Fluid inclusion study of the Mesoproterozoic Nonesuch Formation - biogenic sources and thermal history of oil
Date
2011
Authors
Colbert, Sarah Janette, author
Ridley, John, advisor
Egenhoff, Sven, committee member
Kennan, Alan, committee member
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Abstract
The Nonesuch Formation is part of the Mid-Continent Rift System and is unusual because it contains a relatively high amount of oil that is thought to have formed in situ during the Mesoproterozoic, approximately 1.1 Ga. In this study, primary, pseudosecondary and secondary oil inclusions in samples obtained from Nonesuch Fm outcrop and cores were analyzed using petrography and microthermometry, and one core sample was analyzed by GC-MS. Aqueous inclusions were also studied via petrography and microthermometry. The inclusions studied were hosted in sandstone grains and matrix from parts of the Nonesuch Fm and the upper part of the Copper Harbor Fm, diagenetic calcite nodules from the Marker Bed of the Nonesuch Fm and calcite veins in the Nonesuch and Upper Copper Harbor Fms that formed no later than 30 Ma after deposition. Based on these settings, it can be assumed that all inclusions studied were entrapped during the Mesoproterzoic around the time of deposition of the Nonesuch Fm. The biomarkers detected by GC-MS have a Proterozoic character and the presence of mid-chain substituted monomethyl alkanes as well as 1,2,5-TMN indicates a cyanobacterial hydrocarbon source. Algal biomarkers have also been found in the oil in previous studies. The ratios calculated from the GC-MS data suggest an early- to peak-oil window maturity for the hydrocarbons, which is consistent with data from previous studies. The homogenization temperatures obtained by microthermometry are typically used as an estimate of inclusion entrapment temperature; however, in this study, the wide range of homogenization temperatures along with an inconsistency between these and the GC-MS maturity indicators implied that the thermal history of the oil was more complex. The inclusions were likely trapped during diagenesis at temperatures between 100-130°C, which agrees with the evidence from the maturity ratios and previous work on the Nonesuch Fm, and then reheated by hydrothermal activity after entrapment. The microthermometrical evidence implies that the second period of heating raised temperatures to levels exceeding 250°C, and other studies of the Mid-Continent Rift area suggest that this secondary heating occurred either soon after diagenesis or significantly later, around 200-300 Ma.
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Subject
biomarkers
oil inclusions
Nonesuch
microthermometry
fluid inclusions