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1.
Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ13C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C5, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ13C1: −51.1 to −48.3‰, δ13C2: −37.9 to −25.3‰, δ13C3: −26.0 to −19.2 ‰, respectively. The δD1 values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.  相似文献   

2.
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 (C5–8) 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.  相似文献   

3.
Chemical and isotopic compositions have been measured on 62 microbial gases from Tertiary hemipelagic sediments in the Middle America Trench off Guatemala and from decaying kelp and surf grass currently accumulating in Scripps Submarine Canyon off southern California. Gases from the Middle America Trench have been generated primarily by the reduction of carbon dioxide; methane δ13C varies from −84‰ to −39‰, methane δD varies from −208‰ to −145‰, and carbon dioxide δ13 C varies from −27‰ to +28‰. Gases from Scripps Submarine Canyon have been generated primarily by acetate dissimilation; methane δ13 C varies from −63‰ to −43‰, methane δD varies from −331‰ to −280‰, and carbon dioxide δ13C varies from −17‰ to +3‰.Methane δ13C values as heavy as −40‰ appear to be uncommon for gases produced by carbon dioxide reduction and, in the Middle America Trench, are associated with unusually positive carbon dioxide δ13C values. However, based on the 25‰ intramolecular fractionation between acetate car☐yl carbon and methyl carbon estimated from the Scripps Submarine Canyon data, methane produced by acetate dissimilation may commonly have heavy δ13C values. The δD of methane derived from acetate is more negative than natural methanes from other origins. Microbial methane δD values appear to be controlled primarily by interstitial water δD and by the relative proportions of methane derived from carbon dioxide and acetate.The chemical and isotopic compositions of microbial gas and thermogenic gas overlap, making it difficult to determine the origins of many commercial natural gases from methane δ13C and C2+ hydrocarbon concentrations alone. Measurements of methane δD and carbon dioxide δ13C can provide useful additional information, and together with ethane δ13C data, help identify gases with mixed microbial and thermogenic origins.  相似文献   

4.
A review of the geochemistry of methane in natural gas hydrate   总被引：7，自引：0，他引：7  
The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO2 from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ13CPDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ13C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd  相似文献   

5.
Coalbed methane in the Ruhr Basin, Germany: a renewable energy resource?   总被引：1，自引：0，他引：1  
Around the globe underground hard coal mining leads to a release of methane into the atmosphere. About 7% of the global annual methane emissions originate from coal mining. In the year 2002, 16 countries used coal gas to generate heat and electricity. In many cases, the exact size of coalbed methane reservoirs is not identified. The possibility of a long-term gas production and its profitability at single sites are unknown. To clarify these points, the processes of gas generation as well as the gas-in-place volume have to be determined. Both issues are tackled here for the Ruhr basin. Within this basin, coal gas samples were taken at 13 gas production sites, spread over three samplings within 14 months. There were virtually no changes in the concentrations of gas components at single sites within this period. The isotope composition of methane (δ13C-methane: −40.0 to −57.3‰ vs. PDB) revealed that the produced methane is a mixture of gases of thermogenic and microbial origin. The microbial contribution of methane seems to be more pronounced at sites of active and especially abandoned coal mining than at unmined places. Ethane and propane are of thermogenic origin, with ethane's isotopic composition tending to heavier values (richer in 13C) with time. This time-dependent phenomenon is interpreted as being caused by desorption. In addition, living methanogenic archaea were detected in mine water samples from depths down to 1200 m.  相似文献   

6.
Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C2+ alkanes. The δ13C1 (−38.6‰ to −36.2‰) and δ13C2 values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs.  相似文献   

7.
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.  相似文献   

8.
We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH4, C2H6, and δ13CCH4, and the pore fluids for CH4, C2H6, δ13CCH4, Cl−, SO42−, δ18OH2O, and δDH2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl− concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to  17% dilution of seawater, however the latter was not enriched in CH4. The isotopic compositions of pore water samples suggested the low-Cl− fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH4 enrichment more than 370 μmol/kg, but not depleted in Cl− concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl− concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site (δ13CCH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.  相似文献   

9.
The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low Th values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher Th values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in 18O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons.  相似文献   

10.
A large suite of natural gases (93) from the North West Shelf and Gippsland and Otway Basins in Australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases. About 26% of these gases have anomalous stable carbon isotope compositions in the C1–C4 hydrocarbons and CO2 components, and are interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations between successive n-alkane homologues (up to +29‰ PDB) and isotopically heavy CO2 (up to +19.5‰ PDB). Irrespective of geographic location, these anomalous gases are from the shallower accumulations (600–1700 m) where temperatures are lower than 75°C. The secondary biogenic gases are readily distinguishable from thermogenic gases (74% of this sample suite), which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m. While dissolution effects may have contributed to the high 13C enrichment of the CO2 component in the secondary biogenic gases, the primary signature of this CO2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis. Correlation between biodegraded oils and biodegraded “dry” gas supports the concept that gas is formed from the bacterial destruction of oil, resulting in “secondary biogenic gas”. Furthermore, the prominence of methanogenic CO2 in these types of accumulations along with some isotopically-depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked. It is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis.  相似文献   

11.
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12.
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.  相似文献   

13.
Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO3), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO3). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ18O (+21‰ to +25‰) and δ13C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ18O (+18‰ to +30‰) and δ13C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ13C and δ18O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ13C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ18O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ18O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower 18O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.  相似文献   

14.
This study provided an overview of the geological setting and geochemical characteristics of the Pleistocene shallow gas accumulations in the eastern Qaidam Basin, NW China. The five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km2 and 7.9 trillion cubic feet (tcf) of proven gas reserves. The dominance of methane (mostly more than 99.9%) and the δ13C and δD values of methane (−68.51 to −65.00‰ and −227.55 to −221.94‰, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (<75 °C). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments was made possible by the lake basin’s high altitude (2600–3000 m), high water salinity (>15% TDS) and strong stratification. The deposition and extensive lateral occurrence of lacustrine – shoreline sands/silts in beach sand sheets and sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals are provided by a combination of factors, such as the intermittent vertical variation in the sediment lithologies, hydraulic trapping due to the mudstone water saturation, the hydrocarbon gradient created as the result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400–800 m of muds and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication.  相似文献   

15.
The Deccan Syneclise is considered to have significant hydrocarbon potential.However,significant hydrocarbon discoveries,particularly for Mesozoic sequences,have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks.In this study,near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area.Soil samples were collected from favorable areas identified by integrated geophysical studies.The compositional and isotopic signatures of adsorbed gaseous hydrocarbons(methane through butane) were used as surface indicators of petroleum micro-seepages.An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons(C1-C4) and their carbon isotopes from the western part of Tapti graben,Deccan Syneclise,India.The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases.High concentrations of adsorbed thermogenic methane(C_1 = 518 ppb) and ethane plus higher hydrocarbons(ΣC_(2+) = 977 ppb) were observed.Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents.This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks,beneath Deccan Traps.The carbon isotopic composition of methane,ethane and propane ranges are from-22.5‰ to-30.2‰ PDB,-18.0‰to 27.1‰ PDB and 16.9‰-32.1‰ PDB respectively,which are in thermogenic source.Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks.Prominent hydrocarbon concentrations were associated with dykes,lineaments and presented on thinner basaltic cover in the study area,which probably acts as channel for the micro-seepage of hydrocarbons.  相似文献   

16.
The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ18O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ13C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ13C = − 8‰) and raised precipitation temperatures (δ18O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.  相似文献   

17.
This study aims to assess the hydrocarbon potential of Ganga basin utilizing the near surface geochemical prospecting techniques. It is based on the concept that the light gaseous hydrocarbons from the oil and gas reservoirs reach the surface through micro seepage, gets adsorbed to soil matrix and leave their signatures in soils and sediments, which can be quantified. The study showed an increased occurrence of methane (C1), ethane (C2) and propane (C3) in the soil samples. The concentrations of light gaseous hydrocarbons determined by Gas Chromatograph ranged (in ppb) as follows, C1: 0–519, C2: 0–7 and C3: 0–2. The carbon isotopic (VPDB) values of methane varied between ?52.2 to ?27.1‰, indicating thermogenic origin of the desorbed hydrocarbons. High concentrations of hydrocarbon were found to be characteristic of the Muzaffarpur region and the Gandak depression in the basin, signifying the migration of light hydrocarbon gases from subsurface to the surface and the area’s potential for hydrocarbon resources.  相似文献   

18.
The CO2 gas reservoir sandstones in the Hailaer Basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of CO2. The origins of dawsonite and CO2 in these sandstones were studied by petrographic and isotopic analysis. According to the paragenetic sequence of the sandstones, dawsonite grew later than CO2 charging at 110–85 Ma. The dawsonite δ18O value is 7.4‰ (SMOW), and the calculated δ18O values of the water present during dawsonite growth are from −11.4‰ to −9.2‰ (SMOW). This, combined with the NaHCO3-dominated water linked to dawsonite growth, suggests meteoric water being responsible for dawsonite growth. The δ13C values of gas phase CO2 and the ratios of 3He/4He of the associated He suggest a mantle magmatic origin of CO2-rich natural gas in Hailaer basin. Dawsonite δ13C values are −5.3‰ to −1.5‰ (average −3.4‰), and the calculated δ13C values of CO2 gas in isotopic equilibrium with dawsonite are −11.4‰ to −7.3‰. These C isotopic values are ambiguous for the dawsonite C source. From the geological context, the timing of events, together with formation water conditions for dawsonite growth, dawsonite possibly grew in meteoric-derived water, atmospherically-derived CO2 maybe, or at least the dominant, C source for dawsonite. It seems that there are few relationships between dawsonite and the modern gas phase of CO2 in the Hailaer basin.  相似文献   

19.
The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have 3He/4He ratios of 0.16 to 0.86 Ra, whereas their 40Ar/36Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ34S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ18O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.  相似文献   

20.
Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high δ13C values of C1–C4 gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the δ13CiC4 value is higher than that of the δ13CnC4, the dominant ranges for the δ13C1, δ13C2, and δ13C3 values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The δ13C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the δ13C1, δ13C2 and δ13C3 values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.  相似文献   

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