characteristics of mud diapirs and mud volcanoes and their relationship to oil and gas migration and accumulation in a marginal basin of the northern south china sea mud diapir and mud volcano have the same formation mechanism and similar development and evolution characteristics but different geological features and forms in this study the development and characteristics of a mud diapir in yinggehai basin northwestern south china sea and a mud volcano in southwest taiwan basin northeastern south china sea were investigated the geochemical characteristics of their associated natural gases were also analyzed the results indicated that the yinggehai basin mud diapir is mainly distributed in the deep and large depression of the central basin and presents five lines in echelon arrangement along a northwest direction its development and evolution characteristics are low density and speed and abnormally high temperature and overpressure in contrast the southwest taiwan basin mud volcano is mainly distributed in a deep water slope area of the south sag and tainan kaohsiung onshore mud volcanoes are mainly distributed along the deep and large fault zone and their morphological characteristics are similar to venting volcanoes the seabed morphology of offshore mud volcano seems similar but current research is still insufficient there are many types of associated hydrocarbon gases in mud diapir and mud volcano but the high maturity coal type hydrocarbon gas is the main one the associated non hydrocarbon gas is rich in co2 and n2 which is mainly derived from a co2 source from inorganic crust the source of both diapir and volcano is thick marine miocene and pliocene age mud with hydrocarbon generating potential the mature hydrocarbon generation mud diapir mud volcano migration and deposition into an appropriate reservoir as well as diapir associated structures constitute a unique oil gas migration and accumulation system which thus adds to our understanding of the distribution of such mud diapir volcano type oil gas reservoir

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

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

kinetic nitrogen isotope fractionation associated with thermal decomposition of nh3 experimental results and potential applications to trace the origin of n2 in natural gas and hydrothermal systems abstract ammonia nh3 is the major intermediate phase in the pathway of nitrogen n transfer from the fixed n phases e g in crustal material to free n2 e g in natural gas reservoirs and volcanic gases yet the n isotopic behavior during these n cycling processes remains poorly known in an attempt to contribute to the understanding of n cycling using n isotopes we carried out laboratory experiments to investigate the n isotopic effect associated with thermal decomposition of ammonia 2nh3 n2 3h2 pure nh3 with initial 15 n nh 3 of 2 relative to air standard was sealed into quartz tubes and thermally decomposed at 600 700 or 800 c from 2 hours to 500 days with the progress of the reaction the 15n of the remaining nh3 and the accumulated n2 increased from 2 to 35 and from 20 to 2 respectively the differences of the n isotope fractionations at the three temperatures are not significant modeling using the rayleigh distillation model yielded similar kinetic n isotope fractionation factors n 2 nh 3 of 0 983 0 002 for 600 700 and 800 c applied to geological settings this significant isotope discrimination 17 associated with partial decomposition of nh3 nh4 from crustal sources 15naverage 6 3 can produce mantle like i e 5 or even lower 15n values of n2 this may explain the large variation of 15n 20 to 30 of n2 in natural gas reservoirs it can also possibly explain the extreme 15n depletion of n2 in some volcanic gases this possibility has to be carefully considered when using n isotopes to trace geological n cycling across subduction zones by analysis of volcanic n2

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

thermal regime interreservoir compositional heterogeneities and reservoir filling history of the dongfang gas field yinggehai basin south china sea evidence for episodic fluid injections in overpressured basins the dongfang gas field located in a diapir structure zone is the largest gas field found in the yinggehai basin a strong thermal anomaly caused by hydrothermal fluid flows occurs in the gas field as evidenced from drill stem test and fluid inclusion homogenization temperatures rock eval tmax vitrinite reflectance and clay mineral transformation profiles such a thermal anomaly suggests focused rapid flow of deeply sourced hydrothermal fluids which has shifted the threshold depth to the onset of hydrocarbon generation upward by about 500 m the dongfang gas field shows considerable variation in nitrogen and carbon dioxide content with nitrogen content ranging from less than 5 to 31 2 and carbon dioxide content ranging from less than 1 to 88 9 hydrocarbon gases and associated condensates show high maturities and have been generated most probably from the meishan and sanya formations of miocene age carbon dioxide in gases with co2 content less than 10 is organic in origin whereas carbon dioxide in gases with co2 content higher than 10 is inorganic in origin and has been generated from high temperature decomposition of carbonates most gases display negative d15n values gases with nitrogen content higher than 15 always contain organic co2 co2d13c values lighter than 10o oo and the nitrogen contents decrease as the d13c values for methane and ethane become heavier suggesting an organic origin of the nitrogen gas generated in the catagenetic stage source rock ro 2 0 before the significant thermal decomposition of carbonate took place systematic interreservoir compositional heterogeneities occur in the gas field which along with the thermal regime and fluid inclusion homogenization temperature measurements give a clear suggestion of the reservoir filling history methane dominated gases with relatively high nitrogen content and a small amount of organic co2 accumulated first and carbon dioxide rich gases with hydrocarbon components of higher maturity injected into the reservoir later interreservoir compositional heterogeneities which have the advantage of being unaffected by in reservoir mixing processes can be effective indicators of the field filling history especially when they are studied in combination with fluid inclusion and reservoir diagenesis analyses the short lived transient nature of the thermal effect of fluid flow and the wide variation of the toluene n heptane values seem to suggest episodic fluid injections from the overpressured systems into the reservoirs

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

origin and accumulation of co2 and its natural displacement of oils in the continental margin basins northern south china sea many rich up to 97 by volume natural gas pools have been found in the continental margin basins of the northern south china sea by combining the geochemical data from 53 samples with their geologic backgrounds this study investigated the origins and accumulation mechanism of and discussed the role of in driving oil as it charged the reservoirs the results reveal that the gases in the yinggehai basin originate mainly from the thermal decomposition of both miocene calcareous shales and paleozoic carbonates and that from mantle degassing is only a minor contributor the accumulations in the yinggehai basin are mainly controlled by diapiric faults and episodic thermal fluid movements the gases in the eastern qiongdongnan and western pearl river mouth basins are mainly related to magmatic or mantle degassing and the volatiles from magmatic degassing during the igneous intrusion stage are the most likely major source of in these reservoirs with basement faults providing pathways for upward migration of rich mantle fluids natural displacements of oil by appear to be common in the eastern qiongdongnan and western pearl river mouth basins the flooded oil or gas reservoirs have two common features that the present gas pools or oil bearing structures have residual oils representing prior charge and are close to the basement faults that provide pathways along which the mantle derived rich gas was migrated the oils from prior hydrocarbon reservoirs have been naturally driven out by to form secondary oil reservoirs in the eastern qiongdongnan and western pearl river mouth basins therefore a full understanding of the origin and distribution of cannot just be used to trace hydrocarbon migration pathways but also provide useful information for risk assessment prior to drilling

late miocene provenance change on the eastern margin of the yinggehai song hong basin south china sea evidence from u pb dating and hf isotope analyses of detrital zircons abstract understanding the provenance of the reservoir in a sedimentary basin is of great importance in hydrocarbon exploration and production the yinggehai song hong basin is one of the most important cenozoic petroliferous basins in the south china sea to better understand the provenance characteristics of the upper miocene huangliu formation tortonian messinian on the eastern margin of the basin and test the influence of uplift of the tibetan plateau in the south china sea 172 single zircons from two sandstone samples in the lingtou gas field were dated by u pb chronometer and 54 of these grains were spot analyzed for hf isotopes the results indicate that the upper member of the huangliu formation in this gas field shows two major peaks of u pb ages at ca 250 ma and ca 432 ma along with four subordinate peaks at ca 757 ma ca 1926 ma ca 2529 ma and ca 2775 ma the initial hf isotope ratios ehf t of these zircons are negative to positive for each age group suggesting that existing crustal materials were mixing with new mantle melts in some magmatic episodes in contrast the lower member of the huangliu formation show two major age peaks at ca 98 ma and 248 ma with a subordinate peak at ca 1453 ma the ehf t values are concentrated between 11 6 and 3 4 revealing that they were derived from older crust comparing with the source characteristics of tectonic units surrounding the basin the source of the upper member in the gas field was mainly from the southern yangtze block whereas the lower member was derived from the hainan uplift together with the change of climate and sedimentation rate after 10 ma on the northern margin of the south china sea we attribute the provenance change to the uplift of the tibetan plateau at the same time

the structure and formation of diapirs in the yinggehai song hong basin south china sea abstract the occurrence of shale diapirs in the yinggehai song hong ygh sh basin is well documented as is their association with big petroleum fields in order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high resolution two and three dimensional seismic reflection data as well as drilling data that cover the diapirs in ygh sh basin as many as 18 diapirs were identified and are arranged in six n s striking vertical en echelon zones on seismic reflection sections gas chimney structures diapiric faults and palaeo craters are genetically linked with the process of diapirism here we use geophysical and geological observations to propose a three stage model for diapirism initiation emplacement and collapse during these three stages different diapiric structure styles are formed which we describe in detail these include buried diapirs piercing diapirs and collapsed diapirs we link the diapirism to activity on the offshore continuation of the red river fault as shown on our high resolution seismic reflection data which is also related to a high paleogeothermal gradient caused by crustal thinning we also recognize the role of loading by the very large volume of sediment eroded from the edges of the tibetan plateau and delivered by the red river to the basin

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

origin and migration of hydrocarbon gases and carbon dioxide bekes basin southeastern hungary the bekes basin is a sub basin within the pannonian basin containing about 7000 m of post cretaceous sedimentary rocks natural gases are produced from reservoirs precambrian to tertiary in age located on structural highs around the margins of the basin gas composition and stable carbon isotopic data indicate that most of the flammable gases were derived from humic kerogen contained in source rocks located in the deep basin the depth of gas generation and vertical migration distances were estimated using quantitative source rock maturity carbon isotope relationships for methane compared to known neogene source rock maturity depth relationships in the basin these calculations indicate that as much as 3500 m of vertical migration has occured in some cases isotopically heavy 7 0 co2 is the predominant species present in some shallow reservoirs located on basin margin structural highs and has probably been derived via long distance vertical and lateral migration from thermal decompositon of carbonate minerals in mesozoic and older rocks in the deepest parts of the basin a few shallow reservoirs 2000m contain isotopically light 50 to 60 0 methane with only minor amounts of c2 homologs 3 v v this methane is probably mostly microbial in origin above normal pressures occuring at depths greater than 1800 m are believed to be the principal driving force for lateral and vertical gas migration these pressures are caused in part by active hydrocarbon generation undercompaction and thermal decomposition of carbonates

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

multiphase natural gas migration and accumulation and its relationship to diapir structures in the df1 1 gas field south china sea abstract the dongfang1 1 df1 1 gas field is the largest commercial gas field yet discovered in the yinggehai basin south china sea the gas reservoirs in the field are normally pressured pliocene shallow marine facies fine grained sandstones and the main source rocks are highly over pressured miocene mudstones and shales the natural gases in the reservoirs consist mainly of hydrocarbon gas co 2 and n 2 geochemical data for the natural gases show that the hydrocarbon rich gases except for a small amount of biogas were derived from these source rocks distributed within over pressured zones but gases rich in co 2 mainly originate from the thermal decomposition of miocene calcareous shales and pre tertiary carbonates at higher temperature fluid inclusion data for the reservoirs were used together with the regional geology distribution of different types of reservoirs and the evolution of diapir structures to determine the episodes of gas migration and accumulation in the df1 1 gas field we believe that the field was formed by four episodes of gas migration and accumulation fluid paleopressure simulation software designed and developed by cnooc china limited was used to simulate the paleopressure history of the strata at the top of the present day high pressure zone a depth of 2800 m in the df1 1 gas field it predicts that the first episode occurred before about 2 ma the second and the third episodes between 2 and 0 5 ma and that the co 2 rich gases migrated into the reservoirs after 0 5 ma the formation and evolution of diapir structures in the yinggehai basin had an important influence on the migration and accumulation of natural gases in the df1 1 gas field rapid deposition gas generation in the source rocks and thermal expansion of pore fluids caused over pressuring in the miocene shales in the center of the depression this over pressuring together with active faulting formed diapir structures during 5 0 5 ma the faults and fractures related to the diapir structures provided a pathway for gas vertical migration from the deep source rocks into the df1 1 reservoirs and controlled the geological distribution of different types of natural gases on this basis a model for gas migration and accumulation is suggested for the df1 1 gas field

episodic fluid expulsion from geopressured sediments numerical simulation of fluid explusion from geopressured sediments using a one dimensional finite difference model shows that large volumes of fluid can be transported through a fractured seal during short lived 100 year expulsion events in this model abnormal fluid pressures are maintained by an impermeable seal when the fluid pressure exceeds 85 of the lithostatic pressure a vertical fracture opens in the seal providing a permeable pathway from the geopressured sediments into the overlying section fluid pressure in the seal decreases as fluid flows through the fracture closing the fracture as sediments in the seal compact once the fracture closes the seal remains impermeable until fluid flowing from deeper in the section increases fluid pressure to the fracture criteria a process that takes 10 000 500 000 years fractures emplaced in relatively incompressible seals such as compacted and or cemented sediments remain open for 20 50 years fluid fluxes through the seal integrated over the life of these events are 1 5 107 kg m2 and temperatures in the overlying sediments are increased by 5 20 c these temperature anomalies decay to less than half their original value in 300 years suggesting that the thermal anomalies observed in wells that are attributed to fluid expulsion formed in response to a very recent or currently active event

geochemistry and accumulation of carbon dioxide gases in china we propose the following identification indicators for co2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples the co2 carbon isotope values of 390 samples and the helium isotope composition of 256 samples collected from wells of 16 oil and gas bearing basins and gas seepages in china and reviewing literature on organic and inorganic co2 throughout the world co2 is of organic origin if co2 content 13cco22 is of inorganic origin if dgr13cco2 8 pmil or co2 60 and co2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if 10 pmil 13cco213cco2 values of organic co2 mainly range from 10 to 22 pmil in tectonically stable oil and gas bearing basins where deep faults and magmatism are undeveloped organic co2 occurs almost exclusively in tectonically active oil and gas bearing basins where active faults and magmatism are present or in areas of modern volcanic activity the dgr13cco2 values of inorganic co2 range from 2 to 8 pmil and are distributed along faults the inorganic co2 with a dgr13cco2 value of 0 3 pmil is of metamorphic origin and that with dgr13cco2 values of 6 2 pmil is mantle derived and of magmatic origin in china the dgr13cco2 values of inorganic co2 gas pools range from 3 41 to 8 44 pmil and the helium ratio r ra is 1 which indicates that these gases are typically derived from the mantle

nd sr isotopic mapping of the chinese altai and implications for continental growth in the central asian orogenic belt article i nfo the chinese altai orogen is a typical domain of the central asian orogenic belt and in previous syntheses was divided into six tectonic units or terranes in this paper 41 new and 36 published whole rock nd and sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central altai and units 4 to 6 of the southern altai were used for isotopic mapping the age of the samples used is known from u pb zircon dating end t values range from 4 to 2 in the central altai units 2 and 3 and from 1 4 to 6 in the southern altai units 4 and 5 and up to 8 4 in unit 6 correspondingly nd model ages tdm decrease from 1 6 1 1 ga in the central altai to 1 0 0 5 ga in the southern altai except for a continental fragment in unit 4 isr is elevated in the central units decreasing to b0 705 in the southern units 5 and 6 these results demonstrate that the central altai contains widespread old crustal components in its basement whereas the southern altai comprises a higher proportion of juvenile crust when integrated with published u pb zircon ages our data show that new crust grew southwards in the chinese altai in addition our data suggest that units 2 and 3 are a single tectonic unit whereas the southern altai unit 4 which previously was regarded as a single young accretionary terrane should be subdivided into two units a younger arc and an older continental fragment the granitoids in the chinese altai can be divided into syn and post non orogenic groups the syn orogenic granitoids have ca 50 80 mantle derived component whereas the post non orogenic granitoids contain ca 70 90 the generation of syn and post non orogenic granitoids represents horizontal lateral and vertical crustal growth which contributed ca 18 28 and 7 8 respectively to the area of the chinese altai this work shows that isotopic mapping is an effective approach to elucidate terrane composition in the central asian orogenic belt and thereby better constrain palaeozoic crustal growth

keketuohai mafic ultramafic complex in the chinese altai nw china petrogenesis and geodynamic significance abstract devonian magmatism was very intensive in the tectonic evolutionary history of the chinese altai a key part of the central asian orogenic belt caob the devonian keketuohai mafic ultramafic complex in the chinese altai is a zoned intrusion consisting of dunite olivine gabbro hornblende gabbro and pyroxene diorite the pyroxene diorite gives a zircon u pb age of 409 5 ma variations in mineral assemblage and chemical composition suggest that the petrogenesis of the keketuohai complex was chiefly governed by fractional crystallization from a common magma chamber low sio 2 k 2 o and na 2 o contents negative covariations between p 2 o 5 tio 2 and mg value suggest insignificant crustal assimilation contamination thus the positive e nd t values 0 to 2 7 and slight enrichments in light rare earth elements e g la yb n 0 98 3 64 suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle model calculation suggests that their parental magma was high mg mg 66 tholeiitic basaltic melt the keketuohai intrusion was coeval with diverse magmatism high temperature metamorphism and hydrothermal mineralization which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the chinese altai

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

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

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

geochemistry and origins of natural gases in the yinggehai and qiongdongnan basins offshore south china sea abstract four gas fields and a number of gas bearing structures have been discovered in the yinggehai and qiongdongnan basins offshore south china sea chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins biogenic gas thermogenic gas and gases with mixed origin thermogenic gases produced from the yacheng field are characterized by relatively high contents of benzene and toluene relatively low 13c values of toluene and high abundance of bicadinanes and oleanane in the associated condensates showing good correlation with the coal bearing sequence of the yacheng formation in the qiongdongnan basin in contrast the gases from the df1 1 and ld gas fields contain high amounts of n2 and co2 low to moderate amounts of benzene and toluene with relatively high 13c values of toluene these characteristics correlate well with the miocene neritic shales in the yinggehai basin analyses on potential source rock samples indicate excellent gas source potential for both the coal bearing sequence in the yacheng formation and the miocene neritic shales containing type iii ii2 kerogens as the result of recent rapid subsidence and sedimentation high temperature and overpressure systems are well developed in the yinggehai and qiongdongnan basins the rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata whereas the strongly overpressured systems led to retarded organic matter maturation postponing the time when the source rocks reached peak gas generation results of 1 d modeling indicate that the coal bearing yacheng formation in the yacheng sag reached peak gas generation during mid miocene pliocene time after the deposition of upper oligocene to lower miocene reservoir rocks the lower mid miocene marine shales reached their peak gas generation stage during the pliocene quaternary in the central yinggehai basin supplying abundant gases to charge the pliocene reservoirs more importantly focused episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time the gas source relationship for the yacheng field suggests dominantly short distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the qiongdongnan basin

provenance of upper miocene to quaternary sediments in the yinggehai song hong basin south china sea evidence from detrital zircon u pb ages abstract the yinggehai song hong y sh basin located on the northwestern shelf of the south china sea is one of the world s largest pull apart basins in order to identify the provenance of the upper miocene to quaternary sediments of the basin ten sediment samples were collected from drill cores in the dongfang gas field u pb dating of detrital zircon grains extracted from the ten samples using a laser ablation inductively coupled mass spectrometer la icp ms yields ages ranging from the archean to cenozoic 2898 46 ma to 26 1 ma the zircon u pb age spectra of the upper miocene to quaternary show a similar pattern with major peaks clustered at ca 32 ma ca 99 ma ca 160 ma ca 247 ma ca 422 ma and three broad age groups 600 1000 ma 1700 2100 ma and 2200 2800 ma suggestive of a wide range of igneous rock ages in the source areas the detrital zircon ages provide diagnostic criteria for identification of the various source areas and reveal that the detritus was derived from multiple sources comparison of the results with the rock types and their ages surrounding the potential source areas indicates that the clastic material was derived from three dominant age sources 1 yangtze craton 2 hainan island and 3 indochina block the yangtze craton was a major and continuous source area contribution to the basin comparing the age characteristics and its source areas since 10 ma the largely stable provenance also suggests that if the red river capture ever existed it should have happened before the late miocene

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried