impacts of various toc distributions on hydrocarbon generation abstract this study used numerical basin modeling methods to examine how various total organic carbon content toc distributions in formations with a fixed average toc impact the overall oil and gas generation ability different geological and geochemical conditions such as thickness and burial depth of the source formation thermal gradient organic richness degree of spread and types of kerogen are considered in this investigation the results show that there is a significant impact on the overall oil and gas generation capability especially when the source rocks are in the peak generation windows the magnitudes of oil and gas generation rates can vary by up to 5 10 times in some cases the general rules are that the more dispersion of toc in the formation or the thicker the formation and the greater the thermal gradient the larger the difference of generation rates between the distributions which favor hydrocarbon generation therefore in addition to the average toc richness the toc distribution in a source rock formation must also be considered when the petroleum potential of a basin is evaluated

light hydrocarbon redistribution in a shallow core from the ravnefjeld formation on the wegener halvo east greenland abstract twenty nine core samples extending from the surface to 60 m depth from the upper permian organic rich ravnefjeld formation source rock of east greenland have been analyzed for detailed light hydrocarbon composition in addition complementary organic carbon and rock eval analysis as well as some mineralogical and permeability analyses were carried out the objective was partly to determine if the changes in light hydrocarbon composition reflect generation patterns i e small scale changes in kerogen type and amount or are controlled primarily by redistribution processes a second objective was to discuss possible modes by which hydrocarbon redistribution took place the interval examined in the ravnefjeld formation is overmature 1 75 ro organic carbon contents toc and hydrogen index values range mostly between 1 0 5 5 and 10 20 mg hc g toc respectively the amount of hydrocarbons present in the examined section represent only a fraction of the total amount generated petroleum expulsion efficiency pee exceeding 0 99 was calculated systematic changes of light hydrocarbon composition with depth indicate that effective redistribution has occurred over wide intervals in particular there is good evidence to suggest that there was major loss with chemical fractionation of molecular species by depletion into several zones of fractures and also towards the outcrop surface and that fracture occurrence is the single most important factor in controlling the present day hydrocarbon depth trends in this paper a novel scheme is presented for effective and objective comparison of hydrocarbon depth trends the application of this method to elucidation of the migration mechanisms responsible for fractionation during hydrocarbon loss in the fractured zones is illustrated

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

some pragmatic perspectives in source rock geochemistry abstract conceptual advances in petroleum source rock geochemistry made during the last decade are reviewed and some problem areas in analytical geochemistry are outlined areas of discussion include views of source rocks based on assessment of hydrocarbon expulsion kinetic models of kerogen maturation vitrinite reflectance evolution and a discussion of the data dilemma facing modern source rock geochemistry the increased application of petroleum geochemistry to the quantitative solution of geological problems is stressed

geothermal history and petroleum generation in the norwegian south viking graben revealed by pseudo 3d basin modelling abstract hydrocarbon exploration in the norwegian south viking graben 57 45 60 15 n began in the late 1960s and has now entered a mature phase a map based pseudo three dimensional 3d basin modelling study is performed in order to unravel regional trends in hydrocarbon generation and expulsion and to evaluate the remaining potential of this mature petroleum province generation and expulsion from all major source rock horizons in the area the draupne heather hugin and sleipner formations is simulated using a pseudo 3d model comprising 36 isochronous geological events the map based pseudo 3d model is built from 1d models combined with subsurface and source rock quality maps hydrocarbon expulsion from the middle and upper jurassic source rocks in the area occurred in two major phases the first phase lasted from palaeocene to middle miocene with peak oil and gas expulsion during the early miocene the second quaternary phase of expulsion which supplied 11 and 13 of all oil and gas respectively is related to increased subsidence rates during this period a total of 74 109 sm3 oil and 8 2 1012 sm3 gas has been expelled in the area the frigg area in the north is with 2 9 1012 sm3 gas expelled the most gas prone area and the greater balder area seems to be the most oil prone area with 17 109 sm3 oil expelled the lower syn rift section of the upper jurassic draupne formation dominated oil expulsion 54 of all expelled oil whilst the heather formation dominated gas expulsion with 37 of all gas expelled model predictions were successfully applied to explain the fill and secondary alteration history of the greater balder area norway the modelling supports the draupne formation origin of the oils in the greater balder area as well as the presence of oils deriving from multiple charge phases found in correlation studies based on the modelled cumulative volumes expelled and estimates for in place volumes in the area generation accumulation efficiencies gaes of 1 01 1 05 for oil and 9 67 12 89 for gas have been estimated for the south viking graben in light of comparisons with other petroleum systems and critical evaluation of the elements of the petroleum systems the estimated gaes should be regarded as encouraging for further exploration of the area

enrichment and distribution of shale oil in the cretaceous qingshankou formation songliao basin northeast china abstract the songliao basin is a large scale petroliferous basin in china with a gradual decline in conventional oil production the exploration and development of replacement resources in the basin is becoming increasingly important previous studies have shown that the cretaceous qingshankou formation k2qn has favorable geological conditions for the formation of shale oil thus shale oil in the qingshankou formation represents a promising and practical replacement resource for conventional oil in this study geological field surveys core observation sample tests and the analysis of well logs were applied to study the geochemical and reservoir characteristics of shales identify shale oil beds build shale oil enrichment models and classify favorable exploration areas of shale oil from the cretaceous qingshankou formation the organic matter content is high in shales from the first member of the cretaceous qingshankou formation k2qn1 with average total organic carbon toc content exceeding 2 the organic matter is mainly derived from lower aquatic organisms in a reducing brackish to fresh water environment resulting in mostly type i kerogen the vitrinite reflectance ro and the temperature at which the maximum is release of hydrocarbons from cracking of kerogen occurred during pyrolysis tmax respectively range from 0 5 to 1 1 and from 430 c to 450 c indicating that the k2qn1 shales are in the low mature to mature stage ro ranges from 0 5 to 1 2 and currently generating a large amount of oil the favorable depth for oil generation and expulsion is 1800 2200 m and 1900 2500 m respectively as determined by basin modeling the reserving space of the k2qn1 shale oil includes micropores and mircofractures the micropore reservoirs are developed in shales interbedded with siltstones exhibiting high gamma ray gr high resistivity rt low density den and slightly abnormal spontaneous potential sp in the well logging curves the microfracture reservoirs are mainly thick shales with high rt high ac acoustic transit time high gr low den and abnormal sp based on the shale distribution geochemical characteristics reservoir types fracture development and the process of shale oil generation and enrichment the southern taikang and northern da an are classified as two favorable shale oil exploration areas in the songliao basin

a near uniform basin wide sea level fluctuation of the mediterranean sea abstract a new basin wide oscillation of the mediterranean sea is identified and analyzed using sea level observations from the ocean topography experiment topex poseidon satellite altimeter and a numerical ocean circulation model more than 50 of the large scale nontidal and non pressure driven variance of sea level can be attributed to this oscillation which is nearly uniform in phase and amplitude across the entire basin the oscillation has periods ranging from 10 days to several years and has a magnitude as large as 10 cm the model suggests that the fluctuations are driven by winds at the strait of gibraltar and its neighboring region including the alboran sea and a part of the atlantic ocean immediately to the west of the strait winds in this region force a net mass flux through the strait of gibraltar to which the mediterranean sea adjusts almost uniformly across its entire basin with depth independent pressure perturbations the wind driven response can be explained in part by wind setup

eastern equatorial pacific ocean t s variations with el nino 1 temperature salinity t s relationship variability in the pycnocline of the eastern equatorial pacific ocean nino3 region 5 s 5 n 150 w 90 w over the last two decades is investigated using observational data and model simulation a numerical model simulation using the mitgcm massachusetts institute of technology general circulation model suggests that during el nino years the water in the eastern equatorial pacific ocean becomes saltier by 0 1 to 0 2 and warmer by 0 5 to 1 c on density surfaces within the pycnocline this simulation is consistent with conductivity temperature depth ctd data collected mostly during tropical atmosphere ocean tao mooring maintenance cruises

some mass balance and geological constraints on migration mechanisms oil and gas are not at rest in the sedimentary mantle of the earth they are not in equilibrium whether they are finely dispersed in a potential source rock or whether they are concentrated in a trap in a reservoir rock a wide variety of possible escape mechanisms exists these include diffusion continuous single phase flow solution of oil in gas or gas in oil and solution in water derived from compaction clay diagenesis or meteoric sources the problem is to quantify the possible mechanisms and to rank their relative importance under a given set of physical chemical and geologic conditions the quantitative importance of the various proposed mechanisms can vary by orders of magnitude depending on the physical chemical and geologic conditions during the past decade oil to source correlations have become reliable and the timing of peak generation and concomitant migration has been sufficiently quantified to allow the geologist geochemist to make estimates of when and how much petroleum moved from one location to another combined with a knowledge of the physical chemical and geologic conditions at the time of migration such quantitative descriptions of subsurface petroleum transfer permit an empirical test of the applicability of the various proposed migration mechanisms the application of this technique to selected areas suggests that most of the major commercial oil accumulations of the world left their source rock in a continuous oil phase when bitumen concentrations in the rock are too low for continuous phase flo to exist other migration mechanisms which always are operative will increase in both absolute and relative intensity solution of oil in gas may become significant in thick tertiary delta systems and meteoric water may be a surprising asset in some very specific geologic settings however it is unlikely that solution of oil in water derived from compaction or from dehydration of clay has much to do with the origin of many of the major oil accumulations of the world

diffusion of light hydrocarbons in subsurface sedimentary rocks in a deep core hole penetrating paleocene age strata on spitsbergen island diffusive transport of light hydrocarbons from a siltstone into an adjacent sandstone unit is strongly supported by regular patterns of concentration changes which are controlled by molecular type size and structure individual molecular species are ranked in sequence of increasing effective diffusion coefficients benzene and toluene appear to have anomalously high transportation rates these observations are significant with respect to an evaluation of the role of diffusion processes for hydrocarbon migration

eastern pacific ocean circulation near the onset of the 1982 1983 el nino over a 1 month period october 5 to november 6 1982 during the initial stages of the 1982 1983 el nino hydrographic and velocity profiling measurements were made in the eastern equatorial pacific by two research vessels track lines approximated a square centered near the galapagos islands with approximately meridional sections at 95 w and 85 w and nearly zonal sections at about 5 n and 5 s water mass characteristics and estimated currents of these sections are discussed sea surface temperature nearly everywhere was warmer than 25 c an anomaly of 2 c 3 c the equatorial thermocline represented by the 20 c isotherm was depressed by nearly 50 m at 85 w and more than 100 m at 95 w relative to a historical mean profile across 95 w estimated geostrophic relative to 500 dbar and directly measured near equatorial velocity profiles indicated significant eastward surface flow the south equatorial current south of the equator had disappeared estimated warm water transport t 24 5 kg m 3 indicated an influx into the region which was approximately that required to balance the observed low frequency sea level change at the galapagos islands temperature and salinity properties as well as the transport estimates indicated that this warm water came predominantly from the west

observational evidence for propagation of decadal spiciness anomalies in the north pacific 1 the propagation of density compensated warm salty or cool fresh spiciness anomalies in the north pacific thermocline is investigated using argo profiles for the period 2001 2008 a cool fresh spiciness anomaly on 25 25 5 kg m 3 isopycnals appears in the eastern subtropical north pacific at 120 w 150 w in 2003 2004 with a salinity anomaly of about 0 15 pss 78 this spiciness anomaly migrates southwestward and arrives in the western tropical north pacific at 145 e 175 w in 2008 with the salinity anomaly decreasing to about 0 043 pss 78 two warm salty anomalies are observed to propagate along the same path from 2003 to 2005 and after 2005 the propagation path and speed of the anomalies are in good agreement with advection by the mean geostrophic current in the course of propagation the anomalies are diffused and are subject to high frequency injection of spiciness anomalies especially in the eastern subtropical north pacific

characteristics and quantitative modelling of natural gas migration in a clastic rock series abstract the phase or state of natural gas during migration determines how much is expelled and accumulated phases of natural gas migration include water solution oil solution free gas and diffusion which evolve vertically in a sedimentary stratigraphic section gas expulsion modelling must be based on an evaluation of the migration phase first of all the depth limits of each migration phase are determined then the expelled gas quantities in each range are computed individually and finally added together for oil related gas the oil solution phase is the most important migration phase therefore the expulsion modelling of oil related gas can be treated to a considerable degree as the modelling of oil expulsion a method for direct modelling of oil expulsion is presented the concept of equilibrium concentration is particularly important for gas diffusion modelling in this paper methods of modelling natural gas expulsion are presented through some specific examples such as south china sea basin bohai gulf basin of east china and sichuan basin of southwest china

multi tracer and hydrogeophysical investigation of the hydraulic connectivity between coal seam gas formations shallow groundwater and stream network in a faulted sedimentary basin abstract characterization of geological fault networks in sedimentary basins targeted for coal seam gas development typically use deep penetrating seismic data such methods can reveal the presence of numerous fault zones at depths greater than 100 m but not necessarily show if these extend to the surface or provide a significant degree of hydraulic connectivity between deep and shallower geological formations our combined multi tracer and hydrogeophysical study used transient electromagnetic surveys to image the subsurface in a sedimentary coal seam gas basin gloucester basin nsw australia and indicated that deep faults greater than 200 m from seismic analysis may extend to within metres of the ground surface analysis of environmental tracers in surface water and groundwater samples near fault traces showed the presence of hotspots of deeper groundwater from the coal seams discharging into the stream network via the alluvial aquifer the isotopic composition of methane was extremely variable in groundwater 13c ch4 from 81 6 to 29 2 v pdb and surface water 13c ch4 from 56 0 to 19 9 v pdb suggesting several sources of methane being recycled in the connected alluvial aquifer and stream network a 4he hotspot 1 67 10 6 cm3 stp g 1 in the alluvial aquifer close to a fault line showed high methane 463 g l 1 with a thermogenic signature 13c ch4 of 62 4 and 2h ch4 of 174 7 the gloucester basin which has experienced multiple episodes of complex restructuring displays high dilation tendency of fractures close to fault damage zones with fracture orientations parallel to the orientation of maximum horizontal stress as a result there is increased permeability for wells close to the fault damage zone and with increasing depth these findings resulted in a robust hydrogeological conceptual model of a faulted basin which can be used in a monitoring and modelling framework for improved evaluation of risk associated with unconventional gas developments

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

experiments on adsorption and molecular sieving and inferences on primary migration in upper jurassic claystone source rocks north sea fine pores primarily slit shaped micropores in upper jurassic claystone source rocks were studied by gas adsorption at 20 c approximately 80 of the mesopore and micropore volume is penetrable to water only because of constrictions approximately 3 a wide the pore surface is nonpolar and probably characterized by kerogen covering the clay mineral surfaces water is not structured in the mesopores and micropores but readily forms a liquid phase without distinct monolayer formation n alkanes probably enter the pores end on and become adsorbed parallel with the surface these results apply to all samples studied irrespective of the degree of clay mineral diagenesis three migration types proposed for these claystone source rocks are 1 type a migration which at experimental conditions permits entry and removal of n alkanes isobutane neohexane cyclohexane and benzene in approximately equal amounts 2 type b migration which permits entry and removal of n alkanes isobutane and neohexane in approximately equal amounts but where only 20 of the pore volume and surface area is also open to cyclohexane and benzene and 3 type c migration which is intermediate between type a and type b the occurrence of the migration type is related to the reservoir oil composition in the samples studied tectonics and clay mineral diagenesis appear to have no influence on the migration type migration types may be related to past overpressured conditions but do not appear to be related to current pressures

the termination of the equatorial undercurrent in the eastern pacific abstract the termination of the equatorial undercurrent euc in the eastern tropical pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5 n 10 s 80 w and 100 w three distinctive hydrographic features associated with the euc are used along with dynamic topography to trace the mean path of the euc and to investigate aspects of its seasonal variation these features are 1 the 13 c thermostad 2 the high salinity core and 3 the high dissolved oxygen concentration tongue the thermostad is thickest just south of the equator to the west of the galapagos islands with a decreasing maximum extending southeast to the coast of south america near 7 s the thermostad is poorly developed in the mean near the equator east of the galapagos islands this pattern appears to be produced by convergence of eastward flow in the euc and in the southern subsurface countercurrent as the euc flows southeastward from the galapagos this interpretation is supported by the geostrophic flow calculated from the mean dynamic topography the isolated high salinity core is found to the south of the equator and is traced all the way to the coast of south america the high oxygen tongue associated with the euc splits at the galapagos with the southward projecting branch disappearing rapidly as an identifiable feature the other branch of the tongue continues around the islands to the north and along the equator to the coast of ecuador some evidence of recirculation of euc water in the south equatorial current is seen to the northwest of the galapagos a strong seasonal pulsation of the euc affects these features in a consistent way with the pulse occurring first west of the galapagos and later along the coast the thermostad first reaches maximum development and then a pronounced downwelling of the thermocline occurs rapidly leading to minimum thermostad extent associated wtih this downwelling is an eastward protruding high salinity tongue near the equator which is later seen as an isolated core along the coast at 4 s and 6 s pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the euc and ultimately the peru chile undercurrent the pressure gradient reversal along the equator occurs prior to the annual wind reversal suggesting that remote forcing plays a role in the seasonal pulse of the euc in the eastern pacific

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

the quantification of diffusive hydrocarbon losses through cap rocks of natural gas reservoirs a reevaluation discussion 1 a calculation scheme is presented to estimate the quantities of diffusive gas loss through the water saturated cap rock of natural gas reservoirs over geologic time reexamination of earlier calculations in a 1982 paper by leythaeuser et al revealed that the estimate of the rate of diffusive hydrocarbon losses through the cap rock of the harlingen netherlands natural gas reservoir was based on oversimplified assumptions concerning the boundary conditions at the reservoir cap rock interface according to this recalculation the time required to reduce the present day methane content of the harlingen gas field by one half is approximately 70 m y not the previously published value of 4 5 m y consequently diffusion losses through the cap rock appear to affect the long erm stability of natural gas reservoirs to a lesser extent than suggested by the earlier results

experimental measurements of the diffusion parameters of light hydrocarbons in water saturated sedimentary rocks i a new experimental procedure an automatic experimental set up has been developed for the determination of diffusion parameters of hydrocarbon gases through water saturated rock samples diffusive flow of hydrocarbons through rock slices is monitored by gas chromatography experiments are carried out according to the time lag method yielding diffusion coefficients solubility coefficients and diffusion permeabilities diffusion coefficients down to 10 12 m2 s 1 10 8 cm 2 s 1 may be determined routinely on rock samples of 2 10 mm thickness maximum errors in diffusion coefficients are estimated around 20 and reproducibility was found to range between 10 and 20 specific features of this set up are automatic sampling and data acquistion high sampling frequency and maintenance of water saturation of the rock samples throughout the experiment

formation and migration of hydrocarbons in deeply buried sediments of the gulf of cadiz convergent plate boundary insights from the hydrocarbon and helium isotope geochemistry of mud volcano fluids highlights clay dehydration water expelled from buried sediments drives mud volcanism rise of fluids mediated by crustal scale strike slip faults cross cutting wedge on active accretionary wedge petroleum accumulations were dismantled in neogene 4he enrichment and 13c ch4 50 in fluids reflect an open hydrocarbon system petroleum pools remain on shallow margin microbial gas vented out of active wedge abstract a geochemical study of the composition of hydrocarbon gases and helium isotopes 3he 4he in fluids from mud volcanoes mvs located on and out of the active accretionary wedge of the gulf of cadiz goc provides information on fluid sources and migrations in deeply buried sediments the goc is a tectonically active segment of the africa iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal scale strike slip faults initially built during the miocene gibraltar arc subduction the wedge has since developed toward the w nw in an oblique convergent setting interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge with mvs located along strike slip faults mediating fluid ascent the large excess of radiogenic helium 4he in all goc fluids agrees with a clay mineral dehydration source of water hydrocarbon gases from all deepwater mvs bear methane having similar stable carbon isotope compositions of 50 vpdb whether fluids are highly enriched in methane relative to heavier homologues c2 or not methane ethane propane 10 to 10 000 we suggest that methane with 50 vpdb was largely diffused out of early generating source rocks and became dissolved in the water expelled by the buried sediments consistently low 3he 4he ratios suggest an open hydrocarbon system petroleum accumulations and 3he dissolved in the original sedimentary pore water have mostly escaped into the water column during the major late neogene compressional events at present some mvs vent ch4 rich fluids from dewatering sediments while other structures located on active thrusts additionally vent c2 rich gases generated by active cretaceous source intervals by contrast evaporitic seals preserved petroleum accumulations on the shallow moroccan margin while the westernmost mvs located out of the accretionary wedge vent microbial gas

an overview of pre devonian petroleum systems unique characteristics and elevated risks abstract the radiation of land plants at the onset of the devonian resulted in a change in the character of available biomass which in turn introduced changes to the nature of petroleum systems this study examines the nature of pre devonian petroleum systems prior to this significant evolutionary event conventional hydrocarbon resources have been associated with pre devonian petroleum systems across the globe including the neoproterozoic infracambrian huqf supergroup of the oman basin the riphean vendian sequences of east siberia the cambrian of the murzuq basin the ordovician of the tarim basin and the silurian qusaiba of saudi arabia although individual accumulations can be quite significant e g hassi messaoud has ultimate recoverable reserves of 10 billion barrels the relative importance of pre devonian derived oils is thought to be limited compared to the global conventional resource base with growing interest in unconventional resources the relative importance of these systems is expected to increase as resource plays such as the ordovician utica shale of the appalachian basin the cambrian of china and possibly the silurian of central and eastern europe develop the advanced ages of these systems result in some unique properties and amplify risks that exist in younger petroleum systems differences in the nature of the biomass contributing to pre devonian source rocks may give rise to oils that display unique geochemical characteristics for example there are some precambrian oils where c 29 steranes dominate even though land plants were absent there are also ordovician source rocks dominated by gloeocapsamorpha prisca a primitive prokaryote which yield oils containing limited amounts of c 20 components and nearly lack pristane and phytane the potential for unconventional reservoirs in pre devonian systems may also be highly dependent upon the age of the system as biological evolution influences the availability and nature of biogenic silica an important factor controlling brittleness and fracability literature has shown that biogenic silica from different sources displays varying degrees of resistance to diagenesis which allows the formation of a silica network and influences brittleness for example radiolaria present during the cambrian are more resistant to diagenesis than diatoms which did not evolve until the jurassic risks associated with preservation of hydrocarbons are also amplified in pre devonian petroleum systems many such systems have been exposed to significant thermal stresses resulting in the cracking of oil and wet gas others have had complex tectonic histories potentially resulting in the breaching of seals or changes in pvt conditions that can result in gas loss gas loss may also occur through diffusion from these older reservoirs as a result of long residence times

upper equatorial pacific ocean current and salinity variability during the 1996 1998 el nino la nina cycle the recent el nino la nina cycle exhibited striking patterns of current and salinity variability in the upper equatorial pacific ocean this evolution is described from mid 1996 through 1998 using a remarkable data set of 35 meridional conductivity temperature depth ctd acoustic doppler current profiler adcp sections along with buoy data the sections nominally from 8 s to 8 n between 165 e and 95 w were occupied over the course of 27 months a wide range of current variability was sampled with currents that appeared or disappeared some time during the cycle including an equatorially trapped eastward surface current the equatorial undercurrent the northern branch of the south equatorial current and the north equatorial countercurrent basin wide interannual changes in upper ocean and pycnocline zonal transports were as large as 64 32 106 m3 s 1 changes in the salinity structure included a deep and fresh mixed layer in the central equatorial pacific that built up during the el nino and was then disrupted by upwelled salty water with the onset of la nina a very fresh mixed layer observed in the eastern equatorial pacific late in the el nino and a reduction in the strength of the meridional equatorial salinity gradient within the pycnocline to one third of the usual value during the el nino finally the zonal transports above the thermocline from 5 s to 5 n were well correlated with the rate of change of warm water volume west of the individual ctd adcp sections

primary migration by diffusion through kerogen ii hydrocarbon diffusivities in kerogen abstract laboratory experiments were performed to assess diffusion through kerogen as a mechanism of liquid hydrocarbon transport through fine grained rocks such transport can be important in primary migration within source rocks and in leakage through seals previous work with organic coated model cores has shown that hydrocarbon diffusion through organic matter networks can be substantially greater than hydrocarbon diffusion through aqueous pore space in the present work natural kerogen networks were evaluated experiments with five natural source rocks showed that kerogen increased the experimental hydrocarbon flux over the calculated hydrocarbon flux through pore water in only one of these source rocks the richest 2 2 toc carbonate sample effective diffusivities through kerogen were calculated for the two experimental hydrocarbons through this sample the results were extended with the wilke chang correlation to yield diffusivities of several other hydrocarbons through similar source rocks

marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal abstract detection of chemically identifiable hydrocarbon oil and gas seepage is an important tool in offshore exploration generated and or trapped oil and gas can leak to near surface sediments and ocean in detectable concentrations the seepage surface expression is controlled by hydrocarbon generation regional pressure regimes and migration pathways the migrated hydrocarbon concentration and composition varies due to seepage rates and near surface physical and biological processes that can alter the signal a seepage continuum exists from large volume oil and gas seeps known as macro seepage to lower volume seeps known as micro seepage only detectible with special sampling and analysis understanding the impact of migrated hydrocarbon concentration relative to in situ organic material and locally derived microbial hydrocarbons is critical this paper describes factors that control the near surface geochemical signal as well as the challenges in deciphering the migrated hydrocarbon signature to evaluate subsurface hydrocarbon generation and entrapment

role of diffusion in primary migration of hydrocarbons the following effective diffusion coefficients d cm2 sec were determined for the diffusion of light hydrocarbons through the water saturated pore space of shales methane 2 12 10 6 ethane 1 11 10 6 propane 5 55 10 7 iso butane 3 75 10 7 n butane 3 01 10 7 n pentane 1 57 10 7 n hexane 8 20 10 8 n heptane 4 31 10 8 and n decane 6 08 10 9 on the basis of these new data a deterministic dynamic model was set up to simulate the diffusive transport of light hydrocarbons c1 to c10 from source rocks for eight documented source rock units representing a wid range of geologic conditions maturities of 0 40 to 1 35 mean vitrinite reflectance oil to gas prone kerogens the cumulative amounts of hydrocarbons escaping with time were calculated thus it was shown that diffusion represents an effective process for primary migration of gas but not for oil the rate of mass transport for gas from source rocks with geologic time can be sufficiently high to account for the origin of commercial size gas fields for example a cumulative amount of 109 kg of methane 1 5 109 std m3 or 5 3 1010 scf has escaped by diffusion in 540 000 years from a certain volume 1 000 km2 by 200 m thick of a high mature gas prone mesozoic source rock in western canada the origin of hydrocarbon accumulations with high gas to oil ratios in low mature sediments in geologically young basins early gases and condensates can be explained by an early phase of primary migration predominantly based on diffusion during the initial stages of the accumulation history of those fields extending up to millions of years under certain conditions the reservoir gas changes with geologic time from a methane rich to a wet gas composition at low maturity levels below about 0 6 rm even oil prone source rocks yield methane rich light hydrocarbon mixtures by migration through diffusion compositional trends among reservoir gases of several multiple pay gas fields in louisiana represent evidence for diffusive transport of hydrocarbons the variation in gas composition between the individual pay zones is controlled by increasing distances of diffusive transport of the hydrocarbons from a uniform source rock at depth to their present accumulation sites in the shallow pay zones compounds of high diffusivity are enriched for example in the sligo gas field of louisiana the methane ethane and the iso butane n butane ratios increase from 10 2 to 36 0 and from 0 86 to 1 13 respectively from the deepest to the shallowest of the five productive reservoir sands which are spread over a a depth interval of 5 500 ft 1 676 m diffusion of light hydrocarbons in the subsurface can also have economically adverse effects for example existing gas accumulations can be destroyed by dissipation the rate of this destruction was calculated for the harlingen gas field holland by diffusive loss through 400 m of shale cap rock the initial amount of methane in place of 1 93 109 std m3 6 8 1010 scf is reduced by one half over a period of 4 5 million years this leads us to propose the concept that large gas accumulations can persist through extended periods of geologic time only as dynamic systems reaching some kind of steady state equilibrium between diffusive loss through the cap rock and continuous replenishment from the source rock

understanding of hc migration in sedimentary basins present state of knowledge abstract great advances were made during the decade of 1975 1985 in the understanding of hc migration by establishing that the main mechanisms were polyphasic flow whatever its stage and that migration in solution in water and or by molecular diffusion was insignificant except for methane ethane and benzene research is now focusing on points which still have not been satisfactorily answered such as 1 1 behaviour of source rocks during hc expulsion the main driving force for fluid expulsion has been recognized to be excess fluid pressure within the source rock with respect to its surrounding environment of permeable beds the pressure build up is due mainly to accommodation by the fluids of part of the mechanical stresses which are exerted on the rock mainly sedimentary load but also stresses developed by tectonics to thermal expansion of water and to hc formation from kerogen in response to increasing temperatures however it is not clear whether or not petroleum expulsion proceeds preferentially along oil wet networks kerogen or oil wet mineral surfaces nor it is not clear whether or not microfracturing provoked by an internal fluid pressure higher than allowed by the mechanical strength of source rocks is necessary for hc expulsion 2 2 modifications of the compositions of hc phases e g oil versus gas or the distribution of biomarkers during migration and losses of hydrocarbons along the migration pathways although careful observations of natural sample series and also experimentation have been done no clear picture has emerged for the moment however compositional effects seem to be due more to changes in thermodynamic conditions along migration pathways which provoke inter alia hydrocarbon phase separation retrograde condensation than to differential retention by mineral surfaces it is likely that while careful observations on a large variety of source rock samples are needed to more clearly understand source rocks functioning progress in understanding hc migration will come increasingly from modeling and experimentation models which mathematically simulate natural processes are necessary to help the human mind in handling such complex phenomena and therefore are of precious help to focus research on the most important points they replace migration in its geological and historical dimensions with numerical or conceptual tools while observation only describes the present state models are also the tools by which knowledge on migration can be applied to exploration problems however quantitative applications of models require numerous and precise geological data that are difficult to gather at early stages of exploration in the field of experimentation it will be easier to design crucial experiments now that hc migration mechanisms are better understood thanks to experimentation it should also be possible to validate models however so as to reach such aims substantial progress has to be made in the technology of experimental systems

occurrence and geochemistry of natural gases piceance basin northwest colorado the piceance basin is a hydrocarbon rich province that has natural gas production from reservoirs ranging in age from late jurassic to eocene and large undeveloped resources of natural gas in coal beds and tight sandstone reservoirs of cretaceous age gases from all producing intervals are of predominantly thermal origin and become isotopically heavier delta isotope 13 c 1 51 3 to 29 1 o oo and chemically drier c 1 c 1 5 0 26 to 1 00 with increasing thermal maturity of reservoirs r o 0 45 to 2 40 over a depth range of 1100 to 11 702 ft 335 3567 m scatter in trend is attributed to source rock differences and considerable vertical and lateral migration based on chemical and isotopic composition three major types of gases can be distinguished those generated from mixed type ii and iii kerogens those from dispersed type iii kerogen and those from coal gases generated from mixed type ii and iii kerogens are produced from the upper jurassic morrison formation the lower cretaceous cedar mountain formation the upper cretaceous dakota sandstone the upper cretaceous mancos b producing interval and marginal marine sandstones of the upper cretaceous iles producing interval these gases are associated with minor amounts of oil and probably were generated from kerogen in the marine mancos shale gases generated from dispersed type iii kerogen are produced from nonmarine sandstones of the upper cretaceous williams fork producing inter al and from thermally immature reservoirs in the overlying paleocene and eocene fort union and wasatch formations these nonassociated gases contain large amounts of co 2 and probably were generated from carbonaceous shales in the williams fork producing interval their presence in immature fort union and wasatch reservoirs implies considerable vertical migration the third type of gas is methane rich is produced by devolatilization of humic coal and is generally in coal beds of the cameo fairfield zone of the williams fork producing interval these gases are not the major source for adjacent sandstone reservoirs a fourth distinct type of isotopically light thermogenic gas occurs in immature reservoirs of the eocene green river formation this gas is inferred to have migrated from u identified deeper more mature source rocks

geochemical surface exploration for hydrocarbons in north sea approximately 350 sediment samples were taken in the central part of the north sea and hydrocarbon composition methane c1 and ethane c2 yields and carbon isotope composition of the methane dgr13c1 adsorbed in the sediment were determined methane generation or oxidation by bacterial activity can not be deduced from the analytical data this fact is explained by the assumption of a threshold of methane concentration in sediments below which methane oxidizing bacteria are inactive and that this threshold was not reached in the area investigated background gases in the surface sediments characterized by low c1 yields and dgr13c1 37 ppt can be distinguished from thermal sediment g ses which are obviously derived from a hydrocarbon generating source rock in the subsurface source rock data obtained from several wells situated on a cross section through the investigated area were compared to the geochemical patterns of the sediment gases and used to define profile sections with good dgr13c1 approx 40 ppt yield c1 approx 217 ppb and poor source rock properties dgr13c1 approx 33 ppt yield c1 approx 24 ppb an assessment of the source rock potential of the whole area under investigation has been carried out using the geochemical data of the surface sediment gases and taking into account that the c1 and c2 yields are generally higher in clay sediments than in sand samples the number of producing wells was related to the number of all wells 271 in the areas characterized by surface geochemistry as good or poor source rock areas the comparison showed that the relative number of producing wells is higher in areas geochemically characterized as good hydrocarbons in surface sediments of good north sea areas are not generated by bacteria at shallow depth they are formed thermally from the organic carbon of deep source rocks and migrate to the surface therefore they are indicative of thermal hydrocarbon generation in the subsurface

the effect of interbedding on shale reservoir properties north sea oil is overwhelmingly generated in shales of the upper jurassic basal cretaceous kimmeridge clay formation once generated the oil is expelled and ultimately migrates to accumulate in sandstone or carbonate reservoirs the source rock shales however still contain the portion of the oil that was not expelled as a consequence such shales and juxtaposed non source lithofacies can form the targets for the exploration of unconventional oil in this paper we examine part of the kimmeridge clay formation as a hybrid shale resource system within which hot shale and organic lean sandstone and siltstone intervals are intimately interbedded this hybrid system can contain a greater volume of oil because of the increased storage capacity due to larger matrix porosities of the sand silt interbeds together with a lower adsorptive affinity in the interbedded sandstone the relationship between the estimated volume percentages of sand and mudstone and free oil determined from rock eval s1 yields is used to place limits on the drainage of oil from source mudstone to reservoir sand at the decimeter scale these data are used to determine oil saturations in interbedded sand mudstone sequences at peak oil maturity higher values of free hydrocarbon as evidenced by the s1 value in mudstone suggest that more oil is being retained in the mudstone while higher s1 values in the interbedded sands suggest the oil is being drained to saturate the larger pore spaces high silica content in the interbeds confirms the brittleness in this mudstone sandstone lithofacies an important factor to be considered for fracture stimulation to successfully work in a hybrid system the key points of this hybrid unconventional system are the thickness storage capacity and the possibility to capture a portion of the expelled as well as retained oil

direct measurements of upper ocean currents and water properties across the tropical pacific during the 1990s abstract meridional sections of upper ocean zonal currents potential temperature and salinity are estimated at ten longitudes from 143 e to 95 w using conductivity temperature depth and acoustic doppler current profiler data from 172 synoptic sections taken in the tropical pacific between 138 e and 86 w mostly in the 1990s data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline a mixed layer water property extrema and velocity extrema mean zonal currents potential temperatures and salinities are produced at each longitude the seasonal cycles of these fields are also estimated as well as a simple el nino southern oscillation enso cycle zonal sections along the equator are also presented properties of the near equatorial zonal currents including transports temperatures and salinities are estimated separately from the synoptic sections the seasonal cycles of these quantities and their correlations with the southern oscillation index are investigated the work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data allowing trans pacific estimates of near equatorial current transports and properties including those of the northern branch of the south equatorial current the new guinea coastal undercurrent and the equatorial undercurrent

interpretation of hydrocarbon shows using light c1 c5 hydrocarbon gases from mud log data chromatograph analysis of hydrocarbon gases as reported on mud logs is the first indication of a reservoir s fluid characteristics ratio calculations of these data can enhance the interpretational value of this information giving the well site geologist an immediate indication of zones of interest that need further testing it was found that for an effective interpretation of mud log gas data the results must be plotted on a depth log which can be easily integrated with other data thus a new method of interpreting ratio calculations of c1 c5 gas data is proposed this method can be used at the well site during drilling to give an initial evaluation of the reservoir fluid type and in post well evaluation to enhance wireline and mud log data

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

a novel approach for recognition and quantification of hydrocarbon migration effects in shale sandstone sequences a detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m 1 050 ft through sequences of interbedded source rock type shales 0 84 rm maturity and reservoir sandstones allowed recognition and quantitation of a number of migration effects detailed gas chromatography mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum range hydrocarbons to a much higher degree than the centers of thick shale units for the alkanes expulsion occurs with pronounced compositional fractionation effects shorter chain length n alkanes are expelled preferentially and isoprenoid alkanes are expelled to a lesser degree than their straight chain isomers based on material balance calculations expulsion efficiencies were determined and found to be very high in certain instances for thin interbedded shales they decrease from about 80 around c15 to near zero in the c25 region there is no evidence for significant redistribution of steranes and triterpanes in the two sequences compared to c15 to c25 n alkanes they appear relatively immobile the composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length hence consideration of bulk expulsion efficiencies gives an unrealistic picture furthermore the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk oil migration and without significant fractionation the degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction and the primary migration process appears to have occurred with chromatographic separation the migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes the composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock by this mechanism the origin of accumulations of light oils and gas condensates in low mature sequences bearing predominantly terrestrial derived organic matter can be explained finally the migration effects documented in this study have some consequences for interpretation of geochemical data e g the pristane n c17 ratio a commonly accepted maturity parameter has been shown to be also controlled by the degree of hydrocarbon expulsion

kerogen maturation data in the uinta basin utah usa constrain predictions of natural hydrocarbon seepage into the atmosphere natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits according to some authors it accounts for roughly 5 of the global methane budget i explore a new approach to estimate methane fluxes based on the maturation of kerogen which is the hydrocarbon polymer present in petroleum source rocks and whose decomposition leads to the formation of oil and natural gas the temporal change in the atomic h c ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system which i demonstrate here using the uinta basin of eastern utah as an example obviously this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere after adjusting for biooxidation of natural gas i conclude that the average annual seepage rate in the uinta basin is not greater than 3100 900 tonne yr 1 this is 0 5 0 15 of the total flux of methane into the atmosphere over the basin as measured during aircraft flights i speculate about the difference between the regional 0 5 and the global 5 estimates

effect of water washing on crude oil compositions crude oils from venezuela oklahoma and new mexico were water washed in the laboratory at temperatures from room temperature to 80 c and with water salinities from 0 to 300 000 mg l in experiments lasting from 7 to 338 hours the effects of water washing were determined by gas chromatographic and gas chromatography mass spectrometric analysis of the residual oil these experiments show that water washing is particularly effective for the c15 fraction and hydrocarbons are removed in the sequence aromatics then n alkanes then naphthenes in the c15 fraction no loss of pristane phytane steranes or terpanes occurs but some aromatics and sulfur compounds especially dibenzothiophene are depleted although water washing reduces api gravity water washing is unlikely to produce tar layers the effects of water washing were simulated with a numerical model based on the equation of diffusion predicted oil water ratios were in good agreement with the experimental values for the light ends and the model suggests that water washing is very fast in the subsurface comparison of oils that appeared to have been water washed in nature with the equivalent unwashed oils showed the expected compositional trends water washing is probably the dominant process affecting crude oil composition in the subsurface when water flows past oils under conditions where bacterial degradation is precluded by temperature 80 c or by lack of dissolved oxygen and the temperatures are too low for thermal cracking

migration of c1 c8 hydrocarbons in marine sediments abstract light hydrocarbon c 1 c 8 profiles are compared for three wells of varying maturities two immature dsdp wells site 397 near the canary islands and site 530a near the walvis ridge in the south east atlantic and a mature well the east cameron well in the texas gulf coast primary migration of c 1 and c 2 appears to be occurring in all of the sedimentary rocks examined primary migration of c 3 components becomes important only as fine grained sedimentary rocks enter the catagenetic hydrocarbon generation zone or over short distances in more permeable sections lateral migration along bedding planes was more important than vertical migration in sedimentary rocks of all maturities the lightest methane ethane and propane gases hydrocarbon show greater fractionation than do the c 4 c 8 alkanes which generally show minimal fractionation during the migrational process subsurface diffusion coefficients for these p p b quantities of c 2 c 5 alkanes from immature sediments from dsdp site 530 are estimated to be several orders of magnitude less than values reported in the literature for diffusion of much larger amounts of these compounds from mature water wet sediments into air or sandstones since our calculations suggest light hydrocarbons are present in amounts less than their reported solubilities in pure water at 25 c we postulate that the sediment organic matter has a substantial effect on retarding the movement of these light hydrocarbons

geochemical prospecting in mexico abstract in samples of semiconsolidated sediments at depths of 3 15 and 30 m c 1 c 7 headspace hydrocarbon gases were studied above a great number of structures in a swamp zone of which some were known as oil and or gas producing areas whereas many others were barren of these on oil producing and one barren structure are selected to illustrate this paper the data obtained have shown remarkable differences among the gasometric manifestations encountered in producing and barren areas it was considered important to take samples at different levels and it was found that beyond a depth of 15 m biogenic gases differ from thermogenic ones and gases of vertical migration from a possible horizontal migration the success of distinction between hydrocarbon producing and barren structures based upon this analysis is greater than 80 analysis of the carbon isotopic composition of methane in the adsorbed form from near surface samples taking above one oil producing structure the data of which are presented here yielded 13 c values corresponding to a thermogenic origin of the gas

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

atmospheric methane from organic carbon mobilization in sedimentary basins the sleeping giant abstract the mass of organic carbon in sedimentary basins amounts to a staggering 1016 t dwarfing the mass contained in coal oil gas and all living systems by ten thousand fold the evolution of this giant mass during subsidence and uplift via chemical physical and biological processes not only controls fossil energy resource occurrence worldwide but also has the capacity for driving global climate only a tiny change in the degree of leakage particularly if focused through the hydrate cycle can result in globally significant greenhouse gas emissions to date neither climate models nor atmospheric co2 budget estimates have quantitatively included methane from thermal or microbial cracking of sedimentary organic matter deep in sedimentary basins recent estimates of average low latitude eocene surface temperatures beyond 30 c require extreme levels of atmospheric co2 methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric co2 to values as much as 20 times higher than pre industrial values increased natural gas emission could have been set in motion either by global tectonic processes such as pulses of activity in the global alpine fold belt leading to increased basin subsidence and maturation rates in the prolific jurassic and cretaceous organic rich sediments or by increased magmatic activity such as observed in the northern atlantic around the paleocene eocene boundary increased natural gas emission would have led to global warming that was accentuated by long lasting positive feedback effects through temperature transfer from the surface into sedimentary basins massive gas hydrate dissociation may have been an additional positive feedback factor during hyperthermals superimposed on long term warming such as the paleocene eocene thermal maximum petm as geologic sources may have contributed over one third of global atmospheric methane in pre industrial time variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years but also over geologically short periods of time earth system models linking atmospheric ocean and earth surface processes at different timescales with the sedimentary organic carbon cycle are the tools that need to be developed in order to investigate the role of methane from sedimentary basins in earth s climate

volatile c1c7 organic compounds in an anoxic sediment core from the pettaquamscutt river rhode island u s a abstract surface sediments from an anoxic marine environment the upper basin of the pettaquamscutt river rhode island were analyzed for volatile organic compounds in the c 1 c 7 range the compounds identified included methane ethane alkenes c 2 c 5 carbon disulfide cyclopentane 3 methylpentane methylfuran aldehydes and ketones ethane methylfuran and most of the aldehydes and ketones showed maxima at the sediment water interface methane levels were very high 10 100 times greater than observed in most other surface sediments examined in this laboratory

petroleum retention in the mandal formation central graben norway abstract upper jurassic organic matter rich marine shales of the mandal formation have charged major petroleum accumulations in the north sea central graben including the giant ekofisk field which straddles the graben axis recent exploration of marginal basin positions such as the mandal high area or the sogne basin has been less successful raising the question as to whether charging is an issue possibly related to high thermal stability of the source organic matter or delayed expulsion from source to carrier the mandal formation is in part a very prolific source rock containing mainly type ii organic matter with

on the cause of eastern equatorial pacific ocean t s variations associated with el nino the nature of observed variations in temperature salinity t s relationship between el nino and non el nino years in the pycnocline of the eastern equatorial pacific ocean nino3 region 5 deg s 5 deg n 150 deg w 90 deg w is investigated using an ocean general circulation model the origin of the subject water mass is identified using the adjoint of a simulated passive tracer the higher salinity during el nino is attributed to larger convergence of saltier water from the southern hemisphere and smaller convergence of fresher water from the northern hemisphere

geophysical properties of cap rocks in qiongdongnan basin south china sea abstract based on measured petrophysical properties and statistical methods relationship between petrophysical properties and well log and seismic parameters are established empirically to evaluate the quality of cap rocks in qiongdongnan basin good cap rocks show high density and low sonic travel time four zones can be divided vertically according to displacement pressures in qiongdongnan basin best seal quality is found in the depth range of 2000 4000 m the ya 13 1 structure has excellent cap rock which contributes to its abundant gas accumulation

stable chlorine isotopes in halite and brine from the gulf coast basin brine genesis and evolution stable chlorine isotope data used in conjunction with other geochemical parameters are useful in determining the origin of solute in formation waters with salinity different from that of seawater jurassic salt in the gulf coast basin has 37cl ranges of 0 5 to 0 3 bedded and 0 0 to 0 5 diapiric the values for bedded salt are consistent with a 37cl value of 0 0 for jurassic seawater chloride as in modern oceans the slightly higher values for diapiric salt are possibly caused by incongruent solution of halite formation waters have a 37cl range of 1 9 to 0 7 waters with 37cl 0 2 low salinity water from geopressured aquifers appears to have resulted from shale dewatering low 37cl values in such water are related to low mcl mna a parameter indicating dewatering but show no general relationship to cl br they are consistent with diffusion of chloride from allochthonous brine diffusion can generate domains of water with negative 37cl on a scale of hundreds of meters in 104 106 years and may also generate positive 37cl chloride in residual source brine

anomalies of natural gas compositions and carbon isotope ratios caused by gas diffusion a case from the donghe sandstone reservoir in the hadexun oilfield tarim basin northwest china abstract natural gases in the carboniferous donghe sandstone reservoir within the block hd4 of the hadexun oilfield tarim basin are characterized by abnormally low total hydrocarbon gas contents 13c ethane c2

geochemical characteristics of tight gas and gas source correlation in the daniudi gas field the ordos basin china abstract the molecular composition stable carbon and hydrogen isotopes and light hydrocarbons of the upper paleozoic tight gas in the daniudi gas field in the ordos basin were investigated to study the geochemical characteristics tight gas in the daniudi gas field displays a dryness coefficient c1 c1 5 of 0 845 0 977 with generally positive carbon and hydrogen isotopic series and the c7 and c5 7 light hydrocarbons of tight gas are dominated by methylcyclohexane and iso alkanes respectively the identification of gas origin and gas source correlation indicate that tight gas is coal type gas and the gases reservoired in the lower permian shanxi fm p1s and lower shihezi fm p1x had a good affinity and were derived from the p1s coal measure source rocks whereas the gas reservoired in the upper carboniferous taiyuan fm c3t was derived from the c3t coal measure source rocks the molecular and methane carbon isotopic fractionations of natural gas support that the p1x gas was derived from the p1s source rocks the differences of geochemical characteristics of the c3t gas from different areas in the field suggest the effect of maturity difference of the source rocks rather than the diffusive migration and the large scale lateral migration of the c3t gas seems unlikely comparative study indicates that the differences of the geochemical characteristics of the p1s gases from the yulin and daniudi gas fields originated likely from the maturity difference of the in situ source rocks rather than the effect of large scale lateral migration of the p1s gases

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

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

recent advances in petroleum geochemistry applied to hydrocarbon exploration this paper presents a critical review of recent advances in the study of generation maturation and migration of petroleum the different source potentials for generating hydrocarbons depend on the type of kerogen in addition to the classic types i ii and iii a residual oxidized organic matter is also observed which has no potential for hydrocarbon generation coal is widely considered as a potential source for gas but some coals may generate sizeable amounts of oil as well the development of routine analytical tools particularly pyrolysis makes possible the preparation of geochemical logs that are the basis for screening samples and for interpreting other analyses among optical techniques the major subjects of discussion concern the nature of amorphous organ c matter which may encompass different chemical types of kerogen and the applicability of vitrinite reflectance techniques to type i and ii kerogens following important advances in identification of biological markers in sediments and crude oils these markers are used for oil source rock correlation and proposed for reconstruction of depositional environment and subsequent thermal evolution migration of hydrocarbons is now better understood and the importance of a hydrocarbon phase migration is widely recognized overpressuring of pore fluids is mainly responsible for expulsion of petroleum geologic models of basin evolution subsidence compaction thermal history and geochemical models hydrocarbon generation and migration become progressively available their integration int a comprehensive model should be one of the major developments in future petroleum exploration

advances in soil gas geochemical exploration for natural resources some current examples and practices field studies have demonstrated that gas anomalies are found over buried mineral deposits abnormally high concentrations of sulfur gases and carbon dioxide and abnormally low concentrations of oxygen are commonly found over sulfide ore deposits helium anomalies are commonly associated with uranium deposits and geothermal areas helium and hydrocarbon gas anomalies have been detected over oil and gas deposits gases are sampled by extracting them from the pore space of soil by degassing soil or rock or by adsorbing them on artificial collectors the two most widely used techniques for gas analysis are gas chromatography and mass spectrometry the detection of gas anomalies at or near the surface may be an effective method to locate buried mineral deposits

upper carboniferous source rocks north and south of the variscan front nw and central europe abstract some topical aspects of upper carboniferous source rocks within the three main tectonic settings in nw and central europe are described these are likely to offer an effective tool for reduction of risk in future exploration in the foreland the main geological factors responsible for the nitrogen content of gas accumulations and for the isotopic composition of methane are outlined it will probably be possible to predict the molecular and isotopic composition of gas accumulations in the future in addition the application of the dynamics of diffusion processes to the loss of gas accumulations over geological time offers the possibility of predicting hydrocarbon fill in some areas of the basin in the variscan fold belt the model of thin skin tectonics suggests that the lowermost parautochthonous low rank thrust sheets are more likely to be source rocks than are allochthonous high rank thrust sheets additionally young subsidence is required in order to meet the conditions of gas dynamics the possible source rock potential of the permo carboniferous fills of the intramontane grabens south of the variscan front are reported here for the first time some promising discoveries below the swiss and german molasse basin as well as in the eastern paris basin mean that the intramontane grabens are now an interesting exploration objective

provenance evolution of oligocene pliocene foreland deposits in the dezful embayment to constrain central zagros exhumation history this paper presents a detailed provenance analysis of the 6 5 km thick oligocene pliocene foreland succession in the north dezful embayment in zagros iran using petrography geochemistry and palaeoflow analysis we determined three provenance shifts during the exhumation of the zagros orogen above the ahwaz sandstones sourced from the arabian craton a sharp decrease in sio2 al2o3 zr sc th sc th co and th cr within the gachsaran fm and the appearance of first cycle sands derived from metamorphic and volcanic rocks at the base of the aghajari fm characterize the onset of exhumation rising topography and denudation of the zagros orogenic system during the latest oligocene and constitute the first shift the second shift in the middle aghajari fm is marked by a distinct increase in radiolarian cherts and compatible incompatible elements indicating the exhumation of neotethyan radiolarite ophiolite allochthonous slices in the high zagros zone during the tortonian c 9 ma large quantities of carbonate clasts derived from the nearby growing izeh zone anticlines characterize the messinian c 7 ma bakhtyari conglomerates marking the third shift in conclusion a detailed hands on provenance analysis of the clastic sedimentary sequences in the north dezful embayment enabled us to determine catchment areas and shifts hitherto unstudied thus contributing to the exhumation history of the central zagros orogenic system supplementary material details of the petrography and geochemistry results are available at https doi org 10 6084 m9 figshare c 4905558

regional petroleum alteration trends in barents sea oils and condensates as a clue to migration regimes and processes to date most condensates and gases found in the hammerfest basin exist in distal central basin settings in traps with tight cap rocks of class 1 traps whereas low gas oil ratio gor oils occur systematically in proximal basin settings where cap rocks of class 3 traps prevail multiple fill spill events resulted in the redistribution of oils toward structurally higher basin margins in a systematic evaluation of light hydrocarbon parameters from condensates and oils it was found that oils in general exhibit more traceable alteration effects than do condensates whereas 75 of condensate and 13 3 of oil samples are fractionated 6 25 and 10 respectively show signs of biodegradation long distance migration is indicated for 12 5 of condensate and 50 of oil samples in addition clear evidence is shown for the mixing of recently migrated high gor petroleum phases with older low gor paleo oils in general variation in source specific parameters is surprisingly less pronounced decreasing thermal maturity of entrapped petroleum from the eastern part of the tromso basin toward the masoy nysleppen fault complex is observed whereas high maturities are shown for the nordkapp basin and the finnmark platform in the eastern part of the study area low to medium maturities are recorded for oils from the basin margins of the hammerfest basin alterations in the composition of the petroleums by physiochemical processes and distribution patterns of the petroleums are closely associated with uplift and erosion