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Discussion papers | Copyright
https://doi.org/10.5194/se-2018-34
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 17 May 2018

Research article | 17 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, Offshore Angola

Sutieng Ho1,5, Martin Hovland2, Jean-Philippe Blouet3, Andreas Wetzel4, Patrice Imbert5, and Daniel Carruthers6 Sutieng Ho et al.
  • 1Department of Geosciences, National Taiwan University, P.O. Box 13-318, 106 Taipei, Taiwan
  • 2Center for Geobiology, University of Bergen, Postboks 7803, N-5020 Bergen, Norway
  • 3Fribourg University, Unite of Earth Sciences, Chemin du Musée 6, 1700 Fribourg, Switzerland
  • 4University of Basel, Geological Institute, Bernoullistrassse 32, CH-4056 Basel, Switzerland
  • 5Total-CSTJF, Avenue Larribau, Pau 64000, France
  • 6Compagnie Générale de Géophysique, United Kingdom

Abstract. A new type of gas chimney exhibiting unconventional linear planform has been observed on 3D seismic data offshore Angola, and is termed Linear Chimneys. These chimneys occur in the shallow buried hemipelagic succession which was affected by syn-sedimentary remobilisation processes related to hydrocarbon migrations. Linear Chimneys are oriented parallel to the adjacent faults, within preferentially oriented tier-bound fault networks of diagenetic origin (also known as anisotropic Polygonal Faults, PFs) in the salt-deformational domain. These anisotropic PFs are parallel to salt-tectonic-related structures indicating their submission to horizontal stress perturbations generated by the latter. Only in anisotropic PF areas chimneys and their associated gas-related structures, e.g. methane-derived authigenic carbonates and pockmarks, show linear planforms. In areas without anisotropic PFs where the stress state is isotropic, gas expulsion structures of the same range of sizes exhibit circular geometry. In areas experiencing a transitional stress field, Linear Chimneys follow the trend of weak anisotropic PFs rather than the nearby tectonic structures. Therefore, the development of Linear Chimneys is interpreted to have been predominantly affected by the anisotropic stress field of PFs. The initiation of polygonal faulting formed 40 to 80m below the seafloor and predates Linear Chimneys. The majority of Linear Chimneys nucleated at the lower part of the PF tier below the impermeable, upper portion of PFs, where gas accumulation was facilitated by a regional impermeable barrier. The permeable part of polygonal fault-bound traps is evidenced by PF cells filled with gas. These PF gas traps restricted the leakage points of overpressured gas-charged fluids to occur along the lower portion of PFs and hence, controlling the nucleation sites of chimneys. Gas leaking along the lower portion of PFs pre-configured the spatial organisation of chimneys. Anisotropic stress fields of tectonic and polygonal faults couple with partial impermeability of PFs determined directions of gas migration, linear geometry of chimneys, long term migration pathways and successive leaking events. Methane-related carbonates that precipitated above Linear Chimneys inherited the same linear planform geometry, both structures record the timing of gas leakage, the orientation of palaeo stress and thus can be used as a tool of stress reconstruction in sedimentary successions.

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Following the preliminary works of Ho et al. (2012, 2013), we investigate a newly discovered type of structure in the field of petroleum sedimentology: blade-shaped gas chimneys instead of classical cylindrical ones. These chimneys are caused by overpressured hydrocarbon fluids breaching the cover sediments along preferential directions. These directions are dictated by anisotropic stresses that are induced by diagenetic discontinuities in sediments and pre-existing salt-diapiric structures.
Following the preliminary works of Ho et al. (2012, 2013), we investigate a newly discovered...
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