Soil Gas Geochemistry Research Articles

Applications in utilizing soil gas geochemistry along with geological and geophysical data to construct helium exploration statistical models

Applications in utilizing soil gas geochemistry along with geological and geophysical data to construct helium exploration statistical models

Learning from soil gas change and isotopic signatures during 2012 Emilia seismic sequence

Soil surveys were performed in Medolla (Italy), a peculiar area characterized by spotty high soil temperature, gas vent, and lack of vegetation, to determine the migration mechanisms and spatial behavior of gas species. Hereby we present soil gas measurements and their isotopic ratios measured between 2008 and 2015, including the 2012 Emilia-Romagna seismic sequence. We found that soil gas concentrations markedly changed during the main shocks of May 20 and 29, 2012 (Mw 6.1 and 6.0, respectively), highlighting the presence of a buried fault intersecting the gas vents. We suggest that crustal dilation associated with seismic activity favored the uprising of geogas towards the surface. Changes in the isotopic signature highlight the contribution of two distinct sources, one deeper, thermogenic and another superficial related to organic-rich layer, whose relative contribution varied before, during and after the earthquake. We suppose an increase of microbial component likely due to the ground shaking of shallower layers linked to seismic sequence, which masks the thermogenic contribution. Although the changes we detect are specific for an alluvial plain, we deduce that analogous processes may be active elsewhere, and that soil gas geochemistry represents an useful tool to discriminate the gas migration related to seismic activity.

Study on active tectonic faults using soil radon gas method in Viet Nam

Study on active tectonic faults using soil radon gas method in Viet Nam

Application of diffuse gas flux measurements and soil gas analysis to geothermal exploration and environmental monitoring: Example from the Reykjanes geothermal field, SW Iceland

Soil gas geochemistry and diffuse CO2 flux from soil, together with gas concentrations in 7 production wells and 10 steam vents in the Reykjanes geothermal field (Iceland), were studied with the aim of: 1) studying the potential use of the CO2/He ratio in soil gas to constrain CO2 retention in calcite in the bedrock; and 2) quantifying the effect of 100 MWe power production from the Reykjanes geothermal reservoir on surface activity. The presence of biogenic CO2 in the soil gas prevented the use of the CO2/He ratio for the quantification of CO2 retention in calcite. The CO2/He ratio in about 20 soil gas samples was, however, low and consistent with CO2 retention. Our results suggest that detailed soil gas and soil temperature mapping may provide excellent information for defining drilling targets. The surface activity in the Reykjanes geothermal field increased noticeably after the commissioning of the 100 MWe Reykjanes Power Plant in 2006. When increased steam flow from steam vents is taken into account the total increase in surface activity in Reykjanes between 2004 and 2007 is of the order of 50%.

Diffuse Helium and Hydrogen Degassing to Reveal Hidden Geothermal Resources in Oceanic Volcanic Islands: The Canarian Archipelago Case Study

We report herein the results of soil gas geochemistry studies, focused mainly on nonreactive and/or highly mobile gases such as He and H2, in five mining licenses at Tenerife and Gran Canaria, Canary Islands, Spain, during 2011–2014. The primary objective was to sort the possible geothermal potential of these five mining licenses, thus reducing the uncertainty inherent to the selection of the areas with highest geothermal potential for future exploration works. By combining the overall information obtained by the statistical–graphical analysis of the soil He and H2 data, the spatial distribution of soil gas concentrations and the analysis of selected chemical ratios of the soil gas to evaluate the influence of deep-seating degassing, two of the five mining licenses (Garehagua and Abeque, both located in Tenerife Island) seemed to show the highest geothermal potential. These results will be useful for future implementation and development of geothermal energy in the Canaries, the only Spanish territory with potential high-enthalpy geothermal resources, thus the most promising area for high-enthalpy geothermal installations.

Statistical approach for the geochemical signature of two active normal faults in the western Corinth Gulf Rift (Greece)

Soil–gas measurements of different gas species were performed in two distinct areas of the Corinth Gulf Rift (Greece): the Aigion-Neos Erineos-Lambiri (ANEL) fault zone and the Rion-Patras fault zone. Both zones lie in one of the most seismically active areas of the Euro-Mediterranean region, where a fast-opening continental rift is located. In particular, the geochemical investigations were focused on fault segments and fracture systems previously inferred by geomorphological, lithological and structural studies.In this work the applicability of soil–gas geochemistry surveys for the exploration of buried/hidden faults was tested by using various statistical methods. Moreover, a comprehensive geostatistical treatment of the collected data provided new insights into the control exerted by active structures on deep-seated gas migration towards the surface. In both investigated areas, the highest 222Rn and CO2 concentration peaks correspond with zones where the interaction among fracture and fault segments was inferred by structural and morphological methods. This indicates a clear correlation between the shape and orientation of the anomalies and the different attitude and kinematic behavior of the faults recognized in the two areas. Furthermore, obtained results show that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222Rn in the soil pores.

Gas emission from diffuse degassing structures (DDS) of the Cameroon volcanic line (CVL): Implications for the prevention of CO2-related hazards

In the mid-1980s, lakes Nyos and Monoun violently released massive gas, mainly magmatic CO2 killing about 1800 people. Subsequent geochemical surveys and social studies indicate that lakes Nyos and Monoun event is cyclic in nature and may occur anywhere in the about 37 other volcanic lakes located in the corridor of the Cameroon volcanic line (CVL). This potential threat motivated us to check if, alike Nyos and Monoun, the internal dynamic of the other lakes is also controlled by inputs of deep-seated-derived CO2 and attempt to measure and provide comprehensive insights on the passive gas emission along the CVL. This knowledge shall contribute to the prevention of volcanic lake-related hazards in Cameroon and the refinement of the Global Carbon Cycle. We used in situ fixation and dry gas phase sampling methods to determine CO2 origin and the concentration, and the accumulation chamber technique to measure diffuse CO2 emission from nine lakes and on soil at Nyos Valley and Mount Manenguba Caldera. The results suggest that, although in minor concentrations (compared to Nyos and Monoun), ranging from 0.56mmolkg−1 to 8.75mmolkg−1, the bottom waters of some lakes also contain measurable magmatic CO2 with δ13C varies from −4.42‰ to −9.16‰ vs. PDB. That finding implies that, under certain circumstances, e.g. increase to volcanic and/or tectonic activities along the CVL, the concerned lakes could develop a Nyos-type behavioural scheme. The diffuse gas emission results indicate that the nine surveyed lakes release approximately 3.69±0.37ktkm−2yr−1 of CO2 to the atmosphere; extrapolation to the approximately 39 volcanic lakes located on the CVL yields an approximate CO2 output of 27.37±0.5ktkm−2yr−1, representing 0.023% of the global CO2 output from volcanic lakes. In addition to the precedent value, the gas removal operation in lakes Nyos and Monoun released approximately 2.52±0.46×108molkm−2yr−1 CO2 to the atmosphere from January 2001 to March 2013, more than double the per-area CO2 released by the Yellowstone volcanic system. The CO2 emission from soil was estimated to be 4.57±1.30ktkm−2yr−1; the soil gas geochemistry of the Mount Manenguba Caldera also indicates a dominant magma-derived CO2 (mean δ13C=−8.6‰ vs. VPDB).

Spatial Variations of Soil Gas Geochemistry in the Tangshan Area of Northern China

The concentrations of Hg, Rn, H2, He and CO2 in soil gases at 756 sites were measured in the Tangshan area where Ms 7.8 earthquake occurred in 1976 and is characterized by complex tectonic structures and high seismic hazard. The results showed that, spatial variations of the gaseous anomalies, especially hydrogen and helium have spatial congruence along the tectonic lines, which can be attributed to their deep sources and the migration paths formed by the faults. A better congruence of radon and carbon dioxide is highlighted which indicates that carbon dioxide acts as the carrier gas for radon in this area. Two geochemical anomaly zones of soil gas were found in the area wherein all the studied gases exhibited anomalies or high values, related to the faults and earthquakes.

Soil gas geochemistry in relation to eruptive fissures on Timanfaya volcano, Lanzarote Island (Canary Islands, Spain)

We report herein the first results of an extensive soil gas survey performed on Timanfaya volcano on May 2011. Soil gas composition at Timanfaya volcano indicates a main atmospheric source, slightly enriched in CO2 and He. Soil CO2 concentration showed a very slight deep contribution of the Timanfaya volcanic system, with no clear relation to the main eruptive fissures of the studied area. The existence of soil helium enrichments in Timanfaya indicates a shallow degassing of crustal helium and other possible deeper sources probably form cooling magma bodies at depth. The main soil helium enrichments were observed in good agreement with the main eruptive fissures of the 1730–36 eruption, with the highest values located at those areas with a higher density of recent eruptive centers, indicating an important structural control for the leakage of helium at Timanfaya volcano. Atmospheric air slightly polluted by deep-seated helium emissions, CO2 degassed from a cooling magma body, and biogenic CO2, might be the most plausible explanation for the existence of soil gas. Helium is a deep-seated gas, exhibiting important emission rates along the main eruptive fissure of the 1730–36 eruption of Timanfaya volcano.

Rn, He and CO2 soil gas geochemistry for the study of active and inactive faults

Abstract Two Italian areas, characterized by different seismological histories, were investigated to enhance the basic knowledge of gas migration mechanisms during earthquakes. Sharp variations occur in the movement and concentration of some gaseous species due to the evolution of the local stress regime. The first area (Colpasquale) is located in the central Italian region of Marche and provided a good location to study gas migration in a seismically active region. The area was devastated by a sequence of shallow earthquakes over a 3 month-long period (September–December, 1997). The occurrence of this catastrophic event, as well as the long duration of the “seismic sequence”, presented a unique opportunity to study gas migration in a zone undergoing active displacement. Soil gas surveys were performed 1 day, 1 week, 1 year and 2 years after the main shock (Ms 5.6) in the Colpasquale area. In particular, results highlight a change in the Rn distribution during the three monitoring years indicating a variation of gas migration that may be linked to the evolution of the stress regime. The second study area is located in the Campidano Graben (southern part of Sardinia Island). This area is characterized by seismic quiescence, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. The consistently low values observed for all analyzed gases suggest that the studied area is likely characterized by sealed, non-active faults that prevent significant gas migration. The comparison of data from both studied areas indicate that soil gas geochemistry is useful to locate tectonic discontinuities even when they intersect non-cohesive clastic rocks near the surface and thus are not visible (i.e., “blind faults”).

Radon concentrations in soil gas, considering radioactive equilibrium conditions with application to estimating fault-zone geometry

A calculation method for determining the amount of Rn isotopes and daughter products at the start of measurement (CRAS) is proposed as a more accurate means of estimating the initial Rn concentration in soil gas. The CRAS utilizes the decay law between 222Rn and 220Rn isotopes and the daughter products 218Po and 216Po, and is applicable to α-scintillation counter measurements. As Rn is both inert and chemically stable, it is useful for fault investigation based on the soil gas geochemistry. However, the total number of α particles emitted by the decay of Rn has generally been considered to be proportional to the initial Rn concentration, without considering the gas condition with respect to radioactive equilibrium. The CRAS method is shown to be effective to derive Rn concentration for soil gases under both nonequilibrium conditions, in which the total number of decays increases with time, and equilibrium conditions, which are typical of normal soil under low gas flux. The CRAS method in conjunction with finite difference method simulation is applied to the analysis of two active fault areas in Japan, and it is demonstrated that this combination could detect the sharp rises in 222Rn concentrations associated with faults. The method also allows the determination of fault geometry near the surface based on the asymmetry variation of the Rn concentration distribution when coupled with a numerical simulation of 222Rn transport. The results for the new method as applied to the two case studies are consistent with the data collected from the geological survey. It implies that the CRAS method is suitable for investigating the fault system and interstitial gas mobility through fractures. The present analyses have also demonstrated that high Rn concentrations require the recent and repeated accumulation of 222Rn parents (230Th and 226Ra) in fault gouges through deep gas release during fault movement.

Terrain characteristics of a tonal anomaly remotely detected in an area of hydrocarbon microseepage, Tucano Basin, north-eastern Brazil

This study reports the geological characteristics of a tonal anomaly in an area of hydrocarbon microseepage, identified in Landsat-5 Thematic Mapper (TM) images. A diversified dataset was gathered, which included soil gas geochemistry determinations, soil radioactivity intensity and soil magnetic susceptibility measurements, as well as laboratory soil spectra. Data indicated that the tonal anomaly holds a higher concentration of gaseous hydrocarbons in soils than the surroundings, accompanied by enrichment in radioactive minerals. However, there is no definitive evidence of magnetic mineral enrichment inside the tonal anomaly compared with the surroundings. An intense silicification process, characterized by a cover of fine-grained silicified cobbles and boulders, seems to be responsible for inhibiting the development of diagenetic clay mineralization, as indicated by variations in the depth of the 2200 nm soil spectra absorption band.

Fault detection using soil gas geochemistry : Duddridge, G A; Grainger, P; Durrance, E M Q J Engng GeolV24, N4, 1991, P427–435

Fault detection using soil gas geochemistry : Duddridge, G A; Grainger, P; Durrance, E M Q J Engng GeolV24, N4, 1991, P427–435

Geological Development of the Lonesome Dove II King Sand field

In March of 1989, Marshall and Winston, Inc., drilled Petro-West Corporation's Brady Creek prospect, 5 mi southwest of Eden, Concho County, Texas, along the southern part of the eastern shelf of the Permian basin. The prospect was drilled on the basis of a subsurface, geological anomaly mapped on the lower King Sandstone. The anomaly was confirmed and enhanced by soil gas geochemistry. The McMurtrey No.1 well encountered 38 ft of sandstone, and has an estimated potential of 79 BOPD (pumping). The lower King Sandstone at Lonesome Dove II field is part of a larger northeast-southwest-trending Cisco depositional system. Data from subsequent drilling at Lonesome Dove II (including two conventional cores, samples, sidewall cores, dipmeters, and electric logs) indicates that the lower King Sandstone may have been deposited during a regressive cycles of fluvial-deltaic sedimentation. Characteristics of both fluvial point bar and distributary channel facies have been noted and the development of a distributary model of deposition led to the extension of the field beyond the limits of the original subsurface and geochemical picture. The presence of two or more separate channels or channel systems is indicated by isopach maps and by significant oil gravity, field pressure, and grain size differences.more » These channel systems may have been deposited on a relatively flat-lying deltaic plain as indicated by cross sections showing only slight channel downcutting.« less

Fault detection using soil gas geochemistry

Abstract Soil gas surveys have been carried out at research sites in Great Britain and Italy to test soil gas geochemistry as a site investigation technique for the detection of faults and discontinuities. At a site on Oxford Clay in Gloucestershire, soil gas anomalies of high He, Rn and CO 2 and low O 2 were shown to correspond to the outcrop of a fault, identified by drilling and geophysics. Other apparently random anomalies remained unexplained and lateral migration of gas through superficial horizons complicated interpretation. Using three parallel sample lines at the same distance apart as the sample spacing it has proved not only possible statistically to remove spurious anomalies, but also to enhance and concentrate clusters of high values resulting from gas migration. This method was used to investigate a fault in Neogene clay at Narni in Italy where a gas pathway permeable to Rn and CO 2 was identified, corresponding to a geoelectrical anomaly indicating displaced strata. Further work was carried out over a fault revealed by trenching in the Caithness Flags in Scotland. For each three-line traverse an integrated gas anomaly map was produced to aid interpretation by allowing both sharp and diffuse anomalies to be more easily identified.

Dispersion of soil gas around micron gold deposits

Kesler, S.E., Gerdenich, M,J., Steininger, R.C. and Smith, C., 1990. Dispersion of soil gas around micron gold deposits. In: S.E. Kesler (Editor), Soil and Rock Gas Geochemistry. J. Geochem. Explot., 38:117-132. Experimental weathering of sediment-hosted micron gold (SHMG) ore from the Alligator Ridge deposit indicates that ore and adjacent Pilot Shale wallrock release gas with larger concentrations of COS, CS2 and CH4, and lower CH4/(CH4 + C2H4 ) and CH4 / (CH4 + C2H 6 ) ratios than does barren Pilot Shale. The COS and CS2 are derived from decomposition of disseminated pyrite, which is probably more abundant in mineralized zones, and variations in the CH-gas ratios are thought to reflect increased thermal maturity of organic material in the mineralized zones. Surveys designed to determine whether these gases are present in detectable haloes around SHMG mineralization were carried out over the Yankee prospect, a relatively similar deposit several km south of Alligator Ridge. The surveys included analysis of interstitial soil gas, gases desorbed from soils, and gases desorbed from two sets of Carbotrap collectors, one left on the bedrock interface (at depths of about 30-70 cm below the surface) and another placed in holes drilled about 3 m into bedrock. All of the gases observed in the experiments were detected, as well as organo-sulfur and higher alkane and alkene gases. The surveys indicated systematic variations in gas dispersal related to faults and/or mineralization, with the most consistent patterns being elevated COS values and depressed CH4/(CH4 + C2H4) ratios over mineralization. Similar highs in COS, as well as other gases including SO2 and ethyl mercaptan, were associated with faults, and it was not possible to distinguish these two sources of anomalous gas patterns in all cases. In general, best results were obtained with gases desorbed from bulk soil, and no significant advantage was observed with the use of collectors placed in deep holes. Although all of the survey methods yielded anomalous patterns, none of these patterns are sufficiently strong to merit the use of gas geochemistry as a primary guide in exploration for SHMG deposits.

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Alpers, C.N., Dettman, D.L., Lohmann, K.C. and Brabec, D., 1990. Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin. In: S.E. Kesler (Editor), Soil and Rock Gas Geochemistry. J. Geochem. Explor., 38: 69-86. Stable isotope ratios of oxygen and carbon were determined for CO2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO2 concentrations detected previously by McCarthy et al. (1986). Values of tSl3C in soil gas CO2 from depths between 0.5 and 1.0 m were found to range from - 12.68%0 to -20.03%0 (PDB). Organic carbon from the uppermost meter of soil has ~ t3C between -24.1 and -25.8%0 (PDB), indicating derivation from plant species with the Ca (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C3 and C4 (HatchSlack) plants, together with atmospheric CO2 are the likely sources of carbon in soil gas CO2. Values of t~tsO in soil-gas CO2 range from 32 to 38%0 (SMOW). These ~sO values are intermediate between that calculated for CO2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO2. The 6IS o data indicate that atmospheric CO2 has been incorporated by mixing or diffusion. Any CO2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters. The isotopic composition of soil-gas CO2 taken from directly above the massive sulfide deposit was not distinguishable from that of samples taken 1 to 2 km away. No enrichment of the t~3C value of soil-gas CO2 was observed, contrary to what would be expected if the anomalous CO2 were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C3 plant material were responsible for most CO2 generation both in the vicinity of the massive sulfide and in the background area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO2 anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. ( 1983 ) and McCarthy et al. ( 1986 ). Further work is needed on seasonal and daily variations of CO2 concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.

Tumbleweed Delta-Lower Tannehill Point Bars, Geochemistry and Dipmeter Logs of Dickens and King Counties, Texas: ABSTRACT

The oil-prone Tannehill sandstone trend is an extensive Eastern Shelf clastic depositional system derived from an ancient uplift located several counties away to the east-northeast of Dickens and King counties. The older 100-mi-long lower Tannehill system, preceding the upper Tannehill (Frye) system, trended east-west into King and Dickens counties, where it lies in the domain of the shallow-water-shelf delta distributary, situated between the well-documented prolific point-bar (fluvial) deposits of Baylor, Knox, and eastern King counties, and the massive nonproductive slope-mouth bar (deeper water) deposits of western Kent and western Dickens counties. The individual prolific and shallow sand bodies in the shelf distributary of Dickens and King counties are shelf delta point-bars, with some reworking. Geologists for years have erroneously played the sands and interpreted dipmeter logs in this area as fluvial point-bars. Case histories illustrate the complex stratigraphic traps of varying sizes that are formed, and the problems with dipmeter interpretations. Prospecting techniques involving subsurface geology and soil-gas geochemistry have resulted in wildcat success ratios in excess of 25% and development well success in excess of 85%. Numerous fields with proven total recoverable reserves in excess of 800,000 bbl of oil have been found over the last 10 yr. Themore » premise central to modern-day geochemical soil gas prospecting is that very small amounts (parts per million) of light hydrocarbons move upward continuously (but not always vertically) over time from subsurface hydrocarbon reservoirs toward the ground surface, and such microseepage is detectable by modern instruments sensitive to parts per billion. Case histories illustrate that such unconventional techniques are very successful only in certain geologic provinces, and are to be integrated with subsurface geology and other conventional methods.« less

Soil Radon and Elemental Mercury Distribution and Relation to Magmatic Resurgence at Long Valley Caldera

The response of a large geothermal system to magmatic resurgence was analyzed by a survey of soil gas radon and elemental mercury at 600 sites in the silicic Long Valley Caldera, California. The broad geochemical anomaly over the caldera has superimposed on it a small zone of pronounced radon enrichment and mercury depletion coincident with the surface projection of a postulated dike of rising magma. Soil gas geochemistry studies can complement traditional geophysical and geodetical methods in the evaluation of potential volcanic eruption hazards.