origins of heavy oils in the erlian basin ne china abstract the erlian basin is located in the central asia mongolian fold belt between the siberian and sino korean cratons it is a mesozoic continental rift basin composed of 52 individual fault depressions the main phase of rifting took place during the early cretaceous when a series of fluvial lacustrine sediments were deposited each depression forms an independent sedimentary system and behaves as an independent petroleum system hydrocarbon source rocks are found in the upper arshan and lower tengger formations these are mainly type ii source rocks and are mainly located in oil generation window at the present day a series of oilfields and commercial oil flows have been found in the basin highlighting its good petroleum potential many of these oils are heavy six oil samples from the anan and ten from the jirgalangtu depressions have been subjected to routine geochemical analytical techniques in order to evaluate the origins the methods used include gas chromatography of the saturated and aromatic hydrocarbon fractions gas chromatography mass spectrometry of the saturated hydrocarbon fraction and stable carbon isotope analyses the trace metal elements of the biodegraded oils from the jirgalangtu depression were also analysed by atomic absorption spectroscopy two types of heavy oils primary and biodegraded were identified on the basis of these data the former includes both immature and mature heavy oils a filtering and spill process was proposed to explain the origin of primary mature heavy oils or tar mat in the anan depression the biodegraded oils from the jirgalangtu depression were ranked and classified in terms of the degree of biodegradation using a series of geochemical parameters based on the gas chromatographic concentrations and biomarker fingerprints of gas chromatography mass spectrometry of the saturated hydrocarbon fraction the relationship between oil saturation and porosity indicates that the heavy oils in the jirgalangtu depression were biodegraded after they accumulated

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

the source of highly overmature solid bitumens in the permian coral reef paleo reservoirs of the nanpanjiang depression abstract solid bitumens occur extensively in permian coral reefs of the nanpanjiang depression both potential source rocks and solid bitumens in the study area are highly overmature and have similar bulk carbon isotope values it is difficult to perform an oil source rock correlation study in this area based on only regular molecular geochemical methods and bulk carbon isotope values thus the covalently bound biomarkers released from solid bitumens and source rock kerogens by catalytic hydropyrolysis hypy together with the geological settings were taken into account in this oil source rock correlation study the distribution characteristics of covalently bound biomarkers suggest that the major source rock of the longlin paleo reservoir in the midwest of the depression solid bitumen should be the middle devonian mudstone whereas the source rock of the ziyun paleo reservoir in the north of the depression solid bitumen should be the lower permian source rock however solid bitumens in the ceheng and wangmo paleo reservoirs in the middle of the depression may be mainly sourced from the middle devonian source rock but partly from the permian source rock our bitumen source rock correlation results are also supported by the petroleum geological settings of the study area which indicate that the filling of those paleo reservoirs was controlled by the matching of hydrocarbon generation and trap formation basically the timing of hydrocarbon generation of the middle devonian source rocks matches well with the formation of permian coral reef traps in the middle and midwest portions of the depression but it is earlier than the formation of the permian coral reef trap in the north of the depression we show that our oil source rock correlation study based on covalently bound biomarkers can provide reliable information for petroleum system analysis when highly overmature strata in south china are involved

trace fossils and substrates of the terminal proterozoic cambrian transition implications for the record of early bilaterians and sediment mixing the trace fossil record is important in determining the timing of the appearance of bilaterian animals a conservative estimate puts this time at 555 million years ago the preservational potential of traces made close to the sediment water interface is crucial to detecting early benthic activity our studies on earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic which can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments overall trace fossils can provide a sound record of the onset of bilaterian benthic activity

mesozoic geology of southwestern china indosinian foreland overthrusting and subsequent deformation abstract the southwestern part of the south china block scb records triassic and subsequent deformations and is a key region that provides evidence of the post amalgamation tectonic evolution of southeast asia here we outline the tectonothermal evolution of early mesozoic orogenesis in this region using structural analysis and fission track thermochronology this region is divided into three tectonic units south to north the youjiang fold and thrust belt yftb the qianzhong massif qzm and the thin skinned thrust belt ttb of the southeastern sichuan basin all three of which record three deformation events here termed d 1 d 2 and d 3 d 1 deformation is represented by triassic top to the north thrusts f 1 and fault related folds f 1 that are indicative of n s shortening the intensity of deformation decreases toward the north the subsequent d 2 and d 3 deformations are marked by cretaceous top to the nw thrusting and fault propagation folding and cenozoic ne sw trending normal faults f 3 respectively the temperature time t t path obtained by apatite fission track modeling of samples from the yftb qzm and ttb areas provides evidence of uplift and denudation the d 1 deformation at 230 210 ma is characterized by buried thrusts within the relatively stable yftb and qzm and an increase in depth of the ttb this was coeval with the late triassic northward migration of uplift and denudation that is evidenced by a late triassic stratigraphic gap within the yftb a thin layer of coarse grained continental clastics within the qzm and a thick layer of clastic rocks within the ttb the study area records a triassic thrust system d 1 that progressively migrated northwards and represents a foreland fold and thrust belt that formed during the indochina scb collision this initial deformation was subsequently overprinted by late jurassic to cretaceous nw se thrusting d 2 and the development of cenozoic nw se extensional structures d 3

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

geochemistry of coal measure source rocks and natural gases in deep formations in songliao basin ne china abstract the natural gases developed in deep volcanic rock reservoirs of the songliao basin ne china are characterized by enriched 13 c value for methane and frequently reversal carbon isotopic distribution pattern although many researchers consider such gas type as an abiogenic origin we believe the natural gases have a biogenic origin mainly except little inorganic gases and the reversal carbon isotopic distribution pattern of gases is caused by mixing of different origin gases methane carbon isotopic values for majority samples fall in the range from 24 to 32 which is heavier than typical coal type gases in other chinese basins there are several reasons caused heavy carbon isotope of methane 1 carbon isotopic values of source kerogen are 3 5 heavier than these from other basins 2 source rocks are at extremely high maturity stage with vitrinite reflectance mostly above 3 0 3 portion of gas is derived from basement mudrock or slate with higher maturity the observation on the organic from deep formation reveals that there is a relatively high content for liptinite which reaches approximately 8 to 10 the macerals component of source rock shows that the source rocks have some ability to generate oil small portion of oil was generated from high hydrogen content macerals in coals and shales as proof by oil found in microcrack and in micropore of coal and oil bearing fluid inclusions grown in volcanic reservoir the occurrence of pyrobitumen in volcanic reservoir indicates preexisted oil had been cracked into wet gas and this kind of gas had also been found in gas pools heavy isotopic methane is derived from coal at extremely high maturity stage there may be little inorganic alkane gases in deep layers for their geochemistry and special geological setting of songliao basin artificial mixing experiments of different origins gases confirm that inorganic gas such as gas from well fs1 mixed with other end members can turn out full reversal isotopic distribution pattern based on simple calculations of 13 c 1 values it can be concluded that the deep natural gases in songliao basin are mainly derived from coal

composition and exhalation flux of gases from mud volcanoes in taiwan many mud volcanoes are distributed along the tectonic sutures in southern taiwan and can be divided into five zones based on their relative positions in different tectonic domains most active mud volcanoes are exhaling methane dominated gases nevertheless some gases show unusual carbon dioxide dominated and or nitrogen excess compositions this implies that there are multiple sources for the gas compositions of mud volcanoes in taiwan

geochemistry origin and accumulation of petroleum in the eocene wenchang formation reservoirs in pearl river mouth basin south china sea a case study of hz25 7 oil field abstract the pearl river mouth basin in the south china sea has accumulated 2 km of eocene sediments in its deep basin and has become the exploration focus due to the recent discoveries of the hz25 7 oil field in the eocene wenchang e 2 w formation in this study the geochemical characteristics of potential source rocks and petroleum in the hz25 7 oil field are investigated and the possible origins and accumulation models developed the analytical results reveal two sets of potential source rocks e 2 w and enping e 2 e formations developed in the study area the semi deep to deep lacustrine e 2 w source rocks are characterized by relatively low c 29 steranes low c 19 c 23 tricyclic terpane 24 tetracyclic terpane c 30 hopane trans trans trans bicadinane t c 30 hopane most 30 4 methyl sterane c 29 sterane 0 2 ratios in contrast the shallow lacustrine and deltaic swamp plain e 2 e source rocks are characterized by relatively high c 29 steranes high c 19 c 23 tricyclic terpane 0 6 high c 24 tetracyclic terpane c 30 hopane 0 1 variable yet overall high t c 30 hopane and low c 30 4 methyl sterane c 29 sterane 19 c 23 tricyclic terpane ratios mean value 0 39 low c 24 tetracyclic terpane c 30 hopane ratios mean value 0 07 high c 30 4 methyl sterane c 29 sterane ratios mean value 1 14 and relatively high c 27 regular sterane content of petroleum in the hz25 7 oil field indicate that the petroleum most likely originated from the e 2 w formation mudstone in the huizhou depression one stage of continuous charging is identified in the hz25 7 oil field oil injection is from 16 ma to present and peak filling occurs after 12 ma thin sandstone beds with relatively good connectivity and physical properties porosity and permeability in the e 2 w formation are favorable conduits for the lateral migration of petroleum this petroleum accumulation pattern implies that the e 2 w formation on the western and southern margins of the huizhou depression are favorable for petroleum accumulation because they are located in a migration pathway thus exploration should focus in these areas in the future

petroleum generation and charge history of the northern dongying depression bohai bay basin china insight from integrated fluid inclusion analysis and basin modelling abstract the petroleum generation and charge history of the northern dongying depression bohai bay basin was investigated using an integrated fluid inclusion analysis workflow and geohistory modelling one and two dimensional basin modelling was performed to unravel the oil generation history of the eocene shahejie formation es3 and es4 source rocks based on the reconstruction of the burial thermal and maturity history calibration of the model with thermal maturity and borehole temperature data using a rift basin heat flow model indicates that the upper interval of the es4 source rocks began to generate oil at around 35 ma reached a maturity level of 0 7 ro at 31 30 ma and a peak hydrocarbon generation at 24 23 ma the lower interval of the es3 source rocks began to generate oil at around 33 32 ma and reached a maturity of 0 7 ro at about 27 26 ma oil generation from the lower es3 and upper es4 source rocks occurred in three phases with the first phase from approximately 30 20 ma the second phase from approximately 20 5 ma and the third phase from 5 ma to the present day the first and third phases were the two predominant phases of intense oil generation samples from the es3 and es4 reservoir intervals in 12 wells at depth intervals between 2677 7 m and 4323 0 m were investigated using an integrated fluid inclusion workflow including petrography fluorescence spectroscopy and microthermometry to determine the petroleum charge history in the northern dongying depression abundant oil inclusions with a range of fluorescence colours from near yellow to near blue were observed and were interpreted to represent two episodes of hydrocarbon charge based on the fluid inclusion petrography fluorescence spectroscopy and microthermometry data two episodes of oil charge were determined at 24 20 ma and 4 3 ma respectively with the second episode being the predominant period for the oil accumulation in the northern dongying depression the oil charge occurred during or immediately after the modelled intense oil generation and coincided with a regional uplift and a rapid subsidence suggesting that the hydrocarbon migration from the already overpressured source rocks may have been triggered by the regional uplift and rapid subsidence the expelled oil was then charged to the already established traps in the northern dongying depression the proximal locations of the reservoirs to the generative kitchens and the short oil migration distance facilitate the intimate relationship between oil generation migration and accumulation

the age and emplacement of obducted oceanic crust in the urals from smnd and rbsr systematics the urals contain a 2000 km belt of mafic ultramafic bodies the sm nd and rb sr systematics of two of these bodies the kempersai massif in the south ural mountains and the voykar syninsky ophiolite complex in the polar ural mountains have been examined these data confirm the hypothesis that these bodies represent fragments of pre collision oceanic crust and establish constraints on the nature and timing of events in the uralian orogeny two kempersai gabbros define sm nd internal isochrons of 397 20 my and 396 33 my with e nd t 8 7 0 6 and 8 4 1 3 respectively whole rock samples of pillow basalt diabase gabbros troctolite and a metasediment give sm nd values which lie on this isochron indicating that these rocks are genetically related and have an igneous crystallization age of 397 my whole rock samples of voykar syninsky diabase gabbros and clinopyroxenite give sm nd values which lie on or within 1 e unit of this isochron indicating an age and e nd t virtually identical to those of kempersai e nd t for the kempersai and voykar syninsky mafic samples range from 7 3 to 9 0 with an average value of 8 4 this indicates that the urals ophiolites are derived from an ancient depleted mantle source and are most plausibly pieces of the oceanic crust and lithosphere the fact that a metasediment has the same e nd 397 my as the other samples indicates derivation from an oceanic source with negligible continental input e nd t for the massifs is 1 5 e units lower than the average for modern morbs this may be due to the differential evolution of the morb source over the past 397 my and in conjunction with data for other ophiolites and mesozoic morb suggests that over the past 750 my the source for morb has evolved at a rate less than or equal to its rate of evolution averaged over the age of the earth initial 87 sr 86 sr ratios are highly variable ranging from e sr t 25 2 for a gabbro to 70 3 for a highly serpentinized harzburgite this reflects the effects of seawater alteration which is particularly strong on ultrabasic rocks we conclude that the long belt of mafic ultramafic rocks in the urals which includes the kempersai and voykar syninsky massifs represents segments of siluro devonian oceanic crust our igneous age for kempersai in conjunction with other age constraints suggest that these segments of oceanic crust formed at least 80 my before the collision that produced the urals

a possible lower crustal flow channel in the middle urals based on reflection seismic data a possible lower crustal flow channel in the middle urals based on reflection seismic data

single zircon evaporation ages and rb sr dating of four major variscan batholiths of the urals abstract using single zircon stepwise evaporation 207pb 206pb and whole rock rb sr methods we have dated the main facies of four variscan batholiths of the urals syrostan verkhisetsk murzinka and dzhabyk from different geodynamic environments syrostan and verkhisetsk are subduction related batholiths of the accreted terrains in the west closely related to the main uralian fault syrostan is composed of metaluminous epidote bearing synkinematic gabbros diorites and granodiorites 87sr 86srinit 0 7029 0 7035 dated at 333 3 ma and postkinematic granites 87sr 86srinit 0 7035 0 7038 dated at 327 2 ma whose zircons occasionally have cores with an age of 1800 ma the presence of these relict zircon cores probably indicates the contribution of russian platform materials to subduction related magmas verkhisetsk 250 km north of syrostan comprises deformed amphibole and epidote bearing granodiorites 87sr 86srinit 0 7042 0 7044 dated at 318 3 ma and undeformed granites 87sr 86srinit 0 7044 that record a long history of recurrent partial melting and crystallization from 310 ma to 275 ma murzinka and dzhabyk are continental type batholiths in the east uralian zone dzhabyk in the southern urals postdates the kartali seismic reflector and is composed of several coeval intrusions of metaluminous monzonites 87sr 86srinit 0 7049 and peraluminous granitoids 87sr 86srinit 0 7043 dated at 291 4 ma murzinka 600 km north of dzhabyk also comprises two coeval intrusions of peraluminous granites with different chemical and isotopic compositions 87sr 86srinit 0 7045 and 0 7093 dated at 254 5 ma these data indicate that the age of cessation of movements along the main uralian fault and the transition from subduction to continent continent collision migrated northwards the age of granite magmatism also migrated gradually northwards and eastwards suggesting an oblique convergence with a se movement of the converging continent so that the closure of the uralian palaeo ocean started in the south and then migrated progressively northwards

polygonal faults and their implications for hydrocarbon reservoirs in the southern qiongdongnan basin south china sea the polygonal faults were identified in qiongdongnan basin south china sea by using the technique of time coherent slice and horizon flattening of high resolution 3d seismic data these polygonal faults occur in three tiers of the upper meishan formation and the huangliu formation the faults have lengths of 150 1500 m spacings of 50 3000 m throws of 10 40 m and dips of 50 90 degrees tectonic evolution in the qiongdongnan basin can be divided into a rifted stage and a post rifted stage tectonic faults are widely distributed in the rifted sequences but are not well developed in the post rifted stage few faults in the post rifted sequences might suggest the absence of a migration pathway for hydrocarbon or other fluids however the existence of polygonal faults in the post rifted sequences can serve as the pathway and promote the hydrocarbon migration and accumulation in the qiongdongnan basin during the post rifted stage c 2010 elsevier ltd all rights reserved

gas seepage from tokamachi mud volcanoes onshore niigata basin japan origin post genetic alterations and ch4 co2 fluxes abstract methane and co 2 emissions from the two most active mud volcanoes in central japan murono and kamou tokamachi city niigata basin were measured in from both craters or vents macro seepage and invisible exhalation from the soil mini and microseepage molecular and isotopic compositions of the released gases were also determined gas is thermogenic 13 c ch4 from 32 9 to 36 2 likely associated with oil and enrichments of 13 c in co 2 13 c co2 up to 28 3 and propane 13 c c3h8 up to 8 6 suggest subsurface petroleum biodegradation gas source and post genetic alteration processes did not change from 2004 to 2010 methane flux ranged within the orders of magnitude of 10 1 10 4 g m 2 d 1 in macro seeps and up to 446 g m 2 d 1 from diffuse seepage positive ch 4 fluxes from dry soil were widespread throughout the investigated areas total ch 4 emission from murono and kamou were estimated to be at least 20 and 3 7 ton a 1 respectively of which more than half was from invisible seepage surrounding the mud volcano vents at the macro seeps co 2 fluxes were directly proportional to ch 4 fluxes and the volumetric ratios between ch 4 flux and co 2 flux were similar to the compositional ch 4 co 2 volume ratio macro seep flux data in addition to those of other 13 mud volcanoes supported the hypothesis that molecular fractionation increase of the bernard ratio c 1 c 2 c 3 is inversely proportional to gas migration fluxes the ch 4 emission factor total measured output divided by investigated seepage area was similar to that derived in other mud volcanoes of the same size and activity the updated global emission factor data set now including 27 mud volcanoes from different countries suggests that previous estimates of global ch 4 emission from mud volcanoes may be significantly underestimated

a concentration of exceptionally well preserved large tubular foraminifera in the eocene zumaya flysch northern spain a concentration of exceptionally well preserved large tubular foraminifera occurs on a specimen from the sole of a sandstone bed from the eocene zumaya flysch the foraminifera were probably washed out from the sediment by a weak bottom current orientated and concentrated in a pre existing depression on the ocean floor fortunately subsequently deposited turbidite sand which forms the host bed only disturbed the fossils to an infinitesimal degree

differentiation of strongly peraluminous perphosphorus granites the pedrobernardo pluton central spain the pedrobernardo pluton 295 2 ma initial 87sr 86sr 0 7122 is a layered sheet like granite body with distinct vertical zoning biotite muscovite bearing granites in the lower zone muscovite biotite leucogranites in the middle zone and muscovite bearing aplites and pegmatites in the upper zone the magma was anatectically generated intruded almost free of restite and evolved as a closed system by crystallization in situ geochemical modelling indicates that the lower and middle zones followed an equilibrium crystallization path with no separation between solids and melt and the upper zone followed a fractional crystallization path magmatic fractionation produced leucocratic differentiates with high p concentrations and a p ca ratio higher than in apatite the excess p is located in plagioclase and k feldspar monazite apatite fractionation in the lower and middle zones produced extreme depletions in th u y and the rees and chondrite normalized ree patterns with low lrees hrees and a slight discontinuity between nd and sm about 90 of the th u y and rees reside in accessory minerals especially in monazite biotite and muscovite have very low ree contents with small positive or no eu anomalies k feldspar and plagioclase in both the lower and upper zones have moderate ree contents with positive eu anomalies feldspars from the upper zone in contrast have negative eu anomalies and have very low ree contents compared to available data from literature dsr ba th u y reesbio mineral are lower by one to three orders of magnitude and dreesplag kfsp are also lower by one order of magnitude these huge discrepancies are attributed not only to contamination with ree rich inclusions in mineral concentrates used in previous studies but to real differences in geochemical behavior of trace elements in peraluminous systems with respect to subaluminous systems biotite melt and probably k feldspar melt distribution coefficients for th u y and the rees in peraluminous magmas must be much lower than previously considered

reappraisal of the fossil methane budget and related emission from geologic sources converging evidence from new top down and bottom up estimates of fossil radiocarbon free methane emissions indicates that natural geologic sources account for a substantial component of the atmospheric methane budget comparing emission estimates based on atmospheric 14 ch 4 radiomethane with geologic emissions from seepage including terrestrial macroseeps microseepage marine seeps and geothermal volcanic emissions from the earth s crust shows that such geo ch 4 sources can be conservatively estimated at 53 11 tg yr 1 globally this makes geo ch 4 second in importance to wetlands as a natural methane source such a new appraisal can easily be accommodated within the uncertainty of the global methane budget as recently compiled and recognizes the importance of geophysical out gassing of methane generated within the lithosphere we propose a new coherent contemporary budget in which 30 5 based on atmospheric radiomethane measurements of the global source of 582 87 tg yr 1 has fossil origin both natural and anthropogenic

the post collisional stabilization of thickened crust in the southern urals abstract the urals are characterized by a depression of the moho to a depth of 57 km this structure is interpreted as a relic orogenic root which has been conserved because no significant post collisional processes occurred however there is evidence that voluminous post collisional magmatism affected the lower crust in this paper we use thermal finite element models to quantify the influence of the post collisional magmatism on the stabilization of the root we show that at least 70 of the heat producing elements migrated in granitic melts from the lower crust to the upper crust as a result the crustal heat flow reduced and the lithosphere could stabilize at a thickness of 180 km furthermore we propose that a granulite metamorphic event during the thermal relaxation of the collision zone prevented the 57 km thick crust from delamination these results strongly indicate that post collisional processes were necessary for the stabilization of the uralian crust and lithosphere

the root of the urals evidence from wide angle reflection seismics abstract the existence of a crustal root beneath the urals which would deflect the position of the moho by some 20 km is still largely controversial a french russian project carried out a wide angle reflection seismic experiment across the middle urals to image the moho topography along a 175 km profile running approximately east west north of ekaterinburg new data show a 6 km moho deflection beneath the central part of the orogen the moho reflectivity is variable along the section with very sharp reflections beneath the russian platform 45 km depth and fainter attenuated signals in the root zone 51 km depth even if this crustal root is not as thick as indicated by some previous speculations it makes the urals the only palaeozoic orogen in the world to show such a peculiarity a major ultramafic overload in the upper crust would partly balance the crustal root in accordance with isostatic equilibrium we finally postulate that sharp wide angle reflections from the moho can be considered indicative of a layered lower crust this would apply to the russian platform which maybe gained this structuring during the ordovician extensional regime

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

the late paleozoic porphyry epithermal spectrum of the birgilda tomino ore cluster in the south urals russia abstract the birgilda tomino ore cluster in the east uralian zone south urals russia hosts a variety of late paleozoic porphyry copper deposits birgilda tomino kalinovskoe etc high and low sulfidation epithermal deposits bereznyakovskoe michurino and skarn related base metal mineralization biksizak in carbonate rocks the deposits are related to quartz diorite and andesite porphyry intrusions of the k na calc alkaline series associated to a subduction related volcanic arc we report microprobe analyses of ore minerals tetrahedrite tennantite sphalerite bi tellurides and sulfosalts au and ag tellurides as well as fluid inclusion data and mineral geothermometry on the basis of these data we propose that the birgilda tomino ore cluster represents a porphyry epithermal continuum with a vertical extent of about 2 3 km controlled by temperature decreases and fs2 and fte2 increase from deeper to shallow levels

tectonic and stratigraphic controls of hydrocarbon systems in the ordos basin a multicycle cratonic basin in central china the ordos basin is the oldest and still an important hydrocarbon province in central china it is a typical cratonic basin developed on the archean granulites and lower proterozoic greenschists of the north china block the development of the ordos basin during the paleozoic mesozoic can be divided into three evolutionary stages cambrian early ordovician cratonic basin with divergent margins middle ordovician middle triassic cratonic basin with convergent margins and late triassic early cretaceous intraplate remnant cratonic basin two hydrocarbon systems are present in the basin the paleozoic gas and mesozoic oil systems in the paleozoic gas system the lower ordovician marine carbonates and pennsylvanian lower permian coal measures serve as source rocks the lower ordovician karst modified dolomites and pennsylvanian bauxitic mudstones form a significant reservoir seal association and the pennsylvanian lower permian deltaic sandstones and upper permian lacustrine mudstones form another effective reservoir seal association in the mesozoic oil system the upper triassic lacustrine mudstones are mature source rocks the upper triassic deltaic sandstones and overlying shallow lacustrine and swamp mudstones form a reservoir seal association and the lower jurassic fluvial sandstones and overlying shallow lacustrine and swamp mudstones form another reservoir seal association in both hydrocarbon systems the stratigraphic variations provide the principal traps the ordos basin is characterized by a stable tectonic setting that controlled the distribution of depositional systems and the development of erosional surfaces and ultimately governed the distribution of oil and gas fields and trap types

microseepage of methane to the atmosphere from the dawanqi oil gas field tarim basin china the microseepage of natural gas from subsurface hydrocarbon reservoirs is a widespread process in petroleum basins on a global scale microseepage represents an important natural source of atmospheric methane ch4 to date microseepage ch4 flux data have been obtained from 20 petroleum systems in north america europe and asia while the seasonal variations of gas flux due to soil methanotrophic activity are known the role of geological factors in controlling gas fluxes has been poorly investigated here we present new microseepage data from the dawanqi oil gas field located within the tarim basin china a petroleum system characterized by intense faulting and shallow 700 m reservoirs we measured ch4 fluxes from the ground at 51 sites along three transects by using a closed chamber connected to a portable gas sensor using off axis integrated cavity output spectroscopy our results indicate that the highest ch4 fluxes occur over faults and or shallow reservoirs especially those that were not developed and that have higher fluid pressures microseeping ch4 is thermogenic like that occurring within the dawanqi reservoirs as demonstrated by 13c enrichment 13c from 46 3 to 30 7 in the chamber mean and range microseepage values 17 mg m 2d 1 from 1 4 to 330 mg m 2d 1 are similar to those reported for other petroleum fields with active tectonics our results confirm that dry soil over petroleum fields can be a net source of atmospheric ch4 and its flux is primarily controlled by faulting and reservoir depth and pressure these factors shall be considered in global bottom up seepage emission estimates

a submarine canyon as the cause of a mud volcano liuchieuyu island in taiwan abstract in this paper based on 3 5 khz uniboom and conventional seismic data we propose a model for the creation of liuchieuyu island a near shore mud volcano off the southwestern coast of taiwan in support of this model we also discuss the relationship between a nearby submarine canyon kaoping submarine canyon and the mud diapirs and mud volcanoes in the region seismic data suggest that liuchieuyu was originally a mud diapir with a thick continuous accumulation of sediments on its upper surface this sediment load or overburden prevented unconfined growth of the underlying mud diapir while simultaneously acting as a seal that trapped gas and formed a high pressure zone however kaoping paleo canyon eroded these overlying sediments and the overburdening pressure was reduced in consequence the mud diapir rose up to the sea floor to become a mud volcano the uppermost part of which is present day liuchieuyu island the formation of this island in the path of the paleo canyon also diverted the channel of the canyon to its present location

a bathysiphon foraminifera shell bed from the cretaceous of northern california usa example of a parautochthonous macro skeletal deposit in deep ocean turbidites abstract shell beds consisting of concentrations of minimally transported slightly damaged skeletal remains of indigenous organisms comparable to bedded shelly accumulations of certain shallow marine environments have rarely been reported from truly deep ocean turbidites the general expectation is that shelly accumulations when they do occur ought to be derived from upslope sources and many kilometers away from the site of deposition a cretaceous thin bedded turbidite in the franciscan complex of northern california however hosts a concentration of large specimens of the giant foraminiferan bathysiphon aaltoi reflecting localized transportation and deposition in the original life habitat the tests were derived from a densely populated thicket of the bathysiphonid probably located only a few metres 10s of metres away decimated by a turbidity current that either overflowed an active submarine fan channel or spread outward from a suprafan lobe as such this unusual bathysiphonid rich deposit can be viewed as a kind of deep ocean level bottom shell bed

the sedimentary and tectonic evolution of the yinggehai song hong basin and the southern hainan margin south china sea implications for tibetan uplift and monsoon intensification 1 the yinggehai song hong basin is one of the world s largest pull apart basins lying along the trace of the red river fault zone in the south china sea south of hainan island this basin crosscuts the rifted margin of the northern south china sea in this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins the basins started to open after similar to 45 ma especially after similar to 34 ma the yinggehai basin was folded and inverted in the middle miocene after 21 ma in the north and 14 ma in the south before rapidly subsiding again after similar to 5 ma because of continued tectonism this subsidence has caused shale diapirism especially driven by associated sedimentation in the late pliocene 2 6 2 0 ma extension along the adjacent south hainan margin shows preferential lower crustal extension suggestive of lower crustal flow increasing toward the continent ocean transition during breakup sediment supply is reconstructed to peak in the middle miocene then falls between 14 and 10 3 ma to reach a low in the late miocene however rates rose again in the pliocene pleistocene the red river sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 ma or with large scale river capture away from the red river after similar to 10 ma both lines of evidence point to major uplift in the red river drainage being middle miocene or older the recent preindustrial red river carried much more sediment than the average pleistocene accumulation rate indicating modest sediment buffering onshore at least in recent geologic time

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

hydrocarbon and ore bearing basinal fluids a possible link between gold mineralization and hydrocarbon accumulation in the youjiang basin south china the youjiang basin which flanks the southwest edge of the yangtze craton in south china contains many carlin type gold deposits and abundant paleo oil reservoirs the gold deposits and paleo oil reservoirs are restricted to the same tectonic units commonly at the basinal margins and within the intrabasinal isolated platforms and or bioherms the gold deposits are hosted by permian to triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz pyrite stage i a main stage of auriferous quartz arsenian pyrite arsenopyrite marcasite stage ii and a late stage of quartz calcite realgar orpiment native arsenic stibnite cinnabar dolomite stage iii bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks particularly close to high grade ores but is absent in barren sedimentary rocks bitumen dispersed in the mineralized rocks is closely associated and or intergrown with the main stage jasperoidal quartz arsenian pyrite and arsenopyrite bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages ii and iii mineral assemblages these observations suggest an intimate relationship between bitumen precipitation and gold mineralization in the paleo petroleum reservoirs that typically occur in permian reef limestones bitumen is most commonly observed in open spaces either alone or associated with calcite where bitumen occurs with calcite it is typically concentrated along pore vein centers as well as along the wall of pores and fractures indicating approximately coeval precipitation in the gold deposits aqueous fluid inclusions are dominant in the early stage barren quartz veins stage i with a homogenization temperature range typically of 230 c to 270 c and a salinity range of 2 6 to 7 2 wt nacl eq fluid inclusions in the main and late stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon and co2 rich inclusions the presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase aqueous inclusions in the main stage quartz associated with gold mineralization stage ii typically have a homogenization temperature range of 200 230 c and a modal salinity around 5 3 wt nacl eq homogenization temperatures and salinities of aqueous inclusions in the late stage drusy quartz and calcite stage iii typically range from 120 c to 160 c and from 2 0 to 5 6 wt nacl eq respectively in the paleo oil reservoirs aqueous fluid inclusions with an average homogenization temperature of 80 c are dominant in early diagenetic calcite fluid inclusions in late diagenetic pore and fissure filling calcite associated with bitumen are dominated by liquid c2h6 vapor ch4 ch4 h2o and aqueous inclusions with a typical homogenization temperature range of 90 c to 180 c and a salinity range of 2 8 wt nacl eq it is suggested that the hydrocarbons may have been trapped at relatively low temperatures while the formation of gold deposits could have occurred under a wider and higher range of temperatures the timing of gold mineralization in the youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits among the limited isotopic data the rb sr date of 206 12 ma for au bearing hydrothermal sericite at jinya as well as the re os date of 193 13 ma on auriferous arsenian pyrite and 40ar 39ar date of 194 6 2 ma on vein filling sericite at lannigou may provide the most reliable age constraints on gold mineralization this age range is comparable with the estimated petroleum charging age range of 238 185 ma and the sm nd date of 182 21 ma for the pore and fissure filling calcite associated with bitumen at the shitouzhai paleo oil reservoir corresponding to the late indosinian to early yanshanian orogenies in south china the close association of carlin type gold deposits and paleo oil reservoirs the paragenetic coexistence of bitumens with ore stage minerals the presence of abundant hydrocarbons in the ore fluids and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated migrated and precipitated along with the hydrocarbons in an immiscible gold and hydrocarbon bearing basinal fluid system

polyorogenic deformation history recognized at very shallow structural levels the case of the antola unit northern apennine italy abstract the antola unit is a ligurian unit occurring at the top of the nappe pile of the northern apennines italy structural analysis indicates that the antola unit has been involved in a complex polyphase deformation history developed at a shallow structural level the successive deformation phases belong to both the alpine and apenninic orogenic cycles therefore the antola unit is a good example of a polyorogenic tectonic unit deformed at a very shallow structural level the older part of the structural evolution is assigned to the meso alpine tectonic stage and consists of two deformation phases d1 and d2 characterized by opposite tectonic transport directions the d1 and d2 structures were sealed by the middle eocene miocene deposits of the tertiary piedmont basin the younger part of the deformation history instead can be referred to the northern apennines tectonics and consists of two phases d3 and d4 both involving the tertiary piedmont basin succession the structural data indicate that the d1 deformations developed in mostly unlithified rocks and that these structures can be related to the westward emplacement of the antola unit in the alpine orogenic wedge the d2 phase is characterized by folds and low angle extensional faults and is referable to a gravity driven deformation both the d3 and d4 phases characterized by folds and thrusts are due to the compressive regime related to the development of the northern apennines

sulige field in the ordos basin geological setting field discovery and tight gas reservoirs abstract the sulige gas field in central ordos basin is one of the largest gas fields in china with about 860 7 billion cubic meters bcm of proven gas reserve this field covers an area of about 500 km 2 with largely tight clastic reservoirs the upper carboniferous lower permian deltaic sandstones and upper permian lacustrine mudstones deposited in a stable cratonic tectonic setting form an effective petroleum system that is favorable for gas accumulation in large scale stratigraphic lithologic traps based on wire line log response drilling stem test and core data a generalized classification system was developed for the gas reservoirs of the sulige field thus type i and ii rocks are capable of gas production without natural and or artificial fracturing type iii rocks are capable of gas production after artificial fracturing and type iv rocks are too tight to produce at commercial rates even with artificial fracturing the discovery of the sulige field not only adds large gas reserve in the ordos basin but strongly indicates the overlook potential of both tight sandstone reservoir rock and relatively porous and permeable sweet spots in it

a geochemical study on mud volcanoes in the junggar basin china abstract a comprehensive study was performed to characterize for the first time the mud water and gases released from onshore mud volcanoes located in the southern margin of the junggar basin northwestern china chemical compositions of mud along with the geology of the basin suggest that a source of the mud is mesozoic or cenozoic shale oxygen and h isotope compositions of the released water suggest a local meteoric origin combined with the positive eu anomalies of the water a large 18 o shift of the water suggests extensive interaction with rocks gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons and the high 13 c values 20 vpdb for co 2 gases and dissolved carbonate in muddy water suggest secondary methanogenesis with co 2 reduction after oil biodegradation the enrichments of eu and 18 o in water and the low thermal gradient of the area suggest that the water rock interactions possibly occur deeper than 3670 200 m on the other hand considering the relationship to the petroleum reservoir around the mud volcanoes the depth of the gases can be derived from about 3600 m a depth that is greater than that generally estimated for reservoirs whose gas is characterized by 13 c enriched co 2 oil biodegradation with co 2 reduction likely occurs at a shallower depth along the seepage system of the mud volcano the results contribute to the worldwide data set of gas genesis in mud volcanoes moreover they further support the concept that most terrestrial mud volcanoes release thermogenic gas produced in very deep sediments and may be early indicators of oil biodegradation an important problem in the petroleum industry

gas hydrate and associated free gas in the dongsha area of northern south china sea abstract a large ongoing cold seep named jiulong methane reef was discovered in the dongsha area of northern south china sea scs in 2004 to understand links among seafloor methane seepage subsurface gas hydrate origin of gas source and structural styles controlling the formation and accumulation of gas hydrate high resolution multi channel seismic data were interpreted in this region bottom simulating reflectors bsrs known as the base of gas hydrate stability zone were identified and mapped additionally seismic attributes interval velocity model and acoustic p wave impedance were obtained to better delineate the spatial distribution of gas hydrate hydrate bearing sediments are characterized by weak reflection i e blanking zone above bsrs high velocities and relatively small acoustic impedance differences while enhanced reflection low frequency and low velocities beneath bsrs suggest the presence of underlying free gas beneath solid hydrate a number of faults mud diapirs and a possible submarine landslide were observed in seismic profiles both faults and mud diapirs are useful conduits allowing methane gas to migrate upward and linking free gas zones beneath bsrs hydrate zones and seafloor methane seepage a submarine landslide with a length scale of 10 km is observed in a seismic profile the landslide may trigger dissociation of gas hydrate accounting for the occurrence of jiulong methane seepage seismic observations and estimated heat flow coupled with geochemical analyses suggest that gas source of methane in this region probably originates from mixture of biogenic and thermogenic methane this study would not only help to understand gas hydrate system in the dongsha area but also provide insights to geological hazards associated with the dissociation of gas hydrate

data processing and recognition of seepage and microseepage anomalies of acid extractable hydrocarbons in the south slope of the dongying depression eastern china abstract the successful use of near surface anomalous signals exhibited in all petroliferous basins must lead to great development in petroleum exploration the elementary equations and methods for objective and accurate separation of anomalous signals from background signals are the most critical requirements in these surface geochemical surveys therefore we have worked up a series of fundamental equations and methods for eliminating surface interference and influences of geological conditions on anomaly intensities and recognizing multi type anomalies of uni and multi variates by using statistics fractal geometry wavelet analysis and artificial neural networks these equations illustrate that the threshold i e the boundary between background and micro seepage anomalies is determined by means standard deviations and prior probabilities of background and micro seepage anomalies when multi normality is met and that the traditional equation for the threshold background mean plus one or two standard deviations is not correct in the face of many new equations and methods it is important how to comprehensively use them to resolve the problems in geochemical surveys a geochemical survey in the south slope of the dongying depression eastern china provides a good opportunity to address this issue the anomalies obtained with the traditional equation do not reflect oil gas fields this study reveals three problems in the previous data processing and anomaly recognition 1 the surface interference caused by the variation of soil composition was not identified and removed 2 the influence of caprock thickness on anomaly intensities was not revealed and eliminated and 3 the micro seepage related to oil gas pools and seepage related to faults anomalies were not separated and correctly recognized furthermore the interaction of these problems makes the data processing and anomaly recognition more difficult to ensure effective elimination of the surface interference and caprock thickness influence and correct recognition of uni variate anomalies we iteratively applied the wavelet analysis based methods to eliminate the surface interference and caprock thickness influence and statistical methods with new fundamental equations to recognize uni variate anomalies of seepage and microseepage and then multi variate anomalies of seepage and microseepage were recognized with our fundamental equations and the corresponding methods in the results the seepage anomalies display a string bead shaped pattern and are distributed along faults and the micro seepage anomalies are ring shaped and coincide with oil gas pools sandbodies or traps therefore reprocessing of geochemical data with our fundamental equations and methods combined in an iterative manner can resolve the complicated problems in data processing and anomaly recognition and thus greatly improve the predictive capability of surface geochemical survey

dongfang 1 1 gas field in the mud diapir belt of the yinggehai basin south china sea abstract the dongfang 1 1 df1 1 mud diapir field is the largest gas field discovered in the yinggehai basin up to now with a proven gas reserve of 99 68 10 9 m 3 the gas pay zones are within the pliocene fine sandstones and siltstones of the littoral and sand bar facies with normal pressure regimes the gases include variable amounts of co 2 and n 2 in addition to hydrocarbon gases the hydrocarbon gases with the exception of biogenic methane and nitrogen originated from the marine source rocks within the overpressured lower to middle miocene strata while co 2 is most likely the product of thermal decomposition of the tertiary calcareous mudstones and pre tertiary carbonates through relatively long distance vertical migration the field has received at least four phases of natural gas charging occurring relatively late most likely during the post pliocene himalayan orogeny diapiric faults act as the main conduit for upward movement of fluids released from the deep high pressure system with abnormally high pressures providing the significant driving force for gas migration because of the episodic nature of the deep high pressured fluid release and the diapir fault activity this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation

methane microseepage from different sectors of the yakela condensed gas field in tarim basin xinjiang china abstract methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the earth s surface and it is quite common in commercial petroleum fields while the role of microseepage as a pathfinder in petroleum exploration has been known for about 80 a its significance as an atmospheric ch 4 source has only recently been studied and flux data are currently available only in the usa and europe with the aim of increasing the global data set and better understanding flux magnitudes and variabilities microseepage is now being extensively studied in china a static flux chamber method was recently applied to study microseepage emissions into the atmosphere in four different sectors of the yakela condensed gas field in tarim basin xinjiang china and specifically in a a faulted sector across the luntai fault systems b an oil water interface sector at the northern margin of the field c an oil gas interface sector in the middle of the field d an external area outside the northern gas field boundaries the results show that positive ch 4 fluxes are pervasive in all sectors and therefore only part of the ch 4 migrating from the deep oil gas reservoirs is consumed in the soil by methanotrophic oxidation the intensity of gas seepage seems to be controlled by subsurface geologic settings and lateral variabilities of natural gas pressure in the condensed gas field the highest ch 4 fluxes up to 14 mg m 2 d 1 mean of 7 55 mg m 2 d 1 with higher spatial variability standard deviation 2 58 mg m 2 d 1 occur in the luntai fault sector merhane flux was lower in the oil water area mean of 0 53 mg m 2 d 1 and the external area mean of 1 55 mg m 2 d 1 and at the intermediate level in the gas oil sector mean of 2 89 mg m 2 d 1 these values are consistent with microseepage data reported for petroleum basins in the usa and europe the build up of methane concentration in the flux chambers is always coupled with an enrichment of 13 c from 13 c 1 of 46 to 42 5 vpdb which demonstrates that seeping methane is thermogenic as that occurring in the deep yakela reservoir daily variations of microseepage are very low with minima in the afternoon corresponding to higher soil temperature and higher methanotrophic consumption and maxima in the early morning when soil temperatures are lowest a preliminary and rough estimate of the total amount of ch 4 exhaled from the yakela field is in the order of 10 2 tonnes a 1 the present data can statistically improve the accuracy of the global microseepage flux data set but further surveys are needed in order to understand the frequency of occurrence and spatial variability of positive ch 4 fluxes in soils over petroleum fields

spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the yinggehai basin offshore south china sea abstract about 120 gas seepage vents were documented along the west and southwest coast of the hainan island south china sea in water depths usually less than 50 m the principal seepage areas include the lingtou promontory the yinggehai rivulet mouth yazhou bay the nanshan promontory and the tianya promontory they occur along three major zones reflecting the control by faults and lateral conduits within the basement it is estimated that the total gas emission from these seepage vents is 294 956 m 3 year the seepage gases are characterized by a high ch 4 content 76 heavy 13 c 1 values 38 to 33 and high c 1 c 1 5 ratios 0 95 1 0 resembling the thermogenic gases from the diapiric gas fields of the yinggehai basin hydrocarbon source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried miocene source rocks and sandstone reservoirs in the central depression the 2d basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up dip direction along the sandstone rich strata of huangliu formation to arrive to seabed and form the nearshore gas seepages it is suggested that the seepage gases are sourced from the miocene source rocks in the central depression of the yinggehai basin this migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation

relationship between magnetic anomalies and hydrocarbon microseepage above the jingbian gas field ordos basin china in this study soil magnetic measurements susceptibility and hysteretic parameters and soil hydrocarbon analyses were conducted on samples from three profiles profiles i and ii run across and profile iii runs parallel to the trend of the jingbian gas field in the ordos basin central china to determine the relationship between the magnetic anomalies e g volume specific magnetic susceptibility k and the hydrocarbon seepage environments the results document a strong correlation between magnetic susceptibility and soil gas hydrocarbon concentration furthermore the spatial distribution of k and hydrocarbon anomalies correlate with those of the gas field in addition magnetic minerals in the soils with higher susceptibility are predominantly magnetite with little or no substitution of titanium compared to that of samples with lower susceptibility 7 105 si international unit of susceptibility these results provide strong evidences for the formation of highly magnetic minerals in close association with hydrocarbon seepage recognition of such seepage induced magnetic anomalies can be used to facilitate the exploration for oil and gas in china and elsewhere

origins of taiwan s mud volcanoes evidence from geochemistry abstract the supply of natural gas from mud volcanoes is commonly attributed to the migration of gas bearing fluids from a deeper source area however the relative importance of these gas sources is not well known onshore mud volcanoes in taiwan are mainly located in the foothills of southwestern taiwan and in the coastal range of eastern taiwan generally along structurally weak zones geochemical analyses of gases and sediments from 17 mud volcanoes were undertaken in this study using molecular and stable isotopic compositions hydrocarbon gases were characterized by type and source maturity source rock assessments and nanofossil analyses were carried out to evaluate the potential and to infer the origin of sediments a particular effort was directed towards determining the genetic relationships between mud volcanic gas and associated sediments gases from taiwan s mud volcanoes are predominantly ch4 enriched with the exception of a few mud volcanoes where co2 predominates the origins of hydrocarbon gases in these systems range from microbial to thermally over mature sources while excess co2 may be derived from an inorganic origin as indicated by measured 13 c co 2 values the integration of geochemistry and local geology has yielded models with which to evaluate possible mechanisms for the formation of mud volcanoes

cenozoic qaidam basin china a stronger tectonic inversed extensional rifted basin qaidam basin is a composite sedimentary basin developed on typical continental crust it comprises a jurassic foreland basin and a cenozoic extensional basin the geologic history of the cenozoic qaidam basin can be divided into two megastages on the basis of stratigraphy and structure the first megastage from latest cretaceous to oligocene consisted of two periods of rifting due to upwelling of the hot upper mantle this is demonstrated by two tectono sequences formed by the rifting the synrifting magmatic thermal activity and the control of protobasin development by syndepositional faults the second megastage comprises three tectono sequences that developed in the miocene and pliocene and was a period of structural inversion that consisted of compressive downwarping and reverse faulting this tectonic inversion may be ascribed to the long distance propagation of compressive stress caused by the collisions of the indian and eurasian plates the thick sequence of primary oil source rocks was deposited in the major rifted depression and the restricted drainage graben of the rifted protobasin during the tectonic inversion megastage they were subject to deep burial and prolonged heating a major and a minor oil generating basin have developed the tectonic inversion processes produced several structural features that may contain potential hydrocarbon reservoirs and traps

upper paleozoic petroleum system ordos basin china abstract the ordos basin is a typical lapped basin including three sequences of strata early paleozoic late paleozoic and mesozoic with a total thickness of 4000 6000 m impermeable sealing beds are well developed at the top and base of the upper paleozoic sequence separating it from the mesozoic and the lower paleozoic strata to form an independent petroleum system in this petroleum system the source rocks are widely distributed coals and dark mudstones occurring in the carboniferous lower permian coal measures with a thickness of 10 15 and 40 60 m respectively the reservoirs are mainly early permian tight sandstones mostly with a porosity of 4 8 and a permeability of 0 1 1 0 10 3 m2 the regional cap rock is a 100 150 m thick mudstone in the upper permian strata the structural framework of the basin is a huge asymmetric syncline dipping gently toward the east and north and steeply toward the south and west well data show that gas saturated gas water transition and water saturated zones are developed from the depositional center to the basin edges the gas saturated zone mainly lies in the gently dipping slope area of the shanbei slope toward eastern and northern up dip directions the water gas transitional zone occurs and finally the water saturated zone presenting a reverse relation of water on top of gas an abnormal negative strata pressure is developed in the gas bearing area with a pressure coefficient cp ranging from 0 83 to 0 95 fluid inclusion data indicate that the upper paleozoic gas pool began to develop around the wuqi area at about 150 ma and it extended toward the north and was largely formed at about 120 ma showing there was a regional migration of the gas water interface from south to north during the gas pool formation these characteristics appear to show that the northern and eastern margins of the petroleum system are defined by a regional hydrodynamic regime the critical moment of the petroleum system occurred during the early cretaceous the basin experienced a rapid subsidence and a significant geothermal event resulting in the source rocks being matured into the main stage of gas generation to form a huge amount of gas the gas migrated into the tight sandstone reservoirs and displaced the rock pore water to form the gas pools and developing the regional hydrodynamic trap

relationships between seepage tectonics and subsurface petroleum reserves abstract the relationship between visible petroleum seepage and commercial subsurface reserves has been investigated by a study of the distribution of documented seeps in south east asia this is accompanied and integrated with a broader global study the intensity of flowing seepage is controlled largely by present day tectonics and by the timing of petroleum migration when such influences are reduced by isolating sets of tectonically similar basins correlations are found between numbers of seeps and basin reserves at a sub basinal scale seeps are observed to concentrate over tectonic features such as active diapirs active faults and uplifted basin margins it is relatively rare for seeps to directly overlie major fields although many seeps may be linked to downdip accumulations by long tortuous paths the best seep accumulation relationships are observed in active compressional and diapiric settings particularly those involving shallow reservoirs seeps are rare where traps are buried beneath thick undisturbed overburdens the prime value of visible seeps in most frontier basins lies at the regional level in the clues they give to the nature extent and quantitative hydrocarbon potential of a basin s source system

geochemistry and episodic accumulation of natural gases from the ledong gas field in the yinggehai basin offshore south china sea abstract the ledong gas field consisting of three gas pools in a shale diapir structure zone is the largest gas discovery in the yinggehai basin the gases produced from the pliocene and quaternary marine sandstone reservoirs show a considerable variation in chemical composition with 5 4 88 ch 4 0 93 co 2 and 1 23 7 n 2 the co 2 enriched gases often display heavier methane 13 c values than those with low co 2 contents the 15 n values of the gases range from 8 to 2 and the n 2 content correlates negatively with the co 2 content the high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the lower middle miocene source rocks and the formation of abundant co 2 from the tertiary calcareous shales and pre tertiary carbonates the compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the ld221 gas field is attributed to three phases of gas migration initially biogenic gas followed by thermogenic hydrocarbon gas and then co 2 rich gas the filling processes occurred within a short period approximately from 1 2 to 0 1 ma based on the results of the kinetics modeling geophysical and geochemical data show that the diapiric faults that cut through miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs and that the deep overpressure is the main driving force for vertical and lateral migration of the gases this gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases and is also a favorable place for gas accumulation the proposed multiple sources and multiple phases of gas migration and accumulation model for the ledong gas field potentially provide useful information for the future exploration efforts in this area

geological characteristics and distribution patterns of hydrocarbon deposits in the bohai bay basin east china abstract the bohai bay basin in east china is a meso cenozoic continental fault bounded basin with abundant petroleum resources based on analyses of sedimentation history and tectonic and thermal evolution the vertical and horizontal distribution patterns of oil and gas are discussed in this paper the basin has characteristics of multiple cycles of depression and uplift and each depression has its own sedimentary system and pattern of oil generation the main source rocks are of lower tertiary sediments characterized by multi cyclic sedimentation multiple source reservoir caprock associations were developed abrupt changes of sedimentary facies and the development of numerous faults caused the formation of various types of oil and gas reservoir that constituted multiple oil and gas accumulation zones based on individual structure zones these accumulation zones are mainly distributed around the centre of source rock deposits and are restricted in the areas where source rocks are distributed

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

geology of china and adjacent regions an introduction abstract this introduction outlines the papers collected in the special issue geology of china and adjacent regions the papers focus on scientific issues from archean to cenozoic geology in the areas including northwest pacific to western central orogenic belt concerning tectonics geophysics petrology geochemistry mineral deposit and energy resources a tectonic framework of china and adjacent regions is briefly introduced

geochemistry grouping and origins of crude oils in the western pearl river mouth basin offshore south china sea abstract thirty seven crude oils and 20 source rocks were selected for detailed molecular geochemical and isotopic analyses in order to establish the genetic relationships between the discovered oils and various petroleum source facies in the western pearl river basin offshore south china sea four groups of oils were identified the group i oils distributed in the southwestern wc19 1 field of wenchang b sag are characterized by high abundance of 4 methyl steranes relative to regular steranes low abundance of bicadinanes and high 13 c values showing good correlation with the medium deep lacustrine source facies in the wenchang formation the group ii oils represent the majority of the discovered oil reserves in the study area including those from the qionghai uplift and the northeast block of the wc19 1 oil field these oils have moderate concentrations of c 30 4 methylsteranes abundant bicadinanes and low 13 c values and correlate well with the shallow lake source facies of the wenchang formation in the wenchang b and a sags the group iii oils found in the wenchang a sag and surrounding areas are devoid of c 30 4 methylsteranes with abundant bicadinanes and similar 13 c values to those of the group ii oils these characteristics show close affinity with the coal bearing sequence in the enping formation thus suggesting a coaly source for these oils the group iv oils found only in the wc8 3 field display intermediate chemical compositions between the group ii and group iii oils most likely from mixed sources in the wenchang a sag these oil source genetic relationships suggest dominantly short distance oil migration and thus strong source facies controls on the geographic distribution of oil and gas fields within the western pearl river mouth basin

scratch circles associated with the large foraminifer bathysiphon from deep sea turbiditic sediments of the pagliaro formation palaeocene northern apennines italy abstract scratch circles which are rare sedimentary structures and particularly so in deep sea sediments occur as erosional forms on the lower surface of a turbiditic sandstone bed in the palaeocene pagliaro formation italy northern apennines san nazzaro section it was produced by rotation of a large tubular agglutinated test of the foraminifer bathysiphon originally the tests of bathysiphon stood vertically or obliquely in the sediment in life position they were partly exhumed and broken by bottom currents prior to deposition of the overlying turbiditic bed but remained articulated and anchored in their original position due to elastic organic parts the broken but still articulated parts were rotated by the current and scratched the muddy sediment surface producing concentric furrows the furrows were cast by sand of the turbiditic bed this structure occurs in two morphological variants type a is a set of ridges arranged within a sector of a circle with central angle arc segment s angular distance from 15 to 35 occasionally with a ridge or furrow gnomon structure running along the radius of the circle type b is an almost flat disc with poorly developed concentric ridges within a wide sector of a circle in which the central angle ranges from 100 to 140 the scratch circles determined as flag scratch circles in opposition to full scratch circles new terms are oriented concordantly with small flute casts formed mostly around broken tests of bathysiphon both of them point to the same direction of current flow but the flag scratch circles record flows over longer time periods

the pearl river mouth basin a rift basin and its geodynamic relationship with the southeastern eurasian margin abstract based on multichannel seismic reflection profiles exploration well data and gravity studies the tectonic development of the pearl river mouth basin is discussed in terms of the geodynamic evolution of the southeastern eurasian margin the area around the pearl river mouth basin has been dominated by two episodes of rifting of the attenuated continental crust along the south china margin from the late cretaceous to the early oligocene the rifting brought about ruptures of various magnitudes in various segments of the basin regional normal faults have been produced and resulted in the formation of three major subbasins zhu 1 zhu 2 and zhu 3 which are separated by local uplifts and are characterized by horst and graben structures regional subsidence prevailed throughout the late oligocene recent period and resulted in a tilted continental shelf upon which rests a unified rift basin the pearl river mouth basin a three stage model involving doming rifting and drifting is proposed for the tectonic evolution of the pearl river mouth basin the uplifting and rifting of the continental crust along the south china margin are related to the late episodes of the yanshan orogeny during the late cretaceous early oligocene period when the intensity of rifting decreased the thermal subsidence of the continental margin of south china prevailed during the late oligocene recent period of the himalaya orogeny

methane flux from miniseepage in mud volcanoes of sw taiwan comparison with the data from italy romania and azerbaijan abstract mud volcanoes mvs are considered important methane ch4 sources for the atmosphere gas is not only released from macro seepage i e from craters and visible gas bubbling manifestations but also from invisible and pervasive exhalation from the ground named miniseepage ch4 flux related to miniseepage was measured only in a few mvs in azerbaijan italy japan romania and taiwan this study examines in detail the flux data acquired in 5 mvs and 1 dry seep in sw taiwan and further compares with other 23 mvs in italy romania and azerbaijan miniseepage from the six manifestations in sw taiwan mvs and seeps annually contribute at least 110 tons of methane directly to the atmosphere and represents about 80 of total degassing during a quiescent period combining miniseepage flux and geo electrical data from the wu shan ding mv revealed a possible link between gas flux and electrical resistivity of the vadose zone this suggests that unsaturated subsoil is a preferential zone for shallow gas accumulation and seepage to the atmosphere besides miniseepage flux in chu huo everlasting fire decreases by increasing the distance from the main gas channeling zone and molecular fractionation methane ethane ratio is higher for lower flux seepage consistently with what observed in other mvs worldwide measurements from azerbaijan italy romania and taiwan converge to indicate that miniseepage is directly proportional to the vent output and it is a significant component of the total methane budget of a mv a miniseepage vs macro seepage flux equation has been statistically assessed and it can be used to estimate theoretically at least the order of magnitude of the flux of miniseepage for mvs of which only the flux from vents was evaluated or will be evaluated in future this will allow a more complete and objective quantification of gas emission in mvs thus also refining the estimate of the global methane emission from geological sources

uralian magmatism an overview abstract this paper is an attempt to summarize current knowledge of uralian magmatism focusing on those aspects relevant for understanding its geodynamic evolution the urals consist of three tectonomagmatic domains a suture sector in the west and two n s imbricated island arc continental sectors in the east the suture sector comprises lower palaeozoic mafic ultramafic complexes which show eastward impoverishment in lile thus reflecting the transition of the subcontinental lithospheric mantle of the russian plate to the suboceanic lithospheric mantle of the subducted uralian palaeo ocean the two island arc continental sectors represent the transition from oceanic to continental environments in the middle and south urals collisional magmatism started in the silurian and persisted till the permian migrating progressively eastward and increasing in abundance of lile and 87 sr 86 sr initial magmatic polarity is very similar to that of modern subduction zones and indicates that the subducted slab was dipping eastward during that period the northern and southern island arc continental sectors show many similarities regarding the nature and spatial temporal distribution of magmatism but there are also some important differences which probably indicate somewhat different geodynamic regimes in the northern sector carboniferous tonalite granodiorite batholiths have features compatible with an origin by melting of the oceanic crust in the subducted slab in the southern sector however carboniferous tonalite granodiorite batholiths have features more consistent with a melting event within the lower continental crust above the subduction zone than with melting within the subducted slab upper carboniferous permian granites have high 87 sr 86 sr initial in the north e g 0 7120 in the murzinka batholith but very low 87 sr 86 sr initial in the south e g 0 7045 in the dzhabyk batholith in spite of rocks from both batholiths being equally peraluminous and showing evidence of derivation by anatexis of metasediments the low 87 sr 86 sr initial at dzhabyk might indicate that in the southern sector the accretionary prism grew very rapidly thus allowing involvement in upper palaeozoic melting events

generation and evolution of subduction related batholiths from the central urals constraints on the p t history of the uralian orogen abstract batholiths from the accreted terranes in the urals were generated by repeated episodes of melting and intrusion verkhisetsk the largest and one of the most complex subduction related batholiths from the urals comprises an outer envelope of older tonalites trondhjemites and granodiorites dated at 315 320 ma and equilibrated at 6 kbar intruded by an inner core of younger granodiorites adamellites and granites dated at 275 290 ma and equilibrated at 4 kbar older rocks have a high a1 ttd adakite chemistry 320machur nd 2 5 and initial 87 sr 86 sr 0 7043 with pronounced lateral zoning marked by the increase of lree hree cr ni and mg eastwards they were originated by melting of metabasalts in a subducted slab of young hot oceanic lithosphere with a temperature depth trajectory that intersected the garnet in univariant at 1050 c and 13 kbar thus causing the lateral zoning we believe that such a warm trajectory can be attributed to the oblique subduction of a young lithosphere younger rocks range from na rich metaluminous granodiorites to k rich peraluminous two mica granites with relict epidote crystals and have almost the same isotopic signature as older rocks from which they were generated by anatexis as a consequence of a melting event that occurred throughout the urals at 275 290 ma and produced most of the huge continental type batholiths on the eastern side this melting event is tentatively supposed to be the result of underplating by mafic magmas which also caused the growth of the urals crust from the moho downwards until it reached the increased thickness revealed by seismological studies

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge