Soil Gas Analysis Research Articles

A Critical Review on Soil Gas Analysis: Modern Technologies and Problems

In this review article, the main techniques for spectroscopic studies of gases in field conditions are considered. The issues related to the study of gas emissions from soils and the determination of their concentrations are analysed. The main types of spectroscopy used in portable devices for soil gas analysis, along with their design features and sampling approaches, are provided. Various studies aimed at optimising the operation of devices for analysing gases emitted from the soil, taking into account agronomic, agrochemical, and ecological specifics, are also presented. The effect of using different types of lasers and reflecting elements on the accuracy of optical measurements and the sensitivity to various substances in the gases is analysed.

Advances in Hydrocarbon Speciation for Soil Gas Analysis

In the field of contaminated site remediation, risk analyses can be carried out by the use of appropriate mathematical models, based on simplified hypotheses of real sites. The possibility to have shared and accurate analytical methods is an important challenge for both routine laboratories and supervisory institutions. Although official methods (e.g., APH2009) are already available, they are lacking in some aspects, such as applicability to all the different sampling and analysis media and ease of use in routine analysis. Here, we report our recent advances in the implementation of an empirical method for hydrocarbon speciation in an air matrix, previously developed by the authors and published in 2015. The validation results led to a coefficient of variation (CV%) between 10–15% for the hydrocarbon fractions investigated (C5-C8 aliphatic, C9-C12 aliphatic, C9-C10 aromatic and C11-C12 aromatic) and recovery of ≤20%, in agreement with the required characteristics for the analytical methods. Several real samples were analyzed by the proposed method and the results were compared to those obtained by the official approach. The comparison led to an average bias (that is, the difference between the results from the developed method and those from the conventional one) of about 10%, confirming that the innovative approach is robust, accurate and can be applied to all the analytical media used for air sampling, such as soil gas.

Comparison of Geogases in Two Cenozoic Sedimentary Basins

We investigated fault gases (helium, radon, CO2) in two seismically active Cenozoic sedimentary basins: (a) Meinweg (in 2015), at a tectonically quiescent horst structure in the Lower Rhine Embayment; and (b) Bodanrück (in 2012; Lake of Constance), in the Molasse Basin and part of the seismically active Freiburg–Bonndorf–Bodensee Fault Zone (FBBFZ). Both study areas were selected because recent “GeoBio-Interactions” findings showed that red wood ants (RWA) are biological indicators of otherwise undetected degassing systems. We combined presence/absence data of RWA nests, their spatial pattern analysis (prototype lines), seismicity and known tectonic settings with soil gas analyses (a total of 817 samples) to unveil geochemical anomalies related to tectonic developments unknown so far. Currently, Meinweg can be considered “no ants land” due to the very low background-level of geogas concentrations. Thus, anomalies (Rn-CO2) weakly trending in NE-SW extension direction emerged. This could probably indicate the onset of (re)activation of the NE-SW-trending (Variscan) structures or the development of new fractures as an aftershock process of the 1992 Roermond earthquake that occurred about 15 km to the west. Results at Bodanrück (three RWA clusters and two RWA-free corridors) revealed degassing patterns in NW-SE and NNE-SSW directions in the clusters corresponding to re-activated and recent strike-slip fault systems. No gas anomalies were found in RWA-free corridors. The RWA nest distribution was shown to be a valuable tool for identifying areas of even actively degassing spotty anomalies caused by macro- and microscale brittle deformation masked by sediment cover.

Study of the Long Term Acid Gas Sequestration Process in the Borzęcin Structure: Measurements Insight

Geological sequestration of acid gases, including CO2, is now a growing solution to prevent progressive Earth climate change. Disposal of environmentally harmful greenhouse gases must be performed safely and securely to minimise leakage risk and possible uncontrolled emissions of injected gases outside the sequestration structure. The paper describes a series of research activities at the Borzęcin sequestration site located in western Poland, which were designed to study the migration paths of injected acid gases (mainly mixture of CO2 and H2S) into the water-bearing layers underlying natural gas reservoir. Along with understanding the nature and dynamics of acid gases migration within the sequestration structure, the research was also addressed to assess its leak-tightness and the long-term safety of the entire reinjection facility. As a part of the research works, two downhole sampling campaigns were completed in 2018–2019, where samples of water underlying the Borzęcin reservoir were taken and subsequently studied to determine their physicochemical parameters that were never before examined. Compositions of gas dissolved in downhole brine samples were compared with produced and injected gas. Relevant studies of reservoir water from selected wells were performed, including isotopic analyses. Finally, four series of soil gas analyses were performed on the area surrounding the selected well, which are important for the hazardous gas sequestration safety analysis in the Borzęcin facility. All the above mentioned research activities aimed to acquire additional knowledge, which is valuable for risk assessment of the acid gas sequestration process taking place on the specific example of the Borzęcin site operating continuously since 1996.

Native H2 Exploration in the Western Pyrenean Foothills

AbstractNative hydrogen (H2) may represent a new carbon free‐energy resource, but to date there is no specific exploration guide to target H2‐fertile geological settings. Here, we present the first soil gas survey specifically designed to explore H2 migration in a region where no surface seepage has been documented so far. We choose the Pyrenean orogenic belt and its northern foreland basin (Aquitaine, France) as a test site for our strategy. The presence of mantle rocks at shallow depth (<10 km depth) under the Mauléon Basin connected to the surface by major faults is considered as preliminary requisites for H2 generation and drainage. On this basis, more than 1,100 in situ soil gas analysis (H2, CO, CO2, CH4, H2S, and 222Rn) were performed at ∼1 m depth at the regional scale along a 10 × 10 km grid spanning over 7,500 km2. The analysis campaign reveals several areas of high occurrence to the north of the Mauléon Basin where H2, CO2, and 222Rn concentrations exceed 1,000 ppmv, 10 vol%, and 50 kBq m−3, respectively. Most of these hot spots are located along the North Pyrenean Frontal Thrust and other related faults rooted in the mantle body. These results, together with evidence from the literature of fluid migration at depth, suggest that H2 may be sourced from mantle rocks serpentinization and carried to the surface along major thrusting faults. Traps containing hydrogen remain unidentified up to now but the presence of salt‐related structures (diapirs) near these hot spots is considered encouraging.

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.

Development and deployment of a field-portable soil O2 and CO2 gas analyzer and sampler.

Here we present novel method development and instruction in the construction and use of Field Portable Gas Analyzers study of belowground aerobic respiration dynamics of deep soil systems. Our Field-Portable Gas Analysis (FPGA) platform has been developed at the Calhoun Critical Zone Observatory (CCZO) for the measurement and monitoring of soil O2 and CO2 in a variety of ecosystems around the world. The FPGA platform presented here is cost-effective, lightweight, compact, and reliable for monitoring dynamic soil gasses in-situ in the field. The FPGA platform integrates off-the-shelf components for non-dispersive infrared (NDIR) CO2 measurement and electro-chemical O2 measurement via flow-through soil gas analyses. More than 2000 soil gas measurements have been made to date using these devices over 4 years of observations. Measurement accuracy of FPGAs is consistently high as validated via conventional bench-top gas chromatography. Further, time series representations of paired CO2 and O2 measurement under hardwood forests at the CCZO demonstrate the ability to observe and track seasonal and climatic patterns belowground with this FPGA platform. Lastly, the ability to analyze the apparent respiratory quotient, the ratio of apparent CO2 accumulation divided by apparent O2 consumption relative to the aboveground atmosphere, indicates a high degree of nuanced analyses are made possible with tools like FPGAs. In sum, the accuracy and reliability of the FPGA platform for soil gas monitoring allows for low-cost temporally extensive and spatially expansive field studies of deep soil respiration.

Cover collapse sinkhole over a deep buried carbonate bedrock: The case study of Fossa San Vito (Sarno - Southern Italy)

Sinkholes are widely found in Italy in both carbonate slopes and alluvial plain settings. Since they may occur catastrophically, without showing premonitory signs and resulting in severe economic losses and casualties, they constitute a significant risk in many areas, and even pose a threat to human life. In this paper, we carried out a detailed multidisciplinary study of the Fossa San Vito sinkhole (southern Italy) that is a particular type of cover collapse sinkhole. The latter is a sub-circular, closed depression, with a diameter of ~200 m and with inner walls up to 25 m high, located in the piedmont area placed along the inner border of the Sarno River alluvial plain. The study entailed morphological and stratigraphic analysis by means of field surveys, including nine trenches, four boreholes, and the reinterpretation of stratigraphical data from the literature. In addition, hydrogeological and soil gas analysis were carried out to investigate the possible role of deep-water uprising. The study attempted to identify the genetic mechanism and include the reconstruction of a detailed subsurface stratigraphic model, showing that the infilling of the depression is about 45 m thick and consists of well-laminated, silty-clay lacustrine sediments interbedded with tephra layers. 14C dating of fossil wood found at the base of the lacustrine deposits constrains the age of the collapse at 5483 cal. years B.P. The collapse of the topographic surface involved a volume of about 400,000 cubic metres and originated at depth >80 m.

Linking serpentinization, hyperalkaline mineral waters and abiotic methane production in continental peridotites: an integrated hydrogeological-bio-geochemical model from the Cabeço de Vide CH4-rich aquifer (Portugal)

Continental active serpentinization of ultramafic rocks is today recognized as a key process triggering a sequence of phenomena involving the passage from inorganic, to organic and metabolic reactions. These may have a role in the origin of life, and may explain the occurrence of abiotic hydrocarbons on Earth and other planets. Production of hyperalkaline waters and abiotic methane (CH4) are two critical steps in this sequence. They were described independently by specific hydrogeological and geochemical models. Here, we update and combine these models into a unified scheme using and integrating geological, hydrogeological, hydrogeochemical, gas-geochemical and microbial analyses acquired from 2002 to 2014 in the Cabeço de Vide (CdV) study site, Portugal. The hyperalkaline (pH > 10.5), Na-Cl/Ca-OH mineral water of CdV evolve from groundwater-peridotite interaction (serpentinization) generating hydrogen (H2), which, according to multiple theoretical, laboratory and field evidence, likely reacted with CO2 within metal- (catalyst) rich rocks, abiotically producing CH4 (up to 1.2 mg/L; -24.4°/oo < δ13C-CH4 < -14.0°/oo and -285°/oo < δ2H-CH4 < -218°/oo). The hyperalkaline water hosts hydrogen oxidizing bacteria “Serpentinomonas”, which may explain the paucity of H2 observed in the dissolved gas. The CdV gas-rich mineral waters ascend along a fault at the boundary of the peridotite intrusion. Temporal changes of pH and CH4 concentration result from episodic mixing with shallower Mg-HCO3-type waters. Soil-gas analyses show that methane migrates to the surface along the fault, also independently from the water emergences, consistently with non-aqueous abiotic CH4 production. Our integrated model is generally compatible with observations from other gas-bearing continental serpentinization sites.

Soil δ13C-CO2 and CO2 flux in the H2S-rich Rotorua Hydrothermal System utilizing Cavity Ring Down Spectroscopy

Diffuse soil CO2 flux and stable carbon isotope surveys facilitate interpretation of the nature and structure of hydrothermal systems. Although soil CO2 flux measurements are completed in the field, isotopic carbon in CO2 (i.e., δ13C-CO2) analyses are more commonly performed in a stable isotope mass spectrometry laboratory, thereby, increasing the effort, time and resources required to complete the survey. Cavity Ring Down Spectroscopy (CRDS) overcomes these challenges by enabling both CO2 isotopic and flux analyses in field settings. However, H2S interference has limited CRDS applications in H2S-rich hydrothermal settings. Here, we assess the application of a mobile CRDS system equipped with a simple Cu wire trap to complete δ13C-CO2 and CO2 flux surveying in the relatively high H2S Rotorua Hydrothermal System (RHS), North Island, New Zealand. Laboratory experiments demonstrate that the Cu trap was able to remove H2S (~250 ppmv) enabling accurate and precise δ13C-CO2 measurements. In the RHS, the Cu trap successfully removed H2S concentrations during in situ soil gas analyses. Carbon dioxide fluxes determined using the CRDS were found be 102% of those values produced by a traditional soil gas flux meter (i.e., Licor). Carbon dioxide flux determined by CRDS was similar to previous studies in the RHS; however, zones of high CO2 flux in central Kuirau Park have moved ~100–200 m in the past twelve years. Isoscapes of δ13C-CO2 for the RHS were constructed from our soil gas survey results and were used to assess the apportionment of magmatic-hydrothermal versus biogenic CO2 release. Our findings demonstrate that a CRDS enabled system allows for faster in field survey measurement, lower cost, and reactive sampling for both δ13C-CO2 and CO2 flux measurement in H2S-enriched hydrothermal systems.

14 CO 2 analysis of soil gas: Evaluation of sample size limits and sampling devices

Abstract Radiocarbon (14C) analysis of CO2 respired from soils or sediments is a valuable tool to identify different carbon sources. The collection and processing of the CO2, however, is challenging and prone to contamination. We thus continuously improve our handling procedures and present a refined method for the collection of even small amounts of CO2 in molecular sieve cartridges (MSCs) for accelerator mass spectrometry 14C analysis. Using a modified vacuum rig and an improved desorption procedure, we were able to increase the CO2 recovery from the MSC (95%) as well as the sample throughput compared to our previous study. By processing series of different sample size, we show that our MSCs can be used for CO2 samples of as small as 50 µg C. The contamination by exogenous carbon determined in these laboratory tests, was less than 2.0 µg C from fossil and less than 3.0 µg C from modern sources. Additionally, we tested two sampling devices for the collection of CO2 samples released from soils or sediments, including a respiration chamber and a depth sampler, which are connected to the MSC. We obtained a very promising, low process blank for the entire CO2 sampling and purification procedure of ∼0.004 F14C (equal to 44,000 yrs BP) and ∼0.003 F14C (equal to 47,000 yrs BP). In contrast to previous studies, we observed no isotopic fractionation towards lighter δ13C values during the passive sampling with the depth samplers.

Results from twelve years of continuous monitoring of the soil CO2 flux at the Ketzin CO2 storage pilot site, Germany

The complete cycle of specific processes related to the geological storage of CO2 is investigated in detail at Ketzin since 2004. The scientific monitoring program targets different depths of the involved area and addresses the safety and reliability of the storage. The surface monitoring comprises long-term soil CO2 flux measurements and soil gas analyses. Annual mean values of soil CO2 fluxes ranged from 2.4 to 3.4 µmol m-2 s-1 before the injection started (2005-2008) and from 2.3 to 3.5 µmol m-2 s-1 during and after CO2 injection (2009-2016) and thus do not indicate an upward migration of the injected CO2.

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.

Using 222Rn and carbon isotopes (12C, 13C and 14C) to determine CO2 sources in forest soils developed on contrasting geology in Slovenia

Global carbon estimates have identified abiotic CO2 as a potentially significant source of atmospheric CO2, albeit little is known about its origin. The aim of this study was to identify the origin of soil CO2 using carbon isotopes and 222Rn data. The study involved collecting data from seven Slovenian forest soils developed over bedrock with contrasting geology where different origin of soil CO2 was expected; two sampling sites were located on soils formed above carbonate bedrock, one above metamorphic bedrock and the remainder above clastic sedimentary rocks. Analysis of soil gas including the levels of CO2, carbon isotope measurements (12C, 13C and 14C) and 222Rn activity was recorded at a soil depth of 80 cm. Isotopic analysis revealed that the CO2 was young and there was no difference in the age of soil CO2 above either carbonate or non-carbonate bedrock. The data also suggest that the 13C-enrichment in soil CO2, above carbonate bedrock was a consequence of the mixing of soil CO2 with atmospheric CO2 and/or the ventilation of subterranean CO2 from pores, fissures and cavities. The latter effect was supported by the high 222Rn concentrations observed at these sites. Based on the \(\delta^{13} {\text{C}}_{{{\text{CO}}_{2} }}\) data, photosynthesis prevailed over microbial respiration accounting for the majority (>70 %) of total soil CO2 over non-carbonate bedrock—at least at the time of sampling. Overall, results from this study could represent useful information for global carbon cycle models used to predict the impacts of climate changes.

LNAPL source zone delineation using soil gases in a heterogeneous silty-sand aquifer

Source delineation of hydrocarbon contaminated sites is of high importance for remediation work. However, traditional methods like soil core extraction and analysis or recent Membrane Interface Probe methods are time consuming and costly. Therefore, the development of an in situ method based on soil gas analysis can be interesting. This includes the direct measurement of volatile organic compounds (VOCs) in soil gas taken from gas probes using a PID (Photo Ionization Detector) and the analysis of other soil gases related to VOC degradation distribution (CH4, O2, CO2) or related to presence of Light Non-Aqueous Phase Liquid (LNAPL) as 222Rn. However, in widespread heterogeneous formations, delineation by gas measurements becomes more challenging. The objective of this study is twofold: (i) to analyse the potential of several in situ gas measurement techniques in comparison to soil coring for LNAPL source delineation at a heterogeneous contaminated site where the techniques might be limited by a low diffusion potential linked to the presence of fine sands and silts, and (ii) to analyse the effect of vertical sediment heterogeneities on the performance of these gas measurement methods. Thus, five types of gases were analysed: VOCs, their three related degradation products O2, CO2 and CH4 and 222Rn. Gas measurements were compared to independent LNAPL analysis by coring. This work was conducted at an old industrial site frequently contaminated by a Diesel-Fuel mixture located in a heterogeneous fine-grained aquifer.Results show that in such heterogeneous media migration of reactive gases like VOCs occurs only across small distances and the VOC concentrations sampled with gas probes are mainly related to local conditions rather than the presence of LNAPL below the gas probe. 222Rn is not well correlated with LNAPL because of sediment heterogeneity. Oxygen, CO2, and especially CH4, have larger lengths of diffusion and give the clearest picture for LNAPL presence at this site even when the gas probe is somewhat distant.

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%.

Characterization of geothermal activity along the North American–Caribbean Plate boundary in Guatemala: The Joaquina geothermal field

Structural mapping, chemical reconnaissance of thermal manifestations, soil chemistry including CO2 (soil gas), and shallow temperature measurements were employed in conjunction with petrographic, fluid inclusion, isotopic, and bulk rock chemical analyses of drill cutting samples to identify the characteristics and controls of hydrothermal fluid upwelling and outflow in the Joaquina geothermal system, Guatemala. Evidence is provided for a rapidly-upflowing, meteorically-derived Na-bicarbonate(-sulfate) geothermal fluid (a minority category of geothermal fluid compositions comparable to that found in similar non-volcanic systems). The reservoir temperature, based on the least-diluted samples, is estimated to be 175–185°C. Carbon isotopic analyses of soil gas and sulfur isotopic analyses of sulfide minerals in drill core and cuttings provide insights into the origin of volatiles in the Joaquina system. These data support the notion of CO2, CH4, and sulfur that are products of hydrothermal alteration of organic-rich metasediments of the El Tambor Complex, a unit that likely represents a major component of the stratigraphy in the Joaquina system. Hydrothermal upflow in the region is controlled by interaction between NE-striking sinistral faults and ESE-striking normal faults. Paleo-hydrostatic conditions estimated from fluid inclusion studies indicate a minimum age of 45ka for the commencement of geothermal activity in the region. The natural thermal output of the Joaquina system is estimated conservatively at 32.2MWth. This study represents the first detailed field investigation of a non-volcanic geothermal system in Guatemala, and provides insight into the nature and controls of thermal areas inland from the active volcanic arc.

Nitrous oxide losses from untreated and digested slurry as influenced by soil moisture and application method

Effects of injection depth, slurry composition and soil moisture content on N2O emissions were investigated at laboratory scale. A loamy sand soil was moistened to 61, 64 or 71% of water-filled pore space (WFPS) and packed to the field bulk density. Using a mechanical device, untreated (UN) or anaerobically digested (AD) pig slurry (122 kg N ha−1) was injected to 30 or 60 mm below the soil surface to simulate closed slot injection. Soil with no slurry applied was included as reference. Incubations were carried out at 20 °C for 30 days; emissions of N2O and CO2 were determined several times by a photo-acoustic analyser. Pre-trials were conducted to validate soil packing and gas analysis methods. The experimental setup was found suitable for studying the regulation of N2O emissions from soil. In some cases, increasing WFPS stimulated N2O emissions while in other cases the increase in emission was not statistically significant with increasing WFPS. There were no effects of injection depth and slurry treatment, except that at 71% WFPS N2O emissions were lower from UN compared to AD slurry and the untreated control.

Geo-microbial and geochemical evidences in the near surface soils of Jamnagar sub-basin, Saurashtra, Gujarat, India: implications to hydrocarbon resource potential

The Jamnagar sub-basin of Saurashtra, Gujarat is considered geologically prospective for hydrocarbons by Directorate General of Hydrocarbons (DGH), India. However the major part of the Jamnagar sub-basin is covered by Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. In India, significant hydrocarbon finding in the stratigraphic sequence has not been established, as the major part of the Mesozoic sediments is underlying the Deccan Traps. Detection and mapping of the Mesozoic sediments below the Deccan Trap has been a long-standing complex geophysical problem facing the oil industry. The vast sheet of volcanic cover, which overlies the Mesozoic sediments acts as a geophysical shield and inhibit the effective use of conventional seismic techniques. Surface geochemical techniques based on hydrocarbon microseepage from subsurface accumulations, have potential to evaluate the prospects for petroleum exploration. In view of this, surface geochemical survey for adsorbed soil gas and microbial analysis was conducted in Jamnagar sub-basin, total 150 near surface soil samples were collected for the evaluation of hydrocarbon resource potential of the Basin. Microbial analysis revealed high bacteria counts for methane (1.32×106 cfu/gm), ethane (8.50× 105 cfu/gm), propane (6.86×105 cfu/gm) and butane oxidizing bacteria (5.70×105 cfu/gm) in soil samples of Jamnagar sub-basin. The bacterial concentration distribution maps show three distinct anomalies in the study area, indicating hydrocarbon microseepage at these places. The geochemical investigations in soil samples showed the presence of high adsorbed hydrocarbon gases concentration of methane (C1) 1 to 518 ppb and Ethane plus hydrocarbons (ΣC2+) 1 to 977 ppb respectively. Further, carrying out integrated geo-microbial and geo-chemical studies with adsorbed soil gas showed the anomalous hydrocarbon zones nearby Khandera, Haripur, and Laloi areas which could probably aid to assess the true potential of the Basin. On the basis of microbial prospecting and adsorbed soil gas studies Jamnagar sub-basin study area appears to be a warm zone for hydrocarbon prospects and needs detailed investigation with other geo-scientific methods. Integrated geophysical studies have shown that Jamnagar sub-basin of Saurashtra has significant sediment thickness below the Deccan Traps and can be considered for future hydrocarbon exploration. The corroboration of adsorbed light gaseous hydrocarbons and hydrocarbon oxidizing bacteria suggests its efficacy as one of the potential tool in surface geochemical exploration of hydrocarbons.

Application of Trace Metal Anomalies for Recognition of Petroleum Prospects in Surface Sediments of Kutch and Saurashtra Basins, India

Abstract: Seepage of hydrocarbons, either as macroseepage or microseepage, can set up near-surface oxidation-reduction zones that favour the development of a diverse array of chemical and mineralogic changes. In this study trace metal alterations were mapped that appear to be associated with hydrocarbon microseepages in the oil/gas fields. A total of 210 soil samples were collected from near surface sediments of Kutch and Saurashtra Basins, India. The soil samples were collected from a depth of 2 - 2.5 m. The paper reports the chemical alterations associated with trace metals in soils that are related to hydrocarbon microseepages above the Kutch and Saurashtra Basins, India. The soil samples for trace metals; Scandium (Sc), Vanadium (V), Chromium (Cr), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Barium (Ba) and Strontium (Sr) were analyzed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The concentrations of the trace elements ranged between for Sc (12.69 to 21.91 ppm), V (109.20 to 436.05 ppm), Cr (87.15 to 481.57 ppm), Co (18.25 to 64.31 ppm), Ni (57.55 to 263.15 ppm), Cu (44.88 to 143.96 ppm), Zn (137.60 to 502.31 ppm), Ba (149.27 to 921.46 ppm) and Sr (143.93 to 425.63 ppm) were obtained. It was observed that trace elements concentrations were tremendously increased when compared with normal concentration in the soils. The adsorbed soil gas analysis showed the presence of high concentrations of ∑C2+ (C2H6, C3H8 and n-C4H10) ranged between 1 to 121 ppb respectively. Integrated studies of trace elements over adsorbed light gaseous hydrocarbon (∑C2+) anomalies showed good correlation. The carbon isotopic composition of δ13C1 of the samples ranges between – 36.6 ‰ to -22.7‰ (Pee Dee Belemnite) suggest thermogenic source for hydrocarbon gases. The increase in trace metal concentrations near oil/gas producing areas, suggests that soil chemical change to a reducing environment, presumably due to the influence of hydrocarbon microseepage, which could be applied with other geo-scientific data to identify areas of future hydrocarbon exploration in the frontier areas.