Natural Gas Seeps Research Articles

Extreme methane deuterium, nitrogen and helium enrichment in natural gas from the Homorod seep (Romania)

Methane (CH 4) 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 CH 4 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, δD CH4, 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 CH 4, C 2H 6 and CO 2, and nitrogen isotopes in N 2 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 δD CH4 and δ 13C CH4 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 1 h) in C and H isotope ratios of CH 4, and in the H 2S concentrations which are likely caused by mixing of the deep oxidized CH 4–N 2–H 2S–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 δD CH4 values cannot be used to unambiguously distinguish between biotic and abiotic origin.

Origin and flux of a gas seep in the Northern Alps (Giswil, Switzerland)

AbstractNatural gas seeps in the Alpine region are poorly investigated. However, they can provide useful information regarding the hydrocarbon potential of sedimentary Alpine units and related geofluid migration, typically controlled by pressurized gas accumulations and tectonics. A gas seep located near Giswil, in the Swiss Northern Alps, was investigated, for the first time, for molecular and isotopic gas composition, methane flux to the atmosphere, and gas flux variations over time. The analyses indicated that the gas was thermogenic (CH4 > 96%; δ13C1: −35.5‰ to −40.2‰) and showed evidence of subsurface petroleum biodegradation (13C‐enriched CO2, and very low C3+ concentrations). The source rock in the region is marine Type II kerogen, which is likely the same as that providing thermogenic gas in the nearby Wilen shallow well, close to Lake Sarnen. However, the lack of δ13CCO2 and δ13C3 data for that well prevented us from determining whether the Wilen and Giswil seeps are fed by the same reservoir and seepage system. Gas fluxes from the Giswil seep, measured using a closed‐chamber system, were significant and mainly from two major vents. However, a substantial gas exhalation from the soil occurs diffusely in an area of at least 115 m2, leading to a total CH4 output conservatively estimated to be at least 16 tonnes per year. Gas flux variations, monitored over a 1‐month period by a special tent and flowmeter, showed not only daily meteorological oscillations, but also an intrinsic ‘pulsation’ with periods of enhanced flux that lasted 2–6 h each, occurring every few days. The pulses are likely related to episodes of gas pressure build‐up and discharge along the seepage system. However, to date, no relationship to seismicity in the active Sarnen strike‐slip fault system has been established.

Methane emission from natural gas seeps and mud volcanoes in Transylvania (Romania)

AbstractGas seepage from petroleum basins is the second largest natural source of methane to the atmosphere, after wetlands. The uncertainty in global emission estimates should be reduced by extending the flux database which is fundamental for defining the emission factors and the actual area of seepage adopted for up‐scaling. As a contribution to this goal, we report a new seepage data‐set for the Transylvanian Basin, one of the largest natural gas producing regions of Europe, that is characterized by the widespread occurrence of natural leakages of gas at the surface, including at least 73 mud volcanoes and gas seeps. In this study, methane flux was measured using closed‐chambers, from 12 seepage sites, in correspondence with focused gas vents (mud volcano craters, bubbling pools, and flammable gas leaks), in the soil surrounding the vents, and at 15 sites located far from macro‐seep zones but close to gas fields. Fluxes from individual vents (macro‐seeps) were found to reach orders of kg CH4 m−2 day−1 (up to 12 kg m−2 day−1) and diffuse fluxes from soils (miniseepage) were found to be up to a few g CH4 m−2 day−1. Far from seep zones, positive CH4 fluxes (microseepage) may occur locally, typically on the order of tens to hundreds of mg m−2 day−1. The values, as well as the occurrence of seepage even far from vent zones and in mud volcanoes that are apparently extinct, are coherent with results obtained in other countries. Gas fluxes from macro‐seeps and soils may change seasonally, but the interannual variation of the average emission factor was found to be minimal. The total CH4 output for Transylvania macro‐seeps is estimated conservatively to be around 680 t year−1; the total geo‐CH4 seepage emission from the Transylvania petroleum system could be approximately 40 × 103 t year−1, and at least 100 × 103 t year−1 for all Romanian petroleum systems, that is roughly 10% of the total anthropogenic CH4 emission in the country.

Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles controlled by variable currents

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo‐spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship‐born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long‐lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.

A re-evaluation of Beaufort Sea-Mackenzie Delta basin gas hydrate resource potential: petroleum system approaches to non-conventional gas resource appraisal and geologically-sourced methane flux

Research Article| March 01, 2010 A re-evaluation of Beaufort Sea-Mackenzie Delta basin gas hydrate resource potential: petroleum system approaches to non-conventional gas resource appraisal and geologically-sourced methane flux Kirk G. Osadetz; Kirk G. Osadetz Geological Survey of Canada, 3303-33 Street NW, Calgary, AB T2L 2A7 Search for other works by this author on: GSW Google Scholar Zhuoheng Chen Zhuoheng Chen Geological Survey of Canada, 3303-33 Street NW, Calgary, AB T2L 2A7 Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2010) 58 (1): 56–71. https://doi.org/10.2113/gscpgbull.58.1.56 Article history accepted: 04 May 2010 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kirk G. Osadetz, Zhuoheng Chen; A re-evaluation of Beaufort Sea-Mackenzie Delta basin gas hydrate resource potential: petroleum system approaches to non-conventional gas resource appraisal and geologically-sourced methane flux. Bulletin of Canadian Petroleum Geology 2010;; 58 (1): 56–71. doi: https://doi.org/10.2113/gscpgbull.58.1.56 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract Arctic gas hydrates (GH) sequester large volumes of natural gases. New Beaufort Sea-Mackenzie Delta Basin (BMB) permafrost and geothermal gradient maps motivate the revision of the gas hydrate stability zone (GHSZ) and re-evaluation of BMB GH resources using two independent, petroleum system-based resource models. The first model is both deterministic and spatial, and it is based on the quantitative analysis of both GH occurrences and reservoir characteristics. In addition, a spatial density of structural elements parameter that is well correlated to GH occurrences is used as a proxy for petroleum migration into the GH stability zone for the purpose of guiding the interpolation of GH occurrences between wells. The second GH resource model is probabilistic. Like the deterministic resource model, it permits regional resource characterization as a function of reservoir parameters that are potential proxies for technological and economic supply requirements. The total resource potential mapped deterministically is 8.82 × 1012 m3 raw initial natural gas in place (GIP). Geographic resource variations are illustrated by GHs maps exceeding specified characteristics as illustrated by GH saturation (Sgh), which is a potential proxy for reservoir energy. If the average Sgh is either >30% or >50%, then the inferred GH resource volumes are 6.40 × 1012 m3 and 4.59 × 1012 m3 GIP, respectively. The probabilistic GH resource appraisal predicts an expected total resource = 10.23 × 1012 m3 GIP. Similarly, if the average Sgh is either >30% or >50%, then the respective GH resource volumes inferred probabilistically are expected to be 6.93 × 1012 m3 and 4.20 × 1012 m3 GIP, respectively, which is comparable to the deterministically mapped estimates. Methane sequestered in GHs constrains regional, long-term, geologically-sourced methane flux rates. The BMB long-term regional methane flux sequestered in GHs is certainly <4.20 mg/m2/d and more likely not >0.12 mg/m2/d, whereas a single active thermogenic natural gas seep has a discharge of approximately 3.9 × 103 mg/m2/d. Although additional study is required before the atmospheric contribution from BMB geologically-sourced methane can be assessed, it is clear that:much more thermogenic methane migrates into the GHSZ, or higher, than is trapped in the conventional resource;the current atmospheric natural gas flux at point seeps can exceed greatly the long-term regional flux sequestered in the GHSZ, and;the GHSZ is an imperfect trap. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Growth of a non‐methanotroph on natural gas: ignoring the obvious to focus on the obscure

Methanotrophs are well known for their ability to grow on methane in natural gas environments; however, these environments also contain low concentrations of longer-chain-length gaseous alkanes. This mixture of alkanes poses a problem for organisms that might otherwise grow on alkanes ≥ C2 because methane could inhibit oxidation of growth substrates and lead to an accumulation of toxic C1 metabolites. Here, we have characterized the growth of a C2 -C9 alkane-utilizing bacterium, Thauera butanivorans, in conditions containing high concentrations of methane and small amounts (< 3% of total alkane) of C2 -C4 . During such growth, methanol accumulates transiently before being consumed in an O2 -dependent process that leads to the formation of a proton gradient and subsequent ATP generation. In contrast, formaldehyde-dependent O2 consumption is insensitive to uncouplers and does not lead to significant ATP production. This efficient C1 oxidation process that regains much of the energy loss inflicted by oxidizing methane, coupled with an alkane monooxygenase effective at limiting methane oxidation, allows T. butanivorans to grow uninhibited in natural gas environments. Although longer-chain-length gaseous alkane-utilizing organisms have been previously identified to grow in natural gas seepages, the data presented here represent the first detailed characterization of the physiological effects associated with inadvertent methane oxidation by a non-methanotroph, and suggest the presence of a well-evolved series of biochemical processes that allow them to grow in natural gas deposits without the need for developing the unique metabolic machinery characteristic of methanotrophs.

Earth's Degassing: A Missing Ethane and Propane Source

Current emission inventories require an additional "unknown" source to balance the global atmospheric budgets of ethane (C2H6). Here, we provide evidence that a substantial part of the missing source can be attributed to natural gas seepage from petroliferous, geothermal, and volcanic areas. Such geologic sources also inject propane (C3H8) into the atmosphere. The analysis of a large data set of methane (CH4), ethane, and propane concentrations in surface gas emissions of 238 sites from different geographic and geologic areas, coupled with published estimates of geomethane emissions, suggests that Earth's degassing accounts for at least 17% and 10% of total ethane and propane emissions, respectively.

High pressure autothermal reforming in low oxygen environments

Abstract Recent interest in fuel cells has led to the conceptual design of an ocean floor, fuel cell-based, power generating station fueled by methane from natural gas seeps or from the controlled decomposition of methane hydrates. Because the dissolved oxygen concentration in deep ocean water is too low to provide adequate supplies to a fuel processor and fuel cell, oxygen must be stored onboard the generating station. A lab scale catalytic autothermal reformer capable of operating at pressures of 6–50 bar was constructed and tested. The objective of the experimental program was to maximize H2 production per mole of O2 supplied (H2(out)/O2(in)). Optimization, using oxygen-to-carbon (O2/C) and water-to-carbon (S/C) ratios as independent variables, was conducted at three pressures using bottled O2. Surface response methodology was employed using a 22 factorial design. Optimal points were validated using H2O2 as both a stored oxidizer and steam source. The optimal experimental conditions for maximizing the moles of H2(out)/O2(in) occurred at a S/C ratio of 3.00–3.35 and an O2/C ratio of 0.44–0.48. When using H2O2 as the oxidizer, the moles of H2(out)/O2(in) increased ≤14%. An equilibrium model was also used to compare experimental and theoretical results.

Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera

Research Article| December 01, 2007 Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera K.G. Osadetz; K.G. Osadetz Geological Survey of Canada, 3303 - 33 Street NW, Calgary, AB, T2L 2A7, kosadetz@nrcan.gc.ca Search for other works by this author on: GSW Google Scholar C.A. Evenchick; C.A. Evenchick Geological Survey of Canada, Pacific, 625 Robson Street, Vancouver, BC, V6B 5J3 Search for other works by this author on: GSW Google Scholar F. Ferri; F. Ferri British Columbia Ministry of Energy and Mines and Petroleum Resources, PO Box 9323, 6th Floor, 1810 Blanshard Street, Victoria, BC, V8W 9N3 Search for other works by this author on: GSW Google Scholar B. Mayer; B. Mayer Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4 Search for other works by this author on: GSW Google Scholar L.R. Snowdon L.R. Snowdon Snowdon Research Ltd., 4904 Brockington Road NW, Calgary, AB, T2L 1R6 Search for other works by this author on: GSW Google Scholar Author and Article Information K.G. Osadetz Geological Survey of Canada, 3303 - 33 Street NW, Calgary, AB, T2L 2A7, kosadetz@nrcan.gc.ca C.A. Evenchick Geological Survey of Canada, Pacific, 625 Robson Street, Vancouver, BC, V6B 5J3 F. Ferri British Columbia Ministry of Energy and Mines and Petroleum Resources, PO Box 9323, 6th Floor, 1810 Blanshard Street, Victoria, BC, V8W 9N3 B. Mayer Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4 L.R. Snowdon Snowdon Research Ltd., 4904 Brockington Road NW, Calgary, AB, T2L 1R6 Publisher: Canadian Society of Petroleum Geologists Received: 10 Oct 2007 Accepted: 23 Oct 2007 First Online: 02 Mar 2017 Online ISSN: 2368-0261 Print ISSN: 0007-4802 © The Society of Canadian Petroleum Geologists Bulletin of Canadian Petroleum Geology (2007) 55 (4): 337–341. https://doi.org/10.2113/gscpgbull.55.4.337 Article history Received: 10 Oct 2007 Accepted: 23 Oct 2007 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K.G. Osadetz, C.A. Evenchick, F. Ferri, B. Mayer, L.R. Snowdon; Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera. Bulletin of Canadian Petroleum Geology 2007;; 55 (4): 337–341. doi: https://doi.org/10.2113/gscpgbull.55.4.337 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract The owners of Tatogga Lake Resort informed geologists of persistent flammable natural gas seepage into Tatogga Lake, British Columbia. The gas seep was sampled from anomalously unfrozen openings (UTM zone 9, 440378/6396983, NAD27) when the ice cover was generally 15 to 20 cm thick. Samples of this gas are composed of methane (98.2 %) and carbon dioxide (1.8 %). The stable carbon isotopic composition of both gases is consistent with biogenic generation and possibly slight microbial oxidation. The biogenic origin of this natural petroleum seepage suggests that the gas is not associated with crude oil stains and thermogenic petroleum systems in the Bowser Basin and underlying rocks. The seepage source strata and economic potential remain unidentified and obscured by its underwater occurrence. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characteristics of bleaching of sandstone in northeast of Ordos Basin and its relationship with natural gas leakage

Bleaching of sandstone has significant applications to tracing hydrocarbon pathways and evaluating the scale of natural gas seepage. Bleaching of sandstones in the northeast of Ordos Basin is mainly distributed in the Mesozoic Yan’an Formation. Studying on petrology, major elements, REEs and trace elements of bleached sandstones and comparing with adjacent sandstones, combining with geologic-geochemical evidences of gas seepage in the northeast of the basin, the bleached sandstones are formed in the acid environment and reducing fluids. Characteristics of petrology show that the contents of kaolinite are high and the color of margin of ferric oxide minerals is lighter than that of the center. Major elements of sandstone samples show high contents of Al2O3 and low ratio of Fe3+/Fe2+. The TFe2O3 content of the bleached sandstone is lower than that of red rock. REE data show that bleached sandstones have low ΣREE contents and Eu-depleted and slightly Ce-enriched. Trace elements show that the bleached sandstones enrich in Co, deplete in Sr, and slightly enrich in Zr and Hf which are close to the values for the green alteration sandstones, and slightly lower than ore-bearing sandstones. Geochemical characteristics of oil-bearing sandstone in the northern basin suggest that the oil-shows are formed by matured Carboniferous-Permian coal bed methane escaping to the surface, and natural gas in field could migrate to the north margin of the basin. The δ 13C (PDB) and δ 18O(PDB) values of calcite cement in the study area range from −11.729‰ to −10.210‰ and −14.104‰ to −12.481‰, respectively. The δ 13C (PDB) values less than −10‰ imply the carbon sources part from organic carbon. Comprehensive study suggests that the gas leakage has occurred in the northeastern basin, which is responsible for bleaching of the sandstone on top of the Yan’an Formation.

Constraining past global tropospheric methane budgets with carbon and hydrogen isotope ratios in ice

Upon closer inspection, the classical view of the synchronous relationship between tropospheric methane mixing ratio and Greenland temperature observed in ice samples reveals clearly discernable variations in the magnitude of this response during the Late Pleistocene (<50kyr BP). During the Holocene this relationship appears to decouple, indicating that other factors have modulated the methane budget in the past 10kyr BP. The delta13CH4 and deltaD-CH4 of tropospheric methane recorded in ice samples provide a useful constraint on the palaeomethane budget estimations. Anticipated changes in palaeoenvironmental conditions are recorded as changes in the isotope signals of the methane precursors, which are then translated into past global delta13CH4 and deltaD-CH4 signatures. We present the first methane budgets for the late glacial period that are constrained by dual stable isotopes. The overall isotope variations indicate that the Younger Dryas (YD) and Preindustrial Holocene have methane that is 13C- and 2H-enriched, relative to Modern. The shift is small for delta13CH4 (approx. 1 per thousand) but greater for deltaD-CH4 (approx. 9 per thousand). The YD delta13CH4-deltaD-CH4 record shows a remarkable relationship between them from 12.15 to 11.52kyr BP. The corresponding C- and H-isotope mass balances possibly indicate fluctuating emissions of thermogenic gas. This delta13CH4-deltaD-CH4 relationship breaks down during the YD-Preboreal transition. In both age cases, catastrophic releases of hydrates with Archaeal isotope signatures can be ruled out. Thermogenic clathrate releases are possible during the YD period, but so are conventional natural gas seepages.

Origin of the natural gas seep of Çirali (Chimera), Turkey: Site of the first Olympic fire

The Olympos Mountain, located to the west of the city of Antalya (SW Turkey) in the Çiralı (Chimera) area, is well known since 2500 years ago for the occurrence of gas seeps from ophiolites. The area comprises a large allochthonous slab known as the Tekirova ophiolite and is represented by intensely serpentinized ultramafic rocks. The Chimera gas escapes through fracture zones in the ophiolite. The ophiolitic slab is structurally bounded by a carbonate platform, termed the Beydagları autochthon. Emplacement of the ophiolite nappes onto the carbonate platform took place in the Tertiary (Eocene). This study is aimed at determining the origin and possible source rocks of this gas. Several samples of both gas and the ophiolite were collected at different sites to determine the molecular and isotopic composition of the gases and mineralogical composition of the ophiolites. The seep gases contain hydrocarbons up to C 5, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 20%). The gaseous hydrocarbons are dominated by methane (>91%). The stable carbon isotope ratios of methane, ethane and propane are defined with a δ 13C 1 of −12.5‰ to −11.6‰, δ 13C 2 of −23.5‰ to −22.0‰ and δ 13C 3 of −23.7‰ to −21.0‰, respectively. The δD 1 values of methane range from −129‰ to −96‰. XRD measurements on the ophiolite revealed the presence of brucite, hydromagnesite, aragonite, calcite and lizardite-chrysotile mineral assemblage. Two possible sources are determined for the Chimera gas. The first is an abiogenic process of serpentinization within the Tekirova ophiolite, which would result in heavy methane formation. The second are the Upper Paleozoic to Lower Mesozoic organic-rich shales in which methane to pentane hydrocarbons were thermally generated.

Combining spectral signals and spatial patterns using multiple Hough transforms: An application for detection of natural gas seepages

Object detection in remote sensing studies can be improved by incorporating spatial knowledge of an object in an image processing algorithm. This paper presents an algorithm based on sequential Hough transforms, which aims to detect botanical and mineralogical alterations that result from natural seepage of carbon dioxide and light hydrocarbons. As the observed alterations are not unique for gas seepages, these halos can only be distinguished from the background by their specific spatial pattern: the alterations are present as halos that line up along geological lineaments in the shallow subsurface. The algorithm is deployed in three phases: a prior spectral classification followed by two serialized Hough transforms. The first Hough transform fits circles through spectrally optimal matching pixels. Next, the centers of the detected circles are piped into the second Hough transform that detects points that are located on a line. Results show that our algorithm is successful in detecting the alteration halos. The number of false anomalies is sufficiently reduced to allow an objective detection based on field observations and spectral measurements.

Yampi Shelf, Browse Basin, North-West Shelf, Australia: a test-bed for constraining hydrocarbon migration and seepage rates using combinations of 2D and 3D seismic data and multiple, independent remote sensing technologies

The Yampi Shelf on Australia's North-West Shelf is highly prospective, with two discrete hydrocarbon sources producing dry gas and oil. To reduce exploration uncertainty relating to gas flushing and poor top seal capacity, a study was undertaken to characterise hydrocarbon migration in the area. It used a combination of seismic amplitude and structural data integrated with shipboard water column geochemical sniffer (WaSi) data, satellite Synthetic Aperture Radar or SAR data and aircraft-acquired Airborne Laser Fluorosensor (ALF) data. Data were acquired synchronously and in staged programs, to allow both direct comparison and time-series analysis of results. Massive natural dry gas and oil seepage was detected, though the relative abilities of WaSi, SAR and ALF to detect and characterise this seepage were markedly different. The spatial distribution, concentration, and relative composition of the detected seepage were controlled principally by the regional seal's thickness and capacity, rather than by the inherent composition and flux of the migrating hydrocarbons. WaSi preferentially identified gas seepage, often in basin-ward locations, because the high relative permeability of gas favoured its early leakage, even through thick seals. SAR preferentially identified oil seepage, which was episodic and largely restricted to the basin-margin at the regional zero-edge-of-seal, reflecting the low relative permeability of oil, even through thin seals (it leaked ‘late’). ALF principally detected low-level oil seepage from charged traps, and was hence most useful for trap ranking. The ability of these remote sensing tools, as well as that of seismic data itself, to detect hydrocarbons appears critically dependant upon interplays between the relative sensitivity of the assorted tools to detect various hydrocarbon phases and the capacity of the top seal itself. The study has demonstrated that the interactions between geology and hydrocarbon charge are predictable, and that understanding these interactions is crucial for the reliable interpretation of remote sensing data.

Natural oil and gas seeps on the Black Sea floor

Migration of hydrocarbons to the seafloor in the Black Sea occurs via direct seepages, mud volcanoes, and development of fluidized sediment flows (e.g., diapers). Gas migration occurs on the shelf, continental slope, and abyssal plain. Gas hydrates are spatially related to gas accumulations and are present in shallow subsurface sediment layers. Their distribution is controlled by the activity of mud volcanoes. In regions of methane seepages, specific biogeochemical processes related to the activity of methane-oxidizing bacteria are evident. This activity results in the formation of diagenetic minerals (carbonates, sulfides, sulfates, phosphates and other minerals).

Contribution to atmospheric methane by natural seepages on the Bulgarian continental shelf

A regional estimation of the contribution to atmospheric methane by natural gas seepages on the UK continental shelf was undertaken by Judd et al. (Mar. Geol. 137(1/2) (1997) 165). This paper is the second in the series, and provides an estimation of the atmospheric methane flux from Bulgarian Black Sea continental shelf. Potential gas source rocks include Holocene gas-charged sediments, Quaternary peats and sapropels, and deep-lying Palaeocene and Neogene clays, Cretaceous coals, and other sediments of late Jurassic to early Cretaceous age. These cover almost the whole continental shelf and slope and, together with irregularly developed seal rocks and widespread active and conducting faults, provide good conditions for upward gas migration. A total of 5100 line kilometers of shallow seismic (boomer) and echo-sounder records acquired during the Institute of Oceanology's regional surveys, and several detailed side-scan sonar lines, have been reviewed for water column targets. Four hundred and eighty-two targets were assigned as gas seepage plumes. It is estimated that a total of 19,735 individual seeps exists on the open shelf. The number of seeps in coastal waters was estimated to be 6020; this is based on available public-domain data, specific research, and results of a specially made questionnaire which was distributed to a range of “seamen”. More than 150 measurements of the seabed flux rates were made in the “Golden sands” and “Zelenka” seepage areas between 1976 and 1991. Indirect estimations of flux rates from video and photo materials, and a review of published data have also been undertaken. Based on these data, three types of seepages were identified as the most representative of Bulgarian coastal waters. These have flux rates of 0.4, 1.8, and 3.5 l/min. The contribution to atmospheric methane is calculated by multiplying the flux rates with the number of seepages, and entering corrections for methane concentration and the survival of gas bubbles as they ascend through seawater of the corresponding water depth. The estimation indicates that between 45,100,000 (0.03 Tg) and 210,650,000 m 3 (0.15 Tg) methane yr −1 come from an area of 12,100 km 2.

The geological methane budget at Continental Margins and its influence on climate change

AbstractGeological methane, generated by microbial decay and the thermogenic breakdown of organic matter, migrates towards the surface (seabed) to be trapped in reservoirs, sequestered by gas hydrates or escape through natural gas seeps or mud volcanoes (via ebullition). The total annual geological contribution to the atmosphere is estimated as 16–40 Terragrammes (Tg) methane; much of this natural flux is ‘fossil’ in origin. Emissions are affected by surface conditions (particularly the extent of ice sheets and permafrost), eustatic sea‐level and ocean bottom‐water temperatures. However, the different reservoirs and pathways are affected in different ways. Consequently, geological sources provide both positive and negative feedback to global warming and global cooling. Gas hydrates are not the only geological contributors to feedback. It is suggested that, together, these geological sources and reservoirs influence the direction and speed of global climate change, and constrain the extremes of climate.

Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d−1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4–7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10–15 m3 hr−1 or 1.5–2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure‐dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment‐sea interface. The magnitude of this long‐term change cannot be extrapolated from our tidal data.

Finding Abandoned Sumps, Tanks, And Other Oil Facilities

As oil wells and facilities are abandoned and “cleaned up” in California, commercial and public buildings, recreational facilities, and houses are being constructed on some of those sites. The presence of abandoned wells, former production facilities, sumps, tanks, pipelines, natural seeps, and other oil field artifacts are not always detected during a Phase I environmental survey. Due diligence requires that these oil field remnants be located and a determination made as to whether or not they pose a threat to the public, the environment, or the economic viability of a property. One way to find potential problems is the examination of aerial photographs. The problem with aerials is that they are not always taken at the right time. The records and reports of the California Division of Oil, Gas, and Geothermal Resources are needed to find abandoned wells and the limits of oil fields. U.S. Geological Survey topographic quadrangles sometimes show the locations of sumps, tanks, and oil wells but may not be sufficiently accurate to locate them for environmental testing. Unfortunately, accurate records of the locations of buried sumps and facilities have not always been generated. Anecdotal information usually consists of arm waving, rough pacing, and the lament that after the last merger the old facility maps and diagrams were thrown out. Natural oil and gas seeps, although well documented, can be difficult to locate precisely. The result is that the location of all abandoned facilities is not always possible.

Fractured bedrock aquifers may contribute to atmospheric methane

Along with carbon dioxide (CO2), nitrous oxide (N2O),and chlorofluorocarbons (CFCs), methane (CH4) concentrations in the atmosphere have increased markedly since the industrial revolution [Ledley et al, 1999] .These so‐called greenhouse gases, along with water vapor, are generally considered to be the major players in the current global warming trend. Just how much global warming is due to anthropogenic contributions has not been resolvedsemi; however, recent increases in the atmospheric concentration of methane are undeniable. The bulk of the atmospheric methane increase has been attributed to the burning of fossil fuels and biomass, wetlands and flooded crops, natural gas seeps, and emissions from domesticated animals and landfills.