the effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs abstract thermochemical sulfate reduction tsr is a well known process that can lead to sour h2s rich petroleum accumulations most studies of tsr have concentrated upon gas chemistry in this study we have investigated palaeoformation water characteristics in a deep anhydrite bearing dolomite sour gas reservoir of permian age in abu dhabi using fluid inclusion stable isotope petrographic and gas chemical data the data show that low salinity isotopically distinct water was generated within the reservoir by reaction between anhydrite and methane the amount of water added to the reservoir from tsr indicated by reduced fluid inclusion salinity and water 18o values varied systematically with the extent of anhydrite reaction with methane water salinity and isotope data show that the original formation water was diluted by between four and five times by water from tsr thus we have shown that large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement the salinity of formation water in evaporite lithologies is therefore not necessarily high modelling based upon a typical khuff gas reservoir rock volume suggests that initial formation water volumes can only be increased by about three times as a result of tsr the extreme local dilution shown by the water salinity and 18o data must therefore reflect transiently imperfect mixing between tsr water and original formation water the creation of large volumes of water has important implications for the mechanism and rate of thermochemical sulphate reduction and the interpretation of gas volumes using petrophysical logging tools

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

geochemical relationships of petroleum in mesozoic reservoirs to carbonate source rocks of jurassic smackover formation southwestern alabama algal carbonate mudstones of the jurassic smackover formation are the main source rocks for oil and condensate in mesozoic reservoir rocks in southwestern alabama this interpretation is based on geochemical analyses of oils condensates and organic matter in selected samples of shale norphlet formation haynesville formation trinity group tuscaloosa group and carbonate smackover formation rocks potential and probable oil source rocks are present in the tuscaloosa group and smackover formation respectively extractable organic matter from smackover carbonates has molecular and isotopic similarities to jurassic oil although the jurassic oils and condensates in southwestern alabama have genetic similarities they show significant compositional variations due to differences in thermal maturity and organic facies lithofacies organic facies reflect different depositional conditions for source rocks in the various basins the mississippi interior salt basin was characterized by more continuous marine to hypersaline conditions whereas the manila and conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter petroleum and organic matter in jurassic rocks of southwestern alabama show a range of thermal transformations the gas content of hydrocarbons in reservoirs increases with increasing depth and temperature in some reservoirs where the temperature is above 266 f 130 c gas condensate is enriched in isotopically heavy sulfur apparently derived from thermochemical reduction of jurassic evaporite sulfate this process also results in increased h2s and co2 in the gas and depletion of saturated hydrocarbons in the condensate liquids thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature because some deep 18 000 ft or 5 5 km and hot 320 f or 160 c smackover and norphlet reservoirs contain low sulfur petroleum

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

interaction of coal and oil in confined pyrolysis experiments insight from the yield and composition of gas hydrocarbons abstract isothermal pyrolysis experiments were performed for coal alone oil alone and coal plus oil with oil coal ratios ranging from 0 0065 to 0 1995 at 305 c and 50 mpa for 72 h in confined systems gold capsules the results of these experiments reveal the interaction between coal and oil demonstrating that oil retards the generation of gas hydrocarbons from coal cracking while coal accelerates oil cracking into gas hydrocarbons the yields of gas hydrocarbons vary greater with oil coal ratio in the experiments of coal b plus oil than coal a plus oil because coal a has a higher hi value than does coal b oil cracking rate could increase by up to 10 or even higher times in the experiments of coal plus oil compared with oil alone deduced from the yields and chemical compositions of gas hydrocarbons this result suggests that gas hydrocarbons especially wet gases were largely generated from the cracking of oil or extractable bitumen in the experiments of coal plus oil with oil coal ratio higher than 0 1

distribution and isomerization of c31 c35 homohopanes and c29 steranes in oligocene saline lacustrine sediments from qaidam basin northwest china abstract the 22s 22s 22r ratios for c 31 c 35 homohopanes and the 20s 20s 20r ratio for c 29 steranes were measured by gas chromatography mass spectrometry gc ms on six oligocene saline lacustrine rock samples from a borehole located in the western qaidam basin northwest china these ratios vary significantly and irregularly with burial depth from 2831 m to 3054 m around the threshold of the oil generation window in addition 22s 22s 22r ratios also vary substantially among different homologs within the same sample generally decreasing from c 31 to c 35 homohopanes the distribution patterns of homohopanes for the six samples are unusual such as c 31 32 c 33 c 34 35 or c 31 c 32 c 33 34 35 or c 31 32 c 33 34 35 all of these observations can be mainly ascribed to the release of bound hopanes and steranes from kerogen and other macromolecules the 22s 22s 22r ratios of c 31 c 32 and c 33 homohopanes decreased while that of c 34 homohopanes increased for two samples yhs1 and yhs6 after pyrolysis at low temperatures 180 270 c a major reversal of 20s 20s 20r ratio for c 29 steranes was observed at 240 c for sample yhs1 but at 270 c for sample yhs6 the distributions of homohopanes for these two samples after pyrolysis changed from their original patterns toward the normal pattern c 31 c 32 c 33 c 34 c 35 these results reflect the influences of thermal degradation and direct isomerization of the free biomarkers and the release of bound biomarkers from kerogen and other macromolecules during pyrolysis

geochemical characteristics and origin of natural gas from wufeng longmaxi shales of the fuling gas field sichuan basin china abstract as the first giant shale gas field in china the fuling gas field has recently been regarded as one of the most important regions for natural gas exploration and production in the sichuan basin however the origin of natural gas from upper ordovician wufeng and the bottom of lower silurian longmaxi wl shales in the fuling gas field is poorly understood to limit a comprehensive understanding of gas generation accumulation and exploration in this work based on molecular and stable carbon isotopic composition of a total of 24 gas samples from five shale gas wells in the fuling field we analyzed the geochemical characteristics and gas origin and discussed the cause for the geochemical anomalies carbon isotopic reversals molecular composition results show that gases from the fuling gas field are dry and mainly composed of methane 97 9 98 9 with a very low level of ethane c2h6 propane c3h8 and non hydrocarbon gases mainly co2 and n2 these dry gases are classified as oil associated gas and mainly derived from secondary cracking due to the lack of gas samples across a maturation gradient from immature to late mature the wl gases in the fuling field show an unclear evolution trend between the 13c2 and wetness values however all these samples are located in the isotopic reversal zone carbon isotopes of gaseous alkanes clearly display full isotopic reversals 13c1 13c2 13c3 which is indicative of a relatively high thermal maturity and consistent with the measured vitrinite reflectance and modeled values 3 0 ro the observed complete carbon isotopic reversals in the wl gases are caused by a combination of several mechanisms in which isotope exchange at high temperature is the primary controlling factor other secondary factors include rayleigh type fractionation of c2h6 and c3h8 secondary cracking and gas diffusion mixing of gases at different thermal maturity levels

assessing effects of sulfate minerals on petroleum generation in sedimentary basins using hydrous pyrolysis i light alkanes abstract a series of hydrous pyrolysis experiments were conducted to assess the role of sulfate minerals on the chemical and isotopic compositions of light hydrocarbons generated during thermal degradation of kerogen the shale source rock with low maturity tmax 425 c from eagle ford formation in southeast texas was used in the experiments at 330 c for the duration of 72 h while the abundance of gypsum in each experiment varies from 0 0 5 to 1 wt under experimental conditions the positive effect of gypsum on the yields of gaseous alkanes c1 c4 and co2 has been explicitly observed with an exponential relationship between the yield of each gas and the amount of gypsum while the relative abundance of c2 c4 alkanes has shown an increasing trend with gypsum the gas dryness c1 c1 4 however decreased from 73 1 with no gypsum to 49 6 with gypsum of 1 wt it was attributed to lower overall yields and corresponding higher increased generation rates of c2 c4 alkanes than methane through thermal cracking of kerogen and other organic compounds the carbon isotope value of each alkane reached the highest in the experiment with 1 wt gypsum compared to the experiment without gypsum there was an increase of 11 5 for the 13c value of methane with 1 wt gypsum the extent of this 13c enrichment between experiments with different amount of gypsum became less for c2 to c4 the 13c value of co2 was the highest among generated gases ranging from 4 7 to 2 1 combined with chemical compositions of gases it suggested that enhanced thermal cracking reactions were predominant in generation of light alkanes with the presence of gypsum whereas dissolution of carbonates was the main source of co2 the reactions involving thermochemical sulfate reduction tsr were minor if present under experimental conditions most likely due to the small quantity of gypsum up to 1 wt and neutralization of acidity by carbonates nonetheless the changes in chemical and isotopic compositions of light alkanes which were ultimately caused by the presence of gypsum need to be incorporated into geochemical proxies for assessing source rocks maturation in particular and their generation potential

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

sulfur and carbon isotopic compositions of the permian to triassic tsr and non tsr altered solid bitumen and its parent source rock in ne sichuan basin abstract reservoir solid bitumens from the permian to triassic and potential source rocks in the northeastern sichuan basin were measured for elemental and stable carbon 13c and sulfur 34s isotopic compositions to determine the chemical and isotopic composition of non tsr and tsr altered solid bitumen and to develop proxies to reflect the extent of bitumen alteration by tsr non tsr altered reservoir bitumen samples showed s c atomic ratios 0 03 and had 13c values closer to longtan formation p3l kerogen than to the lower silurian and cambrian kerogens and 34s values slightly more enriched than p3l kerogens the tsr altered solid bitumens have incorporated significant amounts of tsr derived h2s resulting in s c atomic ratios 0 047 0 074 and 34s values 13 4 24 0 being significantly higher than those of the solid bitumen not altered by tsr s c 0 013 0 030 34s 5 8 9 6 and the potential source rock kerogen the incorporation has produced a decrease in n c ratio and negative shift of 2 7 in 13c values that resulted from increasing tsr h2s back reactions with 12c rich n alkanes from the gas phase soluble solid bitumen or paleo oil to generate organic sulfur compounds such as ethanethiol that were incorporated into the solid bitumens which is followed by further aromatization and polymerization the solid bitumens show positive relationships of s c ratios and 34s values to the associated gas h2s h2s c1 6 or tsr extents and thus the s c ratios and 34s values can be used as proxies to reflect tsr extents

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

the effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage a pyrolysis study of type ii kerogen from pingliang shale china high maturity oil and gas are usually generated after primary oil expulsion from source rocks especially from oil prone type i ii kerogen however the detailed impacts of oil expulsion or retention in source rock on further thermal degradation of kerogen at the high maturity stage remain unknown in the present study we collected an ordovician pingliang shale sample containing type ii kerogen the kerogens which had previously generated and expelled oil and those which had not were prepared and pyrolyzed in a closed system to observe oil expulsion or oil retention effects on later oil and gas generation from kerogen the results show that oil expulsion and retention strongly impacts on further oil and gas generation in terms of both the amount and composition in the high maturity stage gas production will be reduced by 50 when the expulsion coefficient reaches 58 and gas from oil expelled kerogen less oil retained is much drier than that from fresh kerogen the oil expulsion also causes n alkanes and gas compounds to have heavier carbon isotopic compositions at high maturity stages the enrichment of 13c in n alkanes and gas hydrocarbons are 1 and 4 6 respectively compared to fresh kerogen oil expulsion may act as open system opposite to the oil retention that influences the data pattern in crossplots of 13c2 13c3 versus c2 c3 13c2 13c3 versus 13c1 and 13c1 13c2 versus ln c1 c2 which are widely used for identification of gas from kerogen cracking or oil cracking these results suggest that the reserve estimation and gas source correlation in deep burial basins should consider the proportion of oil retention to oil expulsion the source rocks have experienced

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

genetic types of natural gas and gas source correlation in different strata of the yuanba gas field sichuan basin sw china abstract the chemical compositions and stable carbon and hydrogen isotopic compositions of natural gases in different strata of the yuanba gas field in the sichuan basin were analyzed to investigate their genetic types and source natural gases from the permian changxing and triassic feixianguan formations have dryness coefficients higher than 0 995 and contain h 2 s in most gas samples having a positive correlation between the h 2 s and co 2 contents the identification of gas origin and gas source correlation from this study indicate that natural gas in the ziliujing formation and most gas samples from the xujiahe formation are coal type gas produced from primary cracking of kerogen and they are self sourced reservoirs in relatively independent systems the natural gases in the 1st and 2nd members of the xujiahe formation were mixed with minor amounts of oil type gas from argillaceous carbonate rocks in the leikoupo formation in addition to the self accumulated oil type gas in the leikoupo formation natural gas mainly came from the overlying or lateral source rocks in the xujiahe formation the h 2 s bearing natural gases in the changxing and feixianguan formations were produced by the cracking of crude oil from source rocks in the underlying wujiaping formation with partial contributions from certain local source rocks in the permian dalong formation the h 2 s bearing natural gas has undergone the thermochemical sulfate reduction tsr which were dominated by heavy hydrocarbons the high 13 c values of co 2 in these gas reservoirs mainly result from the interaction between acidic fluids and carbonate rocks in the reservoirs

hydrocarbon gas generation from pyrolysis of extracts and residues of low maturity solid bitumens from the sichuan basin china abstract two sets of pyrolysis experiments were performed on bitumen a ba and extracted organic matter er from low maturity solid bitumen at heating rates of 2 c h and 20 c h in confined systems gold capsules the main observations can be listed as follows 1 ba and er have high generation potential of hydrocarbons and were a significant secondary hydrocarbon source 2 both have similar evolutionary characteristics of hydrocarbon gas generation and have high c1 c1 5 values than those from an ordovician normal oil ea distributions of the methane generated from pyrolysis of ba and er is far lower than that of ordovician normal oil 3 the evolution of stable carbon isotope values of individual hydrocarbon gases can be divided into three stages plots of ln c1 c2 versus ln c2 c3 13c1 13c2 versus ln c1 c2 and 13c2 13c3 versus ln c2 c3 clearly illustrate the characteristics of hydrocarbon gases generated from ba and er at different thermal maturity stages 4 basin modelling reconstructions of the hydrocarbon generation history of ba and er in sinian and cambrian strata respectively indicate hydrocarbon gases generation from er started in the middle jurassic and ended in the early cretaceous with a maturity equivalent to easy ro 2 2 the final conversion rate of c1 5 and c1 are 0 71 and 0 48 respectively hydrocarbon gas generation from ba started in the early jurassic and ended in the early cretaceous with an easy ro 3 0 the final conversion rate of c1 5 and c1 are 0 90 and 0 81 respectively

coupled alteration of hydrothermal fluids and thermal sulfate reduction tsr in ancient dolomite reservoirs an example from sinian dengying formation in sichuan basin southern china abstract to better understand the fluid activities and their roles in the formation of petroleum reservoirs dolomite was thoroughly studied in the ancient sinian dengying formation in sichuan basin southern china the high homogenization temperatures above 190 c of fluid inclusions positive eu anomaly and o and sr isotopic values of coarse crystalline pore filling dolomite fd in dissolution pore spaces have suggested that the fds are of hydrothermal origin there are also other minerals pyrite organic compounds pyrobitumen and gas components h 2 s co 2 and ch 4 in pore spaces the sulfur isotope compositions of pyrite and h 2 s along with the carbon isotopes of co 2 have shown the involvement of thermochemical sulfate reduction tsr hydrothermal activities during burial diagenesis of the dengying formation provided high temperature environment necessary for tsr to proceed the presence of co 2 and h 2 s in both processes enhanced fluid acidities and consequently porosity and permeability of the dengying formation emplacement of petroleum and acidic fluids helped to maintain pre existing reservoir spaces during deep burial stage overall the coupled hydrothermal activities and tsr provided favorable environments to allow for maintaining high quality reservoirs

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

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

interaction of coal and oil in confined pyrolysis experiments insight from the yields and carbon isotopes of gas and liquid hydrocarbons abstract isothermal confined gold capsule pyrolysis experiments were performed for coal alone oil alone and coal plus oil with oil coal ratios ranging from 0 006 to 0 171 at 315 c 345 c and 375 c respectively and 50 mpa for 72 h in the experiment for coal plus oil the amounts and compositions of hydrocarbon gases are substantially different from those predicted from the results in the experiments for oil alone and coal alone the results of these experiments demonstrate that kerogen and oil do not crack separately in the experiments of coal plus oil the interaction between kerogen and oil components leads to the generation of hydrocarbon gases with oil coal ratio increasing the amounts of individual and total hydrocarbon gases decrease at first and then increase rapidly up to several times those calculated from the yields of these components in the experiments for oil alone and coal alone the c1 c1 5 ratios of hydrocarbon gases decrease and are increasingly lower than those calculated from the yields of hydrocarbon gases in the experiments for oil alone and coal alone the amount and carbon isotopes of individual n alkane demonstrate that the free liquid n alkanes were incorporated into kerogen and replaced the bound liquid n alkanes covalently bonded alkyl groups in kerogen carbon isotopes of hydrocarbon gases further suggest that the bound liquid n alkanes in kerogen preferentially crack into hydrocarbon gases

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

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

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

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

carbon isotope effect during abiogenic oxidation of methane abstract the oxidation of methane during flow over cuo and fe2o3 has been examined in the temperature range of 400 650 c the reaction rate and carbon isotope fractionation are dependent upon the choice of oxide and temperature the activation energy is lower for hematite 8 0 kcal mole 1 than for cupric oxide 16 6 kcal mole 1 the measured ratios of the isotopic rate constants k12 k13 were found to have temperature dependences given by 103 1 2 93 106 t2 8 11 cupric oxide 103 1 7 44 106 t2 6 56 hematite abiogenic oxidation of methane is probably a significant mechanism for fractionating carbon isotopes in nature

origin and carbon isotope fractionation of co2 in marine sour gas reservoirs in the eastern sichuan basin abstract according to gas chemical and carbon isotopic compositions for 75 gas samples from the eastern sichuan basin c 1 c 3 gaseous hydrocarbons are dominant with high dryness coefficient c 1 c 1 3 0 99 1 0 despite the presence of various contents of h 2 s and co 2 a reasonably good positive correlation is exhibited when both contents are 5 the carbon dioxide in the natural gas in the eastern sichuan basin mainly originated from carbonate dissolution by acid gases and thermochemical sulfate reduction tsr the content of co 2 produced at the early stage of acid gas dissolution is commonly 13 c co2 value 2 content exceeds 5 the 13 c co2 values range from 8 to 2 and the co 2 and h 2 s contents demonstrated a positive correlation the majority of co 2 is generated via tsr as well as acid gas dissolution during the late uplift of the basin particularly the co 2 with 13 c co2 value 2 generated via the tsr after it was partially precipitated to form calcite while the co 2 with 13 c co2 value 3 is a mixture of the co 2 produced early via tsr and the co 2 produced by the subsequent carbonate rock dissolution by acid gases

geochemical characteristics and isotopic reversal of natural gases in eastern kopeh dagh ne iran abstract natural gas samples from two gas fields located in eastern kopeh dagh area were analyzed for molecular and stable isotope compositions the gaseous hydrocarbons in both lower cretaceous clastic reservoir and upper jurassic carbonate reservoir are coal type gases mainly derived from type iii kerogen however enriched d values of methane implies presence of type ii kerogen related material in the source rock in comparison upper jurassic carbonate reservoir gases show higher dryness coefficient resulted through tsr while presence of c 1 c 5 gases in lower cretaceous clastic reservoir exhibit no tsr phenomenon carbon isotopic values indicate gas to gas cracking and tsr occurrence in the upper jurassic carbonate reservoir as the result of elevated temperature experienced prior to the following uplifts in last 33 37 million years the 13 c of carbon dioxide and 34 s of hydrogen sulfide in upper jurassic carbonate reservoir do not primarily reflect tsr as uplift related carbonate rock dissolution by acidic gases and reaction precipitation of light h 2 s have changed these values severely gaseous hydrocarbons in both reservoirs exhibit enrichment in c 2 gas member with the carbonate reservoir having higher values resulted through mixing with highly mature completely reversed shale gases it is likely that the uplifts have lifted off the pressure on shale gases therefore facilitated the migration of the gases into overlying horizons however it appears that the released gases during the first major uplift 33 37 million years ago have migrated to both reservoirs while the second migrated gases have only mixed with upper jurassic carbonate reservoir gases the studied data suggesting that economic accumulations of natural gas shale gases deeper than upper jurassic carbonate reservoir would be unlikely

influence of retained bitumen in oil prone shales on the chemical and carbon isotopic compositions of natural gases implications from pyrolysis experiments abstract retained bitumen in oil prone source rocks has a significant impact on late gas generation but such influence is still poorly quantified because it is quite difficult to restore the bitumen content retained in a mature source rock in this study a suite of artificially matured samples were prepared from a moderate maturity shale sample to represent shales with different amount of retained bitumen these samples were then pyrolyzed in sealed gold tubes to investigate how the amount of retained bitumen affects the changes in chemical and carbon isotopic compositions of generated gases and the uncertainty of the evaluation of gas maturity and source identification the results indicate that the decrease in yields of hydrocarbon gases is proportional to the reduction of retained bitumen content however a progressive enrichment in heavier carbon isotope 13c of gaseous hydrocarbons occurs while less amount of bitumen is retained in shales this experimental observation partly explains the heterogeneity of methane carbon isotopes for shale gases that have identical thermal maturity levels although the amount of retained bitumen can significantly change the carbon isotopic ratios of individual gas 13c1 13c2 and 13c3 the difference in carbon isotopic composition between gaseous components such as ethane and methane 13c2 13c1 or propane and ethane 13c3 13c2 are not significantly affected and therefore these differential values can be potentially used as maturity indicators for overmature natural gases exhibiting no carbon isotopic reversals geochemical and carbon isotopic diagrams that are widely used for the identification of oil and kerogen cracking gases are also found to be affected by the retained bitumen content these results illustrate that it is necessary to take into account the retained bitumen content of shales when thermal maturity and genetic origin of natural gases derived from oil prone source rocks are being evaluated

rifting under steam how rift magmatism triggers methane venting from sedimentary basins during opening of a new ocean magma intrudes into the surrounding sedimentary basins heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons these hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming e g during the paleocene eocene thermal maximum petm mound structures at the top of buried hydrothermal vent complexes observed in seismic data off norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation here we present new geophysical and geochemical data from the gulf of california suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker type chimneys rather than mud volcanoes we have evidence for two buried and one active hydrothermal vent system outside the rift axis the vent releases several hundred degrees celsius hot fluids containing abundant methane mid ocean ridge basalt morb type helium and precipitating solids up to 300 m high into the water column our observations challenge the idea that methane is emitted slowly from rift related vents the association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere lending support to the hypothesis that rapid climate change such as during the petm can be triggered by magmatic intrusions into organic rich sedimentary basins

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

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

characteristics of molecular nitrogen generation from overmature black shales in south china preliminary implications from pyrolysis experiments abstract high content of molecular nitrogen n2 is one of the natural gas exploration risks in petroliferous basins where black shales act as source rocks of either hydrocarbon gas or molecular nitrogen in this study two overmature and one low maturity shale samples and their kerogens were investigated to determine the generation characteristics of molecular nitrogen as well as methane and the differences in the release processes of inorganic nitrogen fixed in ammonium bearing minerals and organic nitrogen bound in kerogen the results illustrate that with increasing pyrolysis temperature the yield of methane first increases and then decreases with an inflection temperature of 650 c eqvro 3 4 whereas the yield of molecular nitrogen shows a continuous increase throughout the pyrolysis experiment the molecular nitrogen during the stage of methane generation i e eqvro 3 4 the results also reveal that at the experimental maxima of thermal maturity eqvro 4 9 the generation potential of inorganic molecular nitrogen mg g ninorg is much lower than that of organic molecular nitrogen mg g norg indicating that in our experiments the organic nitrogen in kerogen is more easily to be converted into molecular nitrogen than the inorganic nitrogen in ammonium bearing minerals all these results indicate that molecular nitrogen content in shale gas may change dramatically during thermal evolution and source rocks with exceptionally high maturity and high abundance of organic nitrogen likely lead to a high molecular nitrogen risk particularly in regions of poor preservation conditions of shale gas which may be the main reasons for the high molecular nitrogen content in the lower cambrian shale gas in south china

palaeostress and neotectonic analysis of sheared conglomerates southwest alps and southern apennines abstract geometrical and mechanical characteristics of the deformation of poorly cemented conglomerates are described using striated pebbles for analysis of palaeostresses it is crucial to distinguish radial striation patterns which result from deformation of the matrix around a rigid pebble from unidirectional striation patterns that represent shear zones crossing the conglomeratic material examples of palaeostress determinations from striations of the latter type are given for extensional settings provence and compressional settings southern apennines southwest alps their comparison with fault analyses in brittle rocks that underlie the conglomerates validates their usefulness for palaeostress analyses and suggests that some conglomerates behave as materials containing pre existing surfaces of mechanical anisotropy that fail by sliding on some suitable oriented surfaces these examples show that sheared conglomerates can be used for stratigraphic dating of the deformation for studies of syndepositional deformation and for neotectonic analysis

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

evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes abstract the assessment of gas origin in mud volcanoes and related petroleum systems must consider post genetic processes which may alter the original molecular and isotopic composition of reservoir gas beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation investigated in previous studies we now demonstrate that mud volcanoes can show signals of anaerobic biodegradation of natural gas and oil in the subsurface a large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined due to the very low amount of c 2 in mud volcanoes isotopic ratios of ethane propane and butane generally the best tracers of anaerobic biodegradation are only available in a few cases however it is observed that 13 c enriched propane is always associated with positive 13 c co2 values which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum data from carbon isotopic ratio of co 2 are available for 134 onshore mud volcanoes from 9 countries azerbaijan georgia ukraine russia turkmenistan trinidad italy japan and taiwan exactly 50 of mud volcanoes all releasing thermogenic or mixed methane show at least one sample with 13 c co2 5 pdb thermogenic ch 4 associated with positive carbon isotopic ratio of co 2 generally maintains its 13 c enriched signature which is therefore not perturbed by the lighter secondary microbial gas there is however high variability in the 13 c co2 values within the same mud volcanoes so that positive 13 c co2 values can be found in some vents and not in others or not continuously in the same vent this can be due to high sensitivity of 13 c co2 to gas water rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano however finding a positive 13 c co2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored areas

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

protection of organic matter by mineral matrix in a cenomanian black shale abstract three types of pathways degradation recondensation natural sulphurization and selective preservation are commonly considered for the formation of kerogen dispersed in sedimentary rocks a fourth pathway has been recently put forward however from studies on recent marine sediments the so called sorptive protection mechanism this pathway is based on the adsorption of otherwise labile organic compounds onto minerals thus preventing their diagenetic degradation and promoting their subsequent condensation into kerogen the main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a cenomanian kerogen they provide i evidence on an ancient material for a crucial role of the mineral matrix both in organic matter om preservation during kerogen formation and in kerogen stability once formed ii indications that the dominant protective process likely involves physical protection by minerals resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness rather than om adsorption as molecular monolayers and iii observations of the relatively poor stability of an isolated kerogen contrary to the inertness commonly assumed for fossil macromolecular organic matter

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

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

the effect of organic matter type on formation and evolution of diamondoids abstract a series of anhydrous pyrolysis experiments using sealed gold tubes were performed on three types of kerogen to investigate the effect organic matter type has on the generation and evolution of thermogenic diamondoids based on the compositional variation of pyrolysis products the cracking of kerogens can be divided into three stages oil generation 0 6 1 5 easyro wet gas generation 1 5 2 1 easyro and dry gas generation 2 1 easyro the experimental results indicate that diamondoids were mainly generated in the oil and wet gas generation stages and decomposed in the dry gas generation stage in addition to thermal maturity the formation of diamondoids is also influenced by the type of organic matter type i and iia kerogens produced more diamondoids than type iii kerogen and diamondoids generated from type iii kerogen were dominantly adamantanes therefore the concentration and concentration ratios of diamondoids can be used to assess the maturity of source rocks 1 0 1 5 easyro and determine the type of organic matter 1 0 2 0 easyro isomerization ratios of diamondoids depend mainly on thermal maturity and the type of organic matter has little effect the use of isomerization ratios to determine thermal maturity is best for source rocks at higher maturity levels 1 5 3 0 easyro therefore bivariate diagrams of concentration versus isomerization indices of diamondoids e g dmas mds vs dmai 1 and dmas mds vs tmai 1 can be used to evaluate the source rock maturity over a wider easyro range 1 0 3 0 easyro than single diamondoid parameters as there are differences in the concentration and distribution of diamondoids in the extracts of three source rocks the possibility exists to use diamondoid indices of immature rocks to determine the type of source rock

petroleum generation kinetics for permian lacustrine source rocks in the junggar basin nw china abstract the junggar basin is a major oil producing province in china most of the oil originated from permian lacustrine oil prone source rocks in the basin kinetic parameters for oil and gas generation were obtained from confined pyrolysis experiments using gold capsules at heating rates of 20 c h and 2 c h on kerogen samples from two permian lacustrine oil prone source rocks j23s3 and ss1s3 having hydrogen indices of 663 and 698 mg hc g toc respectively the kinetic properties for oil and gas generation vary substantially between these two source rocks assuming a burial heating rate of 5 c my the temperature for 50 transformation ratio to oil is 132 c for j23s3 and 168 c for ss1s3 the maximum oil yields are about 604 and 770 mg g toc while the maximum yields of total gaseous hydrocarbons are about 306 and 348 mg g toc for j23s3 and ss1s3 respectively gaseous hydrocarbons from the confined pyrolysis experiments include primary cracking components from kerogen and secondary cracking components from the generated oil the secondary cracking gaseous hydrocarbons comprise up to 84 9 and 95 1 of the total respectively for the two source rocks in the highest temperature experiments a practical method estimates the amount of gaseous hydrocarbons generated from these source rocks under geological conditions based on 1 kinetic parameters for gas generation obtained from kerogen confined pyrolysis experiments in the present study 2 the maximum yield of gaseous hydrocarbons from oil confined pyrolysis experiments from previous studies and 3 the maximum amount of expelled oil the maximum amounts of expelled oil range from 509 588 and 724 750 mg g toc with oil expulsion efficiencies in the ranges 84 3 97 4 and 94 0 97 4 for j23s3 and ss1s3 respectively assuming that the amounts of retained oil range from 30 160 and 50 110 mg g toc for the two source rocks after oil expulsion based on the amounts of extracted bitumen consequently the maximum yields of total gaseous hydrocarbons could be in the ranges 53 1 87 0 and 25 5 36 8 mg g toc respectively for j23s3 and ss1s3 in nature using the kinetic parameters for these two source rocks determined from pyrolysis experiments oil and gas generation and expulsion were modeled for source rocks within the lower permian fengcheng p 1f and middle permian lower wuerhe formations p 2w in the central area of the mahu depression the source kitchen for most oilfields in the basin

the influence of rock fabric and mineralogy on thermochemical sulfate reduction khuff formation abu dhabi abstract thermochemical sulfate reduction tsr is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce h2s and calcite in this study of the dolomite hosted hydrocarbon gas reservoirs in the permo triassic khuff formation abu dhabi a geochemically well constrained rock gas system we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of tsr reactions and thus on h2s concentration in the gas phase the controls on the rate on h2s accumulation were tsr reaction kinetics tsr became significant as temperature exceeded 140 c a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane anhydrite dissolution rate once initiated the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide i e suggesting total reaction for each unit of sulfate that dissolves also finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite likely a result of relative surface area suggesting that anhydrite dissolution was the rate limiting step transport rates are unlikely to have been rate limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals transport rate later as tsr proceeded it became transport controlled as calcite growing on the surface of anhydrite crystals began to isolate them from dissolved methane tsr ceased once calcite had effectively armor plated totally isolated the remaining anhydrite finely crystalline anhydrite underwent more extensive and more rapid tsr than coarser anhydrite crystals because these had a greater ratio of surface area to volume allowing more and faster dissolution and requiring more calcite to isolate them from methane localized loss of h2s occurred in the reservoir by reaction with indigenous fe bearing clays consequently reservoirs with a relatively high siliciclastic content have less h2s than would be expected from the advanced state of anhydrite replacement by calcite in order to predict tsr related h2s concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum emplacement history

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

the effect of thermochemical sulfate reduction on formation and isomerization of thiadiamondoids and diamondoids in the lower paleozoic petroleum pools of the tarim basin nw china abstract we attempted to differentiate the effects of thermal chemical alteration tca from thermochemical sulfate reduction tsr on formation and isomeric distribution of thiadiamondoids and diamondoids in oils and condensate from deeply buried cambrian and ordovician carbonate reservoirs in the tarim basin china diamondoids and thiadiamondoids with 1 3 cages were detected in all the oils analyzed total thiadiamondoid concentrations reflect extent of tsr diamondoid concentrations and most of the diamondoid based maturity proxies and the source rock lithology proxy increase with increasing extent of tsr indicating that they are controlled by the extent of the tsr process or reaction with h 2 s to a greater degree than by tca tsr may have oxidized hydrocarbons to oxygenated species and finally to co 2 and newly generated and concentrated diamondoids and organic sulfur compounds the most extensive tsr altered zs1c condensate contains 3 2 wt total dibenzothiophenes 0 4 wt thiadiamondoids and 5 9 wt diamondoids together with higher polymantanes including tetramantanes and pentamantanes in association with polythiadiamondoids including dithiaadamantanes trithiaadamantanes and tetrathiaadamantanes these polythiadiamondoids have been detected only from heavily tsr altered petroleum in this study and north american samples and may be derived from the sulfurization of hydrocarbons including diamondoids under high temperature and highly reducing conditions during advanced stages of tsr the polymantanes may have been generated from alkylation of lower cage diamondoids and subsequent homologation or from free radical reactions this study indicates that tsr may have led to the generation and isomerization of thiadiamondoids and diamondoids and thus diamondoid based maturity and lithology proxies cannot be used to reflect maturity and lithology in the tsr active areas and that the occurrence of polythiadiamondoids can be used to indicate heavy tsr alteration

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

characterization of estonian kukersite by spectroscopy and pyrolysis evidence for abundant alkyl phenolic moieties in an ordovician marine type ii i kerogen abstract the kerogen of a sample of estonian kukersite ordovician was examined by spectroscopic solid state 13c nmr ftir and pyrolytic off line flash methods this revealed an important contribution of long linear alkyl chains in kukersite kerogen the hydrocarbons formed upon pyrolysis are dominated by n alkanes and n alk 1 enes and probably reflect a major contribution of selectivity preserved highly aliphatic resistant biomacromolecules from the outer cell walls of gloeocapsomorpha prisca this is consistent with the abundant presence of this fossilized organism in kukersite kerogen in addition high amounts of phenolic compounds were identified in the pyrolysates series of non methylated mono di and trimethylated 3 n alkylphenols 5 n alkyl 1 3 benzenediols and n alkylhydroxybenzofurans were identified all series of phenolic compounds contain long up to c19 linear alkyl side chains kukersite kerogen is therefore an aliphatic type ii i kerogen despite the abundance of free phenolic moieties this study shows that phenol derived moieties are not necessarily associated with higher plant derived organic matter the flash pyrolysate of kukersite kerogen was also compared with that of the kerogen of the guttenberg oil rock ordovician which is also composed of accumulations of fossilized g prisca similarities in the distributions of hydrocarbons and sulphur compounds were noted especially for the c1 c6 alkylbenzene and alkylthiophene distributions however no phenolic compounds were detected in the flash pyrolysate of the guttenberg kerogen possible explanations for the observed similarities and differences are discussed

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

carbon and hydrogen isotopic reversals in deep basin gas evidence for limits to the stability of hydrocarbons by burruss and laughrey organic geochemistry 41 1285 1296 discussion this discussion reevaluates the data provided by burruss and laughrey 2010 carbon and hydrogen isotopic reversals in deep basin gas evidence for limits to the stability of hydrocarbons organic geochemistry 41 1285 1296 it indicates that gas diffusive migration plays an important role in regulating the isotopic reversals in samples from the silurian and ordovician formations in the appalachian basin carbon and hydrogen isotopic reversals in deep basin gas cannot be attributed to the stability of hydrocarbons in shale gas and tight sand gas accumulations if gas diffusive migration in the subsurface is not considered

study of thermochemical sulfate reduction of different organic matter insight from systematic tsr simulation experiments abstract a series of thermochemical sulfate reduction tsr simulation experiments were carried out involving different organic matter crude oil solid bitumen type ii kerogen type iii kerogen and different sulfate species anhydrite and mgso4 to address the chemical and carbon isotopic variations of the hydrocarbon h2s and co2 the increase of main peak carbons of residual saturated hydrocarbon and the decrease of gaseous hydrocarbons from control treatment to corresponding sulfate treatments suggest that tsr promote the consumption of both liquid and gaseous hydrocarbons the 13c1 3 values generally shift positively from control treatment to sulfate treatments and the 13cethane 13cmethane values in sulfate treatments are higher than that in control treatments the variation of 13c1 3 in tsr is controlled by the isotope fractionation during the generation and consumption of c1 3 in tsr mgso4 is more reactive than anhydrite in tsr a certain amount of h2s is incorporated into solid bitumen as the result of secondary alteration we believe that the reactivity order of different organic matter in tsr is crude oil solid bitumen type ii kerogen type iii kerogen the reactivity of organic matter in tsr depends on the hydrocarbon generation kinetics of each organic matter the dissolution decomposition and precipitation of carbonate control the yield of co2 in sour reservoirs the negative shift of 13co2 with increasing tsr extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons inorganic co2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on 13co2 the h2s yields decrease with ch4 yields and increase with 13ch4 value the 13ch4 values increase with residual amount of gaseous alkane 1 h2s residual alkane h2s suggesting that methane acted as reactant in tsr the 13co2 13ch4 values decrease significantly with increasing temperature and the 13co2 is even more negative than 13ch4 in 450 c mgso4 treatments involving type ii and type kerogen accordingly methane acted as a predominant reactant in 450 c sulfate treatments in our experiment it is possible for natural gas with high gas dryness to experience methane dominated tsr in geological reservoirs

confined pyrolysis of tertiary lacustrine source rocks in the western qaidam basin northwest china implications for generative potential and oil maturity evaluation abstract the parameter s 1 s 2 genetic potential of rock eval analysis is widely used as an evaluation of the genetic potential for the source rocks oligocene miocene saline lacustrine source rocks in the western qaidam basin have low total organic c contents toc most around 0 5 with a few exceptions 1 0 mineral matrix effects are substantial for source rocks with low toc resulting in relatively low s 1 and s 2 peaks based on the results of confined pyrolyses sealed au capsules on 6 oligocene miocene source rocks from the western qaidam basin with toc ranging between 0 48 and 2 22 the relationship between the s 1 s 2 parameter and the maximum amount of extracted bitumen or saturated and aromatic hydrocarbons sa after the confined pyrolysis has been established as follows bitumen mg g rock 1 4924 s 1 s 2 0 3201 r 0 987 or sa saturates aromatics mg g rock 0 7083 s 1 s 2 0 4045 r 0 992 based on these formulas the amounts of hydrocarbons generated from source rocks can be reasonably estimated the typical crude oils with low biomarker maturities in this region appear substantially different to the pyrolysates of these six rocks at 180 300 c but comparable to the pyrolysates at 320 c and higher temperatures based on molecular parameters this result in combination with the physical and gross compositions of the crude oils suggests that the majority of these crude oils were generated from the source rocks during the main oil generative stage possibly at a maturity higher than r o 0 74

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

the fate of co2 derived from thermochemical sulfate reduction tsr and effect of tsr on carbonate porosity and permeability sichuan basin china abstract this article discusses the role of methane in thermochemical sulfate reduction tsr the fate of tsr derived co2 and the effect of tsr on reservoir porosity and permeability and the causes of the anomalously high porosity and permeability in the lower triassic soured carbonate gas reservoirs in the northeast sichuan basin southwest china the lower triassic carbonate reservoirs were buried to a depth of about 7000 m and experienced maximum temperatures up to 220 c before having been uplifted to the present day depths of 4800 to 5500 m but they still possess porosities up to 28 9 and permeabilities up to 3360 md the present day dry gas reservoirs evolved from a paleo oil accumulation and experienced varying degrees of tsr alteration as evidenced from the abundant sulfur rich solid bitumens and varying h2s and co2 concentrations tsr occurred mainly within the oil and condensate wet gas windows with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction sulfur rich solid bitumen and h2s generation and calcite precipitation methane dominated tsr was a rather late event and had played a less significant role in altering the reservoirs intensive h2s and co2 generation during tsr resulted in calcite cementation rather than carbonate dissolution which implies that the amount of water generated during tsr was volumetrically insignificant 13c depleted co2 derived from hydrocarbon oxidation preferentially reacted with ca2 to form isotopically light calcite cements and the remaining co2 re equilibrated with the 13c enriched water rock systems with its 13c rapidly approaching the values for the host rocks which accounted for the observed heavy and relatively constant co2 13c values the carbonate reservoirs suffered from differential porosity loss by tsr involved solid bitumen generation and tsr induced calcite and pyrite precipitation intensive tsr significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents the evaporative platform dolostones and the platform margin shoal dolostones immediately underlying the evaporative facies early oil charge and limited intensity of tsr alteration together with very low phyllosilicate content and early dolomitization accounted for the preservation of anomalously high porosities in the reservoirs above the paleo oil water contact a closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo oil water contact the closed system which is unfavorable for deep burial carbonate dissolution and secondary porosity generation was favorable for the preservation of early formed porosity in deeply buried carbonates especially sucrosic and vuggy dolostones have a high potential to preserve such porosity

chemical structure evolution of kerogen during oil generation abstract kerogen was isolated from the source rock of well l69 zhanhua depression bohai bay basin and an artificial pyrolysis experiment was carried out in a closed gold tube system with a heating rate of 2 c h products were collected at eleven temperature points every ten degrees from 350 c to 450 c the soluble organic matter and residual kerogens from the experiment were quantified by weighing furthermore the residual kerogens were analysed by element analysis x ray photoelectron spectroscopy xps and solid 13c nuclear magnetic resonance 13c nmr spectroscopy to determine the chemical structural of kerogen during oil generation combining the data from the analyses information on the elemental compositions as well as on how functional groups were connected in residues was obtained seven molecular models of initial and residual kerogens were established to trace the structure changes in kerogen with maturity the results suggest that as the temperature increases aliphatic chains and small clusters of aromatic groups break down from kerogens generating soluble hydrocarbons at temperatures of less than 390 c easy ro1 15 at higher thermal maturity a few short aliphatic carbons remain in the structure of residues the number of aromatic groups increases and their size grows with increasing maturity with condensation occurring at aromatic groups at high temperature while these residues have little potential for oil generation

a perspective on the future of physical oceanography the ocean flows because it is forced by winds tides and exchanges of heat and freshwater with the overlying atmosphere and cryosphere to achieve a state where the defining properties of the ocean such as its energy and momentum do not continuously increase some form of dissipation or damping is required to balance the forcing the ocean circulation is thought to be forced primarily at the large scales characteristic of ocean basins yet to be damped at much smaller scales down to those of centimetre sized turbulence for decades physical oceanographers have sought to comprehend the fundamentals of this fractal puzzle how the ocean circulation is driven how it is damped and how ocean dynamics connects the very different scales of forcing and dissipation while in the last two decades significant advances have taken place on all these three fronts the thrust of progress has been in understanding the driving mechanisms of ocean circulation and the ocean s ensuing dynamical response with issues surrounding dissipation receiving comparatively little attention this choice of research priorities stems not only from logistical and technological difficulties in observing and modelling the physical processes responsible for damping the circulation but also from the untested assumption that the evolution of the ocean s state over time scales of concern to humankind is largely independent of dissipative processes in this article i illustrate some of the key advances in our understanding of ocean circulation that have been achieved in the last 20 years and based on a range of evidence contend that the field will soon reach a stage in which uncertainties surrounding the arrest of ocean circulation will pose the main challenge to further progress it is argued that the role of the circulation in the coupled climate system will stand as a further focal point of major advances in understanding within the next two decades supported by the drive of physical oceanography towards a more operational enterprise by contextual factors the basic elements that a strategy for the future must have to foster progress in these two areas are discussed with an overarching emphasis on the promotion of curiosity driven fundamental research against opposing external pressures and on the importance of upholding fundamental research as the apex of education in the field

genetic origins of marine gases in the tazhong area of the tarim basin nw china implications from the pyrolysis of marine kerogens and crude oil abstract although abundant natural gases have been discovered in carboniferous and ordovician reservoirs in the tazhong area central tarim basin their genetic origins is still an open question because of significant variations in chemical and carbon isotopic compositions in the present study three samples representing the possible gas sources were pyrolyzed using sealed gold tubes to investigate their gas potentials and chemical and carbon isotopic signatures the samples were a crude oil a low maturity kerogen and a high maturity kerogen the results show that the chemical and isotopic compositions of gaseous pyrolysates from the three samples are quite different generally the gases from oil cracking are compositionally wetter and isotopically lighter than those from kerogen cracking at similar thermal levels the large quantities of methane experimentally observed in oil cracking gases are mainly associated with the re cracking of wet gases at very high maturity levels methane in gases derived from kerogen especially high maturity kerogen in contrary is mainly generated by the demethylation reaction the diagrams of 13c1 vs 13c2 13c3 and c2 c3 vs 13c2 13c3 along with natural gas plots have proven to be a very effective tool in the genetic interpretation of natural gases from kerogen and oil cracking in the tarim basin their combined application shows that two types of gas source can be identified in the tazhong area the first type is oil cracking gas mainly discovered in carboniferous reservoirs of the central fault horst area but also partly in ordovician reservoirs such as in the wells tz45 tz44b and tz 16 the other is kerogen cracking gas occurring largely in ordovician reservoirs especially in the north slope area the two types of gases however are mixed in many reservoirs as their generation is overlapped in time and or space this mixing process is especially obvious for methane

alteration of solid bitumen by hydrothermal heating and thermochemical sulfate reduction in the ediacaran and cambrian dolomite reservoirs in the central sichuan basin sw china abstract solid bitumen in reservoir rocks is an allochthonous organic matter formed from bitumen or liquid hydrocarbons by various alteration processes in this study two types of altered solid bitumen have been identified in both the ediacaran dengying and cambrian longwangmiao formations in the central sichuan basin using petrographic microthermometric chemical and isotopic data the hydrothermally altered bitumen type 1 is associated with the pore filling saddle dolomite and quartz in which homogenization temperatures of some fluid inclusions are more than 10 c higher than the maximum burial temperature of the dengying formation it has various anisotropic textures with significantly high bireflectance values rbmax rbmin between 3 89 and 9 21 moreover the hydrothermal alteration led to 13c enrichment in the type 1 bitumen compared to the normally matured bitumen derived from the same source on the other hand the thermochemical sulfate reduction tsr altered bitumen type 2 has s c atomic ratios of up to approximately 0 06 the 34s values of type 2 bitumen are close to those of the carbonate associated sulfate and similar to the tsr derived h2s in the same reservoirs a strong negative correlation exists between s c atomic ratios and 13c values in the type 2 bitumen in the dengying formation which is interpreted as a result of an increasing incorporation of the 13c depleted ethanethiol into the bitumen during tsr this interpretation is consistent with the fact that the remaining ethane became isotopically heavier as tsr proceeded in addition the reactive sulfate for tsr was primarily derived from the hydrothermal sulfate such as barite this is evidenced by the replacement of hydrothermal barite by the 34s enriched pyrite and the small differences in 34s value between the barite and the type 2 bitumen or the tsr derived h2s in the same strata we suggest that tsr in the ediacaran and cambrian hydrocarbon reservoirs was probably initiated by hydrothermal activity

constraints of molecular and stable isotopic compositions on the origin of natural gas from middle triassic reservoirs in the chuanxi large gas field sichuan basin sw china abstract the middle triassic leikoupo formation in the sichuan basin is a gypsum bearing stratum the chuanxi cx large gas field has been recently discovered in middle triassic reservoirs with a controversial gas source the gas geochemistry is analyzed in this study to investigate the gas origin and source the h2s bearing dry gas in the cx gas field has an h2s content ranging from 1 07 to 4 09 with a gas souring index gsi of 0 0120 0 0438 the 13c1 13c2 and 2hch4 values range from 31 8 to 30 4 from 32 9 to 29 6 and from 153 to 136 respectively genetic identification based on the geochemical characteristics suggests that the gas is an oil type and derives from the secondary cracking of oil according to the gsi and alkane carbon isotopes the gas experienced weak tsr alteration without ethane potential high quality source rocks include the upper permian mudstones and marlstones as well as the lower cambrian mudstones with a total organic carbon content generally higher than 2 0 a comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the upper permian rather than the lower cambrian source rocks the extremely low development of solid bitumen in the reservoirs suggests the inexistence of large scale paleo oil pools in the leikoupo formation the middle triassic natural gas in the cx gas field originates from the upper permian longtan formation where the gas pools are dominated by direct filling of secondary cracking gas