Methane-related authigenic carbonates : implications for seeps and hydrocarbon plumbing systems
Geophysical surveys allowed characterization of gas-charged sediments and targeting of seepage sites. Acoustic and seismic records revealed that at these locations the release of the hydrocarbon-rich fluids mainly occur from reservoirs sited below the seepage sites or from the dissolution of gas hydrates. Both the size of the minerals and the shape of the authigenic carbonate deposits appear to be related to the seepage conditions and the lithology of the enclosing host sediment. The slabs retrieved on the seafloor or on the subsurface from mud volcano structures consisted of micrite-cemented mud breccia and hemipelagic sedimentary units present. At these sites, finer grained sediments (or gas hydrates) appear to buffer the fluid seepage thereby enhancing the precipitation of authigenic carbonate in the more porous layers, forming slab shaped deposits. Most of the irregularly shaped deposits form the studied pockmarks showed micritic/sparitic calcite- and occasionally aragonite-cemented hemipelagic sediments with associated chemosynthetic fauna. These chemoherms arise when fluids seep through the sediment allowing a diffuse colonization of chemosynthetic fauna on the seafloor that becomes gradually cemented. The formation of tubular features in the subsurface seems to occur when methane-saturated impermeable sediments include weakness zones where the seepage of hydrocarbon-rich fluids is concentrated. Irregularly shaped deposits consisting of sparitic, zoned carbonate, devoid of hemipelagic sediments, are typical of settings where fluids seep through open fractures in more consolidated sediments. There is an association of seepage carbonate deposits, and injected sandstones above hydrocarbon reservoirs. The modelling of this plumbing system suggests that similar pathways can be used for the seepage of hydrocarbon-rich fluids and sand injections.