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A common characteristic of the Gulf of Mexico (GOM) seafloor, hydrocarbon seepage produces asphalt volcanism, brine pools, gas hydrates and authigenic carbonates. We studied authigenic carbonates from two areas: Green Canyon (Louisiana slope, 1000mbsl) and the Chapopote diapir (abyssal plain, southwestern GOM, 2900mbsl). Carbonates consist of oily, matrix-supported limestones with variable porosity and a fine-grained matrix of calcite. They occur in Green Canyon as both concretions formed below the seafloor and as slabs of seabed pavements. In each case, the carbonates are mudstones with a mud matrix consisting of high-Mg calcite, disseminated pyrite and detrital quartz. The carbonate slabs have vug microporosity and contain barite that occurs as vug lining. The precipitation of barite results from a reaction between barium-rich and sulfate-free seep fluids and downwards-diffusing seawater. The …