geochemical evidence of secondary microbial methane from very slight biodegradation of undersaturated oils in a deep hot reservoir a rare finding of early mature undersaturated oils with low gas oil ratios enables us to document secondary microbial methane generation during very slight biodegradation in a deep hot reservoir in the ultradeep water of the gulf of mexico in three studied gas samples methane is enriched in 13 c 13 c is from 63 to 64 relative to pure thermogenic methane estimated 13 c is from 71 to 67 and pure primary microbial methane 13 c is 68 carbon dioxide in gases has 13 c values that negatively correlate with 13 c values of pure thermogenic methane methane is unusually enriched in heavy isotope 2 h relative to associated ethane some extracted oils are depleted in long chain alkyl aromatics these lines of geochemical evidence suggest anaerobic microbial degradation of oil and subsequent reduction of resulting carbon dioxide to methane although specific geobiological details of secondary microbial methane generation are unclear this process may be partially responsible for charging some of the largest gas and gas hydrate fields in the world

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

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

genetic type and maturity of lower paleozoic marine hydrocarbon gases in the eastern tarim basin abstract hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity genetic types and sources of lower paleozoic marine hydrocarbon gases from the eastern tarim basin all of the lower paleozoic gases are relatively dry dryness coefficient 0 9 and belong to the marine sapropelic type of cracked gases these gases are characterized by low 13 c 1 and low 13 c 2 13 c 1 values due to a lack of terrestrial source input thermal simulation experiments on upper proterozoic to lower paleozoic source rocks resulted in a 13 c 1 r o regression equation that was used to discriminate the maturities and sources of the natural gases these natural gases are divided into four genetic types 1 humic coal derived pyrolytic gas 2 lacustrine sapropelic oil cracking gas 3 the marine sapropelic highly mature oil cracking gas and 4 overmature oil cracking gas types 1 and 2 were mainly generated from the upper triassic middle and lower jurassic types 3 and 4 are mainly generated from the lower paleozoic the data can be used to describe multiple sources and multiple generating stages of natural gas

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

compartmentalization and time lapse geochemical reservoir surveillance of the horn mountain oil field deep water gulf of mexico oil is produced at the horn mountain field gulf of mexico mississippi canyon blocks 126 and 127 from middle miocene reservoirs m and j reservoir facies are characterized as sand filled channels and associated overbank deposits and are positioned in combination structural and stratigraphic traps prior to initial production several barriers and baffles were identified in both reservoirs by integrating geological geophysical petrophysical pressure pvt pressure volume temperature relationships and geochemical data and petroleum filling history a compartmentalization risk matrix was developed to facilitate and visualize the integrated evaluation of compartmentalization during production in addition to traditional surveillance technologies we applied time lapse geochemistry tlg to visualize petroleum sweep by monitoring changes in fluid composition and fingerprints across reservoirs in this technology appraisal and preproduction fluid samples are first analyzed to map fluid types across a static reservoir then a surveillance program in which fluid samples are taken from producing wells at regular time intervals is designed and executed the obtained production samples are geochemically fingerprinted and compared with preproduction fluids from the same well and surrounding wells at horn mountain interpretation of geochemical data allowed us to infer oil movement across reservoir m and helped to reevaluate reservoir models and reduce risks in managing reservoir performance in reservoir j an untapped compartment was identified and an additional producer was justified for future drilling time lapse geochemistry results were consistent with and complimentary to other surveillance data available to date our study demonstrates that tlg is a safe and cost effective technology which reduces uncertainties associated with other reservoir surveillance methods and appears to be valuable for reservoir management

distinguishing cambrian from upper ordovician source rocks evidence from sulfur isotopes and biomarkers in the tarim basin abstract the reported source rocks for the abundant petroleum in the tarim basin china range from cambrian to lower ordovician and or upper ordovician in age however the difference between the two groups of source rocks is not well characterized in this study pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of crcl 2 reduction and grinding the kerogen and coexisting pyrite samples were then analyzed for 34 s values results show that the kerogen samples from the cambrian have 34 s values between 10 4 and 19 4 the values are significantly higher than those from the lower ordovician kerogen 34 s of between 6 7 and 8 7 which in turn are generally higher than from the upper ordovician kerogen samples 34 s of between 15 3 and 6 8 the associated pyrite shows a similar trend but with much lower 34 s values this stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil 34 s values from other basins suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite 34 s values the relatively high 34 s values in the cambrian to lower ordovician source rocks are associated with abundant aryl isoprenoids gammacerane and c 35 homohopanes in the extractable organic matter indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification compared with the upper ordovician the cambrian to lower ordovician source rocks show abundance in c 28 20r sterane c 23 tricyclic terpanes 4 23 24 trimethyl triaromatic dinosteroids and depletion in c 24 tetracyclic terpane c 29 hopane thus 34 s values and biomarkers of source rock organic matter can be used for distinguishing the cambrian and upper ordovician source rocks in the tarim basin

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

diamondoid hydrocarbon ratios novel maturity indices for highly mature crude oils abstract diamondoid hydrocarbons have been identified in oil and source rock samples from the tarim yinggehai qiongdongnan and other chinese basins two diamondoid hydrocarbon ratios have been used as novel high maturity indices to evaluate the maturation and evolution of crude oils and condensates in these chinese basins the two ratios 1 ma 1 ma 2 ma mai methyl adamantane index and 4 md 1 md 3 md 4 md mdi methyl diamantane index were found to increase with increasing vitrinite reflectance the initial values for the ratios are ca 50 and 30 respectively diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent based on this study crude oils from the tarim basin are generally believed to be very mature ro 1 1 the cambrian ordovician source beds are the major source rocks of crude oils and natural gases in this basin condensates from the tarim yinggehai and qiongdongnan basins have higher maturities equivalent to ro values of 1 6 2 0 gases and condensates from the yinggehai and qiongdongnan basins are generated mainly from tertiary source rocks below 4000 5000 m

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

genetic origin of sour gas condensates in the paleozoic dolomite reservoirs of the tazhong uplift tarim basin abstract twenty gas condensates six oils and their associated formation water from the tazhong uplift in the tarim basin nw china were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates thermochemical sulfate reduction tsr is the main secondary alteration process relevant to the generation of sour gas condensates those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations the hydrogen sulfide h 2 s concentrations in gas phase show a strong positive correlation to gas oil ratio organosulfur compound osc concentrations in the liquid phase while methane isotopic values are mainly controlled by thermal maturity wet gas components show unusual 13 c enrichment at a low degree of thermal cracking geochemical anomalies occur mainly in the lower ordovician yingshan fm where more h 2 s has been discovered while the upper ordovician lianglitage fm shows less irregularity and h 2 s concentration is much lower generation of sour gas condensate is most likely caused by tsr alteration which could oxidize liquid hydrocarbons and form a large quantity of oscs isotopic enrichment of 34 s in formation water sulfate and elevated osc concentrations provide supplemental evidence of tsr alteration both burial history reconstructions based on fluid inclusion analysis and pvt simulation indicate that in situ tsr reactions mainly occurred around last 10 ma during the late himalayan orogeny and overall tsr extent is relatively low in the tazhong uplift

multiple origins of methane in the earth methane occurrences in the earth s crust are predominantly of biogenic origin i e their ultimate source is biologically formed organic matter methane can also form through inorganic reactions and is consequently termed abiogenic biogenic methanes can either form through bacterial or thermogenic processes bacterial processes follow a co2 reduction and or fermentation pathway the fermentation processes are quantitatively more important in recent fresh sediments and swamps methane formed by co2 reduction however is most common in older sediments and commercial gasfields temperature organic substrate and age may be the major factors controlling the relative importance of the two pathways stable isotope concentrations in thermogenic methanes seem to be controlled by the extent of conversion of organic matter the timing of gas expulsion and trapping the different character of methane in individual sedimentary basins may be a result of the geologic history geothermal methanes are most likely derived from pyrolysis of organic matter abiogenic methane occurs in hydrothermal vents and ophiolite complexes inorganic reactions either surficial or deep seated are the likely source of such methanes a uniform mantle origin of methane is not supported by the observed isotope variations in naturally occurring methanes

younger dryas glaciers and climate in the mourne mountains northern ireland here we present evidence to suggest that the mourne mountains northern ireland were last occupied by glaciers during the younger dryas stadial the margins of these glaciers are marked by moraines chronologically constrained to the younger dryas by schmidt hammer exposure dating reconstructions indicate that these glaciers had equilibrium line altitudes elas ranging from 356 33 m a s l to 570 9 m a s l with a mean of 475 36 m a s l elas rise from west to east probably reflecting the contribution of windblown snow and ice to the accumulation of younger dryas glaciers in the western mournes taking this into consideration a mean climatic ela of 529 4 m a s l is calculated for the mountains as a whole assuming a mean annual sea level air temperature of 8 c and an annual temperature range of 34 c degree day modelling suggests that during the younger dryas accumulation at the climatic ela of glaciers in the mournes was 846 990 mm a 1 this suggests increased aridity relative to present and is consistent with other parts of nw europe where reduced precipitation alongside notable cooling is thought to reflect increased north atlantic sea ice extent during the younger dryas

distinguishing gases derived from oil cracking and kerogen maturation insights from laboratory pyrolysis experiments kerogen separated from an immature type i mudstone from the third section of the tertiary shahejie formation es 3 3 in the dongying depression of bohai bay basin was subjected to stepwise isothermal pyrolysis products were extracted fractionated into chemical groups and re mixed to obtain a synthetic oil with a group composition similar to reservoir oil and a pseudo kerogen composed of 83 kerogen and 17 residual soluble organic matter referred as s oil and p kerogen respectively to distinguish them from a produced reservoir oil and a real kerogen the two samples were pyrolyzed in sealed gold tubes under constant pressure 50 mpa and non isothermal heating 300 600 c conditions and their generated gases were analyzed the two gases are quite different in their chemical and isotopic composition compared with the gas derived from the p kerogen the s oil derived gas is enriched in c2 c5 hydrocarbons in its early cracking stages and c1 c3 hydrocarbons are depleted in d 13 c throughout the cracking stages the maximum wetness c2 5 c1 5 of the s oil gas and the p kerogen gas is 0 62 and 0 35 respectively and the carbon isotopic ratio differences between the two type gases can reach 10 d 13 c1 14

origins of partially reversed alkane δ13c values for biogenic gases in china with increasing molecular weight 13c values of hydrocarbon gases change in two different manners the normal order would be 13c1 13c2 13c3 13c4 partially reversed order is common in gas samples from sedimentary basins in china which can be attributed to one or several of the following four mixing processes a mixing of biogenic and abiogenic gases b mixing of sapropelic and humic sourced gases c mixing of gases from the same types of source rocks with different maturity and d mixing of gases from the same source rock interval of varying maturity

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

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

molecular and stable isotope compositions of natural gas hydrates a revised global dataset and basic interpretations in the context of geological settings abstract a global dataset of molecular and stable isotope compositions of gases released from 209 different specimens of natural gas hydrate is presented and discussed the 26 hydrate bearing areas from 21 geographic regions are grouped into high gas flux hgf settings low gas flux lgf settings and hydrated gas accumulations hga methane ch4 is the most abundant hydrate bound gas while co2 and c2 hydrocarbon gases are frequently present in small amounts non hydrocarbon gases such as h2s are uncommon the stable isotope composition of hydrate bound gases varies significantly e g 13c of ch4 from 74 7 to 39 6 suggesting that gases of both microbial and thermogenic origin form gas hydrates hydrate bound gases are derived from autochthonous located predominantly within the gas hydrate stability zone ghsz and allochthonous located in deep sediments sources the occurrence and concentration of gas hydrates in sediments are controlled not by the origin of gases but rather by their sources which in turn strongly depend on geological setting allochthonous gases microbial and or thermogenic dominate in hgf and hga settings where they are focused in the shallow ghsz along faults within mud volcanoes in permeable carrier beds and other geological features from underlying petroleum systems relatively high concentrations of gas hydrate average 5 15 of pores and locally up to 100 of volume occurring over small areas are typical of hgf and hga settings in contrast autochthonous and diffuse allochthonous gases mostly microbial occur in stratigraphically and structurally simple lgf settings and result in relatively low concentrations of gas hydrate average 2 of pores in the ghsz spread over large areas the major implication of this finding is that successful prediction of resource and geohazard potential of gas hydrates is possible only if regional petroleum systems extending well below the ghsz are properly evaluated in addition to the diagnosis of origins and sources of hydrate bound gases molecular and isotopic data help to better identify hydrate bearing intervals and provide valuable insights into the dynamics of hydrate bearing sites

methane dominated thermochemical sulphate reduction in the triassic feixianguan formation east sichuan basin china towards prediction of fatal h2s concentrations abstract new sour pools have recently found in the lower triassic feixianguan fm carbonate reservoirs in the east sichuan basin in china with h 2 s up to 17 4 by volume a recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of h 2 s in order to assess the origin of fatal h 2 s as well as the cause of petroleum alteration h 2 s concentrations and the isotopes 34 s and 13 c have been collected and measured in gas samples from reservoirs anhydrite pyrite and elemental sulphur 34 s values have been measured for comparison the high concentrations of h 2 s gas are found to occur at depths 3000 m temperature now at 100 c in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs where h 2 s concentrations are greater than 10 its 34 s values lie between 12 0 and 13 2 cdt this is within the range of anhydrite 34 s values found within the feixianguan fm 11 0 to 21 7 average 15 5 3 5 cdt thus h 2 s must have been generated by thermochemical sulphate reduction tsr locally within the reservoirs burial history analysis and fluid inclusion data reveal that the temperature at which tsr occurred was greater than about 130 140 c suggesting that the present depth temperature minimum is an artifact of post tsr uplift both methane and ethane were actively involved in tsr since the petroleum became almost totally dry no alkanes except methane and methane 13 c values become significantly heavier as tsr proceeded methane 13 c difference thus reflects the extent of tsr while it is tempting to use a present day depth control 3000 m to predict the distribution of h 2 s in the feixianguan fm this is an invalid approach since tsr occurred when the formation was buried some 1000 2000 m deeper than it is at present the likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the feixianguan fm so that it is possible to determine which parts of the basin have been hotter than 130 140 c

geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin abstract natural gas from the zhaolanzhuang field of the jizhong superdepression bohai bay basin contains the highest proportions of h 2 s 40 92 among the sour gases encountered in china the gas payzones include the eocene oligocene kongdian formation ek and the es4 member of the shahejie formation the sedimentary sequence consists of halite anhydrite carbonate sandstone and shale interbeds deposited in the evaporative brackish water lacustrine salt lake setting in the deepest part of the jinxian sag the total thickness of evaporites is more than 1000 m of which halite accounts for over 40 various organic rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window with a depth of 2500 3500 m and likely the source for the oil and sour gas in the zhaolanzhuang field the temperatures of the gas reservoirs range from 75 to 100 c too low for significant thermochemical sulfate reduction the co occurrence of abundant elemental sulfur with the sour gas and the 34 s values of the various sulfur containing compounds indicate that the h 2 s gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred

origin of deep strata gas of tazhong in tarim basin china abstract tazhong is located in the interior of the taklamakan desert in the tarim basin deep carbonates are targeted exploration areas with abundant gas and the largest ordovician gas condensate field has been discovered the hydrocarbon is mainly distributed in the reef shoal complex of the upper ordovician lianglitage formation and in the interbedded weathered reservoirs of the lower middle ordovician yingshan formation despite the deep burial depth of the reservoirs and strong heterogeneity the reservoir bodies are large multiple layered and enrichment in hydrocarbons indicates a great exploration potential the ordovician gas condensate reservoir in the tazhong consists mainly of methane with a gas dryness coefficient of 0 91 0 98 the ordovician gas in the tazhong belongs to secondary oil cracking gas the comprehensive analysis of natural gas composition stable carbon isotopes and adamantane content showed that the natural gas was from paleo reservoir cracking at depth basin geothermal history burial history fluid inclusion data and geological analysis show that the gas accumulation occurred 5 ma ago the sulfur isotope of sulfides the presence of 2 thia adamantane and existence of light co 2 carbon isotopes indicate that the h 2 s was formed via tsr the natural gas accumulated through migration along faults

gas venting and subsurface charge in the green canyon area gulf of mexico continental slope evidence of a deep bacterial methane source abstract questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the gulf of mexico continental slope remain unanswered a research submersible was used to sample mixed bacterial and thermal gas 13 c of methane 62 8 d 176 venting to the water column from the gulf slope in green canyon gc 286 the 14 c value of the methane 998 is consistent with fossil carbon another gas vent on gc 185 is 100 methane 13 c 62 9 d 155 and may be from a bacterial source the 14 c 997 of this bacterial methane is also consistent with fossil carbon fossil bacterial methane and thermal hydrocarbons are present in pliocene to pleistocene reservoirs 3509 4184 m of genesis field gc 205 161 160 oil in these reservoirs is biodegraded but gas is not suggesting that gas charge to reservoirs continues presently at 3 4 km depth mixed thermal and bacterial methane may charge the deep reservoirs and fossil methane from depth may ultimately vent on the sea floor at gc 286 and gc 185 results of this study of green canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources

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

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

burial diagenesis and thermochemical sulfate reduction smackover formation southeastern mississippi salt basin the smackover formation in the southeastern mississippi salt basin has been buried from 3 03 to 6 06 km and subjected to temperatures of 100 to 200c pre hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite these phases formed at temperatures of less than 100c the oxygen isotopic composition of calcite ranges from minus 5 to minus 8 4 pdb and carbon isotopic composition ranges from 3 5 to 6 0 pdb the sulfur isotopic composition of late anhydrites ranges from 18 1 to 19 1 cdt post hydrocarbon migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur and precipitation of post bitumen calcite cement these diagenetic phases formed at temperatures above 150c and are present only in the deep part of the basin the oxygen isotopic compositions of calcites that replace anhydrite range from minus 3 0 to minus 4 0 pdb and carbon isotopic compositions range from minus 2 0 to 4 2 pdb the oxygen isotopic compositions of post bitumen calcites range from minus 3 4 to minus 5 6 pdb and carbon isotopic compositions range from minus 1 6 to minus 16 3 pdb two samples of elemental sulfur have sulfur isotopic compositions of 15 8 and 16 1 cdt the high temperature replacement of anhydrite by calcite more the presence of post bitumen calcite cement the sulfur isotopic composition of sulfates and elemental sulfur and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the mississippi salt basin at temperatures as low as 150c less

paleozoic fault systems of the tazhong uplift tarim basin china abstract this report presents interpretations developed from a detailed study of new three dimensional 3 d high resolution reflection seismic data in a portion of tarim basin the tectonic history began with oceanic spreading during the cambrian early ordovician and continues beyond the silurian devonian time of oceanic closure paleozoic faults of the tazhong uplift in the hinterland of tarim basin are capped by thick and undisturbed meso cenozoic sedimentary sequences four paleozoic fault systems have been recognized 1 the cambrian early ordovician extensional faulting 2 the late ordovician nww trending thrust faulting 3 the silurian devonian nne strike slip faulting and 4 the permian plutonism influenced by pre existing fault planes zones of weakness created during cambrian early ordovician extensional faulting influenced subsequent tectonic movements the late ordovician fault system divides the tazhong uplift into several deformation zones their mechanical characteristics vary across the study area with stronger thrusting in the east the silurian devonian strike slip fault system consists of three components main faults subordinate en echelon faults and fault troughs the main faults appear as steeply dipping almost vertical offsets on seismic map view with associated flower structures on seismic profiles together with other levels of faults the permian magmatic plugs have a spotty or bandy distribution and are interpreted to have utilized former faults

methanogenic biodegradation of petroleum in the west siberian basin russia significance for formation of giant cenomanian gas pools approximately 1700 tcf 48 trillion m3 of dry gas 99 methane reserves and resources occur in western siberia mostly in shallow 1500 m 4921 ft cenomanian pools in the northern part of the basin this dry gas constitutes about 11 of the world s conventional gas endowment and about 17 of the annual gas production the origin of the dry gas has been debated extensively over the last 45 yr but remains controversial widely discussed hypotheses on the origin include early mature thermogenic gas from coal primary microbial gas from dispersed organic matter or coal and thermogenic gas from deep source rocks however all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling here i present geochemical and geological evidence that a significant although yet not quantified part of the shallow dry gas in the northern west siberian basin originated from methanogenic biodegradation of petroleum circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many cenomanian gas pools as well as geochemical evidence of heavy to slight biodegradation in jurassic albian reservoirs commonly underlying the cenomanian pools direct evidence includes most importantly 13c enriched co2 in pools with biodegraded oil although data are limited which indicates 40 70 wt conversion of oil derived co2 to secondary microbial methane distinctive hydrocarbon molecular and isotopic compositions of most gases in cenomanian pools average dryness c1 sum c1 c5 is 0 9976 average 13c of methane is 51 8 suggest that they represent mixtures of biodegraded thermogenic gases from deep mainly jurassic source rocks and secondary microbial methane with an occasional small addition of primary microbial methane contribution of early mature coal derived gas is possible in areas with the most significant thermal stress of hauterivian aptian sediments but remains speculative review of petroleum habitats of five representative oil gas condensate fields in western siberia including the world s second largest gas field urengoyskoe suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields recognition of secondary microbial gas in western siberia helps explain the observed dominance of gas in the shallow cool northern part of the basin where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin secondary microbial gas has been recognized worldwide and may 1 represent a volumetrically significant exploration target in shallow reservoirs perhaps more significant than primary microbial gas and 2 indicate effective thermogenic petroleum systems in the deeper sections large volumes up to 66 500 tcf 1884 trillion m3 of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide although a part of that gas accumulated as oil dissolved free and hydrate bound gas most gas apparently escaped into the overburden atmosphere and ocean and could have affected global climate in the geologic past