formation mechanism of tight coal derived gas reservoirs with medium low abundance in xujiahe formation central sichuan basin china although xujiahe formation source rocks in the central sichuan basin china were determined to have low gas generation intensity several large scale gas fields have been found associated with this formation thus origins and formation mechanisms of natural gas in the xujiahe formation have attracted attentions of many researchers in this study geochemistry techniques were deployed to analyze natural gas and formation water in the xujiahe formation tight gas reservoir central sichuan basin by considering geologic background of the gas reservoir its formation mechanism was investigated research results show that the xujiahe formation reservoir contains an independent petroleum system with its natural gas exclusively originated from coal measure source rocks in the xujiahe formation the formation water was determined to be cacl2 type with high salinity h and o isotope values are largely deviated from those of meteoric water line indicating favorable preservation conditions for the formation water the xujiahe formation is composed of multiple coal measure source rocks and superimposed by tight sandstones the well developed formation water provides favorable conditions for generating water dissolved gas structural movements during the himalayan period induced significant uplifting of the central sichuan basin with overlying formations denudated consequently natural gas dissolved in formation water experienced depressurization and exsolution the released natural gas formed free gas phase in structural highs or provided additional natural gas supplies to existing gas reservoirs

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

geochemical identification of marine and terrigenous condensates a case study from the sichuan basin sw china abstract condensates are extensively produced from the marine lower permian maokou formation and the lower triassic jialingjiang formation the main marine condensate producing strata in southern sichuan basin and also in the terrigenous upper triassic xujiahe formation in the central and southern sichuan basin based on the analyses of light hydrocarbons biomarkers and stable carbon isotopic compositions of marine and terrigenous condensates and the light hydrocarbon composition of natural gases in the jialingjiang fm and xujiahe fm in the sichuan basin the following conclusions are drawn a condensates produced from the carbonate reservoirs of jialingjiang fm and the maokou fm marine condensates belong to a single oil family and the condensates from the terrigenous xujiahe fm terrigenous condensates belong to another family b marine condensates were generated from sapropelic organic matter in the late stage of high maturation and the biomarker composition indicates that they were derived from the lower silurian shale and the lower permian carbonate while the terrigenous condensates were generated from mature humic source rocks c most of the marine condensates and gases encountered in the lower permian maokou fm and lower triassic jialingjiang fm were generated from the same source and a few of the gases in the jialingjiang fm contain admixtures of coal derived gas while terrigenous condensates and gases were generated from the xujiahe fm humic source rocks d effects that could be related to evaporative migration and biodegradation are not evident in the examined condensates

carbon isotopes of middle lower jurassic coal derived alkane gases from the major basins of northwestern china abstract coal derived hydrocarbons from middle lower jurassic coal bearing strata in northwestern china are distributed in the tarim junggar qaidam and turpan harmi basins the former three basins are dominated by coal derived gas fields distributed in cretaceous and tertiary strata turpan harmi basin is characterized by coal derived oil fields which occur in the coal measures based on analysis of gas components and carbon isotopic compositions from these basins three conclusions are drawn in this contribution 1 alkane gases with reservoirs of coal measures have no carbon isotopic reversal whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity 2 coal derived alkane gases with high 13 c values are found in the tarim and qaidam basins 13 c 1 19 0 to 29 9 13 c 2 18 8 to 27 1 and those with lowest 13 c values occur in the turpan harmi and junggar basins 13 c 1 40 1 to 44 0 13 c 2 24 7 to 27 9 and 3 individual specific carbon isotopic compositions of light hydrocarbons c 5 8 in the coal derived gases are lower than those in the oil associated gases the discovered carbon isotopic reversal of coal derived gases is caused by isotopic fractionation during migration and secondary alteration the high and low carbon isotopic values of coal derived gases in china may have some significance on global natural gas research especially the low carbon isotope value of methane may provide some information for early thermogenic gases coal derived methane typically has much heavier 13 c than that of oil associated methane and this can be used for gas source rock correlation the heavy carbon isotope of coal derived ethane is a common phenomenon in china and it shed lights on the discrimination of gas origin since most giant gas fields are of coal derived origin comparative studies on coal derived and oil associated gases have great significance on future natural gas exploration in the world

hydrocarbon generation and expulsion characteristics of the source rocks in the third member of the upper triassic xujiahe formation and its effect on conventional and unconventional hydrocarbon resource potential in the sichuan basin abstract the third member of the upper triassic xujiahe formation t3x3 is a typical source reservoir cap unit for natural gas exploration in the sichuan basin however most previous studies mainly focused on the t3x3 as a source rock ignoring its role as a reservoir as a result the evaluation of hydrocarbon resource potential was not accurate this study established a hydrocarbon generation and expulsion model through analyzing the geological and geochemical characteristics of the t3x3 source rocks and calculated the resource potential of conventional gas tight gas and shale gas systematically with combining the lower boundary of buoyancy dominated accumulation lbba the results indicated that the thickness of the source rocks dominated by mudstone was distributed widely with an average value of more than 45 m the sedimentary environment was salty lacustrine deposits they have a high content of total organic carbon toc with an average value of 4 80 wt the kerogen is mainly type iii with minor type ii and the thermal evolution has entered the high to over mature stage the hydrocarbon generation threshold was determined to be 0 5 ro while the hydrocarbon expulsion threshold and the lbba were determined to be 0 83 ro and 1 12 ro respectively the efficiency of hydrocarbon expulsion was relatively high with an average of about 56 also the maximum hydrocarbon generation and expulsion intensity reached to 110 108 m3 km2 and 53 108 m3 km2 the amounts of hydrocarbon generation expulsion and residual associated with the t3x3 source rocks were 125 68 1012m3 70 38 1012m3 and 55 30 1012m3 respectively under the criterion condition of the lbba the conventional gas resource was 7 21 1010m3 the tight gas resource was 294 34 1010m3 and the shale gas resource was 2488 50 1010m3 it indicated good prospects for exploration

stable carbon isotopes of coal derived gases sourced from the mesozoic coal measures in china coal derived large scale gas fields derived from the mesozoic coal measures in china are mainly distributed in the middle lower jurassic coal measures in the tarim junggar and turpan hami basins in northwest china and the upper triassic xujiahe formation coal measure in sichuan basin central china in 2011 the annual production was 21 6 109 m3 and the proved geological reserves were 2485 109 m3 accounting for 21 and 30 of the total in china respectively based on analyses of gas composition and stable carbon isotopes ratios of 203 samples and stable carbon isotopes of 102 co2 samples the following conclusions were made a based on diagnostic plots using the stable carbon isotopic and molecular composition of gas samples alkane gas from the mesozoic coal measures in china is shown to be coal derived b according to the 13c2 vs c2h6 plot of a great number of oil derived and coal derived gases in china it is concluded that gases with 13c2 28 5 are coal derived and those with 13c2 28 5 are oil derived in most cases c among the natural gases from the mesozoic coal measures in china primary coal derived gases with normal carbon isotopic distribution pattern among the c1 c4 alkanes i e 13c1 13c2 13c3 13c4 are dominant d carbon isotopic pattern reversal mainly results from the mixing of coal derived gases having different maturities but the same source and secondly from microbial oxidation of propane e g mu 3 and mu 4 wells in the gumudi gas field junggar basin e co2 in the coal derived gases from the mesozoic coal measures in china has both biogenic and abiogenic origins the biogenic origin is dominant and the abiogenic co2 is mainly found in the kuqa depression in the tarim basin and western sichuan basin f isotopic differences between heavy hydrocarbon gases and methane become less with increasing maturity

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

early generation of 20s 5α h 14α h 17α h and 5α h 14β h 17β h steranes in low salinity lacustrine shales abstract a quantitative sterane biomarker study was conducted on a series of paralic freshwater lacustrine shale samples ranging in maturity from immature to near oil window maturity taken from section 3 of the shahejie formation es3 in the liaohe basin n e china concentrations of 5 h 14 h 17 h 20s and 5 h 14 h 17 h steranes remain nearly constant throughout the sample suite however the decrease in the absolute concentrations of the 20r 5 h 14 h 17 h c 29 steranes with increasing maturity results in an increase in the conventionally defined maturity parameters 20s 20 s 20r and sterane ratios in addition the data suggest that relatively early generation of 5 h 14 h 17 h 20s and 5 h 14 h 17 h steranes has occurred in lacustrine sediments with a vitrinate reflectance 0 3 r o the data provide strong support for the major importance of relative thermal stability of epimers but do not exclude the possibility of isomerization as a viable mechanism for production

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

tsr origin of sulfur in permian and triassic reservoir bitumen east sichuan basin china solid bitumen h2s pyrite and anhydrite samples from the lower triassic feixianguan formation and upper permian changxing formation and source rock kerogen samples from the cambrian ordovician silurian and permian in the east sichuan basin were analyzed for their sulfur content and d 34 s values to determine if the measurements were consistent with a thermochemical sulfate reduction tsr origin of the produced h2s and solid bitumen the results show that the solid bitumen samples have sulfur contents from 2 6 to 12 5 wt and d 34 s values from 13 4 to 22 1 the d 34 s values are significantly heavier than those of potential upper permian source rock kerogens that average 1 3 n 3 suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy h2s of tsr origin from 12 0 to 13 7 interestingly two gases produced from gas to water transition zones during drill stem testing dst show significantly lighter d

fluid inclusion studies of gold bearing quartz veins from the yirisen deposit sula mountains greenstone belt masumbiri sierra leone the auriferous veins at yirisen masumbiri sierra leone occurring mainly in the form of sericitic quartz sulphide lodes and stringers are hosted in metamorphosed volcano sedimentary assemblages invaded by at least two generations of granitic intrusions detailed microthermometric studies of fluid inclusions from the veins coupled with laser raman spectroscopic analysis show that the inclusions contain aqueous fluids of variable salinity 5 to 60 wt nacl equivalent and dense carbonic fluids pure co2 1 08 d 0 88 g cm3 optical observations and analysis on opened inclusions by scanning electron microscopy sem reveal that some of the aqueous inclusions contain a number of daughter minerals halite sylvite ca fe mg and possibly li bearing chlorides and anhydrite nahcolite occurs also in some of the co2 inclusions the sem runs also detected a small amount of electrum suggesting that silver might be a bi product of the mineralisation the aqueous and carbonic fluids remained immiscible throughout the formation and evolution of the hydrothermal veins a few mixed h2o co2 inclusions apparently resulted from accidental trapping of both fluids in the same cavity the wide range of salinities observed in the aqueous inclusions is attributed to the mixing of relatively hot low salinity aqueous fluids and colder high salinity brines the co2 rich and low salinity h2o inclusions are considered to be derived from the metamorphic decarbonation dehydration of the greenstone pile whilst the high salinity brines are believed to be basinal in origin pressure temperature p t conditions of entrapment inferred from the intersection of representative isochores of the immiscible fluids indicate that the formation of the veins started at t 400 c and p about 4 kbar in the presence of the high density co2 and low salinity h2o fluids at about 200 c pressure fluctuations incremental opening of the vein correspond to the trapping of the lower density co2 inclusions and high salinity brines it is proposed that the decarbonation dehydration processes possibly aided by later magmatic processes expelled and mobilised the gold from the greenstone pile and concentrated it in the co2 bearing hydrothermal fluid in the form of au chloride complexes high thermal gradients are believed to have caused the upward migration of this fluid from the bottom of the greenstone pile through structurally controlled conduits we contend that phase separation of the h2o co2 metamorphic fluid aided possibly by some wall rock alteration most probably triggered a decrease in ligand activity and thus precipitation of the gold into lodes percolation of the basinal brines is thought to have remobilised some of the gold together with some silver

generation characteristics of mesozoic syn and post rift source rocks bonaparte basin australia new insights from compositional kinetic modelling abstract potential source rocks on the laminaria high a region of the northern bonaparte basin on the north west shelf of australia occur within the middle jurassic to lower cretaceous early to post rift sequences twenty two representative immature source rock samples from the jurassic to lower cretaceous plover laminaria frigate flamingo and echuca shoals sequences were analysed to define the hydrocarbon products that analogous mature source rocks could have generated during thermal maturation and filled the petroleum reservoirs in the laminaria high region rock eval pyrolysis data indicate that all the source rocks contain type ii iii organic matter and vary in organic richness and quality open system pyrolysis gas chromatography on extracted rock samples show a dominance of aliphatic components in the pyrolysates the plover source rocks are the exception which exhibit high phenolic contents due to their predominant land plant contribution most of the kerogens have the potential to generate paraffinic naphthenic aromatic oils with low wax contents bulk kinetic analyses reveal a relatively broad distribution of activation energies that are directly related to the heterogeneity in the kerogens these kinetic parameters suggest different degrees of thermal stability with the predicted commencement of petroleum generation under geological heating conditions covering a relatively broad temperature range from 95 to 135 c for the upper jurassic lower cretaceous source rocks both shales and coals of the middle jurassic plover formation have the potential to generate oil at relatively higher temperatures 140 145 c than those measured for crude oils in previous studies hence the frigate and the flamingo formations are the main potential sources of oils reservoired in the laminaria and corallina fields apart from being a reservoir the laminaria formation also contains organic rich layers with the potential to generate oil for the majority of samples analysed the compositional kinetic model predictions indicate that 80 of the hydrocarbons were generated as oil and 20 as gas the exception is the lower cretaceous echuca shoals formation which shows the potential to generate a greater proportion 40 of gas despite its marine source affinity due to inertinite dominating the maceral assemblage

gas geochemistry and methane emission from dushanzi mud volcanoes in the southern junggar basin nw china abstract there are many mud volcanoes in the southern margin of the junggar basin northwest china of which the dushanzi area is the most typical and active one emitting large amount of greenhouse gases associated with water and mud the emitted gas is dominated by methane average 90 1 together with other gases such as ethane 4 84 5 46 propane 0 06 0 90 co 2 0 67 1 0 and n 2 2 8 3 3 the carbon 13 c 1 and hydrogen d isotopic ratios of methane are in the ranges of 40 6 to 45 0 and 221 to 249 respectively whereas carbon isotope ratios of ethane 13 c 2 are 25 2 to 27 6 based on 13 c values the released gas is characterized as a thermogenic coal type and possibly originated from the middle low jurassic coal bearing sequences according to the gas source correlation and regional geology helium isotopes show a crustal source the methane flux of dushanzi mud volcanoes from both macro seepage craters vents and micro seepage ground soil exhalation ranged over the orders of magnitude from 0 4 2 7 kg d 1 and 4950 mg m 2 d 1 on average respectively positive ch 4 fluxes from dry soil were widespread throughout the investigated areas the total ch 4 emission from dushanzi mud volcanoes is estimated to be at least 22 6 tons a 1 of which about 89 is from micro seepage surrounding the mud volcano vents

evaluating the sulfur isotopic composition of biodegraded petroleum the case of the western canada sedimentary basin abstract the sulfur isotopic composition 34s of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis however secondary processes could affect the original 34s of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs in this study the different processes that may affect the 34s of in reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the lower cretaceous mannville group western canada sedimentary basin wcsb based on the molecular study these samples appear to have reached various levels of maturity and biodegradation ranging from 0 to 6 5 on the biodegradation scale of peters and moldowan in addition mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the wcsb investigation of the 34s shows a trend that seems a priori correlated to the level of biodegradation however a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed 34s variations have rather to be ascribed to contributions of oils generated by various source rocks alternatively variations of 34s could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation in the case of some specific core samples showing a common origin based on biomarker study 34s variations might not be related to different sources but to secondary sulfur incorporation exchange processes occurring within the reservoir these processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into mannville reservoirs this hypothesis is supported by laboratory experiments showing sulfur exchange incorporation under plausible conditions for shallow reservoirs

retrograde formation of nacl scapolite in high pressure metaevaporites from the cordilleras beticas spain a permo triassic pelite carbonate rock series with interacalated metabasitic rocks in the cordilleras beticas spain was metamorphosed during the alpine metamorphism at high pressures pmin near 18 kbar the rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc of kyanite phengitetalc biotite and of quartz after sulfate minerals and relicts of baryte anhydrite nacl and kcl indicating a salt clay mixture of illite chlorite talc and halite as the original rock the evaporitic metapelites have a whole rock composition characterized by high mg mg ca ratios 0 7 variable alkaline and sr ba contents but are mostly k2o rich 8 8 wt the f 2600 ppm cl 3600 ppm and p2o5 0 24 wt contents are also high the pelitic member of this series is a fine grained biotite rock kyanite phengite talc biotite aggregates in pseudomorphs developed in the high pressure stage albite rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift scapolite rich in nacl ca ca na mol 24 40 and poor in so4 with cl cl co3 ratios between 0 6 and 0 8 formed as porphyroblasts sometimes replacing up to 60 of the rock in a late stage of metamorphism between 10 and 5 kbar near 600 c no reaction with albite is observed and the scapolite formed from biotite by begin gathered al biotite caco 3 nacl sio 2 hfill al poor biotite scapolite mgco 3 kcl hfill mgcl 2 h 2 o hfill end gathered

overpressure compartments in the central paleo uplift sichuan basin southwest china the sichuan basin is a prime gas producing basin in china besides the giant neoproterozoic weiyuan gas field discovered in the central paleo uplift an extra giant gas reservoir in the cambrian in this area has been confirmed in recent years although the lower paleozoic in the southwest weiyuan area is normally pressured it is overpressured in the central area moxi gaoshiti area combined with the seal distribution five pressure systems including three overpressure systems can be divided based on drill stem test mud weight and sonic transit time data overpressures appear at depth interval of 1500 to 4900 m 4921 to 16 076 ft approximately benefiting from the good sealing capacity of the gypsum in the upper and middle triassic high overpressure pressure coefficient r 2 0 has been preserved in the lower triassic and the upper triassic and cambrian are moderately overpressured 1 3 r 1 7 mechanisms for various overpressure systems are different abnormally high sonic transit time in the permian indicates disequilibrium compaction overpressure the analyses of sonic transit time effective stress suggest that disequilibrium compaction is the primary mechanism for the overpressure in the upper triassic xujiahe formation but the cambrian overpressure system is predominantly associated with fluid expansion which mainly resulted from gas generation furthermore we consider that the late thermal cracking of oil to gas is a key factor for gas and overpressure preservation in old strata the high overpressure in the lower triassic marine carbonate rocks was caused by oil cracking and gypsum dehydration mechanisms combining the origin analysis with the burial and hydrocarbon generation histories we constructed the cambrian pore pressure evolution model which is characterized by roughly normal pressure before 200 ma overpressuring from 200 to 90 ma and overpressure releasing since 90 ma

comments by on organic geochemistry 36 1717 1730 abstract in this short communication we reply to the comments of worden and cai worden r h cai c f 2006 geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jinxian sag of bohai bay basin by zhang et al organic geochemistry 36 1717 1730 organic geochemistry 37 on the data and interpretation of zhang et al zhang s c zhu g y liang y b dai j x liang h b li m w 2005 geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin organic geochemistry 36 1717 1730 concerning the origin of an eocene sour gas and high sulfur oil accumulation from the jinxian sag bohai bay basin china we provide clarifications on the current temperature range of sour gas reservoirs and comment on the reliability of 34 s data of various sulfur species used by zhang et al 2005 and cai c f worden r h wolff g a bottrell s wang d l li x 2005 origin of sulfur rich oils and h 2 s in tertiary lacustrine sections of the jinxian sag bohai bay basin china applied geochemistry 20 1427 1444 we conclude that there is no verified evidence in support of bacteria sulfate reduction as the dominant geochemical process that contributed to the sour gases in the eocene reservoirs of the jinxian sag but more geological and sulfur isotope data are still needed to confirm the thermochemical sulfate reduction occurring in deeper parts of the petroleum reservoirs

fluid inclusions in granulite facies metapelites of the hercynian ancient lower crust of the serre calabria southern italy investigations of fluid inclusions in granulitefacies metapelites of southern calabria enable characterization of the fluid composition of these lower crustal rocks and constrain the petrologically deduced retrograde p t path characterized by isothermal uplift prior to isobaric cooling in middle crustal levels fluid inclusions in cordierite garnet and sillimanite have a co2 rich composition inclusions in cordierite rarely contain minor amounts of n2 and h2o and in garnets some co2 ch4 n2 inclusions have been analyzed by raman microprobe quartz reveals the most complex fluid melusion compositions 1 co2 rich 2 co2 ch4 n2 3 ch4 n2 4 h2o mgcl2 cacl2 nacl 5 h2o nacl and 6 h2o co2 the earliest fluid inclusions after peak metamorphism are rich in co2 with minor amounts of n2 and h2o an early co2 h2o n2 fluid composition has been confirmed by detection of co2 h2o and n2 in the channels of the cordierite structure most of the early co2 rich fluid inclusions were modified during the uplift from the lower to the middle crustal level resulting in a density decrease with co2 still dominant the subsequent isobaric cooling led to further modifications of the fluid inclusions high density inclusions around implosion textures or scattered amongst lower density ones must have formed during this cooling episode aqueous inclusions in quartz are mostly formed late and are consistent with trapping during retrograde rehydration

characteristics and petroleum origin of the carboniferous volcanic rock reservoirs in the shixi bulge of junggar basin western china abstract shixi bulge of the central junggar basin in western china is a unique region that provides insight into the geological and geochemical characteristics of large scale petroleum reservoirs in volcanic rocks of the western central asian orogenic belt carboniferous volcanic rocks in the shixi bulge mainly consist of striped lava and agglomerate as well as breccia lava and tight tuff volcanic rocks differ in porosity and permeability striped lava exhibits the highest porosity average 14 2 but the lowest permeability average 0 67 10 15 m among the rock types primary gas pores are widely developed and mostly filled secondary dissolution pores and fractures are two major reservoir storage spaces capillary pressure curves suggest the existence of four pore structure types of reservoir rocks several factors namely lithology pore structure and various diagenesis govern the physical properties of volcanic rocks the oil is characterized by a high concentration of tricyclic terpane a terpane distribution of c 23 21 c 20 and sterane distributions of c 27 28 29 and c 27 c 28 29 oil and gas geochemistry revealed that the oil is a mixture derived primarily from p 2 w source rock and secondarily from p 1 j source rock in the sag west of pen 1 well the gases are likely gas mixtures of humic and sapropelic organic origins with the sapropelic gas type dominant in the mixture the gas mixture is most likely cracked from kerogen rather than oils the carboniferous volcanic reservoirs in shixi bulge share some unique characteristics that may provide useful insights into the various roles of different volcanic reservoir types in old volcanic provinces the presence of these reservoirs will undoubtedly encourage future petroleum exploration in volcanic rocks up to the deep parts of sedimentary basins

hydrocarbon seeps in petroliferous basins in china a first inventory abstract natural hydrocarbon seepage is a widespread phenomenon in sedimentary basins with important implications in petroleum exploration and emission of greenhouse gases to the atmosphere china has vast petroleum oil and gas bearing sedimentary basins but hydrocarbon seepage has rarely been the object of systematic studies and measurements based on the available chinese literature we report a first inventory of 932 hydrocarbon seeps or seepage zones 710 onshore seeps and 222 offshore seeps including 81 mud volcanoes 449 oil seeps 215 gas seeps and 187 solid seeps bitumen outcrops the seeps are located within the main 20 mesozoic cenozoic petroliferous sedimentary basins especially along the marginal regional and local faults the type of manifestations oil gas or mud volcano reflects the type and maturity of the subsurface petroleum system and the sedimentary conditions of the basin oil seeps are particularly abundant in the junggar basin gas seeps mostly developed in the lunpola basin in smaller basins of the eastern guizhou and yunnan provinces onshore taiwan and in the offshore yinggehai basin mud volcanoes developed in basins junggar qaidam qiangtang onshore and offshore taiwan that experienced rapid sedimentation which induced gravitative instability of shales and diapirism in comparison to available global onshore seep data bases china results to be the country with the highest number of seeps in the world the massive gas seepage in china could represent a considerable natural source of methane to the atmosphere and a key process that may drive future hydrocarbon exploration

geochemical comparison between gas in fluid inclusions and gas produced from the upper triassic xujiahe formation sichuan basin sw china abstract natural gas in the xujiahe formation of the sichuan basin is dominated by hydrocarbon hc gas with 78 79 methane and 2 19 c2 hc its dryness coefficient c1 c1 5 is mostly

genetic types of natural gas and gas source correlation in different strata of the yuanba gas field sichuan basin sw china abstract the chemical compositions and stable carbon and hydrogen isotopic compositions of natural gases in different strata of the yuanba gas field in the sichuan basin were analyzed to investigate their genetic types and source natural gases from the permian changxing and triassic feixianguan formations have dryness coefficients higher than 0 995 and contain h 2 s in most gas samples having a positive correlation between the h 2 s and co 2 contents the identification of gas origin and gas source correlation from this study indicate that natural gas in the ziliujing formation and most gas samples from the xujiahe formation are coal type gas produced from primary cracking of kerogen and they are self sourced reservoirs in relatively independent systems the natural gases in the 1st and 2nd members of the xujiahe formation were mixed with minor amounts of oil type gas from argillaceous carbonate rocks in the leikoupo formation in addition to the self accumulated oil type gas in the leikoupo formation natural gas mainly came from the overlying or lateral source rocks in the xujiahe formation the h 2 s bearing natural gases in the changxing and feixianguan formations were produced by the cracking of crude oil from source rocks in the underlying wujiaping formation with partial contributions from certain local source rocks in the permian dalong formation the h 2 s bearing natural gas has undergone the thermochemical sulfate reduction tsr which were dominated by heavy hydrocarbons the high 13 c values of co 2 in these gas reservoirs mainly result from the interaction between acidic fluids and carbonate rocks in the reservoirs

raman spectra of carbonaceous materials in a fault zone in the longmenshan thrust belt china comparisons with those of sedimentary and metamorphic rocks abstract we analyzed micro raman spectra of carbonaceous materials cm in natural and experimentally deformed fault rocks from longmenshan fault zone that caused the 2008 wenchuan earthquake to characterize degree of disordering of cm in a fault zone raman spectral parameters for 12 samples from a fault zone in shenxigou sichuan china all show low grade structures with no graphite low crystallinity and 13c values 24 to 25 suggest that cm in fault zone originated from host rocks late triassic xujiahe formation full width at half maximum values of main spectral bands d1 and d2 and relative intensities of two subbands d3 and d4 of cm were variable with sample locations however raman parameters of measured fault rocks fall on established trends of graphitization in sedimentary and metamorphic rocks an empirical geothermometer gives temperatures of 160 230 c for fault rocks in shenxigou and these temperatures were lower for highly sheared gouge than those for less deformed fault breccia at inner parts of the fault zone the lower temperature and less crystallinity of cm in gouge might have been caused by the mechanical destruction of cm by severe shearing deformation or may be due to mixing of host rocks on the footwall cm in gouge deformed in high velocity experiments exhibits slight changes towards graphitization characterized by reduction of d3 and d4 intensities thus low crystallinity of cm in natural gouge cannot be explained by our experimental results graphite formation during seismic fault motion is extremely local or did not occur in the study area and the cm crystallinity from shallow to deep fault zones may be predicted as a first approximation from the graphitization trend in sedimentary and metamorphic rocks if that case graphite may lower the friction of shear zones at temperatures above 300 c deeper than the lower part of seismogenic zone

the behavior of actinides phosphorus and rare earth elements during chondrite metamorphism new data on the u pu and p distributions in less metamorphosed h chondrites type 3 5 coupled with literature results permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism preferential associations of phosphates with metals and or sulndes in all chondrites strongly indicate an initially siderophile or conceivably chalcophile character for p in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation this oxidation occurred prior to or during chondritic metal silicate fractionation uranium is initially concentrated in chondrule glass at 100 ppb levels with phosphates primarily merrillite in h 3 chondrites being essentially u free 20 ppb as chondrule glass devitrified during metamorphism u migrated into phosphates reaching 50 ppb in nadiabondi h 5 merrillite and 200 300 ppb in merrillite from equilibrated chondrites but froze out before total concentration in phosphates occurred relative 244 pu fission track densities in the outer 5 m of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in bremervorde h 3 give pu mesostasis pu merrillite 0 01 implying total concentration of pu in phosphates similarly no detectable pu 0 1 ppb was found in chondrule mesostasis in tieschitz and sharps whereas direct measurements of tracks in phosphates in h 3 chondrites are consistent with high 10 ppb pu concentrations thus a strong pu p correlation is indicated for ordinary chondrites there is variable pu u fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites therefore the pu u ratio in phosphates appears relatively useless for relative meteorite chronology literature data indicate that the ree are located in chondrules in unequilibrated chondrites most likely in glass thus there may also be strong pu nd fractionation within these meteorites like u the ree migrate into phosphates during metamorphism but unlike u appear to be quantitatively concentrated in phosphates in equilibrated chondrites thus relative ages based on pu nd may be possible for equilibrated chondrites but the same chronological conclusions are probably obtainable from pu concentrations in phosphates i e on the pu p ratio however pu p chronology is possible only for ordinary chondrites so there appears to be no universal reference element to cancel the effects of pu chemical fractionation in all meteorites available data are consistent with but certainly do not prove that variations in pu p represent age differences but if these age differences do not exist then it is conceivable that the solar system 244 pu 238 u ratio important for cosmochronology is still lower than the presently accepted value of 0 007

h chondrites trace element clues to their origin we have analyzed 10 h chondrites for 20 trace elements using rnaa the meteorites included 4 of petrologic type 4 and 2 each of types 3 5 and 6 the data show that h chondrites are not isochemical h3 s are depleted by some 10 not only in fe dodd 1976 but also in the siderophiles os re ir ni pd au and ge moreover the abundance pattern of siderophiles varies systematically with petrologic type as similar fractionations of ree have been observed by nakamura 1974 it appears that both the proportions and compositions of the main nebular condensates varied slightly during accretion of the h chondrites thus the higher petrologic types are independent nebular products not metamorphosed descendants of lower petrologic types abundances of highly volatile elements cs br bi tl in cd ar36 correlate with petrologic type declining by 10 3 from type 3 to type 6 the trends differ from those for artificially heated type 3 s ikramuddinet al 1977b herzoget al 1979 but agree passably with theoretical curves for nebular condensation apparently the low volatile contents of higher petrologic types are a primary feature not the result of metamorphic loss the mineralogy of chondrites suggests that they accreted between 405 k absence of fe3o4 and 560 k presence of fes and the abundances of tl bi and in further restrict this interval to 420 500 k accretion at 1070 100 k as proposed by hutchisonet al 1979 1980 leads to some extraordinary problems volatiles must be injected into the parent body after cooling which requires permeation of the body by 1011 times its volume of nebular gas this process must also achieve a uniform distribution of the less volatile elements rb cu ag zn ga ge sn sb se f without freezeout in the colder outer layers factor analysis of our data shows 3 groupings siderophiles os re ir ni pd au and ge volatiles ag br in cd bi and tl and alkalis rb and cs the remaining 5 elements u zn te se and sb remain unassociated

molecular and carbon isotopic variation in 3 5 years shale gas production from longmaxi formation in sichuan basin china abstract a time series of produced shale gas samples from lower silurian longmaxi formation fm were collected from six production wells during 3 5 years production period from the weiyuan and changning shale gas pilots in sichuan basin china the molecular compositions and carbon isotopes of ch 4 c 2 h 6 c 3 h 8 and co 2 and the hydrogen isotope of methane were analyzed for these samples by an integrated method of gas chromatography combined with mass spectrometry the results showed that the produced shale gases are dominated by ch 4 90 1 99 3 with minor c 2 h 6 0 3 0 8 and non hydrocarbon gases and have low wetness 0 3 0 8 the non hydrocarbon gases are mainly co 2 and n 2 and trace amount of he the longmaxi shale gases display completed reversal pattern of carbon isotopic compositions with carbon number i e 13 c 1 13 c 2 13 c 3 except for wells w201 and w201 h1 with normal patterns between c 2 h 6 and c 3 h 8 i e 13 c 2 13 c 3 and it is a mixture of oil cracking gas and kerogen cracking gas at high over level of thermal maturity some co 2 13 c co2 ranging from 2 5 to 6 0 is mainly derived from the decomposition of carbonate minerals ch 4 concentration and the carbon isotopes of methane and ethane during 3 5 years shale gas production from year 2012 to 2016 displayed small fluctuations which possibly suggested that gas supply from shale matrix to fracture network after hydraulic fracturing is abundant the weiyuan shale gases show larger decrease of ch 4 content than the changning shale gases indicated more adequate supply in the changning area the difference in 13 c 1 values between horizontal and vertical wells on the same drilling site in the weiyuan pilot indicated that the vertical heterogeneity of lower silurian longmaxi shale plays an important role to shale gas production interestingly the shale gases in the weiyuan pilot display much lighter 13 c 1 34 46 to 36 82 and 13 c 2 37 59 to 41 89 values than those 13 c 1 27 18 27 27 13 c 2 33 70 34 14 in the changning pilot the difference in carbon isotopic compositions in weiyuan and changning pilots mainly attributes to mixing between varied proportion of oil cracking gas and kerogen cracking gas because of the regional heterogeneity of the longmaxi marine shale in particular amount of residual oil retained in organic matter rich shale the isotopic difference between weiyuan and changning shale gases doesn t disappear in the period about 3 5 years of shale gas development which further suggests the regional heterogeneity of retained residual oil exists in the longmaxi fm marine shale

melts and fluids an overview of recent advances owing to their capacity to transport mass and heat melts and low viscosity fluids profoundly influence such global processes as planetary heat loss large and small scale planetary differentiation as well as affecting the evolution of oceans and atmospheres as such these materials play a key role in the physical and chemical volution of earth the terrestrial planets and the meteorite parent bodies in this context a review chapter that deals exclusively with recent advances in our understanding of the composition properties origin and volution of melts and fluids is clearly relevant 1 since 1991 a host of research advances has provided earth and planetary scientists with new and unique perspectives for understanding natural melts and low viscosity fluids new instrumentation has provided the basis for several advances and perhaps most notable is the development and application of reaction cells that allow measurement of melt or fluid properties in situ such devices have allowed workers to monitor properties at high pressure and temperature that are typically not preserved when samples are quenched to ambient conditions in addition to the development of new machines tried and true experimental and analytical technologies have also yielded significant new results on melts and fluids largely as a result of their clever application to the solution of what had been longstanding problems in geochemistry and petrology although laboratory based measurements have provided the basis for many recent advances it is also clear that the earth still yields provocative samples for our study and new insights have also been gained concerning the behavior of melts and fluids in natural processes based on recent documentation of previously unobserved melt and fluid compositions along with new technologies clever experiments and unique samples it is also notable that strides have been made in certain research areas enjoying a resurgence of activity following new and provocative developments provoked by controversial theoretical results perhaps the most notable example of this has been an increased experimental effort aimed at evaluating whether equilibrium core segregation can account for the siderophile element abundances in earth s upper mantle

geochemistry of the sinian cambrian gas system in the sichuan basin china abstract based on the outcrops core samples drilling cuttings seismic and geochemical data natural gases of the sinian cambrian gas system were derived from four sets of over mature high quality source rocks the cambrian qiongzhusi source rock is sapropelic with toc of 1 94 ro eq of 1 83 3 90 and a thickness of 140 m on average the sinian 620 541 ma doushantuo source rock is sapropelic with toc of 1 69 ro eq of 2 08 3 83 and a thickness of 11 m on average the 3rd member of the dengying fm is also sapropelic with toc of 1 43 ro eq of 3 16 3 21 and a thickness of 13 m on average the 2nd and 4th members of the dengying fm are dominated by algal dolomite with toc of exhibiting 0 39 ro eq of 1 97 3 46 and a thickness of 246 m on average most of the 13c2 values are lower than 28 and gas dryness is 99 indicating that the natural gas is oil derived with high maturity the high contents of isoparaffins and cycloparaffin suggest that natural gas in the sinian cambrian reservoir was originated from secondary oil cracking the cambrian longwangmiao gas was mainly derived from the cambrian qiongzhusi source rock and the gas within the sinian dengying formation mainly originated from the three sinian source rock layers the area with gas generation intensity superimposed the four sets of source rocks exceeding 6 109 m3 km2 covered 80 of the sichuan basin the proved reserves exceed 1 0 1012 m3 in the sinian cambrian gas system and the accumulated gas resource in the sinian cambrian gas system might exceed 5 1012 m3

low temperature sulphate reduction biological versus abiological sulphate is considered to have been a major source of sulphide in strata bound and stratiform base metal sulphide deposits many of these deposits however appear to have been formed at moderate temperatures 200 c which poses the question by what mechanism s was sulphate reduced to sulphide two modes of reduction have been established experimentally 1 catalysis by sulphate reducing bacteria which at present is only known to occur below ca 100 c and 2 abiological reduction by ferrous iron or organic matter which has only been clearly shown above ca 250 c several attempts have been made to demonstrate abiological reduction below 200 c and some new data are presented here although the results do not exclude the possibility that such a reaction may be geochemically significant there has been no unequivocal demonstration of nett sulphide formation from sulphate at these temperatures recent studies of the microbiology of hydrothermal regions have opened up the prospect of bacterial sulp

origins of partially reversed alkane δ13c values for biogenic gases in china with increasing molecular weight 13c values of hydrocarbon gases change in two different manners the normal order would be 13c1 13c2 13c3 13c4 partially reversed order is common in gas samples from sedimentary basins in china which can be attributed to one or several of the following four mixing processes a mixing of biogenic and abiogenic gases b mixing of sapropelic and humic sourced gases c mixing of gases from the same types of source rocks with different maturity and d mixing of gases from the same source rock interval of varying maturity

metal and bi pb microdistribution studies of an l3 chondrite their implications for a meteorite parent body we find strong localizations relative to bulk of bi and to a lesser extent pb in some of the kamacite grains in khohar other kamacite grains show no such enrichments there are distinctive and correlated differences in the ni contents of the two kamacite populations with the bi pb rich kamacite grains having consistently lower ni levels sometimes unusually low 2 ni than the bi pb poor kamacite which typically have 6 7 ni the bi pb rich kamacite grains are also distinguished on the basis of their etching behavior exhibiting a highly reactive attack which has not been observed previously and which we believe may be due to the fact that the bi pb rich kamacite is finely polycrystalline we conclude that the trace element microdistributions were not established in the nebula nor is it likely that the enrichments occurred with slow cooling in the presence of a vapor phase during the kamacite taenite phase transition rather the bi pb rich kamacite most likely reflect the occurrence of a brief reheating episode or episodes which may have been shock induced and which was followed by rapid cooling we find fine grained metal sulfide intergrowths which testify to such a reheating event and one likely candidate for the site of this event is a hot ejecta blanket at the parent body surface iron oxides are found in our khohar sections we believe that they are not due to terrestrial alteration that they are magnetite and that the magnetite probably originated in the same dynamic event in which the bi pb distributions were established the present data do not allow us to confidently determine whether the event occurred prior to during or after the compaction of this meteorite although the simplest interpretation of the data would indicate the first alternative bulk bi data for khohar has been used for inferring accretion temperatures and this now appears inappropriate

applications of petroleum geochemistry to exploration and reservoir management abstract petroleum geochemistry improves exploration efficiency by accounting for many of the variables that control the volumes of crude oil and natural gas available for entrapment including source rock distribution richness and quality thermal maturity and the timing of generation migration accumulation relative to trap formation it is most powerful when used with other disciplines such as seismic sequence stratigraphy and reservoir characterization four key technology milestones form the basis for most modern applications of geochemistry to exploration these are the concepts and applications of 1 petroleum systems and exploration risk 2 biomarkers stable isotopes and multivariate statistics for genetic oil oil and oil source rock correlation 3 calibrated three dimensional thermal and fluid flow modeling and 4 controls on petroleum composition by secondary processes petroleum geochemistry offers rapid low cost evaluation tools to aid in understanding development and production problems some technology milestones in reservoir geochemistry include 1 assessment of vertical and lateral fluid continuity 2 determination of proportions of commingled production from multiple zones and leaky casing 3 prediction of oil quality in reservoir zones and 4 prediction of gas oil and oil water contact locations as described in the conclusions future research will continue a trend toward predictive geochemistry examples of predictive tools that draw major research support include piston core surveys to assess deepwater petroleum systems prior to drilling and three dimensional basin modeling to predict the regional timing of generation migration and accumulation of petroleum among other research objectives models are needed to better predict the distribution and quality of petroleum in reservoirs

incipient charnockite characterization at the type localities abstract fifty years after the discovery of incipient charnockite orthopyroxene bearing granitic rocks partially replacing amphibole biotite gneiss in southern india this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent and may be a characteristic feature of progressive deep crustal metamorphism the alteration is essentially a metasomatic transformation mass balance calculations for localities in southern india and sri lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite with loss of feo mgo tio 2 and cao as well as h 2 o from the host rock there is concomitant gain of sio 2 no significant change of alkalis can be demonstrated as essential to the transformation though advanced charnockitic alteration at the type locality of kabbal karnataka may also show large increase of k 2 o with removal of mafic constituents very similar orthopyroxene bearing veins in garnet biotite gneiss at the ponmudi type locality in kerala show definite but more subtle metasomatism with loss of feo and mgo and increase of sio 2 and alkalis both the kabbal and ponmudi alterations occurred at mid crustal levels 5 6 kbar and 700 750 c it is postulated that both types of charnockitic alteration may have been produced as open system processes by much the same kind of metasomatizing fluids nearly coeval co 2 rich and ultrasaline fluids based on fluid inclusion evidence the paragneiss alteration is mainly biotite breakdown with some involvement of garnet and leads to a distinct metamorphic facies type the presence of graphite in some of the garnet biotite gneisses at ponmudi would have resulted in lower oxygen fugacity and probably lower h 2 o activity than in the similar kabbal type metamorphism the fluid rock ratios for both types were small about 10 mass percent of the rocks and the fluids whatever their origin were largely rock governed the source of the metasomatizing fluids has not been definitely identified carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities but ultrasaline fluids may be more plausibly explained by metasedimentary sources and calcareous metasediments have been identified as a source of co 2 in some occurrences much remains to be learned about incipient charnockite and its significance in processes of crustal evolution

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

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

comparative study of stable carbon and hydrogen isotopes of alkane gases sourced from the longtan and xujiahe coal bearing measures in the sichuan basin china abstract two sets of coal bearing measures the upper triassic xujiahe formation t 3 x and the upper permian longtan formation p 2 l have been extensively developed in the sichuan basin sw china and some large gas fields related to these coal bearing measures have been discovered the stable carbon and hydrogen isotopic composition of alkane gases sourced from the two sets of coal bearing measures t 3 x and p 2 l gases have been investigated in this study the main controls on the gas composition and isotopic abundances of gases in the sichuan basin have been identified and distinguished on the basis of their molecular and isotopic composition the carbon and hydrogen isotopic compositions of methane 13 c c1 42 5 to 26 8 d c1 173 to 107 vary widely and show a positive correlation with the thermal maturity of the source rocks the 13 c c2 value of the t 3 x and p 2 l gases in the sichuan basin ranged from 28 2 to 21 3 and showed less variations with increasing maturity from r o values of 1 2 to 2 8 however these 13 c c2 values differ greatly from that of oil associated gas in the sichuan basin thus we provide further evidence that the value of 13 c c2 is a useful parameter to distinguish coal derived and oil associated gases the hydrocarbon gases in the xujiahe and jurassic reservoirs in contrast showed little isotopic difference indicating that secondary migration has little influence on their carbon and hydrogen isotope compositions in addition the 13 c values of p 2 l co 2 2 6 to 3 3 may reflect an origin from dissolution of the carbonates in the reservoir by acidic fluids containing h 2 s

a review of alkane gas geochemistry in the xujiaweizi fault depression songliao basin abstract a large suite 172 of gases from the yingcheng fm in the xujiaweizi fault depression has been studied chemically and isotopically the results show that they are mainly type iii kerogen coal type gas and have been generated and accumulated under the influence of hydrothermal fluids both measured and calculated vitrinite reflectance data show that they have been generated mainly at source maturity higher than 2 0 ro many gases show carbon isotopic reversals typically 13 c 1 13 c 2 and 13 c 2 13 c 3 the origin of carbon isotopic reversals is discussed in this paper and the results show that previous interpretations including a mixing and b the occurrence of abiogenic gases are incorrect based on iso butane n butane data wet gas cracking may have occurred but it is not the main cause of carbon isotopic reversals here we propose that a process of abiogenic polymerization of c 2 gases with methane is the main cause of the carbon isotopic reversals in this basin

an experimental study of phase equilibria in the systems h 2 o co 2 cacl 2 and h 2 o co 2 nacl at high pressures and temperatures 500 800 c 0 5 0 9 gpa geological and geophysical applications phase equilibria in the ternary systems h2o co2 nacl and h2o co2 cacl2 have been determined from the study of synthetic fluid inclusions in quartz at 500 and 800 c 0 5 and 0 9 gpa the crystallographic control on rates of quartz overgrowth on synthetic quartz crystals was exploited to prevent trapping of fluid inclusions prior to attainment of run conditions two types of fluid inclusion were found with different density or co2 homogenisation temperature th co2 a co2 rich phase with low th co2 and a brine with relatively high th co2 the density of co2 was calibrated using inclusions in the binary system h2o co2 mass balance calculations constrain tie lines and the miscibility gap between brines and co2 rich fluids in the h2o co2 nacl and h2o co2 cacl2 systems at 500 and 800 c and 0 5 and 0 9 gpa the miscibility gap in the cacl2 system is larger than in the nacl system and solubilities of co2 are smaller cacl2 demonstrates a larger salting out effect than nacl at the same p t conditions in ternary systems homogeneous fluids are h2o rich and of extremely low salinity but at medium to high concentrations of salts and non polar gases fluids are unlikely to be homogeneous the two phase state of crustal fluids should be common for low fluid rock ratios the cation compositions of crustal fluids are buffered by major crustal minerals feldspars and micas in pelites and granitic rocks calcite dolomite in carbonates and pyroxenes and amphiboles in metabasites fluids in pelitic and granitic rocks are na k rich while for carbonate and metabasic rocks fluids are ca mg fe rich on lithological boundaries between silicate and carbonate rocks or between pelites and metabasites diffusive cation exchange of the salt components of the fluid will cause the surfaces of immiscibility to intersect leading to unmixing in the fluid phase dispersion of acoustic energy at critical conditions of the fluid may amplify seismic reflections that result from different fluid densities on lithological boundaries

experimental studies in model fluid systems numerous observations provide strong evidence that fluid fluid coexistence is a widespread phenomenon in many geological processes i oceanic hydrothermal fluids display a wide range in salinity which clearly indicates fluid phase separation of the circulating seawater see foustoukos and seyfried 2007a ii selective enrichment of certain elements and coeval entrapped inclusions of different co existing fluids highlight the role immiscible fluids play in ore formation see heinrich 2007a iii trace element and stable isotope characteristic as well as equilibrium mineral assemblages in contact aureoles prove immiscible fluids to be a ubiquitous feature in most contact metamorphism see heinrich 2007b iv small scale fluid heterogeneity in eclogitic rocks indicate fluid immiscibility to occur in metamorphic rocks up to high or even ultra high pressure see heinrich 2007b unfortunately fluids are principally non quenchable phases and direct quantitative information on fluid properties at p and t is in most cases not available experimental studies are thus central in studying geologic fluids in general and co existing fluids in particular they are still the primary source of quantitative information on physicochemical properties of co existing crustal fluids they allow e g determine i fluid phase relations as function of p t and x ii physical fluid properties like density and viscosity and iii trace element and isotope fractionation between co existing fluids but also between co existing fluids and solids the experimental data can then be used as input in forward modeling approaches the results and or predictions of which may then be tested in natural systems see driesner and geiger 2007 experimental data on co existing fluids in systems of geological relevance already date back to the flrst half of last century e g keevil 1942 since the 1960ies numerous systematic experimental studies on fluid fluid phase relations at crustal p and t conditions were performed e g sourirajan and kennedy

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

identification of c24 and c25 lanostanes in tertiary sulfur rich crude oils from the jinxian sag bohai bay basin northern china abstract unusual short chain lanostanes c 24 and c 25 and c 30 lanostane were identified in sulfur rich crude oils from the jinxian sag bohai bay basin northern china besides the regular steranes c 27 30 a series of 4 methyl steranes c 22 23 c 27 30 4 4 dimethyl steranes c 22 24 c 28 30 short chain steranes c 23 26 abundant pregnanes c 21 22 and androstanes c 19 20 together with sulfur containing steroids 20 thienylpregnanes and thienylandrostanes were detected in the aliphatic and branched cyclic hydrocarbon fraction of these crude oils a literature survey of some long chain sterane analogues e g a nor steranes norcholestanes c 30 steranes lanostanes and pregnanes seems to point to a sponge and or dinoflagellate source 4 methyl 4 4 dimethyl steroids and lanosterols 4 4 14 trimethyl steroids as the basic skeleton of lanostanes can be derived from methanotrophic bacteria thus a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes c 23 26 4 methyl c 22 23 and 4 4 dimethyl steranes c 22 24 as well as lanostanes c 24 25 and c 30 analogues in our oil samples generally the steroids appear to have been extensively sulfurized with sulfur substitution at the c 22 position in the side chain during the early stage of diagenesis which was readily subject to attack by bacterial degradation enzymatic cleavage and or abiotic oxidation as a consequence short chain sterane analogues e g abundant pregnanes and androstanes in this study and short chain lanostanes c 24 c 25 might later be released through cleavage of weak c s bonds at the c 22 carbon in the sulfurized steroids and lanostane sulfides finally the formation of the short chain c 24 c 25 lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir

influence of natural fractures on gas accumulation in the upper triassic tight gas sandstones in the northwestern sichuan basin china abstract the upper triassic xujiahe formation in the northwestern sichuan basin china is a typical tight gas sandstone reservoir that contains natural fractures and has an average porosity of 1 10 and air permeability less than 0 1 md because of compaction and cementation according to outcrops cores and image logs three types of natural fractures namely tectonic diagenetic and overpressure related fractures have developed in the tight gas sandstones the tectonic fractures include small faults intraformational shear fractures and horizontal shear fractures whereas the diagenetic fractures mainly include bed parallel fractures according to thin sections the microfractures also include tectonic diagenetic and overpressure related microfractures the diagenetic microfractures consist of transgranular intragranular and grain boundary fractures among these fractures intraformational shear fractures horizontal shear fractures and small faults are predominant and significant for fluid movement based on the monte carlo method these intraformational shear fractures and horizontal shear fractures improve the reservoir porosity and permeability thus serving as an important storage space and primary fluid flow channels in the tight sandstones the small faults may provide seepage channels in adjacent layers by cutting through layers in addition these intragranular and grain boundary fractures increase the connectivity of the tight gas sandstones by linking tiny pores the tectonic microfractures improve the seepage capability of the tight gas sandstones to some extent low dip angle fractures are more abundant in the t 3 x 3 member than in the t 3 x 2 and t 3 x 4 members the fracture intensities of the sandstones in the t 3 x 3 member are greater than those in the t 3 x 2 and t 3 x 4 members the fracture intensities do not always decrease with increasing bed thickness for the tight sandstones when the bed thickness of the tight sandstones is less than 1 0 m the fracture intensities increase with increasing bed thickness in the t 3 x 3 member fluid inclusion evidence and burial history analysis indicate that the tectonic fractures developed over three periods the first period was at the end of the triassic to the early jurassic the tectonic fractures developed during oil generation but before the matrix s porosity and permeability reduced which suggests that these tectonic fractures could provide seepage channels for oil migration and accumulation the second period was at the end of the cretaceous after the matrix s porosity and permeability reduced but during peak gas generation which indicates that gas mainly migrated and accumulated in the tectonic fractures the third period was at the end of the eogene to the early neogene the tectonic fractures could provide seepage channels for secondary gas migration and accumulation from the upper triassic xujiahe formation into the overlying jurassic formation

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

tem aem observations of cl rich amphibole and biotite and possible petrologic implications cl rich amphibole and biotite 2 9 and 2 8 wt cl respectively were analyzed with high resolution transmission electron microscopy hrtem selected area electron diffraction saed and x ray emission analytical electron microscopy aem the samples are from archean granulite facies metamorphosed iron formations located in the beartooth mountains montana u s a two types of amphibole and biotite were found 1 high cl prograde hastingsite and biotite and 2 retrograde amphibole of grunerite composition containing no detectable amount of cl but intergrown with submicrometer sized slabs ofci rich annite 2 8 wt fe rich talc fe rich clinojimthompsonite and very wide chain silicate tem aem study of prograde hastingsite and biotite suggests that cl is dissolved in the crystal structures and that cl distribution is homogeneous aem analyses show that cl occupies up to about 30 and 15 of the oh positions in hastingsite and biotite respectively calculations of the cl oh exchange reaction for apatite indicate a high 10g ahoo ah o ratio of about 1 8 2 2 suggesting equilibrium with a cl rich fluid speciation and solubility modeling of the equilibrium between an aqueous fluid and the cl rich apatite biotite and hastingsite suggest that reactions with a saline brine of about 25 wt naci could explain the cl enrichment in these hydrous phases whereas the brine may coexist with an immiscible c02 dominated vapor during the granulite facies metamorphism in light of low calculated h20 activities and c02 rich fluid inclusions this explanation is consistent with the results from studies of natural and synthetic fluid inclusions and phase equilibrium experiments reported in the literature the widespread occurrence of cl rich amphibole and biotite in granulite facies rocks thus has significant implications for the nature of high grade metamorphism tem aem observations of submicrometer sized inclusions of cl rich phases within retrograde amphibole may help to explain the commonly observed heterogeneities of electron microprobe cl analyses found in samples from a variety of geologic settings microprobe analyses using typical beam diameters may sample areas with different proportions of submicroscopic intergrowths this will give apparent cl heterogeneities and fortuitous correlations between cl and concentrations of k ai and si although compositional correlations sometimes indicate reciprocal effects that are likely important for amphibole we emphasize the potential danger of artifacts resulting from submicroscopic solid inclusions additional tem aem studies on complex samples are needed to explain the compositional heterogeneities with confidence

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

physical properties of petroleum formed during maturation of lower cambrian shale in the upper yangtze platform south china as inferred from phasekinetics modelling abstract lower cambrian shale in the upper yangtze platform uyp south china is an important source rock of many conventional petroleum fields and was recently recognized as a promising unconventional shale reservoir in this paper hydrocarbon generation kinetics and petroleum physical properties were investigated using the phasekinetics approach di primio and horsfield 2006 and a cambrian shale sample from the georgina basin north territory australia nta as similar paleogeological and sedimentary environments in cambrian are found for the uyp and nta the source rock comprises type ii kerogen and belongs to an organofacies generating paraffinic naphthenic aromatic low wax oil bulk petroleum generation can be described by a single frequency factor a 8 43e 14 1 s and a dominant activation energy at 56 kcal mol which is characteristic for sulphur poor organic matter deposited in an anoxic marine environment onset transformation ratio tr 10 and end tr 90 of bulk hydrocarbon generation was calculated to take place at 120 c and 165 c respectively for an assumed average geological heating rate of 1 5 c ma based on the thermal history of a local model well onset temperature was not reached until the middle triassic 241 ma when sediments were buried more than 2000 m and basalt eruptions caused enhanced heat flows the main generation stage of primary petroleum took place during the middle late triassic and ended in the early jurassic 187 ma for burial depths exceeding 4000 m tr 90 165 c temperatures increased to more than 200 c in the middle late jurassic leading to secondary cracking of primary products hydrocarbons formed at the onset tr 10 of petroleum generation can be characterized by a gas oil ratio gor of 63 sm3 sm3 a saturation pressure psat of 101 bar and a formation volume factor bo of 1 2 m3 sm3 those parameters stay low during primary petroleum generation before 203 ma at temperatures 10 000 sm3 sm3 psat 250 bar and bo 2 0 m3 sm3 during secondary cracking starting roughly at 200 ma 152 c and 3500 m burial assuming zero expulsion the shale reservoir position within the sedimentary basin indicates that bubble point pressure was always below reservoir pressure and fluids in the shale reservoir occurred only as a single undersaturated phase throughout maturation history black oil and volatile oil phases dominated during the primary cracking period whereas wet gas and dry gas phases dominated during the secondary cracking period

experimentally determined limits for h2o co2 nacl immiscibility in granulites synthetic h sub 2 o co sub 2 nacl fluid inclusions with relative salinities r s mass nacl mass h sub 2 o mass nacl of up to 50 wt and mole fractions of co sub 2 x sub co sub 2 ranging from 0 1 to 0 5 formed in spontaneously nucleated forsterite and orthopyroxene hosts are used to define the limits of fluid immiscibility at temperatures of 890 960c and pressures of 6 600 7 500 bar observations of the phase ratios liquid co sub 2 to h sub 2 o nacl liquid in the inclusions at 25c and microthermometric data were used to determine whether the inclusions trapped miscible or immiscible fluids the results of this study show that fluid immiscibility in the system h sub 2 o co sub 2 nacl is possible for fluids with moderate to low salinities 23 wt relative to h sub 2 o and intermediate mole fractions of co sub 2 x sub co sub 2 0 3 for temperatures as high as 940c at 6 800 7 700 bar this result indicates that the h sub 2 o co sub 2 nacl miscibility gap expands rapidly with pressure making fluid immiscibility viable even at the highest grades of metamorphism

interaction of biotite amphibole gneiss with h2o co2 k na cl fluids at 550 mpa and 750 and 800 с experimental study and applications to dehydration and partial melting in the middle crust to constrain effects of chloride bearing h2o co2 fluids on complex natural assemblages during high grade metamorphism and anatexis we report the results of experiments on the interaction of biotite hornblende tonalitic gneiss from the sand river formation limpopo complex south africa with h2o co2 h2o co2 kcl h2o co2 nacl and h2o co2 k na cl fluids at 550mpa 750 and 8008c and varying chloride h2o co2 ratios with molar co2 co2 h2o 1 4 0 5 heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage the equimolar h2o co2 fluid at 7508q also did not significantly influence the phase assemblage addition of kcl to the fluid at 7508q resulted in formation of the clinopyroxene k feldspar ilmenite titanite assemblage after biotite hornblende and plagioclase orthopyroxene accompanied by amphibole appeared only at 8008c as a result of biotite breakdown in the presence of h2o co2 and low salinity h2o co2 kcl fluids increase in the kcl content in the fluid at 8008q resulted in the production of a clinopyroxenebearing assemblage increase of the nacl content stabilized amphibole in an assemblage with either orthopyroxene at low nacl concentrations or clinopyroxene nevertheless clinopyroxene albite is stable only at high salt concentrations comparison of the experimental results with the results of thermodynamic modeling using the gibbs free energy minimization method perple x software showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by kcl and nacl in the h2o co2 fluids and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages no melts formed at 7508c in the presence of the h2o co2 kcl fluids these fluids provoked melting only at 8008c with formation of rhyolitic melts with increasing kcl content of the fluid the melt composition changed to potassic rhyolitic with al2o3513 5 wt cao52 wt k2o na2o47 wt feo feo mgo 40 8 k2o na2o41 and moderate enrichment in cl 0 2 0 6 wt increasing nacl content caused melting at 7508q and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 8008q the experiments support a model for the formation of ferroan a type granite syenite complexes via crustal melting in the presence of h2o co2 salt fluids in extensional tectonic settings they demonstrate a possible link between a type granitoids and mid crustal dehydration zones in amphiboliteto granulite facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity

melting of albite and dehydration of brucite in h 2 o nacl fluids to 9 kbars and 700 900 c implications for partial melting and water activities during high pressure metamorphism the melting reaction albite solid h2o fluid albite h2o melt has been determined in the presence of h2o nacl fluids at 5 and 9 2 kbar and results compared with those obtained in presence of h2o co2 fluids to a good approximation albite melts congruently at 9 kbar indicating that the melting temperature at constant pressure is principally determined by water activity at 5 kbar the temperature t mole fraction x h2o melting relations in the two systems are almost coincident by contrast h2o nacl mixing at 9 kbar is quite non ideal albite melts 70 c higher in h2o nacl brines than in h2o co2 fluids for x h2o 0 8 and 100 c higher for x h2o 0 5 the melting temperature of albite in h2o nacl fluids of x h2o 0 8 is 100 c higher than in pure water the p t curves for albite melting at constant h2o nacl show a temperature minimum at about 5 kbar water activities in h2o nacl fluids calculated from these results from new experimental data on the dehydration of brucite in presence of h2o nacl fluid at 9 kbar and from previously published experimental data indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities buffering and limiting crustal melting and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction related metamorphism low water activity in high pressure temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting but may be due to the presence of low a h2o brines

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

natural gas constituent and carbon isotopic composition in petroliferous basins china abstract there are abundant gas resources in petroliferous basins of china large to midsize gas fields are found in eastern central and western of china however origin constituents and isotopic composition of natural gas in different gas fields are varied distinctly and some present strong chemical secondary alteration and show variation both in age and space based on the systematic analysis of constituents and carbon isotope of a large number of gas samples combined with the geological characteristics this paper classifies the origins of the gases explores the gas isotope characteristics and evolutionary regulation with the variation time and space and further discusses the distinctive geochemistry of the gases in china these gases are dominated by dry gas its methane carbon isotope values range from 10 to 70 ethane from 16 to 52 propane from 13 to 43 and butane from 18 to 34 the carbon isotopes of most gases show the characteristics of humic derived gas and crude oil cracked gas in addition large primary biogenic gas fields have been discovered in the qaidam basin inorganic derived alkane gases have been discovered in deep of the songliao basin half of these gas fields are characterized by the alkane carbon isotope reversal in different degrees research indicates there are several reasons can result in carbon isotope reversal firstly gas charge of different genetic types or different source in one gas reservoir may cause carbon isotope reversal besides high over mature evolution of gas can also lead to the carbon isotopic reversal of alkanes thirdly secondary alteration of hydrocarbons may also result in abnormal distribution of carbon isotope isotope transforms to unusual light and heavy

diagenetic constraints on the heterogeneity of tight sandstone reservoirs a case study on the upper triassic xujiahe formation in the sichuan basin southwest china abstract heterogeneity is an essential feature of tight sandstones because it is directly related to reservoir quality the upper triassic xujiahe formation in the sichuan basin was used as a case study to analyse the diagenetic constraints on the heterogeneity of tight sandstone reservoirs the three main diagenetic processes of interest are mechanical compaction cementation and dissolution a dissolution facies quartz cement facies and carbonate cement facies present sequentially from the centre to the margin of sand bodies abundant feldspars and rock fragments were dissolved to form secondary porosity or filled by carbonate cement authigenic chlorite coatings generated from eogenesis to the deep burial stage are thickest in high porosity samples the 13 c of carbonate cement ranges from 10 56 to 1 15 pdb suggesting multiple sources of carbon including carbonate fragments early inorganic carbonate cement and organically derived co 2 based on 18 o carbonate cementation occurred from 33 c to 110 c microthermometric data of aqueous inclusions indicate that quartz cement precipitated at a temperature range of 120 c 140 c a summary model of the diagenetic constraints on the heterogeneity of sandstones was constructed the spatial distribution of digenetic facies is controlled by differences in fluid flow velocity and ph fluids especially hydrocarbons migrated preferentially within the centre of sand bodies and caused dissolution dissolution generated secondary porosity and provided materials for authigenic mineral precipitation carbonate fragments were better preserved and superimposed with later carbonate cement along the sandstone mudstone boundary quartz overgrowths formed after carbonate cementation and developed in the transition zone between the intervals of carbonate cementation and dissolution consequently the reservoir quality of the xujiahe sandstones decreases gradually moving from the centre to the margin of the sand bodies

where did all the coal gas go isotopic models indicate that reservoired gases in mid norway were generated predominantly from marine source rocks despite the presence of abundant mature coal and terrestrial carbonaceous shales in the are formation there are at least three different possible explanations as to why this may be the case i source models for gas from coals are not valid ii modification of the carbon isotope signature of gas occurs post expulsion iii loss of gas due to poor expulsion efficiencies and complex migration pathways each of these possibilities is discussed in turn the most likely explanation is that loss of gas due to poor expulsion and or complex migration routes in the are formation results in most of the gas not reaching the reservoir however it should also be noted that current isotopic models for gas from coal are less rigorous than those for gas from marine source rocks

fluids and h2o activity at the onset of granulite facies metamorphism abstract evidence is accumulating from experimental petrology and thermodynamic calculations based on detailed descriptions of key natural occurrences that h2o activity at the regional amphibolite facies granulite boundaries in high grade precambrian gneiss terranes coincident with the orthopyroxene isograd is considerably higher 0 3 0 6 than previously thought 0 15 0 3 this revision provides insight into the significance of fluid inclusions in minerals of the incipient charnockites characteristic of the granulite facies transitions in several precambrian gneiss terranes observations on nacl dominated brine inclusions in minerals including some new data in this paper in rocks from such places as the kabbal satnur halagur area of the archean shield of south india have salt concentrations consonant with the stabilization of orthopyroxene relative to calcic amphibole and biotite closely associated co2 dominated fluid inclusions can have substantial h2o contents at typical incipient granulite facies conditions 700 750 c and 0 5 0 7 gpa and can thus have been in equilibrium with the associated brines as immiscible metamorphic fluids observed densities of the best preserved co2 rich fluid inclusions in quartz are consistent with those expected from the equation of state of co2 at incipient granulite facies pressure temperature conditions it seems likely that the role of fluids in granulite facies metamorphism of the deep crust has been considerably underemphasized in discussions still prevalent which concentrate on the role of fluid deficient partial melting as the primary agent to the exclusion of a possible role of metamorphic fluids and metasomatism

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

fluid immiscibility in metamorphic rocks evidence of fluid immiscibility in metamorphic rocks comes in the best of all worlds from fluid inclusion observations in fact fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present in a review paper entitled metamorphic fluids the evidence from fluid inclusions crawford and hollister 1986 summarized the role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks since then tremendous progress has been made a huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all p t conditions thermodynamic databases of minerals solid solutions and fluid mixtures along with the development of gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in p t space significant progress has also been made in understanding reaction kinetics diffusional and convective mass transport and the interpretation of fluid rock interaction during metamorphism given the vast fluid inclusion evidence of immiscibility it would appear however that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered relatively few studies directly link mineral reactions with immiscible fluids therefore this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions questions arise directly from the fluid inclusion record in metamorphic rocks this chapter however is organized the other way round at first it summarizes phase relations of fluid systems pertinent to

sulfur isotope analysis of bitumen and pyrite associated with thermal sulfate reduction in reservoir carbonates at the big piney la barge production complex sulfur isotopes of solid bitumen and associated pyrite from the madison limestone in the big piney la barge production complex were measured using a secondary ion mass spectrometry sims method the solid bitumens a product of thermochemical sulfate reduction yielded 34s values of 18 9 3 9 that are consistent with inferred values for native mississippian sulfate in contrast coarse and fine grain pyrite grains were found to be 34s depleted with values similar to that of the produced h2s 34s 10 we interpret these results to indicate that two different sources of sulfate were involved with tsr within the madison limestone autochthonous anhydrite which is now completely replaced with calcite and permian age sulfate dissolved in the aquifer while checking for inclusions within the bitumen that could lead to erroneous measurement we found the bitumen possesses a 5 m rim and internal worm like features enriched in organic sulfur we hypothesize that the rim is the result of back reaction of the late forming h2s with the solid bitumen and that the 1 m diameter wormy features may result from liquid liquid immiscibility occurring at the high temperatures of formation

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

unequilibrated ordinary chondrites a tentative subclassification based on volatile element content for unequilibrated ordinary chondrites uoc two measures of primitiveness are available volatile content in principle reflecting accretion conditions from the solar nebula and metamorphism reflecting reheating in the parent bodies these two measures do not always correlate and we have therefore developed a tentative classification scheme based on volatile content that complements the searset al 1980 scheme based on metamorphism like the latter it subdivides type 3 chondrites on a scale of 3 0 to 3 9 the notation 3 4 0 indicates a meteorite that is subtype 3 4 according to metamorphism and 3 0 according to volatile content the classification is based mainly on c and xe two elements that are little affected by shock induced reheating and to a lesser extent on ar36 bi in and tl of 22 meteorites considered the majority have concordant classifications 0 2 on the two scales however 5 meteorites are richer in volatiles than their metamorphic grade indicates sharps 3 4 0 alha 77011 3 5 0 ngawi 3 6 3 alha 77299 3 7 4 and mezo madaras 3 7 3 it remains to be seen whether these differences indeed denote a more primitive nature some new clues to the formation of chondrites may eventually come from xe and c their concentrations in uoc s vary by more than 5 but the xec ratio remains nearly constant at 3 4 10 3 of the solar system ratio even the ratios for other chondrite classes differ only slightly from that for uoc s e g c3o 1 5 and e3 4 0 4 either the 4 factors determining this ratio t t p and internal surface area of the carbon varied in complementary fashion or more probably they varied only slightly in the entire source region of chondrites

silicate oxide and sulphide trends in neo archean rocks from the nilgiri block southern india the role of fluids during high grade metamorphism the nilgiri block southern india represents an exhumed section of lower late archean 2500 ma crust the northern highlands of the nilgiri block are characterized by metagabbros with pyroxenite inlayers a two pyroxene granulite zone acts as a transition between the metagabbros and charnockites which are exposed in the central and southern part of the nilgiri highlands thermobarometry results indicate a sw ne regional trend both in temperature similar to 650 800 degrees c and in pressure 700 1100 mpa over the nilgiri highlands in the charnockites composite rutile ilmenite grains are the dominant oxide assemblage in the two pyroxene granulites hemo ilmenite magnetite is dominant with coexisting rutile ilmenite composite grains in a few samples in the vicinity of the boundary with the charnockites in the metagabbros hemo ilmenite magnetite is the dominant oxide assemblage the principal sulphide mineral in the charnockite is pyrrhotite with minor pyrite chalcopyrite exsolution lamellae or blebs in the two pyroxene granulites and the metagabbros the principal sulphide assemblage consists of discrete pyrite grains with magnetite rims and pyrite pyrrhotite chalcocopyrite associations from these observations a specific oxidation trend is seen the northern granulite facies metagabbros and two pyroxene granulites of the nilgiri highlands are highly oxidized compared with the charnockites from the central and southern regions this higher oxidation state is proposed to be the result of highly oxidizing agents probably as so3 in low h2o activity grain boundary nacl saline fluids with a dissolved caso4 component present during granulite facies metamorphism of the metagabbros and two pyroxene granulites eventually these agents became more reducing owing to the inherent buffering of the original tonalite granodiorite granitoids at the graphite co2 buffer such that s took the form of h2s during the granulite facies metamorphism of the charnockites at the same time these saline fluids were also responsible the solid state conversion of biotite and amphibole to orthopyroxene and clinopyroxene in the metagabbro two pyroxene granulite and charnockite

unprecedented occurrence of novel c26 c28 21 norcholestanes and related triaromatic series in evaporitic lacustrine sediments a novel series of c26 c28 21 norcholestanes and their triaromatic analogs have been tentatively identified as major components in eogene evaporitic sediments in jinxian sag of the bohai bay basin northern china these compounds may be viewed as representatives of a new class of molecules whose biological or diagenetic origin remains to be determined their unprecedented high abundance in the evaporitic lacustrine sediments and related oils show important source and or depositional environment controls in addition to thermal maturity

high density co2 n2 inclusions in eclogite facies metasediments of the munchberg gneiss complex se germany the eclogite facies metasedimentary rocks in the munchberg gneiss complex t 630 30 c p 17 24 kbar locally contain co2 n2 rich fluid inclusions of extremely low molar volumes 32 cm3 mol in quartz these fluid compositions are mainly found in rocks intercalated with calcsilicate bands densities were determined from low temperature phase transitions like stable or metastable homogenization l v l partial homogenization s l v s l and the transition s l l l liquid v vapour s solid the high fluid densities are in agreement with eclogite facies pressure and temperature and subsequent amphibolite facies co2 n2 inclusions were not observed in adjacent eclogites nor in non calcareous metasediments these rock types contain predominantly h2o rich inclusions correlating with amphibolite facies conditions the variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite facies metamorphism

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

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

new directions in an established gas play promising dolomite reservoirs in the middle triassic leikoupo formation of the sichuan basin china the discovery of carbonate gas fields in the middle triassic leikoupo formation of the sichuan basin has a complex history in recent years a series of structural fields have been discovered in the western sichuan basin their discovery confirms the immense exploration potential of the leikoupo formation in this study we analyze the characteristics of leikoupo formation exploration plays using exploration wells and test data aiming to provide a reference for further discoveries the leikoupo formation represents the uppermost unit in the sichuan marine carbonate platform succession during its deposition the whole basin was characterized by a restricted and evaporitic platform two classes of reservoirs developed one is pore fracture reservoirs in marginal platform and intraplatform shoals and another is fracture vug reservoirs in the karstic weathering crust of the formation capping unconformity three hydrocarbon accumulation models were established for the leikoupo formation based on the spatial and temporal relationship among the source reservoir and cap rocks two types of exploration plays are present in the leikoupo formation that is shoal including intraplatform shoal and marginal platform shoal dolomite plays and karstic dolomite weathering crust plays including intraplatform shoal karst and marginal platform shoal karst the western sichuan depression in the karstic slope belt presents immense exploration potential because of a proximal hydrocarbon supply charging via an extensive fracture network shoals and karstic reservoir a good seal rock of terrestrial mudstone and potential composite hydrocarbon accumulations in stratigraphic traps making it a promising area for future exploration

predicting co2 occurrence on a regional scale southeast asia example abstract prediction of co 2 occurrence in natural gases is difficult owing to the interplay among basinal sources e g volcanic emanations carbonate hydrolysis and metamorphism organic diagenesis and maturation reservoir processes e g mineralogical transformations and poorly understood physicochemical phenomena e g differential solubility of gas components carbonate buffering phase segregation published approaches to assessing co 2 risk are based either on circumstantial evidence of basinal influences or statistical analysis of reservoir attributes the present study presents the results of empirical statistical and neural network analysis of co 2 abundance in natural gas and discusses potentially influential basinal and reservoir attributes to quantify risk factors for anomalous co 2 occurrences in southeast asia the results of this study indicate that tectonic setting including basement crustal age basement fault density reservoir temperature and reservoir pressure are key elements controlling co 2 abundance assignment of co 2 risk at the basin level can be roughly approximated using the results of this study more accurate assessments of co 2 risk at the basin play field and reservoir levels require more comprehensive geological and geochemical data as well as consideration of potentially influential physico chemical processes

sulfur isotope ratios in petroleum research and exploration williston basin the three major types of crude oil in the williston basin the type i oils of the winnipeg red river system the type ii oils of the bakken madison system and the type iii oils of the tyler pennsylvanian system can be distinguished by their sulfur isotope compositions they have characteristic dgr34s values of 5 8 1 2 parts per thousand ppt 2 8 0 8 ppt and 4 0 0 7 ppt respectively highly mature oils have less typical values type ii oils which have migrated over a distance of some 150 km beyond the region of generation have maintained their characteristic dgr34s values even though sulfur may have been lost this indicates little or no interaction with reservoir sulfates under normal circumstances on the periphery of the basin type ii oils altered by water washing and biodegradation have altered dgr34s values which increase from 2 9 to 9 4 ppt with the increasing degree of crude oil degradation the bakken shales source of the type ii oils have dgr34s distribution patterns in the reduced sulfur typical of marine sediments the dgr34s values for the type ii oils match most closely the dgr34s value of organic sulfur in the black bituminous shales of the lower bakken

fluid rock interaction and its effects on the upper triassic tight sandstones in the sichuan basin china insights from petrographic and geochemical study of carbonate cements abstract tight sandstone gas is an important unconventional hydrocarbon resource but its exploration is made difficult by the compactness and heterogeneity of the sandstone fluid rock interaction and subsequent cementation play key roles in developing this heterogeneity we conducted a petrographic trace element and isotopic study of the upper triassic xujiahe sandstones from the sichuan basin to investigate fluid rock interactions and associated diagenetic effects carbonate fragments and cements are widespread in these sandstones and are the primary control on the nature of the xujiahe sandstone reservoirs the carbonate cements primarily comprise of calcite and are typically distributed around the margins of sandstone bodies and along fine grained beds carbonate minerals in the centre or at the margin of sandstone units have heavy 13ccarb values of ca 0 whereas 13ccarb is markedly depleted in the transition zone with values as low as 10 reflecting a contribution from organic carbon manganese strontium and barium generally become enriched in the carbonate cements with diagenesis and their concentrations correlate with porosity and 13ccarb redox sensitive mn and fluid mobile sr and ba were analysed to reveal the diagenetic processes and environment of carbonate precipitation we propose that fluids flowed preferentially along the central parts of sandstone units associated with mineral dissolution whereas cementations took place near the margins of the units fluid rock interaction especially that involving late hydrocarbon related fluids greatly enhances the heterogeneity of sandstones and can potentially create high quality reservoirs in the central parts of tight sandstone units

juvenile co2in enderbites of tromoy near arendal southern norway a fluid inclusion and stable isotope study the enderbites from tromoy in the central granulite facies part of the proterozoic bamble sector of southern norway contain dominantly co2 and n2 fluid inclusions co2 from fluid inclusions in quartz segregations in enderbites was extracted by mechanical crushing and thermal decrepitation and the 13c measured measurement was also made on samples washed in 10 hcl oxidized with cuo at high temperatures and step wise extracted with progressive heating results between the different techniques are systematic the main results show 13c of 4 5 1 5 for crushing and 7 2 for thermal decrepitation 13c is about constant for co2 extracted at different temperatures and points to a homogeneous isotopic composition due to the presence of carbonate particles and or induced contaminations for the extraction by thermal decrepitation the results for the crushing experiments are assumed the most reliable for fluid inclusion co2 very low values of 13c have not been found in enderbite samples and 13c combined with 18o of the host quartzes 8 11 indicates juvenile values in addition the fluid inclusions were examined by microthermometry and raman analysis and host quartz by acoustic emission and cathodoluminescence co2 fluid inclusions have varying densities with a frequency maximum of 0 92 g cm 3 and generally do not concur with trapping densities at granulite conditions textures show that co2 must have been trapped in fluid inclusions in one early event but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions the present data support a model of intrusion and crystallization of a co2 rich enderbitic magma at granuiite conditions

unique chemical and isotopic characteristics and origins of natural gases in the paleozoic marine formations in the sichuan basin sw china isotope fractionation of deep and high mature carbonate reservoir gases abstract large quantities of natural gases have recently been discovered in the marine carbonate formations in the sichuan basin southern china the origins and isotopic compositions of the gases are poorly understood and highly controversial we examined the chemical and isotopic compositions of the carbonate reservoir gases in marine formations from the sinian to lower triassic as well as shale gas from the silurian and cambrian formation in the sichuan basin it is found that the carbon isotopes of the carbonate reservoir gases had been evolved through three stages corresponding to pre rollover rollover and post rollover in the plot of 13c2 versus wetness which is consistent with that found in shale gas by tilley and muehlenbachs 2013 the unique carbon isotopic compositions indicate that both kerogen and oil cracking gas have contributed to the accumulation of the carbonate reservoir gases in the sichuan basin a mixing of two thermogenic gases with different maturities can cause the 13c reversal between methane and ethane which is commonly observed in the carbonate reservoir gases in the carboniferous permian and triassic formations and the shale gas in the silurian and cambrian formations in the sichuan basin we established two mixing models for quantitatively determining the contribution of each type gas for the natural gases in different formations kinetic extrapolation based on thermal history of the anyue gas field demonstrates that ethane cracking indeed occurred in the deep reservoir formations which may explain the disappearance of the 13c reversal for methane and ethane in the carbonate reservoir gases in the post rollover zone it is proposed that organic inorganic interactions may be responsible for the presence of d isotope rollover for methane in carbonate reservoir gases with ultra high maturity the organic inorganic interactions include thermal reactions between water and residual organic matters or carbons and hydrogen isotope exchange between water and methane and their mechanism needs further investigations in future

observation of non lithophile behavior for u in the hvittis enstatite chondrite fission track radiography shows that u is highly concentrated in cas oldhamite with a concentration of 400 50 ppb the oldhamite u concentrations are equilibrated although intergrain concentration variations of less than 15 would not be detected various approaches to a u material balance do not agree but at least 50 and more likely nearly 100 of the u is concentrated in cas it is likely that cas is the major reservoir for other actinide and lanthanide elements as well consequently enstatite chondrites may provide reliable pu u abundance ratios the highly reducing formation conditions for enstatite chondrites have caused u to deviate from lithophile behaviour but k is observed to remain lithophile although this observation may not be generalizable we propose that discussions of radioactive heating of planetary cores should include u and th and not focus exclusively on k

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