vanadium in ancient sedimentary rocks of marine origin abstract the distribution of vanadium and vanadyl porphyrins in fractions extractable organic inorganic and kerogen of the serpiano marl and the la luna shaly limestone has been determined by employing a variety of geochemical and spectroscopic techniques including atomic absorption atomic emission electronic absorption and electron spin resonance spectroscopies high levels of vanadium and vanadyl porphyrins were found with the major part of the total vanadium located in the kerogen fractions of both rocks in contrast while the kerogen of the la luna rock also contained the major fraction of total vanadyl porphyrins 70 the corresponding value for the serpiano marl is only 30 no vanadyl species were present in the inorganic fraction of either rock it is suggested that the source of vanadium in these rocks is volcaniclastic materials and that the vanadyl porphyrin entities were incorporated into the kerogen structure through abiotic geochemical modifications of biosynthetic pigment chlorophyll

relationships between sulphur organic carbon and iron in the modern sediments of the black sea abstract the ratio of sedimentary s to organic c has been used as a diagnostic criterion for recognizing anoxic environments in the sedimentary record here we examine the relationship between pyrite s and organic c in the modern unit 1 sediments of the black sea to re evaluate this suggested relationship in box cores from shallow oxic or near oxic water pyrite s contents ranging from 0 1 and 2 5 by weight do not correlate with organic c contents in cores from the deep anoxic basin pyrite s concentrations range between 1 0 and 2 5 and are linearly related to organic c contents with an intercept that is not significantly different from zero the s c ratio is around 0 31 in these sediments which is somewhat lower than the ratio of 0 36 considered to be characteristic of normal marine oxic sediments thus the black sea anoxic sediments appear to be depleted in s relative to organic c the degree of pyritization dop of the sediments may be a better measure of anoxic conditions of sedimentation the dop values measured here show that pyrite formation is fe limited the correlation between organic c and pyrite s probably being induced by a positive correlation between fine grained sediment components the original fe source for pyrite formation in the sediments and organic matter such fe limitation is probably characteristic of anoxic basins where pyrite formation can take place in the water column and fe oxyhydroxides do not accumulate in the surface sediments the results on the sample set examined here are at variance with earlier data and interpretations for the black sea in particular the suggested s enrichment of anoxic black sea sediments was not found because of fe limitation

hydrocarbon potential and depositional environment of the lower cretaceous black mudstones and shales in the coastal guangdong province china abstract the mesozoic especially the cretaceous sequences in the northern south china sea scs are important potential targets of oil and gas exploration however they are currently poorly understood we use organic petrology and geochemistry to evaluate the hydrocarbon potential and study depositional environment of the cretaceous black mudstones and shales from the dayawan section in the guangdong province china these mudstones and shales yield total organic carbon toc contents of 0 63 2 76 and most samples meet the requirement for higher toc source rocks organic matters in the mudstones and shales were sourced mainly from higher plants as well as subordinate benthic macroalgae organic petrology and geochemical data suggested that the organic matters were dominated by type iii and minor type ii and generally have low thermal maturity thus we evaluated the black mudstones and shales have a potential for hydrocarbon generation especially gas prone pr ph trace elements and mixture source of organic matter suggested that these black mudstones and shales formed in a coastal lacustrine environment characterized by weakly oxidizing weakly reducing conditions and low salinity a marine transgression developing in the middle part of the section was recognized generally the northern scs which exhibits the more marine influence likely contains source rocks with higher toc contents and better organic matter types the cretaceous source rocks with higher liquid hydrocarbon generation potential may occur locally in the pearl river mouth basin

characterization of tar like material and producible oil in misoa c2 and c3 reservoir sands of the area 2 south ceuta field venezuela abstract here we characterized asphaltic oil occurrences in the misoa c2 and c3 reservoirs within area 2 south of the ceuta field lake maracaibo basin nw venezuela to perform the geochemical and petrological characterization of the c2 and c3 sub units which cover an interval in which tar like material was detected we analyzed core 18 and cutting 123 samples using the rock eval 6 technique in reservoir mode samples were studied under fluorescence light and scanning electron microscopy sem showing the presence of two charges of oils and the relationships within silica cements and clay minerals to evaluate vertical inter reservoir communication and lateral intra reservoir continuity we also studied 12 oil samples from wells in area 2 south biomarkers and isotopic signatures were examined using gas chromatography mass spectrometry gc ms and isotope ratio mass spectrometry irms the results suggest that oil and core extracts belongs to a single family derived from two pulses of hydrocarbon generation from the la luna source rock which was deposited in a marine reducing environment the oil samples are a mixture of a paleobiodegraded oil charge and a later fresh more mature oil recharge the geochemical analyses also evidence that the formation of heavy oil tar zones can be related to biodegradation and in reservoir oil mixing though other formation mechanisms for the tar are also feasible finally the gc fingerprints of the oils are indicative of lateral reservoir continuities across the f 1 and f 2 faults and vertical communication between the misoa c2 and c3 sub units in area 2 south suggesting that tar occurrences do not act as permeability barriers within the two reservoirs studied

spatial and temporal variability of paleocene miocene organofacies of the kura basin eastern azerbaijan and implications for basin evolution and petroleum generation abstract fine grained paleocene miocene strata of eastern azerbaijan record the evolution of the kura basin from an open marine system connecting the south caspian basin to the paratethys of eastern europe to an isolated epicontinental sea tectonic uplift associated with the greater caucasus mountains in azerbaijan resulted in exposure of these strata over much of the northeastern part of the basin allowing detailed observation and sampling over a broad area four hundred paleocene miocene samples collected in eastern azerbaijan demonstrate spatial and temporal geochemical heterogeneity within the predominately mudstone succession indicating a complex evolution of this elongate basin through detailed organic geochemical analysis of samples trends can be seen in the distribution of total organic carbon toc pyrolysis characteristics and biomarker results from a subset of twenty four samples in addition to confirming the well documented regional basin restriction during the oligocene to early miocene deposition of the maikop series heterogeneity of these geochemical indicators across the study area shows spatial variability in oxicity and terrestrial organic input within the basin on a sub regional scale observed geochemical variability suggests a complex evolutionary history of the kura basin during the paleocene miocene and this has important implications for hydrocarbon prospectivity in the region within the 400 collected samples total organic carbon toc values range from 0 3 to 6 3 with oligocene miocene samples showing average values of 1 4 compared to lower toc values found in paleocene eocene strata average 0 3 rock eval pyrolysis shows that the majority of strata are organic lean immature and mixed oil gas to gas prone type ii iii to type iii with a smaller group of latest eocene early miocene samples having better oil prone source potential gas chromatography and biomarker analysis of 24 samples reinforces the immaturity of most samples shows varying levels of terrestrial input in all samples suggests well oxygenated waters prevailed with periodic suboxic to dysoxic anoxic events highlights gross differences between the paleocene eocene and oligocene miocene stratigraphic intervals and shows generally good lateral correlation between samples these interpretations reinforce what is seen through inorganic geochemical evaluation of trace metals bulk composition and stable isotopic ratios and offer more detail as to the evolution of this basin and the implications for oil and gas prospectivity of the region

black shale deposition and early diagenetic dolomite cementation during oceanic anoxic event 1 the mid cretaceous maracaibo platform northwestern south america thin laterally continuous organic rich dolomitic marlstones were deposited in the extended late aptian early albian epicontinental sea of northwestern south america these intervals are the proximal equivalents of thick hemipelagic black shale ammonitic floatstone couplets deposited in the distally stepped differentially subsiding part of the maracaibo platform the marlstones reflect the dynamic conditions resulting from orbital forcing mechanisms and can be genetically related to 1 minor sea level changes 2 proximal turnovers in marine productivity and 3 sudden climate shifts affecting mid cretaceous shoaling upward shallow marine carbonate cyclicity therefore the marlstones may well be linked to the multiple environmental perturbations collectively referred to as oceanic anoxic event 1 the interstitial euhedral dolomite has a medium crystallinity and exhibits unusual textural relations with framboidal pyrite and gypsum the authigenic mineral assemblage also includes quartz ca f apatite and barite which together with the chemical signals of dolomite point to an unsteady climate regime bulk rock biomarker parameters rare earth element geochemistry and iron speciation data point to widespread photic zone anoxia and transient shallow marine euxinia by the time of deposition with climatic instability driving the delivery of oxidized detritus from the hinterlands these conditions led to a schizohaline redox stratified environment favorable to dolomite precipitation in such a depositional setting the bio utilization of fe mn and sulfur for organic matter respiration sustained elevated pore water alkalinity and ph and allowed for the pre compactional growth of interstitial dolomite

paleoenvironmental changes during the late albian oceanic anoxic event 1d an example from the capacho formation southwestern venezuela abstract the late albian early cenomanian oceanic anoxic event 1d oae 1d 103 to 99 5 ma represents a global interval of enhanced organic carbon burial due to widespread oxygen deficiency in the water column and or increased primary productivity the biostratigraphy and geochemistry of organic rich sediments in the la grita member capacho formation in southwestern venezuela were studied to document the paleoenvironmental conditions that governed the deposition of this succession during oae 1d carbon isotope 13ccarb and 13ccorg chemostratigraphy and biostratigraphic constraints show that the la grita member spans the late aptian period rotalipora appenninica zone and that oae 1d is well recorded in this succession a prominent increase in total organic carbon toc values up to 10 is clearly recorded through the onset of oae 1d coinciding with a prominent change in overall redox sensitive proxies suggesting that the la grita member sediments accumulated under anoxic and possibly euxinic bottom water conditions in a partially restricted basin the detrital proxies suggest that the greenhouse climate prevailing during oae 1d induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates overall the carbon and oxygen isotope records toc contents as well as the detrital proxies show cyclic variation during the late albian stage these variations support the hypothesis that orbital forcing likely also modulated the equatorial monsoonal activity during the oae 1d

trace element geochemistry of crude oils and condensates from south african basins abstract crude oil and condensates samples from bredasdorp and orange basins south africa were analyzed for trace elements using inductively coupled plasma optical emission spectrometry the oil and condensate samples were analyzed to classify the oils into family and to determine the origin of the oils and depositional environment of organic matter that yielded the oils using trace elements the concentrations of the elements co cr cu fe mn ni and v determined ranged from 0 10 to 7364 7 ppb iron is the most abundant element in the samples while cobalt is least abundant both concentrations and ratios of nickel and vanadium showed two main families of oils oils of marine origin and oils of terrestrial origin oil samples from the orange basin are mainly of marine source and their organic matter were deposited under anoxic conditions most of the oil samples from the bredasdorp basin are of marine origin with a few samples having terrestrial origin cluster analysis also classified the oil and condensate samples from the two basins into two main families of marine and terrestrial sources

re os geochronology for the nist rm 8505 crude oil the importance of analytical protocol and uncertainty abstract the national institute of science and technology nist research material rm 8505 is a venezuelan crude oil certified for its vanadium concentration it has recently been proposed as a matrix matched standard for interlaboratory comparison of re os isotopic analyses while serving as a benchmark for petroleum based re os method development here the nist rm 8505 oil is characterized using a multi solvent asphaltene precipitation permitting interlaboratory re os data comparison and evaluation of techniques used to derive re os isochron ages we present a detailed discussion of analytical blank sources critical evaluation of analytical uncertainties method improvements and data corrections additionally we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages in addition to nist rm 8505 2 43 0 15 ng g re 30 0 0 7 pg g os we provide the first re os data for nist srm 1634c 1 73 0 01 ng g re 32 5 0 8 pg g os and enviromat scp science 39 1 pg g re 1 01 0 02 pg g os two other commercially available hydrocarbon trace metal standards the application of several asphaltene asph precipitations using different n alkane solvents pentane heptane and decane for the nist rm 8505 significantly improved the spread of data in the 187re 188os 187os 188os space multi solvent asphaltene precipitations yield an asphaltene only model 1 isochron 75 24 ma and the maltene crude oil asphaltene triplets yield a more precise model 1 isochron age of 78 5 2 3 ma with a 187os 188osinitial of 0 877 0 021 our age is considerably older compared to a previously reported model 1 age of 62 7 5 7 ma based on c7 asph crude oil malt triplets liu and selby 2017 and considerably younger than the model 1 age of 98 4 9 5 ma based on asph separates liu et al 2019 the analytical reasons leading to these age differences are discussed in this paper the geological context for the nist 8505 oil is unknown therefore we use trace metal data ni v fe mo etc to derive plausible constraints for the origin of this oil the nist 8505 oil shows trace metal data similar to published data for oils from the urdaneta district the la paz mara and lagunillas oil fields and source rocks of the la luna formation in the maracaibo basin venezuela in this geological context our 79 ma age for the nist 8505 oil implies a late cretaceous early hydrocarbon generation event from older upper cretaceous source rocks and overlaps with the youngest age of potential source rocks in the maracaibo basin however the age of large scale oil generation in the maracaibo basin is significantly younger lasting from the oligocene to the pliocene

a record of long and short term environmental and climatic change during oae3 la luna formation late cretaceous santonian early campanian venezuela abstract the la luna formation was deposited under anoxic dysoxic conditions in a tropical epicontinental sea on the northwest south america margin sedimentological micropaleontological and geochemical evidence provides insights into factors that influenced the sedimentation and controlled the accumulation of organic rich deposits at decimeter and meter scales during the youngest of the cretaceous oceanic anoxic events oae the la luna formation consists of an alternation of black marlstones interbedded with black limestones and black marly limestones the benthic foraminifera assemblages indicate sedimentation in the upper neritic to upper bathyal environment these rocks contain large amounts of organic matter it is interpreted that a combination of warm global and rainy climate and the presence of bathymetric barriers caused poor circulation and low rates of water column ventilation during a high sea level in the early santonian leading to the preservation of carbon rich deposits in this region during the late santonian a cooling trend in global climate increased wind strength and upwelling this change probably reduced runoff causing a weakening of the pycnocline and destabilized the stratification in the water column providing a progressive increase in oxygen in the water column and on the sea floor and a decrease in total organic carbon preservation in a shallower basin these changes and the establishment of full mid and deep water exchange in response to the deepening and widening of the equatorial atlantic gateway could have been important mechanisms for ending the epeiric sea anoxia changes through time in the vanadium nickel fraction planktonic and benthic foraminifera assemblages productivity proxy elements and lithological characteristics support this model superimposed on the general trend variations in calcium carbonate and total organic carbon percentages at the scale of tens of centimeters reveal high frequency cyclic variations which apparently correspond to the main frequencies of orbital forcing this cyclicity is interpreted as a primary depositional signal and is the result of orbital controlled fluctuations in terrigenous dilution and variations in oxygen concentration at the base of the water column variations in the intensity of upwelling can be observed but they did not control the cyclicity

trace element characterisation of cretaceous orange basin hydrocarbon source rocks trace elements in the kerogen fraction of hydrocarbon source rock samples from two wells obtained from the cretaceous units of the orange basin south africa were determined using x ray fluorescence spectrometry in order to determine their distribution and geochemical significances the concentrations of the elements as ce co cu fe mo ni pb and v determined ranged from 0 64 to 47 300 ppm for the samples analysed the total organic carbon toc values indicate that the samples are organic rich but did not show any trend with the distribution of the trace metals except ce mo and pb dendrogram cluster analysis discriminated the samples into three groups on the basis of their level of thermal maturity thermal maturity has a significant effect on the distribution of the trace metals cobalt ni and v ni ratios and cross plots of the absolute values of v and ni indicate that the samples had significant marine organic matter input the v and ni contents and v v ni ratio indicate that the organic matter of the source rocks had been deposited in reducing conditions despite the similarities in the organic matter source input and depositional environment of the organic matter of the samples from the two well cross plots of co ni versus v ni and mo ni versus co ni were able to reveal subtle differences cluster analysis of the samples was also able to reveal the subtle thermal maturity differences of the samples

u mo ratios and δ98 95mo as local and global redox proxies during mass extinction events abstract when used in conjunction with trace metal redox proxies the use of u mo ratios and 98 95 mo values can considerably enhance the interpretation of ocean oxygenation trends both locally and globally this is illustrated with studies from two biogenic chert dominated records of mass extinction the ordovician silurian boundary and the late permian in sections from the yangtze block in south china the latest ordovician mass extinction is well known to coincide with oxygenation episode in the hirnantian stage here revealed by trace metal proxies v v ni v cr ni co that show it sandwiched between anoxic euxinic black shales the predominantly anoxic euxinic ordovician silurian strata provide a record of seawater 98 95 mo values that are seen to gradually rise during the interval from 0 5 to 1 0 the exception to the 98 95 mo stability is the datum of the slight negatively 98 95 mo associated with decreasing of v cr and v v ni ratio suggesting the hirnantian oxygenation event the upper permian chert succession at shangsi reveals an overall better ventilated record in which the best oxygenated strata show the highest u mo values that is interpreted as relative enrichment of u in suboxic conditions at a few levels anoxic euxinic conditions show the 98 95 mo values to be 2 close to modern values implying that latest permian oceans were as well ventilated as those today the well known end permian oceanic anoxic event is therefore suggested to only develop at the end of this period

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

generation and migration of hydrocarbons in the maracaibo basin venezuela an integrated basin study abstract the history of hydrocarbon generation and migration in the maracaibo basin venezuela is closely related to its stratigraphic structural and tectonic evolution during the cretaceous and cenozoic times geochemical evaluation of the possible source rocks within the cretaceous and the tertiary sequences indicates that the argillaceous limestones and calcareous shales of the la luna formation cenomanian coniacian are the most important source rocks in the basin these rocks contain organic carbon ranging from 1 5 to 9 6 c original toc 2 5 10 8 have fluorescent amorphous marine type ii organic matter oil prone and have in the immature stage a h index of 700 mg hc g toc and a h c ratio of 1 5 the beginning of the oil window in these rocks occurs at a relatively low thermal maturity lopatin tti about 10 and the peak of oil generation corresponds to a tti value of 35 geochemistry of oils from the different reservoirs cretaceous paleocene eocene and miocene beds and oil oil and oil source rock correlation studies based on biomarkers have identified three genetic types of oil in the basin a marine type essentially derived from the la luna source a terrestrial type possibly originating either from the cretaceous lisure shales or the paleocene shales and a third type derived from the mixing of the first two types the marine type oil is distributed extensively throughout the basin whereas the terrestrial as well as the mixed marine and terrestrial types are found only in the southwestern part of the basin the marine type oils have been further subdivided based on their alteration and maturity reconstruction of the hydrocarbon generating areas at different geological times using tti maturity diagrams indicates that the generation of oil and gas in the basin took place during two periods the first phase occured during the middle to upper eocene times in the northeastern part of the basin while the second phase occured during the upper miocene to recent times in the central western and southern parts of the basin based on organic geochemical and sedimentological characteristics it is proposed that primary migration of oil from the la luna source rocks took place by expulsion of hydrocarbons through abundant microfractures developed by the pore fluid overpressure during hydrocarbon generation mass balance considerations suggest that the expulsion probably occurred as a hydrocarbon single phase and the expulsion efficiency for oil in la luna source rocks should have been very high 30 50 taking into consideration the development of oil and gas kitchens through time the paleostructures at the time of hydrocarbon generation and the characteristics of oils the migration history of the oil and gas generated from the la luna source has been outlined mixing of oils derived at different times eocene and miocene recent from the la luna source of different areas appeared to have taken place in block iv block 11 lago lamar motatan and bolivar coastal fields

organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in brazilian marginal basins abstract an investigation of bulk isotopic and molecular features of a wide selection of source rocks and oils from brazilian marginal basins ranging from lower cretaceous to tertiary in age provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo environments the methods employed include evaluation of organic carbon contents rock eval pyrolysis vitrinite reflectance measurements determination of carbon isotope ratios elemental and visual kerogen analysis assessment of oils and solvent extracts by liquid and gas chromatography and quantitative biological marker investigations using gc ms for aliphatic hydrocarbons and uv vis spectrophotometry for metalloporphyrins selected data were evaluated using principal component analysis the results enable the recognition and differentiation of seven depositional regimes lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate marine deltaic marine highly anoxic with a predominance of calcareous mudstone lithology and marine anoxic with predominance of siliciclastic lithology diagnostic features in the biological markers from these depositional environments include the carbon number distributions of n alkanes pristane phytane ratios absolute concentrations of carotane higher acyclic isoprenoids c 30 hopane and steranes and the abundance of the latter relative to 4 methylsteranes the occurrence and abundance of particular biological markers such as gammacerane tricyclic and tetracyclic terpanes 18 h oleanane 28 30 bisnorhopane and 25 28 30 trisnorhopane and the relative abundances of nickel and vanadyl porphyrins

discovery of oil bitumen co existing with solid bitumen in the lower cambrian longwangmiao giant gas reservoir sichuan basin southwestern china implications for hydrocarbon accumulation process abstract natural gas in the recently discovered lower cambrian longwangmiao giant gas reservoir of the sichuan basin in southwestern china has been considered to be from oil cracking and sourced from the underlying lower cambrian qiongzhusi shales similar to the underlying neoproterozoic dengying giant gas reservoir however geological considerations and differences in geochemistry between the longwangmiao and dengying gases bitumens imply different scenarios organic petrology geochemistry and trace element analyses of widely occurring bitumen were conducted to evaluate this hypothesis results show the coexistence of solid bitumen and oil bitumen in this reservoir for the first time which implies a new hydrocarbon accumulation process and associated exploration strategies the two types of bitumen were termed type a solid bitumen and type b oil bitumen which were sourced from the lower cambrian qiongzhusi formation shale and the lower silurian longmaxi shale respectively the type a solid bitumen shows little fluorescence and its color under plane polarized light is almost black this indicates that the type a solid bitumen is likely of high maturity it typically occurs in dissolution pores and accounts for approximately 90 of all bitumen in the reservoir in contrast the type b oil bitumen is brown black in color under plane polarized light and yellow green under fluorescence light indicating that it is mature it occurs in the residual spaces of the solid bitumen and represents approximately 10 of the total bitumen content this suggests that type a bitumen charged the reservoir earlier than did the type b bitumen during the indosinian epoch of the triassic oil generated from the mature qiongzhusi formation migrated into the longwangmiao and dengying reservoirs following this the dengying reservoir reached the temperature threshold for oil cracking since the late cretaceous and it is suggested that oil could have started to crack to gas which is the critical moment for the gas formation however the longwangmiao oils had not been cracked at this stage because the reservoir temperature had not reached the threshold later during the himalayan epoch the gas in the dengying reservoir as well as some kerogen cracking gas from the qiongzhusi shales migrated into the longwangmiao reservoir along faults gas gravity drainage and phase separation took place in the longwangmiao reservoir forming the type a solid bitumen and giant gas accumulation later oil generated from the longmaxi shales migrated into the longwangmiao reservoir forming the type b oil bitumen the case here is an example of a multi stage oil and gas accumulation future exploration targets should include middle upper cambrian reservoir rocks

global and local controls influencing the deposition of the la luna formation cenomanian campanian western venezuela bulk and molecular geochemical micropalaeontological and carbon isotopic data are used to address the different local and global factors influencing the environment of sedimentation of the la luna formation cenomanian campanian approximate palaeolatitude 15 n in a single section in western venezuela based on the constructed chronostratigraphic framework oxygen depleted bottom water conditions and black shale deposition started in western venezuela well before the widespread occurrence of organic rich sediments in higher palaeolatitude regions such as the tethys and the north atlantic near or at the cenomanian turonian boundary in the la luna formation palaeoenvironmental conditions that allowed the preservation of organic matter mainly of marine origin prevailed until santonian times in a distal platform facies with very low siliciclastic input changes in lithology appear to reflect the local response to eustatic sea level variations and the presence of a migrating upwelling belt affecting the bioproductivity of silica and carbonate a marked 13corg isotopic excursion is recognised in the middle part of the section and is apparently unrelated to local palaeoenvironmental changes in bioproductivity and oxygen depletion biological marker data show no variations in association with the isotopic excursion being mainly controlled by local fluctuations in organic matter input and preservation

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

euxinia and primary production in late cretaceous eastern equatorial atlantic surface waters fostered orbitally driven formation of marine black shales 1 oceanic anoxic events oae in the cretaceous represent major perturbations in the global climate and ocean system characterized by widespread deposition of organic carbon in the ocean the causes and effects of these events are poorly constrained mainly because of the lack of high resolution marine records here we report the distribution of molecular markers from a millennial scale record of coniacian santonian black shale oae 3 from ocean drilling program site 959 in the deep ivorian basin in the eastern equatorial atlantic highly branched isoprenoids and alkenone derived organic compounds indicate that diatoms and calcareous nanoplankton were important primary producers changes in redox sensitive trace metal accumulation and biomarkers of green sulfur bacteria provide evidence for extreme variations in redox conditions with intervals of lower photic zone euxinia pze accordingly oxygen in the coniacian santonian tropical atlantic was absent as during the cenomanian turonian boundary oae 2 but over more restricted geographic area and more limited time intervals we hypothesize that pze was a common phenomenon typical in large areas of tropical continental margins these conditions fostered sequestration of atmospheric co2 and thus helped cause the positive excursion in 13c of carbonate documented in higher latitude marine records

formation of magnetic minerals at hydrocarbon generation conditions in this paper we report the pyrolysis and formation of magnetic minerals in three source rock samples from the wessex basin in dorset southern england the experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures 5 k coupled with transmission electron microscopy imaging and x ray analysis revealed the formation of nanometre sized 10 nm magnetic particles that varied across the rock samples analysed but more importantly across the pyrolysis temperature range magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 c for all rock samples and then decreased at 300 c before rising again at 320 c the newly formed magnetic minerals are suggested to be primarily pyrrhotite though magnetite and greigite are also thought to be present the sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

early deformation of ultrahelvetic melanges in the helvetic nappes western swiss alps abstract the ultrahelvetic melanges represent an important clue for understanding the tertiary alpine orogeny particularly in the external alps of western switzerland the remarkable architecture of the upper and lower ultrahelvetic units is classically explained by an important phase of gravity sliding and orogenic sliding accompanied by the formation of chaotic complexes and olistostromes wildflysch a phenomenon called diverticulation a structural analysis reveals evidence for early phases of deformation despite strong overprinting by late alpine deformation these early fabrics are observed in foliated pebbly mudstone a tectonic breccia part of the melange matrix in disrupted flysch sequences and in dark shales they attest to brecciation related to a complex pattern of veining with several generations of calcite cement and several stages of failure during lithification and they show the important role of fluids during the deformation they present numerous analogies with the fabrics described in association with thrusts in accretionary prisms considering the tectonic fabrics and the relations among the infra prealpine melanges a tectonic model to explain the emplacement of the ultrahelvetic units is proposed in which the decollements developed in three weak levels the early fabrics evolved within a basal thrust formed in a final stage of the valaisan subduction

vertical variations of organic matter content in guayuta group upper cretaceous interior mountain belt eastern venezuela abstract a geochemical study of the vertical variations of organic matter in the guayuta group querecual and san antonio formations has been conducted on a series of samples taken from a sedimentary sequence outcropping in the interior mountain belt the results indicate that despite very high thermal maturity the organic matter content in the samples is still very high and shows geochemical characteristics related to anoxic paleoenvironmental conditions of sedimentation based on micropaleontological data mineralogical data and trace metal v ni ratio of kerogen the analysis of the sequence shows stronger anoxic depositional conditions toward the base of the section and an increase of clastic sediments toward the top the total organic carbon of the sediments increases toward the base of the sequence in addition there are pseudocyclic patterns and heterogeneities of the organic carbon content distribution at the meter scale the results indicate that the lower part of this sedimentary sequence represents a major anoxic event that can be correlated with other sequences of middle and upper cretaceous age

trace metals and organic geochemistry of the machiques member aptian albian and la luna formation cenomanian campanian venezuela abstract two proposed source rock sequences in the lower aptian albian and the upper cenomanian campanian cretaceous sediments of the perija range maracaibo basin were geochemically analyzed in order to find environmentally controlled characteristic fingerprints trace metals and sulfur concentrations were found to be closely related with the physicochemical conditions during deposition of the rocks based on trace metal analyses and sulfur relationships with toc the aptian albian sequence machiques member shows alternate oxic and anoxic episodes close to the bottom water surface where mixed marine and terrestrial organic matter were deposited the cenomanian campanian sequence la luna formation persistently shows anoxic euxinic conditions that allowed the preservation of marine organic matter ratios of trace metals v mo and zn characterize both cretaceous sedimentary columns however some limitations are visualized in using trace metals as correlation tools in further geochemical studies biomarker ratios are related to the organic matter type and lithology some specific ratios and individual compounds allow a clear differentiation between these two sequences and offer in the context of 13 c an excellent tool for further oil source rock correlation studies

tectonic rotation and diverse remanence carriers revealed by the paleomagnetism of eo oligocene turbidites from two ultrahelvetic units western alps haute savoie france abstract our paleomagnetic study of two ultrahelvetic units from the western alps shows that 1 one unit has experienced an important vertical axis tectonic rotation and 2 in turbidite deposits the nature of the remanence carrier can differ between coarse grained and fine grained intervals samples were collected at close intervals from weakly deformed turbidites in two different areas derochoir and marais characteristic remanent magnetization chrm was measured with alternating field and thermal demagnetization whereas clues for identifying the main remanence carrier were provided by coercivity spectra analysis in the derochoir area the bulk of the section consists of fine grained rocks magnetic intensity values are small directions are scattered and no polarity reversals were detected remanence was probably acquired during late diagenesis or regional tectonic deformation in coarser lithologies depositional chrm has probably been preserved by an early phase of cementation in the marais turbidites remanent magnetization is mainly carried by magnetite although some samples show clustered directions several polarity inversions have been observed suggesting that remanence was acquired soon after sediment deposition magnetic directions values indicate a counterclockwise rotation of about 110 of the marais sediments with respect to the stable european craton this movement which partly affects the underlying subalpine chains is probably related to sinistral strike slip motion along a major regional accident during the late oligocene or miocene

trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks the cretaceous la luna formation of venezuela abstract the late cretaceous la luna formation is the main hydrocarbon source rock formation in venezuela in the lake maracaibo area the formation shows a steep maturation gradient affecting sedimentary organic matter om and clay minerals related to local burial history and thrusting events the influence of the gradient is examined through geochemical organic and inorganic and mineralogical investigations carried out on samples from five cored boreholes in the maracaibo basin the analytical procedure comprises rock eval pyrolysis vitrinite reflectance measurement palynofacies observation major and trace element content determination performed on bulk rock samples or after a sequential leach protocol dop calculation and the examination and analysis of clay mineral assemblages the maturation gradient seems to have had no effect upon the distribution of the trace elements referred to as proxy indicators for palaeodepositional conditions of om rich sediments ba p v mo ni co cu cd cr and mn consequently trace elements appear to be reliable palaeoenvironmental markers they indicate that during the la luna fm deposition the water column must have had variable conditions with high primary production near the surface and the correlative development of euxinic conditions at the bottom this provides further arguments for an upwelling system affecting the northern platform of south america during late cretaceous time the relative scarcity of reactive iron in the depositional environment must have taken an active part in a process involving sulphur and acting on the environmental conditions as well as on om vulcanisation this process favours aliphatic om storage and may be relevant for many other om rich facies or hydrocarbon source rocks

late cretaceous upwelling and anoxic sedimentation in northwestern south america abstract black shales and bituminous limestones were widely deposited in northwestern south america during the maximum transgression of late cretaceous seas these cenomanian to santonian campanian anoxic sediments are distributed in two main subprovinces thick chert and phosphorite accumulated in the trough subprovince landward of a possibly partially emergent cordillera central colombia only minor intercalations of chert were deposited in the platform subprovince lying northward in the maracaibo basin northwestern venezuela faunal evidence depauperate benthic fauna planktic foraminiferal assemblages high dominance clupeoid fish assemblages and sedimentological evidence suggest anoxic condition prevailed throughout the region probably the result of dynamic upwelling in the trough subprovince but the reflection of a global anoxic event within the platform subprovince

rare earth element geochemistry of petroleum source rocks from northwestern niger delta abstract geochemical investigation of forty two rock samples from three fields in the northwestern niger delta was carried out in order to determine their rare earth elements rees content and their geochemical significances the rare earth elements and trace elements in the rock samples were determined using inductively coupled plasma mass spectrometry icp ms and instrumental neutron activation analysis inaa the results indicate that nickel is most abundant trace elements in ml field while cobalt is most abundant in mf and mj fields vanadium had the least concentration in the three fields nickel had enhanced concentrations over vanadium in the three fields the concentrations and ratios of the trace elements indicated that the source rocks in the three fields had strong terrestrial organic matter input and were deposited under oxic conditions gadolinium is the most abundant rare earth element in three fields neodymium had the least concentration in ml and mj fields while praseodymium had the least concentration in mf field the ree distribution patterns of source rocks from the three fields are similar indicating similar genetic origin pearson correlation matrix revealed that europium dysprosium lanthanum and samarium are potential redox and source indicators

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

distribution of major and trace elements in la luna formation southwestern venezuelan basin the la luna formation maraca section maracaibo basin was studied by means of v and ni analysis of the bitumen total organic carbon toc total sulfur st major elements si al fe mg mn ca ti na k p trace elements v ni co cr cu and zn and electron microprobe analysis epma of the whole rock and st major elements si al fe mg mn ca ti na k p trace elements v ni co cr cu zn mo ba u th and rare earth elements la ce nd sn eu th yb lu of the carbonate free fraction the results are discussed based on the organic and inorganic association of trace elements and their use as paleoenvironmental indicators of sedimentation an association between v and organic matter is suggested by means of correlation between v and ni vs toc the use of epma whole rock and v and ni concentrations carbonate free fraction whereas ni is found in the organic matter and the sulfide phase fe is present as massive and framboidal pyrite whereas zn precipitates into a separate phase sphalerite and ni cu and in some cases zn can be found as sulfides associated with pyrite concentrations of v and ni bitumen toc st v ni cr cu and zn whole rock u th mo carbonate free fraction are indicative of changes in the dysoxic sedimentation conditions in the chert layers toc st v ni cu and low zn and v cr 4 in the sequence corresponding to the argillaceous limestone variations in the concentrations of toc st v ni zn cu and cr whole rock can be observed also suggesting variable sedimentation conditions the following is proposed i sedimentation intervals under euxinic conditions associated with high contribution and or preservation of organic matter allowing a high concentration level of v and ni in the organic phase and the accumulation of cu zn and ni in a smaller proportion in the sulfide phase ii sedimentation intervals under anoxic conditions and in the presence of relatively lower h2s which allowed lower concentrations of v and ni in the organic phase and higher concentrations of cu zn and ni in the sulfide phase rare earth elements ree concentrations exhibit a marked increase in ce nd sm eu y and lu for the qm 3 interval relative to post archean average shale paas ree enrichment in shales has been related to the presence of phosphate minerals such as monazite or apatite however these minerals were not detected through xrd or epma in the whole rock or in the carbonate free fraction the association of ree with organic matter is suggested due to the absence of phosphate minerals although assessment of these elements require further analysis 2002 elsevier science ltd all rights reserved

organically bound metals and biomarkers in the monterey formation of the santa maria basin california abstract systematic variations in metal concentration ratios and biomarker ratios are observed with depth in cores of the monterey formation of the union leroy 51 18 well in the santa maria basin california specific metal ratios v v ni and mo mo cr correlate between kerogen and extractable organic matter further the organic matter in the two predominant lithofacies siliceous shales vs phosphatic shales can be distinguished based upon metal concentration ratios e g v v ni and certain biomarker characteristics e g sterane terpane mono triaromatic steroid hydrocarbon ratios maximum 28 30 bisnorhopane hopane ratios occur over a narrow range of v v ni ratios in kerogen concentrates and extractable organic matter this suggests that metal ratios in kerogen concentrates and eom may be useful for oil source rock correlation studies in the monterey further while kerogen parameters metals are discontinuous at the lithofacies boundary extract parameters biomarkers are gradational at this boundary suggesting about 30 ft 9 m of vertical hydrocarbon migration across the lithofacies contact

source characterization of highly mature pyrobitumens using trace and rare earth element geochemistry sinian paleozoic paleo oil reservoirs in south china abstract understanding the relationships between migrated hydrocarbons and source rocks under highly mature conditions i e pyrobitumen source correlation is a critical and challenging issue in the field of petroleum geology and geochemistry based on a case study of three sinian paleozoic paleo oil reservoirs from yunan and guizhou provinces south china we attempt to provide further understanding of this issue using trace and rare earth element ree geochemistry which can characterize the depositional setting organic input and rock provenance of source rocks results show that the bitumens from the three paleo oil reservoirs can be distinguished from each other using post archean australian shale paas normalized trace element distribution patterns ree distribution patterns hierarchal cluster analysis of element concentrations and redox and provenance sensitive elemental ratios e g v ni ni co ni mo zr cr la sc and la co pyrobitumen in the devonian xiaocaoba paleo oil reservoir is derived primarily from devonian source rocks representing a self sourced petroleum system and one stage hydrocarbon charging similarly pyrobitumen in the sinian jinsha yankong paleo reservoir also received a single fluid charge but the source is different and likely generated by contributions from both lower cambrian and sinian sources in contrast the bitumen in ordovician kaili luomian paleo reservoirs are indicated to have contributions from two different hydrocarbon sources and thus experienced two stages of hydrocarbon charging an early charge from lower cambrian source rocks and a later charge from permian source rocks the new trace and rare earth element method used in this study appears to be effective at providing a better understanding of the hydrocarbon source correlation under highly mature conditions the results improve our understanding of the complex hydrocarbon accumulation alteration evolution in the marine strata of south china which is a potential target for petroleum exploration

petrographic and geochemical characterization of the upper cretaceous coal and mudstones of gombe formation gongola sub basin northern benue trough nigeria implication for organic matter preservation paleodepositional environment and tectonic settings abstract under exploration of the upper cretaceous petroleum systems in the northern benue trough nigeria has been attributed largely to lack of subsurface samples and non integration of organic and inorganic geochemical methods recent access to surface sampling from maiganga coalfield revealed a need for detailed evaluation of the sedimentary facies types for better understanding of the hydrocarbon potential of the area detailed maceral point count py gc chnso as well as major oxides and trace element icp ms analysis were used to characterize the gombe formation coals and the mudstones for determination of the organic matter origin preservation paleodepositional environment tectonic setting and paleoclimatic condition during sedimentation evaluation based on the maceral composition tpi gi values plot of trace elements v versus ni and sedimentary succession suggest deposition of predominantly terrigenous organic matter 45 to 73 huminite maceral in an upper deltaic setting for the maiganga facies the maceral analysis associated with py gc studies indicates that the sedimentary facies of gombe formation have potential to generate liquid hydrocarbon this can be estimated from an average of 21 50 liptinite content and the ternary plot of the py gc pyrolysate m p xylene n octene and phenol suggesting presence of type ii and type ii iii kerogen within the basin assessment of the paleodepositional environment based on the trace elements composition indicates a moderate stratified water column and salinity within a relatively sub oxic to oxic conditions during sedimentation the major oxide composition as well as the binary plots of sio2 versus al2o3 k2o na2o and log sio2 versus k2o na2o for the shales and mudstones revealed a predominance of semi arid to slightly humid warm paleoclimatic conditions and a passive continental margin setting this is in accordance with the cretaceous tectonic events of the west and central africa which affected the petroleum system in the entire benue trough

vanadium nickel and sulfur in crude oils and source rocks and their relationship with biomarkers implications for the origin of crude oils in venezuelan basins abstract this work presents a study of vanadium nickel and sulfur concentrations and biomarkers in a suite of crude oils and source rocks from three venezuelan basins maracaibo and eastern basins and barinas sub basin crude oils range from unaltered to altered by biodegradation and source rocks are characterized by having different kerogen types type ii iii or iv and maturity levels early mature to post mature vanadium nickel and sulfur concentrations v ni or v v ni and saturate biomarkers were used to classify the oils and source rocks based on inferred paleo redox environment lithology and maturity of the source rock oils are classified into five groups based on v ni ratios they appear to be related to variations in the paleo redox environment from suboxic dysoxic to anoxic suboxic of source rocks with variable lithological composition and organic matter input regardless of their maturity or biodegradation level these five groups are also classified based on biomarkers related to maturity organic matter type paleo redox environment and source rock lithology in source rocks vanadium nickel and sulfur concentrations together with v ni or v v ni ratios provide information about the paleo redox environment and are related to lithology regardless of their maturity these results also indicate variable paleo redox conditions during the sedimentation of venezuelan source rocks all this suggests that while the main source rocks in venezuelan basins are well established there are still uncertainties regarding their lateral and vertical variations in organic and inorganic facies and paleo redox conditions the presence of other source rocks contributing to the accumulation of hydrocarbons in the venezuela basins is also a possibility

volcanic edifice stability during cryptodome intrusion limit equilibrium analyses were applied to the 1980 mount st helens and 1956 bezymianny failures in order to examine the influence on stability of structural deformation produced by cryptodome emplacement weakening structures associated with the cryptodome include outward dipping normal faults bounding a summit graben and a flat shear zone at the base of the bulged flank generated by lateral push of the magma together with the head of the magmatic body itself these structures serve directly to localize failure along a critical surface with low stability deep within the interior of the edifice this critical surface with the safety coefficient reduced by 25 30 is then very sensitive to stability condition variation in particular to the pore pressure ratio ru and seismicity coefficient n for ru 0 3 or n 0 2 the deep surface suffers catastrophic failure removing a large volume of the edifice flank in the case of mount st helens failure occurred within a material with angle of friction 40 cohesion in the range 105 106 pa and probably significant water pore pressure on 18 may 1980 detachment of slide block i occurred along a newly formed rupture surface passing through the crest of the bulge although sliding of block i may have been helped by the basal shear zone significant pore pressure and a triggering earthquake were required ru 0 3 and n 0 2 detachment of the second block was guided by the summit normal fault the front of the cryptodome and the basal shear zone this occurred along a deep critical surface which was on the verge of failure even before the 18 may 1980 earthquake the stability of equivalent surfaces at bezymianny volcano appears significantly higher thus although magma had already reached the surface weaker materials or higher pore pressure and or seismic conditions were probably required to reach the rupture threshold from our analysis we find that deep seated sector collapses formed by removing the edifice summit cannot generally result from a single slide cryptodome induced deformation does however provide a deep potential slip surface as previously thought it may assist deep seated sector collapse because it favors multiple retrogressive slides this leads to explosive depressurization of the magmatic and hydrothermal systems which undermines the edifice summit and produces secondary collapses and explosive blasts