--> ABSTRACT: Hydrocarbon Leakage Indicators Offshore the Argentinean Continental Margin: A Comparative Study of the Colorado and Malvinas Basins, by Anka, Zahie; Baristeas, Nikolaus ; Loegering, Markus; Rodriguez, Jorge; Marchal, Denis ; diPrimio, Rolando; Vallejo, Eduardo ; #90142 (2012)

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Hydrocarbon Leakage Indicators Offshore the Argentinean Continental Margin: A Comparative Study of the Colorado and Malvinas Basins

Anka, Zahie *1; Baristeas, Nikolaus 1; Loegering, Markus 1; Rodriguez, Jorge 2; Marchal, Denis 2; diPrimio, Rolando 1; Vallejo, Eduardo 2
(1) Section 4.3. Organic Geochemistry, GFZ German Research Centre for Geosciences, Potsdam, Germany.
(2) Petrobras Argentina S.A, Buenos Aires, Argentina.

The Colorado and the Malvinas basins are located in southernmost part of the Atlantic margin of South America. Their stratigraphic infill record the development of the Lower Cretaceous to Present-day volcanic-rifted passive margin of Argentina, which followed the break-up of Gondwana. We have analysed two dense 2D seismic reflection grids that allowed us not only to constraint the main seismo-stratigraphic units, but also to identify the abundance and distribution of hydrocarbon migration/leakage indicators in both basins. These features include: seafloor pockmarks and mud volcanoes, subsurface seismic chimneys and pipes, buried mounded structures, seismic pull-downs and bright/ flat spots. Based upon their relation to structural and stratigraphic elements, they have been classified as structurally- or stratigraphically- controlled.

In the Colorado basin, two populations of seismic chimneys (P1Col and P2Col) have been identified. P1Col is located in the central part of the basin, within one of the main depocentres, and is related to volcanism, as proved by some wells. Some of the features included in P2Col are interpreted as gas chimneys and is supported by migration patterns of 3D petroleum system modelling. Whilst P2Col is found mostly on the slope area, P1Col seems to be rooted in the pre-rift and syn-rift sequences and buried by the top of the Elvira Fm.
(Eocene). In contrast, population P2Col often reaches the seafloor and ends up on possible pockmarks. Some “mega-pockmarks” (diameter up to1200 m) similar to ones reported on the conjugate margin of the Orange basin are also interpreted on the slope. The presence of mound-like structures, both buried and on the seafloor, could represent mud volcanoes towards the southern margin of the basin. A highly vertically-faulted interval has been identified within the Campanian and Paleocene sequences. These faults correspond to a polygonal-fault system.

Similarly, the Malvinas basin is also characterised by the presence of several seismic features interpreted as hydrocarbon indicators which were separated in four populations (CP1 to CP4). CP1 contains seismic anomalies as gas chimneys originating above or in a polygonal faulted Pliocene-Miocene interval accompanied by bright spots and seabed pockmarks. To CP2 belong gas chimneys, pipes, and buried pockmarks located close to the southern transpressional deformation front. CP3 consists of gas chimneys originated both from basal stratigraphic pinch-outs or from a Mid-Cretaceous deltaic-fan front. In CP4 several buried Eocene mounded structures are interpreted as authigenic carbonate mounds possibly derived from oxidation of thermogenic methane that leaked upwards along basement-rooted faults. The gas source for CP1, CP2 and CP4 is most likely leakage from the uppermost Jurassic- Barremian reservoir Springhill Fm, although a biogenic gas source for CP1 can not be totally ruled out, whilst CP3 is possibly sourced from the Mid-Cretaceous sediments of the Middle Inoceramus Fm.

Analysis of the distribution of these leakage indicators is crucial for identifying potential present-day active petroleum systems and seal bypass systems in these basins.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California