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Comparing Stratigraphic Architecture and Petroleum Systems Across the South Atlantic Margin


Comparing Stratigraphic Architecture and Petroleum Systems Across the South Atlantic Margin

Katie-Joe McDonough1, 2, Brian W. Horn1, Kyle R. Reuber1, Friso Brouwer1, 3 Ken G. McDermott1

Extensive offshore areas of the South Atlantic eastern and western continental margins remain underexplored, despite significant recent hydrocarbon discoveries. Recent discoveries and tests attest to functioning petroleum systems on both sides of the conjugate South Atlantic margin, in reservoirs ranging from near-shore shelf carbonates to deep-water siliciclastic reservoirs. South of the salt provinces, offshore Brazil, Uruguay and Argentina host multiple underexplored basins. Exploration wells drilled along the conjugate African margin suggest that Early and Late Cretaceous petroleum systems exist in this area. Synchronous source-reservoir couplets on conjugate South Atlantic margins suggest that petroleum habitats emerging on one margin potentially exist on the other side as well.

We compare the stratigraphic evolution of trans-Atlantic continental margins which began to drift apart during Early Cretaceous with the separation of South America and Africa. A detailed chronostratigraphic interpretation from seismic profiles offshore Brazil and Uruguay is compared to conjugate profiles from northern and southern Namibia, creating a high-resolution stratal architecture interpretation of the post-rift sequences as the margins drifted apart.

Prior to opening of the Atlantic during Early Cretaceous rifting the margins shared a similar geologic evolution. During Aptian/Albian, stratigraphic architecture on each side of the Atlantic began to diverge in conjunction with increasing separation of the continents. Aptian marine anoxic source rocks were deposited on both margins in distal settings, but stratigraphic architecture across the widening ocean became distinct. On the Uruguayan margin, steep shelf margin aggradation gave rise to shelf reservoir buildups as well as slope-failure-related deep water reservoirs. On the Namibian margin, a lower gradient shelf produced high-resolution T-R cycles. Similarly, Uruguayan Turonian shelf profiles were steeper and more aggradational, with less shelf aggradation in Namibia.

Our comparison demonstrates that variation in stratigraphic architecture dictates predictable variation in reservoir/trap configurations. Our evolving understanding of petroleum systems on both sides of the South Atlantic suggests that a joint stratigraphic framework may be applied to depositional history and identification of play types and models.

1. ION E&P Advisors, Houston, TX, United States. 2. KJM Consulting, Pine, CO, United States. 3. I^3 GEO, Denver, CO, United States