Identification and numerical modelling of hydrocarbon leakage in the Lower Congo Basin: Implications on the genesis of km-wide seafloor mounded structures Zahie Anka a, ⁎, Robert Ondrak a , Astrid Kowitz a, 1 , Niels Schødt b a Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Germany b Maersk Olie og Gas AS, Denmark abstract article info Article history: Received 7 March 2012 Received in revised form 7 November 2012 Accepted 19 November 2012 Available online xxxx Keywords: Pockmarks Seismic chimney Gas leakage Carbonate mound Hydrocarbon migration Autigenic carbonate We present a combined approach of interpretation of 2D seismic-reflection data and numerical modelling of hydrocarbon generation and migration across the southern slope of the Lower Congo Basin, in order to investi- gate the factors controlling timing and distribution of hydrocarbon leakage in this area. We identified three main families of past and present-day leakage features: (1) Mid-Upper Miocene seismic chimneys concentrated basinwards and ending up on buried pockmarks, (2) Plio-Pleistocene chimneys, rather clustered to the east of the study area and ending up in seafloor pockmarks, and (3) fewer scattered chimneys identified within the Miocene sequences ending up in shallow enhanced reflectors (“Flat spots”). Stratigraphic and structural ele- ments seem to control the distribution of these features. At least two major events of leakage occurred during the Middle–Late Miocene and intermittently during the Pliocene-Present. External factors as sediment supply are associated to the Miocene leakage event, whilst internal structural elements probably triggered the Pliocene to present-day leakage. A major seabed morphological feature, represented by a margin-paralleled belt of more than 1-km-wide mounds, was identified above growth faults to the east of the study area. Data-constrained 2D HC generation and migration modelling suggests a genetic link between these structures and vertical migration/leakage of thermogenic methane sourced from either currently mature Oligo-Miocene source rocks or secondary cracking and further expulsion from over-mature Upper-Cretaceous source rocks. Hence, the mounds are likely to repre- sent a lineation of methane-derived carbonate build-ups. Despite the natural limitations of a 2D migration model, when combined and calibrated with observations from seismic data, it can be used as a valid tool to assess petroleum migration routes in sedimentary basins. To the best of our knowledge, this is the first integrated approach combining both seismic observations and numerical modelling carried out in the Angola basin. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Leakage of liquid and gaseous hydrocarbons (HC) through focused fluid flow is a process recognized to occur, in varying intensities, along most continental passive margins, where both carbon trapping within the sediments and/or intense carbon release into the hydrosphere and atmosphere may take place e.g. (Hempel et al., 1994; Hornbach et al., 2007; Hovland and Judd, 1988; Roberts and Carney, 1997; Sager et al., 2003). Seafloor features, such as mud volcanoes, pockmarks, carbonate mounds, and polygonal faults have been recognized to be the surface indicators of both dewatering and active leakage of natural gas from deeper sources and reservoirs (e.g. Berndt, 2005; Gay et al., 2007; Hovland, 2005; Leon et al., 2006; Orange et al., 2002). These surface indi- cators are usually linked to subsurface expressions of active hydrocarbon migration as seismic chimneys, pipes, and bright spots (i.e. Andresen et al., 2011; Ben Avraham et al., 2002; Charlou et al., 2004; Graue, 2000). Recognition of similar features in buried sediments can indicate the occurrence of past hydrocarbon leakage events staggered through time, which in turn may be related to particular events on the basin his- tory as sea-level changes, tectonic uplifts and erosions, or even glacial– interglacial cycles. Additionally, since hydrocarbon (HC) seepage can be a source of greenhouse gases (such as thermogenic methane) to the ocean and atmosphere, quantification of this process could help un- derstanding the potential influence of this process on Earth's climate history. On the other hand, submarine cold seeps derived from HC leakage are often associated with the presence of carbonate mounds in deep marine settings (Hornbach et al., 2007; Hovland et al., 2005; Judd and Hovland, 2007). The microbial activity associated with the anaer- obic oxidation of migrating thermogenic fluids (mostly methane) in the bacterial sulphate reduction zone (Suess and Whiticar, 1989) can lead to the precipitation of authigenic carbonates as crusts or con- cretions at the sediment–water interface (Hovland, 1990; Jørgensen Tectonophysics xxx (2013) xxx–xxx ⁎ Corresponding author. Tel.: +49 331 288 1798; fax: +49 331 288 1782. E-mail address: zahie@gfz-potsdam.de (Z. Anka). 1 Now at: Museum für Naturkunde der Humboldt-Universität, Berlin, Germany. TECTO-125693; No of Pages 19 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.11.020 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Please cite this article as: Anka, Z., et al., Identification and numerical modelling of hydrocarbon leakage in the Lower Congo Basin: Implications on the genesis of km-wide seafloor mounded structures, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2012.11.020