stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

variations of natural gas carbon isotope type curves and their interpretation a case study natural gas is dominated by low molecular weight gaseous hydrocarbons c1 c5 whose genetic and diagenetic information is mainly obtained from stable carbon isotope compositions ordos basin is one of the largest natural gas provinces in china by means of examining the carbon isotope compositions of the ordos basin gases altered patterns of the isotopetype curves due to secondary cracking thermochemical sulphate reduction tsr and mixing of gases generated from different sources are recognized and discussed a typical carbon isotope type curve is nearly linear on the natural gas plot chung h m gormly j r squires r m 1988 origin of gaseous hydrocarbons in subsurface environments theoretical considerations of carbon isotope distribution chemical geology 71 97 103 our results show that the isotope type curve pattern of tsr and gas secondary cracking in coal is convex due to catalysis while the isotope type curve of gas secondary cracking in reservoirs is concave the natural gas of yulin suligemiao and wushenqi gas fields is coal derived gas both coal derived gas and mixed gas from oil and gas prone sources exist in the ordovician reservoirs of the jingbian gas field depending on the borehole locations in the ordovician carbonate reservoirs tsr is recorded but uncommon whereas secondary cracking in reservoirs is often observed 2007 elsevier ltd all rights reserved

deep basin gas new insights from kinetic modelling and isotopic fractionation in deep formed gas precursors abstract the formation and origin of high maturity gas from tertiary sediments in the mackenzie delta have been studied using programmed temperature closed system pyrolysis mssv different open system devices and numerical models for geological predictions a sample from the type iii taglu sequence was selected as being representative for one of the major hydrocarbon prone deltaic sediments in this area artificial maturation under closed system conditions was carried out at three different heating rates 0 2 0 7 and 5 0 k min the gas composition as well as carbon isotopes of formed methane were analysed using gc fid and gcirms structural characteristics of the residual macromolecules formed during these experiments were studied at different levels of thermal stress immature 360 412 and 470 c under closed system conditions the taglu sequence forms unusually high amounts of gas at extreme levels of thermal stress the recombination of previously formed hydrocarbons with residual kerogen coal through aromatisation polycondensation reactions is the best explanation for these high temperature features a feature previoulsy reported for the heather formation north sea when the neoformed material becomes active and forms gas at high levels of maturity carbon isotopes measured on methane fall to values that are 10 lighter than those from open system gas formation networks a kinetic model for hydrocarbon formation from this neoformed organic matter has been developed this predicts that the initial phase of the neoformation processes shift from post oil formation conditions at laboratory heating rates to peri oil formation conditions when it comes to geological heating rates this may indicate that recombination reactions through aromatisation polycondensation processes partly influence the major phase of petroleum formation in nature and also justifies an enhancement of gas prospectivity at great depths especially in deep basins where source rock maturities exceed 2 5 ro

transition metal catalysis in natural gas generation evidence from nonhydrous pyrolysis experiment abstract transition metal catalysis has been advocated as a critical mechanism for natural gas generation but its presence and significance in geological settings have long been debated due to conflicting results of laboratory experiments and the lack of reliable field indicator in this study a low rank coal sample from the shahezi formation of the songliao basin was subjected to nonhydrous pyrolysis with and without transition metal the chemical and isotopic compositions of gaseous products in experiments were analyzed to investigate the mechanism and potential indicator of transition metal catalysis in sedimentary basins the chemical and carbon isotopic compositions of hydrocarbon gases generated in pyrolysis experiments revealed that transition metal could catalyze both decomposition of organic matters and methane synthesis such as fischer tropsch type reactions during maturation of source rocks under experimental conditions resembling geological settings catalytic methane has uniform d values unrelated to source rock maturity which is helpful in identification of catalytic natural gases in sedimentary basins transition metal catalysis is probably not a common mechanism for natural gas generation in sedimentary basins due to the positive correlation between dch4 of natural gases and maturities of source rocks observed in most natural cases however transition metal catalysis could well interpret the restricted range of dch4 and carbon isotopic reversal phenomenon of natural gases in the yingcheng formation of the xujiaweizi fault depression whose origin has long been debated so we inferred that the transition metal catalysis may have occurred in deep strata of the xujiaweizi fault depression and played a significant role in natural gas generation our study revealed that restricted range of dch4 of natural gases with different maturities may serve as an effective indicator of transition metal catalysis in sedimentary basins it provides a new perspective to investigate the significance and universality of transition metal catalysis in different geological settings

high resolution global phase velocity distributions we present global phase velocity distributions for fundamental mode love and rayleigh waves in the period range 40 to 150 seconds the models expressed in terms of spherical harmonic expansions up to degree and order 40 have beed derived from 28 479 love wave and 33 662 rayleigh wave measurements the measurements were made using an automatic procedure based on non linear waveform inversion we show that the results are characterized by lateral resolving radii between 500 and 850 km a significant reduction from previous work the improved global resolution is largely due to the inclusion of many more major arc measurements the power spectra of the models are much whiter than has previously been found the results show great similarity to the phase velocity distributions derived from the recent a priori model 3 smac in which the primary relevant features are the crustal thickness and crustal velocity distributions the cooling oceanic lithosphere and the thick 300km lithosphere of the continental cratons the results thus confirm these features differences for example in the strengths of the crust ridge and craton signatures should lead to refinement in the thermal and constitutive parameters upon which the a priori model depends

geochemical characteristics of natural gases related to late paleozoic coal measures in china abstract by 2013 more than 20 gas fields related to late paleozoic coal measures have been discovered in china primarily distributed in the bohai bay ordos sichuan and junggar basins the total proven reserve in these gas fields is more than 3200 109 m3 accounting for 30 1 of the proven gas reserves in china after analyzing the molecular compositions stable carbon isotopes and helium isotopes of 375 gas samples genetic types and origins of the natural gas as well as the secondary alterations it experienced are discussed in this study most of the natural gas related to the late paleozoic coal measures are coal type gas with some of the gases from the sichuan basin and the jingbian gas field in the ordos basin being oil type gases generated from marine mudstones in the permian longtan formation and marine carbonates in the ordovician majiagou formation respectively the equivalent vitrinite reflectance values based on stable carbon isotopes of natural gases vary from 0 8 to 2 5 indicating the mature to over mature stage positive carbon isotope series 13c ch4 13c c2h6 and 13c ch4 13c c3h8 the form of 13c ch4 13c c2h6 is primarily resulted from the mixing of oil type gases of different maturities while the form of 13c ch4 13c c3h8 is primarily resulted from the mixing of coal type gases of different maturities high concentrations of hydrogen sulfide in natural gas from the sichuan basin and the chenghai gas field in the bohai bay basin are resulted from thermochemical sulfate reduction tsr which causes to some extent the fractionation of carbon isotopes by contrast the fractionation of methane carbon isotopes resulting from the tsr effect is not clear carbon dioxide from the bohai bay and junggar basins is primarily biogenic generated via the thermal decomposition of organic matter while that from the ordos basin sichuan basin and chenghai gas field is generated via the thermal decomposition of carbonates carbon dioxide related to the tsr effect has also contributed to the sichuan basin and chenghai gas field

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

new insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs implications for the in situ accumulation of gas cracked from oils previous pressure volume calculations during oil cracking to gas based on the conventional model that presupposes oil cracking to be completed by approximately 150c underestimate the potential for gas accumulation in petroleum reservoirs in this article a compositional kinetic model of gas generation from oil cracking is suggested based on pyrolysis data using sealed gold tubes and the pressure volume changes are recalculated based on the new kinetic model under various geological conditions the kinetic modeling of oil cracking confirms that crude oil begins cracking at about 160c for a heating rate of 2c m y and that the oil cracking process has two distinct stages with significant differences in gas composition the first stage is characterized by dominant c2 5 wet gases whereas the second is characterized by the recracking of c2 5 wet gases to methane and pyrobitumen leading to a progressive increasing dryness of the gas the pressure volume temperature simulations of oil cracking to gas show that initial oil saturation temperature pressure gradients and openness of reservoirs are important geological factors that control gas accumulation in original petroleum reservoirs for a reservoir that is geologically open and saturated with 100 oil gas spills out of the trap at 196c the gas loss at 240c is almost 50 of the total gas far lower than the 75 based on the conventional model of oil destruction with lower oil saturation the gas loss decreases because the gas water contact can shift downward and the gas loss occurs mainly by solution for effectively isolated reservoirs oil cracking readily exceeds lithostatic pressure leading to reservoir fracturing which becomes more obvious when oil saturation decreases the calculated fracturing temperatures for 100 and 50 vol oil saturations correspond to oil destructions of 95 and 86 4 greatly exceeding the value of 1 as suggested by previous studies a conceptual model of gas accumulation and loss in isolated and open geological conditions for a reservoir with 50 oil saturation is suggested on the basis of this model the triassic carbonate gas pool in northeastern sichuan basin was discussed as a typical example for in situ accumulation of gas cracked from reservoired oils the present model infers that the reservoired oils were completely cracked into gas at 87 6 ma and that 75 85 of the gas has been preserved in the original reservoir rocks to form the in situ gas pools with a huge amount of gas resources we believe that gas accumulation from oil cracking in original petroleum reservoirs is much more prospective than previously thought and that gas cracked from oil has great potential in other areas of the sichuan basin and the eastern tarim basin

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

modelling petroleum formation from heterogeneous source rocks the influence of frequency factors on activation energy distribution and geological prediction abstract the application of basin modelling programs permits the prediction of organic matter transformation to oil and gas under variable geological conditions simplified kinetic models are commonly applied with a distribution of activation energies and a single average frequency factor here it will be shown that such simplified kinetic models are not sufficient to correctly predict organic matter transformation in heterogeneous kerogens under geological conditions in the present paper individual pairs of frequency factors and activation energies were calculated from the temperature shift of nine different levels of organic matter transformation ranging from 0 1 to 0 9 tr at different heating rates 0 1 0 7 and 5 0 k min this type of extended kinetic evaluation has been applied on samples from the green river shale the toarcian shale the duvernay formation the draupne formation a type ii s source rock petroleum asphaltenes and a tertiary coal from the mackenzie delta respectively for terrestrial and heterogeneous marine source rocks the frequency factors systematically increase with increasing level of transformation while such an increase was insignificant or absent when homogeneous source rocks were studied by applying geological heating rates it was possible to illustrate the significance of the extended calculation of individual frequency factors the predicted onset temperature of petroleum formation was displaced to 60 70 c in case of lignites coals and type ii s source rocks which was up to 30 40 c lower than predicted by the use of a conventional single frequency factor model to a lesser extend such a shift was also documented for some marine shales e g draupne formation and the duvernay formation the reason for this phenomenon is that an average single frequency factor suppresses the initial formation of petroleum at geological heating rates because high activation energies are needed to maintain the kinetic equation at low generation temperatures in contrast correct activation energies derive from individual frequency factors which keep the activation energies low at low levels of organic matter transformation and result in a more reliable description of the low energy bonds in the heterogeneous organic matter types

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

natural gas composition in a geological environment and the implications for the processes of generation and preservation gases recovered from their location of formation for example gas recovered from drill cuttings are enriched in c2 components relative to gases recovered from reservoir production or testing streams because reservoir gases have been derived through a series of processes including generation from a source with a specific chemical character primary and secondary migration accumulation and selective preservation the compound distribution within these reservoir gases is less likely to be representative of the generation process than in situ gases such as cuttings gas thus rather than comparing compound distributions within reservoir or pipeline gas data to test whether laboratory catalysis experiments for the generation of gas are consistent with geological systems it is better to compare laboratory results of the distribution of gas compounds with gas data for which the opportunity for a wide range of different processes can be precluded the mismatch between compound distributions within cuttings gas recovered from fine grained rocks and those recovered from laboratory experiments disproves the hypothesis of gas generation catalysis in geological systems whereas the distribution of compounds within cuttings gas is consistent with simple non catalytic cracking the generation of natural gas in geological systems has been inferred in some publications to be a universally catalytic process on the basis of observations made during laboratory experiments however this extrapolation has been predicated on the premise that the distribution of compounds within natural gas methane in c1 c4 in reservoirs is different from that observed from non catalytic or simple cracking experiments and that reservoir gas precisely represents natural gas composition as it is generated within source rocks the extrapolation of the inferred kinetic stability of low molecular weight hydrocarbons such as propane determined during high temperature 530 c low pressure 600 mm hg water free experiments to geological time scales yields lower reaction rates than those actually observed for geological systems the inferred catalytically mediated kinetics yield reaction rates that are too high these observations coupled with the proof that transition metal catalysis is not operating in geological environments suggest that the overall kinetic controls under geological conditions are different from those in the dry quartz laboratory experiments

geochemistry of natural gases in deep strata of the songliao basin ne china chemical composition and stable carbon isotopic studies were undertaken for 27 gas samples from deep strata of the xujiaweizi depression in the songliao basin to investigate their origin gas molecular and carbon isotopic compositions show great variety methane is the main component for all studied samples and its content ranges from 57 4 to 98 2 with an average of 90 1 gas wetness ranges from 0 8 to 16 7 with an average of 2 7 the main non hydrocarbon gases are carbon dioxide and nitrogen with an average of 4 0 and 3 2 respectively carbon isotope data suggest that these deep strata gases are mainly coal type gases mixed with minor amounts of associated oil type gases coal type gases are characterized by heavier carbon isotopic values and drier chemical compositions these gases were generated from the lower cretaceous shahezi formation coals interbedded shales with type iii kerogen during the postmature stage of hydrocarbon generation oil type gases are characterized by lighter carbon isotope and higher wetness which were generated from the lower cretaceous shales with type ii kerogen in the shallow strata during the early mature stage of hydrocarbon generation mixing of two different gases causes unusual carbon isotopic distribution patterns with lighter isotopic values in higher numbered carbons in most gases the discovery of coal type gases in the songliao basin provides new prospects for the exploration in this region

constraints on the origins of hydrocarbon gas from compositions of gases at their site of origin it is widely accepted that natural gas is formed from thermal decomposition of both oil in reservoirs and to a lesser extent the organic matter in shales from which the oil was derived1 6 but laboratory pyrolysis experiments on shales do not reproduce the methane rich composition typical of most gas reservoirs7 leading to suggestions7 that other mechanisms such as transition metal catalysis may be important the discrepancy might however instead arise because gas and oil deposits have migrated from their source rocks so that the reservoir composition might not be representative of the composition in the source rocks where the hydrocarbons were generated to address this question we have analysed gas samples coproduced with oils directly from a source rock the bakken shales north dakota usa where the local geology has prevented significant hydrocarbon migration the methane contents of these bakken shale gases are much lower than that of conventional gas reservoirs but are consistent with that from pyrolysis experiments8 9 on these shales thus because these bakken gases form with rather than from oils we argue that compositional differences between gases from source rocks and conventional gas deposits result from fractionation processes occurring after hydrocarbon expulsion from the source rock

crustal structure and upper mantle stratigraphy of the arabian shield 1 analysis of receiver functions from an eight station network on the saudi arabian shield indicates that the moho depths vary between 35 38 km the crust appears to be rather felsic in nature as evidenced by poisson s ratios of 0 25 0 27 a prominent negative phase following the moho conversion is suggestive of a zone of velocity inversion indication of mantle stratification is observed beneath the coastal region of arabia by way of an additional upper mantle discontinuity at about 240 km the conversions from the 410 and 660 km discontinuities are significantly delayed suggesting a slow and hot upper mantle

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

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

lithosphere structure across the dead sea transform as constrained by rayleigh waves observed during the desert experiment summary the interdisciplinary dead sea rift transect desert project that was conducted in israel the palestine territories and jordan has provided a rich palette of data sets to examine the crust and uppermost mantle beneath one of earth s most prominent fault systems the dead sea transform dst as part of the passive seismic component thirty broad band sensors were deployed in 2000 across the dst for roughly one year during this deployment we recorded 115 teleseismic earthquakes that are suitable for a fundamental mode rayleigh wave analysis at intermediate periods 35 150 s our initial analysis reveals overall shear velocities that are reduced by up to 4 per cent with respect to reference earth model prem to the west of the dst we find a seismically relatively fast but thin lid that is about 80 km thick towards the east shallow seismic velocities are low while a deeper low velocity zone is not detected this contradicts the currently favoured thermomechanical model for the dst that predicts lithospheric thinning through mechanical erosion by an intruding plume from the red sea on the other hand our current results are somewhat inconclusive regarding asthenosphere velocities east of the dst due to the band limitation of the recording equipment in jordan

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

surface wave waveform anomalies at the saudi seismic network this paper describes surface wave waveforms observed at the saudi arabian seismic network we identify records corresponding to events occuring in the kuril islands region as anomalous and discuss possible causes for the waveform distortions most likely the waveform anomalies are caused by constructive and destructive interference of laterally refracted wave packets some waveform anomalies are probably of instrumental origin and we suspect a calibration error for the vertical component at station riyd for station afif we find a significant clockwise rotation of the horizontal components with respect to geographic north by approximately 4 degrees

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

comment on natural gas composition in a geological environment and the implications for the processes of generation and preservation lloyd r snowdon 2001 organic geochemistry 32 913 931

modeling of gas generation from the barnett shale fort worth basin texas the generative gas potential of the mississippian barnett shale in the fort worth basin texas was quantitatively evaluated by sealed gold tube pyrolysis kinetic parameters for gas generation and vitrinite reflectance ro changes were calculated from pyrolysis data and the results used to estimate the amount of gas generated from the barnett shale at geologic heating rates using derived kinetics for ro evolution and gas generation quantities of hydrocarbon gas generated at ro 1 1 are about 230 l t 7 4 scf t and increase to more that 5800 l t 186 scf t at ro 2 0 for a sample with an initial total organic carbon content of 5 5 and ro 0 44 the volume of shale gas generated will depend on the organic richness thickness and thermal maturity of the shale and also the amount of petroleum that is retained in the shale during migration gas that is reservoired in shales appears to be generated from the cracking of kerogen and petroleum that is retained in shales and that cracking of the retained petroleum starts by ro 1 1 this result suggests that the cracking of petroleum retained in source rocks occurs at rates that are faster than what is predicted for conventional siliciclastic and carbonate reservoirs and that contact of retained petroleum with kerogen and shale mineralogy may be a critical factor in shale gas generation shale gas systems together with overburden can be considered complete petroleum systems although the processes of petroleum migration accumulation and trap formation are different from what is defined for conventional petroleum systems

significance of coring induced fractures in mesaverde core northwestern colorado several types of mechanically induced fractures occur in mesaverde core from the u s department of energy s multiwell experiment wells two of these types of fractures have significance for the determination of in situ stress orientations scribe line fractures originate at orientation grooves on the surface of the core and are initiated by scribing knives in the core barrel minutes after the core is cut free from the surrounding rock in sandstone core these fractures strike parallel to the maximum in situ horizontal stress direction that has been determined in these wells by anelastic strain recovery measurements coring induced petal and petal centerline fractures form just below the advancing core bit their strike is also controlled by the in situ stress field loc lly however strike may be secondarily influenced by shear stress created by bit rotation reorienting petal fracture strikes in the direction of rotation nevertheless in many situations the orientations of these two types of fractures may be used to determine the orientation of the horizontal in situ stress field if the core is oriented even in unoriented core the relative orientation between in situ stress and natural fractures may be determined

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station

well bore bottom stress concentration and induced core fractures the orientation spacing and shape of drilling induced disking petal and petal centerline fractures in core commonly are remarkably uniform these fractures result from concentrations of in situ stress by the well bore bottom hole cavity and in oriented core their strikes commonly have been used as indicators of the horizontal principal stress directions however an understanding of how these varied fractures are produced has been limited by the lack of detailed knowledge of the distribution of stresses near the bottom hole in this paper we present our result of studying these stress concentrations using full three dimensional finite element modeling for a variety of applied far field in situ stress conditions and as a function of core stub length in nearly all cases purely tensional concentrated stresses are generated within the core by the compressive in situ stresses the directions and magnitudes of these tensions vary with the applied stress indicating the morphology of many of the observed drilling induced core fractures cupped shape disking fractures result from a state of uniform horizontal biaxial stress these fractures also initiate within the rock at the root of the core stub as the horizontal stresses become more anisotropic the point of fracture initiation shifts to the surface of the core and saddle shaped core disks are possible such fractures strike in the direction of the most compressive in situ horizontal principal stress increasing the magnitude of the overburden stress eventually results in petal and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 1 1996 revised manuscript received december 2 1996 final acceptance june 19 1997 2institute of geophysics meteorology and space physics department of physics university of alberta edmonton alberta canada e mail yongyi phys ualberta ca doug phys ualberta ca this work is supported in part by nserc lithoprobe supporting science grant nserc and the alberta oil sand technology and research authority d schmitt acknowledges the assistance of the a v humboldt foundation during revisions of this paper constructive reviews and comments of b r kulander j c lorenz and s l dean were greatly appreciated this paper is lithoprobe submission number 882