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

详细信息    查看全文

• 作者：G. Etiope^a ; ^b ; D. Christodoulou^c ; ^{dchristo@upatras.gr} ; S. Kordella^c ; G. Marinaro^a ; G. Papatheodorou^c
• 关键词：Gas seeps ; Thermogenic methane ; Bubble plumes ; Hydrocarbon biodegradation
• 刊名：Chemical Geology
• 出版年：2013
• 出版时间：15 February, 2013
• 年：2013
• 卷：339
• 期：Complete
• 页码：115-126
• 全文大小：2452 K

文摘

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 (¦Ä¹³C_CH4 ~ ? 34 to ? 36¡ë); (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 (¦Ä³⁴S: + 2.4¡ë) 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.