salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

a wind driven isopycnic coordinate model of the north and equatorial atlantic ocean 1 model development and supporting experiments numerical approximations to the dynamic equations are given which allow basin size ocean circulation models formulated in isopycnic coordinates to accommodate variable bottom topography and irregular coastlines emphasis is placed on computational economy through the use of a split explicit time integration scheme on the proper formulation of the advection and coriolis terms in the momentum equation in case of strongly varying layer thickness and on the correct estimation of the horizontal pressure gradient force in grid boxes truncated by steep bottom slopes the algorithms are tested in a series of two and three layer double gyre experiments in cases of steady forcing leading to a steady circulation we are able to reproduce the expected motionless final state in coordinate layers that are below the direct influence of the wind forcing this includes layers intersecting topographic obstacles a long term 25 30 year vacillation tentatively associated with the outcropping of isopycnals along the edge of the cyclonic gyre in a steadily forced model is documented the paper forms the basis for a subsequent study in which circulation features obtained in a realistic north atlantic setting will be discussed

analysis of secondary migration of hydrocarbons in the ordovician carbonate reservoirs in the tazhong uplift tarim basin china the process and mechanisms of secondary hydrocarbon migration in the tazhong uplift tarim basin were investigated based on the analysis of the regional structure and by integrating geologic hydrodynamic and geochemical parameters parameters successfully analyzed included the fluid potential fluid properties production outputs and diamantane index the results indicated that hydrocarbons migrated into the tazhong uplift from the northern part of the manjiaer depression through a series of injection points ips during four orogenies that is the early caledonian 510 ma the late caledonian 439 ma the late hercynian indosinian 290 ma and the yanshanian himalayan 208 ma a total of six ips were identified at the intersections of the northeast trending faults and the northwest trending flower strike faults the hydrocarbons migrated from the ips into traps along regional trends from northwest to southeast and from northeast to southwest the hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the tazhong uplift with increasing distance from the ips daily hydrocarbon production decreases and the hydrocarbons become progressively heavier and display lower gas oil ratios

preliminary evaluation of trace hydrocarbon speciation and abundance as an exploration tool for footwall style sulfide ores associated with the sudbury igneous complex ontario canada deposits of cu ni platinum group elements pge rich sulfides occur along the north range footwall of the sudbury igneous complex ontario canada the deposits have been modified chemically mineralogically and texturally by multiple syn to postmagmatic hydrothermal events and are hosted mainly within sudbury breccia a contact metamorphosed and hydrothermally altered impact breccia the composition of c1 to c6 unsaturated and saturated hydrocarbons in bulk fluid inclusion volatiles in the matrix of the sudbury breccia was investigated using in line rock crushing gas chromatography gc in order to compare the hydrocarbon composition of this rock type in breccia zones containing economic footwall sulfide deposits at the morrison deposit levack mine north range sudbury ontario with barren breccia zones trace hydrocarbons possibly abiogenic in origin occur throughout the sudbury igneous complex footwall and country rocks investigated however there are differences in the composition and abundance of bulk hydrocarbons released from mineralized and barren sudbury breccia when fluid inclusions are opened volatiles released from samples of sudbury breccia from sulfide mineralized and sulfide barren environments showed the following 1 higher average abundances of light saturated hydrocarbons c1 c4 and an approximately 2 higher total abundance of hydrocarbons mol g of rock basis associated with samples from the mineralized study area and 2 comparable average unsaturated hydrocarbon abundances in both settings but a larger range in unsaturated hydrocarbon abundances in the mineralized study area differences in bulk hydrocarbon abundance and composition reflect differences in the abundance and hydrocarbon content of fluid inclusions trapped in the barren and mineralized sudbury breccia zones in addition the compositional modification of circulating fluids by processes and conditions unique to the mineralized sudbury breccia environment may have occurred prior to inclusion entrapment e g modification by mineral catalysts differences in temperature and oxygen fugacity preliminary spatial analysis of the hydrocarbon distribution within and surrounding the morrison deposit shows that the total hydrocarbon abundance in the sudbury breccia does not change with orebody proximity within a single host breccia zone however volatiles trapped in the sudbury breccia matrix near the top of the footwall deposit where it is physically connected to contact style ni cu pge ores have elevated unsaturated hydrocarbon abundances e g propene compared to other parts of the same breccia zone distal to this zone of physical connection samples of the pge enriched sudbury breccia and mineralized veins containing alteration assemblages characteristic of the footwall ore styles have higher unsaturated hydrocarbon abundances and unsaturated saturated ratios compared to sudbury breccia and vein samples that are pge poor localized enrichments in unsaturated hydrocarbons in sudbury breccia zones may be the result of specific ore minerals and metal complexes involving the pge and group viii metals that acted as catalytic agents modifying the composition of initially light saturated hydrocarbon species prior to entrapment in fluid inclusions to apply the analysis of hydrocarbon compositions in trapped fluid inclusions to discriminating mineralized from barren zones of sudbury breccia samples from targeted areas were compared to samples of barren sudbury breccia background although a large data set is presented here to establish these compositional fields analysis of smaller sample subsets from each environment yields a statistically significant separation between the two environments total hydrocarbon abundance c3 c1 ratios and unsaturated saturated hydrocarbon ratios are useful indicators of sulfide ore proximity and may help delineate regions enriched in pge

some exact solutions to the nonlinear shallow water wave equations these exact solutions correspond to time dependent motions in parabolic basins a characteristic feature is that the shoreline is not fixed it is free to move and must be determined as part of the solution in general the motion is oscillatory and has the appropriate small amplitude limit for the case in which the parabolic basin reduces to a flat plane there is a solution for a flood wave these solutions provide a valuable test for numerical models of inundating storm tides

late paleozoic depositional history of the tarim basin northwest china an integration of biostratigraphic and lithostratigraphic constraints this study provides the first detailed lithostratigraphic and biostratigraphic constraints for improving stratigraphic resolution for hydrocarbon prospecting and exploration in the tarim basin a total of 49 stratigraphic units 38 formations and 11 members ranging in age from the latest devonian to permian are reviewed or redefined in terms of nomenclatures lithology age constraints and lateral distributions based on the detailed field works or newly published data of these the piqiang formation new formation is proposed to include the reefal carbonates of asselian sakmarian age from the northern tarim the subsurface upper paleozoic stratigraphic framework of the desert areas of the basin is also established for the first time the high resolution basinwide stratigraphic correlations reveal that the sedimentation of the basin in the late paleozoic was extremely uneven of these the famennian to changhsingian successions are completely recorded in the southwestern margin areas of the basin here five eustatic sedimentary cycles are well recognizable suggesting the sedimentation was more eustatically controlled and little affected by local tectonism the late paleozoic successions of both kalpin and taklimakan regions are commonly interrupted by major hiatuses at various horizons suggesting that the sedimentation was apparently modified by local tectonism of these the northward movement of the tarim block and its subsequent collision with the yili microcontinent part of the kazakhstan plate may be principally accountable for the discrepancy in the sedimentation of the various regions in the basin in the late paleozoic

the complexity secondary geochemical process genetic mechanism and distribution prediction of deep marine oil and gas in the tarim basin china abstract most paleozoic marine craton basins in china are rich in natural gas except the tarim basin where both oil and gas are prolific various hydrocarbon types have been found in this basin including deep ultra heavy oil heavy oil normal oil volatile oil condensate and natural gas petroleums with diverse types even coexist within a single reservoir generally the ordovician and cambrian reservoirs are deeply buried 5500 8000 m and intensively altered after initial oil and gas accumulations bringing challenges to physical property prediction before drilling and evaluation of exploration targets based on recent exploration discoveries and experimental data we systematically analyzed the origin of oil and gas demonstrated the mechanisms and processes of secondary geochemical alteration e g biodegradation gas washing fractionation tsr and high temperature cracking moreover the preservation mechanism of ultra deep normal oil was also clarified and the distribution patterns of oil and gas were predicted tarim basin has been uplifted and shallowly buried in the early accumulation stage then rapidly and deeply buried in the late stage resulting in various geochemical alteration processes at different burial stages through the reconstruction of complex deep strata oil and gas accumulation histories the spatial distribution of deep petroleum types were predicted this review has provided detailed assessment with case studies in the tarim basin and shed light on the preservation mechanisms and the alteration processes of deep liquid petroleums in addition to massive gas accumulations liquid oils still have great exploration potential in strata deeper than 8000 m in the tarim basin

geochemical characteristics and origin of natural gas in tarim basin china the tarim basin located in northwest china is one of the largest basins in the world that has not been well explored in recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1 1 ratio of oil to gas resource this means that the natural gas resource is very important for the hydrocarbon resource in the tarim basin in this basin most of the discovered gases are condensate associated gas and oil associated gas research on chemical components and isotope composition of carbon and hydrogen of natural gases from the tarim basin shows that all discovered gases in this basin are thermogenic with respect to source organic matter there are two types of gases 1 oil type gas generated from sinian to lower paleozoic marine source rocks with the sapropelic organic matter with 44 5 to 33 8o oo of d13c1 42 0 to 28 1o oo of d13c2 35 4 to 28 4o oo of d13c3 and heavier than 200o oo of dd1 and 2 coal type gas generated from mesozoic terrestrial source rocks with the humic organic matter with 40 5 to 33 1o oo of d13c1 29 7 to 21 3o oo of d13c2 26 3 to 20 3o oo of d13c3 and lighter than 200o oo of dd1

the origin migration and accumulation of the ordovician gas in the tazhong iii region tarim basin nw china abstract the ordovician large scale gas reservoir in the tazhong iii region burial depth 6000 m is a typical deep strata gas reservoir in the tarim basin of nw china the origin migration and accumulation of the ordovician natural gas in the tazhong iii region were investigated by integrating geological geophysical and geochemical parameters the parameters that were successfully analyzed included the gas compositions and carbon isotope ratios production data burial and geothermal histories oil compositions and properties together with the diamantane index the results indicated that the ordovician natural gas in the tazhong iii region originated from cambrian ordovician paleo oil cracking accompanied by a large amount of h2s generated from thermochemical sulfate reduction tsr when the oil cracking gas migrated vertically into the ordovician reservoir through the tazhong i fault and strike slip faults it was preferentially charged in the pinnate fault zones of strike slip faults with high porosity and permeability leading to the enrichment of natural gas and an abnormal distribution of the compositions and properties of hydrocarbon then oil cracking gas intruded into and altered the previous oil reservoir from northeast to southwest through the strike slip faults unconformities and pore fracture reservoir belt leading to the differential accumulation of hydrocarbon in districts of the tazhong iii region the ordovician natural gas migration accumulation model in the tazhong iii region indicates a great exploration potential in the deep or super deep strata of the tarim basin further exploration activities in the ordovician reservoir of the tarim basin should be conducted in the regions where cambrian ordovician source rocks are well developed and the stress fracture zones related deep fault should be emphasized

sources and mechanisms of formation of gaseous hydrocarbons in sedimentary rocks abstract accepted concepts of principal stages of hydrocarbon formation fail to satisfactorily explain the accumulation of supergiant gasfields in west siberia u s s r such as for example urengoy which comprise 30 of the proven gas reserves of the world a new mathematical model has been developed which applies the concept of activation energy distribution to the formation of gases characteristic differences in the molecular structure of humic and sapropelic source organic matter are expressed in different activation energy distributions the model allows the calculation of isotope values for different types of organic matter and their dependence on maturity application of the model to west siberia suggests that the supergiant gas accumulations in urengoy are more likely formed from terrestrial organic matter through thermochemical reactions at moderate depths and are not the result of either deep overmature or bacteriogenic processes the new mathematical model suggests that humic organic matter in general has a high methane generating capacity at comparably low maturities of organic matter equivalent to 0 5 0 7 on the vitrinite reflectance scale

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

geochemistry and origin of natural gas in the petroliferous mahu sag northwestern junggar basin nw china carboniferous marine and permian lacustrine gas systems abstract the mahu sag located in the northwestern junggar basin of nw china is a renowned carboniferous marine permian lacustrine hydrocarbon depocenter that in theory could host both oil and gas however as ongoing hydrocarbon exploration and production are focused solely on oil few gas prospects have been discovered in this paper we discuss the possible occurrence of natural gas deposits based on a comprehensive study of gas geochemistry including gas composition carbon isotopes and compositions of light hydrocarbons c5 c7 and biomarkers in retrograde condensates combined with geological information available for the study area our results indicate that three types of natural gas occur in the region a sapropelic type gas derived from the lower permian fengcheng formation p1f a humic type gas sourced from carboniferous strata c and the overlying lower permian jiamuhe formation p1j and a humic type gas derived from the middle permian lower wuerhe formation p2w the p1f sourced sapropelic type gas accumulated during at least two distinct periods of time yielding low maturity oil associated gases and mature to high maturity kerogen oil cracking gases the carboniferous and p1j sourced humic type gases are mainly high maturity to over mature kerogen oil cracking gases without obvious geochemical evidence for a multi stage accumulation the p2w sourced gas is a mature humic type gas of these three types of gas the former two have the potential to form significant gas deposits and constitute valid exploration targets making the mahu sag a prospective area for gas exploration favorable areas for gas occurrence include deep anticlines hosting both kerogen and oil cracking gases the inferred exploration targets occur at significant depths which is why little drilling has been undertaken in these areas and major gas discoveries have yet to be made

microbial methane in the shallow paleozoic sediments and glacial deposits of illinois u s a abstract methane formed by the microbial decomposition of buried organic matter is virtually ubiquitous in the groundwaters of illinois chemical and carbon isotopic compositions are reported for gas samples collected from over 200 private and municipal water wells and from 39 small gas wells completed in glacial deposits drift gas wells carbon and hydrogen isotopic data for methane carbon dioxide and water show that these gases were formed by the carbon dioxide reduction pathway the same mechanism which has been previously shown to be responsible for microbial methane formation in the marine environment the isotopic composition of methane in these samples can be closely correlated with the chemical composition of the gas and with water chemistry the data are interpreted as indicating that isotopically very light methane is found in waters where the residence time of groundwater in the methanogenesis zone was very short relative to the methane production rate

a numerical scheme for modeling tidal wetting and drying 1 this paper introduces a numerical scheme developed to model the wetting and drying of coastal areas by the tide suitable for an oceanic circulation model using finite differences it is based on existing numerical schemes developed for isopycnic models for which the layer thickness can vanish when they interact with the bottom topography some original features are added and the scheme is optimized to accurately reproduce analytical solutions of an idealized problem the main parameters of this numerical scheme are 1 a critical height which depends on the local bottom slope and the grid step and 2 the rossby number this original numerical scheme is applied to a realistic barotropic configuration of the normand breton gulf where tidal amplitudes are large and where the wet dry front location varies greatly

geochemistry and origins of natural gases in the central junggar basin northwest china abstract the petroliferous central junggar basin in northwest china is predominantly an oil exploration region however its gas exploration also might have good prospects thus to assist in gas exploration the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions carbon isotopes and light hydrocarbons of gases based on the results the gas genetic types are grouped into families and combined with the geological setting e g biomarkers of retrograde condensates and source rock characteristics we show that there are four representative genetic types of gases the first consists of gases derived from permian lacustrine mudstones with type i ii kerogen and type iii kerogen sources in the penyijingxi sag their representative geochemical feature is 13c2 ranging from 31 4 to 24 7 the second is gas sourced from carboniferous tufaceous mudstones of type iii kerogen in the dishuiquan sag whose representative geochemical feature is the heaviest values of 13c1 in the studied samples ranging from 32 0 to 30 4 the third consists of gases sourced from jurassic coals and mudstones in the shawan fukang sag the light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from jurassic source rocks in the southern junggar basin the fourth is gas most likely generated from the degradation of crude oil it is mainly found in the luliang area and has dryness values as much as 0 999 and 13c1 ranging from 54 8 to 43 2 among these four types of gases the first mainly sourced from the permian lacustrine mudstones in the penyijingxi sag is the predominant type

differences between spontaneous and triggered earthquakes their influences on foreshock probabilities 1 in this study we investigate the foreshock probabilities calculated from earthquake catalogs from japan southern california and new zealand unlike conventional studies on foreshocks we use a probability based declustering method to separate each catalog into stochastic versions of family trees such that each event is classified as either having been triggered by a preceding event or being a spontaneous event the probabilities are determined from parameters that provide the best fit of the real catalogue using a space time epidemic type aftershock sequence etas model the model assumes that background and triggered earthquakes have the same magnitude dependent triggering capability a foreshock here is defined as a spontaneous event that has one or more larger descendants and a triggered foreshock is a triggered event that has one or more larger descendants the proportion of foreshocks in spontaneous events of each catalog is found to be lower than the proportion of triggered foreshocks in triggered events one possibility is that this is due to different triggering productivity in spontaneous versus triggered events i e a triggered event triggers more children than a spontaneous events of the same magnitude however further analysis on simulated data shows that such difference might be caused by the events triggered by smaller events below the magnitude threshold of the catalog even if the physical interpretation is unclear a clustering model where spontaneous events and triggered events have different triggering behaviors can be used to assess the risk of foreshocks and to avoid overpredicting

mesozoic hydrogeologic systems and hydrocarbon habitat mandapeta endamuru area krishna godavari basin india a hydrostratigraphic model has been proposed based on the spatial distribution of aquitards aquifers aquicludes recharge discharge areas salinity mineralization hydraulic head diagenesis and biodegradation information overlain with source rock potential nature and dispersal of hydrocarbons in the mandapeta endamuru area krishna godavari rift basin this has resulted in understanding the verticolateral hydrodynamics and accumulation of gaseous hydrocarbons and variance in their geochemical properties three hydrogeologic systems are identified within the mesozoic and the associated gaseous hydrocarbons are characterized by their wetness and fingerprint gastar diagrams temporal distribution of the salinity isolith in the vertical geologic column defines the intensity of meteoric infiltration and saline water percolation in the mandapeta subbasin older formations are found to be less saline than the younger ones indicating salinity inversion reservoirs of higher hydraulic heads are associated with gaseous hydrocarbons the observed variation in hydraulic heads of the mandapeta and gollapalli aquifers is attributed possibly to the intervening red bed aquitard acting as a seal areas of fault conduits are identified that facilitated the upward migration of hydrocarbons while allowing the percolation of infiltrated waters and further causing selective segregation of minerals vertical superimposition of different hydrogeologic systems and relative formation contacts also controlled the diffusion and nature of gaseous hydrocarbons a composite hydrogeologic model has been framed based on studies for understanding the recharge discharge dynamics incorporating seismic inputs

occurrence and distribution of high molecular weight hydrocarbons in selected non marine source rocks from the liaohe qaidam and tarim basins china abstract extracts of 18 potential source rocks from the qaidam liaohe and tarim basins china have been characterized by high temperature gas chromatography htgc and found to contain high molecular weight hydrocarbons hmwhcs above c40 these rocks were originally formed in freshwater lacustrine saline lacustrine and terrigenous bog environments with mudstone calcareous mudstone coaly shale and coal lithologies their ages range from triassic to miocene and maturities from immature to mature covering a vitrinite reflectance range ro for the liaohe and tarim samples of 0 54 0 84 the combined distributions of the n alkane and alkylcyclopentane homologues in the extracts can be used to characterize the nature of the original depositional environment in certain samples the carbon number predominance of the n alkanes changes from an odd even predominance to even odd predominance above c40 possibly suggesting different precursors for the n alkanes above and below c40 concentrations of the hmwhcs were also observed to increase with maturity of the source rocks this is the first comprehensive report on the occurrence of hmwhcs in source rocks and their discovery introduces the possibility of using these compounds for oil source rock correlation purposes in future studies along with the more conventional biomarkers

character origin and occurrence of natural gases in the anadarko basin southwestern kansas western oklahoma and texas panhandle u s a abstract natural gas production in the anadarko basin comes from three geographically separated areas that can be differentiated by age of reservoir and by inferred nature of organic thermal origin of the gases in the central basin non associated gases are produced mainly from upper mississippian and pennsylvanian sandstones gas samples are from reservoirs as much as 6588 m deep gases become isotopically heavier 13 c 1 values range from 49 8 to 33 2 and chemically drier c 2 values range from 1 33 with increasing level of thermal maturity gases were generated mainly from interbedded shales with type iii kerogen during the mature and post mature stages of hydrocarbon generation deviations from the trend are due to vertical migration and mixing of gases generated at different levels of thermal maturity over the past 250 myr in the giant panhandle hugoton field non associated gases are generally produced from permian carbonates at depths of 13 c 1 value is 43 2 mean c 2 value is 14 because organic rich mature source rocks are not present in the area gases probably were generated in the central basin from pennsylvanian or older source rocks during the mature stage of hydrocarbon generation this interpretation implies migration over distances as much as several hundred kilometers in the sooner trend associated gases are produced from silurian devonian and mississippian carbonates at depths as great as 2950 m and were generated from type ii kerogen during the mature stage of hydrocarbon generation associated oil usually correlates with extracts of the upper devonian and lower mississippian woodford shale gases are isotopically lighter mean 1 3 c 1 value is 43 9 and chemically wetter mean c 2 value is 14 than those derived from type iii kerogen at an equivalent level of thermal maturity

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

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

substratum transverse faults in kuqa foreland basin northwest china and their significance in petroleum geology abstract the importance of transverse faults in basin evolution and petroleum geology has been often emphasized however the transverse faults in the kuqa foreland basin the most important gas producing area in china have rarely been studied interpreted seismic sections and earthquake focal mechanism solutions as well as other geological and geophysical data allow us to identify a few widely spaced approximately ns trending transverse strike slip faults separating major structural units and to clarify the geometry and kinematics of these transverse faults hidden below thrust faults in the kuqa foreland basin two major structural domains can be distinguished transverse faults in the northern kuqa foreland basin are mainly nne trending sinistral indicating clockwise rotation of fault bounded blocks in contrast the predominant nw trending dextral faults in the southern kuqa foreland basin imply counterclockwise rotation of fault bounded blocks we propose a tectonic model in which crustal blocks are bounded by strike slip faults in a zone of simple shear rotation about vertical axis the strike slip faulting and thrust faulting in the kuqa foreland basin suggest that some of the convergence between south tianshan and tarim blocks may have been accommodated not only by obvious crustal shortening and thickening along thrust faults but also by rotation and possible lateral movement of the crust along transverse faults controlled by the remote collision of indian block with eurasian block since the miocene these reactivated substratum faults which may inherit from paleozoic deformation control various elements relevant to gas accumulation in the kuqa foreland basin which should expect to be paid more attention in the future these elements include maturity of tertiary and jurassic source rocks a difference in the regional cap of kumugelimu salt beds from east to west reservoir bed properties gas migration channels and traps formation in addition the strike slip faulting of the transverse faults may affect deformation of the sedimentary cover including making the western qiulitag structural belt to form an arcuated structure curved to the ssw

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

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

finite volume model for shallow water flooding of arbitrary topography a model based on the finite volume method is developed for unsteady two dimensional shallow water flow over arbitrary topography with moving lateral boundaries caused by flooding or recession the model uses roe s approximate riemann solver to compute fluxes while the monotone upstream scheme for conservation laws and predictor corrector time stepping are used to provide a second order accurate solution that is free from spurious oscillations a robust novel procedure is presented to efficiently and accurately simulate the movement of a wet dry boundary without diffusing it in addition a new technique is introduced to prevent numerical truncation errors due to the pressure and bed slope terms from artificially accelerating quiescent water over an arbitrary bed model predictions compare favorably with analytical solutions experimental data and other numerical solutions for one and two dimensional problems

applications of mango s light hydrocarbon parameters to petroleum from tarim basin nw china abstract light hydrocarbons in oils from the tarim basin nw china were analyzed by gc the light hydrocarbon parameters proposed by mango revealed the distributions of the oils as two main types marine and terrigenous sources the mango parameter k1 not only displays a remarkable invariance 1 in most of the oils but also shows significant variations ranging from 1 20 to 1 54 in the oils occurring in the eastern part of the tazhong fault uplift eptfu located in the center of the tarim basin this variation of the k1 value may indicate different petroleum systems in the areas which are superimposed in the eptfu parameter k2 proposed by mango shows a significant variation between the two main oil types the marine oils are characterized by relatively low values of k2 average 0 23 and the terrigenous oils by relatively high k2 values average 0 35 with general invariance within the same oil set a plot of p3 p2 n2 vs n 1 6 based on the model proposed by mango can be used not only to discriminate between the two main genetic oils from the tarim basin but also to classify the marine oils from the lunnan area into two sub types which may indicate two sub petroleum systems existing in the area