Soil Gas Analysis Research Articles

Detailed compositional analysis of hydrocarbons in soil gases above multi-horizon petroleum deposits – A case study from western Poland

A surface geochemical survey was carried on in an area with a cluster of petroleum deposits in western Poland. The largest of these accumulations is the Paproć-Cicha Góra dry gas deposit reservoired in the Rotliegend sediments. Other, similar gas deposits are: the Paproć gas field discovered in the top part of the Carboniferous formation and the Paproć W, Paproć N, and Nowy Tomyśl gas fields localized in the Zechstein Limestone reef structures. Moreover, the Paproć gas-condensate and the Jastrzębsko oil deposits were found at shallower depths, in the Main Dolomite formation. This paper aims to analyze the variability of composition and distribution of near-surface gaseous hydrocarbons in relation to hydrocarbon accumulations reservoired in various stratigraphic units at depths. The occurrence of CH4, its heavier homologues and unsaturated hydrocarbons was measured in 267 soil gas samples. The sampling sites were located along six survey lines (A–A, B–B, C–C, D–D, E–E and F–F) at a 200 m spacing. Concentrations of CH4, total alkanes (C2–C4) and total alkenes (C2–C3) reached maxima of 0.26 vol.%, 5.32 ppm and 5.88 ppm, respectively. Soil–gas alkanes heavier than CH4 were interpreted as being derived from subsurface hydrocarbon accumulations. These compounds penetrated to the near-surface zone by means of active and passive microseepage. Source strength of alkanes microseepage from subsurface accumulations to the land surface (determined by the C2H6/C2H4 ratio) is not a factor that substantially changes the patterns of surface anomalies. Analysis of the values of CH4/C2H6 and C2H6/C3H8 coefficients confirmed the presence of compositionally diversified, deep hydrocarbon accumulations. The study revealed that even small accumulations of gas condensate produce very strong surface geochemical signatures, contrasting with dry gas deposits. The reason may be a specific effect of hydrocarbon migration within the existing petroleum system. The Main Dolomite, which is both the source and the reservoir rock in the Fore-sudetic monocline, plays the role of a “filter”, which modifies the flux of gases ascending from deep-seated reservoirs. As a result of such “filtering”, the geochemical signature of hydrocarbons recorded at the surface indicates the dominance of gas condensate and oil–gas accumulations at depths, giving rise to obviously incorrect interpretations. These interpretations were evidenced by compositional analysis of soil gases, which did not explicitly confirm the existence of a dry gas deposit in the Rotliegend reservoir (Paproć-Cicha Góra gas field). On the other hand, the identified anomalies generally reflect the outline of this accumulation. It supports the applicability of surface geochemical surveys to hydrocarbon exploration in the Fore-sudetic Monocline. The presence of the Main Dolomite at shallower depth must be taken into account in the interpretation of data used for positioning of seismically identified Rotliegend structures. The results of this project indicate that the Main Dolomite may obscure the geochemical signature of near-surface gases derived from dry gas accumulations in both the deeper Rotliegend and Carboniferous reservoirs.

LIGHT HYDROCARBONS IN SOIL GAS ABOVE PROSPECTIVE OIL‐ AND GAS‐BEARING STRUCTURES: POMERANIAN SYNCLINORIUM, NW POLAND

The occurrence of methane and heavier homologues and unsaturated hydrocarbons was recorded in 226 soil gas samples recovered from a study area in the Pomeranian Synclinorium, NW Poland. Samples were collected at stations located along three survey lines (I–I', II–II’ and III–III') at a 200 m spacing. Concentrations of methane, total alkanes (C2‐C4) and total alkenes (C2‐C4) reached 13.7 vol. %, 18.4 ppm and 0.56 ppm, respectively. Soil gas alkanes heavier than methane were interpreted to be derived from subsurface hydrocarbon accumulations. These hydrocarbons migrated up into the near‐surface zone along structural discontinuities and fractures which were observed on seismic profiles.The migration rate of hydrocarbons from subsurface accumulations towards the surface was determined by the ethane/ethylene (C2/C2=) ratio. The statistical distribution of the ethane:propane (C2/C3) ratio and plots of the C2/C3+C4 and C1/C2+C3 ratios indicated that accumulations of condensate, gas or oil with a gas cap probably occur in the study area. Variations in normalized values of total alkane C2‐C4 concentrations allowed surface geochemical anomaly zones to be identified. These anomaly zones were evidence for the occurrence of subsurface hydrocarbon accumulations.Hydrocarbon accumulations are most likely to occur beneath the central part of profile II–II' and may also occur in the SW and NE parts of profile III–III' where both the Carboniferous and the Zechstein Main Dolomite are prospective. In these areas, hydrocarbon accumulations may occur in fault‐bounded anticlinal highs.Surface geochemical anomalies also confirm the presence of a non‐commercial hydrocarbon accumulation in the Main Dolomite which was discovered by the D‐1 well, and the possibility of another subsurface accumulation in an adjacent tectonic block. Soil gas analyses combined with seismic data provide evidence for the hydrocarbon prospectivity of the study area.

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.

Light gaseous hydrocarbon anomalies in the near surface soils of Proterozoic Cuddapah Basin: Implications for hydrocarbon prospects

The Proterozoic Cuddapah Basin, South India is grouped under Category IV sedimentary basins of India by Directorate General of Hydrocarbons (DGH), India having possible existence of hydrocarbons. Reconnaissance surface geochemical survey for adsorbed soil gas analysis was conducted in Cuddapah Basin for initial evaluation of hydrocarbon resource potential of the basin. The gas chromatographic analyses of adsorbed soil gases show presence of C 1 to C 4 hydrocarbons. The scatter plots between C 1 to C 4 components depict linear trend indicating that all the gases are derived from the same source and the hydrocarbon ratios of C 3/C 1 ⁎ 1000 between 60–500 and 20–60 suggests thermogenic origin for these gases. Further, XRD analyses demonstrated increased presence of calcite in non-anomalous samples compared to anomalous samples and also lack of correlation between ΣC 2+ and soil TIC-TOC indicates seepage related surface anomalies. An important observation from this study was that the distribution of ΣC 2+ anomalies over Nandhyal and Koilkuntla coincide with the favorable subsurface structural features and thick sediment deposits (6000–9000 m) defined by geophysical methods in these areas, signifying the potential for hydrocarbon generation and entrapment in the basin.

High Purge Volume Sampling—A New Paradigm for Subslab Soil Gas Monitoring

Subslab soil gas sampling and analysis is a common line of evidence for assessing human health risks associated with subsurface vapor intrusion to indoor air for volatile organic compounds; however, conventional subslab sampling methods have generated data that show substantial spatial and temporal variability, which often makes the interpretation difficult. A new method of monitoring has been developed and tested that is based on a concept of integrating samples over a large volume of soil gas extracted from beneath the floor slab of a building to provide a spatially averaged subslab concentration. Regular field screening is also conducted to assess the trend of concentration as a function of the volume removed to provide insight into the spatial distribution of vapors at progressive distances away from the point of extraction. This approach minimizes the risk of failing to identify the areas of elevated soil vapor concentrations that may exist between discrete sample locations, and can provide information covering large buildings with fewer holes drilled through the floor. The new method also involves monitoring the extraction flow rate and transient vacuum response for mathematical analysis to help interpret the vapor concentration data and to support an optimal design for any subslab venting system that may be needed.

Near-surface monitoring for the ZERT shallow CO 2 injection project

As part of a collaborative effort operated by the Zero Emission Research and Technology Center (ZERT), a series of two shallow releases of CO 2 was performed at a test site in Bozeman, MT. The purpose of the experiment was to simulate possible leakage scenarios from a carbon capture and storage operation in order to further develop and verify monitoring technologies used to characterize and quantify the release of CO 2. The project included collaboration with several research groups and organizations. Presented here are the results of soil–gas monitoring conducted by researchers from the National Energy Technology Laboratory, including CO 2 flux measurement, soil–gas analysis, perfluorocarbon tracer monitoring, and soil resistivity measurements. Together, these methods proved to be effective in detecting and characterizing leakage in the near-surface.

The Southern Extension and Reactivations of the Clarendon-Linden Fault System

Gas seeps located near Pike, New York, on the inferred location of the central fault of the Clarendon-Linden Fault System (CLF), were initiated during, or slightly after, the Saguenay earthquake, 1988. Analyses of the gas show that the gas seeps have a Devonian shale source, and a nearby well suggests that the source may be as deep as 330 m. Thus, CLF fractures extending to depths possibly as great as 300+ m were probably opened as a local response to the Saguenay event, allowing the gas to be vented. Published data show that the CLF extends from Lake Ontario to slightly south of Pike. Previously, there were insufficient data available to enable investigators to determine if the fault system continued farther south. Our soil gas survey and analyses of well logs indicate the CLF continues south into Allegany County, which borders the state of Pennsylvania. Closely-spaced wells suggest that the central fault of the CLF is probably a series of step faults, even in units above the Silurian evaporite section, although a monocline cannot be ruled out. Growth-fault geometries of the CLF suggest that the CLF experienced motion during the Taconic and Acadian orogenies. Isopach maps, coupled with the proposed southward extrapolation of the CLF, suggest that in Late Devonian times the CLF motion history recorded the passage of the Appalachian foreland basin axis across the CLF.

Amplified geochemical imaging: an enhanced view to optimize outcomes

Harry S. Anderson of W. L. Gore &amp; Associates describes the benefits of amplified geochemical imaging in a variety of context including environmental and oil and gas applications. Before surgeons begin any delicate operation, they use state-of–the-art imaging to afford an enhanced view into the human body. Without question, the accuracy and detail of a CAT scan or MRI increases their chance of a successful surgical outcome. With a resource so precious, who wouldn’t use the best technology? Likewise today’s earth scientists have sophisticated imaging tools available that provide them with the enhanced view they need to focus their efforts, save time and money, and generate more reserves and profits. 3D seismic imaging is the tool we often think of first in petroleum exploration but recent advances have made possible a complementary technique: amplified geochemical imaging. This advanced geoscience tool is used in diverse applications such as environmental site assessment and pipeline integrity management, as well as petroleum and mineral exploration. Since 1930, earth scientists have used surface geochemical techniques to explore for hydrocarbons. These techniques look for the effects of minute levels of hydrocarbons that migrate through the imperfect seals that cover every reservoir and migrate either as macroseepage via faults or as microseepage vertically through the reservoir overburden. (Klusman, 1993, Coleman et al., 1977) Some of these early techniques were crude and included soil analysis, active soil gas analysis, iodine mapping, and microbial counting. Unfortunately, exploration results using these early geochemical techniques were often disappointing. This is a direct result of several fundamental factors: - Inability of the sampling method to cope with heterogeneous soil characteristics including permeability, moisture, and organic content - Heavy losses in compounds due to sampling techniques (Hewitt and Lukash, 1996) - Monitoring indirect effects (like microbes or iodine) rather than direct effects - Poor sensitivity (ppm rather than ppb or ppt) - Severely limited set of compounds not representative of the target (C1-C6 only) - Failure to use statistical tools to clearly differentiate noise from signal

Using Lidar to Measure Perfluorocarbon Tracers for the Verification and Monitoring of Cap and Cover Systems

Waste site cover systems used to prevent rainfall from reaching the waste need to remain intact throughout the lifetime of the waste site. Monitoring of these covers is needed to ascertain the performance and to determine if any degradation has occurred. Researchers at Brookhaven National Laboratory have used gaseous perfluorocarbon tracers (PFTs) to monitor the integrity of caps and covers for waste disposal sites. Detection of the PFTs currently uses gas chromatography techniques developed at BNL. This paper presents a potential approach to this wide-area screening problem by replacing conventional gas chromatography analysis with laser-based, lidar (Light Detection and Ranging) detection of the PFTs. Lidar can be used to scan the surface of the cover system, looking for fugitive PFTs. If successful this would enable the departure from soil gas analysis and instead look for PFTs in the air just above the soil surface. The advantages of using a lidar platform are multi-fold and include the elimination of soil monitoring ports.

Evaluation of Soil Gas Sampling and Analysis Techniques At a Former Petrochemical Plant Site

Methods for soil gas sampling and analysis are evaluated as part of a research study on soil vapour intrusion into buildings, conducted at a former petro-chemical plant site ("Chatterton site"). The evaluation process was designed to provide information on reliability and selection of appropriate methods for soil gas sampling and analysis, and was based on a literature review of data and methods, and experiments completed as part of the research study. The broader context of this work is that soil gas characterization is increasingly being used for input into risk assessment of contaminated sites, particularly when evaluating the potential intrusion of soil vapour into buildings. There are only a limited number of research studies and protocols addressing soil gas sampling and analysis. There is significant variability in soil gas probe design and sample collection and analysis methods used by practitioners. The experimental studies conducted to evaluate soil gas methods address the permeation or leakage of gases from Tedlar bags, time-dependent sorption of volatile organic compound (VOC)-vapours onto probe surfaces and sampling devices, and analytical and quality control issues for light gas and VOC analyses. Through this work, common techniques for soil gas collection and analysis are described together with implications for data quality arising from the different methods used. Some of the potential pitfalls that can affect soil gas testing are identified, and recommendations and guidance for improved protocols are provided.

Carbon dioxide emissions from soils at Hakkoda, north Japan

A soil gas and CO2 efflux survey was carried out at Hakkoda, north Japan, to investigate the spatial distribution and origin of the gas which caused the accidental death of three Japanese soldiers on 12 July 1997. In the study area, no fumarolic and/or thermal activity is observed. Chemical composition of the gas from areas with high CO2 effluxes (&gt;5,000 g m−2 d−1) and high CO2 contents (&gt;15 vol %) indicate a strong deep sourced contribution for CO2, whereas air and/or air saturated water is suggested for the rest of the discharged gases, which are mainly released from a shallow aquifer. Isotopic analysis of soil gas and hot spring discharges suggests a combination of magmatic degassing and thermal decarbonation, with magmatic degassing being the main contributior to the high CO2 emission. Approximately 74 t d−1 of deep‐seated (magmatic plus thermal decarbonation origin) CO2 were estimated to contribute to the total output of CO2 from this area.

Soils: A global sink of atmospheric carbon tetrachloride

Analysis of soil gas from the world's major biomes indicates that carbon tetrachloride (CCl4) uptake by soils is a ubiquitous process. The global flux of CCl4 from the atmosphere to soils was estimated to be 27 ± 21 kilotons per year (Kt yr−1), corresponding to a partial atmospheric lifetime of 90 years (yr). The total lifetime of CCl4 in the atmosphere, taking into account the stratospheric, oceanic and soil sinks is estimated to be 20 yr. The recently identified oceanic and soil sinks of atmospheric CCl4 suggest that current emission estimates of CCl4 may be underestimated by ∼60%.

Runoff control of soil degassing at an active volcano. The case of Piton de la Fournaise, Réunion Island

Airborne atmospheric and in-situ soil gas analyses demonstrate that distal gas leaks are non-existent at Piton de la Fournaise (PF) despite frequent eruptions. Airborne traverses made at a constant altitude indicated that no detectable large-scale CO 2 degassing occurs from the volcano. In-situ soil gas traverses (CO 2, 4He, 222Rn) across the active rift-zones, the summit crater and the caldera rims, show both a lack of positive anomalies and evidence of strong depleted concentrations for He and CO 2 with respect to air. Depletions in soil He is explained by intense rain water infiltration. A more unexpected result lies in the very low CO 2 concentrations (down to 92 ppm) in bare soils in the summit region. Such a depletion was not previously reported and is attributed to considerable basalt weathering operating as the result of huge runoff. A model has enabled both the runoff and the CO 2 consumption occurring within the basaltic layers to be calculated. Data obtained are consistent with values expected from large-scale erosion models and with the local calculated diffusive flux of CO 2 in soils. The huge runoff, together with the basaltic lithology of the volcano, are found to be the main factor controlling the soil gas signatures at PF.

Detection of buried faults and fractures using soil gas analysis

Abstract Detection of fractures in bedrock, which is essential for hazardous waste site evaluation and for modeling of local ground-water flow in fractured terranes, is difficult where exposure is poor, but may be accomplished using a newly developed method based on soil gas analysis. This method is based upon the accumulation of thermogenic gas in soil above open bedrock fractures. Fractures may provide high permeability pathways through bedrock; if the underlying units contain natural gas, gas will migrate upward through open fractures, producing anomalously high methane concentrations in the soil above. Over ten thousand soil gas analyses from western New York demonstrated that fractures of the Clarendon-Linden Fault System could be traced with soil gas analyses through an area largely covered with unconsolidated glacial deposits. Interpretation of soil gas anomalies as indicators of underlying fractures was confirmed by correlation with outcrop, well log, seismic, and remote sensing data. Samples were collected at a spacing of 10 m, from a depth of approximately 60 cm, using a stainless steel probe driven with a sledgehammer. Analysis of the 60 cc samples was done on-site with a portable gas chromatograph. Ethane/methane ratios were used to distinguish thermogenic gas from biogenic methane. The excellent correlation of linear soil gas anomalies with mapped fractures suggests that where gas-bearing units underlie an area, soil gas analysis may provide a method of mapping faults and fractures where exposure is too poor for conventional mapping.

EXAMINATION OF TREE AND ROOT PERFORMANCE ON CLOSED LANDFILLS IN MERSEYSIDE

Summary A study of the above ground and below ground growth of alder, willow and poplar on four poorly restored landfill sites in Merseyside has been carried out, in conjunction with measurements of physical site characteristics. Soil gas analysis identified the incidence of high methane (CH4) and carbon dioxide (CO2) and low oxygen (O2) concentrations in the root zone at some sites. Statistical analysis revealed that tree roots were inhibited by high soil bulk density. Height growth was also inhibited by soil compaction, and high soil CO2 and low oxygen concentrations. The results demonstrate that satisfactory woodland can only be achieved if adequate physical soil conditions are obtained. However, the study also shows that useful woodlands may develop without a detailed and expensive restoration prescription.

Investigation of fault position and sources of radon by measurement of 238U decay series radionuclide activity in soil samples

North Derbyshire is designated a Radon Affected Area by the National Radiological Protection Board of Great Britain since more than 1% of the housing stock is estimated to have radon levels in excess of the 200 Bq m-3 Action Level. Enhanced radon emissions associated with geological faults make knowledge of their position important in relation to any potential residential or industrial development. A general survey of radionuclides present in the soils of north Derbyshire and their relationship to the underlying geology highlighted the difficulty of identifying the position of geological faults in the field. Using gamma-ray spectroscopy of soil samples the activity of three 238U decay series radionuclides (226Ra, 214Pb and 214Bi), which were taken as evidence of the presence of 222Rn which occurs in the same decay series, was measured to indicate the position of a fault by enhanced activity. The results also provided some evidence of the source of radon emitted at the fault. A comparison of this methodology with the more conventional soil gas analysis method is made.

Field Test of Nonfuel Hydrocarbon Bioventing in Clayey-Sand Soil

A pilot-scale bioventing test was conducted at the Greenwood Chemical Superfund Site in Virginia. The characteristics of the site included clayey-sand soils and nonfuel organic contamination such as acetone, toluene, and naphthalene in the vadose zone. Based on the results of an earlier treatability study, an 80-ft by 80-ft (24-m by 24-m) treatment plot was established in a 35-ft (11-m) vadose zone. Air was injected at a low flowrate for 15 months. Performance monitoring included initial and final soil analysis and periodic soil gas analysis and in situ respiration tests. After beginning aeration, soil gas oxygen levels in the plot rose slowly, reaching 10% at virtually all measured locations in approximately 4 months. In situ respiration rates decreased with time indicating that the site was being cleaned. Soil concentrations of the target contaminants decreased significantly during the test, with > 98% confidence for acetone, naphthalene, benzene, chlorobenzene, and toluene, and > 90% confidence for 1,2-dichloroethane. Almost 80% of the initial mass of the target contamination was acetone. The bioventing system removed approximately 60 kg of acetone. The total oxygen consumed, estimated from respiration rates, was more than 80 times that needed to degrade the mass of contaminants removed as measured by soil analysis. Thus, it is likely that most if not all of the removal of the target compounds was attributable to biodegradation.

Intrinsic bioremediation of gas condensate hydrocarbons

Condensate liquids have been found to contaminate soil and ground water at two gas production sites in the Denver Basin operated by Amoco Production Co. These sites have been closely monitored since July 1993 to determine whether intrinsic aerobic or anaerobic bioremediation of hydrocarbons occurs at a sufficient rate and to an adequate end point to support a no-intervention decision. Ground water monitoring, soil gas analysis, and analysis of soil cores suggest that bioremediation is occurring at these sites by multiple pathways, including aerobic oxidation, sulfate reduction, and methanogenesis. Results of over two years of monitoring of ground water and soil chemistry at these sites are presented to support this conclusion.

Review of direct MS analysis of environmental samples

Direct sampling mass spectrometry (DSMS) is an increasingly popular technique for rapid and sensitive measurement of organic pollutants in air, water, soil, and wastes. Detection limits are typically in the range of 1 ppb for volatile organic compounds (VOCs in water) with little or no sample preparation required and sample analysis times of less than 3 min. Despite the fast sample analysis time and consequent large sample throughput capability, there are numerous situations that would benefit from the ability to continuously monitor the concentration of targeted VOCs in real time. These include incinerator stack emissions, industrial wastewater streams, chemical process streams, vent emissions, exhaust emissions, groundwater and soil remediation process systems, waste-site off gas during remediation, fugitive emissions, hazardous workplace atmospheres, and soil gas analysis with the use of a dynamic depth profiling probe such as a cone penetrometer. During the past year, several pilot studies have been conducted with a DSMS field instrument for various real-time continuous monitoring applications. These studies have included the monitoring of VOCs in an incinerator stack, monitoring of VOCs in pilot-scale photolytic groundwater remediation systems, measurement of automobile exhaust in a moving vehicle, soil gas measurements in conjunction with a cone penetrometer, and in situ measurement of VOCs in groundwater with the use of a special sampling probe. In each instance, the DSMS instrument was configured with special sampling probes that extracted the VOCs from the sample matrix and transported them through an appropriate transfer line into a direct capillary restrictor interface to the ion trap. Targeted compounds were monitored based on unique peaks in the electron impact and proton transfer chemical ionization mass spectra. The real-time detection limits for VOCs in aqueous systems are in the range of 1 ppb, and in gaseous streams detection limits are in the range of approximately 10 ppb by volume. Temporal resolution ranges up to a maximum of 10 full-scan mass spectra per second, which provides the ability to monitor transient events in a sample stream that might be missed by discrete sample collection and analysis. © 1997 John Wiley & Sons, Inc. Field Analyt Chem Technol 1: 251–276, 1997

Some unconventional geophysical methods: A review

The dramatic decrease in oil and gas exploration in the United States since 1986 has renewed the interest of small independent operators in less costly, unconventional techniques. Among these are near‐surface soil gas analysis, radiometric investigations, and magnetometry surveys of shallow Precambrian topography. The purpose of this article is to review the results of original research that I conducted in these areas in the period 1962–80 and to assess the possible value of these methods in modern exploration.