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

wide angle seismic imaging beneath an andesitic arc central north island new zealand 1 structures within the deep crust and upper mantle of a continental back arc region are imaged with seismic wide angle reflection and refraction profiling the data are from a new seismic experiment morc in the central volcanic region cvr of new zealand which is the active continental back arc system behind the hikurangi subduction zone nine 0 4 1 3 ton dynamite shots were recorded on 700 seismographs spaced along a 120 km long array there are two important discoveries from this study a lower crust 15 25 km depth rift pillow of rocks where p wave velocities vp are 6 8 7 1 km s and strong reflections on reversed shots from a limited length 18 km long reflector r3 at a depth of 32 1 km ray tracing shows the r3 reflector is in the upper mantle beneath the eastern margin of the cvr and is spatially linked to the active volcanic andesite arc and active geothermal fields the relative amplitude of the r3 reflections indicates they are best explained by an interface across which there is a 80 drop in the s wave seismic velocity vs and a modest 10 drop in vp such a change in wave speeds is consistent with r3 representing the upper surface of a body where the partial melt content may be as high as 12 on the basis of these new data we propose a model of heat and strain transfer to explain the relative distribution of heat output and andesitic and rhyolitic volcanism within a continental back arc basin

multiple origins of the paleogene natural gases and effects of secondary alteration in liaohe basin northeast china insights from the molecular and stable isotopic compositions abstract the molecular and isotopic c and h compositions of paleogene shahejie formation e 2 3 s and dongying formation e 3 d natural gases from 11 new gas samples as well as the data of 71 gases published from previous studies in the liaohe basin were analyzed and the results were considered in combination with geochemical and distribution characteristics of the source rocks the genetic type gas source correlation and secondary alteration of the natural gases were studied in detail and the source rock maturities of the gases were estimated the following conclusions were reached 1 with the exception of dry gases in the major part of the third member of the shahejie fm e 2 s 3 reservoirs the gases in other reservoirs are mainly wet 2 variable carbon and hydrogen isotopes of the alkane gases are observed in different reservoirs of the basin with 13 c 1 values ranging from 60 7 to 34 2 13 c 2 from 47 3 to 22 3 2 h ch 4 from 261 to 188 and 2 h c 2 h 6 from 269 to 148 3 the natural gases of the basin are of multiple origins and mixing of thermogenic and microbial is observed the mixing ratio in the first member of the shahejie fm e 3 s 1 is the highest among all the reservoirs 4 gases of different reservoirs suffered varying extents of biodegradation which caused partial reversal of the carbon isotope series of methane and its homologues 13 c 1 13 c 2 13 c 3 13 c 4 the reversal of the gases in the shallow reservoirs e 3 s 1 and e 3 d is greater while those in the deep reservoir e 2 s 3 is less 5 humic natural gases of the ed were mainly generated by source rocks at early maturity while the sapropelic ones of the ed and wd were generated during the mature and high mature stages 6 both the source rocks of e 2 s 3 and e 2 s 4 strata could contribute to the gas reservoirs of e 2 s 3 and e 2 s 1 in the west depression sapropelic gases in east depression were generated by e 2 s 3 source rocks while the other gases reservoired in the e 2 s 3 e 3 s 1 and e 3 d strata are probably sourced from the source rock strata of e 2 s 3 and e 3 s 1 2

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

origins of hydrocarbon gas seeping out from offshore mud volcanoes in the nile delta abstract this paper discusses the origin of gas seepages free gas or dissolved gas in ground water or brine sampled with the nautile submarine during the nautinil cruise at the seafloor of the deep water area of the nile turbiditic system on different mud volcanoes and brine pools generally the gas is wet and includes c1 c2 c3 ic4 nc4 co 2 these gas samples show no evidence of biodegradation which is not the case of the gas present in the deep hydrocarbon accumulations at depth it indicates that the gas expelled by the mud volcanoes is not issued from direct leakages from deep gas fields the collected gas samples mainly have a thermogenic origin and show different maturities some samples show very high maturities indicating that these seepages are sourced from great depths below the messinian salt moreover the different chemical compositions of the gas samples reflect not only differences in maturity but also the fact that the gas finds its origin in different deep source rocks carbon dioxide has an organic signature and cannot result from carbonate decomposition or mantle fluids the crustal derived radiogenic isotopes show that the analyzed gas samples have suffered a fractionation processes after the production of the radiogenic isotopes due either to oil occurrence at depth interacting with the flux of gas and or fractionation during the fluid migration

time scales of intra oceanic arc magmatism from combined u th and u th he zircon geochronology of dominica lesser antilles the island of dominica located in the intra oceanic lesser antilles arc has produced a series of intermediate mostly andesitic lava domes and ignimbrites since the early pleistocene u th he eruption ages from centers across the island range from 3 to 770 ka with at least 10 eruptions occurring in the last 80 ka three eruptions occurred near the southern tip of dominica plat pays volcanic complex in the past 15 ka alone zircon u th ages from individual centers range from near eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust overlapping zircon crystallization peaks within deposits from geographically separated vents up to 40 km apart indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously two lava domes from the southern sector of the island display exclusively young zircon rim ages 50 ka with narrow crystallization peaks consistent with the construction of a new magma reservoir the younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward arc parallel over time overall our data support geochemical models for the ongoing construction of a silicic intrusive complex consisting of varying amounts of crystal mush beneath the island u pb zircon ages 1 2 ma indicate that accumulation of this complex is entirely quaternary in age together zircon u th and u pb ages for dominica suggest that the magmatic processes and time scales operating in intra oceanic arcs are similar to those documented for continental arcs

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

geochemical characterization evolution and source of a mesozoic accretionary wedge the torlesse terrane new zealand whole rock major and trace element compositions of quartzofeldspathic permian cretaceous sandstones of the torlesse terrane new zealand display progressive changes which compare well with published petrographic data chemical indices in three permian early late triassic rakaia subterrane petrofacies show small contrasts as do the modes reflecting original source compositions a late triassic rakaia petrofacies displays sharp increases in sio 2 al 2 o 3 k 2 o na 2 o th sc la sc ce sc la n y n and decreases in ti zr and v la consistent with partial cannibalistic recycling and a small influx of lithic volcanic detritus in the post collisional late jurassic early cretaceous pahau subterrane sio 2 al 2 o 3 remains high but other indices decrease to levels seen in the permian due to recycling of the earlier rakaia petrofacies and influx of 10 30 mafic intermediate volcaniclastic detritus derived from inboard volcanogenic terranes the results are confirmed by additional suites including mudstones which cover a larger area but also show that some spatial variation occurs within the terrane as a whole whole rock data and chemical index of alteration indices indicate that the torlesse was derived from a relatively unweathered source with granodioritic bulk composition comparisons with data from a proposed rakaia source in the new england and hodgkinson orogens of eastern australia suggest that the observed chemical and modal compositions cannot be simply derived from that area as presently exposed

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

a laboratory study of the expansion of an irish pyritic mudstone siltstone fill material abstract extensive damage to dwellings in the east of ireland arising from the expansive pyritic fill material used beneath the floor slabs prompted the laboratory study presented in this paper to quantify the expansion and study its influencing factors a novel experimental arrangement in which sample height and water supply were varied was used to test a large quantity of active pyritic infill taken from an affected house in the greater dublin area the expansion of ten separate fill specimens was measured over a 175 day period from december 2010 to may 2011 remarkably consistent results were obtained when the expansions were normalised by the sample height with expansion rates of approximately 2 3 mm year m height of fill for specimens standing in water where the rates were largely insensitive to the amount of water supplied more rapid expansion was noted for one specimen to which no additional water was supplied during the test a steady reduction in ph of the water although relatively small was consistent with pyrite oxidation reactions occurring in the samples during the test and also implies the occurrence of leaching of ions from the pores in the mudstone and also that oxidation reactions may have been occurring in the saturated mudstone changes in expansion were found also to correlate strongly with changes in the ambient temperature in all specimens

origin of the torlesse terrane and coeval rocks south island new zealand the carboniferous to lower cretaceous torlesse terrane and haast schist derivatives constitute the major part of the complexly deformed facies of the eastern province of new zealand strata consist mainly of quartzofeldspathic graywacke and mudstone intercalated with minor but widely distributed conglomerate and volcanics with associated chert and limestone clastic rocks were deposited largely by sediment gravity flow mechanisms in a deep marine environment also present are a few highly fossiliferous shallow marine and terrestrial deposits of limited areal extent which rest unconformably on or in fault contact with torlesse flysch several periods of deformation are recognized and melange is present on both local and regional scales metamorphism ranges from zeolite to greenschist facies the bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages permian atomodesma middle triassic early late triassic torlessia late triassic monotis and late jurassic early cretaceous contacts between major fossil zones are mainly tectonic petrographic analysis permits subdivision of torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones the sandstone petrofacies mainly arkosic together with the composition of conglomerate clasts mainly indurated torlesse rocks indicate that the source terrane was a continental volcano plutonic arc probably part of gondwanaland coupled with autocannibalistic reworking of older uplifted torlesse rocks in contrast to the quartzofeldspathic nature of the torlesse coeval sedimentary rocks of the eastern province are volcanogenic they are thought to represent related forearc basin maitai murihiku terranes and trench complex caples terrane deposits derived from a volcanic island arc brook street terrane three petrofacies are established for maitai murihiku and caples sandstones the petrofacies indicate a common evolving immature to submature volcanic island arc source for these terranes a reconstruction of new zealand s eastern and western provinces is proposed in permian and triassic times the torlesse was deposited in trench slope or borderland basins along a trench transform margin fronting a continental volcano plutonic arc source western province gondwanaland deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of torlesse rock younging outward from the gondwanaland margin at the same time the brook street terrane volcanic arc and associated terranes were forming to the west of the torlesse site separated from gondwanaland by a marginal sea in latest triassic or early jurassic times the torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the gondwana margin the collision event resulted in tectonic thickening of torlesse and caples rocks at the plate interface and metamorphism to haast schist the source was then dominated by older partly metamorphosed torlesse terrane newly uplifted along the collision front closing of the marginal sea behind the brook street terrane in late jurassic early cretaceous times resulted in juxtapositioning with the western province gondwanaland along the median tectonic line

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

the pounamu terrane a new cretaceous exotic terrane within the alpine schist new zealand tectonically emplaced deformed and metamorphosed during collision of the lip hikurangi plateau with zealandia abstract the petrologically renowned haast schist previously acknowledged as having formed by jurassic collision and amalgamation of the exotic rakaia and indigenous caples terranes during late mesozoic orogenesis contains a component of the alpine schist whose protolith was deposited in the mid cretaceous an air fall tuff deposited on the ophiolitic pounamu ultramafic belt pub has an eruption age of 108 ma the pub is in tectonic contact to the east with rakaia terrane derived alpine schist of triassic age and is overlain by turbiditic metasediments that have cretaceous 109 140 ma detrital zircon populations the dominant zircon component of these sediments is permo triassic compatible with autocannibalistic reworking from the torlesse composite terrane reconnaissance analysis of the alpine schist indicates that cretaceous protoliths extend at east 200 km along strike while possible correlatives elsewhere in new zealand suggest that the original basin may have been thousands of kilometres in extent models for the emplacement of these exotic cretaceous sediments the proposed pounamu terrane are explored our favoured interpretation is that the cretaceous alpine schist protoliths were deposited in an ensimatic sedimentary basin north of earlier mesozoic subduction zones multi phase deformation and metamorphism occurred during collision of the lip hikurangi plateau with zealandia resulting in the emplacement of an ophiolite pub soled allochthonous thrust sheet of the pounamu terrane onto the previously metamorphosed rakaia otago schist caples massif based on new and regional evidence this collision and the climactic metamorphism of the pounamu terrane occurred at 72 ma

zircon u th pb dating using la icp ms simultaneous u pb and u th dating on the 0 1 ma toya tephra japan abstract simultaneous u pb and u th zircon dating using laser ablation inductively coupled plasma mass spectrometry la icp ms was performed on the 0 1 ma toya tephra hokkaido japan the method employs the concurrent measurement of 206pb 230th and 238u signals which enables the determination of 238u 206pb u pb and 238u 230th u th ages simultaneously the u pb ages revealed that the toya tephra contains zircons with ages that cluster at 0 1 ma 0 11 0 01 ma error shown as 95 confidence level and xenocrysts with ages 2 ma excluding zircons 2 ma the u th method also gave an isochron age of 0 1 ma 108 19 ka or 0 11 0 02 ma error shown as 2 therefore both u pb and u th methods result in similar ages of 0 1 ma for the toya tephra in agreement with the quartz tl age 104 30 to 118 30 ka of ganzawa and ike 2011 it was revealed that the simultaneous u pb and u th zircon dating technique using la icp ms is easy to apply and is useful for quaternary tephras in that it can give age information on the tephra itself and xenocrystic zircons in a quick and cost effective manner the double dating technique employed here has the merit that zircons yielding quaternary u pb ages can be immediately cross checked by the u th method it was also revealed that the toya tephra magma crystallized zircons and experienced eruptions instantaneously at 0 1 ma which is an important implication to decipher magmatic and eruptive processes for caldera forming large volcanic systems

crystallisation ages in coeval silicic magma bodies 238u 230th disequilibrium evidence from the rotoiti and earthquake flat eruption deposits taupo volcanic zone new zealand abstract the timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent successive eruption deposits in the taupo volcanic zone new zealand the earlier caldera forming rotoiti eruption 100 km3 magma at okataina volcano was followed within months at most by the earthquake flat eruption 10 km3 magma from nearby kapenga volcano both generated non welded ignimbrite and coeval widespread fall deposits the rotoiti and earthquake flat deposits are both crystal rich high silica rhyolites with sparse glass bearing granitoid fragments also occurring in rotoiti lag breccias generated during caldera collapse here we report 238u 230th disequilibrium data on whole rocks and mineral separates from representative rotoiti and earthquake flat pumices and the co eruptive rotoiti granitoid fragments using tims and in situ zircon analyses by sims multiple grain zircon controlled crystallisation ages measured by tims from the rotoiti pumice range from 69 3 ka 350 ka with a pronounced peak at 70 90 ka the weighted mean of isochrons is 83 14 ka in accord with the tims data one glass bearing rotoiti granitoid clast yielded an age of 57 8 ka by tims controlled by th rich phases that however are not apparently present in the juvenile pumices another glass bearing rotoiti granitoid yielded sims zircon model ages peaking at 60 90 ka having a similar age distribution to the pumice age data from pumices are consistent with a published 64 4 ka eruptive age now modified to 62 2 ka but chemical and or mineralogical data imply that the granitoid lithics are not largely crystalline rotoiti rhyolite but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies similarly although the earthquake flat eruption immediately followed and probably was triggered by the rotoiti event age data from juvenile material are significantly different a multiple grain zircon controlled crystallisation age measured by tims from a representative pumice is 173 5 ka while sims model ages range from 70 26 34 ka to 350 ka with a peak at 105 ka these age data coupled with previously published geochemical and isotopic data show that the rotoiti and earthquake flat deposits were erupted from independent unconnected magma bodies

massive vein filling gas hydrate relation to ongoing gas migration from the deep subsurface in the gulf of mexico abstract a gas hydrate mound that contains massive vein filling structure ii gas hydrate occurs on the upper continental slope 540 m water depth of the gulf of mexico southwest of the mississippi delta the mound is located in the green canyon gc block 185 adjacent to jolliet field in gc 184 jolliet field contains oil and gas that filled fault traps caused by salt deformation during late pleistocene holocene time in contrast to reservoir oil in jolliet field which shows bacterial oxidation effects the c 1 c 5 reservoir gas is unaltered by bacterial oxidation disassociated gas is assumed to have recently entered from the subsurface hydrocarbon system vertical migration of gas along faults is ongoing manifested on the sea floor by gas vents gas hydrate complex chemosynthetic communities and by a large gas plume that extends from the vents to the sea surface the isotopic properties of c 1 c 5 hydrocarbons from reservoirs gas vents and gas hydrate correlate closely although outcropping gas hydrate is transiently stable because of variations in seawater temperature the bulk of buried gas hydrate at gc 185 is stable and perhaps increasing in volume because of the copious gas flux the massive accumulation of gas hydrate at the gc 185 site is attributed to the gas that has recently entered the vents largely from jolliet field and to the synchronous activation of fault conduits allowing gas migration to the sea floor synchronous late gas charge and faulting could also explain the wide distribution of gas hydrate across the upper gulf slope

controls on hydrocarbon properties in a paleozoic petroleum system in saudi arabia exploration and development implications the unpredicted discovery of downdip oil in permian carboniferous unayzah sandstone reservoirs along the southern flank of the predominantly gas condensate ghazal field in saudi arabia highlighted the need for a thorough assessment of controls on hydrocarbon properties with the aim to improve prediction and reduce uncertainty primary controls on hydrocarbon property variations condensate to gas ratio here appear to be compartmentalization multiple charging presumably from a lower silurian qusaiba kitchen to the south and east within the late oil to wet gas generation window and dysmigration to the north and west gas on production represents mostly unmixed thermogenic accumulations of variable maturities with no sign of biodegradation or thermal cracking light oil 1 06 1 21 calculated vitrinite reflectance rc in the southern part of the field represent late mature accumulations that apparently escaped flushing by subsequent gas condensate charges generated near the wet gas generation window 1 41 1 55 rc and currently occupying the field to the north constant maturity variations for both gasoline range and heavier components in each sample testify to a lack of mixing between the hydrocarbon phases petroleum inclusions similarly comprise either light oil or gas condensate that coexist with significantly overlapping homogenization temperatures suggesting considerable overlap in their trapping history 135 35 ma less mature petroleum has not been identified in the field possibly because of a lack of trap closure or migration focus prior to the inferred late jurassic filling or because of displacement to updip structures thereby high grading the probability of undiscovered oil west and northwest of ghazal if thermal generation instead of retrograde condensation or phase separation controls this petroleum system then the potential for deeper oil accumulations is limited

thermal history and exhumation of basement rocks from mesozoic to cenozoic subduction cycles central north island new zealand a new thermochronological study of the basement rocks of the central north island new zealand records thermal and exhumation histories related to two subduction cycles since the latest jurassic the basement comprises metasedimentary terranes accreted onto eastern gondwana during mesozoic subduction since the oligocene these terranes have been located on the hanging wall of the hikurangi subduction margin overriding the pacific plate results of zircon fission track 121 264 ma analysis yield detrital or slightly reset ages apatite fission track 19 8 122 ma and u th sm he 10 3 95 2 ma ages are fully reset results from inverse thermal history modeling of the data of individual and stacked samples imply the following 1 after their accretion the basement rocks were cooled and exhumed to shallow depths of the crust in the early cretaceous by 150 135 ma this was followed by an episode of reheating until 100 ma which we interpret to be the result of burial by sedimentation above the accretionary wedge during extensional deformation caused by continued underplating and thickening of the wedge from beneath 2 from 100 ma our data indicate thermotectonic quiescence until the oligocene 27 ma 3 during initiation of the hikurangi margin the fore arc proto axial ranges began to exhume during the late oligocene to early miocene 27 20 ma subduction triggered unroofing removed 1 2 2 8 km of overburden most of which was probably the less lithified passive margin sediments 4 along the western central north island the exhumational response to the hikurangi subduction was limited with a total magnitude of erosion 1 5 km in the cenozoic

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

plutonic lithics in ignimbrites of taupo volcanic zone new zealand sources and conditions of crystallisation abstract plutonic rock fragments occur as rare accessory lithics within some of the ignimbrites erupted from calderas and caldera complexes in central taupo volcanic zone tvz new zealand the two main suites of plutonic lithics are 1 medium to fine grained granitoids and 2 fine grained dolerites and microdiorites the granitoids are mostly medium grained rocks with both equigranular and granophyric intergrowth textures the latter indicating rapid crystallisation at shallow crustal levels biotite is the predominant mafic phase in most granitoid samples although rare hornblende and orthopyroxene bearing granitoids also occur glass occurs in some granitoid samples from rotoiti ignimbrite indicating that these samples were incompletely crystallised prior to eruption dolerite and microdiorite fragments typically contain abundant hornblende and up to 10 vesicular interstitial glass sio2 abundances in the granitoids range between 70 3 and 77 9 they are weakly metaluminous to weakly peraluminous and show an enrichment in large ion lithophile lil elements rb k ba and a depletion in nb relative to morb similar to recent tvz rhyolitic volcanic rocks chemically and petrographically the granitoids fall into two distinct groups high sr granodiorites 120 205 ppm sr and chemically more evolved low sr monzogranites 26 65 ppm sr with an apparent compositional gap between the two eu eu anomalies and strong correlations between al2o3 cao and sr indicate that compositional variation between the two granitoid groups is dominantly controlled by plagioclase fractionation the high and low sr groups coincide with the highest and lowest sr concentrations observed in tvz rhyolites ree data and sr nd pb and o isotope ratios for granitoids are comparable to published data for tvz rhyolites and consequently they are considered to represent crystallised portions of zoned silicic magma chambers associated with quaternary tvz volcanic activity and not fragments of older pre tvz basement most granitoids are regarded as comagmatic with their host ignimbrites but those from rotoiti ignimbrite have different mineral assemblages and whole rock chemistry to the host ignimbrite these fragments appear to be incompletely crystallised portions of an earlier magma system possibly the same system that generated the matahina ignimbrite dolerites and microdiorites are geochemically similar to evolved high alumina basalts erupted from the central tvz and are considered to be intrusive equivalents

stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the ordos basin china ordos basin the second largest sedimentary basin in china contains enormous natural gas resources each of the four giant gas fields discovered so far in this basin i e sulige yulin wushenqi and jingbian has over 100 billion cubic meters bcm or 3 53 trillion cubic feet tcf of proven gas reserves this study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the ordos basin source rocks in the upper paleozoic coal measures are suggested by the generally high 13c values of c1 c4 gaseous hydrocarbons in the gases from the sulige yulin and wushenqi gas fields while the 13cic4 value is higher than that of the 13cnc4 the dominant ranges for the 13c1 13c2 and 13c3 values in these upper paleozoic reservoired gases are 34 to 32 27 to 23 and 25 to 24 respectively the 13c values of methane benzene and toluene in gases from the lower paleozoic reservoirs of the jingbian field indicate a significant contribution from humic source rocks as they are similar to those in the upper paleozoic reservoirs of the sulige yulin and wushenqi gas fields however the wide variation and reversal in the 13c1 13c2 and 13c3 values in the jinbian gases cannot be explained using a single source scenario thus the gases were likely derived dominantly from the carboniferous permian coal measures with some contribution from the carbonates in the lower permian taiyuan formation the gas isotope data and extremely low total organic carbon contents 0 2 toc suggest that the ordovician majiagou formation carbonates are unlikely to be a significant gas source rock thus almost all of the economic gas accumulations in the ordos basin were derived from upper paleozoic source rocks

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

structure and evolution of the wairakei tauhara geothermal system taupo volcanic zone new zealand revisited with a new zircon geochronology abstract the central part of the taupo volcanic zone tvz new zealand is notable for the number 23 and size of its high temperature geothermal systems of these the wairakei tauhara geothermal system collectively represents the largest commercially developed example natural thermal output prior to utilisation i e before 1950 was 530 mw since 1950 drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework within this framework the strata are dominantly rhyolitic pyroclastic rocks and lava bodies with interbedded fluvial and lacustrine deposits until now no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25 4 ka oruanui ignimbrite or to buried ignimbrite that is correlated with the 349 ka whakamaru group this paper presents 33 suites of u pb age data and a limited number of reconnaissance u th disequilibrium age data from magmatic zircon crystals zircons to date the rocks of the wairakei tauhara system the oldest units dated are intermediate lavas from ca 1 7 ma while the youngest locally vented eruptives of tauhara dacite are tentatively dated to ca 60 ka placing the age data within the established drillhole record allows for the recognition of major volcano tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the wairakei tauhara area in contrast to the rotokawa and ngatamariki geothermal systems 10 20 km to the northeast the wairakei tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka leading to major faulting and abrupt variations in thicknesses of units the faulting and deposit thickness variations established in these two events remain major controls on present day permeability of the geothermal system the new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity however from zircon age data from the rhyolite andesite mixed tauhara dacite we propose that the current geothermal system was initiated at ca 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since

evidence from zircon u pb age spectra for crustal structure and felsic magma genesis at taupo volcano new zealand u pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at taupo volcano as well as unexpected crustal structure beneath the taupo volcanic zone ages from the xenolith in a 28 ka rhyolite dome match those from early cretaceous pahau terrane graywackes that crop out 75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process boundaries between graywacke terranes in the north island are thus more complex than implied from map views in contrast ages from zircons in the 20 ka omega dacite match the nearby surficial graywacke of the jurassic kaweka terrane 55 of grains quaternary plutonic sources 39 and pahau terrane graywacke 6 five of 72 grains analyzed contain graywacke cores and pleistocene rims survival of euhedral xenocrystic zircons through the generation processes for omega dacite magma reflects wholesale melting of crustal protoliths including varieties not previously considered for petrogenetic modeling open system behavior and crustal melts being generated and erupted within 1 10 yr

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

geochemistry of petroleum systems in the niuzhuang south slope of bohai bay basin part 2 evidence for significant contribution of mature source rocks to immature oils in the bamianhe field the chemical compositions of 28 representative crude oils and oil sand samples collected from the bamianhe region of the bohai bay basin eastern china were studied in detail in order to understand their genetic relationships biodegradation ranking and thermal maturity level molecular geochemical parameters indicate the presence of one single oil family in the cenozoic reservoirs a number of biomarker features support deposition of the source rocks for these oils under conditions of enhanced salinity stratified water columns the extremely low maturity levels of these immature oils as indicated by sterane isomerization ratios were not supported by other maturity parameters the genetic relationship between the oils and the es4 and es3 members of the eocene oligocene shahejie formation present in the deeper part of the niuzhuang sag but not those along the niuzhuang south slope was suggested using a wide range of molecular parameters variation in the sterane concentrations observed from potential source rocks with different maturity levels in the study area suggests a high probability of migration contamination of mature oils by immature bitumens along migration pathways or from intra reservoir shales variation in the level of oil biodegradation could also create anomalies in the use of gross oil compositions as an indicator of oil maturity although the recent rapid subsidence of the bohai bay basin and the saline depositional environment of one of the potential source rocks favor oil generation at relatively low maturity levels most of the economic oils discovered in the bamianhe region appear to have derived from conventionally mature source rocks but contain lower maturity fluids admixed with them

the occurrence and origin of methane in some groundwater flow systems the origin of methane in selected groundwater flow systems is investigated through carbon isotope geochemical studies combined with hydrogeological and hydrogeochemical information methane is a common trace constituent in these groundwaters and occasionally constitutes a major carbon pool two modes of origin for methane are commonly recognized microbial production biogenic methane and non biological chemical production thermocatalytic methane in the groundwaters sampled the few thermocatalytic methanes encountered have 13c values greater than about 45 whereas the more common normal biogenic methanes have 13c values less than about 60 biogenic methanes from sanitary landfill sites have intermediate 13c values of about 38 to 56 and so can easily be distinguished from normal biogenic methane the carbon source for biogenic methane is usually organic matter present in the aquifer materials the carbon isotope ratios of dissolved methane and total inorganic carbon in sampled groundwaters ar

critical wedge theory and the mesozoic accretionary wedge of new zealand abstract we apply principles of critical wedge theory to explain the jurassic and early cretaceous tectonic development of the mesozoic accretionary wedge of new zealand one of the best exposed subduction complexes on earth by summarizing factors that can lead to drastic changes in wedge balance the wedge has a carboniferous to cretaceous 330 110 ma stratigraphic record with an early cretaceous 145 130 ma hiatus in clastic deposition across the esk head melange near the front of the wedge in the rear of the wedge the age of moderately high pressure metamorphism of the otago schist which represents the forearc high of the wedge occurred near the end of this stratigraphic hiatus the otago schist has a subhorizontal foliation indicative of underplating as the mode of accretion the outer frontal part of the wedge has a subvertical foliation indicative of frontal accretion we propose that in the jurassic and earliest cretaceous the wedge grew by frontal accretion by 135 ma the wedge became too long and was subcritical this resulted in tectonic erosion of the front of the wedge the tectonically eroded material was underthrust underplated beneath the rear of the wedge to form the subhorizontally foliated otago schist colliding seamounts and or oceanic plateaux preserved in the esk head melange may have aided wedge instability the proposed wedge instability and increased horizontal shortening was largely coeval with major changes in the chemistry of subduction related arc magmatism and a phase of shortening and high pressure metamorphism in the arc our model provides a holistic and coherent interpretation explaining the major tectonic features of the long lived new zealand part of the east gondwana subduction zone

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

origin of light hydrocarbons in gases from mud volcanoes and ch4 rich emissions abstract this paper presents new chemical data of hydrocarbon rich gases discharged from mud volcanoes and ch4 rich emissions located in different areas of italy northern apennines and sicily the determination of c5 c10 alkanes cyclics and aromatics was carried out by gc ms while the main gas species and c1 c4 hydrocarbons were analyzed by gc tdc and gc fid respectively methane is by far the most abundant component of all the investigated gas emissions with the exception of part of those discharging close to mt etna volcano which are co2 rich the gas samples collected from the emilia apennine and sicily show d ch4 and 13c ch4 values typical of thermogenic gases most gases from the romagna apennine discharge methane with a biogenic isotopic signature non methane hydrocarbons of biogenic gases are almost exclusively c2 c4 alkanes with minor amounts fraction of mol mol of c4 compounds including few aromatics and cyclics likely deriving from minor thermogenic contribution not evidenced by the 13c ch4 and d ch4 values the etnean gases whose r ra and 13c co2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system show a significant enrichment of aromatics compounds up to 36 of the non methane organic gas fraction which is likely produced by catalytic reforming processes such as dehydrocyclization of alkanes the thermogenic gases from the apennines and southwestern sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several mol mol cyclic compounds are likely formed by i thermal cracking and ii uncompleted aromatization of alkanes occurring at depth 3 km and temperatures not exceeding 120 150 c

natural gas genesis and sources in the zizhou gas field ordos basin china abstract the zizhou gas field is a large tight sandstone gas field in the ordos basin china based on the analysis of the geological background gas components light hydrocarbon composition carbon and hydrogen isotope values of alkane gases and the geochemical correlation between gases of the zizhou gas field and analogs from other fields in the basin we conclude that a most natural gases in the zizhou gas field are dry with a small content of wet gas b the gases have similar carbon isotopic and light hydrocarbon compositions to the analogs from other fields as well as much more positive 13c values and high contents of c5 7 i alkanes and methylcyclohexanes indicating typical coal derived gas c carbon isotopic reversal in some gas samples is due to mixing of coal derived gases of different maturities d the gases are sourced from mature to high mature carboniferous permian coal measures and e the gas reservoirs represent continuous accumulations

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

u pb dating of magmatic and xenocryst zircons from mangakino ignimbrites and their correlation with detrital zircons from the torlesse metasediments taupo volcanic zone new zealand abstract mangakino ignimbrites are a sequence of voluminous eruptive units in the rhyolitic mangakino volcanic centre mvc caldera complex taupo volcanic zone new zealand u pb dating of 113 zircons extracted from four mangakino ignimbrites reveals magmatic and xenocrystic zircons magmatic zircon ages from three ignimbrites are slightly older than k ar and 40 ar 39 ar ages magmatic zircon age spectra are tightly clustered and support a simple crystallisation history for these three ignimbrites it is possible that the assimilation of zircon antecrysts leads to magmatic zircon spectra for the ahuroa ignimbrite being 70 ka older than the 40 ar 39 ar age xenocrystic zircon age spectra correlate with spectra of detrital zircons from the underlying mesozoic torlesse metasediments our data are consistent with significant assimilation of metasediments and the incorporation of zircon prior to eruption systematic variations in the age spectra of xenocrystic zircons in successive mangakino ignimbrites may be related to the structural development of the north island new zealand

production consumption and migration of methane in accretionary prism of southwestern taiwan to systematically quantify the production consumption and migration of methane along a continental margin 210 sediment cores were collected from offshore southwestern taiwan and analyzed for their gas and aqueous geochemistry these data combined with published results were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance to constrain deep microbial and thermogenic methane production rates within the accretionary prism the results showed that methane diffusive fluxes ranged from 2 71 10 3 to 2 78 10 1 and from 1 88 10 1 to 3 97 mmol m 2 d 1 at the sulfate methane transition zone smtz and sediment seawater interfaces respectively high methane fluxes tend to be associated with structural features suggesting a strong structural control on the methane transport a significant portion of ascending methane 50 is consumed by anaerobic oxidation of methane at the smtz at most sites indicating effective biological filtration gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater the flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1 512 43 096 tg myr 1 methane and could be likely accounted for by the sequestration of methane into hydrate forms and clay absorption

geochemistry and possible origin of crude oils from bashituo oil field tarim basin the entire range of hydrocarbons in bashituo oils from devonian and carboniferous reservoirs were investigated to determine their origin as well as their alteration mixing and maturity using gas chromatography gas chromatography mass spectrometry and stable carbon isotope analysis these geochemical studies indicate that the crude oils in the devonian and carboniferous reservoirs share similar c15 molecular marker compositions of marine organic matter origin whereas their light hydrocarbon compositions show distinct differences the carboniferous reservoired oils are characterized by a relatively greater abundance of cycloalkanes and methylcyclohexane which indicate a terrestrial organic matter om contribution based on biomarker and carbon isotope analysis cambrian ordovician j o marine rocks are assumed to be the main source for both the devonian and carboniferous reservoired oils whereas carboniferous rocks with terrestrial om input also contributed to the carboniferous reservoired oils the coexistence of 25 norhopanes evident humps from unresolved complex mixture and intact n alkanes in devonian reservoired oils indicate a mixture of early charged biodegraded oils with late fresh oils corresponding to at least two oil generation episodes by the j o rocks light hydrocarbon indicators suggest a relatively high maturity beyond peak oil generation for the j o sourced late fresh oil whereas c15 molecular marker parameters indicate a maturity equivalent to early peak oil generation for the j o sourced early charged biodegraded oil the maturity of the carboniferous sourced oil is equivalent to peak oil generation the application of the entire range of hydrocarbons is essential when assessing a mixed or altered oil system because light hydrocarbons and biomarkers may yield different source oil correlations and maturities

correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components the natural distribution of carbon isotopes between hydrocarbon gas components is used for 1 determining a gas s maturity 2 correlating a reservoired gas to its source 3 correlating one reservoired gas with another and 4 recognizing gas mixtures calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single continuous diagram independent of source type actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock levels of organic metamorphism ranging from 8 to 13 covering the entire range of oil and wet gas generation at greater maturities the wet gas components are found to undergo thermal degradation losing their usefulness for correlation three examples showing indigenous gas west texas non indigenous gas gippsland basin australia and gas mixtures from multiple sources southeastern alberta illustrate exploration applications

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

tracking deep crust by zircon xenocrysts within igneous rocks from the northern alxa china constraints on the southern boundary of the central asian orogenic belt abstract the southern boundary of the central segment of the central asian orogenic belt caob with the precambrian alxa block is not well constrained due to poor recognition of deep crust statistical analysis of zircon xenocrysts within igneous rocks from the northern alxa and its adjacent regions was applied to resolve this problem we compiled new and previously published geochronological zircon age data obtained by shrimp sims tims and la icp ms for 316 igneous rock samples of which 61 samples contain zircon xenocrysts new and previously published hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study the zircon xenocrysts are mainly contained within permian rocks from the yabulai nurgong honggueryulin ynh zone and igneous rocks from the northwestern margin of the north china craton wncc a few xenocrysts were also found in permian igneous rocks from the zongnaishan shalazhashan zs zone neo proterozoic and paleo proterozoic intrusive rocks in the ynh zone as well as devonian to carboniferous granitoids and volcanic rocks from southern mongolia altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10 discordant xenocryst ages within the permian igneous rocks from the zs zone are mainly around ca 350 ma ca 600 ma and ca 1400 ma the oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the caob 1 1 ga by contrast abundant zircon xenocrysts within permian igneous rocks from the ynh zone show highly variable age populations at ca 2 6 2 1 ga 1 8 1 6 ga 930 750 ma and 460 300 ma zircon xenocrysts from the zs zone have positive e hf t values of 6 3 to 13 9 whereas those from the ynh zone display highly variable e hf t values from 16 1 to 11 6 from the southern caob and the zs zone to the ynh zone the zircon xenocrysts show a significant shift from juvenile to crustal hf isotopic compositions suggesting that the area between the zs and the ynh zones constitutes the southern boundary of the caob in the deep crust within this region our study indicates that statistical analysis or isotopic mapping of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly formed orogenic domain and ancient cratons

natural gas constituent and carbon isotopic composition in petroliferous basins china abstract there are abundant gas resources in petroliferous basins of china large to midsize gas fields are found in eastern central and western of china however origin constituents and isotopic composition of natural gas in different gas fields are varied distinctly and some present strong chemical secondary alteration and show variation both in age and space based on the systematic analysis of constituents and carbon isotope of a large number of gas samples combined with the geological characteristics this paper classifies the origins of the gases explores the gas isotope characteristics and evolutionary regulation with the variation time and space and further discusses the distinctive geochemistry of the gases in china these gases are dominated by dry gas its methane carbon isotope values range from 10 to 70 ethane from 16 to 52 propane from 13 to 43 and butane from 18 to 34 the carbon isotopes of most gases show the characteristics of humic derived gas and crude oil cracked gas in addition large primary biogenic gas fields have been discovered in the qaidam basin inorganic derived alkane gases have been discovered in deep of the songliao basin half of these gas fields are characterized by the alkane carbon isotope reversal in different degrees research indicates there are several reasons can result in carbon isotope reversal firstly gas charge of different genetic types or different source in one gas reservoir may cause carbon isotope reversal besides high over mature evolution of gas can also lead to the carbon isotopic reversal of alkanes thirdly secondary alteration of hydrocarbons may also result in abnormal distribution of carbon isotope isotope transforms to unusual light and heavy

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

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

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