Soil Gas Methane Research Articles

Soil Gas Measurements of Radon, CO2 and Hydrocarbon Concentrations as Indicators of Subsurface Hydrocarbon Accumulation and Hydrocarbon Seepage

Soil gas measurements of radon (222Rn), CO2, and hydrocarbon concentrations, as well as gamma-ray spectrometry, were conducted at two separate locations to estimate the measurement results for known locations of hydrocarbon accumulations in the subsurface and oil seepage on the surface. The aim of the study was to confirm the applicability of the method for identifying migration pathways (e.g., faults) and to detect possible seepages of hydrocarbons to the surface as well as to investigate possible health issue potential about the soil gas analysis results. Site A investigations were performed with a large number of sampling points to provide sufficient spatial coverage to capture the influence of subsurface lithologic variability as well as the influence of the migration pathway on the measured parameters. For the investigation of site B, sampling points were positioned to reflect the situation between the area above producing hydrocarbon fields and areas with no confirmed accumulation. The results presented show that it is possible to distinguish the near-surface lithology (gamma-ray spectrometry), characterize the migration pathway, and indicate the area of oil seepage at the surface. Areas above the known hydrocarbon accumulations generally have elevated radon concentrations and detectable heavier hydrocarbons with sporadic methane in soil gas, which contrasts with the lower radon levels and lack of detectable heavier hydrocarbons in soil gas in the area with no confirmed hydrocarbon accumulation in the subsurface.

Methane in Soil Gas and Its Migration to the Atmosphere in the Dawanqi Oilfield, Tarim Basin, China

Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in hydrocarbon-prone basins. In this study, by analyzing gas concentrations and isotope composition of soil gas, the potentials of CH4 gas transferred to the surface were studied at three measurement transects in Dawanqi oilfield, Tarim Basin, China. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures, and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. Soil gas samples had mean concentrations of 907.1, 62.3, 21.7, 11.0, and 5.8 ppmv for CH4, C2H6, C3H8, C4H10, and C5H12, respectively. The C1/C2+ (13.3 for soil gas and 3.75 for absorbed gas) and gas wetness ratio (12% for soil gas and 26% for absorbed gas) suggested that the hydrocarbons were derived from a thermogenic process. According to isotope composition analysis, the δ13CCO2, δ13CCH4, and δDCH4 values for the soil gas from Dawanqi oilfield varied from -15.5 to -17.2‰, -11‰ to -17‰, and -150 to -189‰, respectively. The extreme 13C enrichment in CH4 is possibly because of the fractionation effects of diffusional migration and methanotrophic oxidation. Soil gas and absorbed gas showed high CH4 concentrations at the edge of the fault block, which indicated that fault was conductive to gas migration. Also, gas migrated from the surface to the atmosphere in the center region of the fault block because of the high permeability and shallow depth of the reservoir in Dawanqi oilfield.

Integrated surface geochemical studies for hydrocarbon prospects in Deccan Syneclise, India

The presented study reports the geochemical investigation for hydrocarbon prospects in western and central regions of Deccan Syneclise to evaluate the near surface manifestations of hydrocarbons due to microseepage; identify the sources of light hydrocarbon gases and integrate the results with geophysical data to demarcate the prospective areas for hydrocarbons. The adsorbed soil gas analysis of soil samples from western and central regions show the presence of C1–C4 hydrocarbons with concentrations of C1 and C2+ varying from 2 to 626ppb, 0–261 ppb and 4–255ppb, 0–114ppb respectively. The propane oxidizing bacteria (POB) ranged between 1.0×102 and 2.65×105cfu/g of soil. The evaluation of compositional ratios of adsorbed soil gases indicates that the gas components are co-genetic and migrated from subsurface hydrocarbon accumulations/organic rich beds. The carbon isotopic compositions of adsorbed soil gas methane from western region of Deccan Syneclise range from −29.87 to −13.09‰ (VPDB) representing near surface oxidation of gases subsequent to migration and are derived from gas producing type III kerogen. The δ13C signatures of soil carbonates from central region of Deccan Syneclise falling in the range of −9.61 to −5.1‰ (VPDB) indicate diagenetic alteration of seeping hydrocarbon gases in the surface soils. The adsorbed light gaseous hydrocarbon, propane oxidizing bacteria and soil carbonate anomalies are representative of samples from Kopargaon, Shirdi, Rampur and Newasa in western region and Edlabad, Popetkheda and Khandwa in central regions of Deccan Syneclise. These anomalies are in good agreement with thick subtrappean Mesozoic sediments of up to 2km delineated by the magnetotelluric (MT) and gravity studies in Deccan Syneclise, which might have sourced the hydrocarbon gases. The anomalous areas are also associated with Barwani-Sukta, Tapti and Gaviligarh faults which might have provided migratory pathways for light gaseous hydrocarbons. The geochemical results of this study combined with the geophysical data show evidence of hydrocarbon potential of the study area.

Methane in soil gas and its transfer to the atmosphere in the Yakela condensed gas field in the Tarim Basin, Northwest China

In this study, by analyzing CH4 concentration and in soil-gas profiles, the potentials of CH4 gas transfer from ground to atmosphere were studied at four representative sectors in the Yakela condensed gas field in the Tarim Basin, Xinjiang, China. These are: 1) the oil-gas interface sector, 2) fault sector, 3) oil-water interface sector, 4) an external area. Variation in CH4 in soil-gas profiles showed that CH4 microseepage resulted from the migration of subsurface hydrocarbon from deep-buried reservoirs to the earth’s surface. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. The lowest level of CH4 at the soil-gas profile was found at the CH4 gas-phase equilibrium point at which the CH4 migration upwards from deep-buried reservoirs and the CH4 diffusion downwards from the atmosphere met. The and ethane, propane in soil gas exhibited thermogenic characteristics, suggesting the occurrence of CH4 microseepage from deep-buried reservoirs. A linear correlation analysis between CH4 concentrations in soil gas and temperature, moisture, pH, Eh, Ec and particle size of soil indicated that both soil Eh and soil temperature could affect CH4 concentration in soil gas while soil pH could indirectly influence soil methanotrophic oxidation via impacting soil Eh.

A New Screening Method for Methane in Soil Gas Using Existing Groundwater Monitoring Wells

Methane in soil gas may have undesirable consequences. The soil gas may be able to form a flammable mixture with air and present an explosion hazard. Aerobic biodegradation of the methane in soil gas may consume oxygen that would otherwise be available for biodegradation of gasoline hydrocarbons. The consumption of oxygen by methane may increase the chance of completing a vapor intrusion pathway for benzene. A protocol was developed to sample soil gas from conventional groundwater monitoring wells that had some portion of their screen in the vadose zone. This protocol was applied at 12 gasoline stations in Oklahoma. The soil gas as collected from the monitoring wells was not flammable, due to low concentrations of oxygen. However, soil gas at five of the 12 sites could form flammable mixtures in air. To allow a simple comparison of the possible effect of methane on vapor intrusion of benzene, characteristics of the sites were matched to computer simulations published by others. At three of 11 sites, the methane in the soil gas might cause unacceptable concentrations of benzene in a hypothetical receptor. Ethanol is readily fermented to methane. The increased use of ethanol in gasoline raises concerns about methane at gasoline spill sites. At five sites with high concentrations of methane, the concentration of 14C in methane was used to determine the source of the methane. At four sites, the majority of the methane was produced from anaerobic biodegradation of petroleum, and not from ethanol or another biofuel. At three sites, the maximum possible contribution of methane from ethanol or another biofuel was 5.1% of total methane. At a fourth site, the maximum contribution of ethanol or another biofuel to methane was 31% of the total methane. At the fifth site, the methane came from a leak of natural gas.

Near surface manifestation of hydrocarbons in Proterozoic Bhima and Kaladgi Basins: Implications to hydrocarbon resource potential

Reconnaissance surface geochemical survey for adsorbed soil gas analysis conducted in Proterozoic Bhima and Kaladgi Basins, have revealed occurrence of anomalous concentrations of light gaseous hydrocarbons i.e. C1 to C4 (CH4, C2H6, C3H8, i-C4H10 and n-C4H10) in the near surface soils. The concentrations of C1 and ΣC2+(C2H6+C3H8+ i-C4H10+ n-C4H10) in Bhima and Kaladgi Basins are in the range of 1–2594 ppb and 1 to 57 ppb and 1–1142 ppb and 1–490 ppb, respectively. The carbon isotopic data of adsorbed soil gas methane in few selected samples are in the range of −29.9 to −39‰ (PDB). The evaluation of adsorbed soil gas data indicates that all the gas components are cogenetic and hydrocarbon ratios of C1/(C2+C3) < 10 and C3/C1*1000 between 60–500 and 20–60 suggest that the adsorbed gases are derived from oil and gas-condensate zones. The carbon isotopic values of methane further support thermogenic origin of these migrated gases. The concentration distribution of C1 and ΣC2+ in the study areas illustrate C1 and ΣC2+ anomalies near Katamadevarhalli, Andola and Talikota in Bhima Basin and near Kaladgi, Lokapur and north of Mudhol in Kaladgi Basin. The hydrocarbon anomalies near the surface coincide with the favourable subsurface structural features and correlate with existing geochemical and geophysical data in these basins suggesting seepage related anomalies.

Soil Gas Methane at Petroleum Contaminated Sites: Forensic Determination of Origin and Source

Hydrocarbon vapors associated with spilled petroleum products arouse regulatory concern and can pose a significant health and safety risk. While petroleum products do not contain a significant amount of methane (CH4), high CH4contents in soil gas near petroleum spills have been reported. While CH4is nontoxic, its accumulation in shallow soil gas represents a potential explosion and asphyxiation hazard, especially in confined spaces. Identifying the source and origin of shallow CH4accumulations is an important part of evaluating potential exposure pathways, selecting appropriate remedial measures, and determining environmental liability. This paper discusses the potential nature and anthropogenic sources for shallow CH4and how integration of geological, geochemical, and land use data can be used to determine its origin and identify its source. Two case studies are presented, one where CH4associated with a gasoline spill is shown to be derived from a natural source rather than the gasoline, and a second where CH4associated with spilled crude oil is shown to be produced in the vadose zone by biodegradation of the oil.

Measurements of Soil Gas and Atmospheric Methane Content on One Active and Two Inactive Landfills in Wood County, Ohio

Methane concentrations (by volume) in soil gas and in the atmosphere were measured over several days in February and March of 1996, at several on-site stations on three solid waste landfills in Wood County, Ohio. The lower detection limit for methane in soil gas was 0.2 percent (2,000 ppm) and in the atmosphere was 0.1 percent (1,000 ppm). The oldest site, Asman9s Landfill (1962–1973), contained no atmospheric methane at or above the detection limit and had the second highest methane content in soil gas of the three landfills. Wales Road Landfill (1950s–1994) had the highest soil gas methane concentration, as high as 96 percent (960,000 ppm) methane, and had some detectible atmospheric methane (as much as 4,000 ppm) on two occasions over one station. Wood County Landfill (1972-present) had the lowest average soil gas methane content of the three landfills, but the highest atmospheric methane (as much as 8,000 ppm), although all of the methane concentrations at or above the detection limit were measured near four passive vents in the landfill, where soil gas methane content was below the detection limit. Methane concentration in soil gas increased with increasing barometric pressure, but the relationship between atmospheric methane concentration and barometric pressure appears to be more complex, from a few data points for atmospheric methane concentration above the Wood County and Wales Road Landfills. The Wood County Landfill would make an excellent test site for a methane imaging experiment that employs airborne multispectral remote sensing.

Methane Associated with a Large Gasoline Spill:Forensic Determination of Origin and Source

Abstract Hydrocarbon vapors associated with spilled petroleum products can arouse regulatory concern and pose a significant health and safety risk. Identifying the source and origin of these vapors is an important part of evaluating potential exposure pathways, selecting appropriate remedial measures, and determining environmental liability. Although gasoline does not contain a significant amount of methane, high methane contents in soil gas near petroleum spills have been reported. An investigation of the distribution and origin of soil gas methane was conducted at the location of a large gasoline spill in an urban area. During routine monitoring of the soil vapor recovery system, methane was observed to comprise a significant component of the influent vapor stream. The methane was originally interpreted to have formed by bacterial degradation of the spilled gasoline. However, the persistence of the methane over time raised new questions about its origin and lead to the current investigation. A survey of the composition of subsurface vapors showed high methane and carbon dioxide concentrations both within and well beyond the limits of the gasoline spill. Site investigations also found that spilled gasoline was not the only subsurface carbon source present. Lake sediments and substantial quantities of wood and sawdust were also encountered. Stable carbon and hydrogen isotope ratios indicate that the methane is biogenic in origin and formed by the acetate fermentation pathway. However, these data did not allow identification of the source of the methane. Carbon-14 (14C) analysis of onsite and offsite methane samples provided the strongest indication of the methane source. Petroleum and its derivative products contain no 14C and are said to have a 14C concentration of 0 percent modern carbon (pMC). Consequently, methane derived from degradation of gasoline also has a 14C concentration of 0 pMC. Methane in soil gas samples from beneath and adjacent to the site had 14C concentrations much higher than 0 pMC. 14C concentration averaged 86 pMC, demonstrating that the spilled gasoline is, at most, a minor contributor to the elevated concentration of methane in the subsurface. It is likely that the underlying lake sediments and/or wood debris are instead the predominant source of the methane. The findings of this investigation lead to a substantial reduction in site liability and costs and show that methane in association with spilled petroleum products is not necessarily derived from degradation of those products.

Landfills as atmospheric methane sources and sinks

Abstract Sanitary landfills are recognized as globally significant sources of atmospheric methane, but field measurements are rare. Existing country-specific landfill emissions have been estimated from solid waste statistics and a series of assumptions regarding methane generation and emission rates. There has been no attempt to reconcile the national and global estimates with limited field data on landfill methane emissions which range over six orders of magnitude (Bogner and Scott, 1995). This paper addresses controlled field measurements of methane emissions at sites in Illinois and California (USA) using a closed chamber technique. Overall, observed rates from various controlled monitoring experiments during 1988–1994 ranged from 0.003 to more than 1000 g CH 4 m −2 d −1 . Rates were related to the presence or absence of gas recovery wells, physical properties of cover soils (texture, moisture, and temperature) relating to their aeration status for diffusional flux, and rates of methane oxidation by indigenous methanotrophs. Surprisingly, at the Illinois site during spring, 1994, the landfill surface was consuming atmospheric methane rather than emitting landfill methane. This was attributed to high capacities for methane oxidation in well-aerated soils which had reduced landfill methane compared to 1993, the result of an effective pumped gas recovery system. Three independent methods confirmed that the landfill cover soils were functioning as a methane sink: (a) static closed chamber measurements yielding negative flux rates (uptake of atmospheric methane); (b) rates of methane oxidation similar to chamber results from in vitro field incubation studies using ambient methane; and (c) a reversal in the soil gas methane concentration gradient at the 25 cm depth. Field verification of landfill cover soils functioning as methane sinks has profound implications for revision of landfill contributions to global methane budgets; furthermore, it should be feasible to develop mitigation strategies incorporating a combination of engineered and natural methanotrophic controls.

Landfills as atmospheric methane sources and sinks

Abstract Sanitary landfills are recognized as globally significant sources of atmospheric methane, but field measurements are rare. Existing country-specific landfill emissions have been estimated from solid waste statistics and a series of assumptions regarding methane generation and emission rates. There has been no attempt to reconcile the national and global estimates with limited field data on landfill methane emissions which range over six orders of magnitude (Bogner and Scott, 1995). This paper addresses controlled field measurements of methane emissions at sites in Illinois and California (USA) using a closed chamber technique. Overall, observed rates from various controlled monitoring experiments during 1988–1994 ranged from 0.003 to more than 1000 g CH 4 m −2 d −1 . Rates were related to the presence or absence of gas recovery wells, physical properties of cover soils (texture, moisture, and temperature) relating to their aeration status for diffusional flux, and rates of methane oxidation by indigenous methanotrophs. Surprisingly, at the Illinois site during spring, 1994, the landfill surface was consuming atmospheric methane rather than emitting landfill methane. This was attributed to high capacities for methane oxidation in well-aerated soils which had reduced landfill methane compared to 1993, the result of an effective pumped gas recovery system. Three independent methods confirmed that the landfill cover soils were functioning as a methane sink: (a) static closed chamber measurements yielding negative flux rates (uptake of atmospheric methane); (b) rates of methane oxidation similar to chamber results from in vitro field incubation studies using ambient methane; and (c) a reversal in the soil gas methane concentration gradient at the 25 cm depth. Field verification of landfill cover soils functioning as methane sinks has profound implications for revision of landfill contributions to global methane budgets; furthermore, it should be feasible to develop mitigation strategies incorporating a combination of engineered and natural methanotrophic controls.

Factors controlling the concentration of methane and other volatiles in groundwater and soil-gas around a waste site

The concentration of methane in groundwater and soil-gas in the vicinity of a waste landfill on an unconfined sand aquifer has been investigated in detail. These data have been used to evaluate techniques which use volatile organic compounds in soil-gas as indicators of groundwater contamination. Simple one-dimensional models of gas advection and diffusion have been adapted for use in the study. Lateral advection of gas in the unsaturated sand was found to be seasonal and was most noticeable in winter when the profile was wet; a mean velocity of 1 m d − was measured from breakthrough of a helium tracer in an injection test. The effects of advection on trace concentrations of methane in soil-gas were limited to within 150–200m from the waste site and resulted from pressure gradients brought about by positive gas pressures in the landfill, and also as a result of ebullition (gas bubbling) from contaminated groundwater. The distribution of methane in soil-gas at shallow (2m) depth gave a general indication of the direction of movement of contaminants with groundwater in close proximity to the landfill. Outside this zone, diffusional transport of methane from groundwater to soil-gas occurred and methane in soil-gas sampled close to the water table was found to be a useful indicator of contaminated groundwater. Modelling the exchange of volatiles between aqueous and gas phases indicates that a wide range of organic compounds, particularly those with Henry's Law constants greater than 2.5 × 10 t-2 kPam 3mol −1, would have potential for use as indicators of pollution, if these were present in groundwater and they behaved relatively conservatively. In general, the principal factors controlling the concentration of these volatiles in soil-gas were the concentration gradient at the water table and capillary fringe and the ratio of diffusion coefficients in the saturated and unsaturated zones.