carbonate source rock with low total organic carbon content and high maturity as effective source rock in china a review abstract carbonate rocks with high maturity and low total organic carbon content are widely distributed in china whether they can be effective source rocks remains controversial but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks dataset reveals that as the depth or maturity increases total organic carbon toc content atomic h c ratio and hydrocarbon generation potential of carbonate source rocks with low total organic carbon csrltoc all decrease because of hydrocarbon expulsion moreover some pyrolysis experiments also indicate that the csrltoc are able to generate and expel a quantity of hydrocarbons the above studies indicate that csrltoc can be effective source rocks for oil gas pools the unique features of carbonate rocks and csrltoc demonstrate that they are easier to expel hydrocarbons and contribute to oil gas reservoirs than shale and thus the lower limit of toc lltoc of effective carbonate source rocks could be smaller than shale 0 5 the lltocs of effective carbonate source rocks with type i ii1 kerogen mainly sapropelic group at the levels of immaturity and low maturity maturity high maturity and over maturity are toc 0 5 0 5 0 3 0 3 0 2 and 0 2 0 1 respectively if the carbonate source rocks are at the high over mature stage we could take 0 2 as the lltoc the present organic poor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods especially carbonate source rocks if type i ii1 kerogen some typical oil and gas fields in china derive partly or mostly from the csrltoc proving that csrltoc could be effective source rock in china

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

petroleum accumulation in the deeply buried reservoirs in the northern dongying depression bohai bay basin china new insights from fluid inclusions natural gas geochemistry and 1 d basin modeling abstract the deeply buried reservoirs dbrs from the lijin shengtuo and minfeng areas in the northern dongying depression of the bohai bay basin china exhibit various petroleum types black oil gas condensates and pressure systems normal pressure overpressure with high reservoir temperatures 154 185 c the pressure volume temperature composition pvtx evolution of petroleum and the processes of petroleum accumulation were reconstructed using integrated data from fluid inclusions stable carbon isotope data of natural gas and one dimensional basin modeling to trace the petroleum accumulation histories the results suggest that 1 the gas condensates in the lijin area originated from the thermal cracking of highly mature kerogen in deeper formations two episodes of gas condensate charging which were evidenced by the trapping of non fluorescent gas condensate inclusions occurred between 29 25 5 ma and 8 6 5 0 ma with strong overpressure pressure coefficient pc 1 68 1 70 resulting in the greatest contribution to the present day gas condensate accumulation 2 the early yellow fluorescent oil charge was responsible for the present day black oil accumulation in well t764 while the late blue white oil charge together with the latest kerogen cracked gas injection resulted in the present day volatile oil accumulation in well t765 and 3 the various fluorescent colors yellow blue white and blue and the degree of bubble filling fv 2 3 72 5 of the oil inclusions in the minfeng area show a wide range of thermal maturity api gravity ranges from 30 to 50 representing the charging of black oil to gas condensates the presence of abundant blue white fluorescent oil inclusions with high grain obtaining oil inclusion goi values 35 8 usually 5 in oil reservoirs indicate that a paleo oil accumulation with an approximate api gravity of 39 40 could have occurred before 25 ma and gas from oil cracking in deeper formations was injected into the paleo oil reservoir from 2 8 ma to 0 ma resulting in the present day gas condensate oil accumulation this oil and gas accumulation model results in three oil and gas distribution zones 1 normal oil reservoirs at relatively shallow depth 2 gas condensate reservoirs that originated from the mixture of oil cracking gas with a paleo oil reservoir at intermediate depth and 3 oil cracked gas reservoirs at deeper depth the retardation of organic matter maturation and oil cracking by high overpressure could have played an important role in the distribution of different origins of gas condensate accumulations in the lijin and minfeng areas the application of oil and gas accumulation models in this study is not limited to the dongying depression and can be applied to other overpressured rift basins

improved gravity field of the moon from lunar prospector an improved gravity model from doppler tracking of the lunar prospector lp spacecraft reveals three new large mass concentrations mascons on the nearside of the moon beneath the impact basins mare humboltianum mendel ryberg and schiller zucchius where the latter basin has no visible mare fill although there is no direct measurement of the lunar farside gravity lp partially resolves four mascons in the large farside basins of hertzsprung coulomb sarton freundlich sharonov and mare moscoviense the center of each of these basins contains a gravity maximum relative to the surrounding basin the improved normalized polar moment of inertia 0 3932 0 0002 is consistent with an iron core with a radius of 220 to 450 kilometers

organic geochemistry of oil and gas in the kuqa depression tarim basin nw china abstract the kuqa depression in the tarim basin nw china contains significant natural gas and condensate resources with only small amounts of black oil this study demonstrates that the primary reason for the accumulation of large natural gas reserves in the kuqa depression is the high maturity level of the jurassic coal bearing sequence that is currently at the peak stage of dry gas generation from the combined stable carbon isotopes and molecular biomarker data it is possible to identify two separate source rocks for the discovered hydrocarbon fluids the gases were primarily from the middle lower jurassic coals and associated clastic rocks and the oils were from the upper triassic lacustrine mudstones peak oil generation from the triassic source rocks occurred during the early miocene 23 12 ma b p these oils migrated laterally over relatively long distances 20 50 km reaching the outer periphery of the depression peak gas generation took place more recently perhaps during the past 5 ma the gases migrated mainly along faults over relatively short lateral distances resulting in accumulations adjacent to the over matured source kitchens different timings for the trap formation along the north and south margins and a late injection of gas into early oil accumulations provided favorable conditions for the formation of evaporative condensates and the preservation of gas pools in the more down dip reservoirs and oil pools in the more up dip locations

modeling hydrocarbon accumulation based on gas origin and source rock distribution in paleozoic strata of the ordos basin china abstract paleozoic gases in the ordos basin china are produced from upper paleozoic tight sandstones ordovician weathered crust and pre salt beneath gypsum deposits dolomite reservoirs their origins have been debated particularly as to whether carboniferous permian c p or ordovician source rocks contributed more gas to the lower paleozoic gas reservoirs the expanded exploration of lower paleozoic gas to the eastern western northern and southern regions of the basin has re initiated the debates in this study geochemical characteristics of gas samples from newly drilled wells in different regions of ordos basin in combination with the source rock evaluation were used to determine the gas origin the carbon and hydrogen isotope of alkane indicate that gas mixing occurred in both upper paleozoic and ordovician weathered crust gases and that tsr occurred in pre salt gases a modified 13c 1 cn plot was suggested to be useful to determine the carbon isotopic patterns of gases generated from the different source rocks the light hydrocarbon characteristic shows that oil type gas mainly distributes in the eastern basin a study of source rock distributions also indicates that the most favorable ordovician source rock is located in the eastern basin based on the distribution of c p coal measures limestones and lower paleozoic carbonate source rock a gas accumulation history was established the results of this study provide applicable method for identifying gas origin in ordos basin moreover the accumulation history proposed in this study is useful for further exploration of the lower paleozoic gas in ordos basin

constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

carbon isotope reversal and its relationship with natural gas origins in the jingbian gas field ordos basin china abstract the geochemical characteristics of natural gases from lower paleozoic jingbian gas field are abnormal and the carbon isotope of most natural gases has reversed which led to a dispute over the source of natural gas in the lower paleozoic the upper paleozoic yulin gas field is adjacent to the jingbian gas field but the geochemical characteristics of the yulin gas field did not appear abnormal besides the ethane carbon isotope of upper paleozoic yulin gas field is much higher than that of lower paleozoic jingbian gas field in order to identify the reasons for the carbon isotope reversal of natural gas in lower paleozoic jingbian gas field and its relationship with natural gas source we conducted the components and stable carbon isotopes of natural gas from upper paleozoic yulin gas field and lower paleozoic jingbian gas field on the basis of these experiments we compared the natural gas components and carbon isotope of 100 samples from both upper paleozoic yulin gas field and lower paleozoic jingbian gas field the results indicate that higher maturity will break the conventional rule when maturity reaches a certain threshold 13cethane value will decrease with maturity although 13cmethane value will also increase with maturity the extent will be relatively smaller this leads to unusual 13cmethane 13cethane in the region with higher maturity in the process of downward migration of coal derived gas from carboniferous permian coal measures component fractionation and carbon isotope fractionation might occur resulting the ethane components and carbon isotope value of lower paleozoic less than those of upper paleozoic which will lead to the carbon isotope reversal the mixing of oil derived gas and coal derived gas at different ratios may also lead to the reversal of carbon isotopes the natural gas types of jingbian gas field are mainly coal derived gas generated from carboniferous permian coal measure strata with a small amount of oil derived gas generated from carboniferous permian marine terrigenous limestone

geochemical characteristics of natural gases in the upper triassic xujiahe formation in the southern sichuan basin sw china abstract the chemical and stable carbon and hydrogen compositions of gases in the upper triassic xujiahe formation xujiahe fm in the southern sichuan basin were analysed and compared with those of gases of the xujiahe gas pools in other areas of the sichuan basin the geochemical characteristics of the xujiahe fm gases in the southern sichuan basin are summarized as follows a gases are dominated by alkane gases and the majority of the natural gas is wet gas b the methylcyclohexane index i mcc6 and cyclohexane index i ch values of the gases are high indicating that the natural gases originated from humic organic matter c the stable carbon and hydrogen isotopes of methane and its homologues in most of the gases display a normal trend i e 13 c 1 13 c 2 13 c 3 13 c 4 indicating that the gases are primary and have not experienced secondary alteration a partial reversal of the carbon isotope trend of a few samples is explained by the mixing of gases from the same xujiahe fm source rock at different maturity levels d the carbon isotope trends of the c 1 c 4 alkanes are very similar to those of the gases produced in the western and central sichuan basin and the light hydrocarbon composition is similar to that in the central sichuan basin both findings support that the source rocks of the gases in the xujiahe fm in the southern sichuan basin are coal measures as in the western and central sichuan basin e the hydrogen isotope values of the alkane gases are higher than those in the western and central sichuan basin indicating a higher salinity of the depositional environment of the xujiahe fm in the southern sichuan basin f a small part of the natural gas in the xujiahe fm in the southern sichuan basin is oil associated gas which was probably derived from sapropelic source rocks of the lower silurian and the lower permian

origin and paleoenvironmental reconstruction of phosphorus bearing sandstones of the cambrian xinji formation southwestern margin of the ordos basin china increasing exploration interest in oil and gas hosted by early cambrian strata has focused research efforts on early cambrian sandstones the origin of phosphorus and the paleoenvironment of phosph

an improved gravity model for mars goddard mars model 1 doppler tracking data of three orbiting spacecraft have been reanalyzed to develop a new gravitational field model for the planet mars goddard mars model 1 gmm 1 this model employs nearly all available data consisting of approximately 1100 days of s band tracking data collected by nasa s deep space network from the mariner 9 and viking 1 and viking 2 spacecraft in seven different orbits between 1971 and 1979 gmm 1 is complete to spherical harmonic degree and order 50 which corresponds to a half wavelength spatial resolution of 200 300 km where the data permit gmm 1 represents satellite orbits with considerably better accuracy than previous mars gravity models and shows greater resolution of identifiable geological structures the notable improvement in gmm 1 over previous models is a consequence of several factors improved computational capabilities the use of optimum weighting and least squares collocation solution techniques which stabilized the behavior of the solution at high degree and order and the use of longer satellite arcs than employed in previous solutions that were made possible by improved force and measurement models the inclusion of x band tracking data from the 379 km altitude near polar orbiting mars observer spacecraft should provide a significant improvement over gmm 1 particularly at high latitudes where current data poorly resolve the gravitational signature of the planet

origin of paleogene natural gases and discussion of abnormal carbon isotopic composition of heavy alkanes in the liaohe basin ne china abstract chemical components stable isotopic compositions and light hydrocarbons are investigated to determine the origin and isotopic anomalies of natural gases in the paleogene of the liaohe basin of northeastern china natural gas in paleogene is dominated by alkanes dry gas predominantly occurs in the shallow layers of the paleogene having burial depth less than 1900 m specifically in the dongying formation e3d and the first member of the shahejie formation e3s1 wet gas mostly occurs in the deep layer primarily in the third member of the shahejie formation e2s3 the relative content of methyl cyclohexane in the western depression wd of the liaohe basin is less than that in the eastern depression ed while the relative contents of alkanes in the c5 7 light hydrocarbons of the wd are higher than those of the ed natural gases are of multiple origins including microbial gas gas generated by microbial action on immature source rock coal derived gas gas generated by humic source rock and oil associated gas gas generated by sapropelic source rock in the wd oil associated gas is dominant while in the ed coal derived gas dominates the abnormally heavy carbon isotopic composition of paleogene heavy alkanes is primarily caused by bacteria oxidation rather than by being mixed with deep source gas natural gases from the e3d and e3s1 suffer the most severe bacterial oxidation the intensity of which gradually declines with the increasing depth the low average heptane and isoheptane values indicate that natural gases are mostly generated in the early mature stage of the source rock natural gases in e2s3 of the wd are mainly derived from e2s4 and e2s3 source rocks while those of e3d and e3s1 are predominantly generated by e2s3 source rock gases in the ed come principally from e2s3 source rock

genetic type and maturity of lower paleozoic marine hydrocarbon gases in the eastern tarim basin abstract hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity genetic types and sources of lower paleozoic marine hydrocarbon gases from the eastern tarim basin all of the lower paleozoic gases are relatively dry dryness coefficient 0 9 and belong to the marine sapropelic type of cracked gases these gases are characterized by low 13 c 1 and low 13 c 2 13 c 1 values due to a lack of terrestrial source input thermal simulation experiments on upper proterozoic to lower paleozoic source rocks resulted in a 13 c 1 r o regression equation that was used to discriminate the maturities and sources of the natural gases these natural gases are divided into four genetic types 1 humic coal derived pyrolytic gas 2 lacustrine sapropelic oil cracking gas 3 the marine sapropelic highly mature oil cracking gas and 4 overmature oil cracking gas types 1 and 2 were mainly generated from the upper triassic middle and lower jurassic types 3 and 4 are mainly generated from the lower paleozoic the data can be used to describe multiple sources and multiple generating stages of natural gas

characteristics and genetic types of the lower paleozoic natural gas ordos basin abstract the source of the lower paleozoic natural gas in the ordos basin has been a highly controversial issue using data obtained from newly drilled wells we present the geochemical characteristics of lower paleozoic natural gas in various regions of the ordos basin the lower paleozoic natural gas is dominated by hydrocarbons of which methane accounts for 40 17 97 24 and the heavier gaseous hydrocarbon c 2 ranges from 0 01 to 9 72 non hydrocarbon gases in the lower paleozoic gas reservoir are mainly co 2 and n 2 2 10 for co 2 and 0 04 48 56 for n 2 higher than those in the upper paleozoic gas reservoir this observation showed that the high co 2 and n 2 content was related to the ordovician marine carbonate rock deposits the carbon and hydrogen isotopes of the methane ethane and propane in the lower paleozoic natural gas of the ordos basin showed various degrees of reversal indicating the mixing origin of natural gas in the central eastern ordos basin coal derived gas from the carboniferous permian c p coal measure and oil associated from the lower ordovician majiagou formation marine source rocks gas coexist as well as mixtures of these two gases for the lower paleozoic natural gas from the western margin of the basin coal derived gas coexist with oil associated gas but no mixing gas in which coal derived gas originated mainly from the c p source rocks and oil associated gas from the lower ordovician kelimoli formation and middle ordovician wulalike formation source rocks in the western margin of the basin the lower paleozoic natural gas in the southern part of the ordos basin was mixing gas and the oil associated gas is the dominant one the natural gas was originated from the lower ordovician majiagou formation source rocks in the central eastern ordos basin and the pingliang formation mudstone and argillaceous limestone at the southern margin of the basin

chemical and isotopic evidence for secondary alteration of natural gases in the hetianhe field bachu uplift of the tarim basin h2s and co2 are found in elevated concentrations in the reservoirs near the carboniferous ordovician unconformity in the hetianhe field of the tarim basin nw china chemical and isotopic analyses have been performed on produced gases formation waters and reservoir rocks to determine the origin of co2 and h2s and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids it is unlikely that h2s and co2 had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source instead h2s and co2 are probably the result of sulphate reduction of the light hydrocarbon gases lhg increasing h2s concentrations and co2 co2 c1 4 values to the west of the hetianhe field occur commensurately with increasingly heavy hydrocarbon gas 13c values however thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low no h2s or rare pyrite was detected in deeper reservoirs where more tsr should have occurred and inferred 34s values of h2s from late stage pyrite in the carboniferous and ordovician reservoirs are as low as 24 9 such low 34s values discount the decomposition of organic matter as a major source of h2s and co2 bacterial sulphate reduction of the light hydrocarbon gases in the reservoir possibly coupled indirectly with the consumption of organic acids and anions is most likely the result is the preferential oxidation of 12c rich alkanes due to the kinetic isotope effect and decreasing concentration of organic acids and anions modern formation water stable isotope data reveal that it is possible that sulphate reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion related unconformity this may account for the apparently stronger influence of bacterial sulphate reduction to the west of the hetianhe field and the consequent greatest decrease of the 13c co2 values and the greatest increase in 13c values of the alkane gases

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

an improved solution of the gravity field of mars gmm 2b from mars global surveyor a spherical harmonic solution of the mars gravity field to degree and order 80 goddard mars model 2b gmm 2b has been developed using x band tracking data of mars global surveyor mgs from october 1997 to february 2000 and altimeter crossovers formed from the mars orbiter laser altimeter mola data between march and december 1999 during the mapping mission mgs was located in a near polar 92 9 inclination and near circular orbit at a mean altitude of 400 km the tracking data from this orbit provide a detailed global and high resolution view of the gravity field of mars mars gravity solutions are stable to 60 60 even without application of a kaula power law constraint the valles marineris is resolved distinctly with lows reaching 450 mgals olympus mons and its aureole are both separately resolved and the volcano has a peak anomaly of 2950 mgals the global correlation of the gmm 2b gravity coefficients with mola derived topography is 0 78 through degree 60 and the correlation remains above 0 6 through degree 62 the global gravity anomaly error predicted from the gmm 2b error covariance through 60 60 is 11 mgal the global geoid error from gmm 2b through 60 60 is 1 8 m mgs orbit quality using gmm 2b as measured by overlapping orbital arcs is 1 m in the radial direction and 10 m in total position

origin of natural gas from the ordovician paleo weathering crust and gas filling model in jingbian gas field ordos basin china abstract the ordos basin the second largest natural gas producing basin in china is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation the origin and potential source of the jingbian gas field one of the five giant gas fields in the basin containing the ordovician paleo weathering crust as reservoir strata remains unclear the question is whether gas is generated from carboniferous permian coal measures or ordovician carbonates this study provides abundant gas geochemical evidence which clarifies its origin and source among 82 natural gas samples collected from the ordovician paleo weathering crust in the jingbian gas field c1 c4 gaseous hydrocarbons are dominant with low amounts of non hydrocarbons co2 h2s and n2 the content of c1 c4 ranges from 79 26 to 99 93 with a gas dryness c1 c1 4 of 0 981 0 999 suggesting that natural gases are dry and possibly originate from a high to over mature source the carbon isotopic compositions of methane and its homologues in natural gases from the ordovician paleo weathering crust vary widely with 13c1 13c2 and 13c3 values of 38 9 to 29 1 37 5 to 22 2 and 30 0 to 20 9 respectively the 13c2 and 13c3 values in the jingbian gas field are much more negative than those of typical coal type gas in sulige wushenqi yulin daniudi and kela2 gas fields the carbon isotopic signature suggests that the natural gas in the ordovician paleo weathering crust in the jingbian gas field most likely originates from ordovician carbonates containing sapropelic organic matter the measured d values of methane range from 177 to 155 and support our hypothesis of the natural gases originating from marine sapropelic organic matter the thermal maturity of the natural gases in the ordovician paleo weathering crust is at a high to over mature stage suggesting that the gases originated from secondary cracking of oil and gas at high temperature and pressure thermochemical sulfate reduction tsr might partially play a role in the alteration of gas dryness and 13c1 in those areas where gypsum is present as an effective cap rock mixing of high to over mature secondary cracking gas from the ordovician source and coal type gas from carboniferous permian coal in the jingbian gas field is minor

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

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

strength of faults on mars from mola topography the stresses which must be maintained on faults bounding the rift topography at tempe fossae the north tempe rift ntr and valles marineris vm on mars are estimated using a simple elastic model and topographic data from the mars orbiter laser altimeter mola the absence of rift flank uplift at the ntr is consistent with an elastic thickness te of 20 km or greater at the time of rift formation the maximum resolved shear stresses on bounding faults due to this topography do not therefore exceed 20 mpa similar to the inferred strength of terrestrial faults elastic thickness estimates at vm are mostly around 50 km or greater therefore for canyon widths of 400 km the bounding faults of vm if present must be able to withstand stresses of up to approximately 100 mpa however if the fault controlled sections of the canyons do not exceed 150 km in width as suggested by geomorphological analysis the fault strength required is only 20 mpa although the maximum resolved shear stresses required to support the topography at vm may need to be greater than the stresses which terrestrial faults can support at least some faults on mars are no stronger than similar features on earth this observation is consistent with the existence of liquid water in the shallow subsurface of mars at the time the faults were active on venus plate tectonics is probably prevented by the frictional resistance to motion across strong faults on mars it is more likely that the large thickness of the elastic layer of the lithosphere and the possible positive buoyancy of the crust are responsible for the observed lack of plate tectonics c 2002 elsevier science usa

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

gravity field of mars a 75th degree and order model radiometric tracking data collected by nasa s deep space network dsn for mariner 9 viking 1 and 2 orbiters and mars global surveyor mgs have been analyzed to determine the gravity field of mars the gravity solution mgs75d is a complete spherical harmonics model to degree and order 75 also parts of the solution are the gravitational constants of mars phobos and deimos the significant improvement in the mgs75d mars gravity model is mainly a consequence of globally distributed data set collected from the mgs mapping orbits the use of optimal weighting and constrained least squares solution techniques and improved force and measurement models in the orbit determination process the gravity anomaly map reveals significantly more detail and larger amplitudes than any previous mars gravity map over the major impact and volcanic areas on mars olympus mons remains the largest known gravity anomaly in our solar system and isidis planitia the highest amplitude mascon there are mascons in the hellas argyre and utopia basins valles marineris has distinctive negative anomalies and all volcanic features are positive gravity anomalies over several craters have also been resolved the overall correlation of the mgs75d gravity anomaly map with topography derived from the mars orbiter laser altimeter mola is very high while the polar regions display relatively poor correlation

helium isotope compositions in sedimentary basins in china abstract the abundance and isotopic composition of he have been determined for a variety of natural gases oil field gases in sedimentary basins in the continent of china the 3 he 4 he and 4 he 20 ne ratios range from 0 004 to 4 99 ra where ra denotes atmospheric 3 he 4 he ratio 1 4 10 6 and from 100 39 000 respectively helium in the gas sample is composed of mantle derived he and crustal radiogenic he the high 3 he 4 he ratios were observed in the sedimentary basins which underwent cenozoic extension in the eastern domain such as the songliao liaohe north china subei sanshui and south china sea basins the basins where mantle derived he is recognized are characterized by the presence of surface and subsurface volcanic rocks high terrestrial heat flow deep faults and thinned crust on the other hand regions which underwent pre tertiary extension such as the jungar tarim tuba caidam and yumen basins in the nw and tectonic loading such as the ordos and sichuan basins in the central domain are marked by almost pure radiogenic 4 he of crustal origin and thus by an apparent absence of mantle derived he which coincides with complete lack of cenozoic volcanics low terrestrial heat flow and thickened crust such a 3 he 4 he distribution pattern clearly indicates that tectonic environment control the distribution of he isotopes in the continent of china

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

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

hemispheres apart the crustal dichotomy on mars the hemispheric dichotomy is a fundamental feature of mars expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands the origin of the dichotomy which may have set the course for most of the subsequent geologic evolution of mars remains unclear internally driven models for the dichotomy form the lowlands by mantle convection plate tectonics or early mantle overturn externally driven models invoke one giant impact or multiple impacts areal densities of buried basins expressed by quasicircular depressions and subsurface echoes in radar sounding data suggest that the dichotomy formed early in the geologic evolution of mars tectonic features along the dichotomy boundary suggest late stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands subsequent deposition and erosion by fluvial aeolian and glacial processes shaped the present day dichotomy boundary

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

characterization of pyrolysates from maceral components of tarim coals in closed system experiments and implications to natural gas generation abstract the macerals vitrinite exinite fusinite and semi fusinite from low maturity coal vrr 0 40 of the manjiaer sag tarim basin were isolated and subjected to isothermal pyrolysis in a sealed stainless steel reactor at temperatures ranging from 250 to 550 c in 50 c intervals for a duration of 72 h gas yields chemical and isotopic compositions vary significantly for the different coal macerals total gas yields predominantly ch4 and co2 from exinite and vitrinite are much higher than those from fusinite and semi fusinite carbon dioxide generated from exinite is slightly enriched in 13c 13 c co 2 range 19 7 to 12 2 as compared to co2 produced from vitrinite 13 c co 2 range 22 4 to 19 1 methane produced from exinite 13 c ch 4 range 35 0 to 30 3 tends to be isotopically lighter than methane generated from inertinite fusinite and semi fusinite 13 c ch 4 range 33 7 to 25 2 interestingly the 13 c ch 4 values of all macerals show a similar evolution pattern with temperature the initial gas is isotopically heavy then becomes lighter at moderate temperatures and finally becomes heavier again this may be an indication of isotopic heterogeneity within the coal precursors hydrocarbon potential of coal closely depends on the maceral composition exinite shows the highest hydrocarbon generation potential followed by vitrinite while inertinite has only a poor hydrocarbon potential with increasing thermal stress n alkane parameters such as oep and cpi decrease while the c 21 c 22 ratio increases and the bimodal pattern of the n alkane distribution changes to a unimodal pattern the pr ph ratio varies irregularly through catagenesis

on the origin of mascon basins on the moon and beyond 1 mascon basins on the moon are large craters that display significant positive free air and bouguer gravity anomalies an important question is why is not every large crater a mascon as less than half have been previously determined to be we detrend the free air topographic and bouguer gravity anomalies and find that most large basins 28 of 41 display mascon characteristics e g strong positive bouguer anomalies narrower than the surface rims negative free air gravity annuli surrounding the central highs generally are absent in the bouguer gravity implicating surface topography we propose that beneath a forming large basin the relatively narrow transient crater drives mantle uplift while upward and inward collapse forms the surface topography furthermore the nonmascon basins are all ancient and heavily degraded indicating a postimpact evolutionary process our results suggest that mascon formation is the standard for large impacts on the moon and by extension on other terrestrial planets

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

geochemical characterization of natural gas a physical multivariable approach and its applications in maturity and migration estimates gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons in this paper graphical representations of physico chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified these diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data from 11 chemical and isotopic ratios a statistical analysis pca yields two very important parameters the first parameter using mainly the c2 fraction of the gas relates to maturity and the second parameter involving the proportions and d13c values of methane indicates mainly segregative migration positive values of the second parameter indicate that gases accumulated far from their source whereas negative values correspond to residual gas pools after leakage of a part of the fluids a tentative reconstruction of the gas history has been performed in two brazilian basins the espirito santo basin and part of the reconcavo basin the espirito santo basin is located on the passive continental margin of the atlantic ocean and the reconcavo basin corresponds to an intracontinental aborted rift in both cases the source rocks are mainly lacustrine with thermal maturities ranging between the oil window and the beginning of the gas window results show that in the reconcavo basin a major fault the mata catu fault acts as a drain for hydrocarbon migration at the basin scale associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration in the espirito santo basin this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites this long distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore

pore structure and fractal characteristics of ordovician majiagou carbonate reservoirs in ordos basin china fractal analysis was performed on carbonate core plug samples from the ordovician majiagou carbonate reservoirs in the ordos basin using mercury intrusion capillary pressure micp nuclear magnetic resonance nmr and the x ray computed tomography ct measurements to improve our understanding of the pore structure characteristics the relationships between pore structure parameters and the fractal dimensions were investigated the pore systems are dominated by secondary intercrystalline pores and enlarged dissolution pores as well as microfractures the fractal curves from micp analysis break into two segments at the swanson s parameter the small pore throat systems can be described by the fractal theory whereas pores connected by relatively large throats greater than the pore throat radius at the pittman s hyperbola s apex are not cylindrical in shape cannot be described by a capillary tube model and tend to have apparent fractal dimensions larger than 3 0 the fact that the entirety of the capillary curve cannot be fit by a single fractal dimension implies that there are multiple pore systems present with different fractal dimensions the ct analysis shows that the pores are dispersed in the three dimensional spaces mainly with elliptical shapes the nmr measurements are sensitive to pore body size and micp probes pore throat dimensions the latter being complementary to the pore body size distribution none of the ct micp and nmr techniques provide right or wrong answers to the pore throat systems but they probe different aspects of the pore systems this study assumes the pore shapes to be spherical in general and then the fractal dimension is calculated from the nmr transverse relaxation time t2 spectrum the fractal dimensions of all the samples are calculated and the accuracy of the fractal model is verified by the high regression coefficients almost all the pore systems can be described by fractal theory and the fractal dimensions are strongly correlated with the t2 values separating the immovable fluid and the free fluid microfractures may bias t2 toward larger values making it hard to derive fractal dimensions from nmr measurements the coexistence of small pores pore radius 50 m results in a heterogeneous pore distribution and a high fractal dimension reservoir quality increases with the complexity degree of the microscopic pore structure conversely samples that are dominated by small pore systems tend to have a lower fractal dimension and a less complex pore structure

an improved jpl mars gravity field and orientation from mars orbiter and lander tracking data abstract the mars gravity field resolution is mostly determined by the lower altitude mars reconnaissance orbiter mro tracking data with nearly four years of additional mro and mars odyssey tracking data since the last jpl released gravity field mro110c and lander tracking from the mer opportunity rover the gravity field and orientation of mars have been improved the new field mro120d extends the maximum spherical harmonic degree slightly to 120 improves the determination of the higher degree coefficients as demonstrated by improved correlation with topography and reduces the uncertainty in the corresponding mars orientation parameters by up to a factor of two versus previously combined gravity and orientation solutions the new precession solution is 7608 3 2 1 mas yr and is consistent with previous results but with a reduced uncertainty by 40 the love number solution k2 0 169 0 006 also shows a similar result to previous studies

an alternative explanation for the buried channels on mars the gravity signal from a sharp boundary on partially compensated long wavelength topography 1 a trough observed in the free air gravity of mars along the edge of tempe terra has been interpreted as a large channel that has been completely buried by sediments here we show that this trough is likely real and not a product of ringing in a truncated harmonic expansion but that it is poorly resolved a detailed examination of the high resolution topographic data displays no evidence for a buried channel instead showing that the trough straddles the highlands lowlands boundary as an alternative we propose this gravity trough in a large way arises from the surface topography due to an effect that occurs when there exists a relatively sharp boundary on a long wavelength partially compensated feature

the gravity field of mars results from mars global surveyor observations of the gravity field of mars reveal a planet that has responded differently in its northern and southern hemispheres to major impacts and volcanic processes the rough elevated southern hemisphere has a relatively featureless gravitational signature indicating a state of near isostatic compensation whereas the smooth low northern plains display a wider range of gravitational anomalies that indicates a thinner but stronger surface layer than in the south the northern hemisphere shows evidence for buried impact basins although none large enough to explain the hemispheric elevation difference the gravitational potential signature of tharsis is approximately axisymmetric and contains the tharsis montes but not the olympus mons or alba patera volcanoes the gravity signature of valles marineris extends into chryse and provides an estimate of material removed by early fluvial activity

origin and carbon isotope fractionation of co2 in marine sour gas reservoirs in the eastern sichuan basin abstract according to gas chemical and carbon isotopic compositions for 75 gas samples from the eastern sichuan basin c 1 c 3 gaseous hydrocarbons are dominant with high dryness coefficient c 1 c 1 3 0 99 1 0 despite the presence of various contents of h 2 s and co 2 a reasonably good positive correlation is exhibited when both contents are 5 the carbon dioxide in the natural gas in the eastern sichuan basin mainly originated from carbonate dissolution by acid gases and thermochemical sulfate reduction tsr the content of co 2 produced at the early stage of acid gas dissolution is commonly 13 c co2 value 2 content exceeds 5 the 13 c co2 values range from 8 to 2 and the co 2 and h 2 s contents demonstrated a positive correlation the majority of co 2 is generated via tsr as well as acid gas dissolution during the late uplift of the basin particularly the co 2 with 13 c co2 value 2 generated via the tsr after it was partially precipitated to form calcite while the co 2 with 13 c co2 value 3 is a mixture of the co 2 produced early via tsr and the co 2 produced by the subsequent carbonate rock dissolution by acid gases

characteristics of the devonian xialei manganese deposit guangxi zhuang autonomous region china abstract the xialei mn deposit occurs in the late devonian famenian wuzhishan formation and consists of three ore horizons composed of three ore types 1 mn carbonate 2 mn silicate mn carbonate and 3 mn oxide mn silicate mn carbonate ore beds consist of mn carbonates and associated rhodonite stilpnomelane actinolite chlorite mn epidote biotite and mn fe antigorite these minerals are mixed with each other and form thin beds bands pelletal and pisolitic grains and horizontal or wavy laminae abundant oolitic and pisolitic grains in mn ore beds show distinct boundaries and nuclei of mn carbonate and mn silicate minerals rarely clastic grains the ore deposit is divided into three zones 1 an inner zone composed of mn carbonate silicate and oxide minerals 2 a transitional zone consisting of mn carbonate and stilpnomelane and 3 an outer zone characterized by mn carbonate argillaceous limestone at the base of the ore beds has a mean 13 c pdb of about 0 0 indicating that the carbon was derived from seawater bicarbonate the 13 c pdb of mn carbonate ranges from 2 8 to 14 3 mean 7 1 indicating that the carbon was derived from both degradation of organic matter and seawater bicarbonate ores formed in a trough on a carbonate platform primary sedimentary mn carbonates precipitated from alkaline and negative eh waters in the diagenetic zone of sulfate reduction which occurred either during shallow burial or at the seafloor oolites and pisolites formed by sedimentary processes on the seafloor in places deposits were reworded and transported by gravity flow processes the region where these sedimentary diagenetic mn deposits formed was intruded by magma and hydrothermal fluids with dissolved metals leached from volcanic and sedimentary rocks ascended along fractures the mn silicate mn carbonate ore may have been produced by contact metamorphism of primary carbonate ore and metal sulfides deposited by hydrothermal fluids which overprinted the primary sedimentary ore

genetic types of natural gas and filling patterns in daniudi gas field ordos basin china abstract the genetic types source precursors and filling pattern of natural gas in the upper carboniferous taiyuan formation lower permian shanxi formation and lower shihezi formation gas reservoirs of daniudi gas field were investigated using chemical composition as well as carbon and hydrogen isotopic compositions geochemical analysis of natural gases in 25 representative wells shows that natural gas in the daniudi gas field is composed predominantly of hydrocarbons with a dryness coefficient of 0 884 0 978 the carbon isotopic values of ethane and propane are higher than 28 and 25 respectively and the hydrogen isotopic values of methane are lower than 180 indicating that natural gas in the daniudi field is a typical coal type gas derived mainly from humic organic matter in the transitional facies of the carboniferous permian age hydrogen isotopic values of ch4 and h2 display a good positive correlation suggesting that both were controlled by thermal maturity when the mixing of ethane generated from mudstone and coal with the same kerogen type and similar thermal maturity occurred the carbon isotopic values of ethane barely reflect the thermal maturity although the fractionation of hydrogen isotopes of ethane is significantly higher than that of carbon hydrogen isotopic values of ethane in natural gas reservoirs evidently are not related to thermal maturity the daniudi natural gas reservoirs represent both self sourced and near source accumulations the natural gas accumulations in the late triassic early jurassic periods are mainly of the self sourced type while accumulations in the late jurassic early cretaceous period comprise both self sourced and near source patterns and the natural gas reservoirs formed after the late cretaceous period are mainly of the near source type

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

helium and neon isotope systematics in carbon dioxide rich and hydrocarbon rich gas reservoirs the isotopic compositions and elemental abundances of helium and neon were measured in three natural gas reservoirs in the pannonian sedimentary basin of hungary kismarja a co2 rich reservoir and szeghalom south and szeghalom north both ch4 dominated reservoirs are located on topographic basement highs close to the derecske sub basin in eastern hungary mantle derived neon has been identified in mixed ch4 co2 reservoirs in the vienna basin austria this study establishes that mantle derived neon and helium are a characteristic feature of gas reservoirs throughout the neogene extensional basins of hungary and austria regardless of the dominant active gas composition 3he4he ratios within these samples are attributable to a two component mixing between mantlederived and crustal radiogenic helium the percent contribution of mantle derived 4he varies from 2 3 to 17 in contrast neon isotopic ratios indicate that the gases contain a significant component of atmosphere derived neon in addition to the mantle and crustal derived components 20ne 21ne and 22ne abundances can be corrected for this atmospheric contribution calculated contributions of mantle and crustal derived 21ne are between 3 6 21 and 1 37 respectively 20ne22nec and 21ne22nec ratios derived for these atmosphere corrected components correlate with measured r ra values and plot along a single two component mixing line between crustal and mantle isotopic endmembers this is consistent with a model in which simple mixing occurs between crustal and mantle endmembers with fixed he ne ratios the mixing line is defined by a hyperbolic constant k where k 4he22ne rad 4 he22ne mntl with a mean value of 67 3 11 8 based on estimated values of 0 47 for 21ne22nerad and 1 62 0 03 107 for 4he21ne rad kennedy et al 1990 values of 7 61 106 for he22ne rad and 11 3 104 for 4he22ne mntl can be calculated for the pannonian basin gases this 4he22ne mntl value is indistinguishable within error from the value of 8 04 104 calculated for rare gases in natural gases from the vienna basin these results clearly establish that the continental expression of mantle derived rare gases in continental extensional systems in austria and hungary is distinct and consistently different from that of gases discharging at the spreading ridges where best estimates of 4he22ne mntl are 8 1 11 3 times higher 9 10 105 staudacher et al 1989 given the remarkable agreement in the continental expression of mantle derived gases throughout the pannonian and vienna basins it is difficult to attribute the observed neon enrichment helium depletion with respect to morb gases to fractionation related to lithospheric transport processes kinetic fraction ation processes involved in transport through the crust might be expected to produce a much wider variation in the observed he ne elemental ratios the consistent order of magnitude neon enrichment observed throughout these gas fields instead implies that mantle derived fluids in these continental extensional systems may be sourced in a region of the mantle distinct from that supplying the mid ocean spreading ridges

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

a study of natural gas origins in china the chinese government has supported the development of the natural gas industry since the sixth five year plan 1981 1985 by studying natural gas and its origin one of the key research projects in technology and science this ongoing research has shown that natural gases in china are composed of three types coal type gas related to coal measures high temperature pyrolytic gas related to paleozoic carbonates and oil type gas which occurs in oil fields related to cenozoic and mesozoic lacustrine sediments each of these three types constitutes about one third of the total observed reserves of natural gas in china since 1990 we have proposed a new genetic theory of natural gas multisource overlap and multistage continuity a new biogenic thermocatalytic transition l zone gas and comprehensively identifying coal type gas which plays an important role in exploring for natural gas

giant impact basins trace the ancient equator of mars 1 it is shown in this paper that five giant impact basins of mars argyre hellas isidis thaumasia and utopia are located on a single great circle this suggests that the projectiles orbital planes coincided with the equator of mars at the time of impact the projectiles were larger fragments of a heliocentric asteroid that broke apart as it entered the roche limit of mars the mass of the asteroid was at least 9 8 1020 kg 1 5 10 3 times the mass of mars

origin of marine sour natural gas and gas filling model for the wolonghe gas field sichuan basin china abstract the chemical and isotopic composition of natural gases from the wolonghe gas field sichuan basin southwest china was investigated to assess the potential gas sources and reconstruct the gas filling history of the reservoirs all natural gases in this field are dominated by gaseous hydrocarbons c 1 c 3 with varied amounts of non hydrocarbon components co 2 h 2 s and n 2 the h 2 s content varies with reservoir intervals it ranges from zero to 1 84 with an average of 0 37 in the carboniferous reservoir from 0 05 to 0 76 with an average of 0 30 in the permian reservoir and from 1 09 to 18 83 with an average of 5 39 in the lower triassic reservoir the gas dryness coefficient c 1 c 1 3 ranges from 0 97 to 1 0 with an average of 0 99 the carbon isotopic compositions of methane and its homologues in the wolonghe gas field vary widely with 13 c 1 13 c 2 13 c 3 in the carboniferous and permian gas reservoirs and 13 c 1 13 c 2 13 c 3 in the lower triassic jialingjiang formation gas reservoirs the d 1 values range from 140 to 100 with an average of 124 5 the 34 s h2s values in the field extend from 5 7 to 31 0 with 34 s h2s values in the lower triassic jialingjiang formation reservoirs being much higher than those in the carboniferous and permian reservoirs the sour natural gases in the field originated from cracking of oil and were sourced from the marine sapropelic organic matter at high maturity levels the natural gases underwent alteration by thermochemical sulfate reduction tsr although the limited extent tsr occurred in the carboniferous and permian gas reservoirs the tsr alteration is likely to have caused the high gas dryness and the 13 c enrichment of ch 4 the present low content of h 2 s in the carboniferous and permian gases would be related to the loss of h 2 s dissolved in water under the reconstruction of these gas reservoirs in contrast the high h 2 s in the jialingjiang formation gases is determined by both the favorable tsr conditions in the lower triassic reservoirs and the favorable preservation conditions provided by the lower middle triassic gypsum the wolonghe natural gas system is characterized by multi pay zones multi source supply and multi type layers the gas generated from silurian source rocks migrated into the carboniferous reservoir through faults and fractures and was trapped beneath the permian liangshan formation shale whereas the natural gas in the permian and lower triassic jialingjiang formation was generated and preserved in its own formations with the overlying gypsum acting as a cap rock

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

mars reconnaissance orbiter radio science gravity investigation 1 the objectives of the mars reconnaissance orbiter mro radio science gravity investigation are to improve knowledge of the static structure and characterize the temporal variability of the martian gravitational field relevant to the planet s internal dynamics the structure and dynamics of the atmosphere and the orbital evolution of spacecraft at mars the investigation will utilize range rate and range measurements from x band and when available ka band tracking systems of the mro spacecraft mro will enable a considerable improvement in the spatial resolution and quality of mars global gravity field the low orbital periapsis of mro 255 km will yield gravity maps suitable for study of regional 102 km structure of the crust and lithosphere the addition of tracking data from the mars global surveyor and mars odyssey spacecraft also currently orbiting mars will be useful in decorrelating errors in spherical harmonic coefficients of the gravity field that will improve the quality of the static field studies of the low degree gravity field combined with measurements of rotational dynamics will permit insight about the structure of mars deep interior changes in the low degree spherical harmonic coefficients of the martian gravity field and in polar mass anomalies will be used to track the seasonal cycle of co2 exchange with the surface measurements of spacecraft drag will be used to estimate density variations in the atmosphere relevant to weather patterns and aerobraking of future spacecraft tracking observations will also be used to improve the ephemeris of mars and the masses of the martian moons

simultaneous estimation of the masses of mars phobos and deimos using spacecraft distant encounters the masses of mars and its satellites phobos and deimos have been estimated from the mariner 9 and viking 1 and 2 orbiter tracking data these spacecraft were sensitive to the gravitational force of mars as well as to its satellites although the satellite masses are eight orders of magnitude smaller than mars their regular effect on the orbits of the spacecraft is evident in the tracking data and has enabled us to derive their masses simultaneously with that of mars our method for estimating the satellite masses uses the many distant encounters of the spacecraft with these small bodies rather than the few close encounters used in previous studies the mass estimate for phobos leads to a mean density of 1530 100 kg m 3 based on a volume of 5748 190 km 3 thomas 1993 while the mass estimate of deimos leads to a poorly constrained mean density of 1340 828 kg m 3 based on a volume of 1017 130 km 3 thomas 1993 our analysis confirms within the bounds of error the anomalously low density of phobos using an independent method and data set if the result is valid within several times the estimated error lc0 then factors other than composition i e porosity a thick regolith and or a significant interior ice content are required to explain the observed mass of this body

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

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

geochemical characteristics and genesis of natural gas in the yan an gas field ordos basin china abstract the yan an gas field in the ordos basin china was discovered in 2014 and contains reserves of up to 100 billion m3 its occurrence in the basin contradicts the distribution model of gas in the north oil in the south upper paleozoic natural gas in the yan an area is dominated by methane with an average dryness coefficient c1 c1 4 of 99 5 13c1 30 8 to 27 5 with minor amounts of co2 3 8 n2 0 5 and ethane 13c2 37 2 to 30 5 and traces of higher hydrocarbons the gas is predominantly coal derived originating from carboniferous permian humic hydrocarbon source rocks and minor oil type gas from sapropelic source rocks the carbon isotope series includes reversals with 13c2

origin and diffusion of the over mature transitional natural gas in multiple lithologic reservoirs a case study of carboniferous permian strata in the southeastern margin of ordos basin abstract to investigate differences in gas origin and diffusion among various lithologies in the continent marine transitional strata of the lower permian shanxi formation p1s and upper carboniferous benxi formation c2b in the southeastern margin of ordos basin 31 natural gas samples were collected from the existing lithologies including mudstone sandstone and coal and their gas components and carbon and hydrogen isotope compositions were measured remarkable differences in the ethane 13c2 and carbon dioxide isotope values 13cco2 within natural gas were found between the shanxi and benxi formations the 13c2 value of natural gas in the shanxi formation is generally less than 30 whereas that of the benxi formation is generally greater than 28 and co2 in the shanxi formation is mainly of organic origin while that in the benxi formation is mainly inorganic the unusually depleted 13c2 values found in the shanxi formation were caused by the addition of oil type gas generated from the carbonate rock in the taiyuan formation which resulted in partial reversal of the carbon isotope values 13c1 13c2 however this incorporation of oil type gas did not occur in the benxi formation which is most likely caused by the tight mudstone at the bottom of taiyuan formation the differences between the shanxi and benxi formations reveal that the gases in the two formations belong to two separate gas systems and this could be attributed to the tight mudstone and limestone existing between the reservoirs which acts as an effective barrier preventing diffusion of gases

the controlling factors of oil and gas generation from coal in the kuqa depression of tarim basin china abstract the organic matter rich rocks in the kuqa depression are mainly coal measures and therefore are gas prone however small amounts of oil also occur and appear to be derived from these coals and associated shales the hydrocarbons have five different modes of occurrence pure dry gas wet gas condensate oil gas mixtures and pure oil the distribution of coal related reservoired petroleum in the kuqa depression of the tarim basin indicates that the existence oil versus gas accumulations are largely influenced by geologic history and timing of generation the oils and gases occur in various geological settings the coal measures have mainly generated gas but in different geological settings different types of oil and gas accumulations occur the traps containing dry gas in the kuqa depression formed late in the geological history and therefore could only capture late gas generated by high maturity coaly source rocks the traps which reservoir wet gas or condensates have a high integrity and formed when the coals had a lower maturity the oil pools mainly occur in areas where seals are poor and in some cases fractured the gas previously existing in these reservoirs was largely dissipated but a small quantity of oil remained to form minor residual oil or oil gas occurrences

geochemistry and occurrence of inorganic gas accumulations in chinese sedimentary basins abstract inorganic gases are commonly seen in eastern china and occasionally in southern china from the shallow water columns above hot and cold springs the gases contain 68 to nearly 100 co2 with 13 c co 2 and 13c1 values in the range of 1 18 to 6 00 and 19 48 to 24 94 respectively all of the 34 large inorganic co2 and one inorganic methane accumulations discovered in china are distributed in eastern parts of the country from both onshore and continental shelf basins no commercial inorganic gas accumulation has been found in central and western china this is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in chinese sedimentary basins a detailed study of gas samples collected from four representative inorganic co2 pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show 13c1 values greater than 10 versus pdb mostly 30 with a positive stable carbon isotope sequence of 13c1 13c2 13c3 13c4 inorganic gases also tend to show less negative 13 c co 2 values 10 than biogenic gases

geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin abstract natural gas from the zhaolanzhuang field of the jizhong superdepression bohai bay basin contains the highest proportions of h 2 s 40 92 among the sour gases encountered in china the gas payzones include the eocene oligocene kongdian formation ek and the es4 member of the shahejie formation the sedimentary sequence consists of halite anhydrite carbonate sandstone and shale interbeds deposited in the evaporative brackish water lacustrine salt lake setting in the deepest part of the jinxian sag the total thickness of evaporites is more than 1000 m of which halite accounts for over 40 various organic rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window with a depth of 2500 3500 m and likely the source for the oil and sour gas in the zhaolanzhuang field the temperatures of the gas reservoirs range from 75 to 100 c too low for significant thermochemical sulfate reduction the co occurrence of abundant elemental sulfur with the sour gas and the 34 s values of the various sulfur containing compounds indicate that the h 2 s gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred

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

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

the global topography of mars and implications for surface evolution elevations measured by the mars orbiter laser altimeter have yielded a high accuracy global map of the topography of mars dominant features include the low northern hemisphere the tharsis province and the hellas impact basin the northern hemisphere depression is primarily a long wavelength effect that has been shaped by an internal mechanism the topography of tharsis consists of two broad rises material excavated from hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary the present topography has three major drainage centers with the northern lowlands being the largest the two polar cap volumes yield an upper limit of the present surface water inventory of 3 2 to 4 7 million cubic kilometers

seasonal and static gravity field of mars from mgs mars odyssey and mro radio science abstract we present a spherical harmonic solution of the static gravity field of mars to degree and order 120 gmm 3 that has been calculated using the deep space network tracking data of the nasa mars missions mars global surveyor mgs mars odyssey ody and the mars reconnaissance orbiter mro we have also jointly determined spherical harmonic solutions for the static and time variable gravity field of mars and the mars k 2 love numbers exclusive of the gravity contribution of the atmosphere consequently the retrieved time varying gravity coefficients and the love number k 2 solely yield seasonal variations in the mass of the polar caps and the solid tides of mars respectively we obtain a mars love number k 2 of 0 1697 0 0027 3 the inclusion of mro tracking data results in improved seasonal gravity field coefficients c 30 and for the first time c 50 refinements of the atmospheric model in our orbit determination program have allowed us to monitor the odd zonal harmonic c 30 for 1 5 solar cycles 16 years this gravity model shows improved correlations with mola topography up to 15 larger at higher harmonics l 60 80 than previous solutions

utopia and hellas basins mars twins separated at birth 1 using topography and gravity data as constraints we formulate spherical harmonic thin elastic shell models to determine the subsurface structure of the hellas and utopia basins for hellas we show that our model is consistent with the elastic thickness results of mcgovern et al 2002 2004 the thin elastic lithosphere at the time of formation implies that hellas is close to isostatic since utopia formed earlier we argue that an isostatic assumption is justified for the utopia basin before it was filled from this supposition we derive a system of equations that allows us to solve for the amount of fill the prefill topography and the amount of flexure due to the fill within the utopia basin an analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling a thinner elastic lithosphere favors a denser fill while a thicker lithosphere will allow for less dense material likewise larger crustal thickness values lead to smaller fill density values the presence of quasi circular depressions interpreted as impact craters within the utopia basin indicates that the majority of the material within utopia was deposited prior to 4 04 4 11 ga the early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could in turn suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice rich material these results are supported using an alternative method of determining the amount of fill and flexure within utopia this model assumes that hellas and utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the utopia basin the volume of material needed to fill utopia is immense on the order of 50 million km3 or more the high density obtained for the fill requires that it contain a large igneous component the source of which is problematic relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly

quantitative evaluation of gas generation from the upper paleozoic coal mudstone and limestone source rocks in the ordos basin china abstract significant quantities of tight gas resources have been discovered in the upper paleozoic interval in the ordos basin central china the gas is believed to have been derived primarily from the upper paleozoic coal mudstone and limestone source rocks an integrated organic geochemistry and basin modeling approach was applied to quantitatively evaluate gas generation from the three types of source rocks source specific gas generation kinetics were first established on the basis of sealed gold tube pyrolysis experiments gas generation was then modeled in 1d and 3d using the source specific kinetics obtained the results show that 1 near the depositional and thermal center e g well zt 1 gas generation occurred much earlier and was greater than that of the sh 360 well which is away from the depositional and thermal center 2 c2 c5 gases in the zt 1 well underwent thermal cracking while gases in the sh 360 well did not experience any secondary thermal alteration 3 hydrocarbon gases were mainly generated during the early cretaceous 135 96 ma and the late jurassic 161 135 ma 4 the cumulative amounts of gas generation from the coal mudstone and limestone source rocks are 147 23 1012 57 85 1012 and 12 73 1012 m3 respectively with coal seams being a major contributor about 67 6 to the upper paleozoic gas system followed by the mudstone about 26 6 and limestone about 5 8 by comparing with previous work and a default kinetics basin model our source specific kinetics basin model is more suitable for simulating gas generation in the upper paleozoic source rocks within the ordos basin

an inversion of gravity and topography for mantle and crustal structure on mars analysis of the gravity and topography of mars presently provides our primary quantitative constraints on the internal structure of mars we present an inversion of the long wavelength harmonic degree 10 gravity and topography of mars for lateral variations of mantle temperature and crustal thickness our formulation incorporates both viscous mantle flow which most prior studies have neglected and isostatically compensated density anomalies in the crust and lithosphere our nominal model has a 150 km thick high viscosity surface layer over an isoviscous mantle with a core radius of 1840 km it predicts lateral temperature variations of up to a few hundred degrees kelvin relative to the mean mantle temperature with high temperature under tharsis and to a lesser extent under elysium and cool temperatures elsewhere surprisingly the model predicts crustal thinning beneath tharsis if correct this implies that thinning of the crust by mantle shear stresses dominates over thickening of the crust by volcanism the major impact basins hellas argyre isidis chryse and utopia are regions of crustal thinning as expected utopia is also predicted to be a region of hot mantle which is hard to reconcile with the surface geology an alternative model for utopia treats it as a mascon basin the utopia gravity anomaly is consistent with the presence of a 1 2 to 1 6 km thick layer of uncompensated basalt in good agreement with geologic arguments about the amount of volcanic fill in this area the mantle thermal structure is the dominant contributor to the observed geoid in our inversion the mantle also dominates the topography at the longest wavelengths but shorter wavelengths harmonic degrees 4 are dominated by the crustal structure because of the uncertainty about the appropriate numerical values for some of the model s input parameters we have examined the sensitivity of the model results to the planetary structural model core radius and core and mantle densities the mantle s viscosity stratification and the mean crustal thickness the model results are insensitive to the specific thickness or viscosity contrast of the high viscosity surface layer and to the mean crustal thickness in the range 25 to 100 km models with a large core radius or with an upper mantle low viscosity zone require implausibly large lateral variations in mantle temperature

constraints of molecular and stable isotopic compositions on the origin of natural gas from middle triassic reservoirs in the chuanxi large gas field sichuan basin sw china abstract the middle triassic leikoupo formation in the sichuan basin is a gypsum bearing stratum the chuanxi cx large gas field has been recently discovered in middle triassic reservoirs with a controversial gas source the gas geochemistry is analyzed in this study to investigate the gas origin and source the h2s bearing dry gas in the cx gas field has an h2s content ranging from 1 07 to 4 09 with a gas souring index gsi of 0 0120 0 0438 the 13c1 13c2 and 2hch4 values range from 31 8 to 30 4 from 32 9 to 29 6 and from 153 to 136 respectively genetic identification based on the geochemical characteristics suggests that the gas is an oil type and derives from the secondary cracking of oil according to the gsi and alkane carbon isotopes the gas experienced weak tsr alteration without ethane potential high quality source rocks include the upper permian mudstones and marlstones as well as the lower cambrian mudstones with a total organic carbon content generally higher than 2 0 a comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the upper permian rather than the lower cambrian source rocks the extremely low development of solid bitumen in the reservoirs suggests the inexistence of large scale paleo oil pools in the leikoupo formation the middle triassic natural gas in the cx gas field originates from the upper permian longtan formation where the gas pools are dominated by direct filling of secondary cracking gas