Publication Date:
2018-05-17
Description:
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.
Electronic ISSN:
1869-9537
Topics:
Geosciences
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