radiogenic helium concentration and isotope variations in crustal gas pools from sichuan basin china abstract helium is an exhaustible natural resource and the main commercial helium was separated from helium rich natural gas the helium in the sedimentary basin is mainly the radiogenic origin of crustal rocks with high level of uranium and thorium data from 242 natural gas samples were used to investigate the distribution and formation mechanism of helium rich gases in the sichuan basin of china all gas samples show remarkably low 3he 4he ratios with ranging from 0 002 to 0 05 ra which represent typical crustal helium the weiyuan gas pool in the southern sichuan basin possesses high levels of helium 1500 18770 ppm average 2785 7 ppm in precambrian and cambrian strata a positive relationship were observed in the helium and nitrogen contents increased with reservoir layer age for all weiyuan gas samples presinian granite was considered as the main helium source for the weiyuan gas pool gaoshiti moxi and weiyuan gas pools belonged to the same ancient gas pool before the late cretaceous but the helium abundance of gaoshiti moxi gas pool is relatively low 200 891 ppm the change of pressure and fracture system caused by the uplift of weiyuan structure in the himalayan orogenesis led to the helium accumulation and enrichment of weiyuan gas pool the shale of the silurian longmaxi formation has high contents of uranium and thorium and generated a large amount of helium while forming large quantities of hydrocarbon gases generally the helium abundance of shale gases and conventional gases generated from the longmaxi formation ranges from 63 to 1500 ppm and some of the gas pool are sufficient for commercial helium resource exploitation therefore the relative quantities of helium and hydrocarbon gases are act as a key factor influencing the formation of helium rich natural gas reservoirs

architecture and facies distribution of organic clastic lake fills in the fluvio deltaic rhine meuse system the netherlands abstract lakes are prominent features in distal zones of delta plains organic clastic lake fills may form in these lakes which include gyttja and lacustrine deltas understanding of the geometrical properties the facies distribution and the origin of organic clastic lake fills is limited knowledge of their development and composition however will improve our fundamental understanding of both sediment distribution in and evolution of distal delta plains furthermore organic clastic lake fills potentially form connectors in hydrocarbon reservoirs this study aims to describe and explain the characteristics of organic clastic lake fills and thereby to contribute to the understanding of delta evolution especially of distal delta plains a field study conducted in the schoonhoven and angstel vecht areas located in the distal part of the rhine meuse delta the netherlands indicates that organic clastic lake fills developed in peat bounded lakes after an avulsion initiated fluvial sedimentation the characteristics of organic clastic lake fills such as geometry sedimentary facies and sedimentation rates were both qualitatively and quantitatively determined by cross sections geomorphogenetic maps and logs the results show that in the angstel vecht area as much as 30 of the fluvial sand volume is stored in organic clastic lake fills this contrasts to the general notion that overbank deposits do not contain significant portions of sand further the lacustrine setting often results in vertically bounded sediment bodies that include gyttja and underlies clastic depositional facies which show a prominent coarsening upward succession it is concluded that the presence of organic clastic lake fills enlarges the reservoir capacity of ancient fluvio deltaic successions as they contribute to the reservoir volume and also can form connectors between isolated channel belt sand bodies

methane flux and origin in the othrys ophiolite hyperalkaline springs greece abstract the occurrence and origin of methane ch4 generated by serpentinization of ultramafic rocks is of current timely interest in planetary geology astrobiology and energy resource exploration as it may contribute in particular to decipher the source of methane on mars the origin of life and the potential of abiotic hydrocarbon synthesis methane of dominant abiotic origin in serpentinized peridotites on continents ophiolites or igneous intrusions has been documented so far with complete carbon and hydrogen isotope composition in six countries in the philippines turkey oman new zealand japan and italy we report the discovery of two new sites in greece at archani and ekkara located in the othrys ophiolite massif portable sensors based on fourier transform infrared spectrometry ftir and tunable diode laser absorption spectroscopy tdlas allowed to realize that out of 21 ophiolitic springs methane is released only by four hyperalkaline ph from 10 7 to 11 3 and calcium hydroxide ca oh type waters all other 17 springs with ph

geochemistry characteristics and genetic types of natural gas in central part of the tarim basin nw china abstract according to the chemical carbon isotopic and hydrogen isotopic compositions and 3 he 4 he ratios of 127 natural gas samples the natural gas in the tarim basin is mainly composed of gaseous alkanes dominated by methane the contents of the gaseous alkanes decrease with the increase of their carbon numbers the 13 c values of ch 4 c 2 h 6 and c 3 h 8 in the platform area of the tarim basin vary in the range of 54 4 24 4 an average value of 39 2 43 1 26 7 an average value of 36 1 and 37 9 21 6 an average value of 31 8 respectively the overall carbon isotopic composition of the alkanes follows the trend of 13 c 1 13 c 2 13 c 3 13 c 4 and in some cases varies with 13 c 1 13 c 2 or 13 c 3 13 c 4 the lower carbon isotope values of methane 13 c 1 13 c 2 may be attributed to thermal sulfate reduction tsr and the higher propane value 13 c 3 13 c 4 was caused by the mixing of the natural gases of the same genetic type formed at different stages the methane hydrogen isotopes 2 h c 1 of the natural gases in the tarim basin are in the range of 195 122 with an average value of 156 the 3 he 4 he ratios of the natural gases are in the range of 2 3 10 8 66 8 10 8 suggesting that there is no deep mantle derived gas in the platform area of the tarim basin the carbon isotope fractionation of alkane gases 13 c 2 13 c 3 1 c 2 vs lnc 2 c 3 and c 2 c 3 vs 13 c 2 13 c 3 of the natural gases indicate that the oil type gas in the tarim basin has different predominant formation processes including kerogen cracking oil cracking oil and gas cracking secondary gas cracking and mixing of the kerogen cracking and oil cracking gases the gas evolved from gas cracking gas to oil and gas cracking gas and oil cracking gas along the direction from the west side of the manjar sag to the central tarim basin the natural gases in some areas of tazhong are the mixtures of the kerogen cracking and oil cracking gases

the puguang gas field new giant discovery in the mature sichuan basin southwest china the giant puguang gas field with a proven original in place gas volume of 350 109 m3 12 36 tcfg was discovered in 2003 in the eastern sichuan fold thrust belt of the mature sichuan basin southwest china the field is a combination structural stratigraphic trap closed by lateral depositional change and fault closure the trap evolved from a paleo oil reservoir originating in the triassic jurassic the entrapment of thermal gas which was derived from lower middle silurian and permian source rocks occurred during deep burial in the jurassic cretaceous tertiary quaternary compression transformed the paleotrap into the present gas reservoir gas is contained in the lower triassic feixianguan and the upper permian changxing reservoirs which consist predominantly of dolomitized oolites deposited in shelf and platform margin shoal and backreef environments reservoir quality is characterized by porosity of 1 29 and permeability of 0 01 9664 md with buried depth greater than 5000 m 16 400 ft the discovery of the puguang field exemplifies the successful application of a new play concept and new technology in a mature basin the discovery resulted from a shift in exploration strategy from structures to stratigraphic traps in reef and shoal dolomites and benefited from advanced high resolution seismic techniques the discovery will not only broaden the exploration scope in the sichuan basin but also provide an excellent analog for exploration in other fold thrust belts worldwide

helium isotopes at the rainbow hydrothermal site mid atlantic ridge 36 14 n the 3he 4he ratio and helium concentration have been measured in the vent fluids and the dispersing plume of the rainbow hydrothermal site on the mid atlantic ridge mar the 3he 4he ratio 7 51 ra and 3he end member concentration 25 pmol kg are in the range of observed values elsewhere on mid ocean ridges pointing to the relative homogeneity of the upper mantle with respect to helium isotope geochemistry 3he is linearly correlated with methane and manganese throughout the plume with ch4 3he and mn 3he ratios identical to those measured in the hot fluids the bulk residence time of the plume in the rift valley estimated from the plume 3he budget is v20 days the 3he flux transported by the plume calculated from current meter data is 12 3 3 nmol s requiring a flux of 490 kg s of high temperature fluid the scaling of the heat flux emitted by the rainbow site to that of 3he using the 3he heat ratio measured in the hydrothermal fluids 9 3 2u10318 mol j indicates a heat output of 1320 mw with a regional spreading rate of v25 mm year we calculate that the annual 3he flux for this section of the mar is of the order of 0 5 0 2 mmol per kilometre of ridge per millimetre of newly formed crust this figure compares well with the flux calculated for the neighbouring lucky strike segment although the uncertainties remain large both fluxes are v40 50 above the world average 0 33 mmol km mm thus supporting earlier suggestions that the intensity of the hydrothermal activity south of the azores is higher than expected from the regional spreading rate

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

methane in serpentinized ultramafic rocks in mainland portugal we report a new case of methane ch4 of apparent abiotic origin in continental serpentinized ultramafic rocks multiple analytical techniques on site and in the laboratory revealed methane and ethane degassing from hyperalkaline ph 11 ca2 oh mineral waters in boreholes drilled in the alter do chao igneous intrusion at cabeco de vide in mainland portugal the c and h isotopic composition of ch4 13c 20 2h 283 suggests a dominant abiotic origin although minor thermogenic contributions cannot be excluded similarly low methane to ethane ratios suggest a predominantly non microbial source consistent with previous microbiological data showing the lack of methanogenic archaea in these waters heavier hydrocarbons co2 and h2 are below detection limits this case study confirms that ch4 from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked to deep circulation of meteoric waters maximum depth of cabeco de vide serpentinized rocks is less than 1 km and present temperatures are likely lower than 50 c serpentinization and related gas formation may have occurred at any time during thermal evolution of the igneous intrusion so gas formation temperature cannot be easily determined this case is an opportunity to test thermometry provided by ch4 isotopologue analyses the existence of methane in continental serpentinized igneous rocks is more widespread than previously thought and petroleum systems with similar serpentinized ultramafics in reservoir rocks may have traces of the observed 13c enriched ch4

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

multiproxy geochemical analysis of a panthalassic margin record of the early toarcian oceanic anoxic event toyora area japan abstract the early toarcian oceanic anoxic event oae was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate and which also had a long lasting impact on marine ecosystems the global significance of the event has been recognised from the widespread occurrence of a 3 7 negative excursion in the carbon isotope 13 c composition of marine organic and inorganic matter and terrestrial plant material this feature of the event is indicative of a pronounced perturbation to the global carbon cycle an inference further supported by widespread evidence for seawater deoxygenation and elevated rates of organic carbon burial nevertheless the precise palaeoenvironmental impacts of this event from sections outside of the boreal and tethyan realms are uncertain here we present the results of a multiproxy geochemical study of an expanded record of the early toarcian event from the northwest panthalassa ocean margin exposed in southwest japan toyora area yamaguchi prefecture our results indicate that in the studied succession organic matter enrichment persisted through the early toarcian event but elemental redox proxies do not support persistent seawater anoxia analyses of terrigenously derived major and trace element abundances coupled with sedimentological observations reveal an increase in coarse grained sediment close to the onset of a 4 excursion in 13 c org and coincident with an inferred increase in terrestrial organic matter flux these observations are consistent with previously published evidence for a marked strengthening of hydrological cycling and increased runoff that occurred contemporaneously with abrupt warming at the onset of the carbon isotope event

geochemical characteristics of gases from the largest tight sand gas field sulige and shale gas field fuling in china abstract this study performed a detailed geochemical analyses of the components stable carbon isotopes of alkane gas and co2 stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field sulige and shale gas fuling field in china the comparative study shows that tight gas from the sulige gas field in the ordos basin is of coal derived origin which is characterized by a positive carbon and hydrogen isotopic distribution pattern 13c1 13c2 13c3 13c4 2h1 2h2 2h3 i e the carbon and hydrogen isotopes increase with increasing carbon numbers carbon dioxide from this field are of biogenic origin and the helium is crust derived shale gas from the fuling shale gas field belongs to oil derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin 13c1

gases and helium isotopes in high temperature solutions sampled before and after odp leg 158 drilling at tag hydrothermal field 26 n mar hydrothermal fluids were sampled for dissolved gases at tag 26 n mar during two alvin dive series in april may 1993 and march 1995 respectively 17 months before and 4 months after ocean drilling program leg 158 total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude even if some increase in h2s c02 and ch4 was noticeable in the 1995 samples no significant difference was observed in helium concentration and helium isotopic ratio 3 he 4 he 8 2 0 1 ra the ch4 3 he ratio found in black smokers sampled in 1993 is around 9 x 10 6 close to ratio found in mar basalts in 1995 this ratio is around 4 times higher due to the ch4 increase the 13 c in c02 measured on two black smokers sampled in 1993 is uniform at 8 4 to 8 8 versus pdb while a 13 c value of 13 is measured at a new site sampled in 1995 located 40 meters east of the main black smokers complex bsc ch4 3 he and 13 c values both point to a mainly magmatic abiogenic origin of the carbon species in the system overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium c02 and ch4 of magmatic origin even if some thermogenic ch4 contribution seems likely in the 1995 samples collected in the new active area we can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the tag mound and temporarily enhanced the hydrothermal circulation by opening new pathways favouring transient input of ch4 rich fluids before coming back to a new steady state

geochemical characteristics of he and co 2 from the ordos cratonic and bohaibay rift basins in china abstract according to the compositional and isotopic analyses of he co 2 and ch 4 of 95 gas samples from the ordos basin and 76 gas samples from the bohaibay basin in china this study carried out a detailed investigation on the geochemical characteristics of helium and co 2 from the cratonic and rift basins especially the cratonic ordos basin and rift bohaibai basin in china compared to the rift bohaibay basin the cratonic ordos basin has lower level of helium and co 2 lower r ra ratio and lower 13 c co2 maximum value when the content of co 2 is 2 associated with the hydrocarbon generation process and inorganic co 2 from the thermal decomposition of carbonates or dissolution of organic acid rift basin can also have co 2 20 and 13 c co2 of 6 2 or 4 9 which is of volcanic magmatic or mantle derived origin while cratonic basin has no such type of co 2 the cratonic basins ordos sichuan have co 2 13 c co2 i e 25 6 0 8 26 4 the rift basins have higher level of co 2 r ra ratios and 13 c co2 values the content of co 2 can reach 100 the r ra ratio can be up to 6 45 and the variation range of 13 c co2 is 34 7 27 bigger variation range of 13 c co2 implies stronger tectonic activities which is not favorable for the gas accumulation while smaller variation range means more stable structure which is favorable for the discovery of large gas fields sample in the cratonic basin in china generally has ch 4 3 he of 10 9 10 12 and r ra 4 3 he of 10 5 10 13 and r ra of 0 1 6 45 it is concluded that methane with ch 4 3 he 10 8 is of inorganic origin and methane with ch 4 3 he 10 11 is of organic origin the regression equation for the ch 4 3 he r ra in the cratonic basin is y 3e 11e 46 28x and it is y 4e 10e 1 791x in the rift basin both ordos and bohaibay basins found gas wells with he 0 05 which has significant commercial values and provides a clue for the exploration of helium rich gas fields traps with r ra 2 is a favorable clue for the discovery of co 2 gas fields seven co 2 gas fields have been found in the bohaibay basin but no systematic development has been carried out and only some liquid co 2 has been injected for tight oil from the huagou co 2 gas field

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

molecular structure and origin of solid bitumen from northern sichuan basin abstract four natural solid bitumen samples were collected from the guangyuan area northwest sichuan basin southwest china to characterize their structure samples were ground to power 120 mesh purified by znbr2 solution to remove mineral and soluble impurities and analyzed using rock eval pyrolysis elemental analysis raman spectroscopy solid bitumen reflectance and solid state 13c nuclear magnetic resonance to determine the maturity degree of samples and obtain structural information results showed that guangyuan solid bitumen samples are in low maturity and three solid bitumen maturity detection methods are compared in this research with respect to the solid bitumen molecular structure less than 10 carbon atoms were determined in aliphatic chains and the solid bitumen aromatic clusters ranged from two to four rings which indicates that the solid bitumen comprises complex geological organic matter such as kerogen four molecular models were established in this research to reflect the basic molecular configuration of natural solid bitumen samples were classified into two types based on the structural information and models and the results imply that the solid bitumen samples may have been derived via different evolutionary processes this phenomenon indicates that structural studies could play an important role in the research of oxidation and carbon isotope changes in solid bitumen in summary this investigation provides an understanding of the evolutionary stages of guangyuan bitumen establishes associated molecular structure models and researches the role of structural information in bitumen source studies as the focus of this study the four molecular models were used to restore the structural characteristics of natural solid bitumen as possible and this study tried to find out the relationship between the solid bitumen structure information and it geology evolution as the carbon isotope may not accurately determine the source of solid bitumen because of oxidation molecular information could be used to research the solid bitumen source and associated evolution

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

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

geochemical characteristics of marine and terrestrial shale gas in china abstract although the annual production of shale gas in china was 13 10 8 m 3 in 2014 a systematic study on geochemical and isotopic characteristics of these unconventional gases has not been well addressed in the present study almost all shale gas samples available in china including marine shale gas from the wufeng longmaxi formation in the sichuan basin o 3 w s 1 l and terrestrial shale gas from chang 7 member t 3 y 7 in the ordos basin were collected and analyzed for their geochemical and isotopic compositions the shale gas from the wufeng longmaxi shale is dry gas with an average methane content of 98 38 and records a highest content of ch 4 in the world 99 59 which is consistent with the very high thermal maturity levels of the gas shales that have equivalent vitrinite reflectance eqvro values between 2 4 and 3 6 the 13 c 1 values are correspondingly heavy and record a heaviest 13 c 1 values 26 7 for the shale gases found in the world as well the average values of 13 c 1 13 c 2 and 13 c 3 for the wufeng longmaxi shale gas are 31 3 35 6 and 47 2 respectively displaying a complete carbon isotopic reversal i e 13 c 1 13 c 2 13 c 3 2 h ch4 and 2 h c2h6 has an average value of 148 and 173 respectively also yielding a hydrogen isotopic reversal i e 2 h ch4 2 h c2h6 the chang 7 shale has an average toc content of 13 81 with vitrinite reflectance vro values between 0 7 and 1 2 the chang 7 shale gas is wet gas with an average methane content of 84 90 and is rich in heavy gaseous hydrocarbons c 2 c 5 the respective values of 13 c 1 13 c 2 and 13 c 3 are 48 7 36 4 and 31 3 displaying a positive carbon isotopic distribution pattern i e 13 c 1 13 c 2 13 c 3 the average 2 h ch4 2 h c2h6 and 2 h c3h8 values are 256 244 and 188 respectively and are characterized by a positive distribution pattern i e 2 h ch4 2 h c2h6 2 h c3h8 the differences in gas wetness and carbon and hydrogen isotopic distribution patterns between the shale gases from the wufeng longmaxi and chang 7 shale are attributed to variations in thermal maturity levels co 2 is present in low content in both the wufeng longmaxi and chang 7 shale gases mostly less than 1 13 c values for the co 2 in the wufeng longmaxi formation are between 8 9 and 9 2 indicating an inorganic origin from the cracking of carbonate mineral in the shales under high temperatures in contrast 13 c values of the co 2 in the chang 7 shale gas range from 8 2 to 22 7 indicating an organic origin from the thermal degradation of organic matter helium in both the wufeng longmaxi and chang 7 shale gases is dominantly of curst origin in terms of their r ra ratios that vary from 0 01 to 0 08 positive carbon isotopic distribution pattern is typical of primitive thermogenic gas however it can be converted into complete or partial carbon isotopic reversal patterns due to secondary alteration the causes that yield carbon isotopic reversal include 1 mixing of gases with the same source but different thermal maturity levels 2 secondary cracking of oil or wet gas 3 formation water involved reactions 4 gas diffusion and 5 carbon isotope exchange between alkane gases at high temperature among them carbon isotopic exchange between alkane gases at high temperature is a key factor nine plots have been drawn based on the shale gases from china usa and canada among them the plot of 13 c 2 versus wetness demonstrates a lying s shape with two inflection points on the gas wetness axis the wetness value of 1 4 represents a critical point from pyrolytic gas primary cracking gas to cracking gas secondary cracking gas and whereas the wetness value of 6 marks the end of oil generation on the diagram of wetness versus 13 c shale gases with wetness values greater than 1 6 are characterized by positive carbon isotopic distribution pattern whereas a complete or partial carbon isotopic reversals are observed for shale gases with wetness values less than 1 6

stable carbon isotopes of alkane gases from the xujiahe coal measures and implication for gas source correlation in the sichuan basin sw china abstract the upper triassic xujiahe formation in the sichuan basin sw china consists of a series of coal measures the first third and fifth members of this formation are dominated by gas prone dark mudstones and coals the mudstones contain type ii and iii kerogens with average organic carbon contents around 1 96 these source rocks are mature in the central sichuan and highly mature in the western sichuan basin characterized by gas generation with subordinate amounts of light oil or condensate oils the source rocks are intercalated with the sandstone dominated second fourth and sixth members of the xujiahe formation thus leading to three separate self contained petroleum systems in the region the proven gas reserves in the xujiahe formation are only less than that of the triassic feixianguan formation and the xujiahe formation has the second largest gas field guang an gas field in the basin gases derived from the xujiahe formation coals generally show a normal stable carbon isotopic trend for c1 c4 n alkanes with the highest 13c2 values among the nine gas pay zones in the basin 20 7 to 28 3 and 13c1 values as low as 43 0 in the central sichuan gas accumulations with an oil leg have also been found in the eastern and southern sichuan where the thickness of the xujiahe formation is significantly reduced gases in these accumulations tend to show low 13c2 values 30 0 to 36 3 characteristic of oil prone source rocks

helium signatures of gases from the sichuan basin china abstract natural gases from the sichuan basin china were analyzed to investigate helium abundances and isotopic composition in order to assess crustal vs mantle inputs into the sinian to jurassic gas reservoirs that occur at depths up to 4900 m in general gases from the sinian and pre sinian strata in south sichuan have relatively high abundances of helium compared to those from the east central and west sichuan implying an increase of crustal helium with time all gas samples demonstrate very low helium isotopic ratios 3 he 4 he designated r with an average of 0 016 ra n 78 where ra is the isotope ratio found in the atmosphere indicative of a crustal origin this is further supported by the relatively stable upper mantle found in west china in contrast the thin and active lithosphere in the east part of china is favorable for the injection of mantle derived volatiles into gas reservoirs this explains why gases from the east part of china generally have r ra 1 while those in the west have r ra 0 1

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

the noble gas isotope record of hydrocarbon field formation time scales abstract noble gases may be considered as the most prominent tracers of natural fluids including hydrocarbons the atmosphere is the only source of 20ne 36ar 84kr 130xe in subsurface environments and their concentrations in pore waters after recharge are known from the solubility data this allows modelling of noble gas partitioning between coexisting gas oil and water phases in the course of hydrocarbon formation migration and storage radiogenic isotopes 4he 21ne 40ar 136xe after being released from source rocks are mixed with air derived noble gases already present in the pore space concentrations of radiogenic species in the pore space of typical hydrocarbon fields are generally so high that they can hardly be accumulated in situ and thus indicate noble gas transfer from ground waters the time bearing ratios 4he 20ne 21ne 20ne 4he 40arair 40ar 40arair in hydrocarbon fields are thus proportional to the time interval between the ground water recharge and noble gases partitioning into the hydrocarbon phase s the recharge partition interval the largest available data set allows the recharge partition intervals to be constrained for a large number of hydrocarbon fields situated in different tectonic settings ancient plates young plates mobile belts these intervals increase systematically with the ages of hydrocarbon source and trap lithologies and are comparable with these ages this important feature valid in general for different hydrocarbon fields implies i local sources of radiogenic noble gas isotopes in ground waters ii relatively recent formation of hydrocarbon fields and iii their short formation time scales in some cases the duration of formation of a hydrocarbon field can be constrained for example nearly constant 21ne 20ne 40ar 40arair ratios measured in samples from the magnus oil field north sea give an accumulation time scale 10 ma it should be emphasized that the above noble gas isotope ratios give the time estimates which are independent of geological reconstructions sometimes the noble gas inventory in a hydrocarbon field and ground waters allows characterization of the source rock volume involved in formation of the field generally this volume exceeds that of the hydrocarbon field rocks by orders of magnitude

stable carbon and hydrogen isotopes of natural gases sourced from the xujiahe formation in the sichuan basin china abstract the compound specific hydrogen and carbon stable isotope ratios of alkanes in natural gases from the xujiahe and jurassic formations in the western sichuan basin were investigated to distinguish between coal derived and oil associated gases all gases were determined to be coal derived and sourced from the xujiahe formation thermal maturities of the source rocks are expected to vary from 0 8 1 8 vitrinite reflectance ro according to the empirical relationship between the measured 13cch4 values and ro which is within the scope of the maturities of source rocks 0 7 2 1 in particular c1 3 alkanes of primary coal derived gases which have not undergone any secondary alteration become more enriched in deuterium with increasing molecular mass and the dch4 values increase with increasing thermal maturity of source rocks in the sichuan basin d values of coal derived gases sourced from terrigenous source rocks of the xujiahe formation are more negative than those of oil associated gases sourced from marine organic matter of the sinian permian and triassic formations

helium and carbon isotopic compositions of thermal springs in the earthquake zone of sichuan southwestern china abstract the origins of gases from hot springs and earthquake activity in the western part of sichuan province southwestern china are discussed in terms of helium and carbon isotopes and energy released by earthquakes 13 c values of co 2 in free and dissolved gases range from 3 34 to 17 09 and 3 he 4 he ratios vary from 1 5 10 8 to 3 63 10 6 the isotopic compositions indicate that carbon dioxide from the hot springs is a mixture of crustal and mantle co 2 and helium is derived from the crust mantle and atmosphere the thermal springs in different earthquake and or fault zones as well as in different parts of the same zone have different values of 13 c and 3 he 4 he the more tectonically active the district is the larger the 13 c and 3 he 4 he values and the greater the frequency of earthquakes this means that more heat energy is derived from the deep earth with upward migration of anatectic fluids in the active tectonic district such as in the kangding district therefore hot spring gases he co 2 etc may be important geochemical markers for determination of seismological and tectonic activity

diagenetic characteristics evolution controlling factors of diagenetic system and their impacts on reservoir quality in tight deltaic sandstones typical example from the xujiahe formation in western sichuan foreland basin sw china abstract deeply buried 3000 5000 m deltaic sandstones of the upper triassic xujiahe formation are important tight gas reservoirs in the sichuan foreland basin china the diagenesis of these tight sandstones was examined using a variety of petrographic and geochemical techniques including thin section description x ray diffraction xrd whole rock chemical analysis scanning electron microscopy sem cathodoluminescence cl imaging electron probe analysis fluid inclusions and isotopic analysis these integrated petrographic and geochemical techniques were used to determine the diagenetic history of the sandstones and its impact on the reservoir quality the tight deltaic sandstones of the t3x2 and t3x4 second and fourth members of xujiahe formation have undergone a significant and complicated series of diagenetic alterations and changes in geochemical composition strong mechanical and chemical compaction together with carbonate cementation destroyed almost all the primary pores and the secondary dissolution pores now dominate the pore space the t3x4 sandstones experienced a more open diagenetic system at near surface and eodiagensis resulted in higher porosity than seen in the t3x2 sandstones both the t3x2 and t3x4 sandstones experienced closed system diagenesis during middle late mesodiagenesis the early diagenetic dissolution which mainly occurred in the open geochemical system produced secondary pores and provided kaolinite and some k needed for the subsequent illitization of kaolinite and k feldspar the late dissolution of k feldspar and illitization of k feldspar in t3x4 sandstones and t3x2 sandstones during the mesodiagenesis produced some effective secondary pores in the closed geochemical system or in the focused fluid flow zone along fractures the diagenetic characteristics size and evolution of open vs closed diagenetic system which were constrained by the depositional environment deep burial depth and tectonic activity can be used to predict the reservoir quality ahead of drilling

extreme methane deuterium nitrogen and helium enrichment in natural gas from the homorod seep romania abstract methane ch 4 in terrestrial environments whether microbial thermogenic or abiogenic exhibits a large variance in c and h stable isotope ratios due to primary processes of formation isotopic variability can be broadened through secondary post genetic processes such as mixing and isotopic fractionation by oxidation the highest and lowest 13 c and 2 h or d deuterium concentrations in ch 4 found in various geologic environments to date are defined as natural terrestrial extremes we have discovered a new extreme in a natural gas seep with values of deuterium concentrations d ch4 up to 124 that far exceed those reported for any terrestrial gas the gas seeping from the small homorod mud volcano in transylvania romania also has extremely high concentrations of nitrogen 92 vol and helium up to 1 4 vol carbon isotopes in ch 4 c 2 h 6 and co 2 and nitrogen isotopes in n 2 indicate a primary organic sedimentary origin for the gas a minor mantle component is suggested by the 3 he 4 he ratio r ra 0 39 both thermogenic gas formation modeling and rayleigh fractionation modeling suggest that the extreme deuterium enrichment could be explained by an oxidation process characterised by a d ch4 and 13 c ch4 enrichment ratio h c of about 20 and may be accounted for by abiogenic oxidation mediated by metal oxides all favourable conditions for such a process exist in the homorod area where increased heat flow during pliocene quaternary volcanism may have played a key role finally we observed rapid variations within 1 h in c and h isotope ratios of ch 4 and in the h 2 s concentrations which are likely caused by mixing of the deep oxidized ch 4 n 2 h 2 s he rich gas with a microbial methane generated in the mud pool of one of the seeps we hypothesize that the unusual features of homorod gas can be the result of a rare combination of factors induced by the proximity of sedimentary organic matter mafic metal rich volcanic rocks and salt diapirs leading to the following processes a primary thermogenic generation of gas at temperatures between 130 and 175 c b secondary alteration through abiogenic oxidation likely triggered by the neogene quaternary volcanism of the eastern transylvanian margin and c mixing at the surface with microbial methane that formed through fermentation in the mud volcano water pool the homorod gas seep is a rare example that demonstrates how post genetic processes can produce extreme gas isotope signatures thus far only theorized and that extremely positive d ch4 values cannot be used to unambiguously distinguish between biotic and abiotic origin

applying isotopic geochemical proxy for gas content prediction of longmaxi shale in the sichuan basin china abstract numerous studies have been conducted on the isotopic geochemical characteristics of shale gas however a systematic study of the relationship between the isotopic geochemical characteristics and gas content has not been addressed to date in this study we investigated the geochemical and isotopic compositions of shale gas samples and the gas content of the lower silurian longmaxi marine shale in the huangjinba area southern sichuan basin china in addition we analyzed the quantitative relationship between the geochemical parameters and the gas content the longmaxi shale gas is a dry gas with a low wetness 0 12 0 67 dominated by methane 89 49 99 00 with a small amount of ethane 0 11 0 61 the average values of 13cmethane 13cethane and 13cpropane are 29 8 35 0 and 33 2 characterized by a partial isotopic reversal 13cmethane 13cpropane 13cethane with a maturity level ro 2 30 2 51 the total gas content of the longmaxi shale varies widely from 0 42 m3 t to 3 88 m3 t correlation analyses revealed that the ethane isotope 13cethane and the magnitude of isotopic reversal 13cmethane 13cethane exhibit strong correlations with the total gas content and the free gas content this study demonstrates that carbon isotopic reversal of shale gas largely depends on the secondary cracking and thermal maturity and residual liquid hydrocarbons content are important factors affecting the magnitude of carbon isotopic reversal based on the quantitative study conducted in this paper we provide a new proxy for predicting the gas content of high maturity shale using an isotopic method