stable carbon isotopes of coal derived gases sourced from the mesozoic coal measures in china coal derived large scale gas fields derived from the mesozoic coal measures in china are mainly distributed in the middle lower jurassic coal measures in the tarim junggar and turpan hami basins in northwest china and the upper triassic xujiahe formation coal measure in sichuan basin central china in 2011 the annual production was 21 6 109 m3 and the proved geological reserves were 2485 109 m3 accounting for 21 and 30 of the total in china respectively based on analyses of gas composition and stable carbon isotopes ratios of 203 samples and stable carbon isotopes of 102 co2 samples the following conclusions were made a based on diagnostic plots using the stable carbon isotopic and molecular composition of gas samples alkane gas from the mesozoic coal measures in china is shown to be coal derived b according to the 13c2 vs c2h6 plot of a great number of oil derived and coal derived gases in china it is concluded that gases with 13c2 28 5 are coal derived and those with 13c2 28 5 are oil derived in most cases c among the natural gases from the mesozoic coal measures in china primary coal derived gases with normal carbon isotopic distribution pattern among the c1 c4 alkanes i e 13c1 13c2 13c3 13c4 are dominant d carbon isotopic pattern reversal mainly results from the mixing of coal derived gases having different maturities but the same source and secondly from microbial oxidation of propane e g mu 3 and mu 4 wells in the gumudi gas field junggar basin e co2 in the coal derived gases from the mesozoic coal measures in china has both biogenic and abiogenic origins the biogenic origin is dominant and the abiogenic co2 is mainly found in the kuqa depression in the tarim basin and western sichuan basin f isotopic differences between heavy hydrocarbon gases and methane become less with increasing maturity

youngest volcanism about 1 million years ago at kahoolawe island hawaii abstract young volcanic deposits in kahoolawe island cutting up through the caldera filling lava colluvium and talus in the west wall of kanapou bay had long been stratigraphically considered the rejuvenated stage products new k ar ages combined with magnetic polarity data show that young volcanism was at about 0 98 1 04 ma and indicate no substantial quiescence between the filling of the caldera and the young volcanism this result and the tholeiitic characteristics of the young deposits suggest they are a component of late shield stage volcanism

age and petrology of alkalic postshield and rejuvenated stage lava from kauai hawaii at the top of the waimea canyon basalt on the island of kauai rare flows of alkalic postshield stage hawaiite and mugearite overlie tholeiitic flows of the shield stage these postshield stage flows are 3 92 ma and provide a younger limit for the age of the tholeiitic shield stage the younger koloa volcanics consist of widespread alkalic rejuvenated stage flows and vents of alkalic basalt basanite nephelinite and nepheline melilitite that erupted between 3 65 and 0 52 ma all the flows older than 1 7 ma occur in the west northwestern half of the island and all the flows younger than 1 5 ma occur in the east southeastern half the lithologies have no spatial or chronological pattern the flows of the koloa volcanics are near primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet bearing source that has about two thirds the concentration of p2o5 rare earth elements and sr of the source of the honolulu volcanics on the island of oahu the same lithology in the koloa and honolulu volcanics is generated by similar degrees of partial melting of distinct source compositions the lavas of the koloa volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt respectively phases that remain in the residue of the honolulu volcanics such as rutile and phlogopite are exhausted during formation of the koloa volcanics at all but the smallest degrees of partial melting the mantle source for kauai lava becomes systematically more depleted in 87sr 86sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage at the same time the lavas become systematically more enriched in incompatible trace elements on a shorter timescale the lavas of the koloa volcanics display the same compositional trends but at a lower rate of change the source characteristics of the koloa volcanics considered along with those of the honolulu volcanics support a mixing model in which the source of rejuvenated stage lava represents large percent melts of a plume source mixed with small amounts of small percent melts of a heterogeneous mid ocean ridge source

isotopic and ree studies of lunar basalt 12038 implications for petrogenesis of aluminous mare basalts we report sr nd and sm isotopic studies of lunar basalt 12038 one of the so called aluminous mare basalts a precise internal rb sr isochron yields a crystallization age of 3 35 0 09 ae and initial87sr 86sr 0 69922 2 2 error limits 1ae 109 years 87rb 0 0139ae 1 an internal sm nd isochron yields an age of 3 28 0 23ae and initial143nd 144nd 0 50764 28 present day143nd 144nd is less than the chondritic value i e nd 0 2 3 0 4 where nd is the deviation of143nd 144nd from chondritic evolution expressed as parts in 104 at the time of crystallization nd 3 2ae 1 5 0 6 we have successfully modeled the evolution of the sr and nd isotopic compositions and the ree abundances within the framework of our earlier model for apollo 12 olivine pigeonite and ilmenite basalts the isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite normalized ree pattern of constant negative slope chondrite normalized la yb 2 2 for this hypothetical magma ocean pattern a plot of i sr versus nd for the apollo 12 basalts clearly shows the influence of varying proportions of olivine clinopyroxene orthopyroxene and plagioclase in the basalt source regions a small percentage of plagioclase 5 in the 12038 source apparently is responsible for low i sr and nd in this basalt aluminous mare basalts from mare crisium luna 24 and by inference mare fecunditatis luna 16 occupy locations on the i sr nd plot similar to that of 12038 implying that some basalts from three widely separated lunar regions came from plagioclase bearing source regions a summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when ree abundances in the lunar magma ocean increased from 20 chondritic to 100 chondritic although there is a general trend from olivine to clinopyroxene dominated source regions with progressive magma ocean evolution significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution as evidenced in particular by the three apollo 12 magma types

origin of lead from green glass of apollo 15426 a search for primitive lunar lead a major obstacle in lunar u pb chronology is the elusiveness of the primary pb isotopic composition and u pb ratio and therefore the pb evolution for the early history of the moon in an attempt to seek the primitive lunar pb isotopic composition green glass from lunar clod 15426 49 was studied for u th pb systematics because it is extremely mg rich and known to be the most primitive among sampled lunar volcanic rocks green glass and yellow impact glass were handpicked washed with water then washed in an acid mixture 0 3 n hbr 0 2 n hf and the washes and residues were analyzed in contrast to the pb from apollo 17 orange glass pb from the surface of the green glass spherules is totally different from that of the spherule interior but is similar to lunar surface lead data for the interior pb yield a 207pb 206pb age of 3 41 ga because of the low pb concentration and high u pb ratio observed for the interior the initial pb was poorly defined nevertheless the data indicate that lunar pb evolved in an environment with 238u 204pb 19 55 which is considerably lower than those for mare basalts around 300 but higher than values for the earth 6 8

postshield volcanism and catastrophic mass wasting of the waianae volcano oahu hawaii the 3 9 to 2 9 ma waianae volcano is the older of two volcanoes making up the island of oahu hawaii exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas the postshield alkalic cap consists of aphyric hawaiite of the palehua member of the waianae volcanics overlain unconformably by a small volume of alkalic basalt of the kolekole volcanics kolekole volcanics mantle erosional topography including the uppermost slopes of the great lualualei valley on the lee side of the waianae range twenty new k ar dates combined with magnetic polarity data and geologic relationships constrain the ages of lavas of the palehua member to 3 06 2 98 ma and lavas of the kolekole volcanics to 2 97 2 90 ma the geochemical data and the nearly contemporaneous ages suggest that the kolekole volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity thus the kolekole volcanics are reduced in rank becoming the kolekole member of the waianae volcanics magmas of the palehua and kolekole members have similar incompatible element ratios and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice however lavas of the kolekole member are less fractionated and appear to have evolved at greater depths than the earlier palehua hawaiites postshield primary magma compositions of the palehua and kolekole members are consistent with formation by partial melting of mantle material of less than 5 10 relative to waianae shield lavas within the section of palehua member lavas an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1 2 this trend is reversed with the onset of eruption of kolekole member lavas where an increase in extent of partial melting is indicated the relatively short time interval between the eruption of palehua and kolekole member lavas appears to date the initial formation of lualualei valley which was accompanied by a marked change in magmatic conditions we speculate that the mass wasting event separating lavas of the palehua and kolekole members may be related to the formation of a large submarine landslide west and southwest of waianae volcano enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1 2 in extent of melting between the youngest palehua magmas and the posterosional magmas of the kolekole member the association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass wasting events and subsequent magmatism in hawaiian volcano evolution

late and deep fermentative methanogenesis as reflected in the h c o s isotopy of the methane water system in deep aquifers of the pannonian basin se hungary abstract h and o isotope ratios and h and c isotope ratios of methane as well as the abundances of some major and minor dissolved components were measured in 26 subsurface waters from se hungary produced from late neogene aquifers within the upper two kilometres in depth mixing with meteoric water of glacial age is of importance the hydrocarbon gas hc gas content of the great majority of the waters is dominated by isotopically light bacterial methane the depth of the onset of thermal gas generation varies from 0 7 to 1 2 km but the amount of thermogenic methane leaving the kerogen was not sufficient to overprint the light isotopic signature of the methane in the waters about two thirds of the waters group i samples are characterised by low sulfate content 20 and methane d values less negative than 260 they also show a direct relationship between the h isotope ratio of the methane and that of the water these waters experienced exhaustive bacterial sulfate reduction followed by a relatively shallow bacterial methanogenesis in a third of the waters group ii samples the methane d values are more negative than 278 the group ii samples which are not affected by an admixture of glacial age meteoric waters contain a relatively high amount of sulfate 35 45 mg l and do not show a direct relationship between the h isotope ratio of the methane and that of the water in group ii waters bacterial sulfate reduction was not exhaustive and the residual sulfate inhibited shallow bacterial methanogenesis as a result of continuing burial the aquifers were heated up to 40 60 c thereby allowing a relatively intensive bacterial acetogenesis followed by acetate fermentation to methane our results show that deep acetate fermentation and early thermal gas generation can occur closely in space and time even partly overlapping with each other

impacts of hydrothermal dolomitization and thermochemical sulfate reduction on secondary porosity creation in deeply buried carbonates a case study from the lower saxony basin northwest germany the role of deep burial dissolution in the creation of porosity in carbonates has been discussed controversially in the recent past we present a case study from the upper permian zechstein 2 carbonate reservoirs of the lower saxony basin in northwest germany these reservoirs are locally characterized by high amounts of carbon dioxide co2 and variable amounts of hydrogen sulfide h2s which are derived from thermochemical sulfate reduction tsr and inorganic sources to study the contribution of these effects on porosity development we combine petrography stable isotope and rare earth and yttrium rey analyses of fracture cements with raman spectroscopy and 13c analyses of fluid inclusions it is shown that fluid migration along deep fault zones created and redistributed porosity fluid inclusion analyses of vein cements demonstrate that hydrothermal fluids transported inorganic co2 into the reservoir where it mixed with minor amounts of tsr derived organic co2 the likely source of inorganic co2 is the thermal decomposition of deeply buried devonian carbonates the rey distribution patterns support a hydrothermal origin of ascending iron and co2 rich fluids causing dolomitization of calcite and increasing porosity by 10 16 along fractures this porosity increase results from hydrothermal dolomitization and dissolution by acids generated from the reaction of fe2 with h2s to precipitate pyrite in contrast hydrothermal dolomite cements reduced early diagenetic porosity in dolomitic intervals by approximately 17 however the carbonate dissolution in the predominantly calcitic host rock results in a net increase in porosity and permeability in the vicinity of the fracture walls which has to be considered for modeling reservoir properties and fluid migration pathways

carbon isotope geochemistry of natural gases from the green tuff basin japan carbon isotopic analyses of c1 c4 hydrocarbons were carried out to investigate the origin and formation mechanism of natural gases commercially produced from volcanic rocks as well as sedimentary rocks in the green tuff basin of northeast japan whose high 3he 4he ratios have suggested a possible magmatic origin 13c pdb values of methane ethane propane and n butane range from 54 4 to 33 3 from 32 5 to 20 9v from 27 7 to 20 6 and from 25 3 to 20 5 respectively all of which fall within the ranges of published data of worldwide gases of biogenie origin most samples show the isotope distribution pattern wherein the 13c values become less negative with increasing molecular weight of the hydrocarbons suggesting that the c2 hydrocarbons were not derived by methane polymerization the 13c values of c2 c4 hydrocarbons are explained by thermogenic breakdown of organic matter whereas the 13c values of methane are more or less lower than those expected from the same origin these results indicate that the gases from the green tuff basin consist of thermogenic hydrocarbons plus variable amounts of isotopically lighter microbial methane significant contributions of abiogenic methane which would be isotopically heavier than the thermogenic methane are not plausible for the gases

origin of oil and gas occurrences in the pliocene sediments of the pannonian basin hungary abstract considerable volumes of oil and gas in the pannonian basin have been discovered in pliocene sediments having good reservoir characteristics the pliocene sediments belonging to delta facies contain quantitatively very good but immature source rocks at depths of 0 5 2 km the origin of the oil and gas accumulations in the pliocene reservoirs is very different due to diverse conditions of hydrocarbon generation and secondary migration the aim of this paper is to establish the origin of oil and gas trapped in pliocene reservoirs in order to have a picture of the migration and accumulation systems different cases examined by means of the geochemical characteristics of oil and gases indicate that the pliocene reservoirs contain indigenous dry gases generated by bacterial and early catagenetic processes and vertically migrated hydrocarbon rich wet gases associated with oil and carbon dioxide rich gases derived from high temperature zones of the earth crust or from volcanic bodies

some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins large volumes of carbon dioxide i e up to 90 v v have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the co2 including mantle degassing metamorphic reactions or magmatic processes the formation of large accumulations of large volumes of co2 in sedimentary basins is discussed with specific reference to published data from the pannonian basin hungary the cooper eromanga basin australia and the south viking graben north sea these basins have occurrences of large volumes i e over 15 v v of isotopically heavy carbon dioxide thought to be of inorganic origin and although they differ somewhat in terms of sedimentary fill all have major structural features close to the gas accumulations the available geochemical data are currently neither sufficient to confirm the specific origin s of the co2 e g mantle derived carbonate metamorphism nor unscramble co2 mixed from two or more sources ongoing studies especially those involving noble gas analyses should provide fuller answers

composition and isotopic constraints on the petrogenesis of alkaline arc lavas lihir island papua new guinea the sio2 undersaturated lavas from lihir island papua new guinea like most arc lavas are highly enriched in sr ba k rb and cs and depleted in hf ta nb and ti relative to ocean floor basalts and oceanic island basalts these alkali rich lavas have arc trace element signatures and nd sr and pb isotopic systematics however they are not a product of present day subduction as this volcanism has tapped mantle which was enriched by prior subduction episodes the narrow range of pb 206pb 204pb 18 74 18 76 and nd 143nd 144nd 0 51297 0 51304 isotopic compositions suggest a cogenetic origin for these lavas during the fractionation of the primitive lihir lavas elements normally considered incompatible i e the light rare earth elements lree rb th and p have high bulk solid melt partition coefficients 0 15 1 5 relatively higher partition coefficients during forma tion of the evolved lavas produced crossing rare earth element ree patterns and primitive lavas have higher incompatible elements abundances than evolved lavas this results from 1 changes in the amount of apatite in the fractionating assemblage and 2 the abnormal partitioning of trace elements into apatite microphenocrysts which nucleate at the crystal liquid interface of rapidly growing clinopyroxene phenocrysts the lihir lavas have lower alkali sr ba k rb cs and lree abundances than other tabar feni lavas they are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands the similarity of sm nd ratios of these undersaturated arc lavas to those of tholeiitic and calc alkaline arc lavas and the moderate chondrite normalized la yb la ybcn 3 7 indicates that there has been limited enrichment of the lree relative to the heavy ree during generation of the arc modified source mantle the alkaline nature of these lavas reflects their generation in a tensional tectonic environment from a fossil arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes

petrology of ultramafic and mafic xenoliths in picrite of kahoolawe island hawaii a picrite lava 22 wt mgo 35 vol ol along the western shore of the 1 3 1 4 ma kahoolawe tholeiitic shield hawaii contains small xenoliths of harzburgite lherzolite norite and wehrlite the various rock types have textures where either orthopyroxene clinopyroxene or plagioclase is in a poikilitic relationship with olivine the mg s of the olivine orthopyroxene and clinopyroxene in this xenolith suite range between 86 and 82 spinel mg s range from 60 to 49 and plagioclase is an75 80 a 87sr 86sr ratio for one ol norite xenolith is 0 70444 in comparison the host picrite has olivine phenocrysts with an average mg of 86 2 range 87 5 84 5 and a whole rock 87sr 86sr ratio of 0 70426 textural and isotopic information together with mineral compositions indicate that the xenoliths are related to kahoolawe tholeiitic magmatism but are not crystallization products of the magma represented by their host picrite rather the xenoliths are crystalline products of earlier primitive liquids feo mgo ranging 1 to 1 3 at 5 9 kbar in the cumulate environment of a magma reservoir or conduit system the presence of ultramafic xenoliths in picrite but not in typical kahoolawe tholeiitic lava 6 9 wt mgo is consistent with replenishment of reservoirs by dense mg rich magma emplaced beneath resident less dense tholeiitic magma mg rich magmas have proximity to reservoir cumulate zones and are therefore more likely than fractionated residual liquids to entrain fragments of cumulate rock

petrogenesis of alkaline basalts from socorro island mexico trace element evidence for contamination of ocean island basalt in the shallow ocean crust postcaldera mafic volcanism on socorro island mexico an island located in the eastern pacific ocean on a segment of a mid ocean ridge spreading center abandoned at 3 5 ma dominantly comprises alkalic basalt hawaiite and mugearite with subordinate mildly alkalic transitional basalt most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50 fractionation of plagioclase clinopyroxene olivine fe ti oxides high field strength element abundances are consistent with this interpretation and the narrow range in zr nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mantle source sr and nd isotopic ratios exhibit relatively narrow ranges 0 7031 0 7032 0 5128 0 5130 respectively despite this a subset of the alkalic basalts has negative ce anomalies and abundances of p2o5 ba y and some rare earth elements in excess of those predicted by fractional crystallization models these chemical features require open system processes the observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir regional mantle heterogeneity is evident from elemental and isotopic data for mildly alkalic transitional basalt from socorro island mugearite from san benedicto island and submarine basalts from near socorro island evidence presented here indicates that compositional variations in ocean island basalts can be introduced by ocean crustal contamination and underscores the importance of considering this source of chemical heterogeneity in basalts before characterizing associated mantle

petroleum geochemistry of the deep part of the drava depression croatia the drava depression is located in the southwestern margin of the pannonian basin the northern zone of the depression is called the deep drava geochemical data imply that in the northwestern part of deep drava the mudstones carbonate fossiliferous mudstones compact marls and siltites of lower miocene to badenian age at the depths of more than 3000 m are petroleum source rocks the organic matter is mostly of terrigeneous origin with local occurrences of hydrogen rich terrigeneous components the source rocks have reached a high level of thermal alteration in the condensate gas window between 3150 and 3800 m hydrocarbons in accumulations are interpreted to be migrated thermogenic condensate associated gases and their compositions are consistent with the facies type and maturity of the source rocks mineralogical composition of reservoir rocks and specific thermodynamic conditions control the formation of co2 and h2s the high co2 concentrations mainly result from the thermal breakdown of carbonates while h2s is the product of thermochemical sulphate reduction from anhydrite in the presence of hydrocarbons

temperature dependent viscoelastic compaction and compartmentalization in sedimentary basins the near surface compaction regime of most sedimentary basins is characterized by hydrostatic fluid pressures and is therefore determined entirely by sediment matrix rheology within this regime compaction is initially well described by a pseudoelastic rheological model with increasing depth precipitation dissolution processes lead to thermally activated viscous deformation the steady state porosity profile of the viscous regime is a function of two length scales the viscous e fold length related to the compaction activation energy and a scale determined by the remaining parameters of the sedimentary process overpressure development is weakly dependent on the second scale for activation energies 20 kj mol application of the steady state model to pannonian basin shales and sandstones indicates a dominant role for viscous compaction in these lithologies at porosities below 10 and 25 respectively activation energies and shear viscosities derived from the profiles are 20 40 kj mol and 1020 1021 pa s at 3 km depth the analytical formulation of the compaction model provides a simple method of predicting both the depth at which permeability limits compaction resulting in top seal formation and the amount of fluid trapped beneath the top seal fluid flow during hydraulically limited compaction is unstable such that sedimentation rate perturbations or devolatilization cause nucleation of porosity waves on the viscous e fold length scale 0 5 1 5 km the porosity waves are characterized by fluid overpressure with a hydrostatic fluid pressure gradient and propagate through creation of secondary porosity in response to the mean stress gradient the waves are a mechanism of episodic fluid expulsion that can be significantly more eycient than uniform darcyian fluid flow but upward wave propagation is constrained by the compaction front so that the waves evolve into essentially static domains of high porosity following cessation of sedimentation yielding mechanisms do not appreciably alter the time and length scale of episodic fluid flow because fluid expulsion is ultimately controlled by compaction the flow instabilities inherent in viscous compaction are similar to and a possible explanation for fluid compartments 2000 elsevier science b v all rights reserved

do co2 charged fluids contribute to secondary porosity creation in deeply buried carbonates abstract carbonate dissolution by acidic co2 charged fluids is one possible mechanism leading to the creation of burial porosity the late permian carbonates of the lower saxony basin nw germany are a suitable target to study the influence of such co2 charged fluids on porosity evolution the so called zechstein 2 carbonates ca2 consist of alternating dolomitic and calcitic intervals and contain locally very high amounts of co2 80 and variable amounts of h2s in reservoir gas and vein cement hosted fluid inclusions composition and isotope data from fluid inclusions suggest that the co2 is sourced from the thermal decomposition of underlying devonian carbonates far reaching fault systems connect the devonian co2 source with the permian ca2 reservoir while thick rock salt deposits above the zechstein 2 carbonates prohibit an influence of surface derived meteoric fluids the aim of the study is to evaluate the contribution of co2 on carbonate dissolution seven wells were preferentially sampled along mineral veins which are interpreted as the preferred migration pathways for the present gas classic petrography methods such as transmitted light microscopy and cathodoluminescence were combined with fluid inclusion analysis and porosity data petrographic analysis reveals no great impact of the massive co2 presence on porosity creation as fluids get quickly saturated with respect to ca2 minor vuggy porosity is associated with fractures in only one of the studied wells but the dominant porosity types are early diagenetic fabric selective oomoldic and intercrystal porosity to evaluate whether the porosity values are higher than usual a comparison with a global porosity dataset is included which however shows no increase in average porosities compared to global values the results of this case study suggest that the contribution of co2 alone on carbonate dissolution and therefore reservoir quality is negligible as long as other processes such as late dolomitization or pyrite formation by thermochemical sulphate reduction derived h2s are absent

isotope techniques for tracing migration of gases in sedimentary basins the isotopic composition of methane is primarily dependent on the stage of maturation of organic matter present in the hydrocarbon source rocks and is not changed during migration the isotopic composition of in situ methane can be estimated from the maturity of the source organic matter migrated methane from deeper more mature sources is recognized by its relative 13 c enrichment compared to indigenous methane examples are presented in this paper to evaluate the migration pattern of gases in sedimentary basins

origin and accumulation of co2 and its natural displacement of oils in the continental margin basins northern south china sea many rich up to 97 by volume natural gas pools have been found in the continental margin basins of the northern south china sea by combining the geochemical data from 53 samples with their geologic backgrounds this study investigated the origins and accumulation mechanism of and discussed the role of in driving oil as it charged the reservoirs the results reveal that the gases in the yinggehai basin originate mainly from the thermal decomposition of both miocene calcareous shales and paleozoic carbonates and that from mantle degassing is only a minor contributor the accumulations in the yinggehai basin are mainly controlled by diapiric faults and episodic thermal fluid movements the gases in the eastern qiongdongnan and western pearl river mouth basins are mainly related to magmatic or mantle degassing and the volatiles from magmatic degassing during the igneous intrusion stage are the most likely major source of in these reservoirs with basement faults providing pathways for upward migration of rich mantle fluids natural displacements of oil by appear to be common in the eastern qiongdongnan and western pearl river mouth basins the flooded oil or gas reservoirs have two common features that the present gas pools or oil bearing structures have residual oils representing prior charge and are close to the basement faults that provide pathways along which the mantle derived rich gas was migrated the oils from prior hydrocarbon reservoirs have been naturally driven out by to form secondary oil reservoirs in the eastern qiongdongnan and western pearl river mouth basins therefore a full understanding of the origin and distribution of cannot just be used to trace hydrocarbon migration pathways but also provide useful information for risk assessment prior to drilling

occurrence and origin of carbon dioxide in the fushan depression beibuwan basin south china sea abstract natural gases produced from the fushan depression beibuwan basin south china sea are generally rich in carbon dioxide the 13 c values of methane in these gases are within the range of 41 18 to 47 00 this together with the linearity of the 13 c values of methane to butane in the natural gas plots indicates that the hydrocarbon gases are of organic origin the 13 c co 2 values of these gases are in the range of 5 1 to 7 5 suggesting an inorganic origin a dominant mantle source for the co 2 in these gases was determined on the basis of a simple binary mixing model using the 13 c co 2 values and 3 he 4 he ratios of associated helium in these gases the primary geological controls on the occurrence of co 2 rich gas accumulations in the fushan depression are discussed following a comparison with other sedimentary basins in the region

hf zr and ree partition coefficients between ilmenite and liquid implications for lunar petrogenesis hf zr and ree partition coefficients between ilmenite and coexisting liquid were determined under near lunar conditions ilmenite was synthesized at 1125 c under low oxygen fugacity fo2 10 12 4 from glass of a high ti mare basalt composition doped with 50 100 g g of ree and hf and 500 1000 g g of zr the partition coefficients d of ilmenite for hf and zr obtained by isotope dilution analysis are 0 41 and 0 33 respectively these values are significantly lower than those of ilmenite 3 0 and 1 8 respectively from a kimberlite megacryst fujimaki et al 1984a dree for the synthesized ilmenite are slightly smaller than those of ilmenite from the kimberlite megacryst and the dlu is 0 056 these results suggest that ilmenite may have played an important role in the lu hf evolution of lunar mare basalts using newly determined dlu and dhf for ilmenite the lu hf evolution or lunar cumulates and the coexisting magma was examined for several different crystallization sequences by a model calculation the lu hf variation trend of most high ti mare basalts is well explained by a small degree less than 5 of partial melting of the cumulates however a higher degree of partial melting is required to explain the variation of very low ti basalts green glass and apollo 12 low ti basalts apollo 15 low ti basalts could not be produced by simple partial melting but may require chromite crystallization as well

in reservoir mixing of mantle derived co2 and metasedimentary ch4 n2 fluids noble gas and stable isotope study of two multistacked fields pannonian basin system w hungary abstract several multistacked gas fields located in very different parts of hungary display a large variation in their gas composition the shallow position of the mantle beneath the basement of the pbs is responsible for the high heat flux it facilitates that the co 2 intensely released from the ascending upper mantle derived melts could reach the basin system in this work the results of a geochemical study of the main components and noble gases of the multistacked repcelak co 2 hc n 2 and mihalyi co 2 fields are presented and discussed the vertical variations of gas composition in the repcelak and mihalyi fields fit well with in reservoir mixing of two end members a mantle related magmatic co 2 fluid and a n 2 rich hc gas fluid of crustal origin however it is very likely that other processes modified to some extent the gas composition of the two fields carbon isotopic composition of the co 2 of the samples studied agrees with both the carbonate decomposition and mantle degassing origins it is very likely that the marked difference in helium isotopic composition between the neighbouring repcelak and mihalyi fields also reflects this heterogeneity

neural network analyses of stress induced overpressures in the pannonian basin summary artificial neural networks can learn relationships between sediment characteristics burial depth composition coordinates and thickness of overlying quaternary deposits and overpressures from well data after which they can interpolate and extrapolate to areas and depths not covered by wells we analyse data from the south eastern part of the pannonian basin we use a neural network for analysing fluid overpressures because of the complex interaction of the key variables making it difficult to derive the functional relationships required for a statistical analysis the optimal topology of the network number of hidden layers and neurons is found by minimizing the network s training and testing errors the optimal design of the network resembles the interactions scheme of the key variables the pannonian basin originally formed in an extensional regime has been in a compressive state of stress since late pliocene causing anomalous subsidence patterns numerical forward modelling of compaction driven fluid overpressures shows that due to an increase in the level of compressive interplate stress the fluid overpressures in the deep subbasins have increased substantially since late pliocene giving rise to a very high overpressure up to 45mpa at present the neural network analyses provide an independent estimate of the current amount of overpressuring in this basin complementing the numerical forward modelling results the overpressure profiles obtained by the two modelling approaches are in excellent agreement showing the same magnitude of overpressures a reversal of the overpressure in the deepest parts of the subbasins and a general decrease of the overpressure from sw to ne

the fate of mantle derived carbon in a continental sedimentary basin integration of c he relationships and stable isotope signatures abstract the isotopic composition and abundances of the rare gases he ne and ar and active gases co2 ch4 have been determined in a series of commercial gas reservoirs in the pannonian and vienna basins of hungary and austria respectively in these zones of continental extension significant components of mantle derived 4he up to 39 8 and 21ne up to 58 are identified the results of this study indicate a major component of mantle derived carbon in these systems as well ranging in composition from close to 100 co2 to ch4 dominated reservoirs with trace concentrations ppm of co2 these gas reservoirs provide a unique opportunity to examine the relationship of the conservative rare gases to the active gas components and to examine the sources and sinks for mantle and crustal derived carbon phases with the exception of kismarja gas field which shows evidence of addition of 3he depleted crustal co2 all gas fields exhibit a trend whereby the most co2 rich samples approach co 2 3 he mntl values similar to morb 2 109 to 7 109 while co2 depleted samples extend to co 2 3 he mntl values as low as 105 the lack of any correlation of co 2 3 he mntl ratios with r ra values indicates the observed trends are not a function of mixing between crustal and mantle derived endmembers but instead reflect progressive loss of the mantle derived co2 carrier phase during volatile transport and emplacement in the continental crust based on stable isotopic signatures from the kismarja field a crustal co2 endmember with an isotopic composition of 6 8 and a mantle co2 endmember with an isotopic composition no more depleted in 13c than 5 0 can be identified one of the few instances where the isotopic signatures of mantle and crustal derived co2 can be reliably distinguished in a continental setting covariation in 13cco2 and 13cch4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent co2 and co 2 3 he mntl ratios can be accounted for by 1 loss of the mantle derived co2 carrier phase 2 addition of crustal derived co2 3 addition of crustal derived thermogenic ch4 and potentially 4 conversion of the co2mntl carrier phase to mantle derived ch4 based on an estimated total mantle 4he flux for the pannonian basin of 4 2 108 atoms m 2 s 1 mantle carbon flux estimates for this basin alone range from 3 108 g c yr to 1 109 g c yr which over the lifetime of the basin is only 4 5 orders of magnitude less than flux estimates based on the total area of the spreading ridges clearly the addition of mantle derived carbon to the crust in areas of continental extension is significant and may have been previously underestimated we demonstrate here that integration of 13c data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings

lu hf constraints on the evolution of lunar basalts very low ti basalts and green glass samples from the moon show high lu hf ratios and low hf concentrations low ti lunar basalts show high and variable lu hf ratios and higher hf concentrations whereas high ti lunar basalts show low lu hf ratios and high hf concentrations kreep basalts have constant lu hf ratios and high but variable hf concentrations using the lu hf behavior as a constraint we propose a model for the mare basalts evolution this constraint requires extensive crystallization of the primary lunar magma ocean prior to formation of the lunar mare basalt sources and the kreep basalts mare basalts are produced by the melting of the cumulate rocks and kreep basalts represent the residual liquid of the magma ocean lu and hf concentrations and the hf isotopic data of lunar rocks suggest that assimilation cannot be accepted as a major process to explain the diversity of the lunar mare basalts the urkreep hypothesis is also unnecessary both high and low ti basalts show enough iron enrichment to be regarded as melting products of the last stage cumulate rocks from the lunar magma ocean the kreep basalts are also rich in iron and may be regarded as the final residual liquid left after the crystallization of the major portion of the primary lunar magma ocean

high precision lu and hf isotope analyses of both spiked and unspiked samples a new approach 1 the functional form of instrumentally produced mass fractionation associated with mc icp ms analysis is not accurately known and therefore cannot be fully corrected by traditional approaches of internal normalization using power linear or exponential mass bias laws we present a method for robust correction of instrumentally produced mass fractionation of both spiked and unspiked samples that can be applied to mass analysis of hf as well as nd sr os etc correction of 176hf 177hf for unspiked samples follows a traditional approach of internal normalization using an exponential law followed by normalization to a standard of known composition such as jmc 475 for spiked samples standards are used to characterize a linear instrumental mass bias coefficient the mass bias coefficient is defined by the slope of a tie line between measured and true values of a standard this approximation results in identical precision and accuracy of measurements for spiked and unspiked samples 0 005 2 external reproducibility the effects of the spike on the 176hf 177hf ratio and calculation of the molar spike sample ratio is determined by a closed form solution modified from the double spike approach used for fe isotope analysis by tims johnson and beard 1999 the measured 176lu 175lu ratios are corrected by doping the sample with er and using the 167er 166er ratio to externally normalize the 176lu 175lu ratio using an exponential law finally spike sample equilibration is confirmed for our sample dissolution protocol through analysis of varying physical mixtures of 1 ga garnet and hornblende where all the data lie on a mixing line within error on a 176lu 177hf 176hf 177hf diagram precision of 176lu 177hf ratios is determined to be 0 2 2 for standards and for physical mixtures of garnet and hornblende

the effects of weathering on rare earth element y and ba abundances in tertiary basalts from southeastern australia tholeiitic basalts showing anomalously high ba y and rare earth element ree concentrations occur within the tertiary newer volcanic province of victoria and these geochemical characteristics are related to incipient alteration accompanying weathering ree and y released during weathering of basalts are retained either in secondary phosphate or clay minerals and in rocks showing very high ba concentrations the ba is present as baso4 leaching experiments suggest that glassy or fine grained rocks are more likely to release ree y and ba while more extensively altered rocks are likely to hold these elements probably because they substitute in or are adsorbed on clay minerals or because secondary baso4 and ree phosphates are resistant to acid leaching samples taken sequentially across a weathering profile in a tholeiitic basalt flow from fresh to intensively altered rock illustrate that in the very earliest stages of alteration extreme variations in the abundances of ba y and ree can occur this is reflected in the broad patterns of trace element behaviour manifested by an extensive analysed sample suite from the victorian newer volcanic province where there is no correlation between anomalous enrichments in ba y or ree and the level of incipient alteration ba anomalies are not necessarily accompanied by abnormal y and ree behaviour and this is probably because the mobility of each trace element is a function of a the ionic radius and charge of the ionic species it forms in solution b local groundwater parameters such as eh ph groundwater flux and concentrations in the groundwater of species such as so42 and c the secondary mineral assemblages developed in a particular situation sr isotopic compositions are not strongly affected by incipient weathering and do not show significant changes until alteration has progressed to the point where plagioclase begins to break down the study illustrates that relatively slight alteration at the earliest stages of weathering can produce drastic changes in the concentrations of ree y and ba and petrographic assessment of the freshness of samples is not precise enough to eliminate prior to analysis cases where these effects may be manifested where weathering has caused changes in abundance patterns these may be recognised by careful comparisons of appropriate elemental ratios e g ba rb throughout a particular analysed sample suite

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

the magma ocean concept and lunar evolution the model of lunar evolution in which the anorthositic plagioclase rich oldest crust of the moon is formed over a period of 300 myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies consideration is given to the classification of lunar rocks the evidence for primordial deep global differentiation constraints on the depth of the molten zone the effects of pressure on mineral stability relationships mainly liquid vs mainly magmifer ocean models and the evidence for multiple ancient differentiation episodes a synthesis of the model of primordial differentiation and its aftereffects is presented and the generalization of the model to the earth and to mars mercury venus and the asteroids is discussed

trace element partitioning between ilmenite armalcolite and anhydrous silicate melt implications for the formation of lunar high ti mare basalts abstract we performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite ilm armalcolite arm and anhydrous lunar silicate melt to constrain geochemical models of the formation of titanium rich melts in the moon experiments were performed in graphite lined platinum capsules at pressures and temperatures ranging from 1 1 to 2 3 gpa and 1300 1400 c using a synthetic ti enriched apollo black glass composition in the cao feo mgo al2o3 tio2 sio2 system ilmenite melt and armalcolite melt partition coefficients d show highly incompatible values for the rare earth elements ree with the light ree more incompatible compared to the heavy ree d la ilm melt 0 0020 0 0010 to d lu ilm melt 0 069 0 010 for ilmenite d la arm melt 0 0048 0 0023 to d lu arm melt 0 041 0 008 for armalcolite d values for the high field strength elements vary from highly incompatible for th u and to a lesser extent w for ilmenite d th ilm melt 0 0013 0 0008 d u ilm melt 0 0035 0 0015 and d w ilm melt 0 039 0 005 and for armalcolite d th arm melt 0 008 0 003 d u arm melt 0 0048 0 0022 and d w arm melt 0 062 0 03 to mildly incompatible for nb ta zr and hf e g d hf ilm melt 0 28 0 05 and d hf arm melt 0 76 0 07 both minerals fractionate the high field strength elements with dta dnb and dhf dzr between 1 3 and 1 6 for ilmenite and 1 3 and 1 4 for armalcolite armalcolite is slightly more efficient at fractionating hf from w during lunar magma ocean crystallisation with dhf dw 12 13 compared to 6 7 7 5 for ilmenite the transition metals vary from mildly incompatible to compatible with the highest compatibilities for cr in ilmenite d 7 5 and v in armalcolite d 8 1 d values show no clear variation with pressure in the small range covered crystal lattice strain modelling of d values for di tri and tetravalent trace elements shows that in ilmenite divalent elements prefer to substitute for fe while armalcolite data suggest ree replacing mg tetravalent cations appear to preferentially substitute for ti in both minerals with the exception of th and u that likely substitute for the larger fe or mg cations crystal lattice strain modelling is also used to identify and correct for very small 0 3 wt melt contamination of trace element concentration determinations in crystals our results are used to model the lu hf ti concentrations of lunar high ti mare basalts the combination of their subchondritic lu hf ratios and high tio2 contents requires preferential dissolution of ilmenite or armalcolite from late stage lunar magma ocean cumulates into low ti partial melts of deeper pyroxene rich cumulates

identifying and quantifying natural co2 sequestration processes over geological timescales the jackson dome co2 deposit usa co2 sources sinks and migration mechanisms in natural co2 gas fields provide critical analogues for developing the safe application of anthropogenic co2 sequestration technologies here we use noble gas and carbon isotopes together with other gases to identify and quantify the origin transport and trapping mechanisms of co2 in the late cretaceous jackson dome co2 gas deposit 98 75 to 99 38 co2 located in central mississippi usa and producing from 5000 m it is one of the deepest commercial co2 gas fields in the world 10 gas samples from producing wells were determined for their noble gas chemical and stable carbon isotope composition he 3 he 4 ratios range between 4 27r a and 5 01r a where r a is the atmospheric value of 1 4 x 10 6 indicating a strong mantle signature similar to co2 deposits worldwide co2 he 3 decreases with increasing groundwater derived ne 20 and he 4 we model several different processes that could account for the jackson dome data and conclude that similar to other co2 dominated deposits a groundwater gas stripping and re dissolution ggs r process best accounts for observed ne 20 ar 36 kr 84 ar 36 co2 he 3 delta c 13 co2 he 4 ne 20 and ar 36 in this context crustal and magmatic co2 components contribute 57 and 43 respectively changes in co2 he 3 across the field show that groundwater contact is responsible for up to 75 loss of original emplaced co2 delta c 13 co2 variance limits the degree of precipitation to be less than 27 with the remaining co2 loss being accounted for by dissolution only a higher degree of dissolution gas loss and evidence for water contact at the reservoir crest compared to the reservoir flanks is used to argue that co2 in this system has not undergone subsequent loss to either dissolution or precipitation since shortly after reservoir filling at over 60 ma c 2012 elsevier ltd all rights reserved

quantification of co2 generation in sedimentary basins through carbonate clays reactions with uncertain thermodynamic parameters abstract we develop a methodological framework and mathematical formulation which yields estimates of the uncertainty associated with the amounts of co 2 generated by carbonate clays reactions ccr in large scale subsurface systems to assist characterization of the main features of this geochemical process our approach couples a one dimensional compaction model providing the dynamics of the evolution of porosity temperature and pressure along the vertical direction with a chemical model able to quantify the partial pressure of co 2 resulting from minerals and pore water interaction the modeling framework we propose allows i estimating the depth at which the source of gases is located and ii quantifying the amount of co 2 generated based on the mineralogy of the sediments involved in the basin formation process a distinctive objective of the study is the quantification of the way the uncertainty affecting chemical equilibrium constants propagates to model outputs i e the flux of co 2 these parameters are considered as key sources of uncertainty in our modeling approach because temperature and pressure distributions associated with deep burial depths typically fall outside the range of validity of commonly employed geochemical databases and typically used geochemical software we also analyze the impact of the relative abundancy of primary phases in the sediments on the activation of ccr processes as a test bed we consider a computational study where pressure and temperature conditions are representative of those observed in real sedimentary formation our results are conducive to the probabilistic assessment of i the characteristic pressure and temperature at which ccr leads to generation of co 2 in sedimentary systems ii the order of magnitude of the co 2 generation rate that can be associated with ccr processes

east molokai and other kea trend volcanoes magmatic processes and sources as they migrate away from the hawaiian hot spot 1 there are geochemical differences between shield lavas from the two parallel trends kea and loa defined by young hawaiian volcanoes the shield of east molokai volcano at greater than 1 5 ma is the oldest volcano on the kea trend sequences of older tholeiitic to younger alkalic basalt that erupted as this volcano evolved from the shield to postshield stage of volcanism are well exposed much younger 0 34 0 57 ma alkalic basalt and basanite erupted during rejuvenated stage volcanism like rejuvenated stage lavas erupted at other hawaiian volcanoes rejuvenated stage east molokai lavas have relatively low 87sr 86sr and high 143nd 144nd such ratios reflect a source component with a long term depletion in abundance of incompatible elements on the basis of positive correlations of 87sr 86sr versus 206pb 204pb and negative correlations of these isotopic ratios with nb zr a smaller proportion of this depleted component also contributed to the late shield postshield lavas erupted at east molokai and the other kea trend volcanoes haleakala and mauna kea at each of these kea trend volcanoes as the volcano moved away from the hot spot the extent of melting and magma supply from the mantle decreased the depth of melt segregation increased and there was an increasing role for a component with long term relative depletion in incompatible elements this depleted component has kea trend pb isotopic characteristics and relatively low 208pb 204pb at a given 206pb 204pb and it is probably not related to oceanic lithosphere or the source of mid ocean ridge basalt the overlap in sr nd and pb isotope ratios of recent kilauea shield lavas and 550 ka mauna kea shield lavas has been used to argue that kea trend shield volcanism samples a vertically continuous geochemically distinct stripe which persisted in the hot spot source for 550 kyr eisele et al 2003 abouchami et al 2005 as kea trend volcanoes migrate away from the hot spot and evolve from the shield to postshield stage there are systematic changes in sr nd and pb isotope ratios however the overlap of sr nd and pb isotope ratios in late shield postshield lavas from mauna kea 350 ka and east molokai 1 5 ma show that the periphery of the hot spot sampled by kea trend postshield lavas also had long term geochemical homogeneity

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

origin and migration of hydrocarbon gases and carbon dioxide bekes basin southeastern hungary the bekes basin is a sub basin within the pannonian basin containing about 7000 m of post cretaceous sedimentary rocks natural gases are produced from reservoirs precambrian to tertiary in age located on structural highs around the margins of the basin gas composition and stable carbon isotopic data indicate that most of the flammable gases were derived from humic kerogen contained in source rocks located in the deep basin the depth of gas generation and vertical migration distances were estimated using quantitative source rock maturity carbon isotope relationships for methane compared to known neogene source rock maturity depth relationships in the basin these calculations indicate that as much as 3500 m of vertical migration has occured in some cases isotopically heavy 7 0 co2 is the predominant species present in some shallow reservoirs located on basin margin structural highs and has probably been derived via long distance vertical and lateral migration from thermal decompositon of carbonate minerals in mesozoic and older rocks in the deepest parts of the basin a few shallow reservoirs 2000m contain isotopically light 50 to 60 0 methane with only minor amounts of c2 homologs 3 v v this methane is probably mostly microbial in origin above normal pressures occuring at depths greater than 1800 m are believed to be the principal driving force for lateral and vertical gas migration these pressures are caused in part by active hydrocarbon generation undercompaction and thermal decomposition of carbonates

deep burial dissolution in an oligocene miocene giant carbonate reservoir perla limestone gulf of venezuela basin implications on microporosity development abstract sedimentary rocks tend to progressively lose porosity with depth due to mechanical and chemical compaction in carbonates this trend is hard to predict since many factors can create porosity in the burial setting the oligocene miocene perla limestone a giant gas reservoir located in the gulf of venezuela basin shows a complex porosity system marked by a strong diagenetic control despite exhaustive depositional facies modelling carried out in the carbonate reservoir inconsistencies remained when distributing petrophysical properties with depositional facies these inconsistencies become more important in areas strongly affected by intense diagenetic processes after the initial investigation burial dissolution was identified as the main diagenetic process affecting porosity in this succession to understand the origin of the burial dissolution process and its distribution along the reservoir detailed petrographic mineralogical and isotopic studies were carried out on recovered cores from different exploration wells results show a series of diagenetic features which were associated with the evolution of the strata including burial dissolution dolomitization and compaction as the main processes after testing several hypotheses the results support an inorganic co2 model as responsible for the burial dissolution process in the perla limestone according to this model deep burial dissolution created pervasive microporosity with subordinated moldic and vuggy pores that enhanced the reservoir quality in localized areas of the carbonate succession these features are located where the ascent of hydrothermal co2 rich fluids funneled by discontinuity surfaces faults fractures stylolites etc reached the perla limestone

relation of natural gas composition to thermal maturity and source rock type in san juan basin northwestern new mexico and southwestern colorado san juan basin is a roughly circular asymmetric structural depression located in northwestern new mexico and southwestern colorado ultimate recoverable reserves of predominantly nonassociated gas 23 tcf 0 65 1012 m3 are present in the structurally low part of the central basin the major producing intervals are low permeability sandstone reservoirs in the upper cretaceous dakota sandstone mesaverde group and pictured cliffs sandstone lesser amounts of oil and or gas are produced from pennsylvanian jurassic and cretaceous rocks along the southern and western flanks of the basin the gases display a trend of becoming isotopically heavier dgr13c1 values range from 48 7 to 31 4 pmil and chemically drier c1 c1 5 values range from 0 75 to 0 99 with increasing depth these changes are assumed to be the result of thermal cracking processes and the gases are interpreted to have been generated during the mature and post mature stages of hydrocarbon generation however there is considerable scatter in the data which is interpreted to result from a difference in source rock type gases generated from nonmarine humic source rocks are isotopically heavier and chemically drier than those generated from marine sapropelic source rocks at equivalent levels of maturity the gases also become isotopically heavier and chemically drier to the northeast following the trend of increasing maturity of all units in that direction the increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the san juan mountains area maximum burial and heat flow occurred during the oligocene which probably coincided with peak hydrocarbon generation lack of oil in the central basin is believed to be the result of two factors first gas in reservoirs such as the dakota sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature wet gas condensate and post mature dry gas stages of hydrocarbon generation second gas in reservoirs such as mesaverde group and pictured cliffs sandstone is nonassociated and probably was generated from nonmarine coaly organic matter during the mature and post mature stages minor amounts of condensate instead of oil may have been generated from nonmarine source rocks during the mature stage

lu hf and sm nd evolution in lunar mare basalts lu hf and sm nd data for mare basalts combined with rb sr and total ree data taken from the literature suggest that the mare basalts were derived by small or 10 degrees of partial melting of cumulate sources but that the magma ocean from which these sources formed was light ree and hf enriched calculated source compositions range fromm lherzolite to olivine websterite nonmodal melting of small amounts of ilmenite or 3 in the sources seems to be required by the lu hf data a comparison of the hf and nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the epsilonhf epsilonnd ratios of low ti mare basalts are much higher than in terrestrial oceanic basalts the results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates alternatively the results may imply that the terrestrial mantle has evolved in two or more stages of evolution and that the net effect was depletion of the mantle during the first approx 1 3 b y followed by enrichment during the last 1 2 b y or simply that there is a difference in lu hf crystal liquid partitioning relative to sm nd between the lunar and terrestrial mantles

melting in the hawaiian plume at 1 2 ma as recorded at maui nui the role of eclogite peridotite and source mixing 1 the volcanoes of maui nui west moloka i east moloka i lana i west maui haleakala and kaho olawe record hawaiian magmatism at 1 2 ma lavas from these volcanoes nearly span the compositional range erupted from all the hawaiian volcanoes over the past 5 myr and represent both the kea and ko olau compositional end members of hawaiian lavas many aspects of major and trace element and isotope compositions of hawaiian shield stage lavas are consistent with ancient recycled oceanic lithosphere in the plume sources of kea and ko olau type magmas lassiter and hauri 1998 blichert toft et al 1999 hypotheses that describe the compositional range of hawaiian lavas as originating from ancient oceanic lithosphere in the hawaiian plume implicitly or explicitly infer lithologic heterogeneity in the plume we present trace element models for the origin of these end members that explicitly address the petrologic complexities of melting eclogite derived from ancient oceanic lithosphere in the plume trace element la nb sm yb sm hf and sm nb major element and isotope compositions of lana i which erupts dominantly ko olau type lavas are consistent with the origin of these lavas in large degree 60 70 melts of ancient upper oceanic crust basalt sediment that mix with plume derived haleakala type melts trace element sm yb hf zr and hf nb and isotope compositions of west maui and east moloka i which erupt dominantly kea type magmas are consistent with an origin in ancient depleted oceanic lithosphere that has been refertilized with moderate degree melts 10 30 of associated crustal gabbro the physical mechanisms melt melt versus melt solid mixing through which the oceanic crustal components melt and mix within the plume lead to the generation of isotopically homogeneous kea type lavas and isotopically heterogeneous ko olau type lavas the volcanoes of maui nui record the exhaustion of the ko olau component and the initiation of the kea component as dominant compositional end members in the hawaiian plume

dongfang 1 1 gas field in the mud diapir belt of the yinggehai basin south china sea abstract the dongfang 1 1 df1 1 mud diapir field is the largest gas field discovered in the yinggehai basin up to now with a proven gas reserve of 99 68 10 9 m 3 the gas pay zones are within the pliocene fine sandstones and siltstones of the littoral and sand bar facies with normal pressure regimes the gases include variable amounts of co 2 and n 2 in addition to hydrocarbon gases the hydrocarbon gases with the exception of biogenic methane and nitrogen originated from the marine source rocks within the overpressured lower to middle miocene strata while co 2 is most likely the product of thermal decomposition of the tertiary calcareous mudstones and pre tertiary carbonates through relatively long distance vertical migration the field has received at least four phases of natural gas charging occurring relatively late most likely during the post pliocene himalayan orogeny diapiric faults act as the main conduit for upward movement of fluids released from the deep high pressure system with abnormally high pressures providing the significant driving force for gas migration because of the episodic nature of the deep high pressured fluid release and the diapir fault activity this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation

helium and neon isotope systematics in carbon dioxide rich and hydrocarbon rich gas reservoirs the isotopic compositions and elemental abundances of helium and neon were measured in three natural gas reservoirs in the pannonian sedimentary basin of hungary kismarja a co2 rich reservoir and szeghalom south and szeghalom north both ch4 dominated reservoirs are located on topographic basement highs close to the derecske sub basin in eastern hungary mantle derived neon has been identified in mixed ch4 co2 reservoirs in the vienna basin austria this study establishes that mantle derived neon and helium are a characteristic feature of gas reservoirs throughout the neogene extensional basins of hungary and austria regardless of the dominant active gas composition 3he4he ratios within these samples are attributable to a two component mixing between mantlederived and crustal radiogenic helium the percent contribution of mantle derived 4he varies from 2 3 to 17 in contrast neon isotopic ratios indicate that the gases contain a significant component of atmosphere derived neon in addition to the mantle and crustal derived components 20ne 21ne and 22ne abundances can be corrected for this atmospheric contribution calculated contributions of mantle and crustal derived 21ne are between 3 6 21 and 1 37 respectively 20ne22nec and 21ne22nec ratios derived for these atmosphere corrected components correlate with measured r ra values and plot along a single two component mixing line between crustal and mantle isotopic endmembers this is consistent with a model in which simple mixing occurs between crustal and mantle endmembers with fixed he ne ratios the mixing line is defined by a hyperbolic constant k where k 4he22ne rad 4 he22ne mntl with a mean value of 67 3 11 8 based on estimated values of 0 47 for 21ne22nerad and 1 62 0 03 107 for 4he21ne rad kennedy et al 1990 values of 7 61 106 for he22ne rad and 11 3 104 for 4he22ne mntl can be calculated for the pannonian basin gases this 4he22ne mntl value is indistinguishable within error from the value of 8 04 104 calculated for rare gases in natural gases from the vienna basin these results clearly establish that the continental expression of mantle derived rare gases in continental extensional systems in austria and hungary is distinct and consistently different from that of gases discharging at the spreading ridges where best estimates of 4he22ne mntl are 8 1 11 3 times higher 9 10 105 staudacher et al 1989 given the remarkable agreement in the continental expression of mantle derived gases throughout the pannonian and vienna basins it is difficult to attribute the observed neon enrichment helium depletion with respect to morb gases to fractionation related to lithospheric transport processes kinetic fraction ation processes involved in transport through the crust might be expected to produce a much wider variation in the observed he ne elemental ratios the consistent order of magnitude neon enrichment observed throughout these gas fields instead implies that mantle derived fluids in these continental extensional systems may be sourced in a region of the mantle distinct from that supplying the mid ocean spreading ridges

origins of co2 in permian carbonate reservoir rocks zechstein ca2 of the nw german basin lower saxony permian carbonate reservoir rocks stassfurt carbonate of the co2 province lower saxony basin nw germany are locally characterised by high amounts of co2 leading to serious problems on exploration in order to receive information on the major co2 sources and co2 migration and accumulation processes mineralogical geochemical and stable isotope analyses c 13 o 18 were carried out on authigenic precipitations within the stassfurt carbonate rocks ca2 these rocks contain early diagenetic e g formed in soft sediment carbonate minerals calcite dolomite and anhydrite in concretions and small cracks in addition large up to 2 cm wide carbonate cemented fractures formed in solid ca2 rocks semi quantitative estimates of the diagenetic mineral content showed variations between the investigated rock cores nine out of 12 ca2 rock cores are derived from wells situated in the area of modem ch4 rich reservoirs and have distinct fracture systems developed in contrast the other three rock cores are from wells characterised by co4 rich reservoirs and lack fractures of similar thickness the assumed relationship between tectonically induced fractures and the gas content of the ca2 reservoir rocks implies co2 degassing of the ca2 rocks at approximately 102 degrees c to similar to 150 degrees c assumed delta 18o pore water variation from 0 parts per thousand to 4 parts per thousand smow ree distribution indicates at least three types of fracture and crack filling carbonates two carbonate generations formed from evolving diagenetic pore waters whereas one type reflects carbonate formation under hydrothermal influence the ages of these cementations derived from rb sr and u pb isotope analyses indicate their fort nation during late stage of or after the inversion of the lower saxony basin in late cretaceous times the stable carbon isotope composition of the authigenic carbonate minerals reflects at least two carbon source endmembers available during diagenesis whereas many early formed carbonates are related to the permian c 13 rich carbon source those carbonates which precipitated or recrystallised during early to late stage diagenesis and or were induced by local tectonic events incorporated c 12 enriched co from organic degradation processes the stable oxygen isotope values of authigenic carbonates probably still reflect low temperatures of mineral formation from permian sea water as it is assumed for non recrystallised carbonates or late stage high temperature up to similar to 150 degrees c precipitation ftoin sea water derived pore waters which may have been locally influenced by o 18 enriched fluids however most of the carbonate samples investigated are recrystallised crystal enlargement of concretionary and fracture carbonate is observed and hence these carbonates are at least partly re equilibrated original o 18 content reduced with surrounding pore waters at elevated temperature similar to the authigenic carbonate at least two sources may explain the concentration carbon isotope relationship of modem co present in the ca 2 reservoir rocks an organic derived co2 endmember source delta c 13 near 20 parts per thousand is present in relatively low concentrations whereas large co concentrations derived from an endmember source with all isotope value near 0 parts per thousand although the latter source is unknown such heavy co sources are most likely attributed to carbonate decomposition processes c 2005 elsevier b v all rights reserved

trace element mobility during spheroidal weathering of basalts and andesites in hawaii and guatemala abstract trace element mobility has been studied in spheroidally weathered corestone shell complexes developed on late tertiary and quaternary volcanic rocks the parent materials are basalts and andesites related to 1 a subduction zone tecuamburro and moyuta volcanoes guatemala and 2 a hot spot hawaiian islands of oahu maui and hawai i the initial major element compositions are largely preserved in the corestones each corestone sample shows major element abundances characteristic of the different magmatic processes through which the lavas were generated however the changes in trace element compositions due to weathering show similar patterns in the corestone shell complexes independent of the tectonic setting and age element mobility within the different corestone shell complex was determined using an isovolumetric reference frame in this method bulk density is chosen as an independent measure of the extent of weathering based on volumetric concentrations the notion of element immobility cannot be applied to all the corestone shell complexes elements commonly assumed to be immobile e g zr ti and al vary in volumetric concentration for some corestone shell complex despite petrographic evidence of only minor weathering two corestones oahu and tecuamburro have become enriched in ree 3 relative to other lava flows from the same volcanoes and display negative ce anomalies the exfoliated shells from these complexes tend to have lower concentrations of ree 3 and have positive ce anomalies the fractionation of ce from the other rare earth elements ree is inferred to be a manifestation of changes in the valence state of ce from trivalent to tetravalent under oxidizing conditions in settings where the development of corestone shell complex is advanced an acidic solution can mobilize trivalent ree leach them out of more extensively weathered materials and transport them to less acidic portions of the regolith undergoing incipient weathering the precipitation of the ree 3 in the rinds and centers of the corestones are likely due to increases of the ph of the weathering solution systematic weathering induced ree behavior enrichment of most ree with lesser ce enrichment resulting in a negative ce anomaly can be discerned even in corestone samples that exhibit major element abundances and petrographic features otherwise similar to unweathered basalts and andesites

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

ages rare earth element enrichment and petrogenesis of tholeiitic and alkalic basalts from kahoolawe island hawaii kahoolawe island hawaii 18 11 km is a basaltic shield volcano with caldera filling lavas seven identified postshield vents and at least two occurrences of apparent rejuvenated stage eruptive we examined 42 samples that represent all stages of kahoolawe volcano stratigraphy for their petrography whole rock major and trace element contents mineral compositions and k ar ages the two oldest shield samples have an average age of 1 34 0 08 ma and four postshield samples 3 are alkalic average 1 15 0 03 ma ages of 1 08 and 0 99 ma for two additional tholeiitic samples probably are minimum ages whole rock major and trace element and mineral compositions of kahoolawe shield and caldera fill laves are generally similar to the lavas forming kilauea and mauna loa tholeiitic shields but in detail kahoolawe shield lavas have distinctive compositions an unusual aspect of many postshield ka hoolawe lavas is anomalously high ree and y abundances up to 200 ppm la and 175 ppm y and negative ce anomalies these enrichments reflect surficial processes where weathering and soil development promoted ree y transport at the weathering front major element abundances mgo 10 6 wt for shield and caldera fill basalts are consistent with fractionation of ol px pl in frequently replenished magma reservoirs in general tholeiitic basalts erupted from late vents are higher in sio2 than the shield lavas and temporal differences in parental magma compositions are the likely explanation alkalic basalts that erupted from vents are comparable in composition to those at other hawaiian volcanoes trace element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source apparent rejuvenated stage basalts i e emplaced after substantial kahoolawe erosion are tholeiitic unlike the rejuvenated stages at other hawaiian volcanoes alkalic kahoolawe like several other hawaiian volcanoes has intercalated tholeiitic and alkalic basalts in the postshield stage but it is the only volcano that appears to have produced tholeiitic rejuvenated stage lavas

geochemistry and episodic accumulation of natural gases from the ledong gas field in the yinggehai basin offshore south china sea abstract the ledong gas field consisting of three gas pools in a shale diapir structure zone is the largest gas discovery in the yinggehai basin the gases produced from the pliocene and quaternary marine sandstone reservoirs show a considerable variation in chemical composition with 5 4 88 ch 4 0 93 co 2 and 1 23 7 n 2 the co 2 enriched gases often display heavier methane 13 c values than those with low co 2 contents the 15 n values of the gases range from 8 to 2 and the n 2 content correlates negatively with the co 2 content the high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the lower middle miocene source rocks and the formation of abundant co 2 from the tertiary calcareous shales and pre tertiary carbonates the compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the ld221 gas field is attributed to three phases of gas migration initially biogenic gas followed by thermogenic hydrocarbon gas and then co 2 rich gas the filling processes occurred within a short period approximately from 1 2 to 0 1 ma based on the results of the kinetics modeling geophysical and geochemical data show that the diapiric faults that cut through miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs and that the deep overpressure is the main driving force for vertical and lateral migration of the gases this gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases and is also a favorable place for gas accumulation the proposed multiple sources and multiple phases of gas migration and accumulation model for the ledong gas field potentially provide useful information for the future exploration efforts in this area

hydrocarbon generation as a mechanism for overpressuring in rocky mountain region overpressured reservoirs are present in the deeper parts of most basins in the rocky mountain region a variety of conditions can cause overpressuring however most high pressures in the region are caused by present day or recently active oil and gas generation or generation in the last few million years in low permeability rock successions that still contain organic matter capable of yielding thermally generated hydrocarbons rocks with above normal pressure range from devonian to tertiary and commonly are associated with low permeability tight reservoirs most of the overpressuring occurs in cretaceous and tertiary rocks overpressuring in rocks older than cretaceous is rare and where present only occurs in strata with very rich hydrocarbon source beds such as the d vonian mississippian bakken formation most organically lean pre cretaceous source beds are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure nearly all overpressured reservoirs and source rocks have temperatures of approximately 200 f 93 c or higher in addition available data indicate that hydrocarbon related overpressuring does not generally occur if vitrinite reflectance ro values are less than 0 6 in oil prone strata or less than 0 7 in gas prone strata overpressured gas bearing strata generally have ro values of 0 8 or higher maximum reservoir pressures are about equal to the fracture gradient pressure in the reservoir rocks some vertical fractures in reservoir rocks are probably caused by rapid expulsion of hydrocarbons from the source beds high pressures in the rocky mountain region means that the rock succession is gas or oil saturated and discrete hydrocarbon water contacts are generally not present classical concepts of hydrodynamics do not appear to be applicable to these overpressured successions the optimum depths for exploration are generally the upper 1 000 2 000 ft 305 610 m of the overpressured succession

evaluation of dissolved light hydrocarbons in different geological settings in romania abstract the paper presents the first systematic geochemical study on the content of natural gas methane heavier alkanes co2 and isotopic composition of methane dissolved in natural springs in romania from various tectonic and geological settings the dissolved methane concentrations in the investigated areas range between 0 08 and 16 359 g l based on isotopic analyses the origin of methane in the gas mixture released by the investigated springs has been inferred the highest concentrations of methane have been measured in the transylvanian basin and carpathian foredeep ethane in relatively high concentrations has been detected in herculane and calimanesti caciulata areas very high concentrations of dissolved co2 were determined in the eastern carpathians volcanic chain area lower amounts of dissolved co2 were detected in the carpathian foredeep southern carpathians transylvanian basin and pannonian basin the measured values of 13c ch4 range between 67 and 4 the methane from the investigated areas is biotic microbial in the transylvanian basin pannonian basin and moldavian platform and thermogenic in the carpathian foredeep we report for the first time the presence of a presumably abiotic methane component in romania from geothermal sources in the areas of ciomadul volcano and herculane area

geochemical variations in lavas from kahoolawe volcano hawaii evidence for open system evolution of plume derived magmas the kahoolawe shield volcano produced precaldera and caldera filling tholeiites and mildly alkalic post caldera lavas that petrographically and compositionally resemble such lavas from other hawaiian shield volcanoes however kahoolawe tholeiites display wide ranges in incompatible trace element ratios e g nb th 9 24 th ta 0 6 1 3 87sr 86sr 0 70379 0 70440 143nd 144nd 0 51273 0 51298 and 206pb 204pb 17 92 18 37 the isotopic variation exceeds that at any other hawaiian shield volcano and spans about half the range for all hawaiian tholeiites quasi cyclic temporal evolution of kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas ratios of highly incompatible trace elements and sr nd and pb isotopic ratios from coherent sub trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends the most mgo rich kahoolawe tholeiites are partial melts of a high nb th 23 5 ascending plume possibly comprising ancient subducted oceanic lithosphere slightly evolved tholeiites experienced combined crystal fractionation and assimilation afc of material derived from a distinct reservoir nb th 9 of asthenospheric derivation the most evolved tholeiites display compositional shifts toward a third component having mid ocean ridge basalt like isotopic ratios but enriched oib like trace element ratios representing part of the lithospheric mantle or melts thereof periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range kahoolawe provides new evidence concerning the nature of the hawaiian plume the distribution of compositional heterogeneities in the suboeanic mantle and the processes by which hawaiian tholeiites form and evolve

fault charged mantle fluid contamination of united kingdom north sea oils insights from re os isotopes 187 os 188 os as expected from a kimmer idge clay formation source however oils from the viking graben with which the brent fi eld is associated and main east shetland basin display unradiogenic 187 os 188 os in this study we seek to demonstrate that these anomalous results are due to interaction with a mantle like fl uid that propagated through the main fault zone of the viking graben and east shetland basin this study enhances our understanding of the re os oil systematics and supports cur rent ideas regarding fault architecture crustal thinning and associated fl uid fl ow and interac tion in the north sea oil system abstract united kingdom north sea oils sourced from the kimmeridge clay formation contain unradiogenic 0 17 to 0 48 and radiogenic 1 04 to 3 34 187 os 188 os values the unra diogenic 187 os 188 os values are spatially associated with the main basin bounding faults of the viking graben and east shetland basin in contrast the radiogenic 187 os 188 os values are associated with north sea basins located farther away from the basin boundary faults we suggest that crustal thinning and strain localization within the viking graben and east shetland basin are suffi cient to have allowed basin bounding faults to propagate to suffi cient depth to act as conduits for mantle derived fl uids to interact with oil this hypothesis is sup ported by previous geochemical data for north sea oil fi elds furthermore we demonstrate the use of os isotopes as an important tracker of crustal scale fl uid dynamics and petroleum migration pathways in extensional basins

carbon isotopes of hydrocarbons and carbon dioxide in natural gases in china abstract 130 natural gases in the continent of china were determined for the abundance and carbon isotopes of hydrocarbons and carbon dioxide the 13c pdb values of methane range from 68 to 24 4 and the largest isotopic variability for hydrocarbons is of 13c1 there is no direct evidence for the presence of abiogenic hydrocarbon in natural gases in china one sample from songliao basin shows a heavier 13c1 24 and reverse isotopic distribution pattern 13c1 13c2 13c3 concerning the occurrence of mantle derived he ne ar and xe in the south songliao basin we propose that the methane mantle derived and heavy hydrocarbons ethane propane butane etc formed from polymerization of the mantle derived methane the 13c values for carbon dioxide range from 13 9 to 13 5 suggesting the multiple origins gases with co2 concentrations greater than 10 have a narrow range of 13c values from 8 to 2 and show higher 1he 4he ratios 0 1 times the atmospheric value suggesting that a mantle derived component is diluted by co2 derived from carbonate and or organic matter positive 13c values 4 14 indicate co2 reduction to ch2 co2 with trace concentration in ch4 rich gases can be attributed to microbial decomposition of organic matter in sedimentary rocks

geochemistry of coalbed gas a review coals are both sources and reservoirs of large amounts of gas that has received increasing attention in recent years as a largely untapped potential energy resource coal mining operations such as ventilation of coalbed gas from underground mines release coalbed ch4 into the atmosphere an important greenhouse gas whose concentration in the atmosphere is increasing because of these energy and environmental issues increased research attention has been focused on the geochemistry of coalbed gas in recent years this paper presents a summary review of the main aspects of coalbed gas geochemistry and current research advances

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

genesis of authigene carbonate minerals in the upper cretaceous reservoir honggang anticline songliao basin a natural analog for mineral trapping of natural co2 storage abstract dawsonite naalco 3 oh 2 as a trace mineral of co 2 is abundant in the upper cretaceous reservoir of the honggang anticline southern songliao basin petrologic and isotopic geochemical characteristics have been investigated by a polarizing microscope a scanning electron microscope sem x ray diffraction xrd cathode luminescence cl fluid inclusions and carbon oxygen isotopic analysis the diagenetic sequence indicated that dawsonite late generation microcrystalline quartz late generation kaolinite late generation calcite and ankerite was formed after co 2 flooding meanwhile as a function of analysis on carbon oxygen isotope the mantle magmatic co 2 provided a carbon source for the precipitation of dawsonite late generation calcite and ankerite the three kinds of carbonate minerals mentioned earlier grew in a progressively evolved condition using a basic thermodynamic fraction equation the total quantity of natural co 2 captured by dawsonite bearing sandstones per cubic meter was calculated to be 99 51 kg

stable carbon isotope geochemistry of adsorbed alkane gases in near surface soils of the saurashtra basin india abstract the stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near surface soil and sediments from the saurashtra basin were characterized for their origin and maturity saurashtra is considered geologically prospective for oil and gas reserves however a major part of the basin is covered by the deccan traps hindering the exploration of mesozoic hydrocarbon targets surface geochemical prospecting based on micro seepage of hydrocarbons from subsurface accumulations could be advantageous in such areas in light of this 150 soil samples were collected from the northwestern part of saurashtra around the jamnagar area where a thick sedimentary sequence of about 2 3 km exists under 1 1 5 km of deccan basalt the concentration of acid desorbed alkane gases from soil samples was found to vary in ppb as methane c 1 3 518 ethane c 2 0 430 propane c 3 0 331 i butane ic 4 0 297 n butane nc 4 2 116 i pentane ic 5 0 31 and n pentane nc 5 0 23 respectively fifteen samples with high concentrations of alkane gases were measured for their 13 c 1 13 c 2 and 13 c 3 compositions using gas chromatography combustion isotope ratio mass spectrometry gc c irms the values for methane varied from 27 to 45 4 ethane from 20 9 to 27 6 and propane from 20 4 to 29 1 versus the vienna peedee belemnite vpdb the carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter likely to be mainly a terrestrial source rock with a partial contribution from a marine source these gases may have migrated to the near surface environment where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity the maturity scale 13 c versus log ro applied to the surface sediment samples of the jamnagar area indicated the source material to be capable of generating oil and gas the detection of thermogenic alkane gases in near surface sediments offers the possibility of hydrocarbons at depth in saurashtra

predicting co2 occurrence on a regional scale southeast asia example abstract prediction of co 2 occurrence in natural gases is difficult owing to the interplay among basinal sources e g volcanic emanations carbonate hydrolysis and metamorphism organic diagenesis and maturation reservoir processes e g mineralogical transformations and poorly understood physicochemical phenomena e g differential solubility of gas components carbonate buffering phase segregation published approaches to assessing co 2 risk are based either on circumstantial evidence of basinal influences or statistical analysis of reservoir attributes the present study presents the results of empirical statistical and neural network analysis of co 2 abundance in natural gas and discusses potentially influential basinal and reservoir attributes to quantify risk factors for anomalous co 2 occurrences in southeast asia the results of this study indicate that tectonic setting including basement crustal age basement fault density reservoir temperature and reservoir pressure are key elements controlling co 2 abundance assignment of co 2 risk at the basin level can be roughly approximated using the results of this study more accurate assessments of co 2 risk at the basin play field and reservoir levels require more comprehensive geological and geochemical data as well as consideration of potentially influential physico chemical processes

dawsonite and ankerite formation in the ldx 1 structure yinggehai basin south china sea an analogy for carbon mineralization in subsurface sandstone aquifers abstract the geochemistry and petrology of the ldx 1 structure of the yinggehai basin a natural analog of a sedimentary carbon storage site was investigated to understand the consequences of the charging of co2 gas in this system the rocks in this structure are dominated by subarkose and sublitharenite sandstones the authigenic minerals formed after co2 injection are dawsonite microcrystalline quartz kaolinite and ankerite dawsonite and ankerite are formed just beneath a co2 bearing anticlinal structure due to the reactions between silicate minerals feldspars and clay minerals and the fluid phase carbon and oxygen isotopic analyses indicate that the main carbon source for dawsonite and ankerite formation was mantle magmatic co2 the aqueous activities of sodium and calcium the partial pressure of co2 ph and temperature are the key factors influencing the stability of the dawsonite and ankerite the presence of the anticlinal structure maintaining a locally high co2 partial pressure in the waters beneath this structure is likely responsible for the observed long term persistence of dawsonite and ankerite in this system

the origin and formation of co2 gas pools in the coal seam of the yaojie coalfield in china abstract high concentrations of co 2 have been observed in the no 2 coal seam in the yaojie coalfield a number of instantaneous co 2 outbursts occurred during coal mining the co 2 concentration range in the eastern coalfield is 18 79 to 96 6 the 13 c co 2 values are mainly in the range of 5 0 to 1 0 pdb suggesting an inorganic origin of co 2 the 3 he 4 he ratios are 0 6 25 9 10 8 and the r ra is 0 0042 0 185 which is a characteristic of crust derived co 2 gas the dynamic thermal metamorphism of the f19 ductile brittle shear zone is believed to result in the release of co 2 from basement marble formations which show an inorganic source of co 2 in the yaojie coalfield the regional geological evolution and multi periodical f19 fault movement control the formation migration and accumulation of the co 2 in addition to the development of co 2 gas pools in the yaojie coalfield the f19 fault played multiple roles in the generation transport and sequestration of gas during the co 2 formation the displacement of ch 4 carbonate generation and pore structure transformation of coal due to a series of physical and chemical effects occurred after co 2 flux into the coal seams