the δ15n of nitrate in the southern ocean consumption of nitrate in surface waters we report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern indian and pacific sectors of the southern ocean focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton the 15n of nitrate increases by 1 2 from deep water into the antarctic summertime surface layer due to kinetic isotopic fractionation during nitrate uptake estimation of the nitrate uptake isotope effect from antarctic depth profiles yields values in the range of 5 6 in east indian sector and 4 5 in the east pacific sector surface transect data from the pacific sector also yield values of 4 5 the major uncertainty in the profile based estimation of the isotope effect involves the 15n of nitrate from the temperature minimum layer below the summertime antarctic surface layer which deviates significantly from the predictions of simple models of isotope fractionation for the subantarctic surface it is possible to distinguish between nitrate supplied laterally from the surface antarctic and nitrate supplied vertically from the subantarctic thermocline because of the distinctive relationships between the 15n and concentration of nitrate in these two potential sources our subantarctic samples collected during the summer and fall indicate that nitrate is supplied to the subantarctic surface largely by northward transport of antarctic surface water isotopic data from the pacific sector of the subantarctic suggest an isotope effect of 4 5 indistinguishable from the antarctic estimates in this sector

application of new technology liquid scintillation spectrometry to radiocarbon dating of tephra deposits new zealand abstract two major technological advances in the radiocarbon dating method have recently enhanced its potential application to tephra studies the advent of accelerator mass spectrometry and the development of new technology liquid scintillation ls spectrometry the new technology ls spectrometer represents a significant refinement of the conventional dating method based on liquid scintillation counting of benzene it improves upon conventional ls counting by allowing spectral analysis and by providing a high degree of counting stability and efficiency in an ultra low level background radiation environment these attributes enable radiocarbon dating with greater accuracy and statistical precision and also allow the determination of both smaller and older samples than previously possible by conventional radiometric methods the new wallac oy quantulus ls spectrometer has been in operation at the university of waikato radiocarbon dating laboratory since 1988 the instrument achieves ultra low background levels by both passive and active forms of shielding the shielding comprises a massive asymmetric passive lead shield surrounding an aticoincident liquid scintillation guard in addition the quantulus contains dual multichannel analyzers which allow spectral analysis and windowless data acquisition the quantulus ls spectrometers at the waikato laboratory have been used to date a variety of carbonaceous materials associated with tephra deposits in new zealand ranging in age from ca 0 1 55 ka in particular we have tested the capability of the quantulus for determining ages of samples that are typically more difficult or impossible to date by conventional methods 1 very young samples e g tarawera tephra erupted ca 100 years ago 2 older samples e g mangaone tephra erupted ca 30 ka and 3 samples containing only sparse carbon e g lake sediments associated with tephra layers aged ca 20 ka or less the quantulus ls spectrometer is both more accurate and precise statistical counting errors are reduced than conventional instruments is capable of extending the limits of detection at both ends of the age scale and has a smaller sample handling ability analyses can be obtained at comparatively low cost such advances are potentially beneficial to tephrochronology and volcanology and a wide range of applications in quaternary research

using natural maturation series to evaluate the utility of parallel reaction kinetics models an investigation of toarcian shales and carboniferous coals germany abstract open system pyrolysis is routinely performed on immature samples in order to determine the kinetic parameters of petroleum generation at both bulk and molecular levels this study tested such predictions for type ii and type iii organic matter by making calibrations with both artificial and natural maturity sequences of toarcian shales posidonia shale 0 48 1 44 rr and carboniferous vitrains 0 74 2 81 rr both natural series showed little or no compositional variability attributable to kerogen type artificially matured samples were prepared by non isothermal heating 0 7 k min of the least mature samples up to end temperatures between 375 and 470 c under either closed or open system conditions measured generation rate versus temperature curves were analyzed assuming a distributed system of activation energies and a single frequency factor in each case the resulting kinetic parameters were then used to assess bulk petroleum formation rates for geological heating conditions in the case of all artificially matured samples measured and predicted bulk petroleum formation rate vs temperature curves for each maturation stage remain within the original envelope defined by the least mature sample despite an upward shift of tmax temperatures this confirms that the reactions taking place during both the pyrolysis measurements and simulated maturation processes are the same involving mainly homolytic cracking a similar pattern of measured and predicted rate curves is reproduced by the natural maturation sequence of the toarcian shales in the maturity range of 0 53 to 1 44 rr suggesting that petroleum generation within these natural systems also results from cracking reactions and therefore that petroleum generation over geological time can be reliably extrapolated from open system pyrolysis of the appropriate immature sample by contrast significant deviations are observed for the natural coal series with measured and predicted rate curves extending beyond the immature envelopes in accordance with the pronounced increase of frequency factors and of protonated aromatic carbon concentrations this behavior is attributed to solid state aromatization reactions which compete with product generation during natural coalification but which are not reproducible to the same extent by experimental heating it is concluded that petroleum generation from vitrinitic coals over geological time cannot be reliably extrapolated from open system pyrolysis of low rank samples

the early toarcian and cenomanian turonian anoxic events in europe comparisons and contrasts two intervals of mesozoic time are demonstrably of sufficient geological brevity to qualify readily for the term anoxic event the earlyfalciferum zone orexaratum subzone of the toarcian and thewhiteinella archaeocretacea interval zone that straddles the cenomanian turonian boundary both periods of time saw regional deposition of anomalously organic rich strata black shales accompanied by a positive 2 pdb carbon isotope excursion in coeval biogenic carbonates and significant faunal change the duration of both events was probably less than half a million years and both were preceded by regional erosion and possible upwelling coincident sea level rise and transgression is likely a european view of these events however spotlights some significant differences whereas thefalciferum zone organic matter is generally more concentrated and more hydrogen rich on the palaeo european shelf than on the tethyan continental margins the reverse holds true for the black shales formed during thewhiteinella archaeocretacea interval zone furthermore the carbon isotope composition offalciferum zone organic matter from both north european and tethyan sites is anomalously negative compared to that developed in thewhiteinella archaeocretacea interval zone differing planktonic organisms may have been involved and their possible diverse ecological requirements may go some way to explaining the nature and distribution of facies deposited during these two events

thermal cracking of kerogen in open and closed systems determination of kinetic parameters and stoichiometric coefficients for oil and gas generation abstract the purpose of the study is to compare artificial maturation of kerogens representative of the main types of organic matter kerogen types i ii ii s and iii in open and anhydrous closed pyrolysis systems the generated compounds are fractionated according to their thermal stability into six chemical classes c 1 c 2 c 5 c 6 c 14 c 15 saturates c 15 aromatics and nsos which include resins and asphaltenes in both systems primary cracking of kerogen can be described by two main reactions oil generation together with early gas generation followed by late gas production mass balances obtained in the two pyrolysis systems are reasonably similar although some secondary cracking of the nsos and underestimation of methane potential due to incomplete pyrolysis may occur in the open system kinetic parameters of thermal cracking are derived from experiments in the open system using an optimization procedure which determines a unique frequency factor and a weight distribution of chemical classes for a discrete series of activation energies the resultant frequency factors are checked with those obtained for specific molecular tracers such as n c 12 and n c 24 alkanes generated from kerogens in open and closed systems under isothermal conditions the results show that using a unique frequency factor as determined by optimization is correct for type i ii and ii s kerogens this factor is more questionable however when the distribution of activation energies is very broad such as for type iii coals based on this study a strategy for determination of kinetic parameters on any new source rock sample is proposed

organic geochemistry of freshwater and alkaline lacustrine sediments in the green river formation of the washakie basin wyoming u s a abstract two members of the green river formation in the washakie basin have been analysed by organic geochemical and organic petrographic techniques and the results placed in a sequence stratigraphic framework one of these members the laney shale was deposited in a hydrologically closed alkaline lake under an arid climate and was characterized by high concentrations of alginite rich organic matter whereas the other the luman tongue was deposited in a hydrologically open freshwater lake under a humid climate and consisted of organic poor profundal lake mudstones and coaly lake margin sediments potential source rocks in both lake types have the potential for generating high wax oil at high subsurface temperatures 150 175 c at a heating rate of 5 k ma but the alkaline system is clearly more prolific the most abundant biological marker hydrocarbons were the 4 methylsteranes in both lake types though dinosteranes were found only in the laney shale oleanane was absent from the luman tongue despite an abundance of angiosperm palynomorphs intraformational heterogeneities were best documented for the laney shale where molecular differences in kerogen type occur at the parasequence level with early transgression maximum transgression and rejuvenation stages of lake history having its own kerogen type and distinctive high wax oil fingerprint thermal lability was shown to be linked to kerogen structure in particular the presence of alicyclic and oxygen containing moieties and these in turn are correlated with total organic carbon content and thence productivity degree of preservation some kerogen components appear to have originated via selective preservation whereas others may have been formed by diagenetic condensation reactions

concepts for estimating hydrocarbon accumulation and dispersion the present volume and composition of trapped hydrocarbons are temporary events controlled by the interacting volumes of trap oil and gas all of which change through time thus both the volume and type of trapped hydrocarbon result from the interacting effects of kerogen amount type and maturity gas and oil migration efficiencies trap growth and the possibility that free gas may displace trapped liquids by estimating the historical evolving effects of each factor we can estimate trapped hydrocarbon volume as well as trap content i e only oil oil leg and gas cap or only gas the procedures that lead to such estimates are 1 estimate the quantities of oil and gas provided by the source within the drainage area 2 estimate the quantities of gas lost by dissolution and diffusion 3 estimate the trap volume and 4 compare hydrocarbon volumes with trap volume to find the limiting volume

holocene vegetation climate and history of a raised bog complex northern new zealand based on palynology plant macrofossils and tephrochronology a holocene history of vegetation climate and ombrogenous mire development is presented from pollen and plant macrofossil analyses of sediments at kopouatai bog a large raised restiad bog in northern new zealand tephra layers of established ages supplemented by numerous radiocarbon dates provide a secure chronology the earliest peats overlying last glacial sediments and dated at c 11700 radiocarbon years bp with extensive accumulation after c 10360 bp are dominated by pollen of warm temperate podocarp angiosperm forest indicating a moist mild early holocene climate the bog began as a series of small soligenous mires within lowland podocarp dominated swamp forest but was mostly oligotrophic by c 8500 bp peat accumulation rates have varied spatially and temporally averaging 0 9 mm yr 1 in central and southern areas the deposition of deltaic muds in the northern part of the bog accompanied a marine transgression c 6500 5000 bp while elsewhere an associated groundwater table rise result

rerewhakaaitu tephra a land sea marker for the last termination in new zealand with implications for global climate change abstract the rerewhakaaiutu tephra erupted from okataina volcanic centre north island new zealand at 14 700 95 14 c yr bp ca 17 600 cal yr bp at a time of rapid re organisation of earth s climate system at the end of the last glacial termination i it provides a distinctive isochron in a range of different environments in north island and in adjacent south pacific ocean sediments terrestrial evidence based on fluvial aggradation and downcutting relationships loess accumulation rates palaeovegetation patterns and buried soil development and mineralogy shows that marked amelioration of climate occurred shortly before the rerewhakaaitu tephra was deposited similarly marine evidence from around this time shows major changes in accumulation rates of sediment and aeolian quartz and in the abundance of various marine organisms while foraminiferal oxygen and carbon isotope records indicate that the arrival of the glacial meltwater signal occurred close to or just after the deposition of the rerewhakaaitu tephra these changes are discussed in relation to controls on climate by oceanic and atmospheric mechanisms the re organisation of climate commencing at ca 15 000 14 500 14 c yr bp ca 18 000 17 400 cal yr bp is detected elsewhere in the southern hemisphere and evidently was linked to orbitally forced warming which is thought to have initiated ice retreat in both hemispheres

prediction of petroleum formation the influence of laboratory heating rates on kinetic parameters and geological extrapolations while major variations in predictions of petroleum formation timing are supposed to be related to the burial history of the source rock and the type of organic matter preserved therein it is often ignored that the laboratory techniques used to obtain reliable kinetic parameters for the description of petroleum formation and its extrapolation to natural conditions can be problematic in the present paper bulk petroleum generation by open system programmed temperature pyrolysis of green river shale toarcian shale and westfalian coal samples has been studied comparatively at heating rates of 0 1 0 7 5 0 15 0 and 25 0 k min in order to evaluate the influence of laboratory heating rates on kinetic parameters and geological extrapolations kinetic parameters were calculated from three sets of pyrolytic generation rate versus temperature curves measured at either 0 1 0 7 and 5 0 0 7 5 0 and 15 0 or 5 0 15 0 and 25 0 k min using both the distributed and the tmax shift model frequency factors and dominant activation energies derived from the tmax shift model are found to decrease systematically in going from sets of slow to fast heating rates for type ii and iii kerogens this decrease of kinetic parameters is confirmed by the distributed model for algal kerogens of type i the distributed model cannot be applied the decrease of kinetic parameters results from increasing delay in heat transfer between the inert gas stream and the sample with increasing rate of heating reducing the sample weight can partly but not fully compensate this effect by using the natural maturation series of westphalian coals and toarcian shales it was possible to illustrate that geological extrapolations from slow heating rate experiments fit much better to natural petroleum formation than those from fast heating rates the latter lead to temperature predictions which are too low by 30 40 c which in turn would result in a 1000 m failure when predicting the oil window depth in an average sedimentary basins

organic matter maturation under the influence of a deep intrusive heat source a natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock toarcian shale northern germany four shallow boreholes were drilled in the hils syncline northern germany in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen type ii bearing source rock the holes penetrated the carbonceous lias shales posidonia shale lower toarcian and part of the adjacent dogger and lias mudstones the maturity of the organic matter in the cores recovered ranges from immature 0 48 r0 to overmature 1 45 r0 due to location of the hils syncline in the vicinity of the vlotho massif which is deep seated intrusive body facies variations of the lias within the short geographical distances in the study area are negligible organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the lias shales kerogen bitumen etc and on the evidence of a uniform initial composition of these sediments in the study area dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations about 50 of the initial kerogen was transformed into oil gas and inorganic compounds during the vitrinite reflectance increase from 0 48 to 0 88 ro and only marginally more during the maturity increase from 0 88 to 1 45 ro only a small portion of the generated material remained in the source rock even at a relatively early stage of generation 0 68 ro expulsion efficiency of oil plus gas reached a value of 86 at the end of the main generation stage 0 88 ro

source rock heterogeneity of the upper jurassic draupne formation north viking graben and its relevance to petroleum generation studies abstract the upper jurassic draupne formation in the north viking graben north sea has been studied to identify lateral and vertical variations in source rock characteristics and their effect on the timing of petroleum generation fourteen source rock samples from the western flank of the graben and five from the eastern flank were analysed using rock eval analysis microscopy open system pyrolysis gas chromatography and kinetic experiments two major organic facies with different petroleum generation characteristics were identified a stratigraphically higher thermally less stable facies corresponding to a type ii kerogen with hi hydrogen index values of 350 mg hc g toc and a stratigraphically lower more stable type ii iii facies with hi values

geochemical effects of petroleum migration and expulsion from toarcian source rocks in the hils syncline area nw germany for mature oil prone source rocks of toarcian age in the hils syncline area nw germany the geochemical effects of primary migration of petroleum are shown expulsion has occurred at high efficiencies primarily via a network of macro and micro fractures the effects of this fracture controlled expulsion with respect to gross and molecular composition of c15 soluble organic matter are documented samples from fractured intervals have expelled total c15 soluble organic matter more efficiently than adjacent unfractured intervals a systematic comparison of samples taken at cm intervals away from macro fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures extracts of shale samples directly at fracture walls are enriched in nso compounds and highly polar asphaltene like fractions the composition of residual petroleum fluids extracted from the fracture fill materials is variable in some cases there is a close match with that of the shale at some distance away from the fractures in other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes rapid heating by a nearby intrusive body at shallow depths when the toarcian sediments still contained about 15 20 pore water has played an important role overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature toarcian sections finally the closely interdependent relationships between petroleum generation and petroleum migration processes are documented especially the evolution of these relationships with maturity progress from immature 0 48 rm to mature 0 68 and 0 88 rm and overmature 1 45 rm stages is shown for example in the maturity interval from 0 68 to 0 88 rm most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum for some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities

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

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

high spatial resolution of pollen and charcoal in relation to the c 600 year bp kaharoa tephra implications for polynesian settlement of great barrier island northern new zealand analysis of high spatial resolution of nine pollen profiles 150 m 6 5 km apart from great barrier island shows that between 7500 and 600 calibrated year bp the island had a low frequency of natural fires compared with elsewhere in the northern north island except for one site which has locally sourced pre kaharoa charcoal source of this charcoal in the awana kaitoke area is uncertain if local pre kaharoa burning did occur at other sites in this area it was patchy occurring at different times in different places and was small scale charcoal was first recorded c 1700 year bp then again after c 1200 year bp pre kaharoa charcoal on great barrier may be interpreted as either an increased frequency of natural fires in the region due to climatic change to drier conditions or small scale localized initial human impact or some combination of these factors major post kaharoa burning in the awana kaitoke area was also patchy commencing at different times in different places the presence of the kaharoa tephra on great barrier island allows the commencement of major sustained polynesian deforestation at awana kaitoke to be reliably dated to c 600 year bp at some sites and possibly up to 50 years later at other sites

determination of gross kinetic parameters for petroleum formation from jurassic source rocks of different maturity levels by means of laboratory experiments abstract solvent extracted core samples of lower toarcian carbonate rich shales posidonia shale lias of the hils syncline area lower saxony basin northern germany covering a wide maturity range vitrinite reflectance rr 0 48 0 88 and 1 45 and a kimmeridge mudstone rr 0 45 are pyrolyzed at three different heating rates 0 1 0 7 and 5 0 k min by application of non isothermal first order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal mol and pre exponential arrhenius factors are obtained by an iteration method hypothetical hydrocarbon generation curves are calculated for the lias assuming three geological heating rates 0 5 5 and 50 k 106a peak hydrocarbon generation temperatures range from 135 to 165 c

microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in toarcian source rocks of different maturities abstract this study is part of a comprehensive research program on the generation and migration of petroleum from the organic rich posidonia shale lower toarcian jurassic of the hils syncline northern germany four cores of the complete posidonia shale with varying amounts of underlying pliensbachian and overlying aalenian mudstones provided the sample base depending upon their distance from a presumed subjacent heat source the vlotho massif the cores display progressive levels of increasing thermal maturation with r 0 values 0 48 0 68 0 88 and 1 45 which span the limits of the oil generation window primary stratigraphic lithologic and petrographic attributes of the four sections are all very similar each includes a lower marlstone member about 5 m thick overlain by 11 31 m of calcareous clay shale clay minerals calcite pyrite kerogen and quartz are the principal constituents thus there is no evidence that depositional conditions of the posidonia shale varied significantly over the sampled area the omnipresence of framboidal pyrite clearly formed from the activity of sulphate reducing bacteria at or near the depositional interface is strong evidence that uniform reducing anoxic conditions were persistent at all localities this conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry organic macerals and other properties are directly related to the variable levels of thermal maturation kerogen from the least mature section r o 0 48 is type ii and composed of alginite a alginite b liptodetrinite bituminite and some terrestrial sporinite vitrinite and inertinite macerals with increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties alginite of the least mature section displays a strong green fluorescence at 0 68 r o alginite macerals exhibit yellow orange fluorescence and are structurally well preserved although geochemical data indicate generation and loss of large amounts of oil at 0 88 r o hydrocarbon generation is almost complete and few primary alginites remain they have been transformed into secondary liptinites designated as metaalginite which have similar fluorescence colours at the highest maturation level 1 45 r o metaalginites are only weakly fluorescent with a shift toward red colours calcite and or bitumen filled bedding plane micro and macrofractures occur exclusively in the organic rich posidonia shale at maturity levels of 0 68 r o and above such features are totally absent in the immature posidonia section and in the adjacent organic poor mudstones this observation indicates that the fractures developed in direct response to overpressuring i e pore pressure exceeding lithologic load related to oil and gas generation in the source rock the fractures undoubtedly provided important expulsion conduits for generated products

accelerator mass spectrometry the new revolution in radiocarbon dating abstract the development of accelerator mass spectrometry ams during the last ten years has created a new revolution in the fields of radiocarbon 14c dating and other types of radioisotope analysis the radiocarbon dating method can be used to determine the ages of many types of carbonaceous material of up to about 50 ka in age an ams instrument has been in operation at the nsf arizona accelerator facility for radioisotope analysis of the university of arizona since 1982 it has been used almost exclusively to measure carbon 14 the greatest advantage of radiocarbon dating by ams is the ability routinely to measure samples containing 1 mg or less of elemental carbon while previous methods have required nearly 1000 times that amount in addition the ability of ams to measure the numbers of individual 14c atoms present rather than having to wait to detect the small numbers of 14c atoms decaying makes the measurement time very much shorter for ams the precision attainable for a one hour counting time for a sample 5 ka in age is about 0 6 or 50 years ams radiocarbon dating is of value to multiple areas of quaternary research these include archaeology oceanography hydrology geology and paleoclimatology while ams dating of archaeological and geological samples was once seen as the biggest application of the method studies of global change atmospheric oceanic climatological changes in the past with potentially important applications to predicting future changes now contribute the largest share of samples for analysis by this laboratory other isotopes that have been analyzed using ams include 10be 26al 36cl 41ca and 129i the research applications of these isotopes are much the same as for 14c but are useful for studying processes of time scales greater than the limit of 14c dating

state of the art of research in kinetic modelling of oil formation and expulsion abstract the last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation this knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation by a combination of observation of homogeneous geochemical series experimental simulation and mathematical modelling it has been now established that parameters governing the quantity and composition of petroleum are temperature type of organic matter time or heating rate catalysis by clay minerals which would produce important amounts of isoalkanes is not necessary to produce petroleum the role of minerals although still controversial is rather to retain the heavy fraction of petroleum pressure seems to have a limited influence within the range for effective source rocks kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental predictive models of thermal cracking i e involving intermediates such as radicals and olefins such models presently apply only to the cracking of simple molecules hexane hexadecane at limited degrees of cracking for this reason kinetic models are still empirically calibrated on pyrolysis experiments and or on natural maturation trends the most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking i e direct formation of mobile compounds from kerogen these are generally calibrated against open system pyrolysis with various temperature programs consideration of natural maturation trends i e decrease in petroleum potential may be used also for calibration but discrepancies of organic matter properties availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods whatever the method main activation energies are found in the range 45 60 kcal mol secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series unknown residence times but only from closed system pyrolysis although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known indeed migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models preliminary studies documenting the interactions between thermal cracking and expulsion are shown from haltenbanken area and paris basin in the last few years a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start indeed high expulsion efficiencies over 50 are systematically found for rich and mature source rocks a further understanding of primary migration requires observations of natural series petrophysical studies of source rocks porosities under 10 permeabilities in the nanodardy range thermodynamic investigation of unconventional multiphase organic systems kerogen liquid s gas and experimental simulation under stress this on going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one

calculation of petroleum masses generated and expelled from source rocks abstract an algebraic scheme for calculating the masses of petroleum formed in mature source rocks has been constructed the scheme exploits routine geochemical measurements and a basic understanding of the behaviour of sedimentary organic matter at elevated temperatures under both laboratory and subsurface conditions application of these techniques to data from a wide range of source rock types has led to the following general conclusions 1 1 major oil and gas generation from kerogen occurs between 120 and 150 c in the subsurface gas generation continues to temperatures 160 c 2 2 oil prone source rocks with average initial potential 5 kgt 1 of rock and average organic carbon contents 1 5 expel 60 90 of the total oil generated 3 3 oil expulsion from leaner source rocks is less efficient most of the oil generated remains in the source rock it is subsequently converted to lighter components at higher temperatures and expelled in the vapour state 4 4 the major loss of organic carbon from rich oil prone source rocks during generation and expulsion means that the petroleum potential and organic carbon content of mature source rocks are much less than their initial immature values hence the algebraic scheme must be used to calculate the initial values before expelled petroleum masses can be determined