Comparison of natural gases accumulated in Oligocene strata with hydrous pyrolysis gases from Menilite Shales of the Polish Outer Carpathians

This study examined the molecular and isotopic compositions of gases generated from different kerogen types (i.e., Types I/II, II, IIS and III) in Menilite Shales by sequential hydrous pyrolysis experiments. The experiments were designed to simulate gas generation from source rocks at pre-oil-cracking thermal maturities. Initially, rock samples were heated in the presence of liquid water at 330 °C for 72 h to simulate early gas generation dominated by the overall reaction of kerogen decomposition to bitumen. Generated gas and oil were quantitatively collected at the completion of the experiments and the reactor with its rock and water was resealed and heated at 355 °C for 72 h. This condition simulates late petroleum generation in which the dominant overall reaction is bitumen decomposition to oil. This final heating equates to a cumulative thermal maturity of 1.6% R_r, which represents pre-oil-cracking conditions. In addition to the generated gases from these two experiments being characterized individually, they are also summed to characterize a cumulative gas product. These results are compared with natural gases produced from sandstone reservoirs within or directly overlying the Menilite Shales. The experimentally generated gases show no molecular compositions that are distinct for the different kerogen types, but on a total organic carbon (TOC) basis, oil prone kerogens (i.e., Types I/II, II and IIS) generate more hydrocarbon gas than gas prone Type III kerogen. Although the proportionality of methane to ethane in the experimental gases is lower than that observed in the natural gases, the proportionality of ethane to propane and i-butane to n-butane are similar to those observed for the natural gases. δ¹³C values of the experimentally generated methane, ethane and propane show distinctions among the kerogen types. This distinction is related to the δ¹³C of the original kerogen, with ¹³C enriched kerogen generating more ¹³C enriched hydrocarbon gases than kerogen less enriched in ¹³C. The typically assumed linear trend for δ¹³C of methane, ethane and propane versus their reciprocal carbon number for a single sourced natural gas is not observed in the experimental gases. Instead, the so-called “dogleg” trend, exemplified by relatively ¹³C depleted methane and enriched propane as compared to ethane, is observed for all the kerogen types and at both experimental conditions. Three of the natural gases from the same thrust unit had similar “dogleg” trends indicative of Menilite source rocks with Type III kerogen. These natural gases also contained varying amounts of a microbial gas component that was approximated using the Δδ¹³C for methane and propane determined from the experiments. These approximations gave microbial methane components that ranged from 13–84%. The high input of microbial gas was reflected in the higher gas:oil ratios for Outer Carpathian production (115–1568 Nm³/t) compared with those determined from the experiments (65–302 Nm³/t). Two natural gas samples in the far western part of the study area had more linear trends that suggest a different organic facies of the Menilite Shales or a completely different source. This situation emphasizes the importance of conducting hydrous pyrolysis on samples representing the complete stratigraphic and lateral extent of potential source rocks in determining specific genetic gas correlations.     

Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: IV Acid–sulfate waters

Many waters sampled in Yellowstone National Park, both high-temperature (30–94 °C) and low-temperature (0–30 °C), are acid–sulfate type with pH values of 1–5. Sulfuric acid is the dominant component, especially as pH values decrease below 3, and it forms from the oxidation of elemental S whose origin is H₂S in hot gases derived from boiling of hydrothermal waters at depth. Four determinations of pH were obtained: (1) field pH at field temperature, (2) laboratory pH at laboratory temperature, (3) pH based on acidity titration, and (4) pH based on charge imbalance (at both laboratory and field temperatures). Laboratory pH, charge imbalance pH (at laboratory temperature), and acidity pH were in close agreement for pH < 2.7. Field pH measurements were predominantly used because the charge imbalance was <±10%. When the charge imbalance was generally >±10%, a selection process was used to compare acidity, laboratory, and charge balance pH to arrive at the best estimate. Differences between laboratory and field pH can be explained based on Fe oxidation, H₂S or S₂O₃ oxidation, CO₂ degassing, and the temperature-dependence of pK₂ for H₂SO₄. Charge imbalances are shown to be dependent on a speciation model for pH values <3. The highest SO₄ concentrations, in the thousands of mg/L, result from evaporative concentration at elevated temperatures as shown by the consistently high δ¹⁸O values (−10‰ to −3‰) and a δD vs. δ¹⁸O slope of 3, reflecting kinetic fractionation. Low SO₄ concentrations (<100 mg/L) for thermal waters (>350 mg/L Cl) decrease as the Cl⁻ concentration increases from boiling which appears inconsistent with the hypothesis of H₂S oxidation as a source of hydrothermal SO₄. This trend is consistent with the alternate hypothesis of anhydrite solubility equilibrium. Acid–sulfate water analyses are occasionally high in As, Hg, and NH₃ concentrations but in contrast to acid mine waters they are low to below detection in Cu, Zn, Cd, and Pb concentrations. Even concentrations of SO₄, Fe, and Al are much lower in thermal waters than acid mine waters of the same pH. This difference in water chemistry may explain why certain species of fly larvae live comfortably in Yellowstone’s acid waters but have not been observed in acid rock drainage of the same pH.     

Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field,Central Mexico

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T_mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T_h)  = 325 ± 5 °C. The boiling zone shows T_h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO₂. Positive clathrate melting temperatures (fusion) with T_h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO₂ (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir.The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ¹⁸O–δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ¹⁸O is due to the rock–water interaction at relatively high temperatures. δ¹³C stable isotope data show a magmatic source with a minor meteoric contribution for CO₂. The initial isotopic value δ³⁴S_RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ³⁴S (FeS₂) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ¹⁸O.     

Thermodynamic model of the formation of ore bodies at the Akchatau wolframite greisen-vein deposit

The Akchatau wolframite deposit in central Kazakhstan is a typical greisen deposit. Extensive geological and geochemical data, including those on numerous geochemical signatures (isotopic composition of O, H, C, noble gases, data on fluid inclusions, REE, and others) allowed us to decipher the physicochemical conditions and main factors that caused metasomatism and ore formation. Physicochemical modeling by the HCh program package (designed by Yu.B. Shvarov) was applied to reconstruct the composition of the greisenizing solution, cooling, boiling, interaction with granites; condensation of the gas phase; and fluid mixing. The predominant species of W transfer, (NaHWO _{4 aq} ⁰ ), and precipitation factors were determined. In small ore bodies, precipitation was caused by a temperature decrease. The precipitation of wolframite in near-vein greisens is related to the interaction of boiling highly mineralized solutions with host granites. Boiling does not affect wolframite precipitation but increases the content and ore potential of the greisenizing fluids, facilitating the formation of high-grade wolframite ores. In the filling veins of these bodies, ore precipitation is related to the dilution of solutions by weakly mineralized exogenic waters and the condensate of the gas phase. Tungsten mineralization of the Akchatau deposit was formed in an oxidizing environment, which is controlled by granite minerals during mobilization of ore components.     

Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints

Chemical (major and trace elements) and isotopic compositions (δD and δ¹⁸O in waters and δ¹³C in CO₂ and ³He/⁴He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO₃, B-depleted groundwaters and rising convective Ca-SO₄(HCO₃), CO₂-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N₂, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N₂, Ne, He-depleted hot rising waters. Boron in the liquid phase and N₂ and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured ³He/⁴He ratios, suggesting a contribution of mantle ³He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of ⁴He in the deeper gases. Paradoxically, there is more relative concentration of ⁴He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ¹³C of CO₂ in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-¹³C depleted organic C, have δ¹³C in CO₂ more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO₂ should generate CO₂ with more positive values of δ¹³C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.     

天然气组分的溶解特征及其意义

通过模拟实验方法研究了天然气组分在地层水中的溶解特征。发现在多组分体系中，重烃气的溶解度随矿化度的增大出现先增后减现象；环烷酸类有机物对长链烃气组有明显的增溶作用，分压及其他条件相同时，天然气组分的溶解能力顺序为：ＣＯ２〉ＣＨ４〉Ｎ２〉Ｃ２Ｈ６〉Ｃ３Ｈ８〉ｎ－Ｃ４Ｈ１０〉ｉ－Ｃ４Ｈ１０〉ｎ－Ｃ５Ｈ１２〉ｉ－Ｃ５Ｈ１２。     

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ¹³C values are found in the Tarim and Qaidam basins (δ¹³C₁: − 19.0 to − 29.9‰; δ¹³C₂: − 18.8 to − 27.1‰), and those with lowest δ¹³C values occur in the Turpan-Harmi and Junggar basins (δ¹³C₁: − 40.1 to − 44.0‰; δ¹³C₂: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C_5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ¹³C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.     

Isotopic signatures of CH4 and higher hydrocarbon gases from Precambrian Shield sites: A model for abiogenic polymerization of hydrocarbons

Previous studies of methane and higher hydrocarbon gases in Precambrian Shield rocks in Canada and the Witwatersrand Basin of South Africa identified two major gas types. Paleometeoric waters were dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilizing the CO₂ reduction pathway. In contrast the deepest, most saline fracture waters contained gases that did not resemble the products of microbial methanogenesis and were dominated by both high concentrations of H₂ gas, and CH₄ and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water-rock reaction in these high rock/water ratio, hydrogeologically-isolated fracture waters. Based on new data obtained for the higher hydrocarbon gases in particular, a model is proposed to account for carbon isotope variation between CH₄ and the higher hydrocarbon gases (specifically ethane, propane, butane, and pentane) consistent with abiogenic polymerization. Values of δ¹³C for CH₄ and the higher hydrocarbon gases predicted by the model are shown to match proposed abiogenic hydrocarbon gas end-members identified at five field sites (two in Canada and three in South Africa) suggesting that the carbon isotope patterns between the hydrocarbon homologs reflect the reaction mechanism. In addition, the δ²H isotope data for these gases are shown to be out of isotopic equilibrium, suggesting the consistent apparent fractionation observed between the hydrocarbon homologs may also reflect reaction mechanisms involved in the formation of the gases. Recent experimental and field studies of proposed abiogenic hydrocarbons such as those found at mid-ocean spreading centers and off-axis hydrothermal fields such as Lost City have begun to focus not only on the origin of CH₄, but on the compositional and isotopic information contained in the higher hydrocarbon gases. The model explored in this paper suggests that while the extent of fractionation in the first step in the hydrocarbon synthesis reaction chain may vary as a function of different reaction parameters, δ¹³C values for the higher hydrocarbon gases may be predicted by a simple mass balance model from the δ¹³C values of the lower molecular weight precursors, consistent with abiogenic polymerization. Integration of isotopic data for the higher hydrocarbon gases in addition to CH₄ may be critical for delineation of the origin of the hydrocarbons and investigation of formation mechanisms.     

Volcanic degassing at Somma–Vesuvio (Italy) inferred by chemical and isotopic signatures of groundwater

A geochemical model is proposed for water evolution at Somma–Vesuvio, based on the chemical and isotopic composition of groundwaters, submarine gas emission and chemical composition of the dissolved gases. The active degassing processes, present in the highest part of the volcano edifice, strongly influence the groundwater evolution. The geological–volcanological setting of the volcano forces the waters infiltrating at Somma–Vesuvio caldera, enriched in volcanic gases, to flow towards the southern sector to an area of high pCO₂ groundwaters. Reaction path modelling applied to this conceptual model, involving gas–water–rock interaction, highlights an intense degassing process in the aquifer controlling the chemical and isotopic composition of dissolved gases, total dissolved inorganic C (TDIC) and submarine gas emission. Mapping of TDIC shows a unique area of high values situated SSE of Vesuvio volcano with an average TDIC value of 0.039 mol/L, i.e., one order of magnitude higher than groundwaters from other sectors of the volcano. On the basis of TDIC values, the amount of CO₂ transported by Vesuvio groundwaters was estimated at about 150 t/d. This estimate does not take into account the fraction of gas loss by degassing, however, it represents a relevant part of the CO₂ emitted in this quiescent period by the Vesuvio volcanic system, being of the same order of magnitude as the CO₂ diffusely degassed from the crater area.     

Description of an aerodynamic levitation apparatus with applications in Earth sciences

Background  

In aerodynamic levitation, solids and liquids are floated in a vertical gas stream. In combination with CO₂-laser heating, containerless melting at high temperature of oxides and silicates is possible. We apply aerodynamic levitation to bulk rocks in preparation for microchemical analyses, and for evaporation and reduction experiments.     

Free hydrogen-hydrocarbon gases from the Lovozero loparite deposit (Kola Peninsula,NW Russia)

The Lovozero nepheline-syenite massif in the north-eastern Fennoscandian Shield, well-known to mineralogists and petrologists, is also interesting with its high contents of hydrogen-hydrocarbon gases in different forms of presence, which is untypical of magmatic rocks. The article systematizes and generalizes little known and unpublished data on the composition, location, character and scale (intensity) of the free gases (FG) emission within a major loparite deposit confined to the massif. СН₄ and Н₂ are dominant in the FG composition. The molecular weight distribution of hydrocarbon gas components corresponds to the classic Anderson–Schulz–Flory distribution with a steep gradient. Carbon and hydrogen of the gases are characterized by rather heavy isotope compositions, becoming lighter from the transition of methane to ethane. The FG volume has been estimated as 0.2–1.6 m³ of gas per 1 m³ of undisturbed rock. The gas recovery of walls in underground workings has been up to 0.2 ml/min/m² for СН₄ and 0.5 ml/min/m² for Н₂ in several years after their heading. The discharge of some shot holes that characterizes the gas emission intensity (1.8–2 m deep and 40 mm in diameter) is up to 300 ml/min, but its 1–2 orders lesser values dominate. The discharge time in some sections varies from several days to 20 years. The overpressure of gases towards the air mainly does not increase 100 hPa, sometimes reaching 120 kPa. It has been defined, that FG distribute irregularly (at the distance of centimeters to hundreds of meters) and their composition and particularly emission intensity perform different temporal fluctuations. The abiogenic origin of FG has been proposed, with FG appearing as a mixture of gases in various proportions: (a) gases remaining in microfissures at the massif's consolidation after the capture by fluid inclusions and those lost during degassing and (b) gases occurred in mechanic-chemical reactions, partial emission and concentration of occluded and diffusely scattered gases under the unstable stress-strain mode of the rock mass. Combustible and explosive hydrogen-hydrocarbon FG can accumulate in the air of underground workings and cause accidents, disrupting the workflow. The background for using characteristics of spatial-temporal variations of the FG emission as precursors of dangerous geodynamic phenomena has been indicated.     

Dissolved helium isotope ratios in ground-waters: a new technique based on gas–water re-equilibration and its application to Stromboli volcanic system

Here, a new technique for the determination of dissolved He isotope ratios in ground-waters is presented. This method is based on the extraction and subsequent equilibrium of dissolved gases in an added “host” gas phase. Ultra pure N₂ is placed in glass flasks (250 cc), containing water samples, that were hermetically sealed after their collection. After shaking in an ultrasonic bath for 10 min, an aliquot of the separated gas phase was removed from the flask for MS analysis. ³He/⁴He ratios are measured by using a modified double collector mass spectrometer (VG 5400-TFT). Helium and Ne concentrations are calculated by comparing the partial pressures of masses 4 and 20 of the samples with those of the air-standard measured by a quadrupole mass spectrometer (QMS;VG Quartz). Using He and Ne equilibrium partitioning coefficients, it is possible to calculate the amount of gas originally dissolved in the water. The technique was tested on both air-saturated waters (ASW) and thermal waters from Stromboli (Aeolian Islands, South Italy), the results of which confirmed good reproducibility (≌5%) and accuracy (≌3%) of the data. The method was then applied to three thermal water samples collected from the same volcanic area and the results compared with those of a fumarolic and a soil gas. The isotope ratios for dissolved He gave values of 4.06–4.23 Ra, which are significantly higher than those previously reported in the literature (3.0, 3.5 and 2.9 Ra) and that measured at the fumarole (3.09 Ra), suggesting a newer and higher isotopic signature for the volcanic system. The proposed method appears to be a useful tool in the determination of ³He/⁴He ratios in ground-water systems, especially when free gases are not available or are dangerous to collect.     

Geochemistry of shallow aquifers and soil gas surveys in a feasibility study at the Rivara natural gas storage site (Po Plain,Northern Italy)

A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural gas (CH₄) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the areal distribution of CO₂ and CH₄ fluxes and CO₂, CH₄, Rn, He, H₂ concentrations both in soils and shallow aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as ²²²Rn, He and H₂ has been studied in order to identify potential faults and/or fractures related to preferential migration pathways and the possible interactions between the reservoir and surface. A geochemical and isotopic characterization of the ground waters circulating in the first 200 m has allowed to investigation of (i) the origin of the circulating fluids, (ii) the gas–water–rock interaction processes, (iii) the amount of dissolved gases and/or their saturation status. In the first 200 m, the presence of CH₄-rich reducing waters are probably related to organic matter (peat) bearing strata which generate shallow-derived CH₄, as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic CH₄ gas leakage from a deeper reservoir have been shown. The δ¹³C(CO₂) both in ground waters and free gases suggests a prevalent shallow origin of CO₂ (i.e. organic and/or soil-derived). The acquisition of pre-injection data is strategic for the natural gas storage development project and as a baseline for future monitoring during the gas injection/withdrawing period. Such a geochemical approach is considered as a methodological reference model for future CO₂/CH₄ storage projects.     

New gas geothermometers for geothermal exploration—calibration and application

Calibration of five gas geothermometers is presented, three of which used CO₂, H₂S and H₂ concentrations in fumarole steam, respectively. The remaining two use $\text{CO}{}_2\text{H}{}_2$ and $\text{H}{}_2\text{S}\text{H}{}_2$ ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO₂, H₂S and H₂ in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO₂. Two buffers are involved for H₂S and H₂ and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm.The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures.Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.     

The chemistry of geothermal waters in Iceland. I. Calculation of aqueous speciation from 0° to 370°C

A computer programme has been developed to calculate the composition and aqueous speciation of geothermal reservoir waters including pH, redox potential and gas partial pressures. The programme is specifically suited to handle geochemical data from wet-steam wells, hot-water wells and boiling hot springs, but it may also be used for non-thermal waters. Solubility data for selected geothermal minerals are incorporated to facilitate the study of $\text{solution}\text{mineral}$ equilibria. The programme may also be used to study chemical changes in water chemistry accompanying boiling, variable degassing and cooling, and how these changes disturb $\text{solution}\text{mineral}$ equilibria.     

Origin of natural waters and gases within the Upper Carboniferous coal-bearing and autochthonous Miocene strata in South-Western part of the Upper Silesian Coal Basin,Poland

The molecular and stable isotope compositions of coalbed gases from the Upper Carboniferous strata and natural gases accumulated within the autochthonous Upper Miocene Skawina Formation of the D?bowiec-Simoradz gas deposit were determined, as well as the chemical and stable isotope compositions of waters from the Skawina Formation and waters at the top of the Upper Carboniferous strata of the Kaczyce Ridge (the abandoned “Morcinek” coal mine) in the South-Western part of the Upper Silesian Coal Basin. Two genetic types of natural gases within the Upper Carboniferous coal-bearing strata were identified: thermogenic (CH₄, small amounts of higher gaseous hydrocarbons, and CO₂) and microbial (CH₄, very small amounts of ethane, and CO₂). Thermogenic gases were generated during the bituminous stage of coalification and completed at the end of the Variscan orogeny. Degassing (desorption) of thermogenic gases began at the end of late Carboniferous until the late Miocene time-period and extended to the present-day. This process took place in the Upper Carboniferous strata up to a depth of about 550 m under the sealing Upper Miocene cover. A primary accumulation zone of indigenous, thermogenic gases is present below the degassing zone. Up to 200 m depth from the top of the Upper Carboniferous strata, within the weathered complex, an accumulation zone of secondary, microbial gas occurs. Waters within these strata are mainly of meteoric origin of the infiltration period just before the last sea transgression in the late Miocene and partly of marine origin having migrated from the Upper Miocene strata. Then, both methanogenic archaebacteria and their nutrients were transported by meteoric water into the near-surface Carboniferous strata where the generated microbial CH₄ saturated coal seams. Waters within the Miocene strata of the D?bowiec-Simoradz and Zab?ocie are of marine origin, and natural gases accumulated within autochthonous Miocene strata of the D?bowiec-Simoradz gas deposit were most probably generated by microbial processes of on organic matter dispersed within the strata, though some contribution of gases migrating from the Carboniferous coal-bearing strata cannot be excluded.     

Stable carbon isotope geochemistry of adsorbed alkane gases in near-surface soils of the Saurashtra Basin,India

The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C₁) = 3–518; ethane (C₂) = 0–430; propane (C₃) = 0–331; i-butane (iC₄) = 0–297; n-butane (nC₄) = 2–116; i-pentane (iC₅) = 0–31 and n-pentane (nC₅) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ¹³C₁; δ¹³C₂ and δ¹³C₃ compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ¹³C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.     

Origin of light volatile hydrocarbon gases in mud volcano fluids, Gulf of Cadiz — Evidence for multiple sources and transport mechanisms in active sedimentary wedges

Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ¹³C–CH₄ and δ²H–CH₄ values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH₄/(C₂H₆ + C₃H₈) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C₂₊ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C₂₊-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH₄ having δ¹³C–CH₄ ~ − 50‰, as at most sites, but markedly lower δ²H–CH₄ values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH₄ is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH₄.     

Geochemical and isotopic approach to maturity/source/mixing estimations for natural gas and associated condensates in the Thrace Basin,NW Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH₄, thermogenic C₂₊ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH₄ is bacteriogenic (Calculated δ¹³C_C1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH₄. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ¹³C_C1–C = −35.80‰; Hamitabat Field) and Group-3 (δ¹³C_1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C₂₊ gases. The Group-2 C₂₊ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ¹³C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C₂₊ gases, produced from one field, were generated from isotopically light marine kerogen (δ¹³C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C₂₊ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ¹³C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R_eq = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.