Abstract
Earth’s hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH4) to the atmosphere. While carbon dioxide degassing is typically associated with extensional tectonics, volcanoes, and geothermal areas, CH4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is known only for some case studies at local scale. Here, we perform a global scale geospatial analysis to assess how the presence of hydrocarbon fields, basin geodynamics and the type of faults control CH4 seepage. Combining georeferenced data of global inventories of onshore seeps, faults, sedimentary basins, petroleum fields and heat flow, we find that hydrocarbon seeps prevail in petroleum fields within convergent basins with heat flow ≤ 98 mW m−2, and along any type of brittle tectonic structure, mostly in reverse fault settings. Areas potentially hosting additional seeps and microseepage are identified through a global seepage favourability model.
Highlights
Earth’s hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH4) to the atmosphere
CO2 degassing from volcanic and geothermal areas may have played a climatic role over geological-time scales[7,8] but it appears to represent a minimal component in the present-day global CO2 atmospheric budget; global geo-CO2 emission[1,4] is three orders of magnitude lower than the total CO2 emissions from natural plus anthropogenic sources[9]
Knowing the type of tectonics favouring hydrocarbon seepage is important in the study of potential methane source regions on other planets as Mars, where recent atmospheric CH4 detections have raised the question on what are the tectonic features that may have released the gas[19]
Summary
Earth’s hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH4) to the atmosphere. CO2 degassing from volcanic and geothermal areas may have played a climatic role over geological-time scales[7,8] but it appears to represent a minimal component in the present-day global CO2 atmospheric budget; global geo-CO2 emission (likely < 1000 Mt yr−1)[1,4] is three orders of magnitude lower than the total CO2 emissions from natural plus anthropogenic sources[9]. Whether a specific type of tectonics and fault (such as extensional tectonics for CO2) is needed for CH4 degassing is unknown Addressing this issue is critical to assess the pathways of CH4 release and to identify potential, not yet inventoried, geo-CH4 emission regions, including those hosting the invisible, diffuse exhalations (microseepage)[6,17,18].
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