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

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

inversion styles in the songliao basin northeast china and estimation of the degree of inversion abstract two inversion patterns fault reactivated and cover folded inversions are recognized in the songliao basin of northeastern china mild inversion with a reactivated fault showing normal separation in all synrift beds moderate partial inversion and total inversion are the common styles of fault reactivated inversion the magnitude of extension by rifting as well as orientation of the synrift faults in relation to the compressional direction during inversion significantly controlled the inversion structures the same amount of shortening may produce a moderate inversion style where the normal slip during the rifting was large but a total inversion in which the initial normal slip was small the cover folded inversions in the songliao basin have either no normal faults or have one or more normal faults that occurred prior to inversion and remained dormant during the inversion the geometry of cover folded inversion may be controlled by deep faults in the basement a graben like basement fault assemblage can result in an antiformal geometry while a horst like basement fault assemblage may control a synformal cover folded inversion an inversion with antiformal geometry of the sedimentary sequences is the major style in the songliao basin a new method of estimating the degree of fault inversion is discussed the degree of fault inversion is the ratio between reverse slip during inversion and normal slip during extension both measured along the fault it can be estimated in most fault reactivated inversions where the top of the basement or the floor of the synrift deposits does not fold during the inversion the horizontal inversion ratio represents the relative amount of shortening and extension of a structure or basin it can be used to analyse the degree of inversion of complex structures or basins however both fault inversion and horizontal inversion ratios are primarily influenced by the amount of extension if undergoing same amount of shortening a structure with a small amount of extension may have a much greater inversion ratio than one with a large amount of extension

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

volcanic rocks as prolific gas reservoir a case study from the qingshen gas field in the songliao basin ne china abstract the qingshen gas field with over 100 billion cubic meters bcms of proven gas reserves was discovered recently in the upper jurassic lower cretaceous strata of the songliao basin northeastern china the stratigraphic relationships and gas chemical compositions indicate mixed gas sources dominantly from the upper jurassic lower cretaceous coals and associated dark mudstones underlying the gas reservoirs with minor contribution from deep inorganic sources the gas reservoirs consist mainly of volcanic rocks rhyolite and tuff with some production also from the conglomerate gas charge into these largely volcanic rock reservoirs appears to coincide with the peak gas generation in the deep upper jurassic lower cretaceous coal measures the presence of several thick lacustrine mudstone beds in the upper cretaceous strata as regional caprocks and relatively mild structural deformation since the late cretaceous are among the favorable conditions for the preservation of large volume of natural gases in these relatively deep gas reservoirs the gas yields are the highest in the paleomorphological highs where fractures and secondary porosity are well developed in the gas reservoirs

evidence for biodegradation and evaporative fractionation in west sak kuparuk and prudhoe bay field areas north slope alaska abstract oils from the west sak kuparuk and prudhoe bay fields on the north slope of alaska display geochemical evidence for a complex petroleum filling history that includes multiple sources and alteration by evaporative fractionation and biodegradation source specific biomarkers in west sak oils indicate generation from the same source rocks in approximately the same proportions as prudhoe bay field oil consistent with the hypothesis that oil spilled from the deeper prudhoe accumulation and remigrated about 10 miles laterally and 5000 feet vertically into the west sak field west sak oils are moderately biodegraded but exhibit anomalously high concentrations of gasoline range saturate and aromatic compounds interpreted as a secondary gas condensate charge that arrived after biodegradation of the spilled prudhoe oil the shallowest west sak oils have less secondary gas condensate charge lower api gravities and reduced concentrations of methylcyclopentane benzene cyclohexane 3 methylhexane heptane methylcyclohexane and toluene relative to the deepest west sak zone a oils the carbon isotopic compositions of these c6 and c7 compounds in the shallowest west sak reservoirs are heavier than those of the deepest west sak oil reservoirs by up to 3 and the isotopic compositions of saturate compounds are altered more than those of aromatic compounds the heavier isotopic compositions of c6 and c7 compounds in the shallow west sak reservoirs are interpreted to result from a kinetic isotope shift caused by light biodegradation of the secondary gas condensate charge west sak field solution gas is also interpreted as biodegraded based upon its dryness 98 methane relatively high isobutane n butane ratio the presence of isotopically heavy carbon dioxide and isotopically light methane and propane that is isotopically heavier than either ethane or butane the isotopic compositions of methane and carbon dioxide in west sak gas suggest that biogenic methane was generated during co2 reduction under anaerobic conditions the isotopic compositions of c6 and c7 compounds in the deepest least biodegraded west sak oils are nearly identical to those of the underlying oils from the kuparuk field but differ from the prudhoe bay field oils the kuparuk field is therefore postulated as the source of the secondary gas condensate charge observed in oils from the west sak field ratios of toluene heptane and heptane methylcyclohexane in kuparuk oils provide independent evidence for loss of gas and condensate from the kuparuk reservoir by the process of evaporative fractionation and the oils subjected to the most evaporative fractionation are isotopically heavier in the c6 c7 range the isotopic compositions of west sak zone a c6 and c7 hydrocarbons suggest that a slight 0 8 isotopic fractionation resulted from evaporative fractionation of the gas condensate charge from the kuparuk field oil accumulation north northeast striking faults that connect the kuparuk and west sak reservoirs provide a potential migration pathway for the secondary gas condensate charge through 3000 feet of intervening shale this model of west sak oil biodegradation subsequent secondary gas condensate charge and further biodegradation has implications for development of north slope west sak oil reserves because heavily biodegraded west sak oils that lack a secondary gas condensate charge may be too viscous to develop by conventional waterflooding methods

numerical simulation of the thermal maturation oil generation and migration in the songliao basin northeastern china abstract the discovery of the giant daqing oil field in the songliao basin led to the realisation of the significant petroleum potential of non marine basins in order to reconstruct the basin evolution and oil formation an integrated organic geochemical basin modelling study along a regional transect across the songliao basin was conducted it provided a regional heat flow evolution model and revealed post orogenic or late syn orogenic maturation in the central depression and pre orogenic maturation in the southeast uplift zone kinetic parameters of petroleum generation for the lacustrine source formations are the basis for the simulation of oil generation and migration in the songliao basin using the principle activation energy peaking at 54 kcal mol and a pre exponential factor of about 4 2 1027 ma 1 the simulation obtained a relatively good match with the measured transformation ratios the qingshankou formation in the west and east central depressions constituted the major source in the basin major oil generation migration and accumulation occurred during the early tertiary in the west central depression the generated oils migrated upwards into the yaojia formation followed by the updip migration into the daqing anticline and towards the local structural high along the west slope in contrast the oil migration in the east central depression was dominated by the downward movement from the lower member of the qingshankou formation followed by the updip migration towards the caoyang anticline the simulated oil accumulations are in good agreement with discovered oil fields implying a potential application of the model for prediction and evaluation of new exploration targets in the basin

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

recognising biodegradation in gas oil accumulations through the δ13c compositions of gas components a large suite of natural gases 93 from the north west shelf and gippsland and otway basins in australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases about 26 of these gases have anomalous stable carbon isotope compositions in the c1 c4 hydrocarbons and co2 components and are interpreted to have a secondary biogenic history the characteristics include unusually large isotopic separations between successive n alkane homologues up to 29 pdb and isotopically heavy co2 up to 19 5 pdb irrespective of geographic location these anomalous gases are from the shallower accumulations 600 1700 m where temperatures are lower than 75 c the secondary biogenic gases are readily distinguishable from thermogenic gases 74 of this sample suite which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m while dissolution effects may have contributed to the high 13c enrichment of the co2 component in the secondary biogenic gases the primary signature of this co2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis correlation between biodegraded oils and biodegraded dry gas supports the concept that gas is formed from the bacterial destruction of oil resulting in secondary biogenic gas furthermore the prominence of methanogenic co2 in these types of accumulations along with some isotopically depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked it is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis

genetic and temporal relations between formation waters and biogenic methane upper devonian antrim shale michigan basin usa abstract controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane natural gas in most conventional deposits has migrated from a source rock to a reservoir rarely remaining associated with the original or cogenetic formation waters we investigated an unusual gas reservoir the late devonian antrim shale in which large volumes of variably saline water are coproduced with gas the antrim shale is organic rich of relatively low thermal maturity extensively fractured and is both source and reservoir for methane that is generated dominantly by microbial activity this hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas providing a unique opportunity to characterize methane generating mechanisms the well developed fracture network provides a conduit for gas and water mass transport within the antrim shale and allows invasion of meteoric water from overlying aquifers in the glacial drift steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases dilute waters grade into dense brines 300 000 ppm over lateral distances of less than 30 km radiogenic 14c and 3h and stable isotope 18o and d analyses of shallow antrim shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20 000 yr bp carbon isotope compositions of methane from antrim shale wells are typical of the established range for thermogenic or mixed gas 13c 47 to 56 however the unusually high 13c values of co2 coproduced with methane 22 and dissolved inorganic carbon dic in formation waters 28 require bacterial mediation the d values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis methane ethane propane ratios and 13c values for ethane indicate 1 the presence of a thermogenic gas component that increases basinward and 2 progressive bacterial oxidation of ethane as the antrim shale subcrop is approached multiple episodes of pleistocene glaciation over northern michigan appear critical to the development of these gas deposits loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water the physical erosion cycle of repeated glacial advances and retreats exhumed the antrim shale around the northern margin of the michigan basin subjecting it to near surface physiochemical and biochemical processes the chemical and hydrologic relations demonstrated in the antrim shale reservoir suggest a dynamic connection between pleistocene glacial history of the midcontinent region and development of recoverable microbially generated natural gas reserves

electromagnetic induction in a spherical earth with non uniform oceans and continents in electric contact with the underlying medium ii bimodal global geomagnetic sounding of the lithosphere summary the vertical electric currents flowing between the inhomogeneous surface layer and the conductive layers of the earth s mantle are substantially dependent upon the transverse resistivity of the earth s crust and upper mantle although these currents do not contribute straightforwardly to a surface magnetic field they change the vortex sub surface currents which contribute to it thus there exists an indirect influence of leakage currents upon the magnetic field this makes it possible to evaluate resistance of the high ohmic layers which could not be evaluated if the earth was laterally homogeneous and only the induction mode of the field was induced this paper presents the first results of the bimodal sounding based on s variation data an estimate of the lithosphere s transverse resistance t 3 x lo9 51 m2 has been obtained

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

significance of microbial processes in gases of the south caspian basin abstract the south caspian basin has been a major petroleum producing province for more than a century while the basin s oil has been the focus of a number of studies the region s gas has been largely understudied this study examines 31 gas samples from 14 fields with the primary purpose of determining their mode s of formation and the role that microbial activity has had in their formation and alteration the basin s natural gas accumulations display significant differences in both molecular and isotopic composition gas wetness ranges from less than 1 at absheron to greater than 12 at guneshli methane carbon isotopic composition ranges from 57 to 37 relative to the pdb standard the stable carbon isotopic composition of the wet gas c 2 components also displays a very broad range e g c 2 ranges from 37 to 20 c 3 ranges from 31 to 10 and n c 4 ranges from 30 to 11 no strong depth related trends were observed in any of the geochemical attributes the absence of a trend implies that the gases have not been generated in situ but have migrated vertically been altered or represent a mixed product mixing is also suggested by the differences in the calculated thermal maturity levels between the c 2 components and methane in some accumulations e g karabagly as much as 55 of the methane may have a biogenic origin these data further indicate that among the gas samples studied those from bakhar appear to be the most mature with thermal maturity values consistent with the condensate window in several fields including guneshli and neftchala the wet gas components display evidence for microbial alteration this is largely manifested in anomalously heavy isotopic compositions of propane n butane and n pentane

coupled feedbacks between mountain erosion rate elevation crustal temperature and density abstract many ancient 1 gyr mountain belts have crust 50 km thick in contrast with isostatic and erosion models that predict crustal attenuation to 30 km in

origin of the neogene shallow gas accumulations in the jiyang superdepression bohai bay basin natural gas resources occur extensively along the east coast of china with a number of large and medium sized gas fields being discovered in recent years gas reservoirs include neogene paleogene and the underlying mesozoic and paleozoic basement of the total proven natural gas reserves in the jiyang superdepression bohai bay basin almost 89 7 is present in the shallow neogene gas pools in traps formed on top of the paleotopographic highs and along the margin of the secondary depressions these gases are closely associated with heavy oils occurring as gas caps or associated gases within the heavy oil pools or in separate gas pools above or updip from the heavy oil pools the gases contain over 95 methane and small quantities of c 2 alkanes nitrogen and carbon dioxide the stable carbon isotopes of methane in these gases are up to 10 more positive than those of the thermogenic gases in the deep paleogene reservoirs with propane more enriched in 13 c than butane this study demonstrated that the majority of the petroleum source rocks in the jiyang superdepression tend to be oil prone and are currently within or shallower than the conventional oil window 0 45 1 0 ro the chemical and carbon isotopic compositions of the gases together with the moderate to severe biodegradation of the associated heavy oils in the shallow neogene strata clearly suggest that the formation of the shallow natural gases in the jiyang superdepression result from the anaerobic degradation of accumulated oils in reservoir

the controls on the composition of biodegraded oils in the deep subsurface part ii geological controls on subsurface biodegradation fluxes and constraints on reservoir fluid property prediction the principal controls on the fluid properties of biodegraded oil systems have been determined by a combination of petroleum geochemistry numerical modeling of oil biodegradation in reservoirs and analysis of oil property data sets from a variety of geological settings petroleum biodegradation proceeds under anaerobic conditions in any reservoir that has a water leg and has not been heated to temperatures more than 80c in most reservoirs with low concentrations of aqueous sulfate methanogenic degradation is a primary mechanism of petroleum degradation whereas in waters containing abundant sulfate sulfate reduction and sulfide production may dominate net degradation of petroleum fractions in reservoirs is primarily controlled by the reservoir temperature the chemical compounds being degraded and relationships between the oil water contact owc area and oil volume the relative volumes of water leg to oil leg prior level of oil biodegradation and reservoir water salinity act as second order controls on the process typically degradation fluxes kilograms of petroleum destroyed per square meter of oil water contact area per year or kg petroleum m2 owc yr1 for fresh petroleum in clastic reservoirs are in the range of 103 104 kg petroleum m2 owc yr1 and increase with decreasing reservoir temperature from zero near 80c to a maximum flux at the owc of less than 103 kg petroleum m2 owc yr1 at a temperature less than 40c at very low reservoir temperatures and with severely degraded oils such as are seen in the near surface canadian tar sands at the present day the net degradation fluxes are much less than maximum values nutrient supply from the aquifer and adjacent shales mostly buffered by mineral dissolution probably provides the ultimate control on the range of degradation flux values oil compositional gradients and resulting oil viscosity variations are common on both reservoir thickness and field scales in biodegraded oil reservoirs and are a defining characteristic of heavy oil fields produced by crude oil biodegradation continuous vertical gradients in the oil columns document episodic degradation for many millions of years suggesting that the time scales of oil field degradation and petroleum charging are similar the flux temperature relationship we have derived coupled with typical reservoir charge histories defines the range of variation of fluid properties seen in many biodegraded oil provinces and identifies oil charge mixing of biodegraded and fresh oils and reservoir temperature history as the primary controls on fluid properties these flux temperature relationships are easily integrated into prospect charge modeling procedures sensitivity analyses show that the limiting factor in fluid property predictions using even this first level approach are ultimately constrained by the accuracy of current oil charge modeling estimates the absence today of any functional geochemical proxies for assessing oil residence time in oil fields and the substantial uncertainty in petroleum charging times estimated by forward basin modeling is a major obstacle to more accurate fluid property predictions that needs to be addressed

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

geochemistry and accumulation of carbon dioxide gases in china we propose the following identification indicators for co2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples the co2 carbon isotope values of 390 samples and the helium isotope composition of 256 samples collected from wells of 16 oil and gas bearing basins and gas seepages in china and reviewing literature on organic and inorganic co2 throughout the world co2 is of organic origin if co2 content 13cco22 is of inorganic origin if dgr13cco2 8 pmil or co2 60 and co2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if 10 pmil 13cco213cco2 values of organic co2 mainly range from 10 to 22 pmil in tectonically stable oil and gas bearing basins where deep faults and magmatism are undeveloped organic co2 occurs almost exclusively in tectonically active oil and gas bearing basins where active faults and magmatism are present or in areas of modern volcanic activity the dgr13cco2 values of inorganic co2 range from 2 to 8 pmil and are distributed along faults the inorganic co2 with a dgr13cco2 value of 0 3 pmil is of metamorphic origin and that with dgr13cco2 values of 6 2 pmil is mantle derived and of magmatic origin in china the dgr13cco2 values of inorganic co2 gas pools range from 3 41 to 8 44 pmil and the helium ratio r ra is 1 which indicates that these gases are typically derived from the mantle

a reversed compositional pseudo gradient in biodegraded oil column from liaohe basin ne china abstract most heavy oils or tar sands are formed by microbial biodegradation and show a compositional gradient toward the base of oil column however a reversed pseudo gradient has been observed from the fourth member of the shahejie formation in shuguang oilfield liaohe basin ne china on the basis of bulk and molecular compositions in a series core extracts from well k037 the bulk compositions show systematic depletion of saturated hydrocarbons and enrichment of polars resins asphaltenes toward the top of the oil column molecular compositions illustrate that these extracts have similar source input depositional environment and thermal maturity and their compositional differences are mainly caused by varying degrees of biodegradation biodegradation removes all n alkanes most isoprenoid alkanes partial alkylnaphthalenes and alkylphenanthrenes at the top of oil column whereas only n alkanes are affected at the bottom reservoir temperature has no crucial impact on biodegradation levels as the whole oil column in the temperature range of 41 44 c is ideal for microbial activities the studied reservoir is not a continuous oil column which is separated by a few barriers and baffles the occurrence of locally high water saturation layers caused multiple small gradients in the mid and lower part of oil column with a normal biodegradation gradient in the lowermost compartmentalized interval the overall reversed compositional pseudo gradient is controlled by a generally upward coarsening sedimentary sequence and the existence of top water situated at a lithology change boundary from coarse sandstones and conglomerates to fine grained sandstones this study provides a new perspective to assess the reservoir fluid heterogeneity and sheds the light on sweet spots identification for heavy oil exploration and development

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

timing of biogenic gas formation in the eastern qaidam basin nw china abstract quaternary biogenic gas from the qaidam basin nw china is regarded as one of the representative examples of early generation biogenic gas gases formed by microbial activity during sedimentation of source rocks however new age determinations of natural gas and formation waters make this generally accepted assumption questionable gas in the qaidam basin is dominantly methane 99 0 and has 13 c ch 4 values in the range of 68 to 65 reflecting the microbial origin carbon and hydrogen isotope values of methane coupled with carbon isotope values of carbon dioxide and dissolved inorganic carbon dic reflect water derived hydrogen in the methane and a dominant co 2 reduction metabolic pathway for the formation of biogenic methane coproduced formation waters have hydrogen and oxygen isotope values similar to surface water from the golmud and ortmuren rivers t test 0 75 but different from the original formation water t test 1 sequence around 1 35 ma bp this corresponds to a regional uplift erosion and water influx event economic accumulations of biogenic gas were thus most likely generated late in the qaidam basin rather than at the same time as sediment deposition fresh water recharge may be critical to enhancing biogenic methane generation by introducing nutrients and or microbes into the reservoir

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried

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

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