a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

permian sedimentary record of the turpan hami basin and adjacent regions northwest china constraints on postamalgamation tectonic evolution the permian marks an important yet poorly understood tectonic transition in the tian shan region of northwestern china between devonian carboniferous continental amalgamation and recurrent mesozoic cenozoic intracontinental orogenic reactivation the turpan hami basin accommodated up to 3000 m of sediment and is ideally positioned to provide constraints on this transition new stratigraphic data and mapping indicate that extension dominated early permian tectonics in the region whereas flexural foreland subsidence controlled late permian basin evolution lower permian strata in the northwestern turpan hami basin consist of coarse grained debris flow and alluvial fan deposits interbedded with mafic to intermediate volcanic sills and flows in contrast lower permian rocks in the north central and northeastern turpan hami basin unconformably overlie a late carboniferous volcanic arc sequence these lower permian strata include possible shallow marine carbonate rocks and thick volcanic and volcaniclastic rocks that are in turn overlain by littoral to profundal lacustrine facies above a regional lower permian upper permian unconformity regional sedimentation patterns record the development of a more integrated sedimentary basin the upper permian is entirely nonmarine and can be correlated east west along the depositional strike of the basin the lower upper permian consists of a broad belt of braided fluvial deposits shed northward these strata are overlain by fluctuating littoral and profundal lacustrine facies and associated fluvial facies the uppermost permian is characterized by shallow lake plain and fluvial environments the early permian association of diffuse volcanism and partitioning of subbasins by normal faulting is consistent with an early phase of lithospheric extension local relationships indicate west east extension in the turpan hami basin along faults oriented normal to late devonian carboniferous collisional sutures within the tian shan the cause of extension in the wake of carboniferous orogenesis remains enigmatic however the temporal and spatial relationships of the two strain regimes suggest that they are genetically related upper permian stratigraphy and unconformities and local late permian triassic contractional deformation record foreland basin development when the turpan hami region became a wedge top basin with respect to the north tian shan fold and thrust belt flexurally induced late permian subsidence is also manifested in the larger junggar basin to the north where 4000 m of strata are preserved in the foredeep region the turpan hami and junggar basins were depositionally connected for much of the late permian when a vast lacustrine system developed across northwestern china this lacustrine paleogeography was only occasionally interrupted possibly by structural damming during uplift of the orogenic wedge

along axis gravity gradients at mid ocean ridges implications for mantle flow and axial morphology along axis gradients in mantle bouguer anomalies mbas were calculated for detailed gravity surveys at the southwest central and southeast indian ridges with half spreading rates of 0 9 2 3 and 3 1 cm yr respectively mba gradients at the very slow spreading southwest indian ridge are 0 4 0 7 mgal km similar to the gradients found at the slow spreading mid atlantic ridge whereas gradients at the central indian and southeast indian ridges are 0 2 0 35 mgal km the results of this study when combined with other published data show that axial mba gradients at ridge segments with an axial high are about 0 1 mgal km independent of spreading rate gradients at segments with a median valley are higher and generally decrease with increasing spreading rate the spreading rate independent mba gradient at ridge segments with an axial high represents the mantle gravity signal due to the focusing of upwelling within segments the larger gravity gradients at segments with an axial valley result from the superposition of intrasegment variations in crustal thickness on this mantle signal the consistent relation between along axis mba gradient and across axis axial morphology suggests that the axial morphology is related to the efficiency of along axis transport of melt

biomarker distributions in australian oils predominantly derived from terrigenous source material oils found in many australian sedimentary basins contain a relatively high input of terrigenous organic source material a study of a large number of these oils has permitted a number of biomarker distributions to be determined that appear to be characteristic for oils derived from terrigenous source material these features include the presence of sesquiterpanes absence of tricyclic terpanes relatively high concentrations of a c24 tetracyclic terpane predominance of c29 steranes presence of two unidentified c27 and c30 terpanes and virtual absence of 18 h oleanane the latter observation is significant since it appears that 18 h oleanane appears only in oils of tertiary age derived from terrigenous source material an attempt has also been made to compare the results for the australian oils with published results for oils derived from terrigenous source material in other parts of the world the number of reports in the literature for these oils is fairly limited but it appears that some of the observations for the australian oils can be extended to them this study is an initial attempt to devise a classification scheme that can be used to determine the origin of an oil unambiguously from biomarker parameters in order for this approach to be successful a relatively large number of biomarker parameters need to be assigned as characteristic of a particular type of source material

upper oligocene lacustrine source rocks and petroleum systems of the northern qaidam basin northwest china our organic geochemical study of oils from the northern qaidam basin defines a family of genetically related oils that contain biomarkers indicative of source rocks deposited in tertiary hypersaline anoxic lacustrine settings although cenozoic outcrop samples from northern qaidam are too organic lean to be of source quality dark laminated upper oligocene mudstones containing gypsum crystals and pyrite from the shi 28 well yield total organic carbon toc and rock eval data indicative of fair to good source rocks organic matter is derived from algae and bacteria and there apparently was little contribution from terrestrial material biomarker data provide a good correlation between the produced oils and the upper oligocene shi 28 core samples hydrocarbons derived from these source rocks are contained in upper oligocene miocene and pliocene reservoirs although eight of the oil samples are from the northwest corner of the basin one sample in this genetic family of hypersaline oils comes from northeast qaidam an area previously believed to only produce oils derived from jurassic freshwater lacustrine source rocks this sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin hypersaline oils and the associated source rocks have low biomarker maturity parameters thermal modeling indicates that hydrocarbon generation probably occurred in northwestern qaidam within the last 3 million years

variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time abstract the analysis of the sterane data of a large set of crude oils 414 derivedfrom marine carbonate 27 and siliciclastic source rocks 14 where influences of terrestrial or lacustrine derived organic matter can reasonably be excluded shows that there are increases in the relative content of c28 steranes and decreases in the relative content of c29 steranes through geological time there are no consistent variations in the relative content of c27 steranes through time with one major exception a proterozoic oil from oman palaeozolic and older crude oils are thus generally characterised by strong predominances of c29 steranes and low relative concentrations of c28 steranes significantly higher proportions of c28 steranes and lower proportions of c29 steranes occur in oils derived from jurassic and upper cretaceous source rocks these changes through time do not appear to reflect the chemical evolution of the sterols of one particular variety of marine organism the increase in c28 steranes may be related to the increased diversification of phytoplantonic assemblages in the jurassic and cretaceous possible sources of the c28 sterols necessary for the observed changes in crude oil steranes includes diatoms coccolithophores and dinoflagellates although the technique does not give an accurate means of determining the age of the source rock of a crude oil it is possible to distinguish younger crudes derived from the upper cretaceous and tertiary from palaeozoic and older crudes

geochemical comparison of fluid inclusion and present day oil accumulations in the papuan foreland evidence for previously unrecognised petroleum source rocks abstract petroleum production in papua new guinea is limited to the papuan fold belt present day oil accumulations in fields such as iagifu hedinia and gobe are thought to be mainly derived from clay rich jurassic marine source rocks containing terrestrially derived organic matter that were deposited under oxic conditions in this study other generative source rocks in the papuan foreland are indicated using fluid inclusion fi oil data from three wildcat wells koko 1 bujon 1 and kimu 1 which were drilled on the bosavi arch detailed organic geochemistry of these fi oils and associated free oils or oil stains suggests that none of these were generated from the aforementioned jurassic source rocks instead at least three different generative source rocks are indicated firstly a marine probably late cretaceous or younger source rock deposited under reducing conditions is indicated by the bujon 1 fi oil this source rock had a moderate contribution from higher plant organic matter including oleanane and or lupane the bujon 1 fi oil has similarities with fi oils from the iagifu 7x and p nyang 2x wells in the papuan fold belt secondly an algal dominated lacustrine source rock is indicated by the biodegraded koko 1 rft crude oil the unbiodegraded fi oil from koko 1 and the bujon 1 oil stains from the toro and imburu formations these lacustrine oils have high abundances of tricyclic terpanes pregnane homopregnane and gammacerane contain carotane and have high c26 c25 tricyclic terpane ratios the source of the lacustrine oils is inferred to contain moderate amounts of terrestrial organic matter indicated by a dominance of c29 steranes and abundant aromatic higher plant biomarkers and could be gypsiferous shales embedded in oligocene miocene carbonates or triassic age sediments deposited on igneous basement during early rifting associated with the break up of gondwana the kimu 1 fi oil was generated from a mature marine suboxic source rock but has a different geochemical composition to free oil from a similar interval which was derived from a carbonate rich source rock none of the fi oils correlate well with oil families previously described in papua new guinea accordingly the oils recovered from fis in this part of the papuan basin support the presence of laterally varying source rock compositions and qualities and indicate that there are opportunities to develop new charge models for features across the papuan foreland furthermore koko 1 bujon 1 and kimu 1 fi oils have different geochemical compositions compared to the associated free oils or oil stains indicating that each of these localities has a multi phase charge history further work to lower exploration risk in the papuan foreland should be directed to understanding the distribution maturity and expulsion history of these newly recognised potential source rocks

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

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

lower middle jurassic nonmarine source rocks and petroleum systems of the northern qaidam basin northwest china the qaidam basin is a nonmarine petroliferous basin on the northeastern margin of the tibet plateau potential source rocks are reported from tertiary saline lacustrine deposits and jurassic freshwater lacustrine or terrestrial strata the jurassic source rock is similar in terms of depositional system lithology age and organic geochemistry to nonmarine jurassic rocks that are thought to be potentially significant hydrocarbon sources in many basins of central asia including the tarim junggar and other important petroleum producing basins these jurassic source rocks were deposited in freshwater lacustrine systems in intracontinental foreland style basins during the early and middle jurassic these lacustrine strata are associated with coals and coaly mudstones with possible secondary importance as hydrocarbon source rocks oils from the northern qaidam basin can be divided into two groups based on molecular indicators of depositional environment and age diagnostic biomarkers those derived from this jurassic freshwater lacustrine source rock and those derived from a tertiary hypersaline lacustrine source rock the correlation of some qaidam oils to a jurassic source establishes a petroleum system involving these nonmarine jurassic source rocks oils of this petroleum system have been produced from tertiary and mesozoic reservoirs in anticlinal and thrust related traps in northeastern qaidam maturation modeling of the jurassic source rock indicates that these oils were generated and expelled during the miocene pliocene in much of northern qaidam the geological context of the basin suggests that drainage within the basin primarily was vertical probably mainly along faults until evaporite or overpressured shale seals were encountered in the overlying cenozoic section documentation of this petroleum system suggests other exploration targets remain underevaluated in northern and southwestern qaidam furthermore the northern qaidam petroleum system is a useful analog to interpret the role of similar jurassic source rocks in other sparsely documented basins of central asia

the character and origin of lacustrine source rocks in the lower cretaceous synrift section congo basin west africa lower cretaceous synrift lacustrine shales from the congo basin west africa have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon the sequence includes a lower active rift section deposited during active faulting and subsidence overlain by an upper late rift section deposited during reduced faulting and subsidence total organic carbon toc averages 2 3 wt throughout the active rift siliciclastic shale section 6 in marls in the lower part of the late rift section and 1 2 in deltaic shales in the upper part of the late rift section organic matter consists of mixed types i and iii kerogen in the active rift shales pure type i kerogen in the late rift marls and a type i and iii mixture in the late rift deltaic shales redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections therefore enhanced anoxia did not trigger deposition of the richest source rocks decreased sedimentation rates in the late rift do not account for the full increase in toc nor the shift on organic matter type the richest source rocks are associated with high rates of organic productivity and chemical sedimentation indicating that flux of dissolved components to the rift lake including nutrients for algae growth was critical we propose that reduced topography associated with the late rift was necessary for efficient cycling of plant derived carbon into soil carbonate and ultimately the rift lake and for enhancing chemical weathering and nutrient flux

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

integrated provenance analysis of a complex orogenic terrane mesozoic uplift of the bogda shan and inception of the turpan hami basin nw china we employ petrographic and advanced geochemical tech niques to better document the evolution of the turpan hami basin based on the unique geologic histories of the arc terranes that served as potential sources of turpan hami deposits first a provenance study of permian through cretaceous sandstone of the turpan hami basin reveals temporal and spatial changes in dominant source ter ranes that provided detritus to the basin volcanic lithic rich upper permian sandstone mean qm 19f18lt63 q p 7lvm89lsm4 q m 48p39k13 followed by more quartzose compositions in triassic sandstone mean qm 41f19lt40 q p 20lvm75lsm5 q m 68p21k11 indicate progressive un roofing of the extinct northern and central tian shan arc terranes to the south of turpan hami a sharp change to sedimentary lithic rich lower jurassic sandstone mean qm 47f16lt37 q p 16lvm42lsm42 qm 75p12k13 overlain by a return to volcanic lithic rich middle ju rassic sandstone mean qm 39f21lt40 q p 14lvm51lsm35 q m 65p21k14 points to the initial uplift and unroofing of the largely andesitic bogda shan to the north which first shed its sedimentary cover as it emerged to become the partition between the turpan hami and southern jung gar basins second geochronological trace element and sm nd isotopic varia tions among granitoids in the late paleozoic tian shan orogenic belt provide a further test of mesozoic uplift of the bogda shan on the basis of previous models of crustal compositions throughout the south central and north tian shan bogda shan and east and west jung gar terranes we infer that isotopically enriched granitic cobbles av erage endi 5 20 50 n 5 6 contained in lower triassic deposits in the north central turpan hami basin were derived from the continen tal crustal central tian shan terrane south of turpan hami and not from the more oceanic north tian shan bogda shan and east and west junggar terranes north of the turpan hami basin we therefore infer that the ancestral bogda shan had not been uplifted by the early triassic and that prior to this time a unified junggar turpan hami basin existed during late permian deposition of extensive lacustrine deposits

oil source rock correlations limitations and recommendations a rigorous defensible and causal oil source rock correlation definitively ties an individual source rock sample to an individual crude oil using genetically based internally consistent parameter matches such a correlation is a relationship established between the two components which is consistent with all known chemical geochemical and geological information and meets three criteria a the relationship must be causal the oil must arise at least in part from the specified source rock s b chemical data used in the correlation must be comparable the elemental molecular and isotopic data derived from the source rock must be of the same type as that derived from the oil and c all available geological data must be supportive clear geological evidence must exist which allows the proposed source rock to have sourced the oil the three points of this definition are satisfied and the correlation is successful if we solve one analytical and three geological problems natural extraction of crude oil from its source rock i e expulsion differs mechanically and temporally from laboratory extraction leading to a correlation problem referred to as extraction differences and representing the single greatest analytical uncertainty in correlation efforts geologically three aspects confound oil source correlations occurrence of multiple source units and or source units at differing maturity levels lateral and vertical depositional variations in source unit s organic matter and lateral and vertical variation in reservoir unit s organic matter these concerns are overcome by taking five actions for each oil source correlation effort 1 select representative samples using statistically defensible methods 2 establish the inherent compositional variability laterally and temporally due to depositional and maturation processes in each prospective source unit 3 assess the extent of migration induced changes in oil composition including post migration changes such as in reservoir alteration 4 support each correlation with migration histories derived from 4d models 5 iterate correlation results with new data gathered from ongoing exploration efforts these five actions will a allow correlation success to be measured more objectively b establish risking parameters for direct use in basin assessment and c provide baseline criteria for use in assessing the reliability of oil source rock correlations although these approaches will improve conventional oil source rock correlation efforts ultimately they may not be needed if inversion methods become more reliable the ability to invert compositional data for crude oil i e predict the age lithology maturity and other characteristics of a source rock solely through chemical analysis of its expelled oil could eventually result in successful oil source rock correlations in which source rock analysis for oil source rock correlation purposes is less critical

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

erosion and deposition by supercritical density flows during channel avulsion and backfilling field examples from coarse grained deepwater channel levee complexes sandino forearc basin southern central america abstract erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans field examples from coarse grained channel levee complexes from the sandino forearc basin southern central america show that cyclic step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion bypass levee construction and channel backfilling during channel avulsion large scale scour fill complexes 18 to 29 m deep 18 to 25 m wide 60 to 120 m long were incised by supercritical density flows the multi storey infill of the large scale scour fill complexes comprises amalgamated massive normally coarse tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones interpreted as deposits of the hydraulic jump zone of cyclic steps the large scale scour fill complexes can be distinguished from small scale channel fills based on the preservation of a steep upper margin and a coarse grained infill comprising mainly amalgamated hydraulic jump zone deposits channel fills include repeated successions deposited by cyclic steps with superimposed antidunes the deposits of the hydraulic jump zone of cyclic steps comprise regularly spaced scours 0 2 to 2 6 m deep 0 8 to 23 m long infilled by intraclast rich conglomerates or pebbly sandstones displaying normal coarse tail grading or backsets these deposits are laterally and vertically associated with subhorizontally stratified low angle cross stratified or sinusoidally stratified sandstones and pebbly sandstones which were deposited by antidunes on the stoss side of the cyclic steps during flow re acceleration the field examples indicate that so called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate grain size distribution and amalgamation the deposits of small scale cyclic steps with superimposed antidunes form fining upwards successions with decreasing antidune wavelengths indicating waning flows such cyclic step antidune successions form the characteristic basal infill of mid fan channels and are inferred to be related to successive supercritical high density turbidity flows triggered by retrogressive slope failures

geochemical characterisation and genetic origin of oils and condensates in the south viking graben norway abstract geochemical analysis of oil and condensate samples from all major fields and discoveries in the norwegian south viking graben as well as source rock extracts of upper and middle jurassic source horizons was carried out to characterise reservoired hydrocarbons and establish genetic relationships the south viking graben is in a mature phase of exploration and this study aims at unravelling regional trends to improve predictions in exploration and will show the significance of the play models in the area seven hydrocarbon families have been identified based on interpretation of molecular and isotopic characteristics and multivariate analysis of biomarker data these families can be linked to three source horizons three families originate from different facies of the draupne formation and are characterised by pr ph ratios below 2 dominance of c 27 regular steranes over the c 29 homologues and carbon isotopic values for saturate and aromatic fraction lighter than 28 and 27 respectively the upper jurassic heather formation and the middle jurassic vestland group each source one group of hydrocarbons typical for heather sourced hydrocarbons are elevated pr ph ratios between 2 35 and 3 08 low ratios of c 27 c 29 regular steranes and average isotopic values of 28 4 13 c sat and 26 8 13 c aro hydrocarbons from a middle jurassic source are characterised by very heavy isotopic composition highest pr ph ratios of the data set and dominance of c 29 regular steranes over the c 27 homologues among other factors the two remaining families represent mixtures from upper and middle jurassic sources and are mainly encountered in the greater sleipner area the available data in this study allowed characterisation of 84 of the recoverable oil resources which proved that the main source rock in the area is the upper jurassic draupne formation the play concept including a draupne source and tertiary reservoir proves to be most prolific with 239 10 6 s m 3 of recoverable oil equalling 88 of the total characterised oil resources oils in these tertiary reservoirs are however likely to be affected by biodegradation based on the available data the zone of biodegradation in the south viking graben appears to be limited to depths above 2140 m and temperatures below 70 c

change of black shale organic material surface area during oxidative weathering implications for rock water surface evolution abstract black shale oxidative weathering plays a significant role in a variety of processes including acid mine drainage and atmospheric co2 control the modeling of weathering is highly dependent on reactive surface area in this study it is shown that black shale oxidative weathering is regulated mainly by the external geometrical surface area of rock polyhedrons and the organic matter s om internal surface area the internal rock surface area decreases dramatically during om dissolution from 15 m2 g to 5 m2 g a linear relationship was found between the decrease of internal rock surface area and quantity of om dissolved optical roughness analyses of black and bleached shale surface area reveal the formation of macropores due to the dissolution of mesoporous and probably microporous om however due to deconsolidation the geometrical external rock polyhedron surface area increases during weathering black shale polyhedrons show a doubling of their external surface area as om decreases this provokes an increase of the shale volume which is easily accessible by fluids the increase of the external rock surface area seems to be self accelerating during weathering the upscaling of external and internal rock surface area evolution during weathering presented in this study demonstrates the possible application of these results to the improved understanding of a chemical transport in a variety of natural systems

lacustrine source rock deposition in response to co evolution of environments and organisms controlled by tectonic subsidence and climate bohai bay basin china three paleogene syn rift intervals from the bohai bay basin the most petroliferous basin in china were analyzed with sedimentological and geochemical techniques to characterize the lateral source rock heterogeneities to reveal the environmental and ecological changes through geologic time and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions the third es3 and first es1 members of the eocene shahejie formation and the oligocene dongying formation ed display widely variable total organic carbon contents hydrogen indices and visual kerogen compositions suggesting changes in organic facies from deep to marginal sediments carefully selected deep lake facies samples from any interval however display fairly uniform biomarker composition these three intervals have distinctly different biomarker assemblages which indicate weakly alkaline freshwater lakes with a moderately deep thermocline during es3 deposition alkaline saline lakes with shallow chemocline during es1 deposition and acidic freshwater lakes with deep unstable thermocline during the deposition of the dongying formation such environmental changes corresponded to changes in subsidence rate and paleoclimate from rapid subsidence and wet climate during es3 deposition through slow subsidence and arid climate during es1 deposition to rapid subsidence and wet climate during ed deposition and resulted in synchronous changes in terrigenous organic matter input phytoplankton community and primary productivity the co evolution of environments and organisms controlled by tectonic subsidence and climate accounted for the deposition and distribution of high quality lacustrine source rocks with distinctly different geochemical characteristics most rift basins experienced changes in subsidence rates and possibly changes in climates during their syn rift evolutions the models constructed in this paper may have important implications for source rock prediction in other lacustrine rift basins

three dimensional gravity study of the mid atlantic ridge evolution of the segmentation between 28 and 29 n during the last 10 m y during the high resolution survey sara segmentation ancienne de la ride atlantique sea beam bathymetry magnetic gravity and seismic reflection data were collected on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to provide off axis information up to approximately 10 m y and to complement a detailed on axis survey carried out in 1988 and 1989 sempere et al 1990 between the atlantis and kane fracture zones a previous gravity study had revealed the existence of bulls eye shaped gravity lows centered on the axial segments and gravity highs centered on the non transform discontinuities lin et al 1990 we carried out a three dimensional calculation of the mantle bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center the off axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities after correcting for the gravity effect of lithospheric cooling away from the ridge the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry off axis bathymetry is characterized by southward pointing v shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously however the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows northward on the western flank and southward on the eastern flank the sense of offset between the gravity and bathymetric traces appears to be related to the right stepping nature of the axial discontinuities

paleoenvironments and paleogeography of the lower and lower middle jurassic coal measures in the turpan hami oil prone coal basin northwestern china detailed facies and paleogeographical analyses of extensive borehole and outcrop data from lower and lower middle jurassic oil prone coal bearing sequences in the turpan hami oil producing basin of northwest china have led to the reconstruction of a basinwide depositional model a total of 20 distinct lithofacies have been identified and grouped into braided fluvial plain meandering fluvial plain braided delta meandering channel delta and lacustrine depositional systems coal forming swamps occur in each depositional system but the preferred sites of accumulation are interdelta bay and lower delta plain environments in the braided delta and meandering channel depositional systems and it is in these sites that major oil prone source rocks are located a series of age specific basinwide paleogeographical maps have been constructed leading to a depositional model for the basin results indicate that the basin experienced cyclic flooding to produce swamps and lakes and that these characterize the deposition of the lower and lower middle jurassic coal measures analyses indicate that both the bogda and harlik mountains were uplifted prior to the early jurassic and a lake separated the two mountain regions during the early and early middle jurassic from these results it is interpreted that major oil prone coal sequences are to be found in the western part of the taibei depression of the basin and thus the full economic potential of the basin has yet to be fully realized

secular and environmental constraints on the occurrence of dinosterane in sediments abstract the distribution patterns of sedimentary a ring methylated steranes have changed markedly over geological time although dinosterane and its isomer 24 ethyl 4 methylcholestane have been tentatively identified in three proterozoic rock units they are either not detectable or occur in low abundance relative to 3 methyl steranes throughout most of the palaeozoic between permian and middle triassic times 260 220 ma ago 4 methyl sterane abundances in marine sediments increased markedly the presence of dinosterane in some middle triassic marine sediments is contemporaneous with the appearance of fossil cysts of uncontested dinoflagellate affinity 4 methyl steranes including dinosterane or their precursor sterenes and sterols then show a continuous presence often in high abundance in marine sediments from the late triassic through to the present day assemblages of 4 methyl steranes and their precursors but with dinosterane absent or in low relative abundance are often the predominant steroids in lacustrine sediments in the cainozoic dinosterane appears to arise predominantly from marine dinoflagellates and as a consequence is a useful biological marker for mesozoic and cainozoic marine organic matter the isomer 24 ethyl 4 methylcholestane is likely to have multiple origins although its very high abundance in tertiary lacustrine sediments and oils compared to older materials suggests that dinoflagellates could also be the source in these cases

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

hydrothermal circulation anhydrite precipitation and thermal structure at ridge axes the thermal structure of the axial region of ridges spreading at 60 mm yr 1 was studied by comparing observations with numerical thermal models data from deep sea drilling project hole 504b constrain the temperature in the pillows and the upper part of the dike complex seismic reflection data constrain the depth to the magma chamber magnetic anomalies constrain the accumulation rate of pillows the thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts in the successful thermal models hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0 8 km above the top of the magma chamber the computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents the computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern anhydrite was precipitated as seawater approached the hot axial zone the anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater the net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off axis circulation and returned to the ocean this is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths alternatively the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean the latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

upper permian as a new play model on the mid norwegian continental shelf investigated by shallow stratigraphic drilling a 750 m thick fully marine succession of sandstones coarse grained turbidites shales and reworked sabkha sediments has been cored on the eastern margin of the mid norwegian shelf the succession has been dated as upper permian lower triassic and is comparable to rocks of the same age exposed onshore east greenland these data demonstrate that the marine depositional basin between greenland and norway extended much farther east than previously thought reddish shallow marine sandstones in the lower 170 m of the cored succession probably represent reworking of older sedimentary rocks present to the east of the drill sites this suggests that upper devonian lower permian sediments were deposited on caledonian basement east of the present day limits of the basin the cored succession also contains source rocks that can be stratigraphically correlated with the oil prone source rocks of the upper permian ravnefjeld formation onshore east greenland some of the cored sandstone intervals were stained with light nonbiodegraded oil that most likely was sourced from upper permian or alternatively lower triassic mudstones reworked fragments of reef building organisms in upper permian turbidites and upper permian carbonates encountered in an exploration well on the nordland ridge indicate that carbonate deposition and reef building occurred on structural highs on the trondelag platform the observations from the cored successions are key elements in a new paleozoic play model on the mid norwegian continental shelf and include the first evidence for the existence of an upper permian source rock in the area with upper permian carbonates or sandstones or triassic jurassic sandstones as reservoir begin page 108

geochemically driven exploration models in uplifted areas examples from the norwegian barents sea the norwegian barents sea with multiple source rock intervals represents a prime example of an overfilled petroleum system however several episodes of uplift and erosion from the paleocene until the pliocene pleistocene have caused the depletion of hydrocarbon accumulations in the region these uplift events were not only potentially catastrophic but have also caused the redistribution of the remaining oil and gas over laterally large distances in the barents sea region this redistribution directed petroleum to distal parts of the various hydrocarbon generating basins thus charging traps which otherwise would not have been reached therefore we expect that discoveries will be made in distal basin settings particularly in traps with partly leaking cap rocks which can bleed off gas and thereby retain oil many oil accumulations in the region represent various mixtures of oils from several different stratigraphic source intervals this suggests that triassic and paleozoic oils may be trapped below the presently drilled targets which are mostly jurassic in the hammerfest basin and older to the north and east deeper exploration targets also stand a higher chance of containing oil because the amount of gas being released from oil during uplift erosion and subsequent pressure release is lower uplift and erosion are followed by a reduction in temperature this is why hydrocarbon generation is believed to have ended in uplifted areas some discoveries in this data set suggest however a significant fresh gas charge

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

organic and isotope geochemistry of the early cretaceous rift sequence in the camamu basin brazil paleolimnological inferences and source rock models abstract the camamu basin located at the eastern brazilian continental margin is characterized by a thick early cretaceous organic rich rift sequence the integration of bulk geochemical isotopic and biomarker data with paleontological and geological information provided the basis for a better assessment of rift lake evolution hydrological regime redox potential salinity etc and its impact on source rock development pyrolysis and visual kerogen analyses reveal the dominance of lipid rich algal bacterial type i kerogen through the entire rift sequence paleolimnological reconstruction suggests that the lower rift sequence morro do barro formation neocomian was deposited in a fault bounded deep lake with fresh to brackish water stable water column stratification and water bottom anoxia high hydrogen indices 700 900 mg hc gtoc and 13c depleted carbon isotopic composition of the organic matter 13c values around 30 might reflect enhanced organic preservation the recycling of light ch4 generated by methanogenesis and the incorporation of isotopically light lipid rich bacterial biomass on the other hand the upper rift sequence rio de contas formation barremian was probably deposited within a broader shallower lake with a higher frequency of water column overturn and a deeper thermocline high organic carbon contents and strong 13c enrichment of the organic matter 13c excursions of up to 23 are interpreted to have resulted from enhanced primary productivity triggered by an increased input recycling of nutrients favored by lake morphology and humid climate

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

the occurrence of bisnorhopane trisnorhopane and 25 norhopanes as free hydrocarbons in some australian shales abstract the triterpenoid hydrocarbons of some west australian shales have been examined by gc ms in addition to the common 17 h 21 h hopanes 17 h 21 h hopanes and 17 h 21 h moretanes 28 30 bisnorhopane 25 28 30 trisnorhopane and 25 norhopanes were identified in the organic extracts in contrast pyrolysates of the solvent extracted sediments contained only the common hopane and moretane series indicating that 28 30 bisnorhopane 25 28 30 trisnorhopane and 25 norhopanes are not bonded to kerogen but rather are present in the sediments as free hydrocarbons

molecular isotopic evidence of environmental and ecological changes across the cenomanian turonian boundary in the levant platform of central jordan abstract we evaluated the structure of planktonic communities and paleoenvironmental conditions throughout the cenomanian turonian oceanic anoxic event oae2 by studying bulk geochemical properties and the molecular isotopic composition of source specific hydrocarbons from organic rich sediments deposited in an intrashelf basin at the levant platform central jordan high concentrations of desmethyl and 4 methylsteranes as well as dinosteranes indicate that marine algae including dinoflagellates were the main primary producing organisms the presence of 2 methylhopanes and 13 c enriched hopanes in addition to isotopically enriched aryl isoprenoids evidenced the contribution of cyanobacteria and green sulfur bacteria respectively additionally variable but fairly low 15 n values during oae2 suggest the occurrence of diazotrophy as a likely important process fueling primary production during oae2 in this stratified anoxic continental platform variations in the relative contribution of biomarkers revealed changes in planktonic communities associated with sea level change and water column stratification oae2 was characterized by strong stratification anoxic bottom waters and a deep chemocline as evidenced by high gammacerane and homohopane indices and the absence of photic zone euxinia pze markers respectively however the presence of isorenieratane and its derivatives in post oae black shales points to a shoaling of the chemocline and to pze this interval was also characterized by an exceptionally high abundance of chlorophyll derived pristane and phytane up to 2 mg g 1 toc likely as a result of highly enhanced primary production and organic matter preservation remarkably this high productivity event co occurs with an exceptionally high abundance of calcispheres reported elsewhere to be part of a global bio event

dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum abstract the steroidal alkanes of a selection of sediments and oils have been examined by gc ms with multiple metastable reaction monitoring specific 4 methyl sterane isomers have been identified by comparison with isomers synthesized from sterols isolated from dinoflagellates an immature marine oil shale and two mature marine oils of triassic to early cretaceous age contained high concentrations of c30 steranes comprising desmethyl 24 ethyl 4 methylcholestane and 4 23 24 trimethylcholestane dinosterane isomers an immature non marine oil shale and two non marine oils of cretaceous to eocene age contained stereoisomers of 24 ethyl 4 methylcholestane as the dominant c30 steranes reaction monitoring analyses in gc ms are particularly suited to unravelling complex distributions of homologous and stereoisomeric steroids encountered in oils and their source rocks

lake type controls on petroleum source rock potential in nonmarine basins based on numerous empirical observations of lacustrine basin strata we propose a three fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated 1 the fluvial lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial deltaic deposits commonly including coal shoreline progradation dominates basin fill resulting in the stacking of indistinctly expressed cycles up to 10 m thick in map view the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts transported terrestrial organic matter contributes to mixed type i iii kerogens that generate waxy oil type i kerogen is hydrogen rich and oil prone type iii kerogen is hydrogen poor and mainly gas prone the luman tongue of the green river formation wyoming and the honyanchi formation junggar basin china provide examples of this facies association which is also present in the songliao basin of northeastern china the central sumatra basin and the cretaceous doba doseo basins in west central africa 2 the fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the world s richest source rocks deposits are regionally extensive in map view having relatively homogenous source facies containing oil prone type i kerogen examples include the laney member of the green river formation wyoming the lucaogou formation junggar basin china the bucomazi formation offshore west africa and the lagoa feia formation campos basin brazil 3 the evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits sublittoral organic rich mudstone facies are relatively thin but may be begin page 1034 quite rich and widespread the highest organic enrichment coincides with the deepest lake stages low input of land plant organic matter results in minimal lateral contrasts in organic content in some cases a distinctive type i s sulfur rich kerogen may generate oil at thermal maturities as low as 0 45 vitrinite reflectance equivalent examples include the wilkins peak member of the green river formation wyoming the jingjingzigou formation junggar basin china the jianghan and qaidam basins china and the blanca lila formation argentina

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

geochemical characteristics of tertiary saline lacustrine oils in the western qaidam basin northwest china abstract based on the systematic analyses of light hydrocarbon saturate aromatic fractions and c isotopes of over 40 oil samples along with related tertiary source rocks collected from the western qaidam basin the geochemical characteristics of the tertiary saline lacustrine oils in this region was investigated the oils are characterized by bimodal n alkane distributions with odd to even c11 c17 and even to odd c18 c28 predominance low pr ph mostly lower than 0 6 high concentration of gammacerane c35 hopane and methylated mttcs reflecting the high salinity and anoxic setting typical of a saline lacustrine depositional environment mango s k1 values in the saline oils are highly variable 0 99 1 63 and could be associated with the facies dependent parameters such as pr ph and gammacerane indexes compared with other tertiary oils the studied tertiary saline oils are marked by enhanced c28 sterane abundance 30 or more of c27 c29 homologues possibly derived from halophilic algae it is noted that the geochemical parameters of the oils in various oilfields exhibit regular spatial changes which are consistent with the depositional phase variations of the source rocks the oils have uncommon heavy c isotopic ratios 24 to 26 and a flat shape of the individual n alkane isotope profile and show isotopic characteristics similar to marine organic matter the appearance of oleanane and high 24 24 27 norcholestane ratios 0 57 0 87 in the saline oils and source rocks confirm a tertiary organic source

oil source correlation in the fulin basin shengli petroleum province east china the fulin basin is located within the giant shengli petroleum province in east china the source rock in the basin is mainly the lower part of the third member es3 of the eocene portion of the shahejie formation it is composed of organic rich dark mudstone and oil shale of lacustrine origin and contains good quality type i ii kerogen these strata reach the oil generative stage at ro 0 4 0 8 the first member es1 of the oligocene portion of the shahejie formation has excellent oil potential in the nearby gunan basin but is immature in the fulin basin and is not an effective source rock the oil and rock samples from both basins can be grouped into three major families based on biomarkers the first family shows low prph 0 3 and notable carotane typical of carbonate evaporite environments this family exists in es1 rocks and the related oils and is early mature 0 2 0 3 and s r s 0 10 0 20 the second family shows high prph 1 a low gammacerane index 0 2 and abundant 4 methyl steranes indicative of freshwater lacustrine conditions like those in es3 this family shows higher maturation 0 30 0 50 and s r s 0 35 0 55 and notable diasteranes the third family is transitional has source and maturity ratios between the above families and occurs in the es3 unit samples from the basin interior based on biomarker similarities between oils and rock extracts all oils in the fulin basin originated from es3 source rocks slight maturation differences indicate that these oils have been derived from different source areas no oil from es1 is found in the fulin basin suggesting es1 source rocks are too immature to generate commercial oil although the nearby gunan basin has much oil generated from es1 source rocks no migrated oil from es1 in this basin reached the kenli faulted zone because of intervening faults which disrupt migration

biological marker compounds as indicators of the depositions history of the maoming oil shale abstract the eocene maoming oil shale from guangdong province occurs as a laterally uniform stratigraphic section typically 20 25 m thick from which the aliphatic hydrocarbon constituents of six representative samples were investigated using gc and c gc ms the sediments evaluated included the basal lignite a vitrinite lens from the overlying claystone and four intervals from the massive oil shale bed as expected the lignite and vitrinite differ markedly from the oil shales the lignite is dominated by bacterial hopanoids and components of higher plant origin including c 29 steroids and triterpenoids such as oleanenes visually the oil shale samples show corroded and degraded phytoclasts spores wispy particles of fluorescent organic material attributable to dinoflagellates and especially in the uppermost sample colonial algal bodies the distributions of biological markers in the oil shales show many features in common notably a dominance of dinoflagellate derived 4 methylsteroids and a significant proportion of higher plant derived n alkanes with marked odd over even carbon number predominance overall they exhibit several features that resemble characteristics of the messel shale the hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales the aliphatic hydrocarbons of the uppermost shale are dominated by a number of c 31 and c 33 botryococcane homologues and other unusual branched alkanes possibly derived from green algae all of the samples are immature overall molecular and microscopic examination of the stratigraphic succession of the maoming oil shale suggests a shallow lacustrine environment within which peats were deposited this lake subsequently deepened to support abundant algal populations especially dinoflagellates culminating in a dominance of botryococcoid algae

the occurence of unusual c27 and c29 sterane predominances in two types of oman crude oil abstract two types of oman crude oils reveal unusual sterane distributions type a which is the more common 74 examples is characterised by a predominance of c 29 iso and normal steranes and generally none or only very low relative concentrations of rearranged steranes the triterpanes are characterised by the predominance of the c 29 17 h 21 h norhopane over the c 30 17 h 21 h hopane and non predominant c 20 c 30 tricyclic terpanes the c 29 steranes of this type of crude were not derived from the c 29 sterols of land plant origin frequently proposed as the source of c 29 steranes since there is good geological evidence that these crudes were generated from a pre cambrian source rock a geological period when land plants did not exist the type b crude oil 11 known examples is characterised by a strong predominance of c 27 iso normal and rearranged steranes relatively lower concentrations of 17 h 21 h hopanes and relatively high concentrations of c 20 c 30 tricyclic terpanes the remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different