Determining Multiple Sources, Facies and Oil-Mixtures of the South Atlantic Margins: Africa and Brazil

Abstract

Multiple source rocks account for the prolific petroleum reserves and potential for new discoveries in the South Atlantic margins. Accurate basin models depend on correct determination of effective sources in a given area. However, classical geochemical technologies often fall short of providing a fine differentiation among the possibilities. We have distinguished as many as four sources in some basins along the Atlantic margins of Brazil and Africa by using Advanced Geochemical Technologies (AGTs). Effective AGTs include Compound Specific Isotope Analysis of Biomarkers (CSIA-B) including hopanes, steranes and alkanes, of Diamondoids (CSIA-D), and the novel Quantitative Extended Diamondoid Analysis (QEDA). Co-sourced oil has been determined and quantified by combinations of such technologies. Some AGTs overcome the difficulties of correlating biodegraded oil, over-mature or cracked oil (condensate) and black oil. We differentiated Paleozoic, Lower and Upper Cretaceous and various Tertiary sources in West Africa and de-convolved mixed oils. In examples from Brazil, CSIA-B shows stark differentiation of Brazilian-margin pre-salt lacustrine oil samples in comparison to Upper Cretaceous marine and late Lower Cretaceous marine oil types. An unusually heavy −18 to −22 ‰ isotope ratio of 4-methyl-24-ethylcholestane compared to C27-C29-steranes at −24 to −29 ‰ suggests that dinoflagellate blooms contributed to the algal lipids component of the pre-salt during source rock deposition. Whereas, in the same oil samples hopane isotope ratios are unusually light, where C29-hopane can reach −40 ‰, indicating a stable chemocline in the water column with methanogenic-methanotrophic ecological conditions established. This condition reverses during transitional marine conditions in which there were blue-green bacterial blooms and hopanes are isotopically heavy in the range of −18 to −22 ‰. Conditions later changed to open marine represented in Late Cretaceous marine oil with ∼-27 to −29 ‰ for both the hopanes and the C27-C29 steranes in the same oils showing prokaryote populations dominated by cyanobacteria and normal marine algal populations. Similar source-differentiating CSIA-B features are observed in the African margin (Angola), except that mixed marine and terrestrial sourced Tertiary oils are also found, in which hopanes again show light isotope characteristics due to the presence of methanotrophic bacteria.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014