overpressure generation and evolution in a compressional tectonic setting the southern margin of junggar basin northwestern china overpressure is widespread in the southern margin of the junggar basin northwestern china pressure measurements in drillstem tests and repeated formation tests and estimates from wire line logs indicate contrasting overpressure values between permeable sandstones and adjacent low permeability mudrocks in addition excess pressure differs among anticlines with similar depth lithologies and geologic age indicating significant lateral changes of overpressure major factors controlling overpressure generation and distribution include rapid sediment deposition pressure compartmentalization by thick mudrocks tectonic stress faulting and folding clay transformation and hydrocarbon generation are believed to be insignificant in overpressure generation in the southern junggar basin numerical modeling of pressure generation and evolution suggests that faulting and stratal tilting associated with folding are the most significant factors in the overpressure generation of a permeable sandstone the extremely high overpressure pressure coefficient up to 2 43 may have been caused by hydraulic adjustment within permeable sandstones associated with structural deformation caused by post miocene intense tectonic activities

petroleum origin and accumulation in basin evolution a quantitative model basin data geologic geophysical geochemical hydrodynamic and thermodynamic can be combined for quantified hydrocarbon prediction a three dimensional deterministic dynamic basin model can be constructed to calculate all the measurable values with the help of mass and energy transport equations and equations describing the physical and or physicochemical changes in organic matter as a function of temperature input data consist of heat flux initial physical and thermal properties of sediments paleobathymetric estimates sedimentation rate and amount and type of organic matter subsequently the model computes pressure temperature physical and thermal properties of sediments maturity of organic matter and the hydrocarbon potential of any source rock as a func ion of space and time thus the complex dynamic processes of petroleum formation and occurrence in a given sedimentary basin can be quantified for example hydrocarbon potential maps for any given source rock and any geologic time slice of the basin evolution can be provided as computer printouts the computer model can be applied to any stage of an exploration campaign and updated as more information becomes available

a deep seismic reflection profile across the nova scotia continental margin offshore eastern canada results from two deep seismic reflection lines are presented when combined these lines span the rifted continental margin off nova scotia from crust unaltered by rifting to the ocean basin these data provide crustal and upper mantle reflection geometry to depths of over 50 km and elucidate the rifting process on this margin which occurred during the mesozoic breakup of pangaea the continental crust below the continental shelf and slope becomes progressively thinner toward the ocean continent boundary in the upper crust normal faults accommodated mesozoic extension and these flatten and terminate at 5 6 s two way time in the lower crust and upper mantle mesozoic rifting may be reflected in dipping events which are interpreted to be normal faults all mesozoic extensional faulting could be controlled by the preexisting fabric of the crust which in this region would be related to appalachian compression within the meguma terrane below the continental rise there is some evidence for magmatic un

upper permian lacustrine oil shales southern junggar basin northwest china upper permian organic rich lacustrine mudstones oil shales that crop out in the southern junggar basin rank among the richest and thickest petroleum source rock intervals in the world with maximum toc values reaching 34 and rock eval pyrolytic yields s 2 up to 200 kg hc t rock lacustrine sedimentary facies define an overall transgressive regressive cycle of approximately 2000 m gross thickness which includes approximately 800 m of source rocks averaging 4 1 toc and 26 2 kg hc t rock basinal facies comprise silicic organic rich laminated lacustrine mudstones and interbedded siltstones organic matter contained in the mudstones ranges in composition from type i to type iii basinal facies were deposited in a deep oxygen deficient stratified lake lake margi facies consist of nonlaminated siliciclastic mudstones rippled dolomitic siltstones and sandstones and minor limestones maximum toc values are approximately 6 desiccation cracks are common in the marginal facies but evaporite minerals are rare or absent biomarker correlation parameters measured from rock extracts exhibit significant stratigraphic variability but strongly support the hypothesis that upper permian lacustrine oil shales charge the giant karamay field in the northwestern junggar basin karamay oils are characterized by high relative abundances of beta carotane this characteristic is restricted to desiccated facies in the outcrop sections however we therefore propose that an abundance of beta carotane indicates elevated environmental salinities during deposition of the oil shales orographic precipitation induced by a paleo tian shan range may have provided fresh water to the southern areas of the basin while more evaporative conditions prevailed to the north outcrop oil seeps confirm that the outcrop sections have attained sufficient thermal maturity to generate oil t max values range from 431 to 451 degrees c vitrinite reflectances range from 0 73 to 0 88 and sterane isomerization ratios range from 0 30 to 0 48 20s 20s 20r matoil registered trademark computer modeling of burial history and thermal maturity of the outcrop sections indicates that approximately 4 5 km of triassic jurassic cover was needed to bring the oil shales to their current maturity the major phase of hydrocarbon generation in this area appears to have preceded the creation of traps by neogene thrusting which may account for the lack of discoveries in the southern junggar basin most of the oil discovered so far is trapped in structures which originated duri g the late paleozoic or mesozoic

moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

distribution and isomerization of c31 c35 homohopanes and c29 steranes in oligocene saline lacustrine sediments from qaidam basin northwest china abstract the 22s 22s 22r ratios for c 31 c 35 homohopanes and the 20s 20s 20r ratio for c 29 steranes were measured by gas chromatography mass spectrometry gc ms on six oligocene saline lacustrine rock samples from a borehole located in the western qaidam basin northwest china these ratios vary significantly and irregularly with burial depth from 2831 m to 3054 m around the threshold of the oil generation window in addition 22s 22s 22r ratios also vary substantially among different homologs within the same sample generally decreasing from c 31 to c 35 homohopanes the distribution patterns of homohopanes for the six samples are unusual such as c 31 32 c 33 c 34 35 or c 31 c 32 c 33 34 35 or c 31 32 c 33 34 35 all of these observations can be mainly ascribed to the release of bound hopanes and steranes from kerogen and other macromolecules the 22s 22s 22r ratios of c 31 c 32 and c 33 homohopanes decreased while that of c 34 homohopanes increased for two samples yhs1 and yhs6 after pyrolysis at low temperatures 180 270 c a major reversal of 20s 20s 20r ratio for c 29 steranes was observed at 240 c for sample yhs1 but at 270 c for sample yhs6 the distributions of homohopanes for these two samples after pyrolysis changed from their original patterns toward the normal pattern c 31 c 32 c 33 c 34 c 35 these results reflect the influences of thermal degradation and direct isomerization of the free biomarkers and the release of bound biomarkers from kerogen and other macromolecules during pyrolysis

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

geochemical characteristics of crude oil in fluvial deposits from maling oilfield of ordos basin china abstract maling oilfield is one of the earliest discovered oilfields in the ordos basin it is also a typical representative of oil reservoirs in fluvial deposits however due to the deficiency of geochemical study on the crude oils and source rocks the interpretation of the oil origin is largely based on the geological situation maling crude oils 21 and source rocks 14 from the basin have been systematically analyzed the distribution of the biomarkers indicates that the crude oils mainly originated from source rocks deposited under a weakly reducing to sub oxic freshwater lacustrine environment besides algae higher land plants have also made an important contribution to the source organic matter comparison of the distributions of the biomarkers suggests that there is a correlation between the maling oilfield oils and the source rocks of the yanchang formation

a new evaluation of palaeo heat flows and eroded thicknesses for the carboniferous ruhr basin western germany palaeo heat flows and thicknesses of eroded carboniferous overburden in the coal bearing ruhr basin western germany were estimated using one dimensional basin modelling techniques thermal and burial histories for 11 localities were calibrated by comparing measured and calculated vitrinite reflectance data based on the assumption of pre tectonic maturation in contrast to former studies the kinetic easy r0 approach applied for the calculation of vitrinite reflectance lopatin s tti algorithm was used in addition for one well in order to demonstrate the impact of the selection of the algorithm for vitrinite reflectance calculation on the simulation results simulated palaeo heat flows for the time of maximum burial most probably in the stephanian range from 64 to 83 mw m2 and decrease towards the south these heat flows are lower than values computed during earlier studies but are still relatively high in comparison to recent foreland basins the regional distribution of palaeo heat flows indicates that the crust beneath the ruhr basin was relatively thin and thickened towards the south at the end of the carboniferous after the stephanian 2200 3500 m of carboniferous overburden were eroded an observed southward increase in eroded thicknesses is caused by an uplift of the area studied which decreased towards the north at the end of the variscan folding large thicknesses of deposited and later eroded carboniferous sediments have to be postulated to explain the maturity data even along the present southern margin of the ruhr basin this indicates that the late carboniferous sediments originally extended far to the south geological arguments suggest that the deposition of carboniferous sediments in the entire ruhr basin continued until the stephanian

source rock potential of lignite and interbedded coaly shale of the ogwashi asaba formation anambra basin as determined by sequential hydrous pyrolysis abstract outcrops in the anambra basin in southern nigeria contain paleogene imo shale marine the neogene paralic ogwashi asaba and the continental benin formations representing equivalents of the subsurface successions in the niger delta basin thirty three samples of lignite and the interbedded coaly shale of the ogwashi asaba formation were investigated petrologically before rock eval screening two selected samples of lignite and coaly shale were subjected to sequential hydrous pyrolysis hp at 330 c for 72 h and at 355 c for 72 h to characterize their oil and gas potential the lignite sample has a rock eval hydrogen index hi of 481 mg g toc and a mean vitrinite reflectance of 0 36 rom the total amount of expelled oil generated in the sequential hp experiments is 259 mg g of original total organic carbon tocorig this expelled waxy oil has abundant high molecular weight n alkanes and an extremely high pristane phytane ratio of 6 5 typical of crude oils generated from coals as observed in some onshore and shallow offshore accumulations of the niger delta the overlying coaly shale has a lower hi of only 191 mg g toc the total amount of expelled oil generated in the sequential hp experiments is only 15 mg g tocorig this oil is not waxy and has a pristane phytane of 2 6 which is more typical of a marine source rock these results are contrary to the idea that coaly shale associated with coal is the main source of oil the greater yields of expelled oil from the coal relative to the coaly shale are attributed to the higher liptinite content in the former and the possibility that the organic matter in the latter was oxidized prior to deposition 13c of the methane generated at 355 c for 72 h is 39 5 for the lignite and 35 0 for the coaly shale this suggests different methane precursors in these two lithologies the data set reveals remarkable differences in the characteristics of the two types of source rocks in the ogwashi asaba formation and their potential to contribute a mixture of hydrocarbons derived from intervals that are stratigraphically only meters apart these results suggest that coal and coaly shale within the thermally mature stratigraphic levels of the agbada formation in the sub surface are potential source rocks for liquids and gaseous hydrocarbons in the niger delta

rifted margin source rock deposition a carbon isotope and biomarker study of a west african lower cretaceous lacustrine section lower cretaceous passive margin sections penetrated in wells peripheral to the deep basinal development of pre salt sediments of the liawenda ponta vermelha troughs in the lower congo coastal basin revealed sustained source potential kerogen type kinetic ea a and carbon isotopic data plus rock extract biomarker signatures confirmed large systematic organofacies variations within these rift phase sediments an inverse correlation between 25 28 30 trisnorhopane and gammacerane abundances suggested three major depositional regimes reflecting salinity and water depth control 13c depleted basal sediments showing strong gammacerane 4 methyl sterane signatures were segregated by a major isotopic excursion this event represented the transition between saline playa lake to deeper water stratified conditions supporting a high level of anoxia for these intermediate sediments sterane and tricyclic diterpane abundances plus sterane hopane ratios had marine connotations and could be interpreted as the result of intermittent marine incursions fore shadowing irreversible oceanic ingression a resurgence of gammacerane abundance in the uppermost sediments typified littoral shallow marine depositional conditions recognition of a possible non ubiquity in lacustrine depositional conditions during the opening of a rifted margin provides further insight into the complexity and attendant controls on hydrocarbon generation in such habitats

modelling petroleum generation of late cretaceous dabut formation in the jiza qamar basin eastern yemen abstract the jiza qamar basin in eastern yemen is still undergoing hydrocarbon exploration and hydrocarbon potential has not been assigned yet in this study subsurface samples from two onshore and offshore wells were collected to characterize the source rocks of dabut formation and to incorporate into basin modelling in order to know and determine the timing of petroleum generation and expulsion of the dabut source rocks based on organic geochemical analysis the dabut sediments have variable toc content in the range of 0 5 2 5 wt indicating fair to good source rock generative potential the organic matter of dabut source rock is dominated by type iii kerogen with minor mixed iii ii kerogen contributions and is thus considered to be mainly gas prone and some oil prone this is supported by hi values in the range of 50 212 mg hc g toc vitrinite reflectance in the range of 0 63 0 88 ro and pyrolysis tmax in the range of 427 454 c generally indicate that the dabut source rocks contain mature organic matter one dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the dabut source rocks in the jiza qamar basin based on the reconstruction of the burial and thermal maturity histories in order to improve our understanding of the hydrocarbon generation potential calibration of the model with measured vitrinite reflectance ro and borehole temperature bht data indicates that the paleo heat flow was constant during the late cretaceous paleogene but increased during the late paleogene neogene and then decreased exponentially from neogene to present day the modelled maturity history predicts that the oil generation 0 5 ro in the dabut source rock began from about 56 ma to 42 ma and the peak hydrocarbon generation 0 86 ro occurred approximately from 14 ma to 13 ma these models also indicate that the main phase of oil generation was at 56 ma to 25 ma whereas the main phase of gas generation began from about 25 ma to 14 ma and still generated in the present day

seismic stratigraphic analysis of the punta del este basin offshore uruguay south america the punta del este basin of offshore uruguay is one of a series of mesozoic sedimentary basins along the eastern seaboard of south america the basin resulted from continental rifting of an older gondwana landmass this rifting led to the initiation of the south atlantic ocean the tectonic and stratigraphic history of the basin can be characterized by a three phase evolution with each phase involving a separate and distinct basin geometry style of faulting and lithostratigraphic succession the rift phase deposited predominantly continental siliciclastics of neocomian age arranged in a number of syndepositional assymetric trap door grabens this phase is overlain by a largely unfaulted succession of continental to marine rocks of the sag basin phase of aptian to maastrichtian age these rocks record an overall marine transgression the passive margin phase is represented by a thick sequence of tertiary sediments that postdate rift related tectonism and correspond to sedimentation along the margin of the newly opened south atlantic ocean the punta del este part of the greater salado basin is relatively unexplored with only two wells having been drilled both in the updip part of the basin these two wells have contributed some understanding of the stratigraphic succession but have failed to adequately assess the basin s true hydrocarbon potential this paper constitutes one of the first detailed comprehensive published accounts of this basin and is a prerequisite for future exploration in the area

mantle convection models of continental collision and breakup incorporating finite thickness plates abstract a two dimensional numerical model has been developed to study mantle convection during continental collision and breakup the model incorporates rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness we systematically investigate the influences of continental width diffusivity thickness and internal heating on continental collision and breakup in addition we consider the influence of different degrees of internal heating in the mantle for each case considered we model a pair of identical continents being carried towards a site of plate convergence by underlying counter rotating mantle convection cells the continents collide at the mid plane of the model to form a motionless rigid conducting supercontinent whereas oceanic plate material continues to recycle through the mantle we find that changes in the mechanical boundary conditions at the upper surface are important factors in initiating flow reversals below the supercontinent more generally our findings based on 144 models are that the following factors favour the initiation of flow reversals below the supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and continental crustal heating furthermore lower percentages of internal heating in the mantle are necessary to sustain and promote the subcontinental flow reversals in our models

thermal maturity history and petroleum generation modelling for the upper jurassic madbi source rocks in the marib shabowah basin western yemen abstract widely exposed in the marib shabowah basin the early upper jurassic madbi formation is divided into two units lam and meem youngest to oldest believed to be major source rocks the madbi source rocks are characterized by relatively high total organic carbon contents toc ranging from 0 5 to 10 0 wt meeting the standard as a fair to excellent source rock generative potential they contain type ii kerogen grading to mixed type ii iii kerogens and type iii kerogen with hi values between 40 and 620 mg hc g toc vitrinite reflectance values range from 0 41 to 2 61 r o indicating sufficient thermal maturity for both oil and gas generation burial thermal history models indicate that the upper jurassic madbi source rocks passed the peak of oil generation stage and convert the oil to dry gas in the late jurassic to late cretaceous time oil generation began immediately after deposition in the late jurassic time 150 145 ma and maximum rates of oil with significant gas have been generated during late cretaceous time 110 65 ma the peak gas generation occurred during the early tertiary the modelled hydrocarbon expulsion evolution suggests that the timing of hydrocarbon expulsion from the madbi source rocks began in the early cretaceous 140 ma persisted to present day the modelling results also indicate that the peak hydrocarbon generation and expulsion from madbi source rocks in the studied area occurs well after deposition of the seal rock and formation of traps ensuring entrapment and preservation of migrated hydrocarbons

organic facies and maturation of jurassic cretaceous rocks and possible oil source rock correlation based on pyrolysis of asphaltenes scotian basin canada abstract this paper establishes source rock characterization and oil oil and oil source rock correlations of selected organic rich shale source rocks and selected light oils and condensates from jurassic and cretaceous formations of the scotian basin multiple source rocks of kerogen types iia oil prone iia iib oil and condensate prone iib condensate and gas prone iii gas prone and iv nonsource are identified in various stratigraphic horizons vitrinite reflectance and t inmax data indicate variable maturation histories in different parts of the basin which are related to differences in lithology related heat conductivity and the presence of older sediments closer to areas of rifting multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families some kerogen type iib rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions a second family of oils could not be correlated with any of the analyzed source rock extracts pyrolysis gas chromatography of asphaltenes from the cohasset a 52 and sable island 3h 58 wells are significantly different in n alkane aromatics and phenol content the asphaltene pyrolyzates of samples from venture b 52 and s desbarres o 76 have a high aromatic content low content of n alkanes and high phenol values s desbarres which is typical of a terrestial source they correlate well with one another and also with sable island light oil the n triumph g 43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons with aromatic phenolic and thiophenic compounds in low concentrations suggesting an algal source algal type organic matter may have made a contribution to the cohasset petroleum as its asphaltene pyrolyzate is also dominated by normal hydrocarbons

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

evidence for unusually thin oceanic crust and strong mantle beneath the amazon fan we used seismic and gravity data to determine the structure of the crust and mantle beneath the amazon fan seismic data suggest that the crust is of oceanic type and is unusually thin 4 km compared to elsewhere in the atlantic we attribute the thin crust to ultraslow seafloor spreading following the breakup of south america and africa during the early cretaceous gravity data suggest that the fan was emplaced on lithosphere that in creased its elastic thickness te and hence strength following rift ing the increase from 10 km to 40 km is greater however than would be expected if te were determined by a single controlling isotherm based on a cooling plate model hence we conclude that the amazon fan has been emplaced on and is supported by un usually thin oceanic crust and strong mantle

the impact of organic fluids on the carbon isotopic compositions of carbonate rich reservoirs case study of the lucaogou formation in the jimusaer sag junggar basin nw china abstract the processes involved in the interaction between organic fluids and carbonates and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates here we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic clastic and organic rich sedimentary sequence within the middle permian lucaogou formation in the junggar basin of nw china the techniques used during this study include drillcore observations thin section petrography scanning electron microscopy sem and electron probe microanalysis and carbon and oxygen isotope analyses oil grades and total organic carbon toc contents represent the amount of oil charging and the abundance of organic fluids within a reservoir respectively and both negatively correlate with the whole rock 13c and 18o of the carbonates in the study area indicating that organic fluids have affected the reservoir rocks secondary carbonates including sparry calcite and dolomite overgrowths and cements are common within the lucaogou formation well developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite rich reservoir rocks in this formation comparing thin section petrology with 13c compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates the net change in whole rock 13c as a result of these secondary carbonates ranges from 1 8 to 4 6 with the secondary carbonates having calculated 13c compositions from 18 6 to 8 5 that are indicative of an organic origin the positive correlation between the concentration of fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the lucaogou formation was relatively closed the correlation between 13c and 18o in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids however good initial correlation between 13c and 18o of whole rock carbonates within the lucaogou formation resulted from the evaporitic sedimentary environment was reduced by organic fluids to some extent consequently the 13c 18o covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water the characteristics and 13c compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic co2 to the reservoir further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic co2 however dissolved carbonate reprecipitated as secondary carbonates meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality although this process did enhance the dissolution of feldspar and increase porosity this indicates that the 13c and 18o of secondary carbonates and their influence on whole rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate rich reservoir systems

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

lacustrine organic facies a biomarker study using multivariate statistical analysis abstract four distinct depositional settings in the devonian orcadian basin of scotland have been characterised by their minerology and organic geochemistry with the help of multivariate statistical analysis the data visualise the organic matter source input the depositional environment and the level of thermal maturity principal component analysis proved invaluable for extracting information from a data set composed of a large number of parameters the thermal maturity of the majority of the samples lies within a restricted range in the oil window and biomarker composition reflects predominantly organic input and depositional environment basin margin deposits are clearly differentiated from deeper water central lake deposits salinity and anoxicity variations are seen within the latter type of deposits

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

source rock characteristics of the lower cretaceous abu gabra formation in the muglad basin sudan and its relevance to oil generation studies organic rich shale and claystone are present within the lower cretaceous abu gabra formation of the muglad basin representing excellent oil source rocks the source rock potential of abu gabra organicrich whole rock samples from two oilfields in the muglad basin was investigated using bulk and quantitative pyrolysis techniques the analytical program included toc pyrolysis sra pyrolysis gc bulk kinetics elemental analysis and kerogen microscopy the results were used to characterize the kerogen composition and type of petroleum generated from these abu gabra organic rich sediments to clarify the structural variability of the organic matter in the abu gabra source rock the investigated abu gabra organic rich sediments proved to contain excellent quality lacustrine kerogens the samples have moderate to high hydrogen index hi values 267e837 mg hc g rock consistent with predominantly oil prone types i and ii kerogen with a minor contribution of type iieiii kerogens this is supported by the abundant liptinitic materials i e lipid rich structured algae structureless amorphous alginite and sporinite and high atomic hydrogen to carbon h c ratio however pyrolysis gc analysis proved that all the investigated abu gabra samples contain homogeneous type i kerogen that produces mainly waxy oil bulk kinetic experiments on these samples indicate that the source rock facies derived from a homogenous organic matter spans an extremely narrow distribution of activation energy ea ranges from 49 kcal mol to 51 kcal mol the magnitude order of frequency factors a also show a narrow distribution range from 10

thermal history of sedimentary basins maturation indices and kinetics of oil and gas generation temperature is the most sensitive parameter in hydrocarbon generation thus reconstruction of temperature history is essential when evaluating petroleum prospects no measurable parameter can be directly converted to paleotemperature maturation indices such as vitrinite reflectance tmax from rock eval pyrolysis spore coloration thermal alteration index tai or concentration of biological markers offer an indirect approach all these indices are a function of the thermal history through rather complex kinetics frequently influenced by the type of organic matter their significance and validity are reviewed besides the problems of identification e g vitrinite and interlaboratory calibration it is important to simultaneously interpret kerogen type and maturation and to avoid difficult conversions from one index to another geodynamic models where structural and thermal histories are connected are another approach to temper ture reconstruction which could be calibrated against the present distribution of temperature and the present value of maturation indices kinetics of kerogen decomposition controls the amount and composition of hydrocarbons generated an empirical time temperature index tti originally introduced by lopatin does not allow such a quantitative evaluation due to several limitations no provision for different types of kerogen and different rates of reactions poor calibration on vitrinite reflectance it is of limited interest unless one has no access to a desk top computer kinetic models based on a specific calibration made on actual source rock samples can simulate the evolution of all types of organic matter and can provide a quantitative evaluation of oil and gas generated examples from the jurassic source rocks of the paris basin monterey formation of california green river shales of utah paleozoic source eds of the algerian sahara and miocene rocks of the mahakam delta indonesia illustrate various aspects of the discussion geological geochemical models are the most efficient way to integrate geological seismic and geochemical data and they should greatly help to reduce the risk in exploration

oil charging model and controlling factors revealed by an online nuclear magnetic resonance nmr system abstract it is very difficult to simulate the charging process of crude oil under experimental conditions due to the low porosity and permeability of tight sandstone however simulations are important for revealing the mechanism of formation of tight oil and the oil water distribution characteristics as well as for enhancing the oil recovery efficiency in this study we established a new method called nuclear magnetic resonance coupling displacement nmr dp to simulate the tight oil charging process online the nmr dp method has the advantages of high precision velocity recyclability and the ability to reflect the distribution of fluids in the micropores of a tight rock we conducted a simulation experiment involving the charging of crude oil into eight sandstone samples from gaotaizi in the qijia longhupao area of the songliao basin the results of this experiment were combined with those of microscopic measurements conducted on the pore structure i e nuclear magnetic resonance nitrogen adsorption high pressure and rate controlled mercury injections and micron computed tomography in this way we clarified the charging process and its controlling factors e g physical properties and wettability and established a tight oil charging model the results indicate that the oil charging saturation was controlled by several factors including the charging pressure physical properties pore throat structure and mineral composition of the rock a smaller radius of the pore throat corresponded to a lower oil charging saturation and higher starting pressure gradient the lower the content of clay and carbonate minerals and the higher the content of feldspar mineral the higher the oil saturation oil mainly occurred in the larger pores while residual water mainly occurred in the smaller pores the charging process was divided into four stages an exponential rapid growth stage a logarithmic slow growth stage a linear slow growth stage and a quasi stationary stage the method established in this paper is expected to be helpful for analyzing the distribution law of oil and water in tight sandstone areas thereby aiding in the prediction of sweet spots and development of tight oil

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

the tectonics of the okhotsk sea abstract the northern and central parts of the okhotsk sea form an epimesozoic platform the hetero aged acoustic basement is represented by deformed geosynclinal rocks from cretaceous to precambrian in age the slightly deformed sedimentary cover levelled the uneven surface of the acoustic basement and this upper paleogene neogene cover filled up the system of the structural basins the general nw se and w e extensions of the taphrogenic horsts and grabens of the acoustic basement were formed due to extension and subsidence of the earth s crust during the late paleogene neogene

continent elevation mountains and erosion freeboard implications 1 to the simplest approximation earth s continental crust is a floating aggregate on the planet s surface that is first attracted to subduction zones and upon arrival thickened by mountain building then producing some extension thickened regions are thinned again by erosion a comparison between 65 ma and the present shows that the modern state is significantly more mountainous an estimated average continental elevation increase relative to average ocean floor depth of about 54 m and sea level decrease relative to the ocean floor of about 102 m add up to a 156 m increase of continent elevation over sea level since 65 ma both are affected most strongly by the roughly 1 7 continent surface area decrease caused by cenozoic mountain building this includes contributions from erosion volumes of sediments in deltas and submarine fans indicate an average thickness of 371 m deposited globally in the ocean basins since 65 ma this relatively large change of continent area over a short span of earth history has significant consequences extrapolating if continent area change exceeded 5 in the past either severe erosion or flooded continents occurred if continent elevation freeboard remains at the present value of a few hundred meters the past continent ocean area ratio might have been quite different depending on earlier volumes of continental crust and water we conclude that along with the ages of ocean basins continental crustal thickening exerts a first order control on the global sea level over hundreds of million years

chemostratigraphy and sedimentary facies analysis of the permian lucaogou formation in the jimusaer sag junggar basin nw china implications for tight oil exploration abstract the middle permian lucaogou formation p2l in the jimusaer sag of the southeastern junggar basin nw china hosts china s first commercial tight shale oil production two tight oil sweet spot intervals have been identified within the p2l formation coupled chemostratigraphic and sedimentary facies analysis reveals that the sweet spot intervals were deposited in deep shallow saline lacustrine to nearshore environments under an overall dry climate setting the sweet spot intervals in the p2l formation comprises several chemostratigraphically and lithologically distinct units deposited under the influence of subtle climatic and environmental changes that have previously not been recognized a total of 11 depositional units have been identified within the two sweet spot intervals based on an integrated prediction error filter analysis infefa of gamma ray logs and environmental parameters derived from chemostratigraphic data the tight oil reservoir sweet spot intervals were found to be controlled by the spatial and temporal distribution of total organic carbon toc and reservoir properties e g porosity and permeability and source reservoir coupling two potential tight oil exploration plays are recognized including those depositional units with porous reservoir beds interbedded with high toc source beds a self generation play and those units with porous reservoir beds adjacent to high toc source beds a near source play

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

global tectonic and climatic control of mean elevation of continents and phanerozoic sea level change abstract mean elevation of all continents as well as mean elevation of individual continents should be at the fundamental level controlled by the global tectonic and climatic systems i propose a first order model considering the interplay of the two factors in controlling mean elevation of continents the model is able to account for the positive correlation between the present day mean elevation and area of individual continents except for antarctica furthermore it can also explain the low sea level during the times of supercontinents finally the model is used to evaluate the variation of continental crust thickness with time

fractal analysis of tight shaly sandstones using nuclear magnetic resonance measurements assessing quantitatively the microscopic pore structures of porous rocks including irregularities of pore shapes and pore size distributions is becoming one of the most challenging efforts nuclear magnetic resonance nmr measurements were used to provide insights into the pore geometry pore size and shape and pore connectivity of the chang 7 tight shaly sandstones in situ permeability 0 1 md in ordos basin the incremental transverse relaxation time t2 distributions for the 100 brine saturated samples display unimodal and bimodal behaviors presenting a geometrical arrangement composed of small to large pore size domains the nmr parameters such as bulk volume irreducible bvi capillary and clay bound water free fluid index ffi movable water the value of t2 separating the bvi from ffi the amplitude weighted mean on a logarithmic scale t2gm and the value of t2 that shows the highest frequency on the t2 spectrum t2peak for each sample were determined then the fractal theory was adopted to quantitatively express the complexity and heterogeneity of the sandstones the results show that only minor primary intergranular porosity remains and variable amounts of micropores and secondary intragranular porosity with poor connectivity occur in the chang 7 tight shaly sandstones assuming spherical pores a new model to calculate the fractal dimension of pore structure from the nmr t2 distributions is proposed the fractal dimensions of all the samples are calculated and the accuracy of the proposed model is verified by the regression coefficients the microscopic pore structures are heterogeneous in these tight sandstones according to the high value of fractal dimensions micropores are the primary causes of heterogeneity in tight sandstones and samples with unimodal t2 distribution behaviors and high content of short components have the highest fractal dimension and heterogeneity the calculated fractal dimension is strongly correlated with t2peak and t2gm therefore the fractal model proposed in this study can be used to calculate the fractal dimensions and evaluate the heterogeneities of the porous rocks satisfactorily the fractal model proposed in this study helps to quantitatively assess the pore structures of tight sandstones using nmr measurements

the effects of accretion erosion and radiogenic heat on the metamorphic evolution of collisional orogens petrological and thermochronological data provide our best record of the thermal structure of deeply eroded orogens and in principle might be used to relate the metamorphic structure of an orogen to its deformational history in this paper we present a two dimensional thermal model of collisional orogens that includes the processes of accretion and erosion to examine the p t evolution of rocks advected through the orogen calculated metamorphic patterns are similar to those observed in the field metamor phic temperatures depths and ages generally increase with distance from the toe of the orogen p t paths are anti clockwise with rocks heating during burial and early stages of unroofing followed by cooling during late stage unroofing the results indicate that peak metamorphic temperatures within the core of a collisional orogen and the distance from the toe of an orogen to the metamorphic core can be related to the relative rates of accretion erosion and plate convergence model orogens displaying high metamorphic temperatures 600 c are associated with low ratios of accretion rate to plate convergence velocity and with high heat flow through the foreland model orogens with metamorphic cores far from the toe of the orogen are associated with high ratios of accretion rate to erosion rate calculated metamor phic gradients mimic steady state geotherms and inverted thermal gradients can be preserved in the metamorphic record suggesting reconsideration of the concept that the metamorphic record does not closely reflect geothermal gradients within an orogen

paleoenvironmental setting mechanism and consequence of massive organic carbon burial in the permian junggar basin nw china abstract the junggar basin in northwestern china is a typical walled sedimentary basin that has a prolonged basin evolution history from the paleozoic to cenozoic and is rich in petroleum resources permian strata reach thousands of meters in thickness and have been intensively studied for their record of regional tectonics and paleoenvironments they have also been exploited for oil and gas for the past several decades in this paper we summarize perspectives on tectonic evolution stratigraphic framework sedimentary environments and the paleoclimate of the permian junggar basin and the adjacent turpan basin further we discuss the mechanism and consequences of massive organic carbon burial based on a compilation of published radiometric dating datasets an updated chronostratigraphic framework was proposed for the junggar basin this framework constrains the ages of two main petroleum source intervals the fengcheng formation and the lucaogou and equivalent formations as early permian and middle permian respectively although a global warming and drying trend was present throughout the permian period the regional climate over the junggar basin was drier in early permian and more humid in the middle late permian with short term climatic fluctuations during the deposition of the two petroleum source intervals dry and temperate climate prevailed and evaporative lacustrine environment dominated organic matter enrichment was promoted by stable salinity stratification and limited dilution by inorganic sediments leading to one of the richest lacustrine source rock deposits in the world the average organic carbon burial rates were 11 2 gc m2 yr for the lucaogou formation and 3 5 gc m2 yr for the fengcheng formation lower than the average organic carbon burial rate in modern lakes however considering the large areas of global tectonic lakes in the permian period organic carbon burial in lacustrine environments could act as a significant carbon sink and impact the global carbon cycle and climate evolution

the crustal structure of the southern caspian region the south caspian basin appears to behave as a relatively rigid aseismic block within the otherwise deforming alpine himalayan orogenic belt this anomalous character suggests that there is a fundamental compositional difference between the crust of the south caspian basin and that of the surrounding region five new estimates of crustal velocity structure beneath the southern caspian region are determined by the teleseismic receiver function method these models show that the crust in turkmenia along the trend of the apshceron balkhan sill kopet dag mountains is 50 km thick in the southwestern part of the caspian basin the crust is 33 km thick and consists of a 13 km thick sedimentary section lying on a high velocity vp 7 1 km s 1 lower crustal section in the southeastern part of the basin the crust is 30 km thick and consists of a 10 km thick sedimentary section overlying a 20 km thick low velocity vp 5 8 km s 1 crystalline crust the receiver function models are combined with velocity models from previous russian deep seismic sounding studies into a 1800 km long ese wnw trending crustal cross section across the kura depression the south caspian basin and the kopet dag mountains the most significant features of this crustal model are the 20 km variation in thickness of cenozoic sedimentary basin deposits the absence of a granitic vp 5 8 6 5 km s 1 crustal layer in the central part of the south caspian basin and 20 km of crustal thinning beneath the central part of the basin the moho beneath the south caspian basin has a broad arch like structure whose western boundary is a relatively narrow zone across which the crust thins rapidly 20 km thinning over a 100 km zone and whose eastern boundary has a more gradual change in crustal thickness 20 km thinning over a 400 km zone the velocity depth profiles derived from the receiver functions are compared to laboratory velocity estimates made at pressures and temperatures appropriate for the lower crust in turkmenia but the seismic p wave data alone do not permit differentiation between the various possible rock types the crustal model developed for the south caspian basin is consistent with the hypothesis that the crystalline crust of the basin is a section of oceanic crust that is being overthrust by the continental crust around much of its margins however we cannot rule out the possibility that the crystalline crust beneath the basin is a section of lower continental crust whose upper crustal section has been removed by erosion or faulting

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

reevaluating the source and accumulation of tight oil in the middle permian lucaogou formation of the junggar basin china abstract the middle permian lucaogou formation in the jimusar sag junggar basin nw china is a world class example of tight oil accumulations in lacustrine sedimentary sequences previous studies have indicated that the main oil accumulation mechanism in this system is near source accumulation within upper and lower sweet spots i e reservoirs here we reveal the complexity of oil source and accumulation in this tight oil system is greater than previously thought a set of mudstones are developed between the upper and lower sweet spots of the lucaogou formation and are thus termed the middle section mudstones they have previously been little studied as few intervened reservoir rocks are developed within the sequences here we report their oil generation and contribution for the first time the mudstones are 97 182 m thick and have total organic carbon contents of 0 67 12 31 wt the kerogen is mainly type ii with small amounts of types i and iii and is in the mature stage these data suggest that the middle section mudstones have good hydrocarbon generation potential oil source correlations reveal a complex history of tight oil migration and accumulation the oils generated in the upper part of the middle section mudstones migrated northward to the basin margins and southeastward to the basin center in contrast oil generated in the lower part of the middle section mudstones migrated westward to the west central and central regions the oil migration resulted in complex tight oil accumulation which reflects the reservoir heterogeneity our data suggest that such reservoir heterogeneity appears to be typical of tight oil accumulations in continental lacustrine basins in the study area future evaluation exploration and exploitation of tight oil should also focus on the middle section mudstones and associated sweet spots in addition to the commonly acknowledged upper and lower sweet spots

characteristics and formation mechanism of multi source mixed sedimentary rocks in a saline lake a case study of the permian lucaogou formation in the jimusaer sag northwest china abstract the permian lucaogou formation in the jimusaer sag of the southeastern junggar basin in china is an uncommon set of mixed sedimentary rocks the mixed sediment content of the lucaogou formation is unusual due to the large amounts of tuffaceous material which makes the system more complex than the commonly mixed siliciclastic carbonate rocks in this study the volcanic material acts as a third end member component in a multi source mixed sedimentary system therefore a new type of mixed siliciclastic carbonate tuffaceous rocks has been introduced methods including core observations thin section analysis grain size measurements and x ray diffraction are used to investigate the characteristics of mineral components sedimentary structures and lithological assemblages in upper and lower sections of the formation these analyses are then used to determine the lateral distribution area of mixed sedimentation sedimentary mixing processes originally described for siliciclastic carbonate systems are used and expanded here for the multi source mixed rocks in a saline lacustrine basin controlling factors on mixed sedimentation included climate change lake level fluctuation tectonic activity and sediment supply rate mentioned in previous studies as well as variations of energy levels in different parts of the lake mixed depositional models for the lower and upper sections of the lucaogou formation in saline lacustrine basin have been established respectively

application of biological markers in the assessment of paleoenvironments of chinese non marine sediments abstract in this study we employ molecular organic geochemical data for 22 representative sediment samples from chinese non marine sedimentary basins in an attempt to characterize and assess the non marine depositonal paleoenvironments the samples investigated include oilshale shale mudstone and sodium sulphate rich sediments ranging in age from cretaceous to tertiary it is evidenced by geological and geochemical data that those sediments studied originate from lacustrine environments ranging from freshwater brackish fresh water to hypersaline the aliphatic fractions of the sediment extracts were analyzed by gc and gc ms and relative abundance of compounds and compound classes of normal acyclic and cyclic alkanes were measured from mass chromatograms the results reveal significant differences in the chemical features of the various sample groups which enable them to be characterized and identified in terms of source and or environment related parameters the diagnostic parameters that allow this assessment are the relative abundances of n alkanes acyclic isoprenoid alkanes and 4 methyl steranes hopane sterane ratio gammacerane and homohopane indexes etc in this study data interpretation was also assisted by pattern recognition through principal component analysis the results show that the sample groups of different environmental origin could be unambiguously distinguished on scores plot

molecular organic geochemistry of the tarim basin northwest china we conducted organic geochemical analyses on the largest suite of oils and source rock extracts from the tarim basin northwest china currently available statistical cluster analysis of the entire suite of tarim oils distinguishes at least seven genetic groups of oils the largest group of oils was collected from the tazhong and tabei uplifts and originated from marine middle upper ordovician anoxic marls that mark slope facies at the margins of structural uplifts two other genetic groups most likely originated from marine middle upper ordovician source rocks but of distinct facies with one an oxic shale rich source west of the bachu uplift and the other an anoxic shale source at tazhong

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

investigation on the pore structure and multifractal characteristics of tight oil reservoirs using nmr measurements permian lucaogou formation in jimusaer sag junggar basin abstract pore structure is a critical parameter to estimate the reservoir quality and evaluate the resource potential however the pore structure and heterogeneity of the unconventional tight oil reservoirs of permian lucaogou formation of jimusaer sag junggar basin are not studied well in this paper the multifractal analysis based on the nmr t 2 distributions was applied to investigate the pore structure and heterogeneity the reservoir quality index rqi and flow zone indicator fzi were calculated using porosity and permeability two parameters t 35 and t 50 were newly defined as relaxation times corresponding to the 35 and 50 saturation on the t 2 reverse accumulative curve respectively the results showed that the rqi and fzi values were extremely low and that the nmr t 2 distributions of water saturated samples had much short relaxation components with no or little long relaxation components indicating a complex and poor microscopic pore structure of tight oil rocks the typical multifractal characteristics such as the large values of singularity strength range revealed that the pore structures of samples were strongly heterogeneous it was also found that the clay contents have an obvious influence on the multifractal parameters and the fzi and new defined t 35 are two good indicators for heterogeneity of pore structure in addition the feasibility and applicability of calculating the multifractal parameters from nmr and other well logs were discussed

petroleum geology of the giant elm coulee field williston basin the elm coulee field of the williston basin is a giant oil discovery in the middle bakken formation devonian mississippian discovered in 2000 horizontal drilling began in the field in 2000 and to date more than 600 wells have been drilled the estimated ultimate recovery for the field is more than 200 million bbl of oil 31 8 million m3 the bakken formation in the field area consists of three members 1 upper shale 2 middle silty dolostone and 3 lower siltstone the total bakken interval ranges in thickness from 10 to 50 ft 3 1 to 15 3 m over the field area the upper shale is dark gray to black hard siliceous slightly calcareous pyritic and fissile the shale consists of dark organic kerogen minor clay silt size quartz and some calcite and dolomite the kerogen consists mainly of amorphous material and the organic material is distributed evenly throughout the shale interval not concentrated in laminations or lenses the upper shale ranges in thickness from 6 to 10 ft 1 8 to 3 1 m over the field area the middle member consists of a silty dolostone and ranges in thickness from 10 to 40 ft 3 1 to 12 2 m the lower member in the elm coulee field consists of brownish gray argillaceous organic rich siltstone burrowing and brachiopod fragments are common in the lower member this facies is equivalent to the lower bakken black shale facies on the northern side of the field and is interpreted to be an updip landward equivalent to the deeper water black shale facies the lower member ranges in thickness from 2 to 6 ft 0 61 to 1 8 m based on the abundance of fossil content and amount of burrowing the members of the bakken formation are interpreted to have been deposited under aerobic middle member common burrows and rare fossils dsyaerobic lower member common fossils and lesser amount of burrows and anaerobic conditions upper member rare fossils and burrows the main reservoir in elm coulee field is the middle member which has low matrix porosity and permeability and is found at depths of 8500 to 10 500 ft 2593 to 3203 m the current field limits cover approximately 450 mi2 1165 km2 the middle bakken porosities range from 3 to 9 and permeabilities average 0 04 md overall the reservoir quality in the middle bakken improves upward as the amount of clay matrix decreases the middle bakken is interpreted to be a dolomitized carbonate shoal deposit based on subsurface mapping and dolomite lithology the main production is interpreted to come from matrix permeability in the field area occasional vertical and horizontal fractures are noted in cores the vertical pay ranges in thickness from 8 to 14 ft 2 4 to 4 3 m the bakken is slightly overpressured with a pressure gradient of 0 53 psi ft 0 02 kpa m horizontal wells are drilled on 640 to 1280 ac 259 to 518 4 ha spacing units long single laterals dual laterals and trilaterals have all been drilled in the field the horizontal intervals are sand gel and water fracture stimulated initial production ranges from 200 to 1900 bopd 31 8 to 302 1 m3 day initial potential rates for vertical wells are generally less than 100 bopd 15 9 m3 day the upper bakken shale probably also contributes to the overall production in the field the exact contribution is unknown but estimated to be less than 20 of the total production the elm coulee field illustrates that the bakken petroleum system has enormous potential for future oil discoveries in the williston basin

thermochronology of the modern indus river bedload new insight into the controls on the marine stratigraphic record 1 the indus river is the only major drainage in the western himalaya and delivers a long geological record of continental erosion to the arabian sea which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains in this study we collected thermochronologic data from river sediment collected near the modern delta u pb ages of zircons spanning 3 gyr show that only 5 of the eroding crust has been generated since india asia collision the greater himalaya are the major source of zircons with additional contributions from the karakoram and lesser himalaya the 39ar 40ar dating of muscovites gives ages that cluster between 10 and 25 ma differing from those recorded in the bengal fan biotite ages are generally younger ranging 0 15 ma modern average exhumation rates are estimated at 0 6 km m y or less and have slowed progressively since the early miocene 20 ma although fission track ft dating of apatites may indicate a recent moderate acceleration in rates since the pliocene 1 0 km m y driven by climate change the 39ar 40ar and ft techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean localized regions of tectonically driven very rapid exhumation e g nanga parbat s karakoram metamorphic domes do not dominate the erosional record

characterization of shale pore system a case study of paleogene xin gouzui formation in the jianghan basin china abstract the pore size classification micropore 50 nm of iupac 1972 has been commonly used in chemical products and shale gas reservoirs however it may be insufficient for shale oil reservoirs to establish a suitable pore size classification for shale oil reservoirs the open pore systems of 142 chinese shales from jianghan basin were studied using mercury intrusion capillary pressure analyses a quantitative evaluation method for i micropores 0 25 nm in diameter ii micropores 25 100 nm mesopores 100 1000 nm and macropores 1000 nm within shales was established from mercury intrusion curves this method was verified using fractal geometry theory and argon ion milling scanning electron microscopy images based on the combination of pore size distribution with permeability and average pore radius six types i vi shale open pore systems were analyzed moreover six types open pore systems were graded as good medium and poor reservoirs the controlling factors of pore systems were also investigated according to shale compositions and scanning electron microscopy images the results show that good reservoirs are composed of shales with type i ii and iii pore systems characterized by dominant mesopores mean 68 12 vol a few macropores mean 7 20 vol large porosity mean 16 83 an average permeability of 0 823 md and an average pore radius r a of 88 nm type iv pore system shales are medium reservoirs which have a low oil reservoir potential due to the developed ii micropores mean 57 67 vol and a few of mesopores mean 20 19 vol poor reservoirs composed of type v and vi pore systems are inadequate reservoirs for shale oil due to the high percentage of i micropores mean 69 16 vol which is unfavorable for the flow of oil in shale pore size is controlled by shale compositions including minerals and organic matter and arrangement and morphology of mineral particles resulting in the developments of shale pore systems high content of siliceous mineral and dolomite with regular morphology are advantage for the development of macro and mesopores while high content of clay minerals results in a high content of micropores

geochemical characteristics of crude oil from a tight oil reservoir in the lucaogou formation jimusar sag junggar basin jimusar sag which lies in the junggar basin is an important tight oil study area in china however the properties and origin of the crude oil and the geochemical characteristics of the tight oil from the lucaogou formation have not yet been studied in the present study 23 crude oil samples from the lucaogou formation were collected for analysis including physical properties density and viscosity bulk composition saturated hydrocarbon gas chromatography mass spectrometry and calculation of various biomarker parameters source rock evaluation and porosity permeability analysis were applied to the mudstones and siltstones to analyze the hydrocarbon generation history of the lucaogou source rocks one dimensional basin modeling was performed the oil filling history was also investigated by means of basin modeling and microthermometry biomarkers of suitable source rocks total organic carbon content is 1 summation of the free and volatile hydrocarbon and remaining hydrocarbon generative potential is 6 mg hc g rock and vitrinite reflectance is between 0 7 and 1 were analyzed for oil source correlation the results indicate that low maturity to mature crude oils originated from the source rocks containing terrigenous organic matter which were deposited in a saline lake the source rocks are of mainly type ii kerogen oil in the lower section of the lucaogou formation is relatively dense and viscous and has elevated nonhydrocarbon content in addition a higher proportion of bacteria and algae is shown to have contributed to the formation of crude oil in the lower section when compared with the upper section of the lucaogou formation oil source correlations demonstrate that not all mudstones within the lucaogou formation contribute to oil accumulation thin section observation indicates organic macerals surround the grains of some siltstones within the porous and permeable subsections which proves the siltstones contribute to hydrocarbon generation crude oils from the upper and lower sections mainly originate from thin bedded mudstones interbedded and siltstones within the porous and permeable subsections the mudstones over or beneath the porous and permeable subsections consist of seals and prevent the vertical movement of oil by capillary forces despite being thicker mudstones between the two porous and permeable subsections make no obvious contribution to the oil accumulation because of their low permeability and distance from the porous and permeable subsections which is consistent with the principle of near source charging for tight oil accumulations lucaogou oil has been charged twice once from 235 to 210 ma and again from 180 to 150 ma coinciding with oil generation time

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

rates of continental erosion and mountain building the first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny the dominant part is played by mechanical erosion which acts most strongly in regions of large topographic expression chemical erosion depends strongly on precipitation or runoff in individual river drainage basins but because most continents have very similar average rainfall chemical erosion is fairly uniform for continental sized areas and will succeed in planing down all continents to a level peneplain if given enough time the exception to this rule is australia which has a very low chemical erosion rate because of its dryness the time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 my depending on the continent and the assumptions made mountain building occurs throughout the geological time scale but at a non uniform rate although there will not be a balance between erosion and mountain building over a short time scale due to the non uniform rate of mountain building the long term situation must be that the two phenomena should balance out it is shown that the freeboard of continents will respond to the long term balance between mountain building and erosion an expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth it is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the proterozoic as mountain building is predicted to decrease on average with time because of the reduction in internal heat generation and as erosion is dependent on the average elevation this average elevation will decrease slowly through time the opposite of what some workers have predicted a more complicated model of mountain building is then investigated in which one component of mountain building has a sinusoidal signal the oscillations in average elevation depend on the period of the sinusoid being smaller for shorter periods finally an average continental elevation is derived using a list of real orogenic events although this list of orogenies is incomplete there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper

oligocene uplift of the western greater caucasus an effect of initial arabia eurasia collision the greater caucasus is europe s largest mountain belt significant uncertainties remain over the evolution of the range largely due to a lack of primary field data this work demonstrates that depositional systems within the oligocene early miocene maykop series on either side of the western greater caucasus wgc display a similar provenance and divergent palaeocurrents away from the range constraining a minimum age for the subaerial uplift of the range as early early oligocene an eocene oligocene hiatus basal oligocene olistostromes and a marked increase in nannofossil reworking also point to initial deformation in the earliest oligocene the initial uplift of the wgc occurred during the final assembly of the tethysides to its south uplift commenced after the late eocene final suturing of northern neotethys and during the initial collision of arabia with the southern accreted margin of eurasia this suggests that compressional deformation was rapidly transferred across the collision zone from the indenting arabian plate to its northern margin

depositional model for a salinized lacustrine basin the permian lucaogou formation jimsar sag junggar basin nw china abstract fine grained sedimentary rocks are well developed in a salinized lacustrine basin in the junggar basin china understanding the sedimentary characteristics and process of these fine grained sediments is essential to find the exploration target for tight oil in this study the sedimentary characteristics and forming mechanism of fine grained sediments in the permian lucaogou formation within the jimsar sag were investigated based on core and thin section observation sem combined with xrd and geochemical data and eleven lithofacies were identified including 1 brecciated dolomitic mudstone 2 intraclastic conglomerate 3 mixed siliciclastic and intraclastic grainstone 4 dolomitic siltstone 5 carbonate packstone and grainstone 6 dolomitic mudstone 7 laminated algae bearing tuffaceous mudstone 8 massive tuffaceous mudstone 9 turbidite dolomitic mudstone to siltstone 10 pebbled sandstone and 11 green mudstone and siltstone the organic constituents in the fine grained sedimentary rocks include sapropelic exinite vitrinite inertinite and secondary macerals which are dominated by unstructured algae and asphaltene the fine grained sedimentary rocks are characterized by high frequency cycles composed of three successive facies associations profundal littoral sublittoral and palustrine which recorded distinct lacustrine expansion and contraction developed mud cracks paleosols and vertical dissolution fractures in the fine grained sedimentary rocks indicate that these facies recorded the depositional process in an ephemeral to perennial lake in an arid climate fluvio deltaic and aeolian transport were main modes for transporting the siliciclastic component that ultimately accumulated in a subaqueous and subaerial environment the dolomite in the littoral zone might develop with the evaporitic concentration of the lake water during the early diagenetic stage the depositional processes in the profundal zone included suspension and turbidity currents multiphase volcanic activities affected the lithofacies and prompted the accumulation of organic matter understating the depositional process of the fine grained sedimentary rocks in the lucaogou formation in the jimsar sag is helpful to understand the depositional environment in the salinized lacustrine basin in the junggar basin

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

geological roles of the siltstones in tight oil play abstract understanding the hydrocarbon accumulation pattern in unconventional tight reservoirs is crucial for hydrocarbon evaluation and oil gas extraction from such reservoirs previous studies on tight oil accumulation are mostly concerned with self generation or from source to reservoir rock over short distances however the lucaogou tight oil in jimusar sag of junggar basin shows transitional feature in between the lucaogou formation comprises fine grain sedimentary rocks characterized by thin laminations and frequently alternating beds the lucaogou tight silt fine sandstones are poorly sorted dissolved pores are the primary pore spaces with average porosity of 9 20 although the toc of most silt fine sandstones after soxhlet extraction is lower than that before extraction they show that the lucaogou siltstones in the area of study have fair to good hydrocarbon generation potential average toc of 1 19 average s2 of 4 33 mg g while fine sandstones are relatively weak in terms of hydrocarbon generation average toc of 0 4 average s2 of 0 78 mg g the hydrocarbon generation amount of siltstones which was calculated according to basin modeling transformation ratio combined with original toc based on source rock parameters occupies 16 72 of oil retention amount although siltstones cannot produce the entire oil reserve they certainly provide part of them grain size is negatively correlated with organic matter content in the lucaogou silt fine sandstones fine grain sediments are characterized by lower deposition rate stronger adsorption capacity and oxidation resistance which are favorable for formation of high quality source rocks low energy depositional environment is the primary reason for the formation of siltstones containing organic matter positive correlation between organic matter content and clay content in lucaogou siltstones supports this view point lucaogou siltstones appear to be effective reservoir rocks due to there relatively high porosity and also act as source rocks due to the fair to good hydrocarbon generation capability

depositional environment and factors controlling β carotane accumulation a case study from the jimsar sag junggar basin northwestern china abstract a combined investigation of bulk geochemistry and molecular geochemistry was carried out on 31 source rock samples from the lower permian lucaogou formation in the jimsar sag of the junggar basin northwestern china geochemical characterization reveals the depositional environments and other factors favorable for the generation and accumulation of carotane an important hydrocarbon precursor many core samples have a total organic carbon content 1 and rock eval s2 values 2 mg hc g rock suggesting relatively high hydrocarbon generation potential biomarkers in the lucaogou source rock extracts are characterized by short to long chain n alkanes a wide range of pr ph ratios 0 74 1 42 and carotane index values carotane cmax 0 16 1 92 high concentrations of c29 sterane relatively low tricyclic terpane content and moderate gammacerane index values 0 14 0 29 this is consistent with a weakly oxidizing to weakly reducing depositional environment with fresh to brackish water conditions rapid input of terrigenous organic matter and a relatively small contribution of aquatic algal bacterial organic matter cross plots of biomarker parameters and the carotane index show that carotane in the lucaogou source rocks is mainly derived from terrigenous organic matter and is found predominantly in sediments deposited in weakly reducing brackish environments it does not appear to have been affected by thermal maturation