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Layer-bound arrays of polygonal compaction faults have long been considered as important migration routes for hydrocarbon fluids leaking to the surface across thick shale sequences. A classic example is the deep offshore of the Lower Congo Basin where numerous fluid-venting structures are present above a Pliocene polygonal fault system. In this paper we present a detailed seismic analysis of a newly recognised system of Quaternary-aged Linear Chimneys and their intersection geometries with pre-existing Pliocene-aged polygonal faults (PF). Most (73%) of the 209 chimneys analysed intersect the lower portions of polygonal faults and almost half of these are rooted in strata below the PF interval. This indicates that fluid (in this case gas) migrated vertically, cross-cutting polygonal faults as it ascended through the tier. This is a strong indicator that PFs did not provide viable migration pathways otherwise chimneys would terminate at the upper tip of the fault, which would be the most likely migration exit point. Only twice in the whole system of Linear Venting Systems did this occur. A sub-set of chimneys stems from or above PF planes but these are restricted to either the lower footwall or from the apex area of hanging wall. At best they are evidence of fluids migrating up the lower part of polygonal faults and exiting deep within the tier, then migrating through most of the tier in their own vertical leakage vents. These results provide strong indicators that at least within this part of the Lower Congo Basin polygonal faults were the least effective/favoured migration pathway and that it was more energy-efficient for migrating gas to hydrofracture its fine-grained overburden than to re-open polygonal faults.Key Words: Linear chimneys; hydrocarbon; polygonal faults (PFs); permeability; methane-related carbonates; Lower Congo Basin; Angola. Résumé : La perméabilité des failles polygonales déduite de la géométrie de leurs intersections avec des cheminées linéaires : une étude de cas dans le bassin du Bas-Congo.-Les réseaux de "failles polygonales" (failles de compaction restreintes à un intervalle stratigraphique) ont été longtemps considérés comme d'importants chemins pour la migration vers la surface des hydrocarbures à travers d'épaisses séries argileuses. Un exemple classique est fourni par l'offshore profond du bassin du Bas-Congo où de nombreuses structures d'échappement de fluides sont présentes au-dessus d'un système de failles polygonales affectant l'intervalle pliocène. Ce papier présente l'analyse sismique détaillée d'un système récemment reconnu de cheminées linéaires d'âge quaternaire, ainsi que de la géométrie de leurs intersections avec des failles polygonales pliocènes (donc préexistantes). La plupart (73 %) des 209 cheminées analysées recoupent le mur des failles polygonales, et près de la moitié d'entre elles s'enracinent dans l'intervalle sous-jacent à l'intervalle faillé. Cette disposition indique que le fluide (gaz en l'occurrence) a migré verticalement, recoupant les failles pol...

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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.

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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 80 m 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.