Focused hydrocarbon-migration in shallow sediments of a pockmark cluster in the Niger Delta (Off Nigeria)

Type Article
Date 2017-01
Language English
Author(s) De Prunele Alexis1, 2, Ruffine Livio1, Riboulot VincentORCID1, Peters Carl A.3, Croguennec Claire1, Guyader VivienORCID1, Pape Thomas3, Bollinger Claire2, Bayon Germain1, Caprais Jean-Claude4, Germain Yoan1, Donval Jean-Pierre1, Marsset TaniaORCID1, Bohrmann Gerhard3, Geli LouisORCID1, Rabiu Abdulkarim5, Lescanne Marc6, Cauquil Eric6, Sultan Nabil1
Affiliation(s) 1 : IFREMER, Dept Ressources Phys & Ecosyst Fond Mer REM, Unite Geosci Marines, Plouzane, France.
2 : Inst Europeen Mer IUEM, Ecole Doctorale Sci Mer, Plouzane, France.
3 : MARUM Ctr Marine Environm Sci, Leobener Str, Bremen, Germany.
4 : IFREMER, Dept Ressources Phys & Ecosyst Fond Mer REM, Unite Ecosyst Profonds, Plouzane, France.
5 : Nigeria Inst Oceanog & Marine Res, Lagos, Nigeria.
6 : TOTAL Explorat Prod, Pau, France.
Source Geochemistry Geophysics Geosystems (1525-2027) (Amer Geophysical Union), 2017-01 , Vol. 18 , N. 1 , P. 93-112
DOI 10.1002/2016GC006554
WOS© Times Cited 12
Keyword(s) anaerobic oxidation of methane, cluster of pockmarks, fluid flow, gas hydrates, pore-water profiles, transient state
Abstract The Niger Delta is one of the largest hydrocarbon basin offshore Africa and it is well known for the presence of active pockmarks on the seabed. During the Guineco-MeBo cruise in 2011, long cores were taken from a pockmark cluster in order to investigate the state of its current activity. Gas hydrates, oil and pore-water were sampled for geochemical studies.

The resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks. There is a clear tendency for gas migration within the hydrate-bearing pockmarks, and oil migration within the carbonate-rich one. This trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks. We also demonstrate that Anaerobic Oxidation of Methane (AOM) is the main process responsible for the depletion of pore-water sulfate, with depths of the Sulfate-Methane Transition Zone (SMTZ) ranging between 1.8 and 33.4 m. In addition, a numerical transport-reaction model was used to estimate the age of hydrate-layer formation from the present-day sulfate profiles. The results show that the sampled hydrate-layers were formed between 21 and 3750 years before present.

Overall, this work shows the importance of fluid flow on the dynamics of pockmarks, and the investigated cluster offers new opportunities for future cross-site comparison studies. Our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades.
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De Prunele Alexis, Ruffine Livio, Riboulot Vincent, Peters Carl A., Croguennec Claire, Guyader Vivien, Pape Thomas, Bollinger Claire, Bayon Germain, Caprais Jean-Claude, Germain Yoan, Donval Jean-Pierre, Marsset Tania, Bohrmann Gerhard, Geli Louis, Rabiu Abdulkarim, Lescanne Marc, Cauquil Eric, Sultan Nabil (2017). Focused hydrocarbon-migration in shallow sediments of a pockmark cluster in the Niger Delta (Off Nigeria). Geochemistry Geophysics Geosystems, 18(1), 93-112. Publisher's official version : https://doi.org/10.1002/2016GC006554 , Open Access version : https://archimer.ifremer.fr/doc/00365/47647/