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

methane released from groundwater the source of natural gas accumulations in northern west siberia the west siberian basin is the largest continental sedimentary basin on earth and contains about one third of the world s conventional reserves of natural gas from gas compositional and isotope data this gas can be described as a mixture of thermogenic gas from lower cretaceous rocks 13c1 36 c2 10 vol and almost pure methane 98 vol with the relatively light stable carbon isotope signature of 51 13c1 on average the proportion of the isotopically light methane increases from the lower cretaceous to the top of the cretaceous hydrocarbon system top of the pokur formation where the main reservoirs are situated the composition and the huge volume of gas is difficult to explain with the established models of gas generation in sediments the hydrogeology of the basin is determined by thick aquifer systems with an almost basin wide lateral continuity the largest hydrodynamic system is the cretaceous aquifer in which 98 of the natural gas is trapped the groundwater within the cretaceous aquifer is saturated with the same isotopically light methane as that found in the reservoirs of the pokur formation based on petrophysieal data a detailed hydrogeological model of the cretaceous aquifer beneath the largest gas accumulation the urengoy gas field has been developed and the contribution of gas release from groundwater was compared with the volume of gas in the reservoirs the potential methane release from groundwater has been evaluated as a result of 1 cooling of sediments in quaternary times 2 the lateral gradient of the hydraulic pressure under the gas field and 3 the pressure and temperature decrease due to cenozoic uplift the role of quaternary cooling is insignificant and according to our calculations the contribution of the effect by groundwater flow is small the volume of isotopically light methane released from groundwater due to a neogene uplift is very large and can explain the entire gas in place the source of this gas must be located south of the gas field province

aspects of hydrocarbon charge of the petroleum system of the yamal peninsula west siberia basin abstract the yamal peninsula is located in the northern portion of the west siberia basin adjacent to the kara sea the sedimentary succession is composed of between 2 5 and 9 km of jurassic through paleogene sediment deposited within lacustrine through deep marine settings although the region is remote and hydrocarbon exploration has been limited a number of discoveries have been made components of the region s petroleum system are however poorly known this study examines aspects of hydrocarbon charge through the integration of new data obtained on core samples from the malygin field and from 10 oils from five fields bovanenkov malygin kharasavey pyasedaysk and new port with published data source rock screening data from the malygin field reveal the presence of organic rich intervals tocs approaching 6 within the cretaceous and jurassic sequences these data also indicate that at the sampling location these intervals are gas prone hydrogen index values less than 300 mg hc g toc within the cretaceous interval the gas prone character appears to be largely depositional in contrast in the jurassic portion of the sequence the hydrogen indices appear to have been reduced as a result of an advanced level of thermal maturity elsewhere on the peninsula where the thermal maturity of the jurassic is less advanced data suggest that the jurassic sequence includes oil prone intervals preliminary review of the oil data suggests that the examined oils belong to a single family i e were derived from a common source and that much of the variation is a result of different alteration and migration histories although the data from this study were incapable of establishing a definitive correlation published information and the lack of an effective cretaceous liquid hydrocarbon source suggests that these oils were derived from the jurassic sequence

giant gas fields of northern west siberia the 66 fields discovered since the 1960s in the northern west siberian basin contain at least 22 trillion m3 777 tcf of proved gas almost one third of the world s reserves half of these fields are giants 85 billion m3 or 3 000 bcf of reserves these include the largest and second largest gas fields in the world urengoy 8 099 trillion m3 or 286 tcf of gas and yamburg 4 81 trillion m3 or 170 tcf of gas as well as most of the other ten largest gas fields in the world the west siberian basin occupies a 3 4 million km2 1 31 million mi2 arctic lowland immediately east of the ural mountains extending north under the kara sea it is a composite basin with mesozoic cenozoic basin fill on top of a paleozoic basin that overlies a crystalline archean proterozoic framework the productive zones in the northern basin are principally in the neocomian section at an average depth of 2 800 m or 9 200 ft and the cenomanian section at an average depth of 1 100 m or 3 600 ft the former contains reservoirs with gas condensate and oil the latter contains two thirds of the region s gas gas in cenomanian reservoirs is almost pure methane hydrocarbons in neocomian reservoirs were generated by thermal maturation of sapropelic organic matter contained principally in the tithonian bazhenov shale methane in the cenomanian section appears to be a combination of thermogenic gas from the bazhenov suite or deeper and biogenic gas generated in the cenomanian section itself although workers disagree over how much gas came from each source continental glaciation during the pleistocene may have been important in concentrating the methane in cenomanian reservoirs regional petroleum geology of the northern basin is reviewed with detailed descriptions of the largest fields urengoy yamburg bovanenko zapolyarnoye and medvezh ye analyses of the size frequency distributions and discovery rates of neocomian and cenomanian pools indicate that future discoveries will probably be much smaller than in the past with new giants most likely to be found in remote areas which are presently untested for transportation and technical reasons into the 1990s the most severe constraint on production from the region will be imposed by available pipeline capacity

new evidence for dislocation creep from 3 d geodynamic modeling of the pacific upper mantle structure laboratory studies on deformation of olivine in response to applied stress suggest two distinct deformation mechanisms in the earth s upper mantle diffusion creep through diffusion of atoms along grain boundaries and dislocation creep by slipping along crystallographic glide planes each mechanism has very different and important consequences on the dynamical evolution of the mantle and the development of mantle fabric due to the lack of in situ observations it is unclear which deformation mechanism dominates in the upper mantle although observed seismic anisotropy in the upper mantle suggests the presence of dislocation creep we examined the thermo mechanical erosion of the lithosphere by thermal boundary layer instabilities in 3 d dynamical models this study demonstrates that the seismically derived thermal structure of the pacific lithosphere and upper mantle imposes an important constraint on the upper mantle deformation mechanism the predominant deformation mechanism in the upper mantle is dislocation creep consistent with observed seismic anisotropy the acceptable activation energy range of 360 540 kj mol is consistent with although at the lower end of those determined from laboratory studies

development of microporosity in a micritic limestone reservoir lower cretaceous middle east abstract abundant intercrystalline microporosity is developed within the matrix of micritic limestones over several hundreds of meters of section in the thamama group lower cretaceous at sajaa field eastern u a e sediments in the upper shuaiba formation uppermost tamama group include muddy rudist algal coralgal and orbitolina dominated lithofacies matrix is composed of euhedral to subhedral micro rhombic low mg calcite crystals with average diameters of about 5 m microporosity development was not related to leaching chalkification of lithified micrite during post depositional emergence of the shuaiba formation features diagnostic of emergence are lacking in core below the upper contact of the unit matrix microporosity is believed to have been an early developed property of micritic limestones trace element compositions of matrix reflect a diagenetic process that involved conversion of a calcitic lime mud into micro rhombic micrite and microspar using a closed system geochemical model the observed matrix oxygen isotope compositions 18 o of 4 to 5 pdb are compatible with dissolution of aptian lime mud 18 o of 2 pdb and reprecipitation of calcite in fluids of marine composition under conditions corresponding to burial depths of 550 800 m porosity of 30 50 at 40 50 c closed system conditions are consistent with textural constraints for limited or arrested cementation in the matrix chemical compaction was responsible for partial occlusion of matrix microporosity as microporosity is decreased in zones of intense stylolitization slight corrosion of matrix crystals and leaching of peloids and orbitolina postdated cementation and chemical compaction during deep burial

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

ecological succession palaeoenvironmental change and depositional sequences of barremian aptian shallow water carbonates in northern oman barremian and aptian shallow water carbonate facies uppermost lekhwair kharaib and shuaiba formations are described from outcrops in northern oman based on facies analysis and bedding pattern three orders of depositional sequences are defined third to fifth order and correlated between sections over the course of three third order sequences covering the barremian to lower aptian a third order depositional pattern is documented that consists of a succession of three distinct faunal assemblages discoidal orbitolinids and calcareous algae were deposited during early transgression microbialites and microencrusters dominate the late transgressive to early highstand facies and a rudist and miliolid dominated facies is typical of the highstand this ecological succession was controlled largely by palaeoenvironmental changes such as trophic level and clay influx rather than sedimentological factors controlled by variations in accommodation space orbitolinid beds and carbonates formed by microbialites and microencrusters seem to be the shallow water carbonate response to global changes affecting late barremian to aptian palaeoclimate and palaeoceanography

geochemical characteristics of oils and source rocks in the yamal peninsula west siberia russia abstract eight oil and condensate samples and four samples of possible source rocks from the yamal peninsula northernmost west siberia have been characterized geochemically in order to determine source rocks and their maturity level the high content of c29 steranes m z 217 in extracts combined with geological data indicate nearshore marine and continental depositional environments for cretaceous argillites jurassic source rocks with a predominance of c27 steranes and pr ph ratios of 2 4 2 6 have been deposited in the deeper marine basin and contain oil prone type ii kerogen a number of parameters 20s 20s 20r steranes m z 217 22s 22s 22r hopane hopane moretane m z 191 ratios of tri and monoaromatic steroids m z 231 m z 253 and methylphenanthrene ratios show an increase in maturity with depth from immature cretaceous source rocks to moderately mature jurassic sources bazhenov and tyumen formations oil oil and oil source rock correlations based on genetic parameters and maturity indicators have demonstrated that fluids found in jurassic and cretaceous reservoirs including heavy biodegraded oil from the cenomanian belong to a single family and could be derived jointly from the bazhenov j3 and tyumen j1 2 formations the origin of naphthenic condensate found at shallow depth is still controversial although earlier work has suggested that biodegradation has been a dominant factor in the formation of naphthenic condensates their formation from cretaceous source rocks with type iii kerogen could also be possible

quantitative characterization of carbonate pore systems by digital image analysis a new method of digital image analysis can quantify pore parameters over more than three orders of magnitude from a submicron to a millimeter scale this porosity characterization does not require knowledge of lithology age burial depth or diagenesis of the sample the method is based on digital analyses of images from thin sections at variable magnifications taken under an optical microscope om and under an environmental scanning electron microscope esem the results help explain variations in permeability for carbonate samples with a variety of complex pore structures the analyses however can be done on any thin sections of other rock types the om images provide macroporosity information whereas the esem images yield information on microporosity the boundary between macroporosity and microporosity is defined at a pore area of 500 m2 which translates to a pore length of approximately 20 m which is roughly the thickness of a thin section and thus the resolution of the om the digitized thin section images are binarized into a macropore and a matrix phase om or a micropore and a solid phase esem a standard digital image analysis program is used to detect all individual pores and to measure pore area and pore perimeter based on these analyses one can calculate for each sample the amount of macroporosity the amount of microporosity within the matrix intrinsic microporosity the shapes of the macropores perimeter over area and the pore size distribution comparison of total porosity determined from plugs indicates that macroporosity and microporosity values based on this methodology match the plug data confirming the validity of the method the combination of macroporosity and microporosity data yields pore size distribution and pore shape information that can explain the distribution of physical properties in particular permeability in parameter sensitivity analyses using neural networks permeability appears to be mainly controlled by the macropore shape in high permeability samples and by the amount of intrinsic microporosity in the low permeability samples

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

free gas and gas hydrates from the sea of marmara turkey chemical and structural characterization gas hydrates and gas bubbles were collected during the marnaut cruise may june 2007 in the sea of marmara along the north anatolian fault system turkey gas hydrates were sampled in the western part of the sea of marmara on the western high and three gas bubble samples were recovered on the western high the central high center part of the sea of marmara and in the cinarcik basin eastern part of the sea of marmara methane is the major component of hydrates 66 1 but heavier gases such as c2 c3 and i c4 are also present in relatively high concentration the methane contained within gas hydrate is clearly thermogenic as evidenced by a low c1 c2 c3 ratio of 3 3 and carbon and hydrogen isotopic data 13cch4 of 44 1 pdb and dch4 of 219 smow a similar signature is found for the associated gas bubbles c1 c2 c3 ratio of 24 4 13cch4 of 44 4 pdb which have the same composition as natural gas from k marmara af field gas bubbles from central high show also a thermogenic origin as evidenced by a c1 c2 c3 ratio of 137 and carbon and hydrogen isotopic data 13cch4 of 44 4 pdb and dch4 of 210 smow whereas those from the cinarcik basin have a primarily microbial origin c1 c2 c3 ratio of 16 600 13cch4 of 64 1 pdb uv raman spectroscopy reveals structure ii for gas hydrates with ch4 trapped in the small 512 and large 51264 cages and with c2h6 c3h8 and i c4h10 trapped in the large cages hydrate composition is in good agreement with equilibrium calculations which confirm the genetic link between the gas hydrate and gas bubbles at western high and the k marmara af offshore gas field located north of the western high we calculate the characteristics of the hydrate stability zone at western high and in the cinarcik basin using the csm gem computer program the base of the structure ii hydrate stability field is at about 100 m depth below the seafloor at the western high site whereas in the cinarcik basin p t conditions at the seafloor correspond to the uppermost range for structure i hydrate formation from microbial gas

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

generation mixing and alteration of thermogenic and microbial gas in oil deposits the case of the alpine foreland basin austria abstract molecular and stable isotope compositions were determined for forty eight gas samples taken from wells producing oil form cretaceous and eocene reservoirs gas was expelled from source rock at various maturity levels 0 6 1 2 rr gas maturities based on stable carbon isotopes are in general agreement with those found in oils nevertheless results revealed that most fields trap methane derived from a source which is not thermogenic shallow northeastern reservoirs trap methane interpreted as secondary microbial in origin the same process is proposed here as source of methane in western deposits moreover those gases are enriched in 2 h isotope suggesting different methanogenesis pathways fields along the southern margin of the alpine foreland basin where reservoir temperature exceeds 80 c host methane generated during primary organic matter degradation thus eocene and lower oligocene layers should be considered as potential source rocks this study gives new insight to filling history of traps and revealed a more complicated geological background and hydrocarbons generation than have been assumed until now

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

thermogenic and secondary biogenic gases san juan basin colorado and new mexico implications for coalbed gas producibility the san juan basin is the most prolific coalbed gas basin in the world with 1992 production exceeding 440 gcf footnote 12 4 billion m3 resources of approximately 50 tcf 1 4 trillion m3 and proved reserves of over 6 tcf 170 billion m3 coalbed gas wells with the highest production initial potential greater than 10 mcf day or 0 28 million m3 day occur in the overpressured north central part of the basin hydrologic analysis indicates that overpressure in the fruitland formation is artesian in origin and represents repressuring that developed during the middle pliocene highly permeable laterally continuous coal beds override abandoned shoreline pictured cliffs sandstones and extend to the elevated recharge area in he northern basin to form a dynamic regionally interconnected aquifer system coal rank and basin hydrodynamics control the composition of fruitland coalbed gases which varies significantly across the basin chemically dry gases in the north central part of the basin coincide with meteoric recharge and regional overpressure the consistency of methane dgr13c values across the basin the presence of isotopically heavy carbon dioxide in coalbed gases and bicarbonate in formation waters and biodegraded n alkane distributions of some coal extracts indicate that coalbed gases in the north central basin are a mixture of thermogenic 25 50 secondary biogenic 15 30 and migrated thermogenic 12 60 gases migrated conventionally and hydrodynamically trapped gases in situ generated secondary biogenic gases and solution gases result in gas content that plot on or above the coal sorption isotherm bacteria transported basinward in groundwater flowing from the elevated northern basin margins metabolized wet gas components n alkanes and organic compounds in the coal and generated secondary biogenic methane and carbon dioxide subsequent to coalification uplift erosion and cooling these gases may be limited to basin margins where shallow depths and structural deformation result in higher permeability or may extend more than 35 mi 56 km basinward from the recharge zone the presence of appreciable secondary biogenic gas indicates an active dynamic flow system with overall permeability sufficient for high productivity basin hydrogeology reservoir heterogeneity location of permeability barriers no flow boundaries and the timing of biogenic gas generation and trap devel pment are critical for exploration and development of unconventional gas resources in organic rich rocks

natural gas formation in the western nile delta eastern mediterranean thermogenic versus microbial abstract the offshore part of the nile delta is an emerging gas and condensate province and one of the most promising areas for future petroleum exploration and supply in northern africa while the nile delta s hydrocarbon potential and its geological and stratigraphic pattern has been the focus of numerous studies the origin of its gas has been poorly studied in order to characterize the natural gas and to determine whether microbial or thermogenic processes are the dominant generation pathways of methane a regional geochemical study was performed on 38 test gases collected from 9 neogene western nile delta discoveries main constituents of the gases are methane 89 4 99 4 up to pentane with traces of carbon dioxide and nitrogen methane carbon isotopic composition ranges from 66 0 to 39 6 displaying a mixed microbial and thermogenic character for most of the gases microbial gas contributions range from 15 to 95 the mixed and the more thermogenic gases are frequently accompanied by light oils and condensates 41 59 api calculated maturities of the thermogenic gas proportions suggest source rock maturities between 0 8 and 1 4 ro which are actually present in the pre miocene section evidence for microbial alteration of petroleum accumulations in the nile delta is largely manifested in anomalously heavy isotopic compositions of both wet gas components propane and n butane and preferential removal of higher n alkanes from condensates the hydrocarbon accumulations affected by biodegradation are trapped within shallow pliocene reservoirs

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

geology and reservoir characteristics of carbonate buildup in giant bu hasa oil field abu dhabi united arab emirates bu hasa field in abu dhabi was the first giant oil field in the arabian gulf to produce from the lower cretaceous shuaiba formation aptian the field has a productive area of about 155 673 ac 63 047 ha and 12 6 billion bbl of oil reserves the formation is composed mainly of rudistid and algal sediments rudistid mounds having overlain and built up the topography of an algal platform the position and elevation of the platform edge combined with rising eustatic sea level created a tendency toward both vertical and lateral growth and regulated the form and distribution of the rudist accumulation the shuaiba formation has good reservoir quality ranging in porosity from 18 25 and with average permeability exceeding 100 md nine reservoir units a i classified by their porosity permeability and lithology can be clearly defined from the logs eleven main lithofacies have been defined in the shuaiba from detailed core descriptions these lithofacies were deposited in three main phases the earliest phase consisted of chalky mud supported sediment with some algal colonies followed by a middle phase characterized by a thick algal platform during this stage of deposition a shallow water carbonate platform developed over the south and central part of the field grading into deeper open marine sedimentation toward the north the late phase was characterized by rudistid buildups mainly caprinids and caprotinids the growth of the thick algal platform in the south and central areas controlled the initial distribution of the main rudistid buildup toward the north small rudistid reefs are sparsely developed and present only n the latest stage on high energy shoals

application of dynamic programming to the correlation of paleoclimate records 1 signal matching is a powerful tool frequently used in paleoclimate research but it is enormously time consuming when performed by hand previously proposed automatic correlation techniques require very good initial fits to find the correct alignment of two records a new technique presented in this paper utilizes dynamic programming to find the globally optimal alignment of two records geological realism is instilled in the solution through the definition of penalty functions for undesirable behavior such as unlikely changes in accumulation rate examples with synthetic and real data demonstrate that the dynamic programming technique produces accurate high resolution results with much less effort than hand tuning or preexisting automated correlation techniques

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

lithospheric heating by mantle plumes summary a broad topographic swell is associated with the hawaiian hotspot a kinematic model is developed assuming that downstream of the hotspot this swell is produced by lithospheric heating by a mantle plume material from the model plume flows radially in the asthenosphere and eventually heats the lower lithosphere upstream from the hotspot the model swell owes its existence mainly to hot material at asthenospheric depths the model predicts extensive lithospheric heating downstream from the hotspot interpreted data to tell if the lower lithosphere behaves in this way are lacking

evolution of the west siberian basin the west siberian basin is one of the largest intracratonic basins of the world and an important hydrocarbon province of russia perhaps the most important geologic event in siberia was the emplacement of basalts around 250 ma i e permo triassic boundary covering an area of about 5 10 6 km 2 this volcanism may be responsible for a mass extinction that occurred around permian triassic time the prebasaltic rifting event was limited to the north northeastern sector of the basin initial basin wide subsidence took place in the jurassic as a result of which the western part of siberia became the west siberian basin bounded by uplifts to the east and to the west one of the surprising aspects of the west siberian basin is the abundance of subvertical faults believed to be result of strike slip movement while intraplate inversions and fault reactivation structures have been observed in many cratons subvertical faults observed in the west siberian basin are unique because of their geometries and abundance the differentiation between the effects of tectonics and eustasy in cratonic basins is simple the global eustatic signal is basin wide with regional and local tectonics playing an overprinting role thus the middle jurassic turonian first second and third order cycles in the west siberian basin were primarily driven by eustasy the middle jurassic turonian series can be subdivided into two second order and 16 third order transgressive regressive cycles within dataset extent fourth order cycles appear to be controlled by delta shifting although extensively studied a number of fundamental questions regarding the origin and evolution of the west siberian basin remain unresolved or poorly documented in the literature

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

three dimensional simulations of plume lithosphere interaction at the hawaiian swell rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date efficient thinning depends on small scale instabilities that convectively remove lithospheric material these instabilities are favored by hotter plumes or stronger temperature dependence of viscosity and a simple scaling independent of rheology controls their onset this scaling allows extrapolation of the results of numerical experiments to the earth s mantle mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small scale convective rolls aligned with the plate motion the unusual variation in heat flow across the hawaiian swell may be due to such instabilities it was found that the spreading of the plume creates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plume origin this isolation has important consequences for the geochemical heterogeneity of the lithosphere and upper mantle

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

asymmetric shallow mantle structure beneath the hawaiian swell evidence from rayleigh waves recorded by the plume network author posting the author s 2011 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 187 2011 1725 1742 doi 10 1111 j 1365 246x 2011 05238 x

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction

national gas hydrate program expedition 01 offshore india gas hydrate systems as revealed by hydrocarbon gas geochemistry abstract the national gas hydrate program expedition 01 nghp 01 targeted gas hydrate accumulations offshore of the indian peninsula and along the andaman convergent margin the primary objectives of coring were to understand the geologic and geochemical controls on the accumulation of methane hydrate and their linkages to underlying petroleum systems four areas were investigated 1 the kerala konkan basin in the eastern arabian sea 2 the mahanadi and 3 krishna godavari basins in the western bay of bengal and 4 the andaman forearc basin in the andaman sea upward flux of methane at three of the four of the sites cored during nghp 01 is apparent from the presence of seafloor mounds seismic evidence for upward gas migration shallow sub seafloor geochemical evidence of methane oxidation and near seafloor gas composition that resembles gas from depth the kerala konkan basin well contained only co2 with no detectable hydrocarbons suggesting there is no gas hydrate system here gas and gas hydrate from the krishna godavari basin is mainly microbial methane with 13c values ranging from 58 9 to 78 9 with small contributions from microbial ethane 52 1 and co2 gas from the mahanadi basin was mainly methane with lower concentrations of c2 c5 hydrocarbons c1 c2 ratios typically 1000 and co2 carbon isotopic compositions that ranged from 70 7 to 86 6 for methane and 62 9 to 63 7 for ethane are consistent with a microbial gas source however deeper cores contained higher molecular weight hydrocarbon gases suggesting a small contribution from a thermogenic gas source gas composition in the andaman basin was mainly methane with lower concentrations of ethane to isopentane and co2 c1 c2 ratios were mainly 1000 although deeper samples were

earth science through thick and thin the sea floor around the hawaiian island chain is unusually shallow new seismic evidence suggests that this up raised swell is partly due to heating and thinning of the lithosphere beneath

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

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

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

effects of compositional and rheological stratifications on small scale convection under the oceans implications for the thickness of oceanic lithosphere and seafloor flattening 1 pressure release melting at mid ocean ridges generate compositional and rheological layering in the oceanic mantle that may control the evolution of the oceanic lithosphere we use dynamic models coupled with melting and petrological models to explore 1 the influence of this layering on the development of small scale convection under the oceans 2 its role in determining the thickness of oceanic lithosphere and 3 its feasibility as responsible for the deviations of seafloor and surface heat flow from predictions by conductive models in mature oceanic lithosphere here we show that the existence of small scale convection is entirely compatible with experimental creep parameters and flow laws and that the viscosity stratification due to melt extraction i e h2o removal is the main factor controlling the plate s thermal evolution its asymptotic thickness and the flattening of seafloor and surface heat flow at ages 70 ma the effects of al rich phase transitions and compositional layering are minor

microbial production and modification of gases in sedimentary basins a geochemical case study from a devonian shale gas play michigan basin an expanded data set for gases produced from the antrim shale a devonian black shale in the michigan basin united states has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid gas liquid systems that discriminate between microbial and thermogenic gas origin in the antrim shale economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network the most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water along the basin margins there is also a systematic enrichment in 13c of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components microbial oxidation accounts not only for the shift in 13c values for ethane but also in part for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed this oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate reducing bacteria the results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water one unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced co2 gas in contrast the 13c value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas in addition the confounding isotopic and compositional overprint of microbial oxidation increasing the values to typically thermogenic values may obscure the distinction between methanogenic and thermogenic gas

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

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

composition and origin of coalbed gases in the upper silesian and lublin basins poland abstract coalbed gases in the upper silesian coal basin uscb are highly variable in molecular and stable isotope composition geochemical indices and stable isotope ratios for coalbed gases from uscb vary within the following ranges ch4 c2h6 c3h8 hydrocarbon index from 122 to more than 10 000 cdmi carbon dioxide methane index cdmi co2 co2 ch4 100 from 0 0 to 21 0 13c ch4 from 79 9 to 44 5 d ch4 from 202 to 153 13c c2h6 from 24 6 to 22 3 13c c3h8 24 7 one sample and 13c co2 from 27 2 to 2 8 only two coalbed gases were collected from the lublin coal basin lcb geochemical indices and stable isotope ratios for these samples show the following values hydrocarbon index more than 10 000 cdmi 4 3 and 34 6 13c ch4 67 3 and 52 5 d ch4 201 one sample and 13c co2 13 7 and 11 9 methane higher gaseous hydrocarbons c2 to c5 and carbon dioxide occurring in the coalbed gases in both the uscb and the lcb were generated during the bituminous stage of the coalification process and probably during microbial reduction of carbon dioxide however depth related isotopic fractionation which has resulted from physical e g diffusion and adsorption desorption processes during gas migration cannot be neglected in both basins the coalification process lasted no longer than several ma and was completed at the end of the variscan orogeny at the turn of carboniferous and permian

upper mantle structure beneath the galapagos archipelago from surface wave tomography 1 we present a rayleigh wave tomographic study of the upper mantle beneath the galapagos archipelago we analyze waves in 12 separate frequency bands 8 50 mhz sensitive to shear wave velocity vs structure in the upper 150 km average phase velocities are up to 2 and 8 lower than for 0 to 4 my old and 4 to 20 my old pacific seafloor respectively laterally averaged vs is 0 05 0 2 km s lower between 75 and 150 km depth than for normal pacific mantle of comparable age corresponding to an excess temperature of 30 to 150c and 0 5 melt a continuous low velocity volume that tilts in a northerly direction as it shoals extends from the bottom of our model to the base of a high velocity lid which is located at depths varying from 40 to 70 km we interpret this low velocity volume as an upwelling thermal plume that flattens against the base of the high velocity lid the high velocity lid is 30 km thicker than estimated lithospheric thickness beneath the southwestern archipelago above the main region of plume upwelling we attribute the thicker than normal high velocity lid to residuum from hot spot melting the thickness of the lid appears to control the final depth of melting and the variability of basalt composition in the archipelago at depths less than 100 120 km plume material spreads in directions both toward and against eastward plate motion indicating that plume buoyancy forces dominate over plate drag forces and suggesting a relatively high plume buoyancy flux b 2000 kg s

a new model linking atmospheric methane sources to pleistocene glaciation via methanogenesis in sedimentary basins methane ch 4 is an important greenhouse gas and amplifier of climate change however the causes of atmospheric ch 4 variations over glacial interglacial cycles remain unresolved we propose that microbial methanogenesis along the shallow margins of sedimentary basins provides a source of atmospheric ch 4 temporally connected with both advance and retreat of continental ice sheets extensive biodegradation of hydrocarbons in the antrim shale formation michigan united states is associated with an active subsurface consortium of fermentative and methanogenic microorganisms this activity was initially stimulated when saline formation waters were diluted by meltwater derived from overriding pleistocene ice sheets during glaciation ch 4 produced by this community accumulated in the shale at a rate of 1 tg ch 4 per 1000 yr as a result of ice coverage and increased hydrostatic pressure we estimate that at present the antrim shale contains only 12 25 of the cumulative mass of ch 4 generated in the shale over the pleistocene indicating that ch 4 that had accumulated during glaciation was subsequently released following ice sheet retreat while release from the antrim shale represents only a small part of the global ch 4 budget when extended to other glaciated sedimentary basins subsurface methanogenesis may generate a substantial previously unrecognized source of atmospheric ch 4 during deglaciation

high resolution seismic constraints on flow dynamics in the oceanic asthenosphere convective flow in the mantle and the motions of tectonic plates produce deformation of earth s interior and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wave speed this deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle including near seafloor spreading centres as new plates are formed via corner flow and within a weak asthenosphere that lubricates large scale plate driven flow and accommodates smaller scale convection seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes seafloor spreading fabric is very strong just beneath the crust mantle boundary the mohorovicic discontinuity or moho at relatively local scales but at the global and ocean basin scales oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere here we use rayleigh waves recorded across an ocean bottom seismograph array in the central pacific ocean the nomelt experiment to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere asthenosphere system in the middle of a plate we find that azimuthal anisotropy is strongest within the high seismic velocity lid with the fast direction coincident with seafloor spreading a minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low velocity zone and then increases with depth below the weakest portion of the asthenosphere at no depth does the fast direction correlate with the apparent plate motion our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis and via pressure driven or buoyancy driven flow within the asthenosphere shear associated with motion of the plate over the underlying asthenosphere if present is weak compared to these other processes

intraplate volcanism with complex age distance patterns a case for small scale sublithospheric convection 1 many volcano chains in the pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions one possible explanation for non hot spot intraplate volcanism is small scale sublithospheric convection ssc and we explore this concept using 3 d numerical models that simulate melting with rheology laws that account for the effects of dehydration ssc spontaneously self organizes beneath relatively mature oceanic lithosphere whenever this lithosphere is sufficiently young and thin ssc replaces the shallow layer of harzburgite which was formed by partial melting at the mid ocean ridge with fresh peridotite this mechanism enables magma generation without any preexisting thermochemical anomalies however the additional effect of melting induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous ssc overturn of the compositional stratification and related magmatism the intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of ssc into very shallow and cooler levels on the one hand such a restriction precludes high degrees of melting but on the other hand it slows asthenospheric cooling and thus prolongs the duration of melting to 25 ma volcanism over such an elongated melting anomaly continues for at least 10 20 ma and occurs on seafloor ages of 20 to 60 ma these seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid ocean ridge melting the long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains ssc thus gives an explanation for previously enigmatic volcano ages along the line islands and the gilbert and pukapuka ridges as well as along the individual subchains of the wakes marshalls and cook australs

paleozoic sourced petroleum systems of the western siberian basin what is the evidence abstract mesozoic petroleum systems in the west siberian basin have been studied extensively and are relatively well understood the two main source systems are the upper jurassic marine bazhenov suite j3 and the lower middle jurassic more terrigenously influenced togur tyumen suites j1 j2 most of the oils known to be generated from these sources are produced from mesozoic reservoirs however there is also production from paleozoic strata the petroleum systems related to these latter reservoirs are much less well understood in particular with respect to source and form the basis of this study the oils studied here are from paleozoic and jurassic reservoirs located in the western part of the basin and the southeastern area distinct sources were recognized on the basis of the general molecular composition of the whole oil and saturate and aromatic fractions in combination with detailed biomarker evaluation and isotopic composition comparison with other oils from the west siberian basin showed that the two main contributions were from the two well known jurassic sources however two samples from the southeast part show evidence of a pre jurassic source which could relate to the proterozoic carbonate petroleum system similar to that sourcing oils of east siberia

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

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

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

new constraints on the origin of the hawaiian swell from wavelet analysis of the geoid to topography ratio abstract analyzing the formation mechanism of hotspot swells enhances our understanding of intraplate volcanism and the underlying geodynamical processes the two main hypotheses for the origin of the archetypal hawaiian swell are thermal lithospheric thinning and dynamic support by an ascending plume any successful model would have to be able to simultaneously explain the swell topography and the corresponding geoid anomaly in simple models of isostatic compensation the geoid to topography ratio gtr is linearly related to the depth of the compensating mass therefore it is often considered a fundamental parameter to assess swell support mechanisms previous estimates for the geoid to topography ratio gtr of the hawaiian swell however are biased towards low values by incomplete removal of the effects of volcanic loading and lithospheric flexure in order to resolve these issues we here apply a continuous wavelet transform which allows resolution of lateral variations of the gtr at various spatial scales in a series of synthetic tests the robustness of this approach and its power to identify the origin of hotspot swells are established with 8 m km on the youngest part of the chain the recovered gtr agrees well with the predictions for dynamic support therefore ruling out thermal rejuvenation as an important mechanism we also find that the depth of the compensating mass decays by 20 km over a distance of 500 km from hawaii to kauai and identify sublithospheric erosion by small scale convection in the ponded plume material as a viable mechanism to support this decay

multiple oil families in the west siberian basin two major oil families are identified in the west siberian basin twenty six of 32 analyzed oils occur in jurassic and cretaceous reservoirs and are derived from anoxic marine upper jurassic bazhenov source rock based on geochemical comparison of oils and source rock extracts these oils are widely distributed both north and immediately south of the ob river and their biomarker ratios indicate a wide range of source rock thermal maturity from early to middle oil window van egan russkoye samotlor sovninsko sovyet olyenye ozynornoye and kogolym to peak oil window srednekhulym yem yegov vostochno surgut khokhryakov fedorov and urengoi to late oil window salym some of these oils have been mildly e g fedorov 75 to heavily e g russkoye biodegraded n the reservoir the bazhenov sourced oils show different compositions that support regional variations of organic facies in the source rock for example the anoxic marine bazhenov facies which generated the oils in the samotlor fedorov and nearby fields was particularly clay poor and sulfur rich six nonbiodegraded highly mature oils show geochemical characteristics that suggest they were derived from clastic rich lacustrine or nearshore marine source rocks dominated by terrigenous higher plant input like those in the lower to middle jurassic tyumen formation although no correlation was observed between the oils and a single rock sample yem yegov 15 from the formation the six oils occur in the tyumen taitym geologiche and cheremshan and fractured basement paleozoic gerasimov yagyl yakh and verchnekombar reservoirs in positions readily accessible to any oil migrating from the tyumen source rock for example at the gerasimov location the tyumen formation lies unconformably on weathered basement paleozoic reservoir rocks most of the probable tyumen sourced oils ar from south of the ob river but the occurrence of geologiche oil to the north suggests that related oils may be widespread in the basin

reaction kinetics of gas generation in selected source rocks of the west siberian basin implications for the mass balance of early thermogenic methane abstract about one third of the actual global recoverable natural gas reserves is accumulated in the west siberian basin in several super giant and mega giant as well as numerous smaller gas fields the elucidation of the nature of this gas its mechanisms of formation and accumulation are still a matter of discussion for instance it is still not yet clear whether the enormous dry gas accumulations in cenomanian reservoirs are of microbial early thermogenic or catagenetic metagenetic origin or if they represent a mixture of these possible formation mechanisms the present contribution provides new results on the thermal gas generation properties of selected source rocks of the cretaceous pokur formation in the west siberian basin eight selected source rock core samples of the pokur formation and a recent peat sample of the same area were submitted to open system programmed temperature pyrolysis in order to quantify the formation of individual gas compounds i e methane as well as c2 and c3 hydrocarbons the rock samples were selected according to geological age and maturity i e from the aptian albian and cenomanian with the most immature samples from the latter organic carbon contents comprise the whole range from 3 to more than 60 the elevated values representing typical lignites a reaction kinetic model system was applied to the generation curves of the gases resulting in activation energy distributions and the corresponding pre exponential arrhenius factors the kinetic data were combined with the temperature history of two selected wells of the urengoy area obtained from basin modelling in order to calculate the gas generation on the geological time scale it was found that even for the low temperatures occurring in the pokur formation early gas generation could have taken place to a certain yet small degree under the assumption of a respective catchment area and the accumulation of this gas in the reservoirs however methane quantities are in the order of magnitude of the urengoy gas accumulation thus early thermogenic methane could at least partially contribute to the gas accumulated in the reservoirs

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

methane sources and production in the northern cascadia margin gas hydrate system abstract the oceanographic and tectonic conditions of accretionary margins are well suited for several potential processes governing methane generation storage and release to identify the relevant methane evolution pathways in the northern cascadia accretionary margin a four site transect was drilled during integrated ocean drilling program expedition 311 the 13c values of methane range from a minimum value of 82 2 on an uplifted ridge of accreted sediment near the deformation front site u1326 1829 mbsl meters below sea level to a maximum value of 39 5 at the most landward location within an area of steep canyons near the shelf edge site u1329 946 mbsl an interpretation based solely on methane isotope values might conclude the 13c enrichment of methane indicates a transition from microbially to thermogenically sourced methane however the co existing co2 exhibits a similar trend of 13c enrichment along the transect with values ranging from 22 5 to 25 7 the magnitude of the carbon isotope separation between methane and co2 ec 63 8 5 8 is consistent with isotope fractionation during microbially mediated carbonate reduction these results in conjunction with a transect wide gaseous hydrocarbon content composed of 99 8 by volume methane and uniform dch4 values 172 8 that are distinct from thermogenic methane at a seep located 60 km from the expedition 311 transect suggest microbial co2 reduction is the predominant methane source at all investigated sites the magnitude of the intra site downhole 13c enrichment of co2 within the accreted ridge site u1326 and a slope basin nearest the deformation front site u1325 2195 mbsl is 5 at the mid slope site site u1327 1304 mbsl the downhole 13c enrichment of the co2 is 25 and increases to 40 at the near shelf edge site u1329 this isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13c enrichment the magnitude of the 13c enrichment of co2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate we suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation during burial thereby reducing the availability of metabolizable organic matter available for methane production this process is reflected in the occurrence and distribution of gas hydrate within the northern cascadia margin accretionary prism our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins

micro rhombic calcite and microporosity in limestones a geochemical study of the lower cretaceous thamama group u a e abstract euhedral micro rhombic calcite forms a microporosity network in limestones of the lower cretaceous thamama group u a e isotopic and trace element data indicate that micro rhomb formation was a two step process associated with diagenesis in meteoric waters the micro rhombic calcite and the micropores are both products of the second event sr fe mn and mg contents of the micro rhombs are all relatively low ranging from 108 to 310 ppm 34 to 308 ppm 3 to 32 ppm and 0 42 to 0 68 mole respectively the low sr and mg contents indicate extensive flushing of these constituents from the rock the low fe and mn contents are probably inherited from the original sediments and indicate the lack of any fe or mn in the pore fluids during micro rhomb formation the 13c of the micro rhombs 3 1 to 4 5 pdm are also primary unaltered by diagenesis the 13o of the micro rhombs 4 5 to 6 0 pdb reflect alteration by mateoric waters a large number of pore volumes and an open water buffered system would be necessary to flush sr and mg yet decreasing 18o with increasing porosity indicates that 18o values in non porous intervals are partially rock buffered two diagenetic events are thus implicated the first flushing sr and mg and lowering 18o in all rocks and the second event actually forming the micro rhombs and further altering 18o as a function of porosity the first diagenetic event was most likely mineralogical stabilization in an unconfined meteoric aquifer immediately after shuaiba deposition if mineralogical stabilization was completed in the first event then calcite to calcite recrystallization is implicated for the second event the second event may have occurred in the unconfined meteoric aquifer or it may have occurred in a confined meteoric aquifer during the early late cretaceous

statistical correlation between geophysical logs and extracted core the precise correlation of extracted core to geophysical borehole logs is often problematic in intervals in which no sufficiently large or distinctive features span the gap between the outer core surface and the borehole wall this impairs our ability to use techniques that require accurate correlation as a prerequisite e g to develop field based upscaling and downscaling relationships of porosity distributions at or below log resolution we propose a new method for such situations in which we correlate a cascade of statistics of core features with those of image logs taken from the borehole wall each statistic is used individually to calculate a likelihood function of possible correlation locations these results are combined using a joint likelihood function and with other prior information using bayesian techniques to bring all available information to apply to the final correlation solution the technique is demonstrated with computerized tomography for a core section and image log data extrac

gas generation and accumulation in the west siberian basin discussion littke et al 1999 present a novel hypothesis for the genesis of the supergiant gas accumulations in the northern west siberian basin through exsolution of gas from formation water some problems are attached to their interpretation the principal of which are discussed in the following paragraphs 1 littke et al 1999 discard supply of thermal gas from the jurassic because of the pressure differential between that section and the overlying cretaceous their figure 4 most unlikely however is that the high pressure in the deeper section is caused by undercompaction given the slow to moderate rate of deposition during the jurassic cretaceous and the long time span since deposition coupled to the abundance of permeable sands providing drains for the compaction water even in the more shaly sections achimov type sands much more likely is that the overpressure is generated by kerogen to oil gas conversion

small scale convection in a plume fed low viscosity layer beneath a moving plate two dimensional simulations using a thermomechanical model based on a finite difference method on a staggered grid and a marker in cell method are performed to study the plume lithosphere interaction beneath moving plates the plate and the convective mantle are modelled as a homogeneous peridotite with a newtonian temperature and pressure dependent viscosity a constant velocity ranging from 5 to 12 5 cm yr 1 is imposed at the top of the plate plumes are generated by imposing a thermal anomaly of 150 to 350 k on a 50 km wide domain at the base of the model 700 km depth the plate atop this thermal anomaly is 40 myr old we analyse 1 the kinematics of the plume as it impacts the moving plate 2 the dynamics of time dependent small scale convection ssc instabilities developing in the low viscosity layer formed by spreading of hot plume material at the base of the lithosphere and 3 the resulting thermal rejuvenation of the lithosphere the spreading of the plume material at the base of the lithosphere characterized by the ratio between the maximum down and upstream horizontal dimensionless velocities in the plume fed sublithospheric layer peup pedown depends on the ratio between the maximum plume upwelling velocity and the plate velocity peplume peplate for fast plate velocities and sluggish plumes low peplume peplate plate motion drags most plume material and downstream flow is dominant as peplume peplate increases an increasing part of the plume material flows upstream ssc systematically develops in the plume fed sublithospheric layer downstream from the plume onset time of ssc decreases with the rayleigh number for vigorous plumes it does not depend on plate velocity for more sluggish plumes however variations in the plume spreading behaviour at the base of the lithosphere result in a decrease in the onset time of sscs with increasing plate velocity in any case ssc results in uplift of the isotherm 1573 k by up to 20 km relative to its initial equilibrium depth at the impact point

thermal and maturation modeling of the urengoy field west siberian basin some special considerations in basin modeling present day temperature and vitrinite reflectance ro profiles are considered key factors for calibration in numerical basin modeling therefore it is important to understand how such profiles are generated basin modeling typically considers factors such as rock consolidation depth variation of petrophysical characteristics changes in heat flow and paleoclimate however our basin evaluations have shown that other processes that influence temperature and ro vitrinite reflectance profiles commonly are inadequately studied modeling within the urengoy field of the west siberian basin has allowed numerical estimates of some of these nonstandard processes first a substantial increase in temperature up to 5 10ic is expected from changes in heat conductivity due to the formation of significant free gas gas condensate and condensate second climate variations creating numerous periods of formation and degradation of permafrost zones for the last 3 4 m y could decrease the modern rock temperatures by 10 17ic in the upper 1 5 km of the sedimentary section and up to 10ic at its base due to the short duration of these two processes their effect on organic matter maturation is minimal nevertheless their roles in the formation of present day temperature profiles necessitate their inclusion in basin modeling third modeling results demonstrated that change in heat conductivity of rocks with dispersed organic matter can increase temperature of sedimentary rocks by 3 5ic and rock maturity by not more than 0 02 ro finally the sharp increase in vitrinite reflectance ro with depth typical for the deep sedimentary units of the urengoy field and many other continental rift basins is possible to explain from hydrothermal activity the influence of these processes on subsurface temperatures and maturation of organic matter can vary significantly depending on lithology organic matter content and the location of the modeled section relative to the ancient rift system the results of this study can help one estimate the relative influence of these parameters

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system

extensive microbial modification of formation water geochemistry case study from a midcontinent sedimentary basin united states the upper devonian antrim shale in the michigan basin is an economically significant source of microbially produced methane along the basin margins where meteoric recharge has been focused oxygen and hydrogen stable isotope compositions of antrim formation waters show that fresh waters recharged from pleistocene glaciation and modern precipitation suppressed basinal brine salinity to great depths and enhanced methanogenesis this paper presents results of integrated elemental and isotope analyses of antrim shale formation waters from the margins and center of the michigan basin focusing on solute sources and geochemical modifications associated with regionally extensive microbial methanogenesis cl br na systematics reveal that salinity is controlled not only by mixing between variable amounts of basinal brine and meteoric water but also by halite dissolution where fluids recharged through underlying devonian carbonate aquifers with localized evaporite deposits divalent cations carbonate system parameters and carbon isotope compositions of dissolved inorganic carbon have been systematically and profoundly altered by microbial methanogenesis large decreases in formation water ca mg and ca sr ratios accompany increasing carbonate alkalinity values in areas with high rates of microbial gas production thermodynamic and reaction path modeling show that these changes are consistent with calcite precipitation during progressive microbial methanogenesis similar variations in fluid chemistry are evident in databases from other sedimentary basins containing black shales and coal beds associated with microbial gas microbial methanogenesis may play an important role in the geochemical evolution of divalent cation relationships in crustal fluids and should be considered in models of formation water origin and evolution

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

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

large aspect ratio convection cells in the upper mantle summary numerical experiments which model a possible large scale flow in the upper mantle are described the lithospheric plates are represented by a constant velocity boundary condition on the upper surface of a twodimensional convecting layer the transition from small scale multiple cell flow to single cell flow is described in terms of the rayleigh number and the peclet number the mean flow field is successfully compared with a simple parameterized model in which flow is driven by the moving upper boundary and by a mean horizontal temperature gradient which must be present in any large aspect ratio cell bathymetry and gravity profiles are calculated for the long cells and it is shown that in the absence of a low viscosity zone a realistic variation of bathymetry and gravity is only obtained if the flow in the cell is being driven predominantly by the horizontal temperature gradient rather than the moving plate mean horizontal temperature gradients in the upper mantle of 0 025 c km are suggested beneath a fast moving plate such as the pacific transverse boundary layer instabilities in the large cells are shown to be governed by a critical boundary layer rayleigh number which linearly increases with the boundary layer pcclet number

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

secondary biological coalbed gas in the xinji area anhui province china evidence from the geochemical features and secondary changes abstract in order to ascertain the origin of the coalbed gas in the xinji area anhui province of china the paper examined the geochemical features and secondary changes of ch4 c2h6 co2 and n2 from the coalbed gas the related gas composition carbon isotope and tracer geochemical data are as follows 0 993 to 1 0 for c1 c1 n 188 6 to 2993 7 for c1 c2

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

onset of small scale instabilities at the base of the lithosphere scaling laws and role of pre existing lithospheric structures summary numerical simulations of 2 d rayleigh benard convection are designed to study the devel opment of convection at the base of the cooling lithosphere a zero temperature is suddenly imposed at the mantle surface which has initially a homogeneous temperature the strongly temperature dependent viscosity fluid is heated from within in order to balance the internal temperature drift resulting from global fluid cooling for a while the lithosphere cools approx imatively as a conductive half space and the lithospheric isotherms depth grows as the square root of age as instabilities progressively develop at the base of the lithosphere lithospheric cooling departs from the half space model we propose two different parametrizations of the age of the first dripping instability using boundary layer marginal stability or quantifying the characteristic timescale of the instability exponential growth as a function of the rayleigh number and of the viscous temperature scale both parametrizations account very well for our numerical estimates of onset times but with a slightly better adjustment of the viscous tem perature scale dependence in the second case the absolute value of the onset time depends on the amplitude and location of initial temperature perturbations within the box and on the initial temperature structure of the thermal boundary layer tbl furthermore thermal perturba tions of finite amplitude located within the lithosphere such as the ones induced by transform faults for example strongly reduce the age of the first dripping instability however the onset time parametrization derived from transient cooling experiments well adjusts the lithospheric age of the first drip instability below a lithosphere cooling perpendicularly to the ridge axis this study emphasizes the role of the initial topography of the lithospheric isotherms on the development of instabilities

petroleum migration alteration and remigration within troll field norwegian north sea troll field represents the largest petroleum discovery within the entire north sea area in oil equivalents with 74 of the accumulated petroleum present as dry gas and 26 as a heavy biodegraded oil leg the field is divided into several provinces based on the distribution of gas and oil and the gas and oil have been suggested to be cogenetic the migration and filling model presented in this paper suggests that the oil and gas represent two different migration phases and that gas migration and filling predate oil emplacement two different oil populations have been characterized and mapped in troll field applying conventional geochemical techniques we suggest that the two oil populations migrated into the structure through two different migration systems oil and gas were subsequently biodegraded within the reservoir the two oil populations have been found in neighboring oil and gas discoveries and an oil oil correlation with these discoveries has been used to determine the location of field filling points and regional migration routes when oil biodegradation terminated fresh oil continued to migrate into the reservoir and mixed with the residue of the biodegraded oil the field was tilted downward to the west in the neogene and oil and gas remigrated within the field with a possible spillage of gas tilting resulted in a dominantly upward movement of the oil phase whereas gas migrated laterally residual oils in the water zone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received july 18 1995 revised manuscript received march 14 1996 final acceptance august 16 1996 2saga petroleum a s kjorboveien 16 p o box 490 n 1301 sandvika norway 3newcastle research group drummond building university of newcastle upon tyne newcastle upon tyne ne1 7ru united kingdom we would like to thank the partners in the troll license who allowed us to publish these results per erling johansen and tone molvik are acknowledged for the analytical work and snorre olaussen nigel mills and terje hellem are thanked for helpful discussions and for assistance with the back tilting of the irap reservoir field model jill sonrier is acknowledged for typing we acknowledge k m wolgemuth john f brint and two anonymous reviewers for helpful comments that improved the paper significantly

gas generation and accumulation in the west siberian basin we appreciate the discussion by murris 2001 and take the opportunity to clarify some of his objections and discuss some of the still open questions in more detail murris s main objections to the conclusions of littke et al 1999 concern first the influence of overpressure in the lower cretaceous and upper jurassic on hydrocarbon distribution and second the model of gas release by exsolution degassing from deep ground water as the main mechanism for accumulating the huge gas fields of northern west siberia the first objective of murris s discussion is how the overpressure in jurassic strata might have influenced the distribution of hydrocarbons in the overlying cretaceous sediments two crucial points regarding this question are the reason and timing of overpressuring and its influence on hydrocarbon movement we agree with murris that the effect of compaction disequilibrium is not sufficient to explain the extent of overpressure in the urengoy area and that hydrocarbon generation in the bazhenov formation has contributed to the buildup of overpressure two dimensional basin modeling along a seismic line through the urengoy field however showed that hydrocarbon generation in jurassic source rocks bazhenov and the tyumen formations did only generate sufficient overpressure to explain the recent situation if the models were laterally completely sealed this sealing appears to be highly unlikely furthermore hydrocarbon generation ceased during the miocene when the tectonic movement started to lift the source rocks into shallower depths where lower temperatures were reached thus any overpressure caused by petroleum generation must have been preserved over a period of about 5 20 m y in addition the overpressure is not restricted to the jurassic bazhenov formation but extends into the cretaceous including the achimov sands uplift might have contributed to the overpressure however maximum pore pressure from the stage of deepest burial might have

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

computer axial tomography in geosciences an overview abstract computer axial tomography cat is one of the most adequate non destructive techniques for the investigation of the internal structure of a large category of objects this method based on the attenuation of x or gamma ray generates digital images which map the numerical values of the linear attenuation coefficient of a section or of the entire volume of the investigated sample depending on the energy of the utilized radiation as well as on the effective atomic number of the sample cat can furnish with a spatial resolution of 0 01 0 5 mm quantitative as well as qualitative information concerning local density or chemical composition of the sample in geosciences cat has been successfully applied to the investigation of bulk density moisture content or macroporosity of the soil soil plant water system oil sand or shales small scale heterogeneities in siltstone or sandstone diamonds in kimberlite mineralization layers in manganese nodules local structure of marine sediments or coal composition the method has also been used for the investigation of the rock mechanical properties rock permeability fracture network two or three phase flow or residual oil distribution in carbonate cores a review of the actual applications of cat in soil science sedimentology coal geology as well as rock mechanics is presented all these applications reveal the great potential of cat in geoscience investigations

biogenic methane generation in the degradation of eastern australian permian coals abstract hydrocarbons extracted from one sydney basin coal have a distribution of aromatic and aliphatic hydrocarbons consistent with a high degree of biodegradation as previously demonstrated for bowen basin coals on the same basis hydrocarbons from two other coals from the sydney basin are less biodegraded and display an additional minor series of alkylaromatic degradation products all these degradation reactions generate co2 which on biogenic reduction to ch4 is reservoired in association with the permian coal seams of the sydney and bowen basins australia variations in access to oxygenated waters nutrients and micro organisms are held to be largely responsible for the extent of biodegradation of coal hydrocarbons and for the reduction of co2

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