correlation of reservoired gases using the carbon isotopic compositions of wet gas components the carbon isotopic compositions of the wet gas components particularly propane isobutane and normal butane have been found to be particularly valuable for correlating one reservoired gas with another the usefulness of these components for correlation results from their carbon isotopic compositions reflecting both the nature of their source and their maturity this source control is strongest for gases derived from the more highly structured types of kerogen i e woody coaly type iii although the wet gas components carbon isotopic compositions of most gases are at least partly controlled by their source for levels of maturity below the point at which thermal destruction of the wet components occurs as a result the wet gas components are found to provide more ositive correlations than is methane alone three exploration examples illustrate the use of the carbon isotopic compositions of the wet gas components for correlation the leduc reef trend of alberta canada the sleipner area in the north sea and the lena field offshore louisiana united states the three examples also illustrate the need to integrate geochemical interpretations with regional geology to obtain a good understanding of the hydrocarbon source

natural seepage of shale gas and the origin of eternal flames in the northern appalachian basin usa abstract natural hydrocarbon gas seeps are surface expressions of petroleum seepage systems whereby gas is ascending through faults from pressurized reservoirs that are typically associated with sandstones or limestones a spectacular eternal flame in western new york state marks a gas macroseep of dominantly thermogenic origin emanating directly from deep shale source rocks which makes this a rare case in contrast to most petroleum seepage systems where gas derives from conventional reservoirs the main flaming seep releases about 1 kg of methane per day and may feature the highest ethane and propane c 2 c 3 concentration ever reported for a natural gas seep 35 vol the same gas is also released to the atmosphere through nearby invisible and diffuse seepages from the ground the synopsis of our data with available gas geochemical data of reservoir gases in the region and the stratigraphy of underlying shales suggests that the thermogenic gas originates from upper devonian shales without intermediation of a conventional reservoir a similar investigation on a second eternal flame in pennsylvania suggests that gas is migrating from a conventional sandstone pool and that the seep is probably not natural but results from an undocumented and abandoned gas or oil well the large flux of the emitted shale gas in new york state implies the existence of a pressurized gas pool at depth tectonically fractured shales seem to express naturally fracked characteristics and may provide convenient targets for hydrocarbon exploration gas production from tectonically fracked systems might not require extensive artificial fracking

multiple origins of the paleogene natural gases and effects of secondary alteration in liaohe basin northeast china insights from the molecular and stable isotopic compositions abstract the molecular and isotopic c and h compositions of paleogene shahejie formation e 2 3 s and dongying formation e 3 d natural gases from 11 new gas samples as well as the data of 71 gases published from previous studies in the liaohe basin were analyzed and the results were considered in combination with geochemical and distribution characteristics of the source rocks the genetic type gas source correlation and secondary alteration of the natural gases were studied in detail and the source rock maturities of the gases were estimated the following conclusions were reached 1 with the exception of dry gases in the major part of the third member of the shahejie fm e 2 s 3 reservoirs the gases in other reservoirs are mainly wet 2 variable carbon and hydrogen isotopes of the alkane gases are observed in different reservoirs of the basin with 13 c 1 values ranging from 60 7 to 34 2 13 c 2 from 47 3 to 22 3 2 h ch 4 from 261 to 188 and 2 h c 2 h 6 from 269 to 148 3 the natural gases of the basin are of multiple origins and mixing of thermogenic and microbial is observed the mixing ratio in the first member of the shahejie fm e 3 s 1 is the highest among all the reservoirs 4 gases of different reservoirs suffered varying extents of biodegradation which caused partial reversal of the carbon isotope series of methane and its homologues 13 c 1 13 c 2 13 c 3 13 c 4 the reversal of the gases in the shallow reservoirs e 3 s 1 and e 3 d is greater while those in the deep reservoir e 2 s 3 is less 5 humic natural gases of the ed were mainly generated by source rocks at early maturity while the sapropelic ones of the ed and wd were generated during the mature and high mature stages 6 both the source rocks of e 2 s 3 and e 2 s 4 strata could contribute to the gas reservoirs of e 2 s 3 and e 2 s 1 in the west depression sapropelic gases in east depression were generated by e 2 s 3 source rocks while the other gases reservoired in the e 2 s 3 e 3 s 1 and e 3 d strata are probably sourced from the source rock strata of e 2 s 3 and e 3 s 1 2

microbial alteration of subsurface natural gas accumulations bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the c1 c4 gaseous hydrocarbons preferentially attacking the wet gas components in most instances during the initial stages of bacterial gas alteration propane seems to be selectively attacked resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumul tions is important for both correlation purposes and maturity estimates

occurrence of minerals within fractures and matrix of selected bowen and ruhr basin coals abstract late permian age samples of black coal from the coeval bandanna formation and baralaba coal measures of queensland s bowen basin as well as carboniferous coal from germany s ruhr basin were investigated this paper presents scanning electron microscope sem analysis of the mode of occurrence of minerals within these coals and characterisation of the natural fractures present within the samples the coal samples located proximal to orogenic fronts in both basins are structurally complex with multiple generations of mineralisation evident stable isotope analyses of carbonates from selected samples indicate that methanogenesis related co2 reduction has occurred at two of the bowen basin sampling locations ankerite precipitated from a magmatic fluid or basinal brine is associated with significant sulphide mineralisation within brecciated ruhr coal proximal to major fault intersections with anticlines and has been meteorically altered to ferroan calcite and sulphates in coal from immediately below the carboniferous cretaceous unconformity similar alteration has occurred within a bowen basin coal sample close to the present day land surface variation of mineralisation within both the cleats and the coal matrix occurs on a fine scale with crystals of multiple mineral species often being present within micrometres of each other

alteration of natural gas during leakage from a north sea salt diapir field oil and gas are leaking from a salt diapir field in the norwegian sector of the central north sea the gas is being biodegraded as it migrates to the surface the seeping gas is pervasive in the sediments above the diapir and clearly visible on shallow and deep seismic records and on wireline logs an exploration well was drilled to test a cretaceous chalk reservoir above the diapir and provided an opportunity to sample the gas along the seepage pathway carbon isotope ratios of gases released from sealed cuttings taken from the gas cloud interval above the diapir were determined to test the hypothesis that biodegradation of leaking hydrocarbons produces secondary methane with a biogenic isotope signature generally high but variable concentrations of co2 were detected all the way from the surface to the salt diapir this results from biodegradation of hydrocarbons and particularly oil the source of oxygen for this process is problematic since most of the migration takes place through shales and there is no obvious aquifer source of dissolved oxygen the effect of biodegradation of gases escaping from this structure results in a comparable increase in 13c for both methane and ethane up to 10 with a lesser effect on propane and less still for butane and pentane this conflicts with published data on such effects which suggest that methane and propane should be most affected with little effect on ethane isotopically light methane was found only in the very deepest samples within the salt of the diapir itself and this is thought to be biogenic gas trapped in the salt at the time of deposition no evidence was found that any secondary biogenic methane is being produced during gas seepage and biodegradation at this site

recognising biodegradation in gas oil accumulations through the δ13c compositions of gas components 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

generation mixing and alteration of thermogenic and microbial gas in oil deposits the case of the alpine foreland basin austria abstract molecular and stable isotope compositions were determined for forty eight gas samples taken from wells producing oil form cretaceous and eocene reservoirs gas was expelled from source rock at various maturity levels 0 6 1 2 rr gas maturities based on stable carbon isotopes are in general agreement with those found in oils nevertheless results revealed that most fields trap methane derived from a source which is not thermogenic shallow northeastern reservoirs trap methane interpreted as secondary microbial in origin the same process is proposed here as source of methane in western deposits moreover those gases are enriched in 2 h isotope suggesting different methanogenesis pathways fields along the southern margin of the alpine foreland basin where reservoir temperature exceeds 80 c host methane generated during primary organic matter degradation thus eocene and lower oligocene layers should be considered as potential source rocks this study gives new insight to filling history of traps and revealed a more complicated geological background and hydrocarbons generation than have been assumed until now

coupled hydrology and biogeochemistry of paleocene eocene coal beds northern gulf of mexico thirty six formation waters gas and microbial samples were collected and analyzed from natural gas and oil wells producing from the paleocene to eocene wilcox group coal beds and adjacent sandstones in north central louisiana usa to investigate the role hydrology plays on the generation and distribution of microbial methane major ion chemistry and cl br relations of wilcox group formation waters suggest mixing of freshwater with halite derived brines high alkalinities up to 47 8 meq l no detectable so4 and elevated 13c values of dissolved inorganic carbon up to 20 5 vienna peedee belemnite vpdb and co2 up to 17 67 vpdb in the wilcox group coals and adjacent sandstones indicate the dominance of microbial methanogenesis the 13c and d values of ch4 and carbon isotope fractionation of co2 and ch4 suggest co2 reduction is the major methanogenic pathway geochemical indicators for methanogenesis drop off significantly at chloride concentrations above 1 7 mol l suggesting that high salinities inhibit microbial activity at depths greater than 1 6 km formation waters in the wilcox group contain up to 1 6 modern carbon a14c to at least 1690 m depth the covariance of d values of co produced h2o and ch4 indicate that the microbial methane was generated in situ with these late pleistocene or younger waters the most enriched carbon isotope values for dissolved inorganic carbon dic and co2 and highest alkalinities were detected in wilcox group sandstone reservoirs that were co2 flooded in the 1980s for enhanced oil recovery leading to the intriguing hypothesis that co2 sequestration may actually enhance methanogenesis in organic rich formations

thermogenic and secondary biogenic gases san juan basin colorado and new mexico implications for coalbed gas producibility the san juan basin is the most prolific coalbed gas basin in the world with 1992 production exceeding 440 gcf footnote 12 4 billion m3 resources of approximately 50 tcf 1 4 trillion m3 and proved reserves of over 6 tcf 170 billion m3 coalbed gas wells with the highest production initial potential greater than 10 mcf day or 0 28 million m3 day occur in the overpressured north central part of the basin hydrologic analysis indicates that overpressure in the fruitland formation is artesian in origin and represents repressuring that developed during the middle pliocene highly permeable laterally continuous coal beds override abandoned shoreline pictured cliffs sandstones and extend to the elevated recharge area in he northern basin to form a dynamic regionally interconnected aquifer system coal rank and basin hydrodynamics control the composition of fruitland coalbed gases which varies significantly across the basin chemically dry gases in the north central part of the basin coincide with meteoric recharge and regional overpressure the consistency of methane dgr13c values across the basin the presence of isotopically heavy carbon dioxide in coalbed gases and bicarbonate in formation waters and biodegraded n alkane distributions of some coal extracts indicate that coalbed gases in the north central basin are a mixture of thermogenic 25 50 secondary biogenic 15 30 and migrated thermogenic 12 60 gases migrated conventionally and hydrodynamically trapped gases in situ generated secondary biogenic gases and solution gases result in gas content that plot on or above the coal sorption isotherm bacteria transported basinward in groundwater flowing from the elevated northern basin margins metabolized wet gas components n alkanes and organic compounds in the coal and generated secondary biogenic methane and carbon dioxide subsequent to coalification uplift erosion and cooling these gases may be limited to basin margins where shallow depths and structural deformation result in higher permeability or may extend more than 35 mi 56 km basinward from the recharge zone the presence of appreciable secondary biogenic gas indicates an active dynamic flow system with overall permeability sufficient for high productivity basin hydrogeology reservoir heterogeneity location of permeability barriers no flow boundaries and the timing of biogenic gas generation and trap devel pment are critical for exploration and development of unconventional gas resources in organic rich rocks

the application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains abstract indicator mineralogy is used to explore for a wide variety of mineral commodities the method utilizes minerals which are sufficiently heavy to be readily concentrated in the laboratory often colourful and possess other useful physical and chemical properties the minerals also must be source specific some indicator minerals are true resistate minerals the others although less resistant are stable in oxidized glacial drift and many non glacial sediments a few such as gold grains are silt sized but most are coarse grained grain size has a major impact on indicator mineral dispersal patterns in glacial drift the coarse grained indicator minerals are of two main types 1 kimberlite indicator minerals kims and 2 metamorphosed or magmatic massive sulphide indicator minerals mmsims kims are enriched in mg and cr and most mmsims are enriched in mg mn al or cr these indicator elements cannot be diagnosed geochemically in anomalous heavy mineral concentrates because the concentrates contain other more plentiful non indicator minerals containing the same elements in the same chemical form chalcopyrite is also a very useful mmsim but the number of surviving grains in a dispersal train is too low for detection by selective geochemical analysis mmsims are derived from three main types of base metal deposits and their associated alteration or reaction zones 1 volcanosedimentary massive sulphides encompassing volcanogenic sedex and mississippi valley subtypes in medium to high grade regional metamorphic terrains 2 skarn and greisen deposits and 3 magmatic ni cu sulphides the variety of mmsims associated with ni cu deposits is astonishing apparently reflecting mineral hybridization related to assimilation of sulphurous sedimentary rocks by ultramafic magmas cr diopside is one of the best indicators of fertile ni cu environments although not necessarily of the actual ni cu deposits heavy indicator mineralogy is much more sensitive than heavy mineral geochemical analysis and offers many exploration benefits in regional exploration programs including 1 sampling efficiencies 2 enlargement of both the bedrock target and dispersal train 3 coverage of a wider range of mineral commodities 4 undiminished sensitivity in areas of overabundant non indicator heavy minerals 5 visual evidence of points of origin of dispersal trains and 6 indications of the economic potential of the source mineralization it is most effective as a reconnaissance exploration tool and is particularly well suited for testing gneissic volcanosedimentary and plutonic terranes where base and precious metal deposits are highly modified and difficult to recognize by other prospecting methods

sources and mechanisms of formation of gaseous hydrocarbons in sedimentary rocks abstract accepted concepts of principal stages of hydrocarbon formation fail to satisfactorily explain the accumulation of supergiant gasfields in west siberia u s s r such as for example urengoy which comprise 30 of the proven gas reserves of the world a new mathematical model has been developed which applies the concept of activation energy distribution to the formation of gases characteristic differences in the molecular structure of humic and sapropelic source organic matter are expressed in different activation energy distributions the model allows the calculation of isotope values for different types of organic matter and their dependence on maturity application of the model to west siberia suggests that the supergiant gas accumulations in urengoy are more likely formed from terrestrial organic matter through thermochemical reactions at moderate depths and are not the result of either deep overmature or bacteriogenic processes the new mathematical model suggests that humic organic matter in general has a high methane generating capacity at comparably low maturities of organic matter equivalent to 0 5 0 7 on the vitrinite reflectance scale

pumping test data analysis in wells with multiple or long screens abstract a computer method is presented to estimate the drawdown and the flow rate development in wells with multiple or long screens the mathematical model involves the following processes unsteady quasi three dimensional laminar flow in a multiple leaky aquifer system laminar and or turbulent radial flow in the skin zone including the screens turbulent axial flow and change of the kinetic energy in the screen pipe and the effect of well storage the numerical accuracy was checked against analytical theis wikramaratna and semianalytical numerical laplace inversion solutions the low error of numerical drawdown and flow rate simulation achieved during testing is acceptable in most of the practical cases two case studies demonstrate the calibration of aquifer and screen parameters the method may also be used as an approximate tool to analyse the three dimensional flow around wells with long partially penetrating screens in thick vertically heterogeneous aquifers analysis of the approximation error and some results of practical application are given

origin and secondary processes in petroleum in the eastern part of the polish outer carpathians a geochemical study of crude oil and natural gas from producing wells and seeps within the lower cretaceous oligocene sandstone reservoirs of the most eastern part of the polish outer carpathians was conducted for the evaluation of their origin migration pathways and the effects of secondary processes biodegradation water washing and evaporative fractionation the petroleum was generated from mixed marine terrigenous organic matter type ii iii kerogen occurring mostly in oligocene menilite beds in the silesian and skole nappes shales within upper cretaceous palaeocene istebna and lower cretaceous ve ovice lgota and spas beds are considered as an additional source of hydrocarbons mainly gaseous biodegradation processes were recorded in numerous oils especially occurring in seeps but the extent of these processes is not severe because steranes and terpanes were not affected all oils are more or less water washed while the evaporative fractionation processes are developed only on a minimal scale the natural gases are primarily of thermogenic origin usually co genetic with oil and often contain components of secondary microbial degradation of both oil and natural gas 13c enriched co2 and 13c depleted ch4 gases with the highest microbial methane contribution have been recorded in petroleum accumulated in dwernik and odyna fields

microbial origin of australian coalbed methane bituminous coal seam gases from the permian sydney and bowen basins australia are characterized by 1 methane ethane ratios greater than or equal to 1000 2 dgr13c and dgrd values for methane of 60 10 pmil pdb and 217 17 pmil smow respectively 3 carbon dioxide contents of less than 5 and 4 dgr13c co 2 ch4 values of 55 10 pmil pdb these data suggest that microbial reduction of co2 rather than traditional thermogenic reactions is mainly responsible for gas composition invasions by co2 of deep seated external origins are readily recognized by their isotopic compositions 7 2 pmil pdb

differential aerobic and anaerobic oxidation of hydrocarbon gases discharged at mud volcanoes in the nile deep sea fan abstract the present study investigates hydrocarbon oxidation processes at isis and amon mud volcanoes mv s in the eastern nile deep sea fan in the water column molecular and carbon isotopic signatures of light hydrocarbons indicate that gases rapidly dissolve in seawater and are partially oxidized in the upper sediments anaerobic oxidation of the light hydrocarbons takes place as clearly shown by their molecular and isotopic composition these processes lead to the presence of a distinct sulfate hydrocarbon interface at 120 145 cm and 20 50 cm below the seafloor for isis and amon mv s respectively in contrast to processes occurring in the water column a clear preferential oxidation of methane propane and n butane over ethane and i butane is observed in the anoxic sediments furthermore for the first time fractionation factors have been determined for the anaerobic oxidation of propane and butane being respectively 4 80 and 0 7 for 13 c and 43 3 for 2 h of propane

geochemistry and origins of natural gases in the central junggar basin northwest china abstract the petroliferous central junggar basin in northwest china is predominantly an oil exploration region however its gas exploration also might have good prospects thus to assist in gas exploration the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions carbon isotopes and light hydrocarbons of gases based on the results the gas genetic types are grouped into families and combined with the geological setting e g biomarkers of retrograde condensates and source rock characteristics we show that there are four representative genetic types of gases the first consists of gases derived from permian lacustrine mudstones with type i ii kerogen and type iii kerogen sources in the penyijingxi sag their representative geochemical feature is 13c2 ranging from 31 4 to 24 7 the second is gas sourced from carboniferous tufaceous mudstones of type iii kerogen in the dishuiquan sag whose representative geochemical feature is the heaviest values of 13c1 in the studied samples ranging from 32 0 to 30 4 the third consists of gases sourced from jurassic coals and mudstones in the shawan fukang sag the light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from jurassic source rocks in the southern junggar basin the fourth is gas most likely generated from the degradation of crude oil it is mainly found in the luliang area and has dryness values as much as 0 999 and 13c1 ranging from 54 8 to 43 2 among these four types of gases the first mainly sourced from the permian lacustrine mudstones in the penyijingxi sag is the predominant type

origin source mixing and thermal maturity of natural gases in the panyu lower uplift and the baiyun depression pearl river mouth basin northern south china sea multiple natural gas fields have been discovered in the baiyun depression and the adjoining panyu lower uplift in the pearl river mouth basin northern south china sea the natural gases are associated with condensate and are characterized by relatively heavy carbon isotopes with methane and ethane 13c values ranging from 44 2 to 33 6 and 30 0 to 25 4 respectively nearly all methane and ethane are derived from oil prone type ii kerogen in the wenchang formation source rock whereas the heavy hydrocarbon gases propane butanes and pentanes are derived from both the wenchang and enping type iii kerogen formations based on an integrated comparison of carbon isotopic compositions of the natural gases typical type i ii and type iii kerogen derived gases and the enping and wenchang kerogens the gases from the eastern parts of the baiyun depression and the panyu lower uplift mainly originate from secondary oil cracking and primary kerogen cracking respectively the gases from the northern slope of the baiyun depression are a mixture of oil cracking and kerogen cracking gases both oil cracking and kerogen cracking gases were mainly generated from the wenchang formation source rock in the maturity range of 1 5 2 5 vitrinite reflectance with a corresponding present day depth range of 5400 6500 m 17 700 21 300 ft the apparent contribution of the wenchang formation to the discovered gas accumulations demonstrates that it is the most important source rock in the area instead of the enping formation the search for more gas derived from oil cracking will be the next natural gas exploration direction in the baiyun depression

origin of light hydrocarbons in gases from mud volcanoes and ch4 rich emissions abstract this paper presents new chemical data of hydrocarbon rich gases discharged from mud volcanoes and ch4 rich emissions located in different areas of italy northern apennines and sicily the determination of c5 c10 alkanes cyclics and aromatics was carried out by gc ms while the main gas species and c1 c4 hydrocarbons were analyzed by gc tdc and gc fid respectively methane is by far the most abundant component of all the investigated gas emissions with the exception of part of those discharging close to mt etna volcano which are co2 rich the gas samples collected from the emilia apennine and sicily show d ch4 and 13c ch4 values typical of thermogenic gases most gases from the romagna apennine discharge methane with a biogenic isotopic signature non methane hydrocarbons of biogenic gases are almost exclusively c2 c4 alkanes with minor amounts fraction of mol mol of c4 compounds including few aromatics and cyclics likely deriving from minor thermogenic contribution not evidenced by the 13c ch4 and d ch4 values the etnean gases whose r ra and 13c co2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system show a significant enrichment of aromatics compounds up to 36 of the non methane organic gas fraction which is likely produced by catalytic reforming processes such as dehydrocyclization of alkanes the thermogenic gases from the apennines and southwestern sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several mol mol cyclic compounds are likely formed by i thermal cracking and ii uncompleted aromatization of alkanes occurring at depth 3 km and temperatures not exceeding 120 150 c

origins of methane discharging from volcanic hydrothermal geothermal and cold emissions in italy abstract we have addressed the origins of methane in fumarolic thermal and cold emissions discharging from a diversity of geological settings spread all over the italian peninsula and at panarea and pantelleria islands fluid discharges from the main geothermal systems of the peri tyrrhenian area including mt amiata larderello latera and manziana show 13c ch4 and d ch4 values 28 v pdb and 160 v smow respectively and c1 c2 concentration ratios 1000 these patterns suggest that ch4 discharged from these systems is primarily generated by abiogenic reduction of co2 and or co methane seems to have at least partially a different origin than the higher hydrocarbons that might be entirely related to the thermal decomposition of organic matter or alternatively to methane polymerization fumaroles from the volcanic systems of solfatara vesuvio panarea and pantelleria have c1 c2 concentration ratios in a wide range from 17 to 4300 but 13c ch4 and d ch4 values 21 v pdb and 120 v smow respectively such isotopic values are very similar to those reported for ch4 discharging from ultramafic hydrothermal systems in agreement with recent evidence provided for solfatara and pantelleria we therefore infer that the ch4 in the volcanic fumaroles is mainly abiogenic too the 13c ch4 from 21 6 to 36 9 v pdb and d ch4 from 113 to 164 v smow values as well as the c1 c2 concentration ratios from 19 to 2240 in the co2 rich fluid discharges located between the geothermal volcanic systems and the apennine sedimentary chain tend to decrease eastwards as the temperatures of the fluid reservoirs progressively diminish suggesting that the ch4 production is mainly related to thermogenic processes according to this general trend the ch4 dominated fluids from mud volcanoes located eastwards of the co2 rich emissions are fed by an almost pure thermogenic source the easternmost fluid discharges from the po valley and the adriatic coast where organic matter evolves at relatively shallow depths in the presence of a low geothermal gradient record the lowest 13c ch4 and d ch4 values i e as low as 69 v pdb and

correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components the natural distribution of carbon isotopes between hydrocarbon gas components is used for 1 determining a gas s maturity 2 correlating a reservoired gas to its source 3 correlating one reservoired gas with another and 4 recognizing gas mixtures calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single continuous diagram independent of source type actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock levels of organic metamorphism ranging from 8 to 13 covering the entire range of oil and wet gas generation at greater maturities the wet gas components are found to undergo thermal degradation losing their usefulness for correlation three examples showing indigenous gas west texas non indigenous gas gippsland basin australia and gas mixtures from multiple sources southeastern alberta illustrate exploration applications

origin and migration pathway of biodegraded oils pooled in multiple reservoirs of the chepaizi uplift junggar basin nw china insights from geochemical characterization and chemometrics methods abstract the chepaizi uplift possesses three sets of oil bearing reservoirs six sets of source intervals multiple episodes of hydrocarbon charging and variable physical properties of crude oils in this study based on geochemical characterization source intervals and the biodegradation rank of the crude oils four oil populations named as group a b c and d d1 d2 were defined the combined results of q cluster analysis qca and discriminant analysis da indicate that group a occurred mainly in lower neogene shawan formation n1s in the western chepaizi uplift and were primarily derived from the middle lower jurassic xishanyao badaowan sangonghe formations j2x j1b j1s and a little of paleogene anjihaihe formation e2 3a of the sikeshu sag group b occurred essentially in upper carboniferous xibeikulasi formation c2x of the western chepaizi and originated mainly from the j1b mudstone of the shawan sag in addition the crude oils of group c occurred mainly in the lower cretaceous tugulu group k1tg of the eastern chepaizi and were primarily derived from carboniferous sourced oil from the chepaizi uplift the crude oils of groups d d1 and d2 are mixed with permian p1f and p2w of shawan sag and carboniferous sourced oil from the chepaizi uplift based on the reconstruction of the migration history the oils of group a in the n1s formation of western chepaizi migrated predominantly from the south sikeshu sag to the north and northeast with wells su1 2 and c27 as the initial filling points the oils of group b in the n1s formation of the western chepaizi travelled predominantly from the southeast shawan sag to the north and northwest the oils of group d1 in the c2x formation of eastern chepaizi migrated mainly from the northeast and southeast shawan sag to the west with well p661 as the initial filling point wells sm011 and c64 are the initial filling points of the k1tg group in the hongche fault belt crude oils of group c in the k1tg group travelled predominantly from the southeast shawan sag and northeast to the west into the eastern chepaizi and from well p68 to the southwest according to the paleo oil preferential filling orientations and paleogeomorphology some favourable targets for petroleum exploration of the chepaizi uplift are forecast in this study

surface geochemical exploration for hydrocarbons in the area of prospective structures of the lublin trough eastern poland abstract the search for crude oil and natural gas deposits is dominated by expensive seismic methods however less expensive and faster surface methods are available which may improve the effectiveness of petroleum exploration one of these methods is the surface geochemical survey the objective of this method is to detect record and analyse the trace amounts of hydrocarbons in soil gas samples our paper presents the results of such research carried out over the prospective structures of the lublin trough in the eastern poland three hundred fifty five soil gas samples were collected from 1 2 m depth below the surface which were analysed by gas chromatographic analysis for their molecular composition sampling sites were arranged along 5 sampling lines with 200 m spacing maximum concentrations of ch4 total alkanes c2 c4 total alkenes c2 c4 h2 co2 and o2 detected in our samples were 0 16 vol 0 95 ppm 6 79 ppm 0 37 vol 5 01 vol and 21 6 vol respectively the results demonstrate that alkanes recorded in the soil gas which are heavier than methane originated from the deeper hydrocarbon accumulations these components migrated to the near surface zone due to their active microseepage along the tectonic discontinuities and cracks in the overburden weak correlation of h2 and co2 with alkanes were found which precludes the application of both components as additional prospective indicators in the study area diverse activity of partial oxidation of alkanes to alkenes perhaps due to different rate of microseepage of hydrocarbons has been determined using the c2h6 c2h4 c2 c2 ratio the presence of numerous tectonic dislocations intensifies the microseepage and increases the amplitudes of geochemical anomalies recorded at the surface normalization of concentrations of methane total alkanes c2 c4 and total alkenes c2 c4 with their of background values which were calculated for separate subsets of c2 c2 ratio values significantly reducing the ranges and the amplitudes of hydrocarbons anomalies it allowed for modification of the anomalous pattern due to diverse activity of partial oxidation of alkanes to alkenes statistical analysis of c2h6 c3h8 c2 c3 ratio values showed that in the study area accumulations of gas condensate and dry gas dominate with the strongest surface geochemical signals the surface geochemical results were integrated with the structural map of the top of devonian formations as well as we when displayed as trend integrated with geological cross sections the surface alkane anomalies generally occur along the major nw se trending chain of structural uplifts located between abramow glinnik and nasutow this zone is most prospective and indicates the highest petroleum potential changes in normalized concentrations of total alkanes c2 c4 allowed the identification of 9 anomalous zones indicating the presence of deep hydrocarbon sources the no 2 anomaly which was localized at the intersection of the b b and the e e sampling lines seems to be the most prospective the accumulations of dry gas or gas condensate likely occur in the anticline formed by devonian and or carboniferous strata this zone should be opened for confirmation by drilling the geochemical lead should now be integrated with all available geological seismic data for prospect development our study demonstrates that the results of surface geochemical surveys combined with the results of geological studies and geophysical surveys provide a valuable source of comprehensive information about the explored area

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the harstad basin southwest barents sea the harstad basin is a structural block on the continental shelf of sw barents sea where gas hydrates likely occurred below the grounded ice sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites we present an integrated study of fluid flow systems in the harstad basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane derived carbonate crusts more than 190 acoustic gas flares were registered in water column many of them in association with pockmarks and carbonate crust fields however weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation in the western harstad basin seeps of microbial methane occur mainly above tertiary formations that are pinching out below the glacial sediments high amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within tertiary deposits in contrast gas seeping in the eastern harstad basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted mesozoic rocks occurring below glacial sediments this spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts with 13 c values typically between 55 and 42 and 40 and 20 vpdb for carbonate crusts associated with microbial and thermogenic fluids respectively u th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor

fluid source and methane related diagenetic processes recorded in cold seep carbonates from the alvheim channel central north sea integrated petrography mineralogy geochronology and geochemistry of cold seep carbonate crusts and free gas from the alvheim channel elucidate diagenetic carbonate precipitation and related seepage histories in the central north sea free gas isotope characteristics coupled with carbonate 13c values as low as 66 vpdb indicate a predominantly microbial methane source with minor thermogenic contribution we estimate that 70 of the carbon sequestered into carbonate precipitates was derived from local oxidation of methane the early stage of crust growth is represented by microcrystalline aragonite and mg calcite 10 to 40 mol mgco3 cementing seafloor sediments consisting of clays quartz feldspar and minor detrital low mg calcite and dolomite typical association of aragonite cement with coarse grained detritus may reflect elevated fluid flow and flushing of fine particles prior to cementation close to the seafloor middle rare earth element enrichment in early generation microcrystalline cements containing framboidal pyrite indicates diagenetic precipitation within the zone of anaerobic methane oxidation contiguous to iron reduction the later generation diagenetic phase corresponds to less abundant radial fibrous and botryoidal aragonite which lines cavities developed within the crusts in contrast to early generation cements late generation cavity infills have rare earth elements and y patterns with small negative ce anomalies similar to seawater consistent with carbonate precipitation in a more open seawater dominated system aragonite u th ages indicate carbonate precipitation between 6 09 and 3 46 kyr bp in the northern part of the channel whereas in the southern part precipitation occurred between 1 94 and 0 81 kyr bp reflecting regional changes in fluid conduit position

stable isotope geochemistry of coal bed and shale gas and related production waters a review coal bed and shale gas can be of thermogenic microbial or of mixed origin with the distinction made primarily on the basis of the molecular and stable isotope compositions of the gases and production waters methane ethane carbon dioxide and nitrogen are the main constituents of coal bed and shale gases with a general lack of c2 hydrocarbon species in gases produced from shallow levels and more mature coals and shales evidence for the presence of microbial gas include 13c ch4 values less than 50 covariation of the isotope compositions of gases and production water carbon and hydrogen isotope fractionations consistent with microbial processes and positive 13c values of dissolved inorganic carbon in production waters the co2 reduction pathway is distinguished from acetate methyl type fermentation by somewhat lower 13c ch4 and higher d ch4 but can also have overlapping values depending on the openness of the microbial system and the extent of substrate depletion crossplots of 13c ch4 versus 13c co2 and d ch4 versus 13c h2o may provide a better indication of the origin of the gases and the dominant metabolic pathway than the absolute carbon and hydrogen isotope compositions of methane in the majority of cases microbial coal bed and shale gases have carbon and hydrogen isotope fractionations close to those expected for co2 reduction primary thermogenic gases have 13c ch4 values greater than 50 and 13c values that systematically increase from c1 to c4 and define a relatively straight line when plotted against reciprocal carbon number although coals and disseminated organic matter in shales represent a continuum as hydrocarbon source rocks current data suggest a divergence between these two rock types at the high maturity end in deep basin shale gas reversals or rollovers in molecular and isotopic compositions are increasingly reported in what is effectively a closed shale system as opposed to the relative openness in coal measure environments detailed geochemical studies of coal bed and shale gas and related production waters are essential to determine not only gas origins but also the dominant methanogenic pathway in the case of microbial gases

carbon isotope fractionation between dissolved and gaseous carbon dioxide abstract fractionation of the stable carbon isotopes between dissolved and gaseous carbon dioxide has been measured at temperature 25 c by two methods in the first method the open system conditions and different methods of co 2 sampling were arranged in the second method the closed system conditions and co 2 gas extraction were used the results obtained by these methods are very consistent gaseous co 2 is enriched in heavy isotope 13 c by 1 03 0 02 permil in comparison to dissolved carbon dioxide

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

deep gases discharged from mud volcanoes of azerbaijan new geochemical evidence abstract this paper presents new geochemical data of hydrocarbon rich gases released from some mud volcanoes of azerbaijan methane is considerably the most abundant component of all the sampled gases which show d ch 4 and 13 c ch 4 values likely related to a dominant thermogenic source these gases are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several mol mol a similar gas composition has recently been found to characterize many mud volcanoes of the northern apennines and sicily italy the data of the azerbaijan mud volcanoes corroborate the notion that cyclic compounds can be considered reliable tracers for hydrocarbon gas production at considerable depths and temperatures up to 120 150 c which correspond to a 6 5 8 3 km depth range assuming an average geothermal gradient of 18 c km this depth interval is consistent with both the depth of potential source rocks imaged seismically beneath some mud volcanoes and the results of previous estimates that used the 13 c 12 c values of methane and ethane such deep sourced gases and material fluidized clayey mass and rock fragments ascend into the core of anticlines and accumulate at shallower reservoirs where fold parallel outer arc faults or fold orthogonal fractures may penetrate and transfer the fluids to the surface finally the basically equivalent composition of the different hydrocarbon groups c 2 c 10 alkanes aromatics and cyclic determined in the gases sampled in both azerbaijan and italy manifests the lack of evident relationships between the chemistry of light hydrocarbons and the type of source rock

geochemical characterization of the nirano mud volcano italy abstract the nirano mud volcano is located in the western sector of the modena apennine margin italy it represents one of the most spectacular phenomena of sedimentary volcanism in the entire italian territory and is among the largest in europe here numerous aligned gryphon clusters and seeping pools constantly burst gas and mud inside a morphological depression besides the obvious surface expressions of these emission spots until now the type and amount of gas released in the rest of the large nirano caldera zone remained unknown an extensive geochemical soil gas survey o2 n2 co2 ch4 222rn he h2 and light hydrocarbons and exhalation fluxes co2 and ch4 was carried out inside the mud volcano field with the aim of identifying soil degassing distribution and to estimate the micro and macro seepage budget for both co2 and ch4 soil gas data highlight the presence of two zones characterized by high concentrations and flux values these enhanced seepage zones are located in the sw and ne sectors of the mud volcano suggesting that the enhanced gas emissions present in the peripheral zones are controlled by caldera collapse structures the most significant co2 flux up to 91 g m 2 d 1 and 222rn anomalies are located in the central part of the crater in correspondence of a morphological escarpment here we infer the presence of a buried tectonic system of collapsed terraces that facilitate fluids degassing in contrast ch4 fluxes show a scattered distribution and low values mean 221 mg m 2 d 1 overall the ch4 degassing budget is low 27 09 t km 2 y 1 when compared with other italian mud volcanoes this could be related to a relative low emission activity during the period of the geochemical survey and to a more homogeneous dilution of surface distribution of the emission point s chemical and isotopical composition of the gas discharged from the active gryphons is methane dominated and the thermogenic signature ranging from 41 to 47 suggests a deep reservoir source this conclusion is supported by noble gas measurements he ne ar kr xe conducted in the pore water phase of the emitted mud indicating a secondary gas exchange occurring at a depth of a few kilometers the geochemical anomalies found in this study successfully predicted the occurrence of new degassing phenomena towards the ne sector of the caldera indeed recently i e after the survey data acquisition new manifestations of mud and gas emissions appeared in the north eastern edge of the caldera

geochemistry of gas and water discharge from the magmatic hydrothermal system of guallatiri volcano northern chile this work presents the first chemical and isotopic 13c co2 13c ch4 3he 4he 20ne 40ar 36ar 18o and d data for fluid discharges from guallatiri volcano a remote and massive stratovolcano which is considered as the second most active volcano of the central volcanic zone cvz in northern chile fumarolic gases had outlet temperatures of between 80 2 and 265 c and showed a significant magmatic fluid contribution marked by the occurrence of so2 hcl and hf that are partially scrubbed by a hydrothermal aquifer the helium isotope ratios 3 2 were relatively low compared to those of other active volcanoes in cvz possibly due to contamination of the magmatic source by 4he rich crust and or crustal fluid addition to the hydrothermal reservoir geothermometry in the h2o co2 co h2 ch4 system suggests equilibrium temperatures of up to 320 c attained in a vapor phase at redox conditions intermediate between those typical of hydrothermal and magmatic environments thermal springs located 12 km northwest of the volcano s summit had outlet temperatures of up to 50 1 c neutral to slightly basic ph and a sodium bicarbonate composition typical of distal fluid discharges in volcanic systems cold springs at the base of the volcanic edifice showing a calcium sulfate composition were likely produced by interaction of shallow meteoric water with co2 and h2s rich gases a geochemical conceptual model was constructed to graphically represent these results which can be used as an indication for future geochemical monitoring and volcanic hazard assessment

a combined geochemical and isotopic study of the fluids discharged from the montecatini thermal system nw tuscany italy abstract the thermo mineral fluids discharges of montecatini terme northern apennines tuscany italy have been exploited since the roman times and despite the fact that this thermal complex is one of the biggest in europe the most recent geochemical investigations were published almost 40 years ago to fill this gap in this paper a detailed geochemical and isotopic investigation on the main thermal springs and wells from the montecatini thermal system mts is presented the chemical and isotopic features of the montecatini waters suggested that they are mainly controlled by water rock interaction processes between meteoric water permeating at depth from the surrounding reliefs up to 800 m a s l and the triassic evaporites burano formation belonging to the tuscan sedimentary series the local stratigraphic and tectonic framework favors an efficient recharge of the hydrothermal reservoir by the meteoric precipitation from a large catchment area and this aspect plays a fundamental role for the longevity of the montecatini thermal spas notwithstanding the huge amount of thermal water exploited the 3h values indicated that the thermal waters are likely related to a relatively long 50 years fluid circulation pattern approaching the surface thermal and saline waters mix with cold and low tds total dissolved solids waters hosted in short shallow aquifer s whose chemistry is dictated by the interaction of rain waters with silico clastic rocks of low solubility geothermometric estimations in the f so42 hco3 system suggested the occurrence of a main fluid reservoir at t 80 95 c and pco2 0 5 bars such co2 pressure is consistent with values estimated for other thermal springs from central southern tuscany being co2 basically supplied by a deep source nevertheless 13c co2 and 13c tdic values were lower than those expected for a mantle thermometamorphic co2 source this can be explained by i isotopic fractionation occurring during calcite precipitation and or ii mixing with biogenically derived gases occurring at relatively shallow depth

extreme methane deuterium nitrogen and helium enrichment in natural gas from the homorod seep romania abstract methane ch 4 in terrestrial environments whether microbial thermogenic or abiogenic exhibits a large variance in c and h stable isotope ratios due to primary processes of formation isotopic variability can be broadened through secondary post genetic processes such as mixing and isotopic fractionation by oxidation the highest and lowest 13 c and 2 h or d deuterium concentrations in ch 4 found in various geologic environments to date are defined as natural terrestrial extremes we have discovered a new extreme in a natural gas seep with values of deuterium concentrations d ch4 up to 124 that far exceed those reported for any terrestrial gas the gas seeping from the small homorod mud volcano in transylvania romania also has extremely high concentrations of nitrogen 92 vol and helium up to 1 4 vol carbon isotopes in ch 4 c 2 h 6 and co 2 and nitrogen isotopes in n 2 indicate a primary organic sedimentary origin for the gas a minor mantle component is suggested by the 3 he 4 he ratio r ra 0 39 both thermogenic gas formation modeling and rayleigh fractionation modeling suggest that the extreme deuterium enrichment could be explained by an oxidation process characterised by a d ch4 and 13 c ch4 enrichment ratio h c of about 20 and may be accounted for by abiogenic oxidation mediated by metal oxides all favourable conditions for such a process exist in the homorod area where increased heat flow during pliocene quaternary volcanism may have played a key role finally we observed rapid variations within 1 h in c and h isotope ratios of ch 4 and in the h 2 s concentrations which are likely caused by mixing of the deep oxidized ch 4 n 2 h 2 s he rich gas with a microbial methane generated in the mud pool of one of the seeps we hypothesize that the unusual features of homorod gas can be the result of a rare combination of factors induced by the proximity of sedimentary organic matter mafic metal rich volcanic rocks and salt diapirs leading to the following processes a primary thermogenic generation of gas at temperatures between 130 and 175 c b secondary alteration through abiogenic oxidation likely triggered by the neogene quaternary volcanism of the eastern transylvanian margin and c mixing at the surface with microbial methane that formed through fermentation in the mud volcano water pool the homorod gas seep is a rare example that demonstrates how post genetic processes can produce extreme gas isotope signatures thus far only theorized and that extremely positive d ch4 values cannot be used to unambiguously distinguish between biotic and abiotic origin