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Zhou Z.,University of Manchester | Zhou Z.,ETH Zurich | Ballentine C.J.,University of Manchester | Schoell M.,GasConsult International Inc. | Stevens S.H.,Advanced Resources International Inc.
Geochimica et Cosmochimica Acta | Year: 2012

CO 2 sources, sinks and migration mechanisms in natural CO 2 gas fields provide critical analogues for developing the safe application of anthropogenic CO 2 sequestration technologies. Here we use noble gas and carbon isotopes, together with other gases, to identify and quantify the origin, transport and trapping mechanisms of CO 2 in the Late Cretaceous Jackson Dome CO 2 gas deposit (98.75% to 99.38% CO 2). Located in central Mississippi, USA, and producing from >5000m, it is one of the deepest commercial CO 2 gas fields in the world. 10 gas samples from producing wells were determined for their noble gas, chemical and stable carbon isotope composition. 3He/ 4He ratios range between 4.27R a and 5.01R a (where R a is the atmospheric value of 1.4×10 -6), indicating a strong mantle signature. Similar to CO 2 deposits worldwide, CO 2/ 3He decreases with increasing groundwater-derived 20Ne (and 4He). We model several different processes that could account for the Jackson Dome data, and conclude that, similar to other CO 2 dominated deposits, a Groundwater Gas Stripping and Re-dissolution (GGS-R) process best accounts for observed 20Ne/ 36Ar, 84Kr/ 36Ar, CO 2/ 3He, δ 13C(CO 2), 4He, 20Ne and 36Ar. In this context, crustal and magmatic CO 2 components contribute 57% and 43%, respectively. Changes in CO 2/ 3He across the field show that groundwater contact is responsible for up to 75% loss of original emplaced CO 2. δ 13C(CO 2) variance limits the degree of precipitation to be less than 27%, with the remaining CO 2 loss being accounted for by dissolution only. A higher degree of dissolution gas loss and evidence for water contact at the reservoir crest compared to the reservoir flanks is used to argue that CO 2 in this system has not undergone subsequent loss to either dissolution or precipitation since shortly after reservoir filling at over 60Ma. © 2012 Elsevier Ltd.

Etiope G.,Italian National Institute of Geophysics and Volcanology | Etiope G.,Babes - Bolyai University | Schoell M.,GasConsult International Inc. | Hosgormez H.,Istanbul University
Earth and Planetary Science Letters | Year: 2011

The emission of abiotic methane (CH 4) into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries, the Philippines, Oman, New Zealand, and Turkey. Serpentinization produces large amounts of hydrogen (H 2) which in theory may react with CO 2 or CO to form hydrocarbons (Fischer-Tropsch Type synthesis, FTT). Similar mechanisms have been invoked to explain the CH 4 detected on Mars, so that understanding flux and exhalation modality of ophiolitic gas on Earth may contribute to decipher the potential degassing on Mars. This work reports the first direct measurements of gas (CH 4, CO 2) flux ever done on onshore ophiolites with present-day serpentinization. We investigated the Tekirova ophiolites at Çirali, in Turkey, hosting the Chimaera seep, a system of gas vents issuing from fractures in a 5000m 2 wide ophiolite outcrop. At this site at least 150-190t of CH 4 is annually released into the atmosphere. The molecular and isotopic compositions of C 1-C 5 alkanes, CO 2, and N 2 combined with source rock maturity data and thermogenic gas formation modelling suggested a dominant abiotic component (~80-90%) mixed with thermogenic gas. Abiotic H 2-rich gas is likely formed at temperatures below 50°C, suggested by the low deuterium/hydrogen isotopic ratio of H 2 (δD H2: -720‰), consistent with the low geothermal gradient of the area. Abiotic gas synthesis must be very fast and effective in continuously producing an amount of gas equivalent to the long-lasting (>2 millennia) emission of >100tCH 4yr -1, otherwise pressurised gas accumulation must exist. Over the same ophiolitic formation, 3km away from Chimaera, we detected an invisible microseepage of abiotic CH 4 with fluxes from 0.07 to 1gm -2d -1. On Mars similar fluxes could be able to sustain the CH 4 plume apparently recognised in the Northern Summer 2003 (10 4 or 10 5tyr -1) over the wide olivine bedrock and outcrops of hydrated silicates in the Syrtis Major and Nili Fossae; just one seep like Chimaera or, more realistically, a weak, spatially sporadic microseepage, would be sufficient to maintain the atmospheric CH 4 level on Mars. © 2011 Elsevier B.V.

Etiope G.,Istituto Nazionale di Geofi sica e Vulcanologia | Etiope G.,Babes - Bolyai University | Schoell M.,GasConsult International Inc.
Elements | Year: 2014

Abiotic gaseous hydrocarbons comprise a fascinating, but poorly understood, group of Earth fluids generated by magmatic and gas-water-rock reactions that do not directly involve organic matter. At least nine different inorganic mechanisms, including Fischer-Tropsch type reactions, occur over a wide range of temperatures. Trace amounts (typically parts per million by volume) are formed in volcanic and geothermal fluids, but considerable amounts of methane, reaching 80-90 vol%, are now recognized in an increasing number of sites in Precambrian crystalline shields and serpentinized ultramafi c rocks. Surface manifestations of abiotic gas related to serpentinization release gas directly to the atmosphere in ways that are similar to seepages of ordinary biotic gas from petroliferous areas. Abiotic methane is more widespread than previously thought. It also likely exists in sites undergoing active serpentinization and may be present in petroleum systems in the vicinity of serpentinized rocks.

Etiope G.,Italian National Institute of Geophysics and Volcanology | Etiope G.,Babes - Bolyai University | Ehlmann B.L.,California Institute of Technology | Ehlmann B.L.,Jet Propulsion Laboratory | Schoell M.,GasConsult International Inc.
Icarus | Year: 2013

We evaluate, based on terrestrial analogs, the potential flux, origin and isotopic signature of methane (CH4) from serpentinized or serpentinizing rocks on Mars. The Tekirova ophiolites, in Turkey, have been shown to release, either via focused vents or through diffuse microseepage, substantial amounts of CH4 which could be produced via catalyzed abiotic methanation (Sabatier reaction) at low temperatures (<50°C). Serpentinized ultramafic rocks on Mars are likely to have necessary chemical constituents for methane production and fractures for release of gas to the atmosphere, similar to those on Earth. A simple, first-order estimation gas-advection model suggests that methane fluxes on the order of several mgm-2d-1, similar to microseepage observed in terrestrial ophiolites, could occur in martian rocks. High temperature, hydrothermal conditions may not be necessary for abiotic CH4 synthesis on Mars: low temperature (<50°C) methanation is possible in the presence of catalysts like ruthenium, rhodium or, more commonly, chromium minerals, which occur in terrestrial ophiolites as in martian mantle meteorites. The terrestrial analog environment of abiotic microseepage may thus explain production of methane on Mars in the ancient past or at present. The wide range of martian 12C/13C and D/H ratios and the potential secondary alteration of CH4 by abiotic oxidation, as observed on Earth, could result in large isotope variations of methane on Mars. CH4 isotopic composition alone may not allow definitive determination of biotic vs. abiotic gas origin. Using our terrestrial vs. martian analysis as guide to future Mars exploration we propose that direct methane and ethane gas detection and isotopic measurements on the ground over serpentinized/serpentinizing rocks should be considered in developing future strategies for unraveling the source and origin of methane on Mars. © 2012 Elsevier Inc.

Etiope G.,Italian National Institute of Geophysics and Volcanology | Etiope G.,Babes - Bolyai University | Baciu C.L.,Babes - Bolyai University | Schoell M.,GasConsult International Inc.
Chemical Geology | Year: 2011

Methane (CH4) in terrestrial environments, whether microbial, thermogenic, or abiogenic, exhibits a large variance in C and H stable isotope ratios due to primary processes of formation. Isotopic variability can be broadened through secondary, post-genetic processes, such as mixing and isotopic fractionation by oxidation. The highest and lowest 13C and 2H (or D, deuterium) concentrations in CH4 found in various geologic environments to date, are defined as "natural" terrestrial extremes. We have discovered a new extreme in a natural gas seep with values of deuterium concentrations, δDCH4, up to +124‰ that far exceed those reported for any terrestrial gas. The gas, seeping from the small Homorod mud volcano in Transylvania (Romania), also has extremely high concentrations of nitrogen (>92 vol.%) and helium (up to 1.4 vol.%). Carbon isotopes in CH4, C2H6 and CO2, and nitrogen isotopes in N2 indicate a primary organic sedimentary origin for the gas (a minor mantle component is suggested by the 3He/4He ratio, R/Ra~0.39). Both thermogenic gas formation modeling and Rayleigh fractionation modeling suggest that the extreme deuterium enrichment could be explained by an oxidation process characterised by a δDCH4 and δ13CCH4 enrichment ratio (δH/δC) of about 20, and may be accounted for by abiogenic oxidation mediated by metal oxides. All favourable conditions for such a process exist in the Homorod area, where increased heat flow during Pliocene-Quaternary volcanism may have played a key role. Finally we observed rapid variations (within 1h) in C and H isotope ratios of CH4, and in the H2S concentrations which are likely caused by mixing of the deep oxidized CH4-N2-H2S-He rich gas with a microbial methane generated in the mud pool of one of the seeps.We hypothesize that the unusual features of Homorod gas can be the result of a rare combination of factors induced by the proximity of sedimentary organic matter, mafic, metal-rich volcanic rocks and salt diapirs, leading to the following processes: a) primary thermogenic generation of gas at temperatures between 130 and 175°C; b) secondary alteration through abiogenic oxidation, likely triggered by the Neogene-Quaternary volcanism of the eastern Transylvanian margin; and c) mixing at the surface with microbial methane that formed through fermentation in the mud volcano water pool. The Homorod gas seep is a rare example that demonstrates how post-genetic processes can produce extreme gas isotope signatures (thus far only theorized), and that extremely positive δDCH4 values cannot be used to unambiguously distinguish between biotic and abiotic origin. © 2010 Elsevier B.V.

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