propagation tectonics and multiple accretionary processes of the qinling orogen abstract the qinling orogen was built through collision between the north china and south china blocks previous detailed geological geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono lithostratigraphy which are from north to south the southern sector of the north china block north qinling belt south qinling belt and northern sector of the south china block separated by the kuanping shangdan and mianlue sutures according to the petrology geochemistry and geochronology of ophiolitic melanges and related magmatic rocks as well as the features of sedimentary units we think that the north china block the north qinling belt and the south china block were originally independent continental units while the south qinling belt had been the northern part of the south china block these units experienced three episodes of accretionary tectonic processes and amalgamation from south to north the neoproterozoic accretion took place along the luonan luanchuan fault and kuanping ophiolitic melange belt as a result of southward subduction and subsequent collision between the north qinling and north china blocks during ca 1 0 0 8 ga related to the formation of the supercontinent of rodinia the paleozoic accretion occurred along the shangdan suture resulted from northward subduction of oceanic lithosphere in the early paleozoic and subsequent continental subduction in the late paleozoic late triassic accretion took place along the mianlue suture between the south qinling and south china blocks due to northward subduction of the mianlue oceanic lithosphere during the permian early triassic and subsequent collision in the late triassic after the late triassic collision along the mianlue suture the whole qinling mountain range entered the phase of intense intracontinental deformation

geochemical variations among eastern saudi arabian paleozoic condensates related to different source kitchen areas abstract large reserves of gas and condensate are found in paleozoic reservoirs khuff unayzah and jauf in the ghawar area of eastern saudi arabia the source rock for these hydrocarbons is the basal organic rich hot shale of the qusaiba member of the silurian qalibah formation this is the same prolific source rock that generated the sweet light crude oil and condensate found in the unayzah reservoirs in central saudi arabia thermal maturity modeling of the source rock identifies two kitchen areas to the east and west of the ghawar structure the west kitchen was well into the oil window by 210 ma while the east kitchen was just entering the oil window large areas of both kitchens were in the wet gas window by 125 ma and by 90 ma most of the area north of ghawar was generating dry gas in eastern saudi arabia paleozoic oil and condensate are found in early formed structures carboniferous and early triassic growth whereas late cretaceous and tertiary structures contain only dry gas the geochemistry of the oils and condensates indicates two groups each generated from a different hydrocarbon kitchen suggesting that the two kitchens differ slightly in organic facies within each group variations among hydrocarbons are related to the thermal maturity of the source rock during generation within the khuff reservoir gas souring is caused by thermochemical sulfate reduction

influential factors and model of shale pore evolution a case study of a continental shale from the ordos basin abstract pore characteristics have a significant influence on the occurrence and enrichment of shale gas therefore these characteristics are key parameters in shale reservoir evaluation and have important research significance taking a continental shale sample from the ordos basin china as an example this research used low pressure co2 n2 adsorption high pressure mercury intrusion porosimetry mip vitrinite reflectance ro analysis hydrocarbon generation measurements x ray diffraction analysis and argon ion polishing field emission scanning electron microscopy aip fesem in conjunction with a thermal simulation experiment to study pore characteristics through the above analyses this study achieved a whole aperture dynamic characterization of the shale pores and discussed the characteristics of the shale pore distribution and evolution through geological history then the factors influencing pore evolution were analysed and a pore evolution model was established the results showed that the micropores and mesopores provided the majority of the surface area in the sample while the mesopores and macropores provided the majority of the pore volume in the sample during the thermal simulation experiment pore evolution was dominated by the hydrocarbon generation by organic matter the transformation of minerals and compaction according to the pore evolution characteristics and the main influential factors during the thermal simulation experiment the pore evolution model can be divided into four stages the characteristics of the micropores mesopores macropores and the influential factors differ in each stage

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

late gas generation potential for different types of shale source rocks implications from pyrolysis experiments abstract gas generation from shale source rocks typically occurs via cracking of both kerogen and retained oils such that it is difficult to predict and compare gas generation potentials of different shales because they are related not only to the kerogen type but also to the oil expulsion efficiency in this study five different shale kerogen samples were pyrolyzed in sealed gold tubes to investigate how kerogen type and oil expulsion efficiency affect their gas generation after oil window maturity the results illustrate that the maximum extractable organic matter eom and gas generation potentials of different original shale kerogens o kerogen in a closed system vary widely in the range of 229 790 mg g tocok and 308 594 ml g tocok respectively however the gas yields of different residual shale kerogens r kerogen with a starting equivalent vitrinite reflectance eqvro value of approximately 1 22 are quite similar and vary between 131 and 145 ml g tocrk pyrolysis experiments also reveal that the late gas generation potential eqvro 1 22 of shale is mainly controlled by the amount of retained eoms rather than kerogen type when the shale source rocks containing types i and ii kerogens have the same amount of retained eoms their gas generation potentials are quite similar under most geological conditions the late gas generation potentials of shale source rocks normalized to the toc of matured shale at 1 22 eqvro tocshale vary approximately in the range of 180 300 ml g tocshale to reach a gas content of 3 m3 ton shale for the present day overmature shale gas exploration in the lower palaeozoic shales of south china a conservative present day toc tocpd value of 2 0 is proposed as a screening parameter that can eliminate the risk of insufficient gas generation potential as much as possible

triterpane and sterane biomarkers in the ya13 1 condensates from qiongdongnan basin south china sea triterpanes and steranes in condensates from the ya13 1 gas field qiongdongnan basin were monitored the ya13 1 condensates have unusual biomarker distributions dominated by terpanes and steranes derived from higher plants anomalously abundant 1 got oleanane and remarkably abundant bicadinanes are present in the ya13 1 condensates whereas the 17alpha hopane contents are extremely low taraxastane and significantly abundant 17alpha diahopanes occur in the condensates in addition a number of unknown c 29 and c 30 pentacyclic triterpanes including previously unreported compounds were detected in the condensates some of which are significantly abundant the unknown compounds may be terrestrial biomarkers c 29 homologues are relatively predominant among the regular and rearranged steranes the diasterane concentrations are markedly higher than those of regular steranes the maturity of the ya13 1 condensates is relatively high at the peak to late oil generation stage corresponding to 0 85 1 10 ro based on sterane and terpane and including bicadinane maturity parameters i e t t 1 r and 2t r bicadinane ratios the above maturity assessment result is different from that based on diamondoid maturity parameters ro 1 60 1 70 org geochem 25 1996 179 which can be explained by a contribution of hydrocarbons from two sources at different depths the ya13 1 condensates were probably generated from the yacheng and lingshui coal bearing source rocks buried both in the qiongdongnan basin 3400 5000 m and in the yinggehai basin 5000 m the possible contribution of lower maturity hydrocarbons from the yacheng and lingshui formations 3400 4100 m in the qiongdongnan basin to the ya13 1 gases and condensates should not be neglected c 2003 elsevier science b v all rights reserved

new paleomagnetic results from the ediacaran doushantuo formation in south china and their paleogeographic implications abstract a new paleomagnetic pole is obtained from the top of member 3 of the doushantuo formation in the jiulongwan section of the yangtze gorges area in south china a total of 191 samples from 23 sites were collected and subjected to stepwise alternating field af and thermal demagnetization after the removal of a soft component sc of viscous magnetic remanence acquired in present geomagnetic field a high temperature component hc likely carried by magnetite was isolated the hc includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great circle analysis both the vectors only and the combined vectors and arc constraints of the hc passed a reversal test this is the first ediacaran paleomagnetic remanence from the south china block scb that passes a reversal test and it is interpreted as a primary remanence the hc vectors were thus used to calculate the paleomagnetic pole and in combination with arc constraints were used for polarity interpretation the vectors give a mean direction of d 75 2 i 41 0 95 2 5 n 147 after bedding correction and a corresponding paleomagnetic pole at 23 9 n 187 0 e dm dp 3 0 1 8 this pole differs significantly from previously published results of the doushantuo formation which may have been subjected to paleozoic remagnetization our results provide a 23 5 1 8 n paleolatitude for the yangtze gorges area of the scb and suggest several alternative paleogeographic options for the ediacaran cambrian amalgamation of east gondwana depending on chronostratigraphic correlations

an investigation of the in reservoir conversion of oil to gas compositional and kinetic findings from closed system programmed temperature pyrolysis abstract the thermal alteration of reservoired petroleum upon burial was simulated by closed system non isothermal pyrolysis samples of a medium gravity oil from the norwegian north sea central graben were pyrolysed in microscale sealed glass or quartz vessels at heating rates of 0 1 0 7 and 5 0 k min 1 up to temperatures varying from 300 to 650 c the composition of the oil and gas in each vessel was determined by a single step on line gas chromatographic analysis with increasing rate of heating the onset of all oil degradation reactions was shifted to higher temperatures the same successive compositional changes occurred in each case increase in the total yield of gc detectable compounds significant gas c1 c4 generation accompanied by a decrease in yield of heavy components aromatisation and attainment of maximum gas yield and finally a cracking of the c2 gas components the stable final mixture consisted of methane aromatic hydrocarbons pyrobitumen and possibly hydrogen kinetic modelling of the oil to gas conversion resulted in a narrow gas potential vs activation energy distribution between 66 and 70 kcal mol assigning 35 of the total gas potential 460 mg per g of oil to an energy of 66 kcal mol and 29 to 67 kcal mol pre exponential factor 1 1 1016 s 1 by extrapolation to natural maturation conditions the onset of gas generation is predicted to occur between 160 and 190 c for geological heating rates between 0 53 and 5 3 k ma 1 predictions from the model are in accordance with the observed preservation of liquid hydrocarbons in a deep hot 165 c petroleum reservoir from the saga 2 4 14 well norwegian continental shelf

effect of maturation on the indigenous δd signatures of individual hydrocarbons in sediments and crude oils from the perth basin western australia abstract we measured the stable hydrogen isotopic composition d of selected aliphatic hydrocarbons in nine sediments of various maturity and two crude oils all from the northern perth basin on shore western australia the sediments and crude oils are from the lower triassic hovea member sapropelic interval of the kockatea shale the n alkanes pristane and phytane from two immature sediments have d values that represent the expected isotopic compositions of their precursors phytane is enriched in deuterium d relative to pristane in all sediment extracts and crude oils this is attributed to either different sources or to different isotopic effects during their derivation from phytol with increasing maturity pristane and phytane become enriched in d while the n alkanes generally remain at a constant isotopic composition a more rapid enrichment of d in isoprenoids relative to n alkanes with increasing maturity suggests that hydrogen isotopic exchange is occurring via a carbocation mechanism the phytane diastereomer ratio for each of the sediments correlates linearly with the progressive enrichment of d in phytane and therefore maturity suggesting that significant hydrogen exchange occurs at the chiral carbons of phytane the d values of pristane and phytane from the crude oils are similar or more positive relative to the d values of the n alkanes the average d values of pristane and phytane for the sediments correlate well with equivalent vitrinite reflectance values as does the conventional biomarker maturity parameter t s t m indicating that d values may be useful for establishing maturity over a wider range than conventional biomarker parameters the results provide further evidence that care must be taken not to over interpret d values of sedimentary hydrocarbons in samples of high thermal maturity

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

primary migration by diffusion through kerogen i model experiments with organic coated rocks abstract laboratory experiments were performed to assess diffusion through kerogen as a mechanism of hydrocarbon transport through fine grained rocks such transport can be important in primary migration within source rocks and in leakage through seals to test the concept of diffusion through organic matter networks model experiments were performed in which hydrocarbon diffusion was measured through austin chalk cores that had been coated with a monolayer of fatty acids hydrocarbon fluxes through the coated cores were compared to hydrocarbon fluxes through uncoated austin chalk cores results showed that the organic coating enhanced transport through the core by a hundredfold over diffusion through its water filled pore space alone

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea ii molecular composition of alkylated naphthalenes phenanthrenes benzo and dibenzothiophenes abstract variations with depth in the quantitative distributions of di and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the upper jurassic age kimmeridge clay formation brae field area north sea reveal in part the effects of depletion with hydrocarbon expulsion trends of increasing depletion with approach to the nearest shale sandstone contacts as documented by a decrease in carbon normalized yields are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes but less drastic than previously observed for long chain alkanes cf part i leythaeuser et al 1988 lowest yields consistently were encountered at the outermost 1 2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for c15 to c30n alkanes 80 95 than for c2 and c3 alkylnaphthalenes 30 40 in contrast c2 and c3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post expulsion selective replenishment from the adjacent oil accumulation expulsion was not associated with any detectable fractionation among c1 and c3 alkylnaphthalenes or c1 and c2 alkylphenanthrenes as evidenced by a mass balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early mature through to mature stages of thermal evolution it was verified that the methylphenanthrene index mpi 1 and methyldibenzothiophene ratio mdr of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum

thermal cracking of kerogen in open and closed systems determination of kinetic parameters and stoichiometric coefficients for oil and gas generation abstract the purpose of the study is to compare artificial maturation of kerogens representative of the main types of organic matter kerogen types i ii ii s and iii in open and anhydrous closed pyrolysis systems the generated compounds are fractionated according to their thermal stability into six chemical classes c 1 c 2 c 5 c 6 c 14 c 15 saturates c 15 aromatics and nsos which include resins and asphaltenes in both systems primary cracking of kerogen can be described by two main reactions oil generation together with early gas generation followed by late gas production mass balances obtained in the two pyrolysis systems are reasonably similar although some secondary cracking of the nsos and underestimation of methane potential due to incomplete pyrolysis may occur in the open system kinetic parameters of thermal cracking are derived from experiments in the open system using an optimization procedure which determines a unique frequency factor and a weight distribution of chemical classes for a discrete series of activation energies the resultant frequency factors are checked with those obtained for specific molecular tracers such as n c 12 and n c 24 alkanes generated from kerogens in open and closed systems under isothermal conditions the results show that using a unique frequency factor as determined by optimization is correct for type i ii and ii s kerogens this factor is more questionable however when the distribution of activation energies is very broad such as for type iii coals based on this study a strategy for determination of kinetic parameters on any new source rock sample is proposed

the effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage a pyrolysis study of type ii kerogen from pingliang shale china high maturity oil and gas are usually generated after primary oil expulsion from source rocks especially from oil prone type i ii kerogen however the detailed impacts of oil expulsion or retention in source rock on further thermal degradation of kerogen at the high maturity stage remain unknown in the present study we collected an ordovician pingliang shale sample containing type ii kerogen the kerogens which had previously generated and expelled oil and those which had not were prepared and pyrolyzed in a closed system to observe oil expulsion or oil retention effects on later oil and gas generation from kerogen the results show that oil expulsion and retention strongly impacts on further oil and gas generation in terms of both the amount and composition in the high maturity stage gas production will be reduced by 50 when the expulsion coefficient reaches 58 and gas from oil expelled kerogen less oil retained is much drier than that from fresh kerogen the oil expulsion also causes n alkanes and gas compounds to have heavier carbon isotopic compositions at high maturity stages the enrichment of 13c in n alkanes and gas hydrocarbons are 1 and 4 6 respectively compared to fresh kerogen oil expulsion may act as open system opposite to the oil retention that influences the data pattern in crossplots of 13c2 13c3 versus c2 c3 13c2 13c3 versus 13c1 and 13c1 13c2 versus ln c1 c2 which are widely used for identification of gas from kerogen cracking or oil cracking these results suggest that the reserve estimation and gas source correlation in deep burial basins should consider the proportion of oil retention to oil expulsion the source rocks have experienced

kinetic study of marine and lacustrine shale grains using rock eval pyrolysis implications to hydrocarbon generation retention and expulsion abstract the kinetic parameters of marine and lacustrine shale grains 4 mm were retrieved by using rock eval pyrolysis in comparison with finely ground powder

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon stratigraphic and chemical correlations of tertiary volcanic units in eastern oregon confirm that the steens basalt represents the earliest eruptions of the columbia river flood basalt province field correlations are supported by major and trace element analyses and confirmed by 4 0 ar 3 9 ar dates within the basalt of malheur gorge situated between steens mountain and the southernmost extent of the previously mapped columbia river basalt group the lowest unit correlates with the steens basalt and the conformably overlying middle and upper units correlate with the imnaha and grande ronde basalt formations of the columbia river basalt group new dates indicate that imnaha and grande ronde basalt formations on the columbia plateau 90 of the columbia river basalt group erupted between 16 1 and 15 0 ma these were immediately preceded by the steens basalt a plagioclase phyric tholeiite that erupted above the calculated position of the yellowstone hotspot at 16 6 ma in eastern oregon the flood basalt tholeiites of steens mountain and malheur gorge form a voluminous but brief interlude 16 6 15 3 ma superimposed on the low volume calc alkalic to mildly alkalic volcanism associated with continuing eocene to present east west extension

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

a comparison of pyrolysis products with models for natural gas generation abstract hydrous pyrolysis using progressive heating from 150 to 365 c is used to study the generation and isotopic composition of hydrocarbon gases and carbon dioxide different source rocks from the draupne heather dunlin and brent formations in the north sea a miocene brown coal from germany a coal from the middle jurassic ness formation in the northern north sea and a rich marine type source from the late jurassic kimmeridge clay formation onshore uk are studied the experimental results are related to vitrinite reflectance as a maturity reference parameter and to existing models for natural gas generation hydrous pyrolysis simulates the observed isotopic composition of natural systems fairly closely the largest deviations are found for methane which is most sensitive to fractionation effects the obtained results mainly reflect variations in source rock type and initial isotopic composition different maturity dependent trends are found between coal derived and marine type ii derived gases trends in the experimental data fit well with basic assumptions of existing models for natural gas generation

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

new mesozoic paleomagnetic results from the northeastern sichuan basin and their implication abstract this paper presents the results of a paleomagnetic study undertaken on samples from lower triassic and lower cretaceous sediments within the northeastern sichuan basin china in an area close to the daba mountains stepwise thermal demagnetization was used to isolate the characteristic higher temperature component htc for these samples these htc t1 d 53 7 i 21 6 95 6 5 k1 d 28 9 i 25 7 95 6 6 passed both fold and reversal tests and indicate that these sediments have undergone sequential clockwise rotations relative to the sichuan basin these detected tectonic rotations were probably caused by the obstruction of the hannan massif during mesozoic thrusting within the southern daba mountains the northeastern sichuan basin and the central yangtze fold belt formed part of a continuous belt of basins the northern yangtze basin and acted as a coherent tectonic domain during the early middle jurassic the daba mountains and the central yangtze fold belt subsequently underwent rapid uplift and cooling around the late jurassic and early cretaceous this suggests that tectonic rotations within both the central yangtze fold belt and the daba mountains were very probably caused by intra continental deformation over a long period of time rather than a single tectonic event

jurassic paleomagnetic constraints on the collision of the north and south china blocks this paper reports a paleomagnetic study of jurassic rocks from the ordos basin in the north china block ncb a recent remagnetization and a high temperature and or high coercivity component with dual polarities have been isolated the middle jurassic pole 74 n 233 e a95 5 is roughly consistent with previous results and the lower jurassic pole 82 n 286 e a95 7 is located between the late triassic and middle jurassic poles these results combined with paleomagnetic results from adjacent blocks suggest that 1 the pole for the ncb suffered an angular displacement of 47 8 between the late triassic and middle jurassic 2 the scb underwent final accretion to the ncb in the middle jurassic and 3 accretion of the ncb to siberia was not complete until the late jurassic and possibly even until the early cretaceous

tectonic evolution of the north qinling orogen from subduction to collision and exhumation evidence from zircons in metamorphic rocks of the qinling group abstract the north qinling block nqb is an important segment of the qinling orogen in central china here we report the results from sims geochronology and oxygen isotopes as well as la mc icpms hf isotopic analyses on zircon grains from a suite of metamorphic rocks felsic gneisses garnet plagioclase amphibolites and retrograde eclogite dikes in the qinling group of the nqb the age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483 501 ma followed by 454 470 ma retrograde metamorphism these results are generally coeval with the periods of 500 480 ma for peak metamorphism and 460 420 ma for retrograde metamorphism previously obtained from the hp uhp metamorphic rocks of the nqb during the prograde and retrograde metamorphism widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons the fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low th u ratios positive ehf t 5 and extremely 18o 10 01 13 91 values in metamorphic zircons in contrast the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high u and th contents and th u ratios and enriched hf ehf t 5 42 to 0 18 and oxygen isotope composition 18o around 8 geochronological and geochemical features of the magmatic cores of the clear core rim textured zircons demonstrate that the protoliths of the gneisses were intermediate acid volcanic rocks erupted before neoproterozoic 800 ma which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes with the oldest magmatic zircon formed at 789 ma the protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism combined with the previous studies the qinling group experienced overall subduction in the early paleozoic the nqb as represented by the qinling group was most likely a discrete micro block in the neoproterozoic and underwent deep subduction in the cambrian 483 501 ma and exhumation in ordovician 454 470 ma we propose that the nqb preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading and micro continent formation to deep subduction and exhumation

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

correlation between hydrogen isotope ratios of lipid biomarkers and sediment maturity abstract we investigated the influence of thermal maturity on the hydrogen isotope ratios of sedimentary hydrocarbons to prove that the isotope ratio of hydrocarbons mirrors paleoclimate signatures d values from n alkanes and acyclic isoprenoids of two sediment sections kupferschiefer ks 258 ma and posidonienschiefer ps 184 ma with different maturation history were investigated both covered thermal maturity from 0 48 to 1 3 rc vitrinite reflectance and reflectance calculated from mpi1 sediment burial up to 4500 m caused thermal maturation of organic matter in the ks horizon from the early zechstein basin of poland whereas contact metamorphic thermal maturation originated in the early toarcian ps posidonienschiefer of the north german vlotho massif the d values of the extracted n alkanes positively correlate with thermal maturity in the ks y 56 mpi1 x 160 vsmow and in the ps y 104 mpi1 x 200 vsmow the d values of isoprenoids i e pristane phytane were even more enriched with increasing maturity y 179 mpi1 x 341 vsmow in the ks y 300 mpi1 x 415 vsmow in ps these results explain why isotope ratios of n alkanes and isoprenoids in mature sediments are generally enriched in d and do not have the expected isotopic difference between n alkanes and isoprenoids of 190 moreover the correlation between sediment maturity parameters and d values suggests that after correction the d values of n alkanes can be used to reconstruct climate and environment in the geological past

characteristics and origin of the gas and condensate in the yinggehai basin offshore south china sea evidence for effects of overpressure on petroleum generation and migration three superimposed pressure systems have developed in the yinggehai basin the strongly overpressured rocks have abnormally low vitrinite reflectance ro and rock eval tmax values and display abnormal c15 hydrocarbon distributions such organic maturity anomalies were confirmed to be the result of overpressure retardation commercial gas fields with carbon dioxide content up to 93 and gas shows with nitrogen content more than 50 have been found in the basin methane and carbon dioxide dominated gases have low c2 hydrocarbon contents less than 4 and heavy methane 13c and d values and the associated condensates have high diamondoid indices and very low c25 saturated hydrocarbon concentration at the same time the methane and carbon dioxide dominated gases have high branched and cyclic light hydrocarbon contents and associated condensates have abnormally low heptane and isoheptane values low sterane isomerization values and abnormally high concentrations of isoprenoid hydrocarbons relative to adjacent n paraffins these distinct characteristics seem to result from the effects of overpressure on both hydrocarbon generation and migration

the effect of source and maturity on the stable isotopic compositions of individual hydrocarbons in sediments and crude oils from the vulcan sub basin timor sea northern australia abstract recent work has demonstrated the effect of maturation on the stable hydrogen isotopic compositions d of individual sedimentary hydrocarbons n alkanes pristane and phytane in a series of marine source rocks from the perth basin western australia there was an enrichment in deuterium d in the hydrocarbons with increasing maturity attributed to isotopic exchange associated with thermal maturation an initial large 115 biologically derived difference between the d values of n alkanes and isoprenoids gradually decreased as pristane and phytane became enriched in d while the n alkanes generally remained at constant isotopic composition this work has now been extended to include a series of late jurassic sediments from the lower vulcan formation of the vulcan sub basin timor sea northern australia where the d values of n alkanes and isoprenoids show similar trends to those observed for the perth basin the enrichment in d in isoprenoids correlates strongly with traditional maturity parameters and is shown to be related to the epimerisation of pristane and phytane pristane and phytane extracted from a post mature paqualin 1 sediment are significantly enriched in d relative to the n alkanes indicating that d enrichment persists at very high maturity more so for regular isoprenoids than n alkanes this supports the notion that hydrogen h d exchange causes the observed shift in d values and not free radical hydrogen transfer a mechanism is proposed which can account for both h d exchange and epimerisation of pristane and phytane in the sedimentary environment pristane is enriched in d relative to phytane throughout the vulcan sub basin sequences attributed to a lower relative algal input to the isoprenoids and indicating that they exchange hydrogen at similar rates during maturation crude oils and condensates from the sub basin were also analysed to evaluate their source and thermal maturity and the results complement previous molecular and stable carbon isotopic analysis the d values of n alkanes and regular isoprenoids largely support the previous classification of vulcan sub basin crude oils and condensates into two groups group a having a marine source affinity and group b a terrigenous source affinity some oils and condensates are suggested to be a mixture of sources a and b or a and other as yet unknown sources tenacious 1 crude oil formerly a group a oil contains n alkanes with more positive d values than other group a oils and is suggested to have been mixed with another source of more mature hydrocarbons the group a crude oils and condensates show an upward inflection in the n alkane d profile from n c 11 to n c 15 which is suggested to represent an addition of d enriched lower molecular weight n alkanes from a more mature wet gas condensate to an initial oil charge the small differences between the d values of the n alkanes and regular isoprenoids in the crude oils and condensates indicate that significant h d exchange has taken place implying that the samples were generated from mature source rocks

diamondoid hydrocarbon ratios novel maturity indices for highly mature crude oils abstract diamondoid hydrocarbons have been identified in oil and source rock samples from the tarim yinggehai qiongdongnan and other chinese basins two diamondoid hydrocarbon ratios have been used as novel high maturity indices to evaluate the maturation and evolution of crude oils and condensates in these chinese basins the two ratios 1 ma 1 ma 2 ma mai methyl adamantane index and 4 md 1 md 3 md 4 md mdi methyl diamantane index were found to increase with increasing vitrinite reflectance the initial values for the ratios are ca 50 and 30 respectively diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent based on this study crude oils from the tarim basin are generally believed to be very mature ro 1 1 the cambrian ordovician source beds are the major source rocks of crude oils and natural gases in this basin condensates from the tarim yinggehai and qiongdongnan basins have higher maturities equivalent to ro values of 1 6 2 0 gases and condensates from the yinggehai and qiongdongnan basins are generated mainly from tertiary source rocks below 4000 5000 m

understanding of reservoir gas compositions in a natural case using stepwise semi open artificial maturation abstract in a study of the gas distribution and composition in different reservoirs of multilayered gas fields in south east asia two types of variations with depth of the 13c values of gases in particularly methane were observed in the normally pressured zones the 13c values of methane increase steadily with depth whilst in some overpressured zones they first decrease with depth and then follow a regular trend in order to explain these patterns these data were compared to those obtained by artificial maturation of a coal using confined pyrolysis experiments were conducted in perfectly closed and in semi open conditions the free gases recovered were quantified by gas chromatography and the carbon isotopic composition of individual compounds c1 c5 hydrocarbons determined the evolution of the 13c versus pyrolysis temperature obtained for the artificially generated methane is very similar to that of the natural gas the use of the 13c2 13c3 versus c2 c3 mol diagram eage 58th conference and technology exhibition 1996 chem geol 147 1998 249 allows recognition of two trends in the artificial maturation data related to the degree of opening of the experimental systems comparison of the data provide evidence that the closed pyrolysis results are very similar to those observed in high pressured reservoirs while those obtained in the normal pressured reservoirs follow the values of semi open pyrolysis the gas distributions and carbon isotope composition in the reservoirs can therefore be explained in terms of degree of opening of the system high pressure reservoirs can be considered as fairly closed systems with in situ gas generation while normally pressured zones are more open and subject to important lateral migration the composition of the gas richness in methane is controlled by the degree of opening of the system thus our findings do not support the hypothesis that the generation of methane rich gases in nature is controlled by transition metal catalysis

experimental investigation of the compositional variation of petroleum during primary migration anhydrous non isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from permian through miocene age the objectives of this study were to investigate molecular and compound class fractionation during petroleum generation and expulsion and to highlight causes of compositional variations due to primary migration from source rocks molecular compositions of n alkanes triterpanes methylphenanthrenes and fatty acids of the original samples initial bitumen the samples after pyrolysis residual bitumen and the expelled products were compared acyclic alkanes were preferentially expelled with compositions depending on source kerogen and partial evaporative loss from the open experimental system expulsion no substantial molecular fractionation within various compound classes or preferential primary migration of n alkanes relative to acyclic isoprenoids occurred cyclic triterpanes and aromatic hydrocarbons were found only in small concentrations in the expelled bitumen indicating preferential retention relative to acyclic alkanes palmitic c16 and stearic c18 acids occurred as dominant compounds in the initial and residual bitumen in the expelled bitumen they constitute virtually the only fatty acid species indicating that expulsion efficiency is strongly dependent on concentration and generation intensity

regional deformational features of the south china block inferred from middle triassic palaeomagnetic data summary middle triassic red purple sandstones were sampled at four localities in the northeastern part of the south china block scb to detect regional deformational features most of the samples gave a characteristic remanent magnetization with unblocking temperatures up to 680 c the primary nature of this magnetization was ascertained by a positive fold test and a reversal test the tilt corrected mean direction from the yueshan area 30 6 n 116 8 e which is located at a distance of 25 km from the eastern part of the tan lu fault shows a counter clockwise deflection of 67 3 with respect to north however mean directions from the nanzhang 31 5 n 111 7 e tongling 30 9 n 117 9 e and nanjing 32 0 n 118 9 e areas which are located between 75 and 90 km from the northeastern boundary of the scb indicate a northeasterly deflection in declination such as d 38 2 i 25 1 95 6 4 d 32 1 i 25 2 95 9 7 and d 34 9 i 27 8 95 5 4 corresponding to palaeopoles of 50 5 n 220 5 e dp dm 3 7 6 9 55 6 n 231 8 e dp dm 5 6 10 4 and 53 9 n 229 1 e dp dm 3 2 5 9 respectively comparison with the previously reported permo triassic palaeomagnetic results from the scb suggests that very consistent palaeopoles are observed within a stable belt up to 1800 km along the northern margin of the scb covering the changxing nanjing tongling nanzhang and emeishan areas this type of behaviour implies that an area located between 70 and 225 km from northern margin of the scb forms a stable platform where very little relative rotation has been detected however the inner region of this block especially the mesozoic fold thrust belt experienced a clockwise rotation of 11 5 5 0 to 30 3 9 2 with respect to the stable outer region later than the middle triassic areas close to the tan lu fault were subjected to post triassic counter clockwise rotation by 90 with respect to the stable tectonic belt we thus conclude that the northern stable belt of the scb suffered no oroclinal bending during its suturing with the north china block

distinguishing gases derived from oil cracking and kerogen maturation insights from laboratory pyrolysis experiments kerogen separated from an immature type i mudstone from the third section of the tertiary shahejie formation es 3 3 in the dongying depression of bohai bay basin was subjected to stepwise isothermal pyrolysis products were extracted fractionated into chemical groups and re mixed to obtain a synthetic oil with a group composition similar to reservoir oil and a pseudo kerogen composed of 83 kerogen and 17 residual soluble organic matter referred as s oil and p kerogen respectively to distinguish them from a produced reservoir oil and a real kerogen the two samples were pyrolyzed in sealed gold tubes under constant pressure 50 mpa and non isothermal heating 300 600 c conditions and their generated gases were analyzed the two gases are quite different in their chemical and isotopic composition compared with the gas derived from the p kerogen the s oil derived gas is enriched in c2 c5 hydrocarbons in its early cracking stages and c1 c3 hydrocarbons are depleted in d 13 c throughout the cracking stages the maximum wetness c2 5 c1 5 of the s oil gas and the p kerogen gas is 0 62 and 0 35 respectively and the carbon isotopic ratio differences between the two type gases can reach 10 d 13 c1 14

middle triassic paleomagnetic results from central hubei province china and their tectonic implications twenty one sites consisting of 4 to 7 samples per site were drilled from the redbeds of the middle triassic badong formation from central hubei province china these samples were subjected to progressive thermal demagnetization which revealed two components of magnetization the low temperature a component was probably acquired at a late stage of folding and is similar in direction to the late mesozoic overprint widely observed in the eastern yangtze block the high temperature b component is prefolding with dual polarity unlike the prefolding paleomagnetic directions observed in eastern sichuan western hubei and northwestern hunan the b component resolved from central hubei indicates no significant rotation with respect to other parts of the yangtze block since one of the two sampled sections is in the strongly deformed foreland fold thrust belt of the qinling fold belt this would seem to negate the possibility that the differential rotations observed in the border area between sichuan hubei hunan and guizhou are associated with the collision of the yangtze block with the north china block

primary migration by diffusion through kerogen ii hydrocarbon diffusivities in kerogen abstract laboratory experiments were performed to assess diffusion through kerogen as a mechanism of liquid hydrocarbon transport through fine grained rocks such transport can be important in primary migration within source rocks and in leakage through seals previous work with organic coated model cores has shown that hydrocarbon diffusion through organic matter networks can be substantially greater than hydrocarbon diffusion through aqueous pore space in the present work natural kerogen networks were evaluated experiments with five natural source rocks showed that kerogen increased the experimental hydrocarbon flux over the calculated hydrocarbon flux through pore water in only one of these source rocks the richest 2 2 toc carbonate sample effective diffusivities through kerogen were calculated for the two experimental hydrocarbons through this sample the results were extended with the wilke chang correlation to yield diffusivities of several other hydrocarbons through similar source rocks

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

magnetic fabric and weak deformation in sandstones of accretionary prisms of the flysch and klippen belts of the western carpathians mostly offscraping indicated abstract the magnetic fabric of sandstones in thrust sheets of the flysch belt of the western carpathians is basically sedimentary in origin in minority of specimens slightly affected by deformation which is generally very weak corresponding to the earliest deformation stage and to types i ii in terms of the magnetic fabric classification of the accretionary prisms by pares et al 1999 and aubourg et al 2004 respectively this indicates an absence of the return flow during the subduction process the deformation is weaker in the thrust sheets located at the margins of the flysch belt the danice and silesian thrust sheets in the west and the bile karpaty and oravska magura thrust sheets in the east than in the fore magura raca and bystrica thrust sheets located in the centre of the flysch belt in the danice and silesian thrust sheets the weak deformation reflects their sedimentation on the northern european slope of the flysch belt basin and relatively short transport during their thrusting over the foredeep in the bile karpaty and oravska magura thrust sheets that are located near the subduction zone the weak deformation is explained by assuming early offscraping and frontal accreting of these thrust sheets in the pieniny klippen belt the deformation of the palaeogene sandstones peri klippen palaeogene is stronger than that in the marginal thrust sheets danice silesian bile karpaty and oravska magura thrust sheets of the flysch belt but weaker than that in the central thrust sheets fore magura raca and bystrica thrust sheets of the flysch belt this is in an apparent contradiction with strong macroscopic deformation of mesozoic rocks of the pieniny klippen belt but understandable realizing that the sediments of the peri klippen palaeogene were deformed only during the neoalpine orogenies in palaeogene and neogene after the main deformation late cretaceous in the pieniny klippen belt

genetic origins of marine gases in the tazhong area of the tarim basin nw china implications from the pyrolysis of marine kerogens and crude oil abstract although abundant natural gases have been discovered in carboniferous and ordovician reservoirs in the tazhong area central tarim basin their genetic origins is still an open question because of significant variations in chemical and carbon isotopic compositions in the present study three samples representing the possible gas sources were pyrolyzed using sealed gold tubes to investigate their gas potentials and chemical and carbon isotopic signatures the samples were a crude oil a low maturity kerogen and a high maturity kerogen the results show that the chemical and isotopic compositions of gaseous pyrolysates from the three samples are quite different generally the gases from oil cracking are compositionally wetter and isotopically lighter than those from kerogen cracking at similar thermal levels the large quantities of methane experimentally observed in oil cracking gases are mainly associated with the re cracking of wet gases at very high maturity levels methane in gases derived from kerogen especially high maturity kerogen in contrary is mainly generated by the demethylation reaction the diagrams of 13c1 vs 13c2 13c3 and c2 c3 vs 13c2 13c3 along with natural gas plots have proven to be a very effective tool in the genetic interpretation of natural gases from kerogen and oil cracking in the tarim basin their combined application shows that two types of gas source can be identified in the tazhong area the first type is oil cracking gas mainly discovered in carboniferous reservoirs of the central fault horst area but also partly in ordovician reservoirs such as in the wells tz45 tz44b and tz 16 the other is kerogen cracking gas occurring largely in ordovician reservoirs especially in the north slope area the two types of gases however are mixed in many reservoirs as their generation is overlapped in time and or space this mixing process is especially obvious for methane

influence of retained bitumen in oil prone shales on the chemical and carbon isotopic compositions of natural gases implications from pyrolysis experiments abstract retained bitumen in oil prone source rocks has a significant impact on late gas generation but such influence is still poorly quantified because it is quite difficult to restore the bitumen content retained in a mature source rock in this study a suite of artificially matured samples were prepared from a moderate maturity shale sample to represent shales with different amount of retained bitumen these samples were then pyrolyzed in sealed gold tubes to investigate how the amount of retained bitumen affects the changes in chemical and carbon isotopic compositions of generated gases and the uncertainty of the evaluation of gas maturity and source identification the results indicate that the decrease in yields of hydrocarbon gases is proportional to the reduction of retained bitumen content however a progressive enrichment in heavier carbon isotope 13c of gaseous hydrocarbons occurs while less amount of bitumen is retained in shales this experimental observation partly explains the heterogeneity of methane carbon isotopes for shale gases that have identical thermal maturity levels although the amount of retained bitumen can significantly change the carbon isotopic ratios of individual gas 13c1 13c2 and 13c3 the difference in carbon isotopic composition between gaseous components such as ethane and methane 13c2 13c1 or propane and ethane 13c3 13c2 are not significantly affected and therefore these differential values can be potentially used as maturity indicators for overmature natural gases exhibiting no carbon isotopic reversals geochemical and carbon isotopic diagrams that are widely used for the identification of oil and kerogen cracking gases are also found to be affected by the retained bitumen content these results illustrate that it is necessary to take into account the retained bitumen content of shales when thermal maturity and genetic origin of natural gases derived from oil prone source rocks are being evaluated

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 2 maturity assessment abstract the thermal maturity of forty three crude oil samples collected from the cratonic portion of the tarim basin nw china has been assessed using a variety of parameters while compound ratios of diamondoids alkylated naphthalenes methyl phenanthrenes and methyl dibenzothiophenes showed consistent trends with increasing maturity a large discrepancy was observed for the maturity levels of the oils determined from these parameters equivalent vitrinite reflectance values calculated from methylphenanthrene index 1 mpi1 methyldibenzothiophene ratio mdr and methyldiamontane index mdi were in the range of 0 66 0 97 1 11 1 64 and 0 89 1 74 ro respectively while we caution that some of the discrepancy may be related to calibration problems the difference observed from different compound classes could be real reflecting the multiple charge to petroleum accumulations as the diamondoid hydrocarbons were possibly generated at later maturation stages than most aromatic hydrocarbons as diamondoids with methyl substitution on the bridgehead positions are more stable than those with substitution on other positions several compound ratios based on ethyl admantanes dimethyl admantanes trimethyl admantanes and dimethyl diamantanes also correlated with the established maturity indicators such as mdr and mdi while condensates with high maturity levels represent the late charge hydrocarbons some of them with relatively low maturity levels may be affected by evaporative fractionation processes the normal gravity oils cover a wide maturity range consistent with multiple charges and in reservoir mixing while the waxy oils with high apparent maturity levels may be the products of wax precipitation due to a later gas injection into an oil column heavy oils with low apparent maturity levels could be the residue of oil that has lost its light end

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

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high maturity shales abstract this article reviews the abnormal characteristics of shale gases natural gases produced from organic rich shales and discusses the cause of the anomalies and mechanisms for gas enrichment and depletion in high maturity organic rich shales the reported shale gas geochemical anomalies include rollover of iso alkane normal alkane ratios rollover of ethane and propane isotopic compositions abnormally light ethane and propane 13 c values as well as isotope reversals among methane ethane and propane these anomalies reflect the complex histories of gas generation and associated isotopic fractionation as well as in situ mixing and accumulation of gases generated from different precursors at different thermal maturities a model was proposed to explain the observed geochemical anomalies gas generation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso alkane normal alkane ratios and ethane and propane 13 c values normal trend simultaneous cracking of kerogen retained oil and wet gas and associated isotopic fractionation at higher maturity caused decreasing iso alkane normal alkane ratios lighter ethane and propane 13 c and corresponding conversion of carbon isotopic distribution patterns from normal through partial reversal to complete reversal relatively low oil expulsion efficiency at peak oil generation low expulsion efficiency at peak gas generation and little gas loss during post generation evolution are necessary for organic rich shales to display the observed geochemical anomalies high organic matter richness high thermal maturity high degrees of kerogen gas and oil gas conversions and late stage the stage of peak gas generation and post generation evolution closed system accounted for gas enrichment in shales loss of free gases during post generation evolution may result in gas depletion or even undersaturation total gas content lower than the gas sorption capacity in high maturity organic rich shales

geochemical characterization of gases from the mississippian barnett shale fort worth basin texas molecular and isotopic compositions of 51 gas and 7 condensate samples produced from the barnett shale were determined by gas chromatography isotope ratio mass spectrometry to investigate their origin and maturity at the time of generation additionally two hydrous pyrolysis experiments were performed to calibrate maturity values predicted for gas generation molecular and carbon and hydrogen isotopic compositions of the gases indicate that gas produced from the barnett is thermogenic to estimate generation maturity additional analyses were performed on the condensates and available core samples diamondoid indices combined with vitrinite reflectance ro measurements isotopic and molecular composition and hydrous pyrolysis experiments suggest that gas produced from the barnett shale was generated within the condensate wet gas window 1 3 2 0 ro at a later stage than hydrocarbons accumulated in shallower reservoirs the origin of the gas is thermogenic probably derived from both kerogen cracking and secondary cracking of previously generated nonmigrated hydrocarbons additionally the occurrence of a methane isotope reversal in group 1 gases corroborates that in situ oil or gas cracking is occurring in the easternmost part of the study area

multistage growth of garnet in ultrahigh pressure eclogite during continental collision in the dabie orogen constrained by trace elements and u pb ages abstract major and trace elements for garnet and other minerals and u pb ages and trace elements for zircon as inclusions in garnet or as matrix were determined on thin sections of uhp eclogite from the western part of the dabie orogen the results provide temporal constraints on growth of garnet during continental subduction zone metamorphism major elements show homogeneous distribution across garnet grains with weak or no zonation of ca and almost flat distribution patterns of mg and fe however their ree compositions display two groups of patterns relative to chondrite group i garnet is characterized by steep patterns from lree to hree whereas group ii garnet shows enrichment of mree relative to lree and hree these two groups of garnet also have different parageneses of mineral inclusions group i garnet always contains abundant inclusions of amphibole epidote allanite rutile plagioclase quartz zircon whereas group ii garnet occasionally contains inclusions of omphacite rutile apatite zircon la icpms u pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet four types of zircon were identified based on their cl images ree distributions and u pb ages types i and ii zircons are metamorphosed after protolith magmatic zircon with type i being modified by solid state recrystallization and type ii by dissolution recrystallization types iii and iv zircons are metamorphic and grew in the stability field of garnet they gave consistent 206pb 238u ages of 239 6 ma and 222 4 ma respectively in association with groups i and ii garnets this provides a temporal constraint on the two stage growth of garnet respectively predating and postdating the peak uhp metamorphic event therefore we conclude that there are two stages of zircon and garnet growth during continental collision which occurred during the final subduction from hp to uhp phase and the early exhumation from uhp to hp eclogite facies respectively

carbon isotope composition of lower palaeozoic kerogen effects of maturation abstract the carbon isotope composition of type i kerogen from a marine organic rich black shale deposit of late cambrian age the alum shale formation of southern scandinavia varies from 28 to 30 pdb the thermal maturity is rated by hi oi index t max pi and vitrinite reflectance values and published conodont alteration data the results range from immature conditions with regard to oil generation in central sweden to overmature and submetamorphic conditions in southern sweden denmark and norway in the immature areas the shale is found to be extremely hydrogen rich and oil prone hydrocarbon potential up to 85 kg per ton rock the observed 13 variations are not correlated with maturity it is therefore concluded that thermal maturation processes have no influence on the isotopic composition of a purely marine kerogen the observed range in lower palaeozoic 13 kerogen values 27 to 31 is in accordance with published results from other sedimentary basins

effects of oil expulsion and pressure on nanopore development in highly mature shale evidence from a pyrolysis study of the eocene maoming oil shale south china abstract the influence of oil expulsion efficiency on nanopore development in highly mature shale was investigated by using anhydrous pyrolysis 425 600 c on solvent extracted and non extracted shales at a pressure of 50 mpa additional pyrolysis studies were conducted using non extracted shales at pressures of 25 and 80 mpa to further characterize the impact of pressure on pore evolution at high maturity the pore structures of the original shale and relevant artificially matured samples after pyrolysis were characterized by using low pressure nitrogen and carbon dioxide adsorption techniques and gas yields during pyrolysis were measured the results show that oil expulsion efficiency can strongly influence gas generation and nanopore development in highly mature shales as bitumen remained in shales with low oil expulsion efficiency significantly promotes gaseous hydrocarbon generation and nanopore diameter 10 nm remains flat at high maturation in addition high pressure can promote the development of micropores and fine mesopores in highly mature shales

advances in research of asian geology a summary of 1 5m international geological map of asia project abstract the international geological map of asia at a 1 5 000 000 scale igma5000 is the first digital asian geological map under the standard of the commission for the geological map of the world cgmw major advances that have been achieved in compiling the map are manifested in the following understandings 1 large amounts of mesozoic volcanic rocks occurring in the eastern asian coastal area are mainly cretaceous instead of upper jurassic lower cretaceous most of the carboniferous permian volcanic rocks in central asia seem not to be arc volcanics but the product of an extensional stage the basal boundary of the meso neoproterozoic jixian section in china is not dated at 1 8 ga as defined previously but less than 1 68 ga 2 the most significant neoarchean tectono thermal events in the sino korean craton and the indian craton took place at 2 5 ga rather than at 2 7 ga the basement of the yangtze craton was finally formed at 0 75 0 8 ga which is 0 2 0 3 ga later than the greenville orogenic cycle geologically south china is identified to be an early paleozoic caledonian foldbelt the qinling belt where no oceanic basin was developed in triassic times is not an indosinian collisional orogen but a continental crust subduction one when pangea was formed indo australian gondwana had been joined to paleo asia and between them there was no oceanic basin i e no paleo tethys which continued from paleozoic to mesozoic a huge indosinian orogenic belt existed on the southern margin of paleo asia to the north of the zagros himalayas 3 asia is a composite continent consisting of three major cratons the siberian indian and arabian and three huge orogenic belts with a number of minor cratons and numerous microcontinents included the main body of the asian continent took its shape during the mesozoic the orogenic belts belong respectively to three global tectonic domains the paleo asian tethyan and pacific the small cratons such as sino korea yangtze tarim and sibumasu are thought to be affiliated to the tectonic transform zone between gondwana and siberia they had been situated on the northern margin of gondwana before the disappearance of the paleo asian ocean and were lying on the southern margin of paleo asia after the closing of the paleo asian ocean and then the opening of the tethys the fact that ophiolites in asia appear to get progressively younger in age from north to south throws light on the phanerozoic evolutionary process of the dispersion of gondwana and the accretion of asia accompanied by a southward migration of its orogenic belts

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

isotopic compositions and biomarkers in crude oils from the tarim basin oil maturity and oil mixing abstract hydrogen and carbon isotopic compositions of n alkanes as well as biomarkers in various types of oil from the tarim basin were analyzed data were used to discuss the occurrence and formation of marine oils from this old composite basin of which we have limited understanding first hydrogen and carbon compositions of n alkanes clearly classify the collected oils into two distinct groups supporting the reported oil classifications based on biomarkers group i the majority of oil samples is characterized by relative enrichment in d and n alkane d values vary from 110 to 75 group ii includes two heavy oils and d values of n alkanes range from 145 to 142 second variations in the 13c value of n alkanes 2 3 show good correlations with the maturity indices indicating that kinetic isotopic fractionation is one control on the 13c value of n alkanes in the maturation process d values of n alkanes in most oils are also evidently influenced by the oil maturity finally by combining the isotopic compositions with biomarkers of oils two distinct oil mixing processes were recognized in the oils from the tabei and tazhong uplifts in this old composite basin the former process is the mixing of oils from the same source at different maturity stages while the latter involves the mixing of oils from distinct sources at different maturity stages biodegraded oils may contribute to the total oils but their proportions are minor

different response of δd values of n alkanes isoprenoids and kerogen during thermal maturation abstract this study investigates the extent of post depositional alteration of d values of n alkyl lipids isoprenoids and kerogen isolated from a continuous 450 m core that covers the transition from thermally immature to early mature sediments in the lacustrine kissenda formation lower cretaceous gabon basin large variations in d values up to 40 for n c 17 and up to 30 for n c 29 alkanes as well as up to 10 for kerogen in closely spaced samples are evident throughout the core and remain preserved even at the bottom of the section d values of individual n alkanes show a slight overall d enrichment with depth and a general trend of increasing d values with increasing n alkane chain length characterizes all samples particularly in those below 600 m depth hydrogen isotopic compositions of kerogen samples overlap with those of n alkanes throughout the section d values of pristane and phytane are more negative than those of n c 17 alkane by as much as 120 at shallow depths but increase dramatically and approach d values of n c 17 alkane in the samples closest to the oil window integration of analytical and computational results indicates that 1 n alkanes and isoprenoids have the potential to preserve the original biological signal before the onset of oil generation 2 isomeric and structural rearrangements taking place at the beginning stages of oil generation do not influence significantly the d values of n alkanes and kerogen however these processes have a major effect on the isotopic composition of isoprenoids causing isotopic d enrichment up to 90

new mesozoic apparent polar wander path for south china tectonic consequences a new paleomagnetic study in the south sichuan basin south china block has been carried out on sediments of early triassic to middle jurassic age detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components a low temperature component component a falls close to the local present earth field direction a middle temperature component component b is found both in limestone and red beds of early triassic age in limestones of middle triassic age and finally in red beds of jurassic age the component b directions failed the fold test but show a significant increase of the ks kg ratio at 60 when a progressive stepwise unfolding is applied the directions obtained at 60 unfolding are grouped best and yield a pole position at 76 5 n 174 1 e a95 3 7 that is close to the late cretaceous pole of the south china block scb or the early tertiary pole of the eurasian plate characteristic directions are finally obtained component c for early triassic and early to middle jurassic sediments only both with reversal and fold tests they yield pole positions at 44 1 n and 217 2 e a95 4 9 and 79 n and 227 e a95 4 5 respectively the early triassic result well fits other data of the same age from the scb the revised late permian to cretaceous paleopole data provide a new mesozoic apparent polar wander path for south china the successive pole positions from late permian to early triassic indicate motion of the south china block as a whole rather than local rotations linked to important strain within the block the early to middle jurassic pole is statistically indistinguishable from the compiled north china block ncb poles and indicates that final suturation between the two blocks occurred by the end of the triassic which is coherent with the age of ultra high pressure metamorphism in the dabie sulu areas of eastern china the asian plate including the mongol central asian block ncb and scb however was probably separated from siberia by the mongol okhotsk ocean during jurassic time

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity abstract this paper reports isotopic variations in n alkanes isoprenoids and terpenoids from different source rock types as a function of maturity by analysis of these compounds in hydrous pyrolysis products and oil at high pyrolysis temperatures n alkanes isoprenoids and terpenoids in the pyrolysis products become depleted in 12 c the most marked depletion occurs in the terpenoids particularly in the c 29 c 31 hopanes these compounds are isopically very light in the immature unheated samples of the lacustrine green river shale tertiary and a guatemalan carbonate source rock there is a marked depletion in 12 c at 330 c and higher pyrolysis temperatures for these two samples moderately mature samples of the toarcian paris basin shale and a jurassic coal from the hammerfest basin showed less or no 12 c depletion fairly uniform values were recorded for the hopanes in pyrolysates of all samples heated at temperatures of 350 c and above the isotopic composition of n alkanes and isoprenoids are fairly similar to the hopanes in samples pyrolysed at the higher temperatures it appears therefore that with increasing maturity n alkanes isoprenoids and terpenoids become fairly similar in their carbon isotopic composition the isotopic compositions within the different compound groups in the different source rocks are sufficiently different for this technique to be used in source rock oil correlation

change in hydrogen isotope composition of n alkanes pristane phytane and aromatic hydrocarbons in miocene siliceous mudstones with increasing maturity abstract the variation in the hydrogen isotope composition d h ratio values of n alkanes pristane phytane and aromatic hydrocarbons with increasing maturity was investigated using a set of immature to mature miocene siliceous mudstones the d h values of aromatic hydrocarbons change at the earliest stage vitrinite reflectance r o 0 4 0 5 by way of higher values in immature rocks a dramatic increase in values is observed for pristane and phytane during the mature stage r o 0 6 0 7 attributable to hydrogen exchange with sediment porewater the decrease in the values of short chain n alkanes and the disappearance of carbon number predominance in n alkane d h values with increasing maturity is a result of the thermal cracking of aliphatic structures in bitumen and kerogen the long chain n alkanes derived from higher plant wax can maintain their initial values even in the zone of extensive oil generation r o

geochemistry of petroleum systems in the niuzhuang south slope of bohai bay basin part 3 estimating hydrocarbon expulsion from the shahejie formation abstract based on the distribution and geochemical characteristics of potential source rocks in the niuzhuang and guangbei sags and the niuzhuang south slope bohai bay basin the quantities of hydrocarbons expelled from the eocene oligocene shahejie formation were evaluated using a mass balance approach the results confirm that most of the hydrocarbons in the study area were generated and expelled from the shahejie formation source rocks in the niuzhuang sag the algae rich saline brackish water lacustrine es 4 oil shales of the shahejie formation from the bamianhe to wangjiagang region along the niuzhuang south slope exhibit excellent potential for early hydrocarbon generation but they are not the major source for proven oil reserves in the bamianhe region the source rocks for most of the known oil reserves in the study area occur in the es 4 mudstones in the deep part of the niuzhuang sag thus the commonly assumed origin of so called immature oils from source rocks shallower than the conventional oil window in this area needs revision

mesozoic intracontinental orogeny in the qinling mountains central china abstract the qinling orogenic belt has been well documented that it was formed by multiple steps of convergence and subsequent collision between the north china and south china blocks during paleozoic and late triassic times following the collision in late triassic times the whole range evolved into an intracontinental tectonic process the geological geophysical and geochronological data suggest that the intracontinental tectonic evolutionary history of the qinling orogenic belt allow deduce three stages including strike slip faulting during early jurrassic n s compressional deformation during late jurassic to early cretaceous and orogenic collapse during late cretaceous to paleogene the strike slip faulting and the infills in early jurassic along some major boundary faults show flower structures and pull apart basins related to the continued compression after late triassic collision between the south qinling belt and the south china block along the mianlue suture late jurassic to early cretaceous large scale of n s compression and overthrusting progressed outwards from inner of qinling orogen to the north china block and south china block due to the renewed southward intracontinental subduction of the north china block beneath the qinling orogenic belt and continuously northward subduction of the south china block respectively after the late jurassic early cretaceous compression and denudation the qinling orogenic belt evolved into late cretaceous to paleogene orogen collapse and depression and formed many large fault basins along the major faults

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

experimental investigation of oil generation retention and expulsion within type ii kerogen dominated marine shales insights from gold tube nonhydrous pyrolysis of barnett and woodford shales using miniature core plugs abstract although oil retention has recently emerged as a key topic of unconventional shale resource assessment oil retention and expulsion controls in organic rich shales during thermal maturation remain poorly constrained this study presents an experimental comparison of oil generation retention and expulsion in two immature type ii kerogen dominated marine shales the mississippian barnett shale and the upper devonian lower mississippian woodford shale mainly with respect to the combined effects of the organic macerals and rock fabric involved in both cases miniature core plugs drilled from the given samples were isothermally pyrolyzed at 130 to 425 c for 72 h under a confining pressure of 68 mpa during gold tube nonhydrous pyrolysis corresponding to the thermally immature early stage of the oil window the main stage of the oil window the late stage of the oil window the main stage of oil cracking to wet gas and the late stage of oil cracking yields of generated oil retained oil and expelled oil for the two studied samples were systematically quantified on the basis of mass balance calculation of measured oil and gas yields as well as rock eval analyses on pyrolyzed subsamples through the six stages of petroleum formation investigated the principal difference in oil generation was observed in the two studied samples with approximately 38 to 68 greater yields of generated oil equivalent to 130 mg oil g toco for the woodford shale when it evolved into the main and late stages of the oil window these elevated yields of generated oil for the woodford shale were compensated for by additional oil generation resulting from conversion of abundant type i kerogen like algae such as tasmanites and leiosphaeridia which lag in onset and have a shorter period of petroleum generation upon maturation as a response to the difference in oil generation oil retention was found to be significantly enhanced for the woodford shale at equivalent stages with 0 2 to 1 7 times more free oil equivalent to 24 105 mg oil g toco and 0 7 to 3 9 times more sorbed oil equivalent to 58 76 mg oil g toco being retained than that of the barnett shale although this effect was not pronounced for oil expulsion in contrast to the barnett shale relatively low expelled oil yields and expulsion efficiencies both indicate highly limited oil expulsion in the woodford shale implying that the woodford shale whose mineral composition and lithofacies are similar to those of the barnett shale may have a relatively low permeability rock fabric to prevent oil from being expelled furthermore not only significantly higher oil saturation index osi values but also a wider range of maturity at which the oil crossover effect osi 100 mg g toc occurs is expected for the woodford shale when extrapolation to a geological setting occurs these data suggest that the presence of abundant type i kerogen like algae and relatively low permeability rock fabric in the woodford shale are critical to significant oil retention during oil generation and expulsion which jointly raise the possibility of potential commercial shale oil within type ii kerogen dominated marine shales

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

hydrogen isotopic compositions of individual alkanes as a new approach to petroleum correlation case studies from the western canada sedimentary basin isotopic compositions of carbon bound hydrogen in individual n alkanes and acyclic isoprenoid alkanes from a number of crude oil samples were measured using gas chromatography thermal conversion isotope ratio mass spectrometry the precision of this technique is better than 3 for most alkanes compared to the large range of d variation among the samples up to 160 the oils were selected from major genetic oil families in the western canada sedimentary basin with source rocks ranging in age from ordovician and possibly cambrian to cretaceous the hydrogen isotopic composition of alkanes in crude oils is controlled by three factors isotopic compositions of biosynthetic precursors source water d values and postdepositional processes the inherited difference in the lipid s biosynthetic origins and or pathways is reflected by a small hydrogen isotopic variability within n alkanes but much larger differences in the d values between n alkanes and pristane phytane the shift toward lighter hydrogen isotopic compositions from paleozoic to upper cretaceous oils in the wcsb reflects a special depositional setting and or a minor contribution of terrestrial organic matter the strong influence of source water d values is demonstrated by the distinctively lower d values of lacustrine oils than marine oils and also by the high values for oils with source rocks deposited in evaporative environments thermal maturation may alter the d values of the alkanes in the oil to some extent but secondary oil migration does not appear to have had any significant impact the fact that oils derived from source rocks that could be of cambrian age still retain a strong signature of the hydrogen isotopic compositions of source organic matter and source water indicates that d values are very useful for oil source correlation and for paleoenvironmental reconstructions

simple kinetic models of petroleum formation part iii modelling an open system abstract the movement of petroleum from organic rich source beds to potential carrier beds is the net result of two processes expulsion the release of generated petroleum from kerogen within the source bed and primary migration flow of a petroleum phase through the inorganic void networks of the source bed and all other intervening fine grained rocks expulsion is by far the more important process within the source bed orgas a simple zero dimensional model of the source rock as an open system explicitly models expulsion concurrent with oil and gas generation and oi lgas cracking as a storage threshold problem oil and gas are retained adsorbed absorbed within kerogen until their concentrations exceed the respective sorptive capacity of the residual organic carbon 0 1 and 0 02 g g ck 1 the initial hydrogen index hl 0 controls the important balance between potential sorbate petroleum and sorbant residual carbon using simple geochemical measurements the initial organic carbon is divided into initial oil inert kerogen and oil and gas generative reactive kerogen components reactive kerogens are assigned to one of five global kinetic organofacies with pre determined kinetic parameter sets governing separately rates of oil and gas generation kinetic parameters governing the rates of oil cracking within the source rock are predicted from hl 0 the two most important determinants of source rock behaviour are organofacies and hl 0 at a reference constant heating rate of 2 c ma 1 the projected oil expulsion threshold for typical organofacies members increases in the order a de 100 110 120 135 c f typically expels no oil an order of magnitude increase decrease in heating rate increases decreases these thresholds by ca 15 c typical cumulative gas mass fractions in their ultimate expulsion products are a f 0 25 0 3 0 25 0 65 and 1 0 g g 1 a value of 200 mg hc g c 1 is an approximate minimum hl 0 prerequisite for oil expulsion irrespective of absolute petroleum potential p 0 as a family coals vary dramatically in their expulsion behaviour as their hl 0 ranges both above and velow this value primary migration is efficient for expulsive source rocks movement within the source bed is limited only at low p 0 a source rocks total genetic potential p 0 thickness becomes important during migration outside the source bed it potentially restricts the primary migration distance and the excess of petroleum available for secondary migration and entrapment we evaluate primary migration approximately using a small migration loss factor no more than about 10 saturation of porosity or 1 mg g 1 coupled with an understanding of fluid potential gradients which determine migration direction within the stratigraphic architecture ultimately kerogen degradation and oil to gas cracking in the source rock are the rate limiting processes in petroleum accumulation

dating of subduction and differential exhumation of uhp rocks from the central dabie complex e china constraints from microfabrics rb sr and u pb isotope systems abstract the correlation of deformation fabrics and metamorphic reactions with geochronologic data of uhp metamorphic rocks demonstrate that the multistage subduction and exhumation history of the central dabie complex requires rapid subduction and rapid initial exhumation moreover these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems thus our age data do not simply reflect a thermal cooling history in the investigated section the maximum age for uhp is given by the 244 3 ma 2 u pb age of a pre uhp titanite phenocryst that survived uhp metamorphism and subsequent tectonometamorphic events a minimum age for uhp is set by the 238 1 ma 2 238 u 206 pb mineral isochron age of titanite and cogenetic epidote these minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust in the period of ca 238 218 ma the uhp terrain records ht metamorphism local partial melting and extensive pervasive strain below the eclogite jd grt stability field exhumation was polyphase with a first phase of fast exhumation succeeded by episodes of ht metamorphism and concomitant deformation at deep mid crustal level between 238 and 218 ma slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca 209 204 ma the resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep which was the dominant deformation mechanism in quartz feldspar rocks in combination with fluid influx

effect of thermal maturation on chemical structure and nanomechanical properties of solid bitumen abstract an abundance of organic matter is present as solid bitumen in highly mature shales changes in chemical structure of solid bitumen during catagenesis could alter the way its mechanical properties evolve this study considers a series of solid bitumen samples artificially produced from the pyrolysis at 420 618 c of soluble organic matter which originated from maoming shale to examine alternations in their chemical structure nano morphological and mechanical properties during maturation solid state 13c nuclear magnetic resonance spectroscopy and elemental analysis were employed to characterize the structural changes young s modulus and microstructures were determined by atomic force microscopy the results show that both structural including c h atomic ratio and carbon aromaticity and mechanical young s modulus variations in solid bitumen exhibit two staged evolutions i e wet gas stage and dry gas stage during maturation in the wet gas stage easyro 1 85 2 64 the c h ratio and carbon aromaticity increase substantially with maturation and the solid bitumen is relatively compliant with an average young s modulus varying from 2 7 0 6 to 3 0 0 6 gpa when maturity reaches to dry gas stage easyro 2 64 4 59 structural parameters do not change greatly whereas solid bitumen becomes much stiffer with modulus rising sharply to 7 2 1 1 gpa at easyro 3 49 and further gently to 8 8 1 4 gpa at easyro 4 59 concomitantly great variations are observed on surface morphology and the degree of heterogeneity in solid bitumen is recorded to be increasingly high along thermal simulation given the strong correlation between structural parameters and young s modulus we suggest that the stiffening of solid bitumen is extensively affected by its structural condensation due to the loss of aliphatic carbons and the increase of aromaticity

role of water in hydrocarbon generation from type i kerogen in mahogany oil shale of the green river formation abstract hydrous and anhydrous closed system pyrolysis experiments were conducted on a sample of mahogany oil shale eocene green river formation containing type i kerogen to determine whether the role of water had the same effect on petroleum generation as reported for type ii kerogen in the woodford shale the experiments were conducted at 330 and 350 c for 72 h to determine the effects of water during kerogen decomposition to polar rich bitumen and subsequent bitumen decomposition to hydrocarbon rich oil the results showed that the role of water was more significant in bitumen decomposition to oil at 350 c than in kerogen decomposition to bitumen at 330 c at 350 c the hydrous experiment generated 29 more total hydrocarbon product and 33 more c 15 hydrocarbons than the anhydrous experiment this is attributed to water dissolved in the bitumen serving as a source of hydrogen to enhance thermal cracking and facilitate the expulsion of immiscible oil in the absence of water cross linking is enhanced in the confines of the rock resulting in formation of pyrobitumen and molecular hydrogen these differences are also reflected in the color and texture of the recovered rock despite confining liquid water pressure being 7 9 times greater in the hydrous experiments than the confining vapor pressure in the anhydrous experiments recovered rock from the former had a lighter color and expansion fractures parallel to the bedding fabric of the rock the absence of these open tensile fractures in the recovered rock from the anhydrous experiments indicates that water promotes net volume increase reactions like thermal cracking over net volume decrease reactions like cross linking which results in pyrobitumen the results indicate the role of water in hydrocarbon and petroleum formation from type i kerogen is significant as reported for type ii kerogen

isotopic reversals with respect to maturity trends due to mixing of primary and secondary products in source rocks abstract carbon isotopic composition 13 c reversals with respect to maturity trends have been reported for ethane and propane in barnett shale gas in the fort worth basin u s a this trend also exists in some conventional gas reservoirs such as in the ordos basin china and appalachian basin u s a fractionation during mass transport is unlikely to be a significant mechanism because its effect is minor under geological conditions it would have caused more obvious reversal for methane than for ethane and propane and mass transport can not account for the occurrence of the trend both inside and outside source rocks this paper demonstrates that the reversal may result from mixing of indigenous primary gas generated directly from kerogen and secondary gas generated from oil and condensate within source rocks and presents deconvolution results using end members derived from a kinetic isotope fractionation model the results including the wetness and mixing ratio of primary and secondary gases are consistent with geological conditions condensates may be the major precursor of the secondary gas in shale when the reversal occurs a complete trend of 13 c variations in methane and ethane with maturity is proposed that is based on the indigenous mixing mechanism

modeling of gas generation from the barnett shale fort worth basin texas the generative gas potential of the mississippian barnett shale in the fort worth basin texas was quantitatively evaluated by sealed gold tube pyrolysis kinetic parameters for gas generation and vitrinite reflectance ro changes were calculated from pyrolysis data and the results used to estimate the amount of gas generated from the barnett shale at geologic heating rates using derived kinetics for ro evolution and gas generation quantities of hydrocarbon gas generated at ro 1 1 are about 230 l t 7 4 scf t and increase to more that 5800 l t 186 scf t at ro 2 0 for a sample with an initial total organic carbon content of 5 5 and ro 0 44 the volume of shale gas generated will depend on the organic richness thickness and thermal maturity of the shale and also the amount of petroleum that is retained in the shale during migration gas that is reservoired in shales appears to be generated from the cracking of kerogen and petroleum that is retained in shales and that cracking of the retained petroleum starts by ro 1 1 this result suggests that the cracking of petroleum retained in source rocks occurs at rates that are faster than what is predicted for conventional siliciclastic and carbonate reservoirs and that contact of retained petroleum with kerogen and shale mineralogy may be a critical factor in shale gas generation shale gas systems together with overburden can be considered complete petroleum systems although the processes of petroleum migration accumulation and trap formation are different from what is defined for conventional petroleum systems

paleomagnetic constraints of the lower triassic strata in south qinling belt evidence for a discrete terrane between the north and south china blocks

chemical structure evolution of kerogen during oil generation abstract kerogen was isolated from the source rock of well l69 zhanhua depression bohai bay basin and an artificial pyrolysis experiment was carried out in a closed gold tube system with a heating rate of 2 c h products were collected at eleven temperature points every ten degrees from 350 c to 450 c the soluble organic matter and residual kerogens from the experiment were quantified by weighing furthermore the residual kerogens were analysed by element analysis x ray photoelectron spectroscopy xps and solid 13c nuclear magnetic resonance 13c nmr spectroscopy to determine the chemical structural of kerogen during oil generation combining the data from the analyses information on the elemental compositions as well as on how functional groups were connected in residues was obtained seven molecular models of initial and residual kerogens were established to trace the structure changes in kerogen with maturity the results suggest that as the temperature increases aliphatic chains and small clusters of aromatic groups break down from kerogens generating soluble hydrocarbons at temperatures of less than 390 c easy ro1 15 at higher thermal maturity a few short aliphatic carbons remain in the structure of residues the number of aromatic groups increases and their size grows with increasing maturity with condensation occurring at aromatic groups at high temperature while these residues have little potential for oil generation

effects of thermal maturation on stable organic carbon isotopes as determined by hydrous pyrolysis of woodford shale acquiring crude oils that have been expelled from the same rock unit at different levels of thermal maturation is currently not feasible in the natural system this prevents direct correlation of compositional changes between the organic matter retained in a source rock and its expelled crude oil at different levels of thermal maturation alleviation of this deficiency in studying the natural system requires the use of laboratory experiments natural generation of petroleum from amorphous type ii kerogen in the woodford shale may be simulated by hydrous pyrolysis which involves heating crushed rock in contact with water at subcritical temperatures 374 c four distinct stages of petroleum generation are observed from this type of pyrolysis 1 pre oil generation 2 incipient oil generation 3 primary oil generation and 4 post oil generation the effects of thermal maturation on the 13c values of kerogen bitumen and expelled oil like pyrolysate from the woodford shale have been studied through these four stages of petroleum generation similar to the natural system the kerogens isolated from the pyrolyzed rock showed no significant change in 13c this suggests that the 13c value of kerogens may be useful in kerogen typing and oil to source rock correlations 13c values of bitumens extracted from the pyrolyzed rock showed an initial decrease during the incipient oil generation stage followed by depletion during the primary and post oil generation stages this reversal is not favorable for geochemical correlation or maturity evaluation saturated and polar components of the bitumen show the greatest 13c variations with increasing thermal maturation the difference between the 13c of these two components gives a unidirectional trend that serves as a general indicator of thermal maturation and is referred to as the bitumen isotope index bii 13c values of the expelled pyrolysates show a unidirectional increase with increasing thermal maturation the constancy and similarity of 13c values of the aromatic components in the expelled pyrolysates and bitumens with increasing thermal maturation encourages their use in oil to oil and oil to source rock correlations isotopic type curves for expelled pyrolysates indicate that they may be useful in oil to oil correlations but have a limited use in oil to source rock correlations

kinetics of oil group type generation and expulsion an integrated application to dongying depression bohai bay basin china pyrolysis of two kerogens isolated from the e2 3s 3 3 and e2 3s 1 4 source rocks in the niuzhuang sag dongying depression bohai bay basin china was performed in a confined system the products were extracted with solvent and separated using micro column chromatography into group type fractions saturates aromatics resins and asphaltenes with the kerogen residue in each case undergoing swelling with a variety of solvents the kinetics for generation and retention of crude oil and its group type fractions from the kerogens were studied and the kinetic parameters applied to modeling generation and retention of crude oil and its fractions from the e2 3s 3 3 and e2 3s 1 4 source rocks on the basis of burial and thermal history of the niuzhuang sag the results show that the normal oil was generated at about 4 26 ma and 24 85 ma ago but expelled at about 3 96 ma and 17 46 ma ago respectively from e2 3s 3 3 and e2 3s 1 4 source rocks the current proportions of the expelled saturates aromatics and nsos are about 60 15 and 25 respectively

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence