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1.
Gas associated with Carboniferous coal seams and younger Miocene sediments in the SW Upper Silesian Basin in Czech Republic shows wide compositional variation. Coal-related gas extracted from cross-measure degassing boreholes, as well as gas released during canister desorption of coal samples from three different mines was analyzed to evaluate the genetic origin and the influence of sorption/desorption processes on the gas composition. Analyses comprised the compositional and stable carbon and isotope composition of coal-related gases. The isotopic composition of gas from cross-measure boreholes indicates thermogenic origin in the southern part of the basin and microbial CO2 reduction and mixed type origin in the northern part. Gas from canister desorption shows similar origins, but larger compositional and isotopic variation. No consistent isotopic fractionation due to desorption could be observed. Differences in geochemical composition of thermogenic gases could indicate a contribution of gas migrated from deeper formations below the Carpathian overthrust.  相似文献   

2.
The origin of the combustible gases in groundwater from glacial-outwash and fractured-bedrock aquifers was investigated in northern Tioga County, Pennsylvania. Thermogenic methane (CH4) and ethane (C2H6) and microbial CH4 were found. Microbial CH4 is from natural in situ processes in the shale bedrock and occurs chiefly in the bedrock aquifer. The δ13C values of CH4 and C2H6 for the majority of thermogenic gases from water wells either matched or were between values for the samples of non-native storage-field gas from injection wells and the samples of gas from storage-field observation wells. Traces of C2H6 with microbial CH4 and a range of C and H isotopic compositions of CH4 indicate gases of different origins are mixing in sub-surface pathways; gas mixtures are present in groundwater. Pathways for gas migration and a specific source of the gases were not identified. Processes responsible for the presence of microbial gases in groundwater could be elucidated with further geochemical study.  相似文献   

3.
In this study, compositions and δ13C and δ2H isotopic values of hydrocarbon gases from 5 mines in the Witwatersrand basin, South Africa, support the widespread occurrence of microbially produced methane in millions of years-old fissure waters. The presence of microbial methane is, to a large extent, controlled by the geologic formations in which the gases are found. Samples from the Witwatersand Supergroup have the largest microbial component based on δ13C and δ2H signatures and CH4/C2+ values. Based on mixing between a microbial CH4 component and a more 13C-enriched and 2H-depleted C2+-rich end member, conservative estimates of the % contribution of microbial CH4 to the gas samples range from >90% microbial CH4 at Beatrix, Masimong, and Merriespruit, to between 5 and 80% microbial CH4 at Evander, and <18% microbial CH4 at Kloof. The Witwatersrand basin’s history of thermal alteration of organic-rich ancient sedimentary units suggests a thermogenic origin for this 13C-enriched end member. Alternatively, the potential for an abiogenic origin similar to hydrocarbon gases produced by water-rock interaction at other Precambrian Shield mines is discussed. Microbial methane is predominantly found in paleo-meteoric fissure waters with δ18O and δ2H values that fall on the meteoric waterline, and have temperatures between 30 to 40°C. In contrast, fissure waters with a larger component of nonmicrobial hydrocarbon gases show a trend towards more enriched δ18O and δ2H values that fall well above the meteoric waterline, and temperatures of 45 to 60°C. The enrichment in 18O and 2H in these samples, and their high salinity, are similar to the isotopic and compositional characteristics of saline groundwaters and brines produced by water-rock interaction at Precambrian Shield sites elsewhere. The reported 100 Ma ages of fissure waters from the Witwatersrand and Ventersdorp formations suggest that these microbial hydrocarbon gases are the product of in situ methanogenic communities in the deep subsurface of the Witswaterand basin. Small subunit ribosomal RNA genes were amplified using archaeal-specific primer sets from DNA extracts derived from several of these waters. Fissure waters with a high proportion of microbial methane also contained sequences resembling those of known methanogens.  相似文献   

4.
柴达木盆地北缘晚石炭世地层   总被引：2，自引：0，他引：2  
本文对柴达木盆地北缘晚石炭世地层进行了划分、对比,分2个组,下部为克鲁克组,上部称扎布萨朵秀组。克鲁克组自下而上分为4个段。第2段建立(竹蜓)类Eostaffella su-bsolana-Pseudowedekindellina prolixa组合带;腕足类1—2段为Productus concinus-Choristitesyanghukouensis组合带;3段(竹蜓)类为Profusulinella-Pseudostaffella qinghaiensis组合带,4段(竹蜓)类为Fusulina-  相似文献   

5.
南岭地区在晚三叠世-早侏罗世沉积了一套海陆交互相含煤地层,其中双壳类化石十分丰富,对其详细的生物地层 学研究可以为这套地层的划分和对比、识别该区侏罗系与三叠系界线提供重要的古生物资料。本文对湖南浏阳石康、湖南 宜章下坪和广东揭西灰寨三条剖面采集的海相和非海相双壳类化石进行了系统研究。结果显示研究区上三叠统-下侏罗统 具有4个双壳类生物带,即：半咸水相Unionites? emeiensis 顶峰带、海相Retroceramus subinconditus-Hiatella 组合带、海相 Ryderia guangdongensis-Parainoceramus jinjiensis组合带和半咸水相-陆相Kija ovata-Waagenoperna mytiloides组合带。对各生物 带代表化石在区域上的时代延限分析表明,Unionites ?emeiensis 顶峰带时代延限主要为瑞替期, Retroceramus subinconditus-Hiatella 组合带主要为赫塘期,Ryderia guangdongensis-Parainoceramus jinjiensis 组合带主要为辛涅缪尔期,但有 可能向上延伸至土阿辛期。经与海相同类生物带的对比,Kija ovata-Waagenoperna mytiloides 组合带时代延限大致为赫塘期 至土阿辛期。根据综合对比分析认为南岭地区半咸水相-陆相T-J 界线应位于Unionites? emeiensis 顶峰带与Kija ovata-Waagenoperna mytiloides 组合带之间,这与四川盆地陆相T-J界线（位于Modiolus weiyuanensis-Unionites? emeiensis 组合 带与Pseudocardinia （=Kija） kweichouensis-Cuneopsis sichuanensis 组合带之间） 和新疆准噶尔盆地陆相T-J 界线（位于 Waagenoperna-Yananoconcha（=Kija）组合带与下伏郝家沟组之间） 大致相当。  相似文献   

6.
The origin of the combustible gases in groundwater from glacial-outwash and fractured-bedrock aquifers was investigated in northern Tioga County, Pennsylvania. Thermogenic methane (CH4) and ethane (C2H6) and microbial CH4 were found. Microbial CH4 is from natural in situ processes in the shale bedrock and occurs chiefly in the bedrock aquifer. The δ13C values of CH4 and C2H6 for the majority of thermogenic gases from water wells either matched or were between values for the samples of non-native storage-field gas from injection wells and the samples of gas from storage-field observation wells. Traces of C2H6 with microbial CH4 and a range of C and H isotopic compositions of CH4 indicate gases of different origins are mixing in sub-surface pathways; gas mixtures are present in groundwater. Pathways for gas migration and a specific source of the gases were not identified. Processes responsible for the presence of microbial gases in groundwater could be elucidated with further geochemical study.  相似文献   

7.
据西藏自治区大地构造演化特征,从整个青藏高原构造单元分布特点考虑,本文将西藏自治区地层区划结合构造单元和含煤地层一并考虑,将其划分为三个构造-地层大区:羌塘-三江构造-地层大区、班公湖-双湖-怒江构造-地层大区、冈底斯-喜马拉雅构造-地层大区。从区域地层、沉积构造及其古生物化石组合等特点综合分析,得出西藏赋煤区聚煤作用具有时代多、分布广、煤层层数多、厚度薄和稳定性差的总体特点。区内含煤地层包括下石炭统、上二叠统、上三叠统、中侏罗统、下白垩统和古近系等。最主要煤系是下石炭统马查拉煤系、上二叠统妥坝煤系、上三叠统土门煤系、下白垩统多尼煤系。  相似文献   

8.
《Applied Geochemistry》2001,16(7-8):895-910
Coalbed gases in the Lower Silesian Coal Basin (LSCB) of Poland are highly variable in both their molecular and stable isotope compositions. Geochemical indices and stable isotope ratios vary within the following ranges: hydrocarbon (CHC) index CHC=CH4/(C2H6+ C3H8) from 1.1 to 5825, wet gas (C2+) index C2+=(C2H6+ C3H8+ C4H10+ C5H12) / (CH4+ C2H6+ C3H8+ C4H10+ C5H12) 100 (%) from 0.0 to 48.3%, CO2–CH4 (CDMI) index CDMI=CO2/(CO2+ CH4) 100 (%) from 0.1 to 99.9%, δ13C(CH4) from −66.1 to −24.6‰, δD(CH4) from −266 to −117‰, δ13C(C2H6) from −27.8 to −22.8‰, and δ13C(CO2) from −26.6 to 16.8‰. Isotopic studies reveal the presence of 3 genetic types of natural gases: thermogenic (CH4, higher gaseous hydrocarbons, and CO2), endogenic CO2, and microbial CH4 and CO2. Thermogenic gases resulted from coalification processes, which were probably completed by Late Carboniferous and Early Permian time. Endogenic CO2 migrated along the deep-seated faults from upper mantle and/or magma chambers. Minor volumes of microbial CH4 and CO2 occur at shallow depths close to the abandoned mine workings. “Late-stage” microbial processes have commenced in the Upper Cretaceous and are probably active at present. However, depth-related isotopic fractionation which has resulted from physical and physicochemical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglected. The strongest rock and gas outbursts occur only in those parts of coal deposits of the LSCB which are dominated by large amounts of endogenic CO2.  相似文献   

9.
The natural gases in the Upper Paleozoic strata of the Ordos basin are characterized by relatively heavy C isotope of gaseous alkanes with δ 13C1 and δ13C2 values ranging mainly from ?35‰ to ?30‰ and ?27‰ to ?22‰, respectively, high δ13C excursions (round 10) between ethane and methane and predominant methane in hydrocarbon gases with most C1/(C1-C5) ratios in excess of 0.95, suggesting an origin of coal-derived gas. The gases exhibit different carbon isotopic profiles for C1-C4 alkanes with those of the natural gases found in the Lower Paleozoic of this basin, and believed to be originated from Carboniferous-Permian coal measures. The occurrence of regionally pervasive gas accumulation is distinct in the gently southward-dipping Shanbei slope of the central basin. It is noted that molecular and isotopic composition changes of the gases in various gas reservoirs are associated with the thermal maturities of gas source rocks. The abundances and δ13C values of methane generally decline northwards and from the basin center to its margins, and the effects of hydrocarbon migration on compositional modification seem insignificant. However, C isotopes of autogenetic calcites in the vertical and lateral section of reservoirs show a regular variation, and are as a whole depleted upwards and towards basin margins. Combination with gas maturity gradient, the analysis could be considered to be a useful tool for gas migration.  相似文献   

10.
New isotopic and chemical data on the sodium bicarbonate water and associated gases from the Razdolnoe Spa located in the coastal zone of Primorsky Kray of the Russian Far East, together with previous stable isotope data (δ18O, δD, δ13C), allow elucidation of the origin and evolution of the groundwater and gases from the spa. The water is characterized by low temperature (12 °C), TDS – 2.5–6.0 g/L, high contents of B (∼5 mg/L) and F (4.5 mg/L) and low contents of Cl and SO4. Water isotopic composition indicates its essentially meteoric origin which may comply with an older groundwater that was recharged under different (colder) climatic conditions. Major components of bubbling gases are CH4 (68 vol%), N2 (28%) and CO2 (4%). The obtained values δ13C and δD for CO2 and CH4 definitely indicate the marine microbial origin of methane. Thus the high methane content in the waters relates to the biochemical processes and presence of a dispersed organic matter in the host rocks. Based on the regional hydrogeology and the geological structure of the Razdolnoe Spa, Mesozoic fractured rocks containing Na–HCO3 mineral water and gases are reservoir rocks, a chemical composition of water and gases originates in different environmental conditions.  相似文献   

11.
Based on the analytical data of over 30 gas samples, combined with geochemical and geological backgrounds, the composition and distribution characteristics of shallow biogenetic gases in the Baise Basin, a Tertiary residual basin in southern China, were extensively investigated, and the origin and formation mechanism tentatively approached. The shallow gases are primarily composed of gaseous hydrocarbons, generally accounting for over 90%. The abundances of methane and C2＋ homologues show a relatively wide range of variation, mainly 50%-100% and 0%-50%, respectively, depending on the mixing proportions between biogenetic and thermogenic gases. A highly negative carbon isotope is the significant signature for the shallow gases with δ^13C1 values of -55‰ to -75‰. According to molecular and isotopic compositions and light hydrocarbon parameters, the shallow gases in the basin can be classified into three types of origins： biogenetic gas, biogenetic/thermogenic mixed gas, and oii-biodegraded gas. They exhibit regular distribution both spatially and temporally, and are believed to be associated with the maturity of adjoining gas source rocks and biodegraded oil accumulation. The Baigang and Nadu source rocks can be considered to have experienced early and late gas generation during early burial and after basin uplift respectively. A late accumulation mechanism of multiple gas sources is put forward for the formation of the shallow gas reservoirs, which is responsible for the variations in chemical and isotopic composition of the gases in depth profile.  相似文献   

12.
金陵山一带发现的晚石炭世纺锤虫类计有8属15种,其中新种3个,未定种3个,可称为Sphaeroschwagerina-Pseudofusulina动物群,与新疆柯坪地区康克林组Pseudoschwagerina带的下两个亚带对比。  相似文献   

13.
In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH4 (0.01–50.55 mol%) with low δ13C–CH4 values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ13C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH4 to ethane and propane ratios (3–35 mol%; C1/C2 + C3) and high δ13C and δD values of CH4 (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH4 was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ13C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO4 values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ13C–DIC values (<−22‰) and relatively high SO4 concentrations (up to 12.31 mmole/L) in many brine samples point to SO4 reduction, which likely limits microbial CH4 generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH4 in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH4 are present at shallow depths along Lake Erie and in western NY.  相似文献   

14.
In the western fold-and-thrust belt of the southern Urals, the Kübler and Árkai indices determined on shales, slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east. The metamorphic grade varies strongly within the different tectonic segments, which are separated by major thrusts. The increase of diagenetic and incipient metamorphic grade from the footwall to the hanging wall of all major Upper Palaeozoic thrusts indicates a pre-Permo/Triassic origin. West of the Avzyan thrust zone, the diagenetic to incipient metamorphic grade is related to the Palaeozoic basin development and reached the final grades in Late Carboniferous to Early Permian times. East of the first Avzyan thrust in the Yamantau anticlinorium, the diagenetic to lower greenschist metamorphic grade is possibly of Neoproterozoic origin and might be related to the development of the Neoproterozoic basin at the eastern margin of the East European Craton. The eastern part of the Yamantau anticlinorium was exhumed below 200 °C in the Late Carboniferous or Early Permian. The diagenetic grade of the autochthonous Palaeozoic sedimentary units increases toward the stack of Palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the Palaeozoic nappes. Within the Timirovo thrust sheet, the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes. The upper anchizonal metamorphic grade of the Upper Devonian slates of the Zilair nappe results from the deformation process related to the Lower Carboniferous nappe emplacement.  相似文献   

15.
Taking insight into genetic mechanisms of coalbed methane (CBM) can provide an effective approach for evaluating the value of CBM resources. In this study, the geo-temperature and the thermal subsidence history were used to investigate the effect of the present geothermal field characteristic on the genetic mechanisms of CBM at the Huaibei Coalfield. The results showed that the Permian coal strata in the study areas had a relatively low geo-temperature (< 50°C), high vitrinite reflectance (Ro,max; 0.75%-1.2%) and a coal rank typical of intermediate-high metamorphic bituminous. Comprehensive analyses of the characteristics of the present geothermal field indicate that the CBM at the Huaibei Coalfield are dominated by secondary biogenic gases. Furthermore, the genetic mechanism towards CBM was further proposed based on the tectonic evolution history: (1) Tectonic thrusting contributed to Ro,max values ranging from 0.5% to 3.0%, with maximum geo-temperatures of 140–180°C, which resulted in the generation of thermogenic CBM. (2) An extensional regime contributed to gradual uplift of the Permian coal-bearing strata, with the gradual escape of CBM at burial depths greater than 700m. (3) A large number of faults and hydrodynamic environments greatly promoted the microbial degradation of the early thermogenic gases, resulting in generation of secondary biogenic gases.  相似文献   

16.
The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH4, 0–93% CO2, and 1–23.7% N2. The CO2-enriched gases often display heavier methane δ13C values than those with low CO2 contents. The δ15N values of the gases range from −8 to −2‰, and the N2 content correlates negatively with the CO2 content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO2 from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO2-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.  相似文献   

17.
《Applied Geochemistry》2005,20(11):2017-2037
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 δ13CC1–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 (δ13CC1–C = −35.80‰; Hamitabat Field) and Group-3 (δ13C1–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 (Req = 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.  相似文献   

18.
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   

19.
A series of geochemical anomalies of Pt and Pd were found in 1 358 recombined samples from a geochemical stream sediment survey in eastern Yunnan (云南) Province, China. Chemical optical emission spectroscopy, X-ray fluorescence analysis, and inductively coupled plasmas atomic emission spectrometry analyses of 22 elements and chemical compositions of 21 samples from coal-bearing strata from the Late Paleozoic, Mesozoic, and Cenozoic show Pt and Pd concentrated to some extent in coal rocks, with Pd/Pt相似文献   

20.
To determine the origin, maturity, formation mechanism and secondary process of marine natural gases in Northeastern Sichuan area, molecular moieties and carbon isotopic data of the Carboniferous and Triassic gases have been analyzed. Typical samples of marine gas precursors including low-maturity kerogen, dispersed liquid hydrocarbons (DLHs) in source rocks, residual kerogen and oil have been examined in a closed system, and several published geochemical diagrams of gas origins have been calibrated by using laboratory data. Results show that both Carboniferous and Triassic gases in the study area have a thermogenic origin. Migration leads to stronger compositional and weak isotopic fractionation, and is path dependent. Carboniferous gases and low-H2S gases are mainly formed by secondary cracking of oil, whereas high-H2S gases are clearly related to the TSR (Thermal Sulfate Reduction) process. Gases in NE Sichuan show a mixture of heavy (13C-enriched) methane in comparison to the lower maturated ethane of Triassic gas samples, suggesting a similar source and maturity for ethane and propane of Carboniferous gases, and a mixture of heavy ethane to the propane for Triassic gases. Based on the data plotted in the diagram of Chung et al. (1988), the residual kerogen from Silurian marine shale and palaeo oil reservoirs are the main source for Carboniferous gases, and that the residual kerogen from Silurian and Permian marine rocks and Permian paleao oil reservoirs constitute the principal source of Triassic gases.  相似文献   

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