Assessment of Formation and Evolution of Organic Compounds in Sedimentary Basins: Geochemical and Carbon Isotopic Constraints

Geochemical and stable carbon isotope data of crude oil and natural gas have long been used effectively in evaluating generation sources and pathways, the extent of degradation, and the reconstruction of sedimentary paleo-environment. This thesis consists of three individual studies focused on understanding the detailed processes during the formation and evolution of oil and natural gas using geochemical (including biomarkers) and carbon isotope analyses.
The first study evaluates the paleo-environment of the Monterey Formation in the Santa Maria Basin using carbon isotopes of saturates, aromatics, resins, and asphaltenes (SARA). The values become higher as molecular complexity increases, with minor variations in aromatics. Combined with lithofacies analysis, the results show that the depositional environment was likely to be a deep marine silled basin under anoxic conditions. This study has confirmed previous work that was solely relied on lithofacies analysis and provided another approach for constructing sedimentary environment using carbon isotopes.
The second study utilized carbon isotope compositions of SARA fractions and biomarkers to identify the origin and extent of degradation of tar balls in the beach environment. A set of tar balls were collected from the shore of Elmer's Island, Louisiana, where oils from the Deepwater Horizon oil spill have been observed. A variety of biomarker parameters have been assessed. The results suggested most tar balls were related to the Deepwater Horizon oil spill by comparison with previously reported data. The use of carbon isotope of SARA fractions and biomarkers for delineating associated degradation processes suggests their potential as molecular tracers for oil spill events. The third study investigated the chemical and carbon isotopic compositions of light alkanes under different redox conditions during petroleum generation. A series of hydrous pyrolysis experiments were conducted at 310 and 350 °C under redox conditions created by dissolved H2 and MnO2, respectively. The C1-C4 alkanes from experiments with dissolved H2 are depleted in 13C and enriched in 13C with MnO2. The yields were higher with dissolved H2 and lowered with MnO2. The results suggest that the thermal maturation of hydrocarbons from hydrous pyrolysis can be changed by redox conditions.