stable carbon isotopes of coal derived gases sourced from the mesozoic coal measures in china coal derived large scale gas fields derived from the mesozoic coal measures in china are mainly distributed in the middle lower jurassic coal measures in the tarim junggar and turpan hami basins in northwest china and the upper triassic xujiahe formation coal measure in sichuan basin central china in 2011 the annual production was 21 6 109 m3 and the proved geological reserves were 2485 109 m3 accounting for 21 and 30 of the total in china respectively based on analyses of gas composition and stable carbon isotopes ratios of 203 samples and stable carbon isotopes of 102 co2 samples the following conclusions were made a based on diagnostic plots using the stable carbon isotopic and molecular composition of gas samples alkane gas from the mesozoic coal measures in china is shown to be coal derived b according to the 13c2 vs c2h6 plot of a great number of oil derived and coal derived gases in china it is concluded that gases with 13c2 28 5 are coal derived and those with 13c2 28 5 are oil derived in most cases c among the natural gases from the mesozoic coal measures in china primary coal derived gases with normal carbon isotopic distribution pattern among the c1 c4 alkanes i e 13c1 13c2 13c3 13c4 are dominant d carbon isotopic pattern reversal mainly results from the mixing of coal derived gases having different maturities but the same source and secondly from microbial oxidation of propane e g mu 3 and mu 4 wells in the gumudi gas field junggar basin e co2 in the coal derived gases from the mesozoic coal measures in china has both biogenic and abiogenic origins the biogenic origin is dominant and the abiogenic co2 is mainly found in the kuqa depression in the tarim basin and western sichuan basin f isotopic differences between heavy hydrocarbon gases and methane become less with increasing maturity

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

petroleum accumulation in the deeply buried reservoirs in the northern dongying depression bohai bay basin china new insights from fluid inclusions natural gas geochemistry and 1 d basin modeling abstract the deeply buried reservoirs dbrs from the lijin shengtuo and minfeng areas in the northern dongying depression of the bohai bay basin china exhibit various petroleum types black oil gas condensates and pressure systems normal pressure overpressure with high reservoir temperatures 154 185 c the pressure volume temperature composition pvtx evolution of petroleum and the processes of petroleum accumulation were reconstructed using integrated data from fluid inclusions stable carbon isotope data of natural gas and one dimensional basin modeling to trace the petroleum accumulation histories the results suggest that 1 the gas condensates in the lijin area originated from the thermal cracking of highly mature kerogen in deeper formations two episodes of gas condensate charging which were evidenced by the trapping of non fluorescent gas condensate inclusions occurred between 29 25 5 ma and 8 6 5 0 ma with strong overpressure pressure coefficient pc 1 68 1 70 resulting in the greatest contribution to the present day gas condensate accumulation 2 the early yellow fluorescent oil charge was responsible for the present day black oil accumulation in well t764 while the late blue white oil charge together with the latest kerogen cracked gas injection resulted in the present day volatile oil accumulation in well t765 and 3 the various fluorescent colors yellow blue white and blue and the degree of bubble filling fv 2 3 72 5 of the oil inclusions in the minfeng area show a wide range of thermal maturity api gravity ranges from 30 to 50 representing the charging of black oil to gas condensates the presence of abundant blue white fluorescent oil inclusions with high grain obtaining oil inclusion goi values 35 8 usually 5 in oil reservoirs indicate that a paleo oil accumulation with an approximate api gravity of 39 40 could have occurred before 25 ma and gas from oil cracking in deeper formations was injected into the paleo oil reservoir from 2 8 ma to 0 ma resulting in the present day gas condensate oil accumulation this oil and gas accumulation model results in three oil and gas distribution zones 1 normal oil reservoirs at relatively shallow depth 2 gas condensate reservoirs that originated from the mixture of oil cracking gas with a paleo oil reservoir at intermediate depth and 3 oil cracked gas reservoirs at deeper depth the retardation of organic matter maturation and oil cracking by high overpressure could have played an important role in the distribution of different origins of gas condensate accumulations in the lijin and minfeng areas the application of oil and gas accumulation models in this study is not limited to the dongying depression and can be applied to other overpressured rift basins

late and deep fermentative methanogenesis as reflected in the h c o s isotopy of the methane water system in deep aquifers of the pannonian basin se hungary abstract h and o isotope ratios and h and c isotope ratios of methane as well as the abundances of some major and minor dissolved components were measured in 26 subsurface waters from se hungary produced from late neogene aquifers within the upper two kilometres in depth mixing with meteoric water of glacial age is of importance the hydrocarbon gas hc gas content of the great majority of the waters is dominated by isotopically light bacterial methane the depth of the onset of thermal gas generation varies from 0 7 to 1 2 km but the amount of thermogenic methane leaving the kerogen was not sufficient to overprint the light isotopic signature of the methane in the waters about two thirds of the waters group i samples are characterised by low sulfate content 20 and methane d values less negative than 260 they also show a direct relationship between the h isotope ratio of the methane and that of the water these waters experienced exhaustive bacterial sulfate reduction followed by a relatively shallow bacterial methanogenesis in a third of the waters group ii samples the methane d values are more negative than 278 the group ii samples which are not affected by an admixture of glacial age meteoric waters contain a relatively high amount of sulfate 35 45 mg l and do not show a direct relationship between the h isotope ratio of the methane and that of the water in group ii waters bacterial sulfate reduction was not exhaustive and the residual sulfate inhibited shallow bacterial methanogenesis as a result of continuing burial the aquifers were heated up to 40 60 c thereby allowing a relatively intensive bacterial acetogenesis followed by acetate fermentation to methane our results show that deep acetate fermentation and early thermal gas generation can occur closely in space and time even partly overlapping with each other

geochemistry characteristics and genetic types of natural gas in central part of the tarim basin nw china abstract according to the chemical carbon isotopic and hydrogen isotopic compositions and 3 he 4 he ratios of 127 natural gas samples the natural gas in the tarim basin is mainly composed of gaseous alkanes dominated by methane the contents of the gaseous alkanes decrease with the increase of their carbon numbers the 13 c values of ch 4 c 2 h 6 and c 3 h 8 in the platform area of the tarim basin vary in the range of 54 4 24 4 an average value of 39 2 43 1 26 7 an average value of 36 1 and 37 9 21 6 an average value of 31 8 respectively the overall carbon isotopic composition of the alkanes follows the trend of 13 c 1 13 c 2 13 c 3 13 c 4 and in some cases varies with 13 c 1 13 c 2 or 13 c 3 13 c 4 the lower carbon isotope values of methane 13 c 1 13 c 2 may be attributed to thermal sulfate reduction tsr and the higher propane value 13 c 3 13 c 4 was caused by the mixing of the natural gases of the same genetic type formed at different stages the methane hydrogen isotopes 2 h c 1 of the natural gases in the tarim basin are in the range of 195 122 with an average value of 156 the 3 he 4 he ratios of the natural gases are in the range of 2 3 10 8 66 8 10 8 suggesting that there is no deep mantle derived gas in the platform area of the tarim basin the carbon isotope fractionation of alkane gases 13 c 2 13 c 3 1 c 2 vs lnc 2 c 3 and c 2 c 3 vs 13 c 2 13 c 3 of the natural gases indicate that the oil type gas in the tarim basin has different predominant formation processes including kerogen cracking oil cracking oil and gas cracking secondary gas cracking and mixing of the kerogen cracking and oil cracking gases the gas evolved from gas cracking gas to oil and gas cracking gas and oil cracking gas along the direction from the west side of the manjar sag to the central tarim basin the natural gases in some areas of tazhong are the mixtures of the kerogen cracking and oil cracking gases

geochemistry of shallow tropical marine sediments from the arafura sea australia abstract tropical shelves have not been the focus of many biogeochemical studies this study presents new data from sediments collected in the arafura sea including headspace gas and lipid contents this allows assessment of modern depositional environments and reconstruction of environmental changes that have affected this tropical carbonate shelf during the last glacial transgression biomarker compositions from surface sediment and core samples from the upper 2 5 m revealed that marine algal input dominated the recently deposited organic matter over the survey area in a region closest to the shore high taraxerol abundance indicated a strong input of mangrove material to samples below 2 5 m indicating strong mangrove development associated with rising sea level during the transgression following the last glacial maximum when sea level stabilized around 6000 bp biomarker data indicate that the terrigenous contribution decreased and that palaeo channels formed during erosion of the continental shelf were infilled with marine dominated material sediments within these palaeo channels contained elevated concentration of dissolved ch4 at the base of the cores the source of the gas was determined to be microbial in origin based on depleted 13c isotopic composition 96 and very low concentration of wet gas components

stable carbon isotope systematics of methane ethane and propane from low permeability hydrocarbon reservoirs abstract we have reassessed the stable carbon isotope systematics of methane c1 ethane c2 and propane c3 in more than 500 natural gas samples from low permeability hydrocarbon reservoirs around the world with the purpose of providing new tools for thermal maturity assessment of natural gas that is increasingly produced from such reservoir types world wide a low permeability reservoir resembles a semi closed system and we found that the stable carbon isotope distribution in c1 c3 alkanes differs partially from what has been previously observed in natural gas from conventional hydrocarbon accumulations e g c isotope distributions dominated by rayleigh distillation and kinetic isotope effects in a low permeability reservoir isotope exchange may play a more prominant role in the carbon isotope distribution driving the system towards an even isotopic distribution eid of 6 between methane and ethane and ethane and propane at ro of 1 5 at higher maturity ethane and propane depleted in 13c are formed as a consequence of thermal cracking of wet gas components and possibly free radical decomposition polymerization reactions which leads to the occurrence of isotope reversals 13cc2

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

some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins large volumes of carbon dioxide i e up to 90 v v have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the co2 including mantle degassing metamorphic reactions or magmatic processes the formation of large accumulations of large volumes of co2 in sedimentary basins is discussed with specific reference to published data from the pannonian basin hungary the cooper eromanga basin australia and the south viking graben north sea these basins have occurrences of large volumes i e over 15 v v of isotopically heavy carbon dioxide thought to be of inorganic origin and although they differ somewhat in terms of sedimentary fill all have major structural features close to the gas accumulations the available geochemical data are currently neither sufficient to confirm the specific origin s of the co2 e g mantle derived carbonate metamorphism nor unscramble co2 mixed from two or more sources ongoing studies especially those involving noble gas analyses should provide fuller answers

the origin and accumulation of multi phase reservoirs in the east tabei uplift tarim basin china abstract a giant deep strata oil gas field with complex fluid characteristics was recently discovered in the hade yuke area of the east tabei uplift tarim basin nw china the deep fluids show a lateral co existence of several gas condensate and oil phase reservoirs characterized through the integration of complimentary geochemical e g bulk composition biomarker and stable carbon isotope and geological data typical features of the gas and condensate reservoirs which were mainly distributed in the eastern parts of the study site included high gor gas to oil ratio hydrocarbon gases with relatively heavy 13c values and crude oils with high wax content in contrast oil reservoirs which mostly occurred in more westerly locations had lower gor values lighter 13c values and lower wax content indicating gradual change of fluid characters the petroleum accumulations derive from two hydrocarbon charge events an early oil charge in the late hercynian and a later gas charge in the late himalayan which migrated from east to west leading to a gradual variation in fluid phase and characteristics alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi phase reservoirs presently in place secondary condensates were formed from the retrograde condensation of paleo reservoirs our re construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west

regional scale porosity and permeability variations in upper devonian leduc buildups implications for reservoir development and prediction in carbonates upper devonian carbonate rocks of the rimbey meadowbrook reef trend in the subsurface of central alberta are characterized by a wide range of porosity and permeability values dolostones show the highest values of absolute and average horizontal and vertical permeability for all rock types horizontal permeability averages tens of times to several hundred times the vertical permeability if considered irrespective of burial depth limestones and dolomitic limestones are more porous than dolostones there is an overall decrease of porosity and permeability in leduc formation carbonates with increasing burial depth and thermal maturity along the reef trend at relatively shallow burial depths 2000 m dolostones are significantly more porous and permeable than limestones dolostones appear to retain their porosity and permeability much better than limestones during burial indicating that dolostones are more resistant to porosity and permeability reducing processes particularly pressure solution thus reservoir quality in dolostone reservoirs decreases less severely with depth in addition porosity enhancement in dolostones by leaching in the burial environment during and after dolomitization is important although these significant relationships arenot new concepts they appear to be underappreciated the better porosity and permeability of dolostones relative to limestones at greater depths is of economic significance for exploration in upper devonian carbonates of the deep alberta basin and probably other carbonate areas where reservoirs are mostly found in dolostones and where limestones have a low reservoir potential and pose a considerable exploration risk the best reservoir potential in the leduc formation of the deep alberta basin occurs in dolomitized buildups that were connected to a regional conduit system

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

the lower cretaceous chinkeh formation a frontier type play in the liard basin of western canada the lower cretaceous liard basin in western canada covers an area of 9500 sq km 3668 sq mi but is relatively unexplored despite its size the present day expression of the basin which formed during the latest cretaceous to early tertiary trends north south and is delineated by the outcrop of the coarse clastic upper cretaceous dunvegan formation the lowermost cretaceous unit herein named the chinkeh formation is up to 32 m 105 ft thick and unconformably overlies older paleozoic strata the chinkeh formation contains four major lithotypes 1 conglomeratic breccia interpreted as debris flow or talus deposits 2 interbedded coal carbonaceous shale rooted sandstone and conglomerate interpreted as nonmarine valley fill or channel deposits 3 conglomeratic lag related to marine transgression and 4 upward coarsening sandstone interpreted as abandoned shoreline deposits the lower part of the chinkeh formation consists locally of an angular chert breccia overlain by medium to coarse grained sandstone and conglomerate with coal and rooted beds the uppermost chinkeh formation consists of a basal conglomeratic layer overlain by an upward coarsening fine to medium grained well sorted marine sandstone with bioturbated shale sediment was recycled repeatedly with the dominant sediment source being from the east with perhaps some western sources cretaceous strata in the liard basin have good petroleum source rock and reservoir potential and hydrocarbons may be present in sandstone of the chinkeh formation potential play types include stratigraphic traps formed by incised valley deposits and shallow marine sandstone pinching out laterally into marine shales of the garbutt formation a potential structural play may occur along the bovie fault zone where reservoirs may abut against a shale seal on the eastern side of the fault most of lower cretaceous strata falls within the hydrocarbon generation window with increasing maturity to the southwest pyrobitumen is locally abundant and constitutes 30 of the bulk rock volume in one sandstone potential source rocks include the lowermost garbutt formation and underlying triassic oad and grayling formations the chinkeh formation sandstone has porosity values of 8 18

origin and accumulation of petroleum in deep precambrian reservoir in baxian sag bohai bay basin china abstract the baxian sag is a mature exploration area the exploitation of deep sedimentary formation is becoming increasingly important as the further development potential of shallow plays faces growing challenges the discovery of the niudong gas condensate field from deep precambrian reservoirs indicates that potentially significant hydrocarbon resources remain to be discovered in deeper stratigraphic units this study was undertaken to better understand the generation and accumulation of hydrocarbons in deep reservoirs through integrated petroleum geochemistry fluid inclusion analysis and source rock analysis geochemical parameters of condensate and gas suggest that condensate and gas in precambrian reservoir have similar thermal maturity and are directly derived from gas prone source rock at high maturity levels the regional structure section and hydrocarbon distribution indicate that gas condensate in the nd1 well is more likely derived from lacustrine source rock in the paleogene shahejie formation refined source kitchen analysis indicates that lacustrine source rocks in the shahejie formation display a high degree of heterogeneity including organo facies c d e and f differences in the nature and composition of the expelled petroleum exist between organo facies due to variations in kerogen composition the orderly spatial distributions of gas oil ratio gor api gravity and gas carbon isotopes denote that the various fronts expelled hydrocarbons at different maturity levels from different organo facies this reflects the dynamic scenario of petroleum migration and accumulation from kitchen to shallow reservoirs the precambrian reservoir in nd1 well has experienced multiple charging with hydrocarbon expulsion at different maturity levels while hydrocarbons of lower maturity have arrived into the shallower reservoirs condensate and gas expelled at high maturation levels have efficiently accumulated in deeper prospects

carbon isotope geochemistry of petroleum associated gases in krishna godavari basin india abstract this paper presents the results of a comprehensive study of the compositional and stable carbon isotopic characteristics of 23 natural gases from 13 structural areas from different tectonic blocks of the krishna godavari basin the 13 c ethane and 13 c propane values of reservoired associated gases have been used in furthering an understanding of origin and occurrence of these hydrocarbons three aspects have been discussed 1 genetic correlation of gases 2 evaluation of maturity level of the source of these gases by james method and 3 delineation of major hydrocarbon generation centres for reservoired petroleum the isotopic data reveal a multiplicity of hydrocarbon generation centres in terms of inter basinal and inter structural within a basin variations in maturation and organic facies the study illustrates the use of gas isotopic data for predicting the source of petroleum and for developing a model of source accumulation relationships across the basin in the west godavari basin oil has been generated at a maturity of 1 0 vr o which corresponds to sources in the vicinity of these accumulations in the east godavari basin hydrocarbons have been generated at maturities of 0 55 1 4 vr o which correspond to sources adjacent to reservoirs in the godavari offshore basin oil has been generated at 0 78 vr o which corresponds to a deeper source and medium distance upward migration the validity of the methodology of isotopic gas to gas and gas to source correlation for modelling the origin and migration of petroleum is confirmed by the hydrocarbon occurrence pattern in the krishna godavari basin

source rock evaluation and nature of hydrocarbons in the khalda concession shushan basin egypt s western desert abstract organic geochemical studies were undertaken on 241 jurassic cretaceous ditch cuttings samples and 9 gases and 10 crude oils from some productive mesozoic pay zone intervals in the khalda concession shushan basin northern western desert egypt we aim to investigate source rock characteristics origin of gases nature of oils and their inferred source rocks and to establish genetic relationships the middle jurassic khatatba formation and older rocks contain organic rich mature coaly and carbonaceous shales which are the main source for hydrocarbons in egypt s western desert including the shushan basin which generally contains kerogen type iii with some contribution of type iii ii kerogen molecular and isotopic composition indicates that the khalda gases are of thermogenic origin a mature 0 6 2 0 ro source rock with terrigenous organic matter kerogen type iii was identified as a precursor of wet thermogenic gases in the shushan basin geochemical results indicate that the oil samples were derived from a source rock deposited mainly under oxic conditions and containing terrigenous organic matter biomarker data indicate that paraffinic oils and condensates from the shushan basin were derived from mature source rocks within the oil window the light hydrocarbon data suggests that the analyzed oils can be classified into two groups as a result of evaporative fractionation and maturation this signal of a secondary reservoir transformation which is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to understand variations in the chemical composition and origin of oils

gas geochemistry of the macuspana basin mexico thermogenic accumulations in sediments impregnated by bacterial gas a gas survey was performed in the macuspana basin south of mexico two distinct gas families were identified family 1 is represented by homogeneous and purely thermogenic gas associated with condensates expelled at the end of the oil window and family 2 is composed of a mixture of bacterial dry gas and thermogenic gas derived from source rocks at high maturity chemical and isotopic parameters as well as noble gas data provided information about the sources the proportions of mixing and directions of hydrocarbon migration of these two families considering the risk associated with further oil and gas exploration in the basin deeper zones present higher exploratory interest for family 1 gas fields and associated oil accumulations in contrast the bacterial gas fields should be considered a less promising target for future oil and gas exploration

origin of natural gas from the ordovician paleo weathering crust and gas filling model in jingbian gas field ordos basin china abstract the ordos basin the second largest natural gas producing basin in china is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation the origin and potential source of the jingbian gas field one of the five giant gas fields in the basin containing the ordovician paleo weathering crust as reservoir strata remains unclear the question is whether gas is generated from carboniferous permian coal measures or ordovician carbonates this study provides abundant gas geochemical evidence which clarifies its origin and source among 82 natural gas samples collected from the ordovician paleo weathering crust in the jingbian gas field c1 c4 gaseous hydrocarbons are dominant with low amounts of non hydrocarbons co2 h2s and n2 the content of c1 c4 ranges from 79 26 to 99 93 with a gas dryness c1 c1 4 of 0 981 0 999 suggesting that natural gases are dry and possibly originate from a high to over mature source the carbon isotopic compositions of methane and its homologues in natural gases from the ordovician paleo weathering crust vary widely with 13c1 13c2 and 13c3 values of 38 9 to 29 1 37 5 to 22 2 and 30 0 to 20 9 respectively the 13c2 and 13c3 values in the jingbian gas field are much more negative than those of typical coal type gas in sulige wushenqi yulin daniudi and kela2 gas fields the carbon isotopic signature suggests that the natural gas in the ordovician paleo weathering crust in the jingbian gas field most likely originates from ordovician carbonates containing sapropelic organic matter the measured d values of methane range from 177 to 155 and support our hypothesis of the natural gases originating from marine sapropelic organic matter the thermal maturity of the natural gases in the ordovician paleo weathering crust is at a high to over mature stage suggesting that the gases originated from secondary cracking of oil and gas at high temperature and pressure thermochemical sulfate reduction tsr might partially play a role in the alteration of gas dryness and 13c1 in those areas where gypsum is present as an effective cap rock mixing of high to over mature secondary cracking gas from the ordovician source and coal type gas from carboniferous permian coal in the jingbian gas field is minor

shelf sequence and characteristic bounding surfaces in a wave dominated setting latest pleistocene holocene examples from northeast japan abstract this paper presents modern models for coastal to shelf sequences in a microtidal and wave dominated setting based on detailed analyses of latest pleistocene holocene sequences on the pacific side of northeast japan the latest pleistocene holocene sequences on the pacific side of northeast japan created in response to sea level changes for the last 30 kyr can be assigned to lowstand transgressive and highstand systems tracts related to sea level changes and show characteristic bounding surfaces and depositional systems these basically support the sequence architecture model of vail however transgression and regression occurred almost simultaneously with rise and fall of sea level in the study areas and so inflection points of sea level curves where the absolute slope or the rate of change is greatest are less significant for these high frequency sequences the characteristic bounding surfaces are a sequence boundary a transgressive surface which is the boundary between the lowstand and the transgressive systems tracts a ravinement surface which is found mainly in deposits of the transgressive systems tract and a maximum flooding surface which is the boundary between the transgressive and the highstand systems tracts the sequence boundary was created in response to falling sea level fluvial entrenchment shifted seaward with falling sea level therefore the boundary is a time transgressive unconformity and is not an isochronous boundary the transgressive surface is a nearby isochronous boundary that occurred when the shoreline had started to move landward from its maximum seaward excursion the ravinement surface is a diachronous surface created by shoreface erosion during transgression the shoreface erosion shifted landward with rising sea level and so the erosion surface created is time transgressive this surface is the most distinct bounding surface in the shelf sequence these characteristics of the bounding surfaces support and clarify the model of thorne and swift 1991 the transgressive systems tract on the shelf can be divided into two backstepping wedges separated by the ravinement surface the lower wedge consists of coastal plain and fluvial systems and the upper wedge is composed of transgressive sand and or sand ridges both the wedges show a backstepping depositional pattern which can be recognized in seismic records and cores these wedges correspond to the backstep backbarrier wedge and backstep shelf wedge of thorne and swift 1991 respectively

the long term fate of co2 in the subsurface natural analogues for co2 storage abstract co 2 is a common gas in geological systems so that planned storage of co 2 in the subsurface may do no more than mimic nature natural co 2 has a wide number of sources that can be at least partly identified by carbon stable isotope geochemistry three pairs of case studies with different reservoir characteristics and co 2 contents have been examined to assess the natural impact of adding co 2 to geological systems carbonate minerals partially dissolve when co 2 is added simply because the co 2 dissolves in water and forms an acidic solution therefore carbonate minerals in the subsurface are not capable of sequestering secondary co 2 the addition of co 2 to a pure quartz sandstone or a sandstone in which the supply of reactive aluminosilicate minerals has been exhausted by excess natural co 2 addition will have no consequences the co 2 will simply saturate the water and then build up as a separate gas phase the addition of co 2 to carbonate cemented sandstone without reactive aluminosilicate minerals will induce a degree of carbonate mineral dissolution but no solid phase sequestration of the added co 2 when co 2 is naturally added to sandstones it will induce combined aluminosilicate dissolution and carbonate cementation if the aluminosilicate minerals contain calcium or magnesium or possibly iron examination of a co 2 filled porous sandstone with abundant reactive aluminosilicate minerals that received a huge co 2 charge about 8000 to 100 000 years ago reveals minimal evidence of solid phase sequestration of the added co 2 this indicates that either dissolution of reactive aluminosilicates or precipitation of carbonate minerals is relatively slow it is very likely that the slow dissolution of reactive aluminosilicates is the rate limiting step solid phase sequestration of co 2 occurs only when reactive aluminosilicates are present in a rock and when the system has had many tens to hundreds of thousands of years to equilibrate the two critical aspects of the behaviour of co 2 when injected into the subsurface are 1 that the rock must contain reactive ca and mg aluminosilicates and 2 that reaction to produce carbonate minerals is extremely slow on a human timescale the reactive minerals include anorthite zeolite smectite and other fe and mg clay minerals such minerals are absent from clean sandstones and limestones but are present in dirty standstones lithic arenites which are mineralogically immature and some mudstones the analysis of geological analogues shows that injection of co 2 into carbonate bearing rocks that do not contain reactive minerals will induce dissolution of the carbonate whether it is a matrix cement rock fragment fault seal or part of a top sealing mudstone

a comparison of pyrolysis products with models for natural gas generation abstract hydrous pyrolysis using progressive heating from 150 to 365 c is used to study the generation and isotopic composition of hydrocarbon gases and carbon dioxide different source rocks from the draupne heather dunlin and brent formations in the north sea a miocene brown coal from germany a coal from the middle jurassic ness formation in the northern north sea and a rich marine type source from the late jurassic kimmeridge clay formation onshore uk are studied the experimental results are related to vitrinite reflectance as a maturity reference parameter and to existing models for natural gas generation hydrous pyrolysis simulates the observed isotopic composition of natural systems fairly closely the largest deviations are found for methane which is most sensitive to fractionation effects the obtained results mainly reflect variations in source rock type and initial isotopic composition different maturity dependent trends are found between coal derived and marine type ii derived gases trends in the experimental data fit well with basic assumptions of existing models for natural gas generation

quantification of methane emissions at abandoned gas wells in the central north sea as a result of extensive hydrocarbon exploration the north sea hosts several thousand abandoned wells many believed to be leaking methane however how much of this greenhouse gas is emitted into the water column and ultimately reaches the atmosphere is not known here we investigate three abandoned wells at 81 93m water depth in the norwegian sector of the north sea all of which show gas seepage into the bottom water the isotopic signature of the emanating gas points towards a biogenic origin and hence to gas pockets in the sedimentary overburden above the gas reservoirs that the wells were drilled into video analysis of the seeping gas bubbles and direct gas flow measurements resolved initial bubble sizes ranging between 3 2 and 7 4mm in diameter with a total seabed gas flow between 1 and 19 tons of ch4 per year per well estimated total annual seabed emissions from all three wells of 24 tons are similar to the natural seepage rates at tommeliten suggesting that leaky abandoned wells represent a significant source of methane into north sea bottom waters however the bubble driven direct methane transport into the atmosphere was found to be negligible 2 due to the small bubble sizes and the water depth at which they are released

shallow marine deposits associated with the 2011 tohoku oki tsunami in sendai bay japan little is known about offshore tsunami deposits although they have the potential to improve palaeotsunami reconstructions especially in areas with limited preservation of onshore tsunami deposits we analysed 44 sediment cores collected in 2012 at water depths of 14 30 m in sendai bay north eastern japan to identify the deposits of the 2011 tohoku oki tsunami with the aid of 134cs released from the fukushima daiichi nuclear plant the preserved tsunami deposits in the inner shelf are interbedded with very fine sand and mud showing similar features to storm deposits in the shoreface a distinct layer of coarse sand overlies fine sand appearing to have originated as beach sand transported offshore by the tsunami backwash the coarse sand fades offshore around the boundary between the shoreface and inner shelf many of the core sites show evidence that surface sediment was reworked probably by post tsunami storm waves suggesting that further reworking would make the ultimate preservation of the tsunami deposits unlikely

long term performance of a mudrock seal in natural co2 storage the ability of mudrock seals to prevent co 2 leakage is a major concern for geological storage of anthropogenic co 2 the long term performance of a mudrock seal which provides a natural analogue in the north sea miller oil fi eld has been evaluated this mudrock seal is immediately above a natural co 2 rich reservoir the paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with co 2 during 4 km of burial a well defi ned linear trend of upward decreasing 13 c traces the progressive penetration of free phase co 2 causing dissolution and reprecipitation of carbonate minerals the co 2 was emplaced ca 70 80 ma and has only penetrated 12 m vertically in this case the infi ltration rate is estimated as 9 8 10 7 g cm 2 yr 1

mixing of thermogenic natural gases in northern appalachian basin stable isotopic studies were undertaken to investigate the origin of 20 oil associated and non associated gases from western and central new york state in the northern appalachian basin gas wetness c2 c5 c1 c5 methane dgr13c and methane dgrd values range from 0 8 to 26 9 47 9 to 30 7 pmil and 305 to 143 pmil respectively corresponding ethane and propane dgr13c values range from 40 9 to 32 0 pmil and 34 9 to 27 9 pmil these data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature equivalent vitrinite reflectance ro approximately 0 5 to post mature equivalent ro 2 0 inferred gas maturities generally in rease with increasing reservoir age from late to middle devonian marcellus and overlying rocks middle devonian to latest silurian oriskany onondaga akron silurian lockport herkimer medina to ordovician queenston and black river oil and gas in many late to middle devonian reservoirs may have migrated from interbedded organic rich shales in other reservoirs particularly those of ordovician and silurian age unusual isotopic reversals methane dgr13c ethane dgr13c suggest that gas generated locally from equivalent age rocks within the oil window mixed with post mature gas migrated from deeply buried sources to the south and southeast the post mature gas was probably sourced from middle to late ordovician shales regional migration of middle ordovician gas may have occurred along the middle ordovician knox unconformity gas was presumably channeled into overlying silurian and devonian reservoirs along faults and fractures

microfractures due to overpressures caused by thermal cracking in well sealed upper devonian reservoirs deep alberta basin microfractures 3 this condition is favored by high pore pressures but failure occurs before pore fluid pressures reach the lithostatic pressure radial and random microfractures are rare and of uncertain origin

geochemical characteristics of gases from the largest tight sand gas field sulige and shale gas field fuling in china abstract this study performed a detailed geochemical analyses of the components stable carbon isotopes of alkane gas and co2 stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field sulige and shale gas fuling field in china the comparative study shows that tight gas from the sulige gas field in the ordos basin is of coal derived origin which is characterized by a positive carbon and hydrogen isotopic distribution pattern 13c1 13c2 13c3 13c4 2h1 2h2 2h3 i e the carbon and hydrogen isotopes increase with increasing carbon numbers carbon dioxide from this field are of biogenic origin and the helium is crust derived shale gas from the fuling shale gas field belongs to oil derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin 13c1

understanding of reservoir gas compositions in a natural case using stepwise semi open artificial maturation abstract in a study of the gas distribution and composition in different reservoirs of multilayered gas fields in south east asia two types of variations with depth of the 13c values of gases in particularly methane were observed in the normally pressured zones the 13c values of methane increase steadily with depth whilst in some overpressured zones they first decrease with depth and then follow a regular trend in order to explain these patterns these data were compared to those obtained by artificial maturation of a coal using confined pyrolysis experiments were conducted in perfectly closed and in semi open conditions the free gases recovered were quantified by gas chromatography and the carbon isotopic composition of individual compounds c1 c5 hydrocarbons determined the evolution of the 13c versus pyrolysis temperature obtained for the artificially generated methane is very similar to that of the natural gas the use of the 13c2 13c3 versus c2 c3 mol diagram eage 58th conference and technology exhibition 1996 chem geol 147 1998 249 allows recognition of two trends in the artificial maturation data related to the degree of opening of the experimental systems comparison of the data provide evidence that the closed pyrolysis results are very similar to those observed in high pressured reservoirs while those obtained in the normal pressured reservoirs follow the values of semi open pyrolysis the gas distributions and carbon isotope composition in the reservoirs can therefore be explained in terms of degree of opening of the system high pressure reservoirs can be considered as fairly closed systems with in situ gas generation while normally pressured zones are more open and subject to important lateral migration the composition of the gas richness in methane is controlled by the degree of opening of the system thus our findings do not support the hypothesis that the generation of methane rich gases in nature is controlled by transition metal catalysis

experimental constraints on the chemical evolution of large icy satellites abstract we derive experimental constraints on the interior structure and mineralogy of large icy satellites by reacting material of chondritic chemistry with water at a pressure of 1 5 gpa temperatures between 300 c and 800 c and a range of oxygen fugacities our results document the existence of three chemical processes that probably occur in large icy satellites as a result of high pressure hydrothermal processing 1 the formation of low density hydrated silicates 2 the alloying of iron and sulfur to form fes dominated cores and 3 the instability of organic material relative to carbonates we construct new internal models of the thermal and structural state of ganymede and infer that the magnetic field of this body arises from convection within a mostly iron sulfide core simple thermochemical calculations are conducted to further explore the likely effects of composition and oxygen fugacity on the high pressure chemistry undergone by organic material within icy satellites both experimental and calculated results show that primordial organics are likely to have been largely oxidized to carbonates through hydrothermal processing early in ganymede s history potentially sterilizing ganymede s h 2 o layer

geochemical characterization of natural gas a physical multivariable approach and its applications in maturity and migration estimates gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons in this paper graphical representations of physico chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified these diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data from 11 chemical and isotopic ratios a statistical analysis pca yields two very important parameters the first parameter using mainly the c2 fraction of the gas relates to maturity and the second parameter involving the proportions and d13c values of methane indicates mainly segregative migration positive values of the second parameter indicate that gases accumulated far from their source whereas negative values correspond to residual gas pools after leakage of a part of the fluids a tentative reconstruction of the gas history has been performed in two brazilian basins the espirito santo basin and part of the reconcavo basin the espirito santo basin is located on the passive continental margin of the atlantic ocean and the reconcavo basin corresponds to an intracontinental aborted rift in both cases the source rocks are mainly lacustrine with thermal maturities ranging between the oil window and the beginning of the gas window results show that in the reconcavo basin a major fault the mata catu fault acts as a drain for hydrocarbon migration at the basin scale associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration in the espirito santo basin this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites this long distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore

evidence for deeply buried oil prone source rocks in the baiyun depression pearl river mouth basin northern south china sea carbon isotope analyses of fluid inclusion gases trapped in quartz grains in nine sandstone core samples from eight wells in the baiyun depression and the panyu lower uplift were performed to determine the source and thermal maturity of the paleo natural gases two dimension thermal and hydrocarbon generation history modelling of the source rocks coupled with the carbon isotopic results of the fluid inclusion gases were used to understand the natural gas accumulation process and to locate the active gas source rocks in the baiyun depression the fluid inclusion gases were generated from high thermal maturity oil prone wenchang formation source rocks 1 7 3 1 ro with most ro values between 2 1 ro and 2 7 ro the mixing of high thermal maturity wenchang formation sourced gas with relatively low thermal maturity enping formation sourced oil associated gas resulted in relatively lighter carbon isotopes of the reservoired gases compared with the fluid inclusions gases gas mixing could have occurred during the main enping formation sourced oil charge period based on the hydrocarbon generation history modeling results this study confirms the presence of oil prone semi deep water and deep water lacustrine facies source rocks in the wenchang formation of the baiyun depression which are widely developed with great thicknesses based on the seismic facies interpretations however successful oil and gas exploration in the deep water area should consider the distribution of active gas source rocks in the wenchang formation because the depth of the base of the active source rock greatly decreases from shallow water to deep water areas which increases exploration risk in deep water and ultra deepwater areas

gas geochemical evidences for biodegradation of shale gases in the upper triassic yanchang formation ordos basin china abstract fractionation of chemical components and hydrogen carbon isotopic compositions associated with biodegradation of shale gases were determined from typical shale profiles of the upper triassic yanchang formation ordos basin china the results show that the light hydrocarbons c1 c5 of the shale gases were partially degraded by microorganisms and the degradation degree decreased with depth in the profile of well yy22 however the degradation degree of wells fy2 is basically unchanging at different depths and there is no obvious evidence for biodegradation in well yy18 biodegradation of c1 and c2 can occur and the degradation of hydrocarbon gas does not occur in any obvious sequence but instead there are only small differences between the relative degradation rates the relative degradation rates in descending sequence were c3 n c4 n c5 i c5 i c4 c2 c1 with intensification of degradation 13c1 of shale gas showed no obvious change whereas a marked alteration took place in 2hc1 the 13c values of c3 and n c4 did not alter significantly during early biodegradation thus use of exceptional heavy isotopic compositions of those two components to recognize moderate levels of biodegradation is limited in contrast the 13c and 2h values of c2 became more positive even at a low degree of biodegradation hence these are effective parameters for identifying biodegraded gases with a low degree of degradation no obvious change of isotopic composition was observed for the n c5 and i c5 during degradation due to the low relative degradation rates but high content of c1 and c2 biodegradation of c1 and c2 will severely destroy and decrease the gas content of shale gas there is a positive relationship between gas content and total organic carbon toc of shale samples without biodegradation in contrast samples that have experienced biodegradation show an obvious decrease in gas content with toc

geochemical characteristics and origins of natural gas in the zhu iii sub basin pearl river mouth basin china abstract the zhu iii sub basin is an important petroliferous region in the south china sea where hydrocarbon distribution shows strong heterogeneity with oil primary produced from the wenchang b depression and gases from the wenchang a depression however a mixture of oil and gases are produced from the surrounding qionghai and shenhu uplifts the origin generation history and migration pathway of oil have been intensively studied but no study have been done directly on the gas which restricts future petroleum exploration in this study natural gas molecular compositions were measured on 46 samples stable carbon isotopic composition were measured on 39 samples and light hydrocarbon composition were measured on 30 samples results indicate that natural gases in the zhu iii sub basin are mainly composed of hydrocarbon gases with abundant co2 and n2 the hydrocarbon gases are all thermogenic gases which are mainly derived from the oligocene enping formation e3e 35 30 ma the contribution of the eocene wenchang formation e2w 56 35 ma to natural gases increases from the wenchang a depression to the south boundary fault sbf and surrounding uplifts this is caused by the early generated natural gases with more contribution from the e2w and dissolved in oils that migrated to the surrounding qionghai and shenhu uplifts natural gases and oil in the surrounding uplifts and near the sbf therefore avoided secondary cracking due to the high gas migration rate through the sbf and lateral sandstone carrier beds whereas natural gases in the wenchang a depression mainly originated from the secondary cracking of oil the equivalent vitrinite reflectance values of source rocks when the natural gases were generated varied from 0 6 to 1 5 based on carbon isotope differences between hydrocarbons indicating mature to high mature stage co2 with abundance less than 10 mainly originated from organic matter decomposition co2 with abundance more than 15 was sourced from the deep mantle and crust and mainly concentrated in the deep layers near the sbf which cuts into the basement n2 is probably the product of in situ reservoir alteration such as biodegradation in shallower layers our study on the geochemical characteristics and origins of the natural gas will enable a better understanding of petroleum system in the zhu iii sub basin and provide theoretical basis for optimizing future petroleum exploration strategy detailed study on shallow layers in the qionghai and shenhu uplifts and deep layers near the sbf are needed because exploration risks are relatively high in these plays

geochemistry of low molecular weight hydrocarbons in hydrothermal fluids from middle valley northern juan de fuca ridge abstract hydrothermal vent fluids from middle valley a sediment covered vent field located on the northern juan de fuca ridge were sampled in july 2000 eight different vents with exit temperatures of 186 281 c were sampled from two areas of venting the dead dog and odp mound fields fluids from the dead dog field are characterized by higher concentrations of nh3 and organic compounds c1 c4 alkanes ethene propene benzene and toluene compared with fluids from the odp mound field the odp mound fluids however are characterized by higher c1 c2 c3 and benzene toluene ratios than those from the dead dog field the aqueous organic compounds in these fluids have been derived from both bacterial processes methanogenesis in low temperature regions during recharge as well as from thermogenic processes in higher temperature portions of the subsurface reaction zone as the sediments undergo hydrothermal alteration carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the odp mound are greater than those at the dead dog field maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes benzene and toluene and carbon dioxide and methane the calculated temperatures for alkene alkane equilibrium are consistent with differences in the dissolved cl concentrations in fluids from the two fields and confirm that subsurface temperatures at the odp mound are hotter than those at the dead dog field temperatures calculated assuming benzene toluene equilibrium and carbon dioxide methane equilibrium are similar to observed exit temperatures and do not record the hottest subsurface conditions the difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot low salinity vapor with a cooler seawater salinity fluid because of the disparate temperature dependence of alkene alkane and benzene toluene equilibria the mixed fluid records both the high and low temperature equilibrium conditions these calculations indicate that vapor rich fluids are presently being formed in the crust beneath the odp mound yet do not reach the surface due to mixing with the lower temperature fluids

relation of natural gas composition to thermal maturity and source rock type in san juan basin northwestern new mexico and southwestern colorado san juan basin is a roughly circular asymmetric structural depression located in northwestern new mexico and southwestern colorado ultimate recoverable reserves of predominantly nonassociated gas 23 tcf 0 65 1012 m3 are present in the structurally low part of the central basin the major producing intervals are low permeability sandstone reservoirs in the upper cretaceous dakota sandstone mesaverde group and pictured cliffs sandstone lesser amounts of oil and or gas are produced from pennsylvanian jurassic and cretaceous rocks along the southern and western flanks of the basin the gases display a trend of becoming isotopically heavier dgr13c1 values range from 48 7 to 31 4 pmil and chemically drier c1 c1 5 values range from 0 75 to 0 99 with increasing depth these changes are assumed to be the result of thermal cracking processes and the gases are interpreted to have been generated during the mature and post mature stages of hydrocarbon generation however there is considerable scatter in the data which is interpreted to result from a difference in source rock type gases generated from nonmarine humic source rocks are isotopically heavier and chemically drier than those generated from marine sapropelic source rocks at equivalent levels of maturity the gases also become isotopically heavier and chemically drier to the northeast following the trend of increasing maturity of all units in that direction the increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the san juan mountains area maximum burial and heat flow occurred during the oligocene which probably coincided with peak hydrocarbon generation lack of oil in the central basin is believed to be the result of two factors first gas in reservoirs such as the dakota sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature wet gas condensate and post mature dry gas stages of hydrocarbon generation second gas in reservoirs such as mesaverde group and pictured cliffs sandstone is nonassociated and probably was generated from nonmarine coaly organic matter during the mature and post mature stages minor amounts of condensate instead of oil may have been generated from nonmarine source rocks during the mature stage

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

sources of hydrocarbon gases in mud volcanoes from the sorokin trough ne black sea based on molecular and carbon isotopic compositions abstract molecular and stable carbon isotope properties of hydrocarbon gases methane through pentanes and sometimes hexanes from seven sediment cores collected from five mud volcanoes mvs in the sorokin trough ne black sea suggest that these gases are initially derived from the comparable hydrocarbon pools and are likely initial products of non microbial oil cracking processes our results state that dry characteristics of gas and 13 c depleted signatures of methane are result of a high admixture of secondary microbial gas formed due to subsequent microbial anaerobic degradation of redeposited hydrocarbons in the shallow reservoirs the wet gas components in all mvs and gas hydrates are related to each other the compositional variations in c 2 content appear to result from a complex of secondary processes such oil cracking in the deep subsurface migration and mixing of resulted gaseous and liquid hydrocarbons biodegradation of possibly redeposited hydrocarbons forming shallow reservoirs and additional alterations of hydrocarbon gases in the surface sediments due to currently active microbial processes such as aom c 2 consumption etc our data show that the most unaltered gas is in the mud breccia from the kazakov mv the gas mixture possibly represents the original properties of the hydrocarbons trapped in the deep subsurface of the sorokin trough analysis of the hydrocarbon gas data complemented with published maturity characteristics of organic matter from maycopian rock clasts and mud breccia matrix implies that the original source of gases is likely to be located below the maycopian shale formation

marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal abstract detection of chemically identifiable hydrocarbon oil and gas seepage is an important tool in offshore exploration generated and or trapped oil and gas can leak to near surface sediments and ocean in detectable concentrations the seepage surface expression is controlled by hydrocarbon generation regional pressure regimes and migration pathways the migrated hydrocarbon concentration and composition varies due to seepage rates and near surface physical and biological processes that can alter the signal a seepage continuum exists from large volume oil and gas seeps known as macro seepage to lower volume seeps known as micro seepage only detectible with special sampling and analysis understanding the impact of migrated hydrocarbon concentration relative to in situ organic material and locally derived microbial hydrocarbons is critical this paper describes factors that control the near surface geochemical signal as well as the challenges in deciphering the migrated hydrocarbon signature to evaluate subsurface hydrocarbon generation and entrapment

gas accumulations in oligocene miocene reservoirs in the alpine foreland basin austria evidence for gas mixing and gas degradation two petroleum systems are present in the eastern austrian sector of the alpine foreland basin whereas oil and thermogenic gas in mesozoic and eocene reservoir rocks have been generated beneath the alps in lower oligocene source rocks relative dry gas in oligocene miocene clastic rocks deposited in the deep marine basin axial channel system puchkirchen channel is interpreted as microbial in origin detailed investigations of the molecular and isotope composition of 87 gas samples from 86 wells representing all producing fields with oligocene and miocene reservoir rocks suggest that the presence of pure microbial gas is rare and limited mainly to the northern basin flank e g kk field all other fields contain varying amounts of thermogenic gas which has been generated from a source rock with oil window maturity a relation with the underlying thermogenic petroleum system is obvious upward migration occurred along discrete fault zones e g h field or through low permeability caprocks local erosion of lower oligocene sediments the principal seal for the thermogenic petroleum system as well as a high percentage of permeable rocks within the puchkirchen channel favored upward migration and mixing of thermogenic and microbial gas all gas samples in oligocene miocene reservoirs are biodegraded biodegradation and the formation of secondary microbial gas resulted in gas drying therefore the gas samples analyzed in this study are relative dry despite significant contributions of thermogenic hydrocarbons biodegradation probably continues at present time the degree of biodegradation however decreases with depth

petroleum geochemistry of oil and gas from barbados implications for distribution of cretaceous source rocks and regional petroleum prospectivity petroleum produced from the barbados accretionary prism at woodbourne field on barbados is interpreted as generated from cretaceous marine shale deposited under normal salinity and dysoxic conditions rather than from a tertiary source rock as previously proposed barbados oils correlate with some oils from eastern venezuela and trinidad that are positively correlated to extracts from upper cretaceous la luna like source rocks three distinct groups of barbados oils are recognized based on thermal maturity suggesting petroleum generation occurred at multiple levels within the barbados accretionary prism biodegradation is the most significant process affecting barbados oils resulting in increased sulfur content and decreased api gravity barbados gases are interpreted as thermogenic having been co generated with oil and show mixing with biogenic gas is limited gas biodegradation occurred in two samples collected from shallow reservoirs at the woodbourne field the presence of cretaceous source rocks within the barbados accretionary prism suggests that greater petroleum potential exists regionally and perhaps further southeast along the passive margin of south america likewise confirmation of a cretaceous source rock indicates petroleum potential exists within the barbados accretionary prism in reservoirs that are deeper than those from woodbourne field

natural gas genesis and sources in the zizhou gas field ordos basin china abstract the zizhou gas field is a large tight sandstone gas field in the ordos basin china based on the analysis of the geological background gas components light hydrocarbon composition carbon and hydrogen isotope values of alkane gases and the geochemical correlation between gases of the zizhou gas field and analogs from other fields in the basin we conclude that a most natural gases in the zizhou gas field are dry with a small content of wet gas b the gases have similar carbon isotopic and light hydrocarbon compositions to the analogs from other fields as well as much more positive 13c values and high contents of c5 7 i alkanes and methylcyclohexanes indicating typical coal derived gas c carbon isotopic reversal in some gas samples is due to mixing of coal derived gases of different maturities d the gases are sourced from mature to high mature carboniferous permian coal measures and e the gas reservoirs represent continuous accumulations

genetic types of natural gas and filling patterns in daniudi gas field ordos basin china abstract the genetic types source precursors and filling pattern of natural gas in the upper carboniferous taiyuan formation lower permian shanxi formation and lower shihezi formation gas reservoirs of daniudi gas field were investigated using chemical composition as well as carbon and hydrogen isotopic compositions geochemical analysis of natural gases in 25 representative wells shows that natural gas in the daniudi gas field is composed predominantly of hydrocarbons with a dryness coefficient of 0 884 0 978 the carbon isotopic values of ethane and propane are higher than 28 and 25 respectively and the hydrogen isotopic values of methane are lower than 180 indicating that natural gas in the daniudi field is a typical coal type gas derived mainly from humic organic matter in the transitional facies of the carboniferous permian age hydrogen isotopic values of ch4 and h2 display a good positive correlation suggesting that both were controlled by thermal maturity when the mixing of ethane generated from mudstone and coal with the same kerogen type and similar thermal maturity occurred the carbon isotopic values of ethane barely reflect the thermal maturity although the fractionation of hydrogen isotopes of ethane is significantly higher than that of carbon hydrogen isotopic values of ethane in natural gas reservoirs evidently are not related to thermal maturity the daniudi natural gas reservoirs represent both self sourced and near source accumulations the natural gas accumulations in the late triassic early jurassic periods are mainly of the self sourced type while accumulations in the late jurassic early cretaceous period comprise both self sourced and near source patterns and the natural gas reservoirs formed after the late cretaceous period are mainly of the near source type

gas maturity and alteration systematics across the western canada sedimentary basin from four mud gas isotope depth profiles abstract carbon isotope and molecular compositions of mississippian to upper cretaceous mud gases have been examined from four depth profiles across the western canada sedimentary basin wcsb the profiles range from the shallow oil sands in the east r 0 0 25 to the very mature sediments in the overthrust zone to the west r 0 2 5 in the undisturbed wcsb 13 c 1 13 c 2 and 13 c 2 13 c 3 cross plots show three maturity and alteration trends 1 pre cretaceous gas sourced from type ii kerogen 2 cretaceous colorado group gas and 3 lower cretaceous mannville group biodegraded gas a fourth set of distinctly different maturity trends is recognized for lower cretaceous gas sourced from type iii kerogen in the disturbed belt of the wcsb displacement of these latter maturity trends to high 13 c 2 values suggests that the sampled gas was trapped after earlier formed gas escaped probably as a result of overthrusting unusually 13 c enriched gas 13 c 1 34 13 c 2 13 and 13 c 3 0 from the gething formation in the disturbed belt is the result of late stage gas cracking in a closed system in general gas maturity is consistent with the maturity of the host sediments in the wcsb suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale the deep basin portion of the wcsb is an exception here extensive cross formational homogenization of gases has occurred in addition to updip migration along the most permeable stratigraphic units

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

hydrothermal hydrocarbon gases 1 genesis and geothermometry various sources for hydrothermal ch4 have been proposed over the years while c isotope studies have narrowed the possibilities enough higher hydrocarbon gas data now exist both to supplement the isotopic data and to permit additional deductions regarding origins comparison of typical c1 c6 data for gases of various origins from sedimentary and crystalline rocks and hydrothermal systems reveals certain characteristics apart from isotopic differences hydrothermal hydrocarbons differ from sedimentary hydrocarbons mainly in possessing tendencies towards a relative excess of ch4 higher normal iso ratios for butane and pentane and relatively high amounts of c6 gases despite these differences consideration of the evidence indicates that hydrothermal hydrocarbon gases in most cases originate like sedimentary basin gases by thermal degradation of organic matter in the relatively shallow subsurface the principal characteristic of these hydrothermal gases excess ch4 appears to have a geothermometric function the following empirical relationship has been derived t c 57 8 log ch4 c2h6 96 8 which fits moderately well a range of geothermal fields worldwide this gas geothermometer may be particularly applicable during geothermal exploration in areas where there is little direct knowledge of subsurface conditions

effects of deep co 2 on petroleum and thermal alteration the case of the huangqiao oil and gas field abstract the co2 rich natural gas from the huangqiao oil and gas field of the subei basin is produced along with crude oil the co2 content ranging from 80 56 to 99 30 and the 3he 4he ratio of 0 94 10 6 to 4 67 10 6 indicates that this includes a substantial fraction of mantle derived co2 in the huangqiao region although the positive carbon isotopic series of alkane gases i e 13c1 the crude oil of the huangqiao region is light or condensate oil with low densities of 0 7933 0 8255 g cm3 saturated hydrocarbons comprise 90 06 97 37 the crude oil while the aromatic non hydrocarbon and asphaltene fractions are very small the n alkanes range from c13 to c34 with highest abundances at c16 and c17 the gas chromatogram of the crude oil shows an unresolved hump or unresolved complex mixture ucm distribution of a complete n alkane series non odd even predominance and major n alkane peaks on the highest positions of the ucm distribution this may indicate that deep mantle derived co2 fluid extracted and accumulated the oil now found in the reservoirs or source rocks

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high maturity shales abstract this article reviews the abnormal characteristics of shale gases natural gases produced from organic rich shales and discusses the cause of the anomalies and mechanisms for gas enrichment and depletion in high maturity organic rich shales the reported shale gas geochemical anomalies include rollover of iso alkane normal alkane ratios rollover of ethane and propane isotopic compositions abnormally light ethane and propane 13 c values as well as isotope reversals among methane ethane and propane these anomalies reflect the complex histories of gas generation and associated isotopic fractionation as well as in situ mixing and accumulation of gases generated from different precursors at different thermal maturities a model was proposed to explain the observed geochemical anomalies gas generation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso alkane normal alkane ratios and ethane and propane 13 c values normal trend simultaneous cracking of kerogen retained oil and wet gas and associated isotopic fractionation at higher maturity caused decreasing iso alkane normal alkane ratios lighter ethane and propane 13 c and corresponding conversion of carbon isotopic distribution patterns from normal through partial reversal to complete reversal relatively low oil expulsion efficiency at peak oil generation low expulsion efficiency at peak gas generation and little gas loss during post generation evolution are necessary for organic rich shales to display the observed geochemical anomalies high organic matter richness high thermal maturity high degrees of kerogen gas and oil gas conversions and late stage the stage of peak gas generation and post generation evolution closed system accounted for gas enrichment in shales loss of free gases during post generation evolution may result in gas depletion or even undersaturation total gas content lower than the gas sorption capacity in high maturity organic rich shales

origin and migration of oil and natural gas in the western part of the ukrainian outer carpathians geochemical and geological approach abstract we established the generation and migration processes as well as the influence of secondary processes of oil and natural gas accumulated in lower cretaceous oligocene strata of the western part of the ukrainian outer carpathians based on the results of organic geochemical analyses of oil and natural gas related to the geological setting and source organic matter the oligocene menilite beds from the boryslav pokuttya nappe are the main source rock for oil and natural gas accumulated in the fields of the boryslav pokuttya and skiba nappes co sourcing of oils and natural gas accumulated in the skiba nappe in the study area by both menilite and spas beds of this nappe cannot be excluded in some cases e g novoshidnytsya field oil and natural gas originated within the oligocene menilite beds of the boryslav pokuttya nappe migrated through thrusts and fissure zones and accumulated in the lower cretaceous reservoirs of the skiba nappe oil from small vovche field and pohar seep of the krosno nappe was generated by menilite source rocks of this tectonic unit and migrated to the traps sporadically oils underwent secondary processes mainly biodegradation and occasionally water washing hydrocarbon gases and carbon dioxide were generated mainly during low temperature thermogenic processes from type ii kerogen and partly during the microbial co2 reduction process only natural gas from the mala volosyanka oil and gas seep of the krosno nappe was documented to have undergone biodegradation

origin of marine sour natural gas and gas filling model for the wolonghe gas field sichuan basin china abstract the chemical and isotopic composition of natural gases from the wolonghe gas field sichuan basin southwest china was investigated to assess the potential gas sources and reconstruct the gas filling history of the reservoirs all natural gases in this field are dominated by gaseous hydrocarbons c 1 c 3 with varied amounts of non hydrocarbon components co 2 h 2 s and n 2 the h 2 s content varies with reservoir intervals it ranges from zero to 1 84 with an average of 0 37 in the carboniferous reservoir from 0 05 to 0 76 with an average of 0 30 in the permian reservoir and from 1 09 to 18 83 with an average of 5 39 in the lower triassic reservoir the gas dryness coefficient c 1 c 1 3 ranges from 0 97 to 1 0 with an average of 0 99 the carbon isotopic compositions of methane and its homologues in the wolonghe gas field vary widely with 13 c 1 13 c 2 13 c 3 in the carboniferous and permian gas reservoirs and 13 c 1 13 c 2 13 c 3 in the lower triassic jialingjiang formation gas reservoirs the d 1 values range from 140 to 100 with an average of 124 5 the 34 s h2s values in the field extend from 5 7 to 31 0 with 34 s h2s values in the lower triassic jialingjiang formation reservoirs being much higher than those in the carboniferous and permian reservoirs the sour natural gases in the field originated from cracking of oil and were sourced from the marine sapropelic organic matter at high maturity levels the natural gases underwent alteration by thermochemical sulfate reduction tsr although the limited extent tsr occurred in the carboniferous and permian gas reservoirs the tsr alteration is likely to have caused the high gas dryness and the 13 c enrichment of ch 4 the present low content of h 2 s in the carboniferous and permian gases would be related to the loss of h 2 s dissolved in water under the reconstruction of these gas reservoirs in contrast the high h 2 s in the jialingjiang formation gases is determined by both the favorable tsr conditions in the lower triassic reservoirs and the favorable preservation conditions provided by the lower middle triassic gypsum the wolonghe natural gas system is characterized by multi pay zones multi source supply and multi type layers the gas generated from silurian source rocks migrated into the carboniferous reservoir through faults and fractures and was trapped beneath the permian liangshan formation shale whereas the natural gas in the permian and lower triassic jialingjiang formation was generated and preserved in its own formations with the overlying gypsum acting as a cap rock

origins of taiwan s mud volcanoes evidence from geochemistry abstract the supply of natural gas from mud volcanoes is commonly attributed to the migration of gas bearing fluids from a deeper source area however the relative importance of these gas sources is not well known onshore mud volcanoes in taiwan are mainly located in the foothills of southwestern taiwan and in the coastal range of eastern taiwan generally along structurally weak zones geochemical analyses of gases and sediments from 17 mud volcanoes were undertaken in this study using molecular and stable isotopic compositions hydrocarbon gases were characterized by type and source maturity source rock assessments and nanofossil analyses were carried out to evaluate the potential and to infer the origin of sediments a particular effort was directed towards determining the genetic relationships between mud volcanic gas and associated sediments gases from taiwan s mud volcanoes are predominantly ch4 enriched with the exception of a few mud volcanoes where co2 predominates the origins of hydrocarbon gases in these systems range from microbial to thermally over mature sources while excess co2 may be derived from an inorganic origin as indicated by measured 13 c co 2 values the integration of geochemistry and local geology has yielded models with which to evaluate possible mechanisms for the formation of mud volcanoes

transgressive sand sheet covering the shelf and upper slope off sendai northeast japan abstract a thin sand sheet deposited during the last major transgression from 14 000 to 6000 yrs b p covers the shelf and upper slope off sendai northeast japan the sand sheet overlies the sediments deposited during the last low sea level stand and is partly overlain by modern offshore muds this sheet is regarded as palimpsest sediment formed during the transgression in water depths of 50 m the sand sheet is 50 150 cm thick and consists of alternations of poorly sorted thin sand layers and mud layers that are several millimeters to a few centimeters thick it bears mixed environment molluscan shells and small amounts of glauconite scour and fill structures are frequently found gravel with shell fragments is common near the base of the sheet

abundance and carbon isotopic composition of neo pentane in australian natural gases abstract here we report the abundance and carbon isotopic composition of neo pentane or 2 2 dimethylpropane in australian natural gases derived from source rocks of late proterozoic to early tertiary age in australian natural gases neo pentane abundance is low ranging from 6 to 295 ppm the 13cneo pentane shows a strong source control and together with the other gaseous hydrocarbons a maturity effect albeit less pronounced than the source effect in severely biodegraded natural gases neo pentane abundance is similar to that in non biodegraded gases as a result of a counter balance between preservation of neo pentane relative to other wet gases and the addition of biogenic methane nevertheless neo pentane can become the most abundant pentane isomer with neo pentane iso n pentane ratios up to 9 9 here the 13cneo pentane is identical between biodegraded and non biodegraded natural gas from the same source providing strong evidence that neo pentane is resistant to biodegradation even within severely biodegraded gases where the carbon isotopic composition of all the other gaseous hydrocarbons has been altered the carbon isotopic composition of neo pentane ranges from 13c 50 8 to 24 4 vpdb and is typically isotopically more depleted in 13c up to 9 7 than iso pentane and n pentane this wide carbon isotopic variability between the pentane isomers most likely results from the interplay between decomposition isomerisation and methylation mechanisms for neo pentane formation in biodegraded gases the iso pentane and n pentane become progressively enriched in 13c resulting in much larger isotopic differences 13cneo pentane iso pentane values of up to 16 6 in this paper we show how the carbon isotopic composition of neo pentane is related to source rock type and age and demonstrate the usefulness of neo pentane in gas gas correlation particularly in biodegraded accumulations

molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the gulf of cadiz ne atlantic investigations of molecular and carbon isotopic variability of hydrocarbon gases from methane through butanes pentanes have been performed on six mud volcanoes from two fluid venting provinces located in the gulf of cadiz ne atlantic the main aims were to define the basic gas types to describe their geochemical characteristics in relationship to their sources and to determine the secondary effects due to migration mixing and microbial alteration hydrocarbon gas data reveal two groups of gases despite the different maturation characteristics both gas groups are allochthonous to the erupted mud breccia and represent a complex of redeposited secondary migrated mixed and microbially altered hydrocarbons it may possibly imply the presence of hydrocarbon accumulations in the deep subsurface of the gulf of cadiz

bed thickness characteristics of inner shelf storm deposits associated with a transgressive to regressive holocene wave dominated shelf sendai coastal plain japan holocene inner shelf storm deposits preserved beneath the sendai coastal plain facing the pacific coast of north eastern japan were formed during a transgressive regressive cycle the evolution of the holocene wave dominated depositional system along the sendai coast is reconstructed using 76 ams accelerator mass spectrometers 14c ages and the origin of bed thickness variations in the inner shelf storm deposits is explored the holocene succession is 30 m thick and overlies latest pleistocene to early holocene non marine deposits above a transgressive ravinement surface it comprises transgressive ravinement and inner shelf deposits and regressive inner shelf shoreface and coastal plain deposits the inner shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds the average preservation interval of a single storm bed is shortest during the transgression 5 7 20 6 years and then increases to a maximum during the early regression 83 3 250 0 years decreasing to 7 7 31 3 years with shoreline progradation average accumulation rates decreased during the transgression and then increased during the regression but the sand mud ratio varies little reflecting inefficient sediment segregation downdip on the inner shelf the vertical pattern of sand layer thicknesses also shows no relationship to position within the cycle although small scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and or random storm processes the average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression this is probably because of the low preservation potential of thin beds associated with frequent low magnitude storms during this period of low accumulation rates and extensive reworking this preservation bias and the nature of the sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits

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 of oil and natural gas in the central part of the ukrainian outer carpathians geochemical and geological approach the generation and migration processes as well as the influence of secondary processes on oil and natural gas accumulated in lower cretaceous lower miocene strata of the central part of the ukrainian outer carpathians are discussed type ii and ii iii kerogens from the oligocene lower menilite beds of the boryslav pokuttya nappe generated oil and natural gas mainly during the middle stage of the oil window these hydrocarbons accumulated in the fields of the boryslav pokuttya and skiba nappes water washing is the dominant secondary process occurring in many accumulations regardless of reservoir rock whereas biodegradation is insignificant and is clearly evident only in natural gas from one seep in the abandoned starunia ozocerite mine all hydrocarbon gases are thermogenic and were generated from mixed organic matter containing various proportions of type ii and iii kerogens only one natural gas accumulation in the pivnichna dolyna field contains some microbial hydrocarbon contribution carbon dioxide mainly originated from thermogenic kerogen decarboxylation and sporadically from microbial processes molecular nitrogen was mainly generated during low temperature thermal transformation of organic matter and could also partly originate from nh4 rich illites of the clayey facies of the oligocene lower miocene menilite beds the integration of the geochemical data with geological setting and maturation history of source rocks enabled us to reconstruct hydrocarbon migration paths revealing the complicated trap filling process

oil bearing co2 ch4 h2o fluid inclusions oil survival since the palaeoproterozoic after high temperature entrapment the 2 45 ga fluvial quartz arenites and conglomerates of the matinenda formation at elliot lake canada contain evidence for two episodes of oil migration with entrapment of oil bearing fluid inclusions the first episode was during diagenesis the second during a subsequent sub greenschist facies metamorphic event at ca 2 2 ga at temperatures exceeding 250 c remnants of the migrating hydrocarbon fluids are preserved as radiogenic pyrobitumen nodules occupying former inter granular pore spaces and as complex fluorescing oil bearing fluid inclusions the latter include two dominant types 1 aqueous oil inclusions with a minor oil phase located within original detrital grains and clasts and within rare quartz overgrowths and 2 aqueous carbonic inclusions with a minor oil phase light hydrocarbons and solids located within microfractures in detrital quartz grains and clasts the majority of type 1 inclusions were trapped with a high salinity fluid up to 25 wt nacl equivalent at relatively low temperatures between 80 and 200 c and pressures of less than 2 kbar early in the burial history of the matinenda formation type 2 inclusions were trapped during metamorphism with a much lower salinity aqueous carbonic fluid 1 2 to 17 wt nacl equivalent at temperatures between 280 and 350 c and pressures between 1 and 1 5 kbar under these conditions oil may have been miscible in the mixed fluid 13c of 25 5 and 24 9 for the pyrobitumen nodules and fluid inclusion hydrocarbon gas indicate a biogenic origin for the petroleum 13c of 16 2 for the bulk inclusion gas suggests a value of 12 for the carbonic phase hence a mix of inorganic co2 and organic co2 from either thermal alteration of organic matter or in situ chemical oxidation of hydrocarbons this study has implications for the depth and temperature at which oil may be present suggesting that deep old reservoirs may not be completely barren of petroleum the fact that hydrocarbons in the matinenda formation fluoresce indicates that aromatic compounds are able to survive temperatures of up to 350 c in a suitable geological environment

fault charged mantle fluid contamination of united kingdom north sea oils insights from re os isotopes 187 os 188 os as expected from a kimmer idge clay formation source however oils from the viking graben with which the brent fi eld is associated and main east shetland basin display unradiogenic 187 os 188 os in this study we seek to demonstrate that these anomalous results are due to interaction with a mantle like fl uid that propagated through the main fault zone of the viking graben and east shetland basin this study enhances our understanding of the re os oil systematics and supports cur rent ideas regarding fault architecture crustal thinning and associated fl uid fl ow and interac tion in the north sea oil system abstract united kingdom north sea oils sourced from the kimmeridge clay formation contain unradiogenic 0 17 to 0 48 and radiogenic 1 04 to 3 34 187 os 188 os values the unra diogenic 187 os 188 os values are spatially associated with the main basin bounding faults of the viking graben and east shetland basin in contrast the radiogenic 187 os 188 os values are associated with north sea basins located farther away from the basin boundary faults we suggest that crustal thinning and strain localization within the viking graben and east shetland basin are suffi cient to have allowed basin bounding faults to propagate to suffi cient depth to act as conduits for mantle derived fl uids to interact with oil this hypothesis is sup ported by previous geochemical data for north sea oil fi elds furthermore we demonstrate the use of os isotopes as an important tracker of crustal scale fl uid dynamics and petroleum migration pathways in extensional basins

geochemistry and habitat of natural gases in po basin northern italy three types of natural gases can be distinguished in the po basin 1 bacterial gases dgr13cch4 60 ppt dgrdch4 180 to 200 ppt c2 0 2 which are found predominantly in pliocene pleistocene reservoirs 2 mixed gases dgr13cch4 60 to 50 ppt dgrdch4 180 to 200 ppt c2 0 2 to 5 which are predominantly in reservoirs of messinian upper miocene age but also in a few places in older and younger reservoirs 3 thermogenic gases dgr13c 50 ppt dgrd 200 to 150 ppt c2 5 which commonly are associated with oil and are produced predominantly from miocene and mesozoic reservoirs a quantitative assessment of gas in place for these three gas types is 1 80 2 10 and 3 10 respectively the areal distribution and amount of the three gas types are controlled strictly by geologic factors synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area where 70 of the bacterial gas in place is produced here bacterially formed methane is produced from depths of up to 4 500 m 15 000 ft and is the deepest bacterial gas so far reported in the literature in the pedealpine homocline lower sedimentation rates and a weak deformation of pliocene pleistocene sediments prevented appreciable accumulations only 15 of the bacterial gas in the basin is produced in this area thermogenic gases are found in the vicinity of the apennines where strong tectonic movements have favored its migration from deeper mesozoic sources into tertiary reservoirs or into deep mesozoic carbonate reservoirs of the pedealpine homocline

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

15 9 gamma gas field offshore norway new trap type for north sea basin with regional structural implications the 15 9 gamma gas field lies in the central north sea midway between norway and the united kingdom the field was discovered in 1981 and by mid 1982 3 additional confirmation wells had been completed the reservoir is provided by a complex of paleocene submarine fan sandstones the sandstones decrease in thickness across the field from 150 m 492 ft in the northwest to less than 50 m 164 ft in the southeast the sandstones pinch out entirely a few kilometers beyond the field the volume of gas in place is on the order of 65 109 m3 2 3 tcf the trap in the paleocene sandstones is interpreted as being formed by a mid tertiary compressive phase which reactivated preexisting basement faults detailed structural analysis suggests that wnw e e trending faults have suffered repeated strike slip offset associated with jurassic transtension and mid cretaceous and middle tertiary transpression the writers suggest that the wnw ese trending basement faults represent a northwestward continuation of the tornquist line a fundamental fracture zone bounding the east european scandinavian platform which had previously been considered as terminating at the line of the oslo graben about 250 km 155 mi to the southeast

late pleistocene coastal sediments drainage patterns and sand ridge systems on the shelf off sendai northeast japan abstract seismic reflection profiles on the shelf off sendai northeast japan show drainage patterns coastal sequences and shoreface connected sand ridges formed during the last lowstand the present shelf break coincides with storm wave base at the lowstand sediments supplied by river systems in the latest pleistocene were deposited on the shelf and the upper slope the sediments were molded into sand ridges on the shelf the ancient coastal plain that formed during this period was widely eroded by shoreface erosion during the following transgression and is recognized on the middle shelf

increased fluid flow activity in shallow sediments at the 3 km long hugin fracture in the central north sea the north sea hosts a wide variety of seafloor seeps that may be important for transfer of chemical species such as methane from the earth s interior to its exterior here we provide geochemical and geophysical evidence for fluid flow within shallow sediments at the recently discovered 3 km long hugin fracture in the central north sea although venting of gas bubbles was not observed concentrations of dissolved methane were significantly elevated up to six times background values in the water column at various locations above the fracture and microbial mats that form in the presence of methane were observed at the seafloor seismic amplitude anomalies revealed a bright spot at a fault bend that may be the source of the water column methane sediment porewaters recovered in close proximity to the hugin fracture indicate the presence of fluids from two different shallow 500m sources i a reduced fluid characterized by elevated methane concentrations and or high levels of dissolved sulfide up to 6 mmol l 1 and ii a low chlorinity fluid cl 305 mmol l 1 that has low levels of dissolved methane and or sulfide the area of the seafloor affected by the presence of methane enriched fluids is similar to the footprint of seepage from other morphological features in the north sea

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

anomalies of natural gas compositions and carbon isotope ratios caused by gas diffusion a case from the donghe sandstone reservoir in the hadexun oilfield tarim basin northwest china abstract natural gases in the carboniferous donghe sandstone reservoir within the block hd4 of the hadexun oilfield tarim basin are characterized by abnormally low total hydrocarbon gas contents 13c ethane c2

hydrocarbon system analysis in a rift basin with mixed marine and nonmarine source rocks the south viking graben north sea the south viking graben of the north sea is a prolific oil and gas province that has recoverable reserves of approximately 176 x 109 sm3 standard cubic meters 6 2 tcf gas 58 x 106 t 412 million boe natural gas liquids and 12 5 x 106 sm3 78 million bbl of black oil contained in the main fields sleipner vest sleipner ost and volve these fields are located primarily within block 15 9 in the norwegian sector and extend into neighboring blocks 15 6 and 16 7 the principal source of black nonvolatile oil is the late jurassic draupne formation which has a predominance of marine algal organic matter the lower section of the draupne formation together with the heather formation are organically leaner and contain a mixture of marine algal and terrigenous organic matter resulting in a potential to generate both oil and gas the middle jurassic hugin and sleipner formations contain humic coals and coaly shales with potential to generate gas and some light liquids these coals contain on average 80 90 vitrinitic woody material with occasional enrichment of resinite high resinite concentrations can lead to an overprediction of oil potential as they contribute significantly to the hydrogen index hi but generate primarily low molecular weight aromatic compounds all source rock facies types have reached maturities sufficient to generate oil and gas basin modeling suggests that onset of oil and gas generation started during the latest cretaceous early paleocene these source rocks have continued to yield oil and gas to the present day in many parts of the catchment area for the sleipner fields detailed geochemical analyses identified five main oil and condensate families family a comprises condensates and oil located in the northernmost part of sleipner vest and dagny generated from a marine clastic source with a predominance of type ii algal organic matter family b is condensates in the middle to southern part of sleipner vest generated from a source or a contribution from several source facies with mixed terrigenous higher plant organic matter and marine algal material family c consists of condensates reservoired in the jurassic of sleipner ost except well 15 9 a15 generated from a mixed algal terrigenous source but with a higher contribution of hydrocarbons from a marine algal source as compared with the sleipner vest family b family d comprises condensates in the jurassic triassic of loke and gungne in well 15 9 a15 from the crest of the sleipner ost structure as well as condensates within the paleocene sands of sleipner ost these condensates have a mixed terrigenous higher plant and marine algal signature and are derived from pre upper draupne formation sources family e is the black oil present in the jurassic volve field derived from a marine calcareous shale with type ii to ii s organic matter the source rock for this oil is unique in the greater sleipner area and is likely located in the isolated subgraben between the northern parts of sleipner vest and sleipner ost the hydrocarbon gases are broadly similar and are interpreted to have been generated from coals and coaly shales of the sleipner and hugin formations as well as those parts of the pre upper draupne section that have a predominance of terrigenous higher plant organic matter

origin of hydrocarbon and non hydrocarbon h2s co2 and n2 components of natural gas accumulated in the zechstein main dolomite carbonate reservoir of the western part of the polish sector of the southern permian basin abstract the origin and migration of hydrocarbon and non hydrocarbon hydrogen sulphide carbon dioxide and molecular nitrogen components of natural gas accumulated within the zechstein pz2 stassfurt main dolomite ca2 carbonate reservoir in the western part of the polish sector of the southern permian basin spb were established based on molecular and isotopic compositions 13c ch4 c2h6 c3h8 n c4h10 i c4h10 n c5h12 i c5h12 and co2 2h ch4 15n n2 and 34s h2s related to geological settings and 1 d modelling of generation processes hydrocarbon gases were generated during both microbial and thermogenic processes primary microbial processes ended at the turn of the late permian and early triassic when oil and thermogenic gas generation processes began the reconstruction of the thermal evolution and transformation of organic matter in this part of the spb shows that thermogenic processes ended in the late jurassic in the grotow peninsula zone c and in the late cretaceous in the western part of the wielkopolska platform and gorzow platform zones a and b the presence of the microbial component in analysed accumulations indicates that traps within the ca2 carbonate strata were formed and sealed by the overlying impermeable anhydrites and salts of pz2 stassfurt pz3 leine and pz4 aller cycles preventing its escape accumulation of natural gas in ca2 traps took place in at least two stages and had a cumulative character first microbial gases accumulated then thermogenic gases associated with oil were generated from type ii kerogen in the oil window an insignificant methane component was generated from type iii kerogen and migrated to ca2 reservoir from carboniferous strata h2s was most probably generated during both microbial msr and thermochemical sulphate reduction tsr of anhydrites in the pz1 werra cycle and additionally from thermal decomposition of organic sulphur components of oil and kerogen in the ca2 carbonate rocks msr ended during the late permian tsr was temporarily halted as a result of early cretaceous uplift and probably resumed on a minor scale after the late cretaceous co2 was predominantly generated during thermogenic processes of kerogen decarboxylation carbonate dissolution processes and tsr and msr together with h2s a lesser contribution of co2 from microbial fermentation was also observed to a large extent n2 was mainly generated during thermal transformation of type ii kerogen dispersed in the ca2 carbonate strata and probably partly migrated from carboniferous high maturity type iii kerogen to ca2 reservoirs through fault systems

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

isotopic fingerprints of shallow gases in the western canadian sedimentary basin tools for remediation of leaking heavy oil wells abstract thousands of wells have been drilled in northeastern alberta and adjacent saskatchewan in order to develop the heavy oil reservoirs of the lower cretaceous mannville group in a large number of these wells vertical migration of gas from unknown sources to the surface via well casings and surrounding soils represents a serious environmental problem mud samples from new wells drilled in the area were collected at approximately 50 m intervals from the surface to the completion depth and the gases contained within the muds mud gases were analyzed for 13c 12c composition gases from the various mannville group sands do not show unique isotopic signatures they comprise bacterial methane 13cc1 70 to 60 pdb with trace c2 components that are residues from biodegradation processes gases from each of the overlying upper cretaceous colorado group shale units are however isotopically distinct the 13c of methanes in the shales have values equivalent to those within the mannville sands but the 13c of the c2 components are unambiguously different c2 gases from the shallower colorado shales are incipient thermal gases and fit the isotopic maturity diagram of james a t 1983 aapg bull 67 1176 1191 and also of chung h m gormly j r squires r m 1988 chem geol 71 97 103 we believe these gases may include the most immature suite of thermogenic gases reported 13cc2 63 to 32 13cc3 42 to 22 13cnc4 37 to 23 carbon isotope fingerprints from mud gases are used to identify specific source depths of migrated gases comparisons between the isotopic characteristics of the shallow gases and the migrated gases collected near surface infer that most of the migrated gases are actually generated in the colorado group shales and not the mannville sands in which the wells were completed the ability to estimate the source depth of the migrated gases greatly aids remediation of leaking wells within the western canadian sedimentary basin wcsb

geochemical characterization of secondary microbial gas occurrence in the songliao basin ne china abstract a suite of natural gases from the northern songliao basin in ne china were characterized for their molecular and carbon isotopic composition gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation with very high dryness c1 c2 100 high c2 c3 and i c4 n c4 ratios high nitrogen content and variable carbon dioxide content isotopic values show wide range variations 13 c ch 4 from 79 5 to 45 0 13 c c 2 h 6 from 53 7 to 32 2 13 c c 3 h 8 from 36 5 to 20 1 13 c n c 4 h 10 from 32 7 to 24 5 and 13 c co 2 from 21 6 to 10 5 a variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures system openness also was considered for shallow biodegraded gas accumulations gases from the daqing anticline are relatively wet with 13c enriched methane and 13c depleted co2 representing typically thermogenic origin gases within the longhupao da an terrace have variable dryness 13c enriched methane and variable 13c of co2 suggesting dominant thermogenic origin and minor secondary microbial methane augment the puqian ao nan uplift contains relatively dry gas with 13c depleted methane and 13c enriched co2 typical for secondary microbial gas with a minor part of thermogenic methane gas accumulations in the western slope are very dry with low carbon dioxide concentrations some gases contain 13c depleted methane ethane and propane indicating low maturity primary microbial origin recognition of varying genetic gas types in the songliao basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems

isotopic reversal rollover in shale gases produced from the mississippian barnett and fayetteville formations abstract ethane propane and carbon dioxide show reversed carbon isotopic maturity trends in natural gas produced from the barnett and fayetteville shales at thermal maturities greater than 1 5 vre at this high level of thermal maturity the iso butane to n butane ratio also reverses suggesting wet gas cracking has occurred generating more gas molecules in the same volume resulting in overpressure and increased stabilized production rates in the barnett hydrothermal fluids from the nearby ouachita thrust front apparently enhanced the maturity of the fayetteville and barnett shales water hydrocarbon reactions at these high maturities may account for the isotopic reversals

origin of near surface hydrocarbon gases bound in northern barents sea sediments abstract in the southern part of the norwegian barents sea extensive hydrocarbon exploration and drilling has led to several oil and gas discoveries yet little is known on the petroleum systems and potential hydrocarbon reservoirs in the northern norwegian barents sea if hydrocarbons generated by thermogenic processes deep in the subsurface migrate to the surface traces of these hydrocarbons can remain in the near surface sediments and provide indirect information on potential petroleum systems near surface hydrocarbon prospecting is a method frequently used in hydrocarbon exploration of frontier areas where no direct geochemical information from drilling is available to improve knowledge of subsurface structures and the evolution of potential petroleum systems in the northern barents sea seismic data and near surface sediment samples were collected analysis of bound hydrocarbon gases extracted from these sediments revealed concentrations significantly above background in areas along the hornsund knollega fault complex as well as near margins of the olga basin generally the compositional and stable carbon isotope signatures of bound gases indicate thermogenic origins from source rocks of oil window maturity for near surface gases with anomalous high concentrations whereas for sediments with low concentration of bound gases mixture of thermogenic and microbial gas is indicated amount and composition of bound gas extracted from source rock samples from spitsbergen indicate that contribution of transported material may have influenced the bound gas in near surface marine sediment of the barents sea in areas where presence of mature organic matter is indicated in the knolegga fault complex near the western barents sea margin high concentrations of thermogenic gas in near surface sediments are associated with fault bound basins and most likely originate from the paleocene eocene torsk formation in the olga basin higher bound gas concentrations occur near the southern border of the basin corresponding to sub cropping late jurassic early cretaceous shales whereas elevated concentrations in the northern olga basin are associated with reactivated faults reaching close to the surface sediments above the center of the basin show significantly lower bound gas concentrations these observations indicate that the jurassic shales act as a regional seal for hydrocarbons and that reactivated faults at the basin margin represent pathways for migration to the surface 1d basin and petroleum systems modeling for the olga basin indicates that early to middle triassic sediments reached oil window maturity and represent the most likely source for thermogenic near surface gas in that area