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Sources and Reservoir History of Mixed Oil and Heavy Oil Determined by Advanced Geochemical Technologies

Abstract

An analytical scheme is proposed for paleo-reconstruction of any oil deposits including heavy, medium and light oil. Applications benefit both conventional and unconventional petroleum system modeling. Routine geochemical analysis is often insufficient to determine mixed-source components and timing of migration critical to developing correct basin models and opening up new play concepts in “mature” basins. Asphaltenes, biomarker acids and the recalcitrant diamondoid hydrocarbons are used as probes for the history of sequential charging and accumulation of heavy oil deposits. The quantitative importance of oil that has reached various levels of biodegradation is shown by biomarker acids analysis. Heavy oil usually results from a sequence of charging. The first oil to reach the reservoir becomes severely biodegraded, is overprinted with fresh oil that becomes moderately biodegraded, is often further overprinted by fresh oil that becomes lightly biodegraded, and might eventually be overprinted by more oil that remains non-biodegraded. Biomarker acids characteristic of each level of biodegradation remain intact showing the quantitative importance of each sequential charge or portion of the charging sequence. Biodegraded oil can be entirely hidden by fresh oil overprinting. Biomarker acids provide a means to determine when this occurs. Asphaltenes and diamondoids in the earliest charges to reach a reservoir remain unaltered by even severe biodegradation. Asphaltenes are isolated from the oil and the biomarkers and diamondoids released from asphaltenes by hydrous pyrolysis are used to conduct oil-to-oil and oil-to-source rock correlations. Comparison with diamondoid correlation using the whole oil provides a way the correlate both the earliest and most recent charges. Multiply-sourced oil accumulations are suggested if the fingerprints fail to match between the asphaltene pyrolysis oil and the whole oil. Thus, mixed co-sourced oil accumulations can be revealed and their components determined by these asphaltene-diamondoid-biomarker based methods. Asphaltenes in both biodegraded and non-biodegraded oil can be used in the same way to reveal and correlate co-sources. Examples from mixed petroleum systems in Venezuela and Colombia are used to illustrate this scheme which has been employed worldwide.