Offshore and onshore seepage of thermogenic gas at Katakolo Bay (Western Greece)

Abstract

Considerable seepage of natural gas occurs throughout the Katakolo Bay, both at sea and on land, along the Ionian coast of Peloponnesus (Western Greece). Explosive levels of CH4 and toxic concentrations of H2S accumulating in the ground, pose a severe hazard for humans and tourist infrastructures. A wide offshore and onshore gas survey, including marine remote sensing, underwater exploration by a towed instrumented system, compositional and isotopic analyses, and flux measurements of gas, allowed us to assess that: (a) gas seepage takes place along two main normal faults; (b) offshore side-scan sonographs recorded at least 823 gas bubble plumes over an area of 94,200 m2, at depths ranging from 5.5 to 16 m; (c) offshore and onshore seeps release the same type of thermogenic gas (δ13CCH4~−34 to −36‰); (d) offshore gas showed increased stable carbon isotopic ratio of CO2 and propane, which suggests enhanced biodegradation of hydrocarbons; (e) isotopic data combined with thermogenic gas generation modeling and maturity plots, suggest that the gas is related to a deep Petroleum System with Jurassic carbonate reservoirs, Triassic source rocks, and Triassic evaporites; (f) H2S (δ34S: +2.4‰) is produced by thermochemical sulfate reduction in deep anhydrites, in contact with hydrocarbon-rich carbonates; (g) due to the shallow depth, more than 90% of CH4 released at the seabed enters the atmosphere, consistent with theoretical bubble dissolution models, with a mean plume output of 0.12 kg d−1; total offshore CH4 output was estimated in the range of 33 to 120 t y−1; and (h) in the onshore area at least 50 gas vents in the harbor and a large seep on the adjacent Faros hill, emit in total about 89 t CH4 y−1. Katakolo results to be one of the biggest thermogenic gas seepage zones in Europe.