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Llanos Basin: Unraveling Its Complex Petroleum Systems With Advanced Geochemical Technologies

Abstract

Llanos Basin petroleum systems combine complex geology with complex petroleum geochemistry consisting of multiple source rocks and charges at different maturities. In addition, many of these charges were biodegraded to varying degrees. We have shown that all of these factors can converge in a single oil accumulation. Using advanced geochemical technologies (AGTs) we have determined that certain oilfields consist of multiply co-sourced accumulations with as many as four sources, thermally cracked light oil mixed together with normally-maturated black oil, and multiply-charged reservoirs culminating in a wide range of biodegradation severities in the same oil. AGTs were applied to see through biodegradation and maturity issues to determine source affinities and unravel mixtures. The most critical AGTs are diamondoid-based, including compound specific isotope analysis (CSIA) and quantitative extended diamondoid analysis (QEDA), which are unaffected by biodegradation and high maturity. Through these diamondoid methods five oil sources and their mixtures were differentiated. By the addition of taxon-specific biomarker analysis and CSIA of biomarkers (CSIA-B), source rock age and depositional environment were constrained. The oils are of five source-types: (1) Cretaceous terrestrial-marine shale mix (2) Cretaceous marine-terrestrial carbonate mix, (3) Tertiary terrestrial shale, and (4) and (5) both Cretaceous marine shale, but from distinct facies. Hydrous pyrolysis of asphaltenes introduced another dimension to this study. Asphaltenes are both highly resistant to biodegradation and preferentially represent low-maturity oil charges. QEDA, CSIA and biomarker parameters applied to the pyrolysates revealed similarities and differences between sources for the maltenes and the asphaltenes, exposing and unraveling the co-sources with a high level of confidence. Biomarker Acids Analysis (BAA) was used to characterize oil biodegradation history and the relative importance of each charge pulse to the reservoirs. It was determined that most oil reservoirs have multiple charge-pulses with different levels of biodegradation. Losses of liquid oil due to biodegradation were estimated using High Temperature Simulated Distillation (HT-SimDis). Results were found to directly correlate to oil gravity. Since very little oil is necessary to run HT-SimDis it could be a useful tool for field development to estimate oil gravity from cores and oil shows.