids are present in the upper part of the section. Limited cementation, overcompaction, and arrested carbonate diagenesis resulted from the effects of interstitial asphalt which formed as a result of intraformational biodegradation of Oil Creek oil. Stratigraphic variation in petrologic characteristics suggests that oil introduction into Dry Branch sediments occurred as a single event within 10,000 years of deposition of uppermost part of section, coinciding regionally with the Desmoinesian Ouachita orogenic pulse. Diagenesis has resulted in moderate isotopic depletion of altered fossils (-4 per thousand delta 18 O; -3.5 delta 13 C) relative to unaltered skeletal aragonite (delta 18 O = 0 to -1 per thousand ; delta 13 C = 3.7 to 5.0 per thousand ). Petrographic and stable isotopic study shows that two phases of early carbonate cementation affected Dry Branch carbonates: 1) early clear spar that is more depleted in delta 18 O (-5 per thousand ) than the altered fossils; 2) later spar which contains asphalt inclusions. Microspar and poikilotopic sparry cement present in the upper Dry Branch are anomalously depleted in delta 13 C (< -20 per thousand ). This is close to the isotopic compositional range of asphalt in the section, indicating that organic carbon derived from oil biodegradation was present in the porewater during diagenesis. Carbonate geochemical and petrographic evidence, and the early post-depositional tectonic introduction of oil, indicate that subsurface water was a principal diagenetic agent.

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