Origin and distribution of hydrogen sulfide in the Yuanba gas field,Sichuan Basin,Southwest China

The Yuanba gas field in the Permian Changxing Formation (P₂c), which exhibits wide variations in its hydrogen sulfide (H₂S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ¹³C values, which are smaller than the δ¹³C values of the host dolostone, indicate that the H₂S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P₂c's gases is generally lower than 0.1. The ethane δ¹³C values increase as the GSI increases, although no obvious increase was observed in the methane δ¹³C values. The calcite cements' δ¹³C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P₂c's reservoirs, and dissolved sulfate anions (SO₄²⁻) were mainly enriched during dolomitization. Insufficient dissolved SO₄²⁻ most likely caused the lower H₂S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO₄²⁻ in residual water in paleo-oil zones, SO₄²⁻ from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO₄²⁻ and can produce more H₂S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H₂S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H₂S concentrations in a single reservoir. Carbon dioxide (CO₂), which has a relatively heavy δ¹³C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO₂ and inorganic fluid systems, and carbonate decomposition.     

TSR-altered oil with high-abundance thiaadamantanes of a deep-buried Cambrian gas condensate reservoir in Tarim Basin

The first exploratory well, the ZS1C well, with 158,545 m³ daily gas production was discovered in 6861–6944 m deep strata of the Cambrian gypsolyte layer of the Tarim Basin, China in 2014. The discovery opens a new target for the Cambrian-reservoired oil and gas exploration, and directly leads to large-scale oil and gas exploration of the deep-reservoired Cambrian oil and gas fields in the Basin. Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry and a comprehensive two-dimensional gas chromatography–flame ionization detector revealed the presence of abundant adamantane compounds, 2-thiaadamantanes and 2-thiadiamantanes, and a large amount of sulfur-containing compounds in the condensate oil. The formation of organic sulfur-containing compounds, such as 2-thiaadamantanes, is an indication of sulfur incorporation from the gypsum in the stratum into oil and gas in the course of TSR. This reservoir has apparently suffered severe TSR alteration because (1) High content of H₂S, (2) H₂S sulfur isotopes, (3) CO₂ carbon isotopes, and others abundant data to support this findings. Similar sulfur isotopic composition of H₂S, oil condensate and the gypsum in the Cambrian strata indicate that the produced condensate is experienced TSR alteration. Therefore, the deep-accumulated Cambrian oil reservoir has experienced severe TSR alteration, and accumulated natural gas and condensate contains high sulfur content.     

Origin of high H2S concentrations in the Upper Permian Changxing reservoirs of the Northeast Sichuan Basin,China

Intense thermochemical sulfate reduction (TSR) and up to 18% H₂S are found in the Upper Permian Changxing Formation (P₃ch) in the northeast (NE) Sichuan Basin, China, despite that rare gypsum or anhydrite was found in this formation. Here, we present new concentration data of carbonate-associated sulfate (CAS) from carbonate host rocks, C, O, and Sr isotope data for TSR-related calcites, and S isotope data for sulfur compounds obtained during this study. These data along with spatial-temporal changes in palaeogeopressure conditions, hydraulic conductivity and the physical capacity indicate that the H₂S was generated locally from TSR within the P₃ch reservoirs. We propose that the reactive sulfates were derived from CAS released during dolomitization and recrystallization of earlier dolomite within the P₃ch Fm. and from the cross-formational migration of evaporative brines from the Lower Triassic Feixianguan Formation (T₁f) to P₃ch Fm. Our calculation shows that the two sources could provide enough SO₄²⁻ for the generation of H₂S within the P₃ch reservoirs. Early downward migration of sulfate-rich evaporative brines from the T₁f formation occurred in near-surface and shallow burial diagenetic settings (mainly <1000 m). The evaporative brines seeped into porous grainstones and displaced preexisting seawater, causing pervasive dolomitization within the P₃ch Fm. Subsequently, TSR calcites precipitated from the pore water have high Sr concentrations (up to 7767 ppm), close to the T₁f TSR calcites, and ⁸⁷Sr/⁸⁶Sr ratios mainly from 0.7074 to 0.7078, which are significantly higher than those of Late Permian seawater but within the range of early Triassic seawater.     

Geochemical characteristics and isotopic reversal of natural gases in eastern Kopeh-Dagh,NE Iran

Natural gas samples from two gas fields located in Eastern Kopeh-Dagh area were analyzed for molecular and stable isotope compositions. The gaseous hydrocarbons in both Lower Cretaceous clastic reservoir and Upper Jurassic carbonate reservoir are coal-type gases mainly derived from type III kerogen, however enriched δD values of methane implies presence of type II kerogen related material in the source rock. In comparison Upper Jurassic carbonate reservoir gases show higher dryness coefficient resulted through TSR, while presence of C₁C₅ gases in Lower Cretaceous clastic reservoir exhibit no TSR phenomenon. Carbon isotopic values indicate gas to gas cracking and TSR occurrence in the Upper Jurassic carbonate reservoir, as the result of elevated temperature experienced, prior to the following uplifts in last 33–37 million years. The δ¹³C of carbon dioxide and δ³⁴S of hydrogen sulfide in Upper Jurassic carbonate reservoir do not primarily reflect TSR, as uplift related carbonate rock dissolution by acidic gases and reaction/precipitation of light H₂S have changed these values severely. Gaseous hydrocarbons in both reservoirs exhibit enrichment in C₂ gas member, with the carbonate reservoir having higher values resulted through mixing with highly-mature-completely-reversed shale gases. It is likely that the uplifts have lifted off the pressure on shale gases, therefore facilitated the migration of the gases into overlying horizons. However it appears that the released gases during the first major uplift (33–37 million years ago) have migrated to both reservoirs, while the second migrated gases have only mixed with Upper Jurassic carbonate reservoir gases. The studied data suggesting that economic accumulations of natural gas/shale gases deeper than Upper Jurassic carbonate reservoir would be unlikely.     

In-reservoir mixing of mantle-derived CO2 and metasedimentary CH4–N2 fluids – Noble gas and stable isotope study of two multistacked fields (Pannonian Basin System,W-Hungary)

Several multistacked gas fields, located in very different parts of Hungary, display a large variation in their gas composition. The shallow position of the mantle beneath the basement of the PBS is responsible for the high heat flux. It facilitates that the CO₂ – intensely released from the ascending upper-mantle derived melts – could reach the basin system. In this work the results of a geochemical study of the main components and noble gases of the multistacked Répcelak (CO₂–HC–N₂) and Mihályi (CO₂) fields are presented and discussed. The vertical variations of gas composition in the Répcelak and Mihályi fields fit well with in-reservoir mixing of two end-members, a mantle-related magmatic CO₂ fluid and a N₂-rich HC-gas fluid of crustal origin. However, it is very likely that other processes modified to some extent the gas composition of the two fields. Carbon isotopic composition of the CO₂ of the samples studied agrees with both the carbonate decomposition and mantle degassing origins. It is very likely that the marked difference in helium isotopic composition between the neighbouring Répcelak and Mihályi fields also reflects this heterogeneity.     

Geochemistry,origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin,South China Sea

The Qiongdongnan Basin, South China Sea has received huge thickness (>12 km) of Tertiary-Quaternary sediments in the deepwater area to which great attention has been paid due to the recent discoveries of the SS22-1 and the SS17-2 commercial gas fields in the Pliocene-Upper Miocene submarine canyon system with water depth over 1300 m. In this study, the geochemistry, origin and accumulation models of these gases were investigated. The results reveal that the gases are predominated by hydrocarbon gases (98%–99% by volume), with the ratio of C₁/C_1-5 ranging from 0.92 to 0.94, and they are characterized by relatively heavy δ¹³C₁ (−36.8‰ to −39.4‰) and δD_CH4 values (−144‰ to −147‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The C_5-7 light hydrocarbons associated with these gases are dominated by isoparaffins (35%–65%), implying an origin from higher plants. For the associated condensates, carbon isotopic compositions and high abundance of oleanane and presence of bicadinanes show close affinity with those from the YC13-1 gas field in the shallow water area. All these geochemical characteristics correlate well with those found in the shales of the Oligocene Yacheng Formation in the Qiongdongnan Basin. The Yacheng Formation in the deepwater area has TOC values in the range of 0.4–21% and contains type II_b–III gas-prone kerogens, indicating an excellent gas source rock. The kinetic modeling results show that the δ¹³C₁ values of the gas generated from the Yacheng source rock since 3 or 4 Ma are well matched with those of the reservoir gases, indicating that the gas pool is young and likely formed after 4 Ma. The geologic and geochemical data show that the mud diapirs and faults provide the main pathways for the upward migration of gases from the deep gas kitchen into the shallow, normally pressured reservoirs, and that the deep overpressure is the key driving force for the vertical and lateral migration of gas. This gas migration pattern implies that the South Low Uplift and the No.2 Fault zone near the deepwater area are also favorable for gas accumulation because they are located in the pathway of gas migration, and therefore more attention should be paid to them in the future.     

The impact of organic fluids on the carbon isotopic compositions of carbonate-rich reservoirs: Case study of the Lucaogou Formation in the Jimusaer Sag,Junggar Basin,NW China

The processes involved in the interaction between organic fluids and carbonates, and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates. Here, we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic–clastic and organic-rich sedimentary sequence within the middle Permian Lucaogou Formation in the Junggar Basin of NW China. The techniques used during this study include drillcore observations, thin section petrography, scanning electron microscopy (SEM) and electron probe microanalysis, and carbon and oxygen isotope analyses. Oil grades and total organic carbon (TOC) contents represent the amount of oil charging and the abundance of organic fluids within a reservoir, respectively, and both negatively correlate with the whole-rock δ¹³C and δ¹⁸O of the carbonates in the study area, indicating that organic fluids have affected the reservoir rocks. Secondary carbonates, including sparry calcite and dolomite overgrowths and cements, are common within the Lucaogou Formation. Well-developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite-rich reservoir rocks in this formation. Comparing thin section petrology with δ¹³C compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates. The net change in whole-rock δ¹³C as a result of these secondary carbonates ranges from 1.8‰ to 4.6‰, with the secondary carbonates having calculated δ¹³C compositions from −18.6‰ to −8.5‰ that are indicative of an organic origin. The positive correlation between the concentration of Fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the Lucaogou Formation was relatively closed. The correlation between δ¹³C and δ¹⁸O in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids. However, good initial correlation between δ¹³C and δ¹⁸O of whole rock carbonates within the Lucaogou Formation (resulted from the evaporitic sedimentary environment) was reduced by organic fluids to some extent. Consequently, the δ¹³C–δ¹⁸O covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water.The characteristics and δ¹³C compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic CO₂ to the reservoir. Further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic CO₂. However, dissolved carbonate reprecipitated as secondary carbonates, meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality, although this process did enhance the dissolution of feldspar and increase porosity. This indicates that the δ¹³C and δ¹⁸O of secondary carbonates and their influence on whole-rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate-rich reservoir systems.     

Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C₁/_C2+ from all the seepage sites were in the range of 1,500–50,000, while δ¹³C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO₂. δ¹³C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ¹³C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.     

Methane-dominated thermochemical sulphate reduction in the Triassic Feixianguan Formation East Sichuan Basin, China: towards prediction of fatal H2S concentrations

New sour pools have recently found in the Lower Triassic Feixianguan Fm carbonate reservoirs in the East Sichuan Basin in China with H₂S up to 17.4% by volume. A recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of H₂S. In order to assess the origin of fatal H₂S as well as the cause of petroleum alteration, H₂S concentrations and the isotopes, δ³⁴S and δ¹³C have been collected and measured in gas samples from reservoirs. Anhydrite, pyrite and elemental sulphur δ³⁴S values have been measured for comparison. The high concentrations of H₂S gas are found to occur at depths >3000 m (temperature now at 100 °C) in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs. Where H₂S concentrations are greater than 10% its δ³⁴S values lie between +12.0 and +13.2‰ CDT. This is within the range of anhydrite δ³⁴S values found within the Feixianguan Fm (+11.0 to +21.7‰; average 15.5±3.5‰ CDT). Thus H₂S must have been generated by thermochemical sulphate reduction (TSR) locally within the reservoirs. Burial history analysis and fluid inclusion data reveal that the temperature at which TSR occurred was greater than about 130–140 °C, suggesting that the present depth-temperature minimum is an artifact of post-TSR uplift. Both methane and ethane were actively involved in TSR since the petroleum became almost totally dry (no alkanes except methane) and methane δ¹³C values become significantly heavier as TSR proceeded. Methane δ¹³C difference thus reflects the extent of TSR. While it is tempting to use a present-day depth control (>3000 m) to predict the distribution of H₂S in the Feixianguan Fm, this is an invalid approach since TSR occurred when the formation was buried some 1000–2000 m deeper than it is at present. The likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the Feixianguan Fm so that it is possible to determine which parts of the basin have been hotter than 130–140 °C.     

Contribution of siderite–water interaction for the unconventional generation of hydrocarbon gases in the Solimões basin,north-west Brazil

Hydrocarbon gases with unconventional carbon isotopic signatures were observed in the Solimões sedimentary basin in north-west Brazil. Siderite contents measured with a new Rock-Eval methodology in the drill-cuttings samples of the Famenian source rock were found to decrease with the increase of gas maturity and with the occurrence of the gas isotopic anomalies. Triassic diabase intrusions induced heating of the source rock, which likely resulted in the gradual oxidative dissolution of siderite as suggested by the observation of etch pits on the siderite surfaces. It is proposed that ferrous iron from the carbonate was involved in a redox reaction with water producing ferric iron and H₂, then reducing CO₂ and yielding an inverse correlation between siderite content and gas maturity. Alternatively, hydrogenation of highly mature kerogen by H₂ derived from siderite could explain the production of ¹³C-rich CH₄. Mass balance considerations suggest that these mechanisms may account for a significant fraction of the hydrocarbon gases generated from the Famenian source rock in the Solimões basin.     

Gas geochemistry of the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: Implications for gas hydrate exploration in the Arctic

Gases were analyzed from well cuttings, core, gas hydrate, and formation tests at the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well, drilled within the Milne Point Unit, Alaska North Slope. The well penetrated a portion of the Eileen gas hydrate deposit, which overlies the more deeply buried Prudhoe Bay, Milne Point, West Sak, and Kuparuk River oil fields. Gas sources in the upper 200 m are predominantly from microbial sources (C₁ isotopic compositions ranging from −86.4 to −80.6‰). The C₁ isotopic composition becomes progressively enriched from 200 m to the top of the gas hydrate-bearing sands at 600 m. The tested gas hydrates occur in two primary intervals, units D and C, between 614.0 m and 664.7 m, containing a total of 29.3 m of gas hydrate-bearing sands. The hydrocarbon gases in cuttings and core samples from 604 to 914 m are composed of methane with very little ethane. The isotopic composition of the methane carbon ranges from −50.1 to −43.9‰ with several outliers, generally decreasing with depth. Gas samples collected by the Modular Formation Dynamics Testing (MDT) tool in the hydrate-bearing units were similarly composed mainly of methane, with up to 284 ppm ethane. The methane isotopic composition ranged from −48.2 to −48.0‰ in the C sand and from −48.4 to −46.6‰ in the D sand. Methane hydrogen isotopic composition ranged from −238 to −230‰, with slightly more depleted values in the deeper C sand. These results are consistent with the concept that the Eileen gas hydrates contain a mixture of deep-sourced, microbially biodegraded thermogenic gas, with lesser amounts of thermogenic oil-associated gas, and coal gas. Thermal gases are likely sourced from existing oil and gas accumulations that have migrated up-dip and/or up-fault and formed gas hydrate in response to climate cooling with permafrost formation.     

Sulfur Isotopes in the Upper Part of the Black Sea Anoxic Zone

New data are reported on the sulfur isotope composition and concentration of sulfide and sulfate in the upper part of the Black Sea anoxic zone as a function of the potential water density. The observations were performed at a station with the coordinates 44.489° N and 37.869° E three times a week every two days. A local negative deficiency in sulfate concentration up to 1.7% related to the sulfate reduction processes was recorded. This anomaly in sulfate concentration was short-lived and did not affect the sulfur isotope composition. In the upper part of the anaerobic zone, the δ³⁴S(SO₄) value varied from 21.2 to 21.5‰, which could have occurred from mixing of water masses from the oxic zone (21.1‰) and the Bottom Convective Layer (23.0 ± 0.2‰). The sulfur isotope composition of sulfide ranged from ?40.8% at a depth of 250 m to ?39.4‰ at the upper boundary of the anoxic zone with a H₂S content of only 2.7 μM. Two models (mass balance and fractionation of sulfur isotopes using the Rayleigh equation) are considered to explain the differences in δ³⁴S(H₂S) values observed.     

Mineralogical-geochemical features of sulfide ores from the Broken Spur hydrothermal vent field

A representative collection of hydrothermal sediments was sampled practically from all the hydrothermal mounds of the Broken Spur hydrothermal vent field from the Mir manned submersibles during three cruises of R/V Akademik Mstislav Keldysh. Mineral associations characteristic of different morphological types of sulfide ores from hydrothermal pipes, plates, and diffusers are assessed. Particular attention is paid to the distribution of minor elements and their distribution patterns determined by the mineralogical zonation. The measured isotopic value of the sulfur in the sulfide minerals appeared to vary from 0.4 to 5.2‰, which indicates their similarity with the ores from the Snake Pit vent field and is related to the dilution of hot ore-bearing solutions by seawater and reduction of the water sulfate ions to H₂S with a heavy isotopic composition.     

Organic geochemistry of marine source rocks and pyrobitumen-containing reservoir rocks of the Sichuan Basin and neighbouring areas,SW China

Three bitumen fractions were obtained and systematically analysed for the terpane and sterane composition from 30 Paleozoic source rocks and 64 bitumen-containing reservoir rocks within the Upper Sinian, Lower Cambrian, Lower Silurian, Middle Carboniferous, Upper Permian and Lower Triassic strata in the Sichuan Basin and neighbouring areas, China. These bitumen fractions include extractable oils (bitumen I), oil-bearing fluid inclusions and/or closely associated components with the kerogen or pyrobitumen/mineral matrix, released during kerogen or pyrobitumen isolation and demineralization (bitumen II), and bound compounds within the kerogen or pyrobitumen released by confined pyrolysis (bitumen III). In addition, atomic H/C and O/C ratios and carbon isotopic compositions of kerogen and pyrobitumen from some of the samples were measured. Geochemical results and geological information suggest that: (1) in the Central Sichuan Basin, hydrocarbon gases in reservoirs within the fourth section of the Upper Sinian Dengying Formation were derived from both the Lower Cambrian and Upper Sinian source rocks; and (2) in the Eastern Sichuan Basin, hydrocarbon gases in Middle Carboniferous Huanglong Formation reservoirs were mainly derived from Lower Silurian source rocks, while those in Upper Permian and Lower Triassic reservoirs were mainly derived from both Upper Permian and Lower Silurian marine source rocks. For both the source and reservoir rocks, bitumen III fractions generally show relatively lower maturity near the peak oil generation stage, while the other two bitumen fractions show very high maturities based on terpane and sterane distributions. Tricyclic terpanes evolved from the distribution pattern C₂₀ < C₂₁ < C₂₃, through C₂₀ < C₂₁ > C₂₃, finally to C₂₀ > C₂₁ > C₂₃ during severe thermal stress. The concentration of C₃₀ diahopane in bitumen III (the bound components released from confined pyrolysis) is substantially lower than in the other two bitumen fractions for four terrigenous Upper Permian source rocks, demonstrating that this compound originated from free hopanoid precursors, rather than hopanoids bound to the kerogen.     

Transformation of suspended particulate matter into sediment in the Kara Sea in September of 2011

The biogeochemical processes participating in the transformation of the particulate matter into sediment along the Yenisei River-St. Anna Trough (Kara Sea) meridional profile were studied using hydrochemical, geochemical, microbiological, radioisotope, and isotope methods. The water-sediment contact zone consists of three subzones: the suprabottom water, the fluffy layer, and the surface sediment. The total number, biomass, and integral activity of the microorganisms (dark ¹⁴CO₂ assimilation) in the fluffy layer are usually higher than in the suprabottom water and sediment. The fluffy layer shows a decrease in the oxygen content and the growth of the dissolved biogenic elements. It was provided by the particulate organic matter supporting the vital activity of the heterotrophs from the overlying water column and by the flux of reduced compounds (NH₄, H₂S, CH₄, Fe²⁺, Mn²⁺, and others) from the underlying sediments. The C_org isotopic composition of the fluffy layer and the sediments is 2–4 ‰ heavier than that of the particulate matter and sediment due to the presence of the isotopically heavy biomass of microorganisms. A change in the isotopic composition of the C_org in the fluffy layer and surface sediment as compared to the C_org of the particulate matter is a widespread phenomenon in the Arctic shelf seas and proves the leading role of microorganisms in the transformation of the particulate matter into sediment.     

Greenhouse gases generated from the anaerobic biodegradation of natural offshore asphalt seepages in southern California

Significant offshore asphaltic deposits with active seepage occur in the Santa Barbara Channel offshore southern California. The composition and isotopic signatures of gases sampled from the oil and gas seeps reveal that the coexisting oil in the shallow subsurface is anaerobically biodegraded, generating CO₂ with secondary CH₄ production. Biomineralization can result in the consumption of as much as 60% by weight of the original oil, with ¹³C enrichment of CO₂. Analyses of gas emitted from asphaltic accumulations or seeps on the seafloor indicate up to 11% CO₂ with ¹³C enrichment reaching +24.8‰. Methane concentrations range from less than 30% up to 98% with isotopic compositions of –34.9 to –66.1‰. Higher molecular weight hydrocarbon gases are present in strongly varying concentrations reflecting both oil-associated gas and biodegradation; propane is preferentially biodegraded, resulting in an enriched ¹³C isotopic composition as enriched as –19.5‰. Assuming the 132 million barrels of asphaltic residues on the seafloor represent ~40% of the original oil volume and mass, the estimated gas generated is 5.0×10¹⁰ kg (~76×10⁹ m³) CH₄ and/or 1.4×10¹¹ kg CO₂ over the lifetime of seepage needed to produce the volume of these deposits. Geologic relationships and oil weathering inferences suggest the deposits are of early Holocene age or even younger. Assuming an age of ~1,000 years, annual fluxes are on the order of 5.0×10⁷ kg (~76×10⁶ m³) and/or 1.4×10⁸ kg for CH₄ and CO₂, respectively. The daily volumetric emission rate (2.1×10⁵ m³) is comparable to current CH₄ emission from Coal Oil Point seeps (1.5×10⁵ m³/day), and may be a significant source of both CH₄ and CO₂ to the atmosphere provided that the gas can be transported through the water column.     

Bitumen biomarkers in the Mid-Proterozoic Ilímaussaq intrusion, Southwest Greenland - A challenge to the mantle gas theory

GC and GC/MS/MS analysis on rock extracts has shown that the bitumen in the peralkaline Ilímaussaq intrusion, previously assumed to be abiogenic, is biotic in origin. A biotic origin is in accordance with previously published stable carbon isotopic data on bituminous matter in the rocks. The biomarker distribution in the bitumen, including the less common bicadinanes, resembles that of oil seeps on the central West Greenland coast 2200 km farther north, whose source rocks and migration history are relatively well established. We use a recent re-construction of the subsidence and later exhumation of the West Greenland coastal region during the Mesozoic-Cenozoic (Japsen et al., 2006a, b) to anticipate that hydrocarbons migrated from deeper parts of the basin offshore west of Greenland. The rocks of Ilímaussaq were probably more deeply invaded than the surrounding granites due to their higher proportion of corroded minerals, which may explain why bitumen has not been observed elsewhere in the area.Hydrocarbon gases (C1-C5) present in fluid inclusions were also analysed, after having been released by treatment with hydrochloric acid that resulted in an almost complete disintegration of the Ilímaussaq intrusion rocks. The acid extraction method proved generally more efficient than the crushing procedure applied by others, but gave similar results for the chemical composition of the gas (CH₄: 88-97%) and isotopic ratios (δ¹³C₄CH: −1.6 to −5.0‰; δ¹³CC₂H₆: −9.2 to −12.5‰), with the exception of hydrocarbons hosted in quartz, which showed significantly lower isotopic values for methane (Graser et al., 2008). Previous researchers have suggested an abiotic origin for these hydrocarbon gases, but we suggest a biotic origin for the majority of them, not just those in quartz, assuming that the isotopic ratio of the constituents have changed due to loss of gas by diffusion. The assumption of gas loss via diffusion is supported by published studies on micro-fissures in minerals typical of the Ilímaussaq and field investigations showing diffusive loss of gas from the peralkaline Khibina and Lovozero massifs on the Kola Peninsula, Russia, which are, in many respects of mineralogy and hydrocarbon content, similar to the Ilímaussaq intrusion.Both the hydrocarbon gases and bitumen in the Khibina and Lovozero massifs have been cited as prime examples of a deep mantle source, although the carbon isotopic ratio of the bitumen clearly pointed to an organic origin. The trends in carbon isotopic ratio of methane released with time from freshly exposed rocks also supports our hypothesis of ¹³C enrichment of the methane remaining within the rock. Thus, there is good evidence that the hydrocarbons in the Kola alkaline massifs are mostly biotic in origin, in which case the probability of finding economic hydrocarbon accumulations from a deep mantle source seems exceedingly small.     

Saddle dolomite and calcite cements as records of fluid flow during basin evolution: Paleogene carbonates,United Arab Emirates

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately ⁸⁷Sr-rich basinal NaCl–MgCl₂–H₂O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ¹⁸O_V-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ¹⁸O_V-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE–SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately ⁸⁷Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ¹⁸O_V-PDB and δ¹³C_V-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.     

Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH₃Cl, N₂O, and CH₄ within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH₃Cl, N₂O, and CH₄ during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C₂H₆, 5 times for C₃H₈, and 20 times for n-C₄H₁₀. The δ¹³C values of both C₂H₆ and C₃H₈ remained almost constant while concentrations increased, whereas that of n-C₄H₁₀ increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ¹³C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C₂H₆ and C₃H₈ were produced during the autolysis of diatoms cells, whereas n-C₄H₁₀ was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.     

Geochemical characteristics of gases from the largest tight sand gas field (Sulige) and shale gas field (Fuling) in China

This study performed a detailed geochemical analyses of the components, stable carbon isotopes of alkane gas and CO₂, stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field (Sulige) and shale gas (Fuling) field in China. The comparative study shows that tight gas from the Sulige gas field in the Ordos Basin is of coal-derived origin, which is characterized by a positive carbon and hydrogen isotopic distribution pattern (δ¹³C₁ > δ¹³C₂ > δ¹³C₃ > δ¹³C₄; δ²H₁ > δ²H₂ > δ²H₃), i.e., the carbon and hydrogen isotopes increase with increasing carbon numbers. Carbon dioxide from this field are of biogenic origin and the helium is crust-derived. Shale gas from the Fuling shale gas field belongs to oil-derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin (δ¹³C₁ < δ¹³C₂ < δ¹³C₃; δ²H₁ < δ²H₂ < δ²H₃), i.e., the carbon and hydrogen isotopes decrease with increasing carbon numbers. Such complete isotopic reversal distribution pattern is due to the secondary alteration like oil or gas cracking, diffusion and so on under high temperature. In that case, positive carbon or hydrogen isotopic distribution pattern will change into complete isotopic reversal as the temperature increases. Carbon dioxide is of abiogenic origin resulting from the thermal metamorphism of carbonates and helium is crust-derived.