formation of lava tubes and extensive flow field during the 1991 1993 eruption of mount etna detailed mapping during the 1991 1993 eruption of mount etna has shown that there is a relationship between tumuli ephemeral vents lava tubes and their parent lava flows during this eruption many tubes formed in stationary inflated a a lava flows ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows many of which subsequently developed new tubes the resulting complex network of tubes ephemeral vents and secondary flows was responsible for most of the widening thickening and lengthening of the 1991 1993 etna lava flow field the supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel fed lava flow our observations suggest that lava tubes play a more important role in the formation of extensive a a flow fields on etna than has previously been recognized

topographic control on lava flow paths at mount etna italy implications for hazard assessment 1 assessment of the hazard from lava flow inundation at the active volcano of mount etna italy was performed by calculating the probability of lava flow inundation at each position on the volcano a probability distribution for the formation of new vents was calculated using geological and volcanological data from past eruptions the simulated lava flows from these vents were emplaced using a maximum expected flow length derived from geological data on previous lava flows simulations were run using downflow a digital elevation model based model designed to predict lava flow paths different eruptive scenarios were simulated by varying the elevation and probability distribution of eruptive points inundation maps show that the city of catania and the coastal zone may only be impacted by flows erupted from low altitude vents 2000 m preferentially inundate the northeast and southern sectors of the volcano as well as the valle del bove eruptions occurring in the summit area 3000 m elevation pose no threat to the local population discrepancies between the results of simple hydrological models and those of the downflow model show that hydrological approaches are inappropriate when dealing with etnean lava flows because hydrological approaches are not designed to reproduce the full complexity of lava flow spreading they underestimate the catchment basins when the fluid has a complex rheology

analysis of light hydrocarbons in soil gases lost river region west virginia relation to stratigraphy and geological structures analyses of 471 near surface soil gas samples for light hydrocarbons c1 c4 c2l c3l and h2 from the lost river gas field in hardy county west virginia reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases which occur directly over the faulted eastern limb of the whip cove anticline compositional changes in the soil gases data clearly define major changes in the maturity and locations of potential source beds grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil gas constituents large concentrations coupled with high saturate to olefin ratios further confirms that this active seepage is near macroseep levels variations in soil gas compositional trends separate the soil gas data into two domains with oilier compositions to the west and gassier compositions to the east although the composition of the shallow soil gases above the lost river gas field are oilier than the reservoir gases they occur directly over the eastern faulted limb of the producing anticlinal structure suggesting that the dry gases from the oriskany reservoir are probably mixed with oilier gases from organic rich strata among devonian shales the eastern anomalies are much gassier and are very similar to the oriskany gases produced by the lost river gas field the eastern anomalies directly overlie near vertical beds of devonian and older age formations that are likely conduits for deeper mature thermal gases

hydrocarbons from near surface sediments of the barents sea north of svalbard indication of subsurface hydrocarbon generation abstract the barents sea is considered as an important target for oil and gas exploration but the petroleum potential of its shelf and slope regions is unknown here we present results of a research cruise to the northern hinlopen margin at the transition to the southern nansen basin and the eastern yermak plateau multichannel reflection seismic data acquisition heat flow measurements and geochemical analyses of near surface sediments obtained by gravity coring were conducted to study the northern barents sea shelf and the early opening of the nansen basin and decipher their petroleum potential seismic data indicate high thicknesses of up to 2000 m of cenozoic sediments heat flow density values in the study area range between 67 and 108 mw m 2 the sediment samples were analysed for bulk geochemistry and sorbed hydrocarbon gases and for two sites for extractable hydrocarbons data from extractable n alkanes n c 25 and bulk hi and oi from rock eval organic matter demonstrate predominantly terrigenous organic material most likely derived from ice transported allochthonous sediments none of the sediments revealed substantial amounts of methane in pore waters arguing against active hydrocarbon seepage in the studied areas however thermogenic gases sorbed to the sediment matrix clay minerals organic matter and or carbonates were found in concentrations of up to 600 ppb on sediment wet wt basis for the samples from the northern hinlopen margin and particularly from the adjacent nansen basin a paleo fluid flow of thermogenic gas is indicated and accompanied by higher n alkanes with a modal petroleum like distribution 13 c values of methane ethane and propane and gas compositions point at a mainly marine source rock origin of all studied gases with early oil window maturities of the associated rocks 0 6 0 9 vr from this data an admixture of type iii derived thermogenic gases is indicated for some of the yermak plateau sediments for which also the lowest abundances of sorbed gases 50 100 ppb were observed gas geochemical characteristics in the samples with low gas abundances can partially be explained by an input of gases through ice transport of allochthonous hydrocarbons which were bound to mature organic matter for a site on the northern hinlopen margin ne of svalbard right at the southern termination of the nansen basin a different situation is indicated in this area the highest concentrations of sorbed gases most likely derived from sediments with an early oil window maturity and a marine kerogen type ii typical isotopic distribution at this location a pseudo well was constructed from 2d seismic data for reconstruction of thermal and maturity evolution the simulation results indicate that an early to middle eocene source rock would be in the early oil window since the early miocene a possible source rock here and in the circum arctic region could have been formed by azolla algae and other flourishing primary producers

geochemical characteristics of natural gases related to late paleozoic coal measures in china abstract by 2013 more than 20 gas fields related to late paleozoic coal measures have been discovered in china primarily distributed in the bohai bay ordos sichuan and junggar basins the total proven reserve in these gas fields is more than 3200 109 m3 accounting for 30 1 of the proven gas reserves in china after analyzing the molecular compositions stable carbon isotopes and helium isotopes of 375 gas samples genetic types and origins of the natural gas as well as the secondary alterations it experienced are discussed in this study most of the natural gas related to the late paleozoic coal measures are coal type gas with some of the gases from the sichuan basin and the jingbian gas field in the ordos basin being oil type gases generated from marine mudstones in the permian longtan formation and marine carbonates in the ordovician majiagou formation respectively the equivalent vitrinite reflectance values based on stable carbon isotopes of natural gases vary from 0 8 to 2 5 indicating the mature to over mature stage positive carbon isotope series 13c ch4 13c c2h6 and 13c ch4 13c c3h8 the form of 13c ch4 13c c2h6 is primarily resulted from the mixing of oil type gases of different maturities while the form of 13c ch4 13c c3h8 is primarily resulted from the mixing of coal type gases of different maturities high concentrations of hydrogen sulfide in natural gas from the sichuan basin and the chenghai gas field in the bohai bay basin are resulted from thermochemical sulfate reduction tsr which causes to some extent the fractionation of carbon isotopes by contrast the fractionation of methane carbon isotopes resulting from the tsr effect is not clear carbon dioxide from the bohai bay and junggar basins is primarily biogenic generated via the thermal decomposition of organic matter while that from the ordos basin sichuan basin and chenghai gas field is generated via the thermal decomposition of carbonates carbon dioxide related to the tsr effect has also contributed to the sichuan basin and chenghai gas field

the 2004 2005 etna eruption implications for flank deformation and structural behaviour of the volcano the development of the 2004 2005 eruption at etna italy is investigated by means of field surveys to define the current structural state of the volcano in 2004 2005 a fracture swarm associated with three effusive vents propagated downslope from the se summit crater towards the se such a scenario is commonly observed at etna as a pressure increase within the central conduits induces the lateral propagation of most of the dikes downslope nevertheless some unusual features of this eruption slower propagation of fractures lack of explosive activity and seismicity oblique shear along the fractures suggest a more complex triggering mechanism a detailed review of the recent activity at etna enables us to better define this possible mechanism in fact the nw se trending fractures formed in 2004 2005 constitute the southeastern continuation of a n s trending fracture system which started to develop in early 1998 to the east of the summit craters the overall 1998 2005 deformation pattern therefore forms an arcuate feature whose geometry and kinematics are consistent with the head of a shallow flank deformation on the e summit of etna similar deformation patterns have also been observed in analogue models of deforming volcanic cones in this framework the 2004 2005 eruption was possibly induced by a dike resulting from the intersection of this incipient fracture system with the se crater a significant acceleration of this flank deformation may be induced by any magmatic involvement the central conduit of the volcano is presently open constantly buffering any increase in magmatic pressure and any hazardous consequence can be expected to be limited a more hazardous scenario may be considered with a partial or total closing of the central conduit in this case magmatic overpressure within the central conduit may enhance the collapse of the upper eastern flank triggering an explosive eruption associated with a landslide reaching the eastern lower slope of the volcano

recent advances in petroleum isotope geochemistry abstract organic matter is isotopically heterogeneous lipids tend to be depleted in the heavy carbon and hydrogen isotopes as compared to carbohydrates and lignite the c and h isotopic composition of kerogens is determined by many factors such as environmental conditions during formation and deposition and variability of sources of organic matter stable isotope concentrations in mature kerogens are not significantly altered during maturation and hence reflect the variability of the organic matter during deposition isotope variations in kerogens therefore reflect to some extent the paleogeographic situation and can be used for paleoreconstruction a basin facies with anoxia and a marginal facies with detrital input can be differentiated isotopically in the toarcian of central europe coals are isotopically heterogeneous due to different isotopic composition of macerals inertinite and vitrinite tends to be enriched in deuterium as compared to exinite also boghead coals tend to be depleted in dueterium which possibly reflects various lipid contents of respective precursor materials petroleums can be genetically classified through the patterns of covariance of their compound classes carbonate and shale sourced petroleums may be additionally differentiated through their deuterium content the general world wide applicability of such genetic differentiations needs still to be proved carbon and hydrogen isotope variations in methane are genetically controlled bacterial methane in freshwater environments is characteristically depleted in deuterium even complex origins of natural gases can be unraveled using compositional and isotopic variations new analytical developments led to the application of carbon isotope analyses of gaseous hydrocarbons desorbed from sediments for geochemical surface exploration a discussion reveals that processes such as isotope effects during degassing and bacterial oxidation as well as indigenous formation of hydrocarbons may completely obscure the results

near surface evidence of migration of natural gas from deep reservoirs and source rocks gaseous hydrocarbons migrate to the surface in small quantities from deep source rocks and natural gas or crude oil accumulations this process leads to anomalous concentrations of hydrocarbons in near surface sediments the hydrocarbon anomalies amount and composition are useful in exploration because they may point to oil and gas reservoirs the origin of hydrocarbon anomalies in shallow sediments is not a simple one methane in near surface sediments may be caused by a combination of bacterial production of methane leakage from reservoirs and leakage of methane from thermal maturation of organic matter in source rocks organic geochemistry especially carbon isotope techniques have been used to recognize secondary fractionation processes caused by degassing or bacterial oxidation examples of geochemical surveys in onshore and offshore areas indicate secondary fractionations so small that gases from near surface sediments and from reservoirs can be correlated by the geochemical data the application of stable carbon isotopes is a significant step toward improving geochemical hydrocarbon surface exploration methods

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

forecasting lava flow paths by a stochastic approach 1 a stochastic model named downflow is presented to forecast possible lava flow paths with the aim of hazard assessment and mitigation the model relies on the fact that lava flow emplacement is in many cases controlled by topography the potential inundation area of the flow is determined by considering a number of steepest paths over stochastic perturbations of the original topography since the code requires very short computational time and few input data the model proved to be a very useful tool to obtain real time predictions during the initial phase of an eruption the necessary topographic information can be obtained by photogrammetry radar altimetry or laser scanning techniques downflow has been successfully applied to a number of flows including the september 2004 lava flow where updated topographies were available

geochemistry and accumulation of carbon dioxide gases in china we propose the following identification indicators for co2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples the co2 carbon isotope values of 390 samples and the helium isotope composition of 256 samples collected from wells of 16 oil and gas bearing basins and gas seepages in china and reviewing literature on organic and inorganic co2 throughout the world co2 is of organic origin if co2 content 13cco22 is of inorganic origin if dgr13cco2 8 pmil or co2 60 and co2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if 10 pmil 13cco213cco2 values of organic co2 mainly range from 10 to 22 pmil in tectonically stable oil and gas bearing basins where deep faults and magmatism are undeveloped organic co2 occurs almost exclusively in tectonically active oil and gas bearing basins where active faults and magmatism are present or in areas of modern volcanic activity the dgr13cco2 values of inorganic co2 range from 2 to 8 pmil and are distributed along faults the inorganic co2 with a dgr13cco2 value of 0 3 pmil is of metamorphic origin and that with dgr13cco2 values of 6 2 pmil is mantle derived and of magmatic origin in china the dgr13cco2 values of inorganic co2 gas pools range from 3 41 to 8 44 pmil and the helium ratio r ra is 1 which indicates that these gases are typically derived from the mantle

molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the gulf of cadiz ne atlantic investigations of molecular and carbon isotopic variability of hydrocarbon gases from methane through butanes pentanes have been performed on six mud volcanoes from two fluid venting provinces located in the gulf of cadiz ne atlantic the main aims were to define the basic gas types to describe their geochemical characteristics in relationship to their sources and to determine the secondary effects due to migration mixing and microbial alteration hydrocarbon gas data reveal two groups of gases despite the different maturation characteristics both gas groups are allochthonous to the erupted mud breccia and represent a complex of redeposited secondary migrated mixed and microbially altered hydrocarbons it may possibly imply the presence of hydrocarbon accumulations in the deep subsurface of the gulf of cadiz

biogenic gas in fluid inclusions from sandstones in contact with oil mature coals this study was initiated to investigate if coals on the norwegian offshore continental shelf nocs expel petroleum and in which form the results revealed that equally isotopically light methane c1 was released from fluid inclusions in sandstones and from adjacent coal 60 9 to 72 7 the analyzed samples were collected from cored northern north sea and mid norwegian shelf wells in the depth interval 3924 5095 m 12 874 16 716 ft the vitrinite reflectance ro values of the coals range between 0 53 and 1 12 with most values between 0 8 and 1 0 only minor c2 fractions were released showing much heavier gas isotope signatures similar to values seen in our in house nocs gas isotope database for comparable depths the similar light c1 isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal and that no isotope fractionation occurs during release of the gas in nature traditional isotope interpretation schemes suggest the c1 to have a biogenic origin whereas recently published data also show the possibility for an early mature thermogenic origin the isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample these did not form simultaneously but during multiple events potentially covering several million years the release of isotopically light c1 proves gas presence in the sandstone at the time of fluid inclusion formation we therefore speculate that significant volumes of isotopically light c1 have been expelled from the analyzed coals over time the expelled isotopically light c1 may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values thus explaining the isotopically lighter than expected gas accumulations on the nocs e g troll frigg and draugen fields

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

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

model for hydrocarbon microseepage and related near surface alterations despite recent advances in surface and near surface geochemical methods there continues to be a need for improved understanding of the scientific basis relating these methods to petroleum deposits we have reviewed the literature pertinent to the several methods we have used and integrated that information with some 100 man years of our experience to contribute to that end evidence indicates that hydrocarbon micro seepage from reservoirs most probably involves buoyant colloidal size microbubbles of light hydrocarbons principally methane through the butanes ascending relatively rapidly through a water filled network of fractures joints and bedding planes bacterial consumption of these hydrocarbons creates carbon dioxide and hydrogen sulfide which may alter sediments producing detectable modifications in geomorphic seismic magnetic and radiometric properties we have used combinations of 1 airborne microwave sensing and laboratory analyses of soil gas hydrocarbons 2 shallow source aeromagnetic and soil magnetic susceptibility measurements 3 aerial and surface gamma ray measurements and 4 geomorphology to find productive oil and gas deposits results suggest that using these methods can substantially increase the probability of wildcat success and reduce finding costs in selected geologic settings however these surface methods cannot reveal depth size or quality of reservoirs or even if producible hydrocarbons will be found

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

the role of organic matter in the genesis of the el soldado volcanic hosted manto type cu deposit chile solid bitumen residual petroleum occurs intimately associated with sulfides in several manto type cu deposits of central chile el soldado 32 degrees 38 s 71 degrees 04 w is one of the largest manto type deposits 130 million metric tons1 5 cu and is hosted by rhyolites and andesites of the lower cretaceous lo prado formation as a result of basinal burial in the early cretaceous back arc basin petroleum was generated from organic rich shales in the underlying lower lo prado formation and migrated into primary and structural porosity in the upper lo prado formation migration was controlled by extensional basinal faults low temperature 90 degrees c biodegradation of petroleum developed an early stage i assemblage of mostly framboidal pyrite or sphalerite or chalcopyrite geopetal structures related to pressure degassing of semi solid petroleum suggest that migration occurred while the strata were horizontal at ca 130 ma continued basinal burial led to thermal degassing increased maturation and solidification of the petroleum low grade regional pumpellyite prehnite metamorphism of the sequence was followed at ca 110 to 103 ma by the influx of high temperature ca 300 degrees c cu rich fluids concurrently with emplacement of the cretaceous batholith into the basin basinal fluids were focused into a now tilted sequence by a brittle shear north south to north northwest fault system copper sulfides stage ii assemblage replaced preexisting pyrite and bitumen forming zoned orebodies with a central chalcocite hematite zone followed outward by bornite chalcocite chalcopyrite bornite and pyrite chalcopyrite zones the mineralizing fluids altered the bitumen thermally and chemically as follows 1 reflectance r o increased up to ca 5 percent in the core of the orebodies against a background of ca 3 percent 2 anisotropy nongranular microscale and domain developed and the bitumen was locally graphitized 3 chlorine or cu or fe was incorporated into the bitumen affecting its reflectance the source of cu was most likely the overlying oxidized volcanic pile the cu was transported as chloride cucl complexes precipitating where the cucl rich solutions reacted with bitumen adsorption reduction and pyrite from the stage i assemblage the study demonstrates that in the lower cretaceous basin of chile degraded petroleum reservoirs can be important controls for metallic mineralization derived from hydrothermal solutions of different sources particularly if biodegradation generated pyrite

hydrocarbon gas seeps of the convergent hikurangi margin north island new zealand abstract two hydrocarbon gas seeps located about 13 km apart have distinctive molecular and isotopic compositions these seeps occur within separate tectonic melange units of narrow parallel trending and structurally complex zones with incorporated upper cretaceous and palaeogene passive continental margin deposits which are now compressively deformed and imbricated along the convergent hikurangi margin of north island new zealand at brookby station within the coastal high the seeping hydrocarbon gas has a methane ethane ratio of 48 and 13 c and d values of methane of 45 7 and 188 respectively relative to the pdb and smow standards within the complex core of the elsthorpe anticline at campbell station seep gas has a methane ethane ratio of about 12000 and the methane has 13 c and d values of 37 4 and 170 respectively the source of the gases cannot be positively identified but the gases probably originate from the thermal decomposition of organic matter in tectonically disturbed upper cretaceous and or lower tertiary sedimentary rocks of passive margin affinity and reach the surface by migration along thrust faults associated with tectonic melange the geochemical differences between the two gases may result from differences in burial depths of similar source sediment

origin of near surface hydrocarbon gases bound in northern barents sea sediments abstract in the southern part of the norwegian barents sea extensive hydrocarbon exploration and drilling has led to several oil and gas discoveries yet little is known on the petroleum systems and potential hydrocarbon reservoirs in the northern norwegian barents sea if hydrocarbons generated by thermogenic processes deep in the subsurface migrate to the surface traces of these hydrocarbons can remain in the near surface sediments and provide indirect information on potential petroleum systems near surface hydrocarbon prospecting is a method frequently used in hydrocarbon exploration of frontier areas where no direct geochemical information from drilling is available to improve knowledge of subsurface structures and the evolution of potential petroleum systems in the northern barents sea seismic data and near surface sediment samples were collected analysis of bound hydrocarbon gases extracted from these sediments revealed concentrations significantly above background in areas along the hornsund knollega fault complex as well as near margins of the olga basin generally the compositional and stable carbon isotope signatures of bound gases indicate thermogenic origins from source rocks of oil window maturity for near surface gases with anomalous high concentrations whereas for sediments with low concentration of bound gases mixture of thermogenic and microbial gas is indicated amount and composition of bound gas extracted from source rock samples from spitsbergen indicate that contribution of transported material may have influenced the bound gas in near surface marine sediment of the barents sea in areas where presence of mature organic matter is indicated in the knolegga fault complex near the western barents sea margin high concentrations of thermogenic gas in near surface sediments are associated with fault bound basins and most likely originate from the paleocene eocene torsk formation in the olga basin higher bound gas concentrations occur near the southern border of the basin corresponding to sub cropping late jurassic early cretaceous shales whereas elevated concentrations in the northern olga basin are associated with reactivated faults reaching close to the surface sediments above the center of the basin show significantly lower bound gas concentrations these observations indicate that the jurassic shales act as a regional seal for hydrocarbons and that reactivated faults at the basin margin represent pathways for migration to the surface 1d basin and petroleum systems modeling for the olga basin indicates that early to middle triassic sediments reached oil window maturity and represent the most likely source for thermogenic near surface gas in that area