Downslope-shifting pockmarks: interplay between hydrocarbon leakage, sedimentations, currents and slope’s topography

Abstract

Pockmarks in Pliocene-Quaternary continental slope deposits offshore Angola show features related to: (1) fluid leakage craters that formed repeatedly, (2) authigenic methane-derived carbonates that indicate the (former) presence of hydrocarbons and (3) erosional–depositional structures that are clearly related to current activity. Depending on topography, the pockmarks show differing development: “Advancing Pockmarks” preferentially developed on regional slopes or inclined topography (> 2.5°–3°). They arranged in a chain-like pattern and mimic the outline of buried turbidite channels below. These pockmarks and their infill migrated downslope in response to shifting vents. “Nested Pockmarks” occur in gently sloping areas (< 2°). Their isolated conical infill records slope-parallel migration within a specific depth range pointing to the influence of contour currents. Both pockmark types are long-lived and they record preferential fluid migration along specific pathways, which developed at the downcurrent sidewalls of pockmarks due to flow separation initiating “cavity flow” within the pockmarks. The durable specific migration paths include pockmark sidewalls, vertically stacked erosional-interface of sediment waves, or entire pockmark bodies. The vertical extent of both pockmark types from End Miocene to the present-day seafloor documents various intensities of episodic fluid bursts followed by periods of quiescence and fill.

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Acknowledgements

We thank Total S.A. for providing data, funding and its partner for publication permission, and the Ministry of Science and Technology of Taiwan for the Grant MOST1052914I002069A1. Our work is based on and extended from S. Ho’s PhD. The scientific work was fully carried out in Total S.A. and under its direction. S. Ho thanks Benoit Paternoster for his supervision on Geophysics. S. Ho also thanks Cardiff University and JA Cartwright for his great interest in this work and general support. Thanks also for the advices and the enormous support from Gordon Lawrence, David Hutchings, Ludvig Löwemark, and Char-Shine Liu.

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Appendix

Appendix

See Figs. 11, 12, 13, 14, 15.

Fig. 11
figure11

The studied pockmarks outcrop on the modern seafloor. a Advancing pockmark (AP). (i) Cross-section of an AP [see (ii) for location]. (ii) Dip map of modern seafloor shows the topmost depressional topography of the AP, which suggests the recent activity of the AP. b Nested pockmark (NP). (i) Cross-section showing a NP with its top partly visible on the modern seafloor [see (ii) and (iii) for location]. (ii) Seabed dip map showing the top of NP constituting the floor of a modern pockmark and suggesting recent activities of the NP. (iii) Time horizon of 5.3 Ma superposed on its dip map showing the position of the NP’s primary crater

Fig. 12
figure12

Height versus horizontal distance between topmost and basal deepest centre of advancing pockmarks

Fig. 13
figure13

Detailed seismic interpretation of an advancing pockmark (AP). (Top) Interpreted seismic section of an AP. Outline for middle portion of AP. Dotted line at stoss side represents initial volume of infill before removed by erosion. (Bottom) Outline of upper part and middle part of AP

Fig. 14
figure14

Fluid venting structures in association with the pockmark trails and channel complexes. a Chimneys rooting within the levees of Channel Complex 2. (i) Two chimneys (black arrows) above levee terminating in depressions on modern seafloor, [see (ii)]. (ii) amplitude map of present-day seafloor with 2 chimneys terminating in shallow depressions and associated positive high-amplitude anomalies (black arrows). Positive high-amplitude anomalies are interpreted as methane-derived authigenic carbonates possibly in association with gas hydrate. b Negative high-amplitude anomalies (NHAAs; black arrows) at 5.3 Ma horizon above polarity inversion at top of Channel Complex 1. (i) Cross-section of downslope side of Channel Complex 1 showing NHAAs and polarity inversion above and at channel surface (horizon 6H). (ii) NHAAs showing elongate and round shapes on horizon 5.3 Ma amplitude map (orange domains) being superimposed with blue contours indicating location of polarity inversion below NHAAs. (iii) Polarity inversion showing elongate and round shape on amplitude horizon 6H (black domains). (c) Amplitude anomalies at upslope side of Channel Complex 1. (i) Seismic profile showing BSR that define upper boundary of NHAA infills within BCs and intersect bottom of VSPs above. Chimneys originate from channel complex or NHAA infills of BCs and penetrate BSR and VSPs above. (ii) Amplitude map of Lower Pliocene across base of VSPs (P1) and top of BCs (P0) showing planar geometry of NHAA infill inside pockmarks

Fig. 15
figure15

The 3D view of a seismic section shows basal craters (BCs) in troughs of sediment undulations located in the upslope part of Channel Complex 1. BCs expressed by green colour 3D-horizon intersecting seismic record showing some sediment undulations (labelled corresponding to Fig. 5c). The 3D drawing shows the internal structure of the undulations and the interpreted gas migration pathways along the inclined stacked of erosional interfaces

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Ho, S., Imbert, P., Hovland, M. et al. Downslope-shifting pockmarks: interplay between hydrocarbon leakage, sedimentations, currents and slope’s topography. Int J Earth Sci (Geol Rundsch) 107, 2907–2929 (2018). https://doi.org/10.1007/s00531-018-1635-5

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Keywords

  • Pockmark migration
  • Pockmark infill
  • Hydrocarbon leakage
  • Angola