Distribution of hydrocarbon leakage indicators in the Malvinas Basin, offshore Argentine continental margin

The Malvinas Basin is located in the southernmost Argentinian continental margin. Despite the lack of commercial hydrocarbon accumulation discoveries, the presence of thermogenic gas in gravity cores and seafloor oil slicks points to the existence of an active petroleum system in this basin. Based on the analysis of over 1000 20 industrial seismic-reflection profiles, covering the shelf and upper-slope of the Malvinas Basin offshore the southernmost Argentinian margin, we document the presence of buried and present-day features including subsurface seismic chimneys, seabed and buried pockmarks, and buried-mounded structures which are probably indicators of long-term leakage history of both liquid and gaseous hydrocarbons since the Eocene to the Present. Based on their distribution and likely controlling factors, these leakage features were classified into four areas of leakage: area I to area IV. Area I is located in the centre of the basin and contains seismic anomalies as pipes originating above or in a polygonal faulted Pliocene-Miocene interval accompanied by bright spots and seabed pockmarks. Area IIa/b is located in the south of the basin and contains pipes and buried pockmarks located close to the southern transpressional deformation front. Area III is located to the east of the basin and consists of pipes hosted in a Mid-Cretaceous deltaic-fan. Area IV, located in the western part of the basin, consists of buried Eocene mounded structures located near the Rio Chico High and above basement highs and faults. They are interpreted as authigenic carbonate mounds possibly derived from oxidation of thermogenic methane that leaked upwards along basement-rooted faults. A reversed-polarity seismic reflection showing a lineation of bright spots has been identified at an average depth of 170 m below seafloor in water depths of about 500 m. We interpret this reflection as a bottom simulating reflector (BSR), which enables us to estimate a geothermal gradient of 23.9 +/- 2.0 degrees C/km for the area. Near and above the thrust faults of a transpressional deformation front, the vertical pipes in area II cross-cut possible hydrate deposits, suggesting that there is a current breaching of these deposits due to tectonically-driven upward focused fluid flow and heat transport. The gas source for the features observed in areas I, IIa/b and IV is most likely leakage from the uppermost Jurassic-Barremian reservoir Springhill Fm., although a biogenic gas source for leakage indicators in area I cannot be ruled out. The leakage indicators in area III are possibly sourced from the Mid-Cretaceous sediments of the Middle Inoceramus Fm. (C) 2012 Elsevier B.V. All rights reserved.