Articles | Volume 9, issue 6
Solid Earth, 9, 1437–1468, 2018
Solid Earth, 9, 1437–1468, 2018

Research article 12 Dec 2018

Research article | 12 Dec 2018

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

Sutieng Ho1,5,6, Martin Hovland2,6, Jean-Philippe Blouet3,6,7, Andreas Wetzel4,6, Patrice Imbert5,6, and Daniel Carruthers8 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, 5020 Bergen, Norway
  • 3Unit of Earth Sciences, Fribourg University, Chemin du Musée 6, 1700 Fribourg, Switzerland
  • 4Geological Institute, University of Basel, Bernoullistrassse 32, 4056 Basel, Switzerland
  • 5Total-CSTJF, Avenue Larribau, Pau 64000, France
  • 6Fluid Venting System Research Group, Nancy 54000, France
  • 7Department of Geosciences, Université Libre de Bruxelles, Avenue Franklin Roosevelt 50, 1050 Brussels, Belgium
  • 8CGG MCNV, GeoSolutions, Llandudno, North Wales, LL30 1SA, UK

Abstract. A new type of gas chimney exhibiting an unconventional linear planform is found. These chimneys are termed Linear Chimneys, which have been observed in 3-D seismic data offshore of Angola. Linear Chimneys occur parallel to adjacent faults, often within preferentially oriented tier-bound fault networks of diagenetic origin (also known as anisotropic polygonal faults, PFs), in salt-deformational domains. These anisotropic PFs are parallel to salt-tectonic-related structures, indicating their submission to horizontal stress perturbations generated by the latter. Only in areas with these anisotropic PF arrangements do chimneys and their associated gas-related structures, such as methane-derived authigenic carbonates and pockmarks, have linear planforms. In areas with the classic isotropic polygonal fault arrangements, the stress state is isotropic, and gas expulsion structures of the same range of sizes exhibit circular geometry. These events indicate that chimney's linear planform is heavily influenced by stress anisotropy around faults. The initiation of polygonal faulting occurred 40 to 80 m below the present day seafloor and predates Linear Chimney formation. The majority of Linear Chimneys nucleated in the lower part of the PF tier below the impermeable portion of fault planes and a regional impermeable barrier within the PF tier. The existence of polygonal fault-bound traps in the lower part of the PF tier is evidenced by PF cells filled with gas. These PF gas traps restricted the leakage points of overpressured gas-charged fluids along the lower portion of PFs, hence controlling the nucleation sites of chimneys. Gas expulsion along the lower portion of PFs preconfigured the spatial organisation of chimneys. Anisotropic stress conditions surrounding tectonic and anisotropic polygonal faults coupled with the impermeability of PFs determined the directions of long-term gas migration and linear geometries of chimneys. Methane-related carbonates that precipitated above Linear Chimneys inherited the same linear planform geometry, and both structures record the timing of gas leakage and palaeo-stress state; thus, they can be used as a tool to reconstruct orientations of stress in sedimentary successions. This study demonstrates that overpressure hydrocarbon migration via hydrofracturing may be energetically more favourable than migration along pre-existing faults.

Short summary
A newly discovered type of hydrocarbon leakage structure is investigated following the preliminary works of Ho (2013; et al. 2012, 2013, 2016): blade-shaped gas chimneys instead of classical cylindrical ones. These so-called Linear Chimneys are hydraulic fractures caused by overpressured hydrocarbon fluids breaching cover sediments along preferential directions. These directions are dictated by anisotropic stresses induced by faulting in sediments and pre-existing salt-diapiric structures.