Print this pagePredicting Petroleum Origin, Type and Quality: Geochemistry from Work
Geochemistry has always played a critical role in petroleum exploration. Source rocks characterization and oil typing are essential parts of any exploratory project. Whereas in mature basins large databases allow well refined geochemical correlations, exploration frontiers require more predictive information, combined with basin modeling. This study presents the general workflow of the organic geochemistry techniques that Petrobras is applying to exploration in frontier areas.
In Brazilian marginal basins, the expansion of exploration from onshore basins, to offshore areas and deep to ultradeep water areas, provided a change in geological context, which is reflected in different petroleum systems. Provinces where lacustrine oil accumulations dominate change laterally to marine-sourced oil fields, reflecting changes in the depositional system of petroleum source rocks, thermal regimes and different migration pathways. Typical end-members are well characterized, but mixing in varying proportions of oils from two or more source rocks require more refined analytical techniques and interpretation of source-related parameters, including quantitative analyses of biomarkers, diamondoids and carbon and hydrogen isotopic ratios of individual compounds. Gas origin can be assessed not only by thermal evolution parameters, but also by hydrogen isotopes, which reflect the source input, revealing different origins for the liquid and gas phases in some fields. Impregnation of immature bitumen and mud additives may interfere in geochemical interpretations.
Lack of data in frontier areas make it necessary the generation of quantitative data, which can calibrate basin modeling software integrated to seismic interpretation. Organic facies modeling can help source rock prediction in undrilled areas. In shallow leads, low temperatures suggest high biodegradation risk, but the timing of reservoir filling and the charge history must be evaluated. In deep prospects, not only the kinetics of kerogen cracking needs to be understood, but also the kinetics of oil to gas cracking. Compositional kinetics are currently being applied. The occurrence of non-hydrocarbon gases must also be considered.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil