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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 80m 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., SCOPUS: ar.j, info:eu-repo/semantics/published

Methane cold seep systems typically exhibit extensive buildups of authigenic carbonate minerals, resulting from local increases in alkalinity driven by methane oxidation. Here, we demonstrate that modern seep authigenic carbonates exhibit anomalously low clumped isotope values (Δ47), as much as ∼0.2‰ lower than expected values. In modern seeps, this range of disequilibrium translates into apparent temperatures that are always warmer than ambient temperatures, by up to 50 °C. We examine various mechanisms that may induce disequilibrium behaviour in modern seep carbonates, and suggest that the observed values result from several factors including kinetic isotopic effects during methane oxidation, mixing of inorganic carbon pools, pH effects and rapid precipitation. Ancient seep carbonates studied here also exhibit potential disequilibrium signals. Ultimately, these findings indicate the predominance of disequilibrium clumped isotope behaviour in modern cold seep carbonates that must be considered when characterizing environmental conditions in both modern and ancient cold seep settings., The geochemistry of methane cold seep carbonates is often used to reconstruct environmental conditions. Loyd et al. find disequilibrium clumped isotope compositions in modern seep carbonates that suggest temperatures up to 50°C too high, raising doubt on seep carbonate temperature reconstructions.

Some 31 separate mounds have been identified from seismic reflection records located in a relatively narrow east-west swath across the Porcupine Basin off western Ireland in water depths ranging from 650m to 1000m. The isolated reef-like features are found on a generally smooth and unlithified seabed. Their size is impressive, often being over 1km across and over 100m high.

Shallow geophysical anomalies at Nyegga on the continental slope off Mid-Norway were discovered in the 1980’s. The anomalies were first recognized to be associated with a regional BSR and acoustic ‘bright spots’ associated with the scar of the Storegga Slide. Subsequently, surface depressions, mounds, and sub-surface columnar features, ‘pipes’ (chimneys) were discovered. ROV surveys documented large crusts and blocks of methane derived carbonates, irregular shaped pockmark craters, and an exotic fauna which included giant sea spiders, ophiurids, stalked crinoids, basket stars, and various kinds of fish. The academic research at Nyegga culminated with the acquisition of geochemical samples, high-resolution 3D-seismic tomography (2006) and P-cable 3D-reflection seismic data. In summary, the data suggest that there is both pervasive and focused fluid flow occurring at Nyegga. The migrating fluids involve thermogenic hydrocarbons. However, despite there being abundant topographic, geophysical, and geochemical evidence of active seepage, no water column ‘flares’ have so far been reported from the area. Analysis of buried and surfacing ‘chimneys’ suggests that they first form by vigorous escape of over-pressured fluids followed by continuous micro-seepage. The most likely forcing of the fluid escape flux variation seems to be associated with former cyclic glaciations.