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

Abstract

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.

Giant seabed polygons and underlying polygonal faults in the Caribbean Sea as markers of the sedimentary cover extension in the Grenada Basin

“…Clausen et al (1999) have suggested that PF developed within a broadly polygonal array due to gravitational sliding influenced by far-field tectonic stresses, even if Wrona et al (2017) have recently suggested that such processes played no or an imperceptible role in the growth of this specific North Sea system. However, the hypothesis that layer-parallel displacements of these faults are accommodated by regional extension of the host strata has been confirmed in various basins, such as in the Lower Congo Basin (Gay et al, 2004) or in the Angola basin (Ho et al, 2018), and more recently by field studies showing that a radial extension may account for PFS development (Antonellini & Mollema, 2015;Petracchini et al, 2015).…”

“…> Classical normal faults usually propagate parallel to the principal stress σ1, while hydraulic fractures open in parallel to the intermediate horizontal stress σ2 and against the minimum principal stress σ3 (Cosgrove, 1995). However, PFS do not follow such behavior due to the anisotropy of unlithified cohesive fine-grained sediments and the anisotropic stress attributed to perturbations of the regional stress field can control the PF orientations (Ho et al, 2018). Volumetric contraction is a spontaneous process forming polygons by joints growing from an initiation point in all directions at the same time, such as in cooling lava flows (Aydin & DeGraff, 1988).…”

“…This study also reported that faults are not systematically parallel to the general trending anticline axis suggesting an evolving pattern of PFS influenced by structural paleo-stresses conditions. So, the development of PFS at the seabed could thus be indicative of the present-day strain state within the sedimentary column (Tuckwell et al, 2003;Ho et al, 2018;Jitmahantakul et al, 2020).…”

“…In the NGB, the strain ellipses show a similar minor axis orientation of N40° in both areas A and B, meaning that all polygons are contracting following the same major direction. It is now commonly admitted that such direction of contraction is parallel to the minimum principal stress σ3 (Petracchini et al, 2015;Ho et al, 2018), which represent a direction of extension within the sedimentary column. So, we assume here that the PFI is currently in extension following one major direction of N40°.…”

“…Recent studies have attempted to link PFS with structural context using topology of the faults (Morley & Binazirnejad, 2020 and references therein) and/or rose diagram orientations (Jimahantakul et al, 2020 and references therein). Even if the fault pattern can be influenced by the slope of underlying units, causing different amounts of horizontal stress anisotropy within the faulted units (Gay et al, 2004;Li et al, 2020), the fault orientations are generally used as a direct marker of extension related to local paleo-stress fields during PFS formation (Laurent et al, 2012, Ho et al, 2018. The large extent of acquired data from north to south in the Grenada Basin gives the opportunity to investigate whether these polygonal faults are related to the current state of stress in the sedimentary cover by using a novel approach involving the centre-to-centre method.…”