charging of the neogene penglai 19 3 field bohai bay basin china oil accumulation in a young trap in an active fault zone the penglai 19 3 pl19 3 oil field the largest offshore oil field in china was found in shallow reservoirs 700 1700 m 2297 5577 ft within the most active fault zone in east china the pl19 3 anticline was not finally formed until about 2 0 ma and is cut by densely distributed faults source rock and crude oil samples from the pl19 3 field were analyzed to determine the origin and formation mechanisms of this large oil field three organic rich oil prone source rock intervals exist in the bozhong subbasin each of which has a distinct biomarker assemblage oil samples from different wells have different biomarker associations and three source related oil classes were identified within the pl19 3 field based on biomarker compositions and multivariate analysis of the data the pl19 3 field displays considerable compositional heterogeneity the compositional heterogeneity within the field and comparison between oil samples from the pl19 3 field and those from nearby structures suggest three field filling directions which is consistent with the results of migration pathway modeling the pl19 3 field was charged in the north by oil generated from dongying formation source rocks in the eastern bozhong depression and bodong depression in the southeast by oil generated from shahejie formation source rocks in the miaoxi depression and in the northwest by oil generated from shahejie formation source rocks in the central bozhong depression oil charge from multiple source rock intervals and multiple generative kitchens and focusing of oil originating from a large area of the bozhong depression into the same trap resulted in rapid oil accumulation in the pl19 3 structure and the formation of this large oil field in a very young trap within an active fault zone

dating reservoir filling a case history from the north sea abstract secondary petroleum inclusions in reservoir sandstones in the froy field and the rind discovery are used in combination with burial history modelling reservoir oils and core extracts to shed light on how and when these structures received their hydrocarbon charges analysis of normal alkane and biomarker distributions in these three data sets fluid inclusions core extracts and drill stem tests dsts provide information on the changes in organic facies and maturity of petroleum in the various reservoir strata over time the geochemistry of core extracts and dst oils in rind and froy reveal that the maturity of the rind petroleum system is higher and also of slightly less anoxic facies compared to the oil present in the froy system biodegraded oil is found today in a small sub compartment of the froy field using the burial history of the froy and the assumption that biodegradation effectively comes to a halt at 70 80 c we tentatively conclude that the filling of this sub compartment and the biodegradation of the oil must have occurred earlier than 30 40 m y b p and before oil from the upper jurassic draupne shales more recently homogeneously filled the main froy structure at 30 40 m y b p the froy structure was at a depth of 1 5 2 km compared to the present depth of 3 5 4 km and it is more than likely that neither the heather nor the draupne formations were mature in the paleo drainage area of the field at this time still the stratigraphically deeper dunlin formation could have been mature and the geochemical signatures of the now biodegraded oil correlates with known signatures from the dunlin formation in this region in the rind discovery no fluorescent petroleum inclusions are observed in k feldspar overgrowths however the number of inclusions in quartz and plagioclase is larger than that observed in the shallower froy field from the mean homogenization temperatures of the petroleum inclusions which are close to the present reservoir temperature filling of the froy field and main rind structure is suggested to have been a recent event a presently dry compartment penetrated by well 25 2 15r2 in the rind discovery contains high residual oil saturations indicating loss of oil from this structure either by direct cap rock leakage or displacement of oil by gas and subsequent cap rock leakage numerous petroleum and coexisting water inclusions were found in the presently dry overpressured structure and it is clear from measurements of homogenization temperatures that the water and the petroleum inclusions are not both saturated with respect to gas due to the lower solubility of hydrocarbon gas in water compared to in oil it is more likely that water inclusions are closer to saturation than are the petroleum inclusions using the difference in homogenization temperatures in these two systems and the burial history we tentatively suggest that the cap rock leakage occurred later than 10 m y b p such information on time for reservoir filling and seal failure when used to calibrate basin modelling should facilitate better models for understanding movement of hydrocarbon fluids in basins

conflicting sterane and aromatic maturity parameters in neogene light oils eastern chepaizi high junggar basin nw china abstract the saturated and aromatic hydrocarbon maturity characteristics of crude oils from the eastern chepaizi high in the junggar basin of northwest china were analyzed in detail the results show that sterane maturity parameters for light oils from the neogene shawan formation differ from other maturity parameters sterane c 29 20s 20s 20r and c 29 ratios indicate low maturity while aromatic hydrocarbons indicate high maturity we infer that light oils from the shawan formation have higher maturity than cretaceous and jurassic crude oils and were generated at equivalent vitrinite reflectance of 0 9 1 2 it can be concluded that the shawan light oils originated mainly from deeply buried highly mature jurassic source rocks southeast of sikeshu sag southwest of shawan sag and west of huomatu anticline belt during upward migration crude oils generated from jurassic source rock dissolved less mature cretaceous bitumen resulting in a mixture with apparently conflicting parameters phase fractionation during migration also could have contributed to the anomalously low sterane maturity parameters

dibenzothiophenes in solid bitumens use of molecular markers to trace paleo oil filling orientations in the lower cambrian reservoir of the moxi gaoshiti bulge sichuan basin southern china abstract the moxi gaoshiti mg bulge of the sichuan basin in southwestern china is unique because of its enormous gas field in old cambrian and proterozoic marine strata where oils in carbonate reservoirs have been nearly completely cracked to solid bitumen and natural gas because of high to post mature thermal maturity levels numerous solid bitumen bearing dolomites obtained from boreholes in the mg bulge provide an ideal opportunity to trace the major filling orientations for the lower cambrian paleo oil reservoirs biomarker analysis was conducted on solid bitumens through quantitative detection with gas chromatography mass spectrometry results show that all solid bitumen samples in the mg bulge exhibit similar compositions of molecular markers thereby implying that they belong to the same oil population and that paleo oils should be derived from the same source kitchen bed low pristane phytane and dibenzothiophene dbt phenanthrene ratios as well as the predominance of dbt among dbt dibenzofuran and fluorene are observed in the solid bitumen samples which suggest that related paleo oil reservoirs likely originated from a highly reducing marine shale environment all isopleth maps of the dbt concentrations 4 methyldbt 1 methyldbt 4 6 dimethyldbt 1 4 1 6 dimethyldbt dmdbt and 2 6 3 6 dmdbt 1 4 1 6 dmdbt ratios consistently indicate that the overall orientation of oil migration in the mg bulge is primarily from the west to east direction with the strongest filling point located at approximately the mx12 well the north to west direction is an additional filling orientation these indicators show that those paleo oils that had been cracked to pyrobitumens mainly originated from a source kitchen in the western part of the mg bulge a hydrocarbon source kitchen in the northern part of the mg bulge may also have developed but further research on this is needed the recognition of these solid bitumen molecular signatures particularly the consistent results using dbt concentrations and alkyl dbt thermal maturity indicators suggests a potential approach that can be applied to the study of paleo oil reservoirs

geosynthesis of dibenzothiophene and alkyl dibenzothiophenes in crude oils and sediments by carbon catalysis sediment extracts and crude oils have been shown to contain methyl substituted biphenyls and dibenzothiophenes with isomer distributions suggesting a geochemical relationship between the two compound classes laboratory simulation experiments have shown that carbon catalyses the reaction between surface adsorbed sulfur and biphenyl to form dibenzothiophene similarly the methyl substituted biphenyls reacted to yield corresponding methyl dibenzothiophenes we suggest that the widespread distribution of dibenzothiophene and alkylated dibenzothiophenes in sediments and crude oils is the result of a catalytic reaction of biphenyl ring systems and surface adsorbed sulfur on the surface of carbonaceous material

recognising biodegradation in gas oil accumulations through the δ13c compositions of gas components a large suite of natural gases 93 from the north west shelf and gippsland and otway basins in australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases about 26 of these gases have anomalous stable carbon isotope compositions in the c1 c4 hydrocarbons and co2 components and are interpreted to have a secondary biogenic history the characteristics include unusually large isotopic separations between successive n alkane homologues up to 29 pdb and isotopically heavy co2 up to 19 5 pdb irrespective of geographic location these anomalous gases are from the shallower accumulations 600 1700 m where temperatures are lower than 75 c the secondary biogenic gases are readily distinguishable from thermogenic gases 74 of this sample suite which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m while dissolution effects may have contributed to the high 13c enrichment of the co2 component in the secondary biogenic gases the primary signature of this co2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis correlation between biodegraded oils and biodegraded dry gas supports the concept that gas is formed from the bacterial destruction of oil resulting in secondary biogenic gas furthermore the prominence of methanogenic co2 in these types of accumulations along with some isotopically depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked it is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis

geochemical application of tricyclic and tetracyclic terpanes biomarkers in crude oils of nw china composition and distribution of tricyclic terpanes tetracyclic terpanes and hopane in crude oils from tarim turpan ordos and qaidam basins nw china were investigated and some new source dependent parameters were developed and evaluated c 19 and c 20 tricyclic terpanes are more abundant in terrigenous oils with c 24 tetracyclic terpane dominant relatively higher carbon number tricyclic terpanes especially c 23 tricyclic terpane are often the dominant homologues in crude oils from saline lacustrine and marine sources on the whole the relative content of c 23 tricyclic terpane of crude oils follow the order of saline lacustrine oils marine oils freshwater lacustrine oils terrigenous oils for c19 23 tricyclic terpane some new parameters based on the tricyclic hopane and tetracyclic tricyclic terpane as well as tetracyclic hopane ratios could be used to distinguish the oils and source rocks with similar maturity deposited in different sedimentary environments c 2015 elsevier ltd all rights reserved

microbial cycling migration and leakage of light alkanes in the nile delta tertiary fan abstract we present a geochemical dataset which helps unravel the complex set of processes by which light alkanes are sourced charged mixed leaked and altered in the plio pleistocene nile delta thermogenic gas derives from a sub messinian source and mixes with microbial hydrogenotrophic methane rich gas in the plio pleistocene section there is strong isotopic evidence for both microbial generation and microbial degradation of ethane and propane but these signals are only recorded where the rates of biological processes outstrip the rate of thermogenic charge high resolution gas geochemistry profiles through four 500 1100 m sequences of mixed channel channel levee mass flow deposits and hemipelagites shed light on migration and leakage pathways we see limited geochemical evidence for migration along or across faults which cross cut mud rich sequences lateral migration along sands silts delivers gas to structural highs where leakage occurs vertically along focussed pathways through heterogeneous mud rich sediments we propose that in situ generation of microbial gas within mud rich sequences facilitates percolation of migrating gases that replenishes c2 components and sustains the biodegradation pathway in syntrophic microbial communities high resolution changes in gas composition suggest an unequilibrated system which is being actively charged in some structures rates of charge and of microbial alteration of gas are broadly similar in others rates of charge exceed rates of microbial alteration our results have important implications for the understanding of both basin wide fluid flow and the ecology of the deep biosphere

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

the influence of facies and depositional environment on the occurrence and distribution of carbazoles and benzocarbazoles in crude oils a case study from the gulf of suez egypt abstract two major hinge zones separate the gulf of suez gos of egypt into three tectonic sub provinces namely the northern central and southern sectors based on bulk and molecular parameters the crude oils from the gos can be subdivided into three groups according to their geographical origin from the three sectors the geochemical characteristics of the oils are consistent with inferred depositional environments of anoxic marine carbonate facies in the central sector compared to a more oxic environment that has received an increased proportion of clastic input in the southern sector while the northern is intermediate in character most of the oils cover a relatively narrow maturity range 0 86 0 98 r c in the peak oil window the biomarker parameter ts ts tm is controlled by variations of facies and depositional environment but not by maturity concentrations of pyrrolic nitrogen compounds in the gos crude oils are in general relatively low but show a large range of variability in crude oils of the northern sector a strong predominance of c 4 and c 5 carbazoles over the lower homologues is observed towards the south the relative abundance of carbazole and the methylcarbazoles increases successively variations within the methylcarbazoles and c 2 carbazoles are rather small and therefore are not useful for a straightforward classification of oil families however atypical patterns in some of the oils particularly from the northern sector give evidence for contributions from yet unidentified local sources which might not be of general importance in the gos petroleum province benzo b carbazole is near or even below detection limit in the gos crude oils the benzocarbazole a a c ratio shows a consistently positive correlation with api gravity pr ph and ts ts tm and a negative correlation with the sulphur content based on these observations it is inferred that facies and depositional environment of the relevant source rocks are the major factors influencing the benzocarbazole distributions in the central sector of the gos in which oils are predominantly sourced from carbonates deposited under strongly reducing conditions a a c values are mainly below 0 5 in the southern sector where source rocks were deposited under less reducing conditions and received significant clastic input as well as in the northern sector the a a c values are mainly above 0 5

biodegradation of tar sand bitumens from the ardmore and anadarko basins carter county oklahoma abstract a series of tar sand bitumens collected from a single shallow well in the ardmore basin carter county oklahoma has been characterized by various geochemical techniques particular emphasis has been placed on determination of variations in biomarker distributions both hydrocarbons and porphyrins as a function of biodegradation the tar sand bitumens have been severely biodegraded and most of the n alkanes low molecular weight cycloalkanes isoprenoid alkanes c27 c29 steranes light aromatics and sulfur compounds have been removed in addition the hopane distributions have been altered to differing degress with the pseudohomolugoues above c30decreasing in concentration prior to the c20 21 the triaromatic steroid hydrocarbons were also altered with preferential removal of 20r epimers c20 21 and c27 28 diasterane and c30 sterane distributions appear to be relatively unaffected by biodegradation the high resistance of tricyclic terpanes the c24 tetracyclic terpane and monoaromatic steroid hydrocarbons to biodegradation indicate that the distributions of these compounds are well suited to serve as correlation parameters in heavily altered samples it was also observed in this study that the main ring structure of the porphyrin molecule was not affected by these very high levels of biodegradation previously hypothesized changes in porphyrin structure due to biodegradation have included n alkyl side chain cleavage and possible conversion from dpep to etio type porphyrins an investigation has also been made with an active seep collected from sulphur in the anadarko basin oklahoma and the results show that the relative rate of biodegradation of various classes of biomarkers differs between these two locations although it was proposed that the original oil responsible for these two seeps is derived from the same source rock formation

application of new diterpane biomarkers to source biodegradation and mixing effects on central llanos basin oils colombia llanos basin oils have been commonly attributed to the prolific upper cretaceous age source formations outcropping in the eastern cordillera recently however variable oleanane indices have been reported for llanos oils suggesting a contribution from tertiary source sequences largely on the basis of the presence of high relative abundance of oleanane a tertiary age diagnostic biomarker a homologous series of 25 norhopanes in many central llanos basin oils indicates that heavy biodegradation is particularly common to the initial oil charging reservoirs from upper cretaceous marine sources one homologue 20s 25 norhomohopane c30 25 norhopane co elutes with oleanane and may thus contribute to the peak attributed to oleanane also confusing the source identification is the possibility that some cretaceous facies also contain oleanane three plant diterpanes which may be isomers of fichtelite and are common to the central llanos oils have been observed in an eocene source rock sample but not in cretaceous rocks or oils providing additional strong evidence for a tertiary contribution to some llanos basin oils in mixed oils with low to moderate oleanane contents and with a strong co eluting c30 25 norhopane the fichtelite isomers are the more reliable indicators of tertiary source input

origin of beatrice oil by co sourcing from devonian and middle jurassic source rocks inner moray firth united kingdom commercial hydrocarbons in the moray firth area of the north sea occur in reservoirs ranging in age from devonian to tertiary with the bulk of the reserves located in upper jurassic and lower cretaceous clastic rocks most hydrocarbon discoveries in this area have been made in the outer moray firth primarily in the witch ground graben a single oil field beatrice has been developed in the inner moray firth area the unusual location and chemical composition of beatrice compared to piper and other north sea oils generated from the kimmeridge clay have contributed to the longstanding controversy on its origin oil source potential in the inner moray firth has been recognized in lacustrine devonian dolomitic siltstones middle jurassic paralic coals upper jurassic marine shales and other rocks however prior to this study only the upper jurassic kimmeridge clay was recognized as a significant effective source rock in the main graben area of the north sea biomarker and stable carbon isotope analyses conclusively show that beatrice oil could not have been generated from the classic kimmeridge clay source rock but is a mixture of products derived from effective devonian and middle jurassic source rocks this work demonstrates the power of a multiparameter geochemical approach in solving this difficult oil to source rock correlation problem use of this approach can lead to the recognition of new exploration plays in previously well explored basins

the impact of severe biodegradation on the molecular and stable c h n s isotopic compositions of oils in the alberta basin canada abstract although the effects of biodegradation on the composition and physical properties of crude oil have been well studied effects of in reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood the alberta basin in western canada is one of the world s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils the basin hosts moderately to severely biodegraded petroleum regionally distributed and in single more or less continuous oil columns in this study a series of oil samples from the alberta heavy oil and oil sands provinces with varying degrees of biodegradation were analyzed to assess the impact of progressive biodegradation on the molecular and c h n and s isotopic compositions of oils the results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10 toward the alberta saskatchewan border on the peters and moldowan scale of biodegradation abbreviated pm 2 to pm 10 and from tens to hundreds on the manco scale within single reservoirs increasing biodegradation was observed from top to bottom of the oil columns at all sites studied the whole oil stable isotopic compositions of the samples varied in the ranges 13 c 31 2 to 29 0 2 h 147 to 133 15 n 0 3 4 7 and 34 s 0 4 6 4 the maximum differences between values of samples within single oil columns were 13 c 1 4 2 h 7 15 n 1 7 and 34 s 1 0 regional variations in the isotopic compositions of oil samples from different wells averaged values from top to bottom were 1 2 for 13 c 12 for 2 h 4 1 for 15 n and 5 5 for 34 s and hence generally significantly larger variations were seen than variations observed within single oil columns especially for n and s it appears that even severe levels of biodegradation do not cause observable systematic variations in carbon nitrogen or sulfur isotope composition of whole oils this indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies

origin and occurrence of 25 norhopanes a statistical study abstract the alkane fraction of more than 200 rocks biodegraded oils and non biodegraded oils have been analysed by means of computerized gc ms in order to investigate the effect of natural biodegradation on the occurrence of demethylated hopanes i e 17 25 norhopanes the results obtained indicate that 25 norhopanes are preexisting biomarkers the concentration of which is enhanced by selective biodegradation of more readily degradable homologs i e regular hopanes rather than by demethylation of hopanes in reservoirs however the use of 25 norhopane enrichment as a palaebiodegradation indicator in apparently non biodegraded oils is still valuable providing the initial background content in the corresponding source rocks is known furthermore 25 norhopanes appear to be diagnostic of specific environmental conditions marine and lacustrine source rocks dysoxic and not very hypersaline lastly one other novel bacterially resistant rearranged hopanoic compound namely a c29 neohopane is applicable for both biodegradation and maturation evaluation

circum arctic petroleum systems identified using decision tree chemometrics source and age related biomarker and isotopic data were measured for more than 1000 crude oil samples from wells and seeps collected above approximately 55n latitude a unique multitiered chemometric multivariate statistical decision tree was created that allowed automated classification of 31 genetically distinct circum arctic oil families based on a training set of 622 oil samples the method which we call decision tree chemometrics uses principal components analysis and multiple tiers of k nearest neighbor and simca soft independent modeling of class analogy models to classify and assign confidence limits for newly acquired oil samples and source rock extracts geochemical data for each oil sample were also used to infer the age lithology organic matter input depositional environment and identity of its source rock these results demonstrate the value of large petroleum databases where all samples were analyzed using the same procedures and instrumentation

mixed oils derived from multiple source rocks in the cainan oilfield junggar basin northwest china part ii artificial mixing experiments on typical crude oils and quantitative oil source correlation abstract there are five groups of crude oils in the east junggar basin four of which are derived from single sources viz permian triassic jurassic and carboniferous the crude oils in the cainan field are of a special type in that they have little correlation with any of the four known source rocks in the basin based on their distribution in the reservoir formations and their carbon isotopic and biomarker compositions it is concluded that these crude oils are mixtures of hydrocarbons from permian triassic and jurassic source rocks a mixing calculation using whole oil stable carbon isotope ratios of typical end member crude oils suggests that the contribution of jurassic source rocks to the cainan crude oils averages 50 65 a second calculation based on absolute concentrations of biomarkers indicates that the jurassic contribution averages 62 while those of the triassic and permian source rocks average 15 and 23 respectively artificial laboratory mixing of three typical permian triassic and jurassic oils indicates that the average contribution of the jurassic source rocks is 65 whereas those of the triassic and permian rocks are around 15 and 20 respectively in summary using whole oil stable carbon isotope ratios and biomarker parameters it can be concluded that the jurassic source makes a contribution of 55 70 while the triassic and permian sources each contribute 10 30 therefore the crude oils in the cainan oilfield are mainly derived from jurassic source rocks in the fukang sag

cross stratigraphies from a seismically active mud lens off peru indicate horizontal extensions of laminae missing sequences and a need for multiple cores for high resolution records abstract marine laminated sediments in dysoxic areas of the ocean floor are an excellent archive for high resolution climate reconstructions while the existence of discontinuities produced by natural events such as underwater landslides slumps strong bottom currents and or bioturbation is usually acknowledged for long records the extent of their influence on high resolution sequences is usually not considered in the present work we show strong evidence for multiple stratigraphic discontinuities in different gravity and box cores retrieved off pisco peru covering the last 600 years chronostratigraphies are largely based on cross correlation of distinct sedimentary structures determined by x ray image analysis and validated using 210 pb 241 am and 14 c profiles as well as proxy records the cross correlation of distinct stratigraphic layers allows for chronostratigraphic tie points and clearly shows that some sedimentary sequences are continuous across scales of tens of kilometers indicating that regional processes often determine laminae formation some differences in laminae thickness were found among cores which could be explained by different sedimentation rates spatially variable deposition of diatom blooms changes in silica dissolution and partial deposition erosion caused by bottom currents using multiple stratigraphic tie points provides clear evidence for laminated sequences present in some cores to be missing in other cores moreover instantaneous depositions from upslope were identified in all the cores disrupting the continuity of the sediment records these discontinuities instantaneous deposits and missing sequences may be due to slumps possibly triggered by earthquakes and or erosion by strong bottom currents in spite of the missing sequences in some cores a continuous composite record of the last six centuries was reconstructed from spliced sequences of the different cores which provides a well constrained temporal framework to develop further high resolution proxies in this region the present work shows that paleoreconstructions developed from single cores particularly in areas with strong seismic activity and or strong bottom currents are subject to both temporal gaps and instantaneous depositions from upslope both of which could be misinterpreted as abrupt climate changes or anomalous climate events we stress the need for multiple cores to determine the stratigraphic continuity and chronologies for high resolution records

a novel microbial hydrocarbon degradation pathway revealed by hopane demethylation in a petroleum reservoir abstract the origin of 25 norhopanes is controversial despite their abundance in many biodegraded oils in a biodegraded oil reservoir we found concomitant and directionally opposite changes with depth in the concentrations of certain 17 hopanes and their 17 25 norhopane counterparts demonstrating microbially induced demethylation of hopanes these observations suggest a hitherto unknown biodegradation mechanism and provide insight into the poorly understood process of natural petroleum biodegradation

evolution of tricyclic alkanes in the espirito santo basin brazil abstract the distributions of tricyclic isoprenoid alkanes were characterized in the neocomian sequence from the espirito santo basin brazil by high resolution gas chromatography mass spectrometry and co injection with synthetic standards where available a suite of homologous components ranging from c 20 to c 28 were present throughout the sequence as a mixture of stereoisomers whose relative abundance changed with depth the epimerization rates were similar for the various tricyclic compounds irrespective of side chain length achieving equilibrium ratios before the main stage of oil generation this new maturity parameter should be useful as a supplement to sterane hopane measurements

hydrocarbon generation potential of triassic mudstones in the junggar basin northwest china triassic mudstones in the junggar basin of northwest china especially those of the upper triassic baijiantan formation which formed during a lake flooding event are the most important set of cap rocks in the basin these mudstones presumably have hydrocarbon generation potential although this potential has not been documented here we provide new organic geochemical and geological background data on these mudstones and discuss their hydrocarbon generation potential geochemical analyses were performed on available samples collected from more marginal shallow lake facies in the central basin area these analyses indicated that these rocks contain about 1 0 total organic carbon toc the organic matter type is predominantly type iii kerogen derived mainly from terrestrial higher plants the organic matter underwent maturation in uplifted areas and reached higher maturity levels in depressions within the basin thus the triassic mudstones may have a potential for hydrocarbon generation during the lake flooding period especially for gas and hence deserve attention in future exploration more research needs to be conducted on the generation potential of these units including oil besides gas as few deep lake facies samples from depressions in the basin are currently available for investigation

mechanisms for oil depletion and enrichment on the shijiutuo uplift bohai bay basin china the shijiutuo uplift is a major uplift to the north of the bozhong depression the largest generative kitchen in the bozhong subbasin bohai bay basin although the n35 2 trap on this uplift contains a medium size oil accumulation and the q32 6 trap contains china s third largest offshore oil accumulation the q31 1 trap between the n35 2 and q32 6 traps with very similar evolution history was confirmed to be dry biomarker associations of crude oil and source rock samples were analyzed and three dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift multiple parameter oil source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source related oil classes almost all oils from the shijiutuo uplift are derived from the eocene shahejie formation whereas oils found between the shijiutuo uplift and the bozhong depression either are derived from or have important contributions from the oligocene dongying formation variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the shahejie and then dongying formations in the bozhong depression which is reasonably supported by petroleum migration pathway modeling oil charge from two oil prone source rock intervals and more importantly focusing of oil originating from a large area of the bozhong generative kitchen into the same trap accounted for oil enrichment and formation of china s third largest offshore oil field in the q32 6 structure the complexity and primary control of the sealing surface top surface of the carrier bed morphology on the positions of migration pathways caused the q31 1 trap to be shielded from migration of oil originating from the bozhong depression resulting in oil depletion in this trap shadows to petroleum migration may occur because of the three dimensional behavior of petroleum migration and two dimensional migration modeling may be misleading in predicting petroleum occurrences

carbon isotope composition of marine crude oils 1 a histogram of isotope 13 c values of 621 post ordovician marine oils shows a trimodal distribution within this distribution four groups of oils can be recognized on the basis of isotope 13 c values in conjunction with pristane phytane ratios and sulfur contents oils with isotope 13 c values of 32 0 to 28 0 o oo are mainly marine shale oils older than oligocene oils with isotope 13 c values of 28 0 to 23 5 o oo are mainly deltaic oils of varying geologic age or mesozoic carbonate oils and oils with isotope 13 c values heavier than 23 5 o oo are mainly marine shale oils of miocene age the isotope 13 c variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric co 2 concentration since 25 ma which has resulted in decreased isotope fractionation by marine plankton during photosynthesis deltaic oils are mostly derived from terrestrial organic matter and their isotope 13 c values reflect the time invariant average isotope 13 c value of land plant carbon at about 25 o oo mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded processes that occur after oil generation generally have smaller effects on isotope 13 c values of oils as a result isotope 13 c val es of oils are useful for determining oil oil and oil source rock relationships and in conjunction with other geochemical properties can indicate the possible age and depositional environment of source rocks

organic geochemistry of venezuelan extra heavy crude oils 2 molecular assessment of biodegradation abstract the extent of biodegradation of extra heavy crude oils from the eastern venezuelan basin orinoco tar belt has been assessed using biological marker compounds which include steranes triterpanes aromatic steroids and phenanthrenes analyzed by gc ms and petroporphrins analyzed by hplc the analyses indicate that biodegradation takes place in two regions namely surface or close to surface and subsurface with a different pattern of biodegradation taking place at each type of location some oils show patterns of biomarkers which may be the combined result of degradation at both main sites of biodegradation however two empirical biodegradation parameters are proposed to achieve this classification the classification of crude oils in terms of degree of biodegradation is further complicated by the identification of mixed crudes

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

fractionation of benzocarbazoles between source rocks and petroleums the concentrations of the a and c isomers of benzocarbazoles in a number of crude oils and source rock bitumens were obtained by in house methods and from reported published data the changes in benzocarbazole concentrations and the benzocarbazole a a c ratio relating to source maturity facies and migration are described with increasing source maturity in north sea oils and source rocks up to 0 85 vitrinite reflectance equivalent benzocarbazole concentrations increase due to generation followed by a decrease above 0 85 vitrinite reflectance equivalent due to expulsion dilution by newly generated hydrocarbons comparison of benzocarbazole concentrations between reservoired petroleum and related source petroleum revealed a concentration bias in favour of source petroleum where concentrations of benzocarbazoles reached up to 10 times that found in equivalent reservoired petroleum the differences in benzocarbazole concentrations between related oils and source rocks indicate that the major fractionation is introduced during primary migration and expulsion because when secondary migration is limited as in the miller field we see extensive benzocarbazole fractionation between reservoired petroleum and associated source petroleum the relationships between source rock toc content and petroleum benzocarbazole concentrations and a a c ratios indicate that source organic matter exerts a strong influence on benzocarbazole distributions the introduction of a reservoir source fractionation parameter rsfp defined as the ratio between the concentrations of benzo a benzo c carbazole in reservoired petroleum versus the concentration of benzo a benzo c carbazole in source petroleum enables description of the relationship between reservoired petroleum and source petroleum from different petroleum systems petroleum deriving from clastic source rocks demonstrated high levels of benzocarbazole fractionation between reservoired petroleum and source petroleum during primary and secondary migration compared to petroleum deriving from carbonate systems the rsfp parameter may also be used to examine fractionation effects during migration between different compound types the more polar benzocarbazoles are fractionated more strongly than dibenzothiophene and phenanthrene between reservoired petroleum and source petroleum whereas sometimes source maturity signals do dominate the benzocarbazole signals in migrated oils in other circumstances often associated with clastic source rocks no clear maturity signal is transferred to migrated oils

a practical biodegradation scale for use in reservoir geochemical studies of biodegraded oils existing scales widely used to describe the extent of biodegradation of petroleum have insufficient resolution to usefully characterize many heavy oil and bitumen occurrences including the volumetrically dominant heavily and severely biodegraded oil accumulations in the foreland basins of western canada and venezuela in these and other deposits existing classifications or descriptions of the biodegradation level may vary only slightly yet oil may vary in viscosity by orders of magnitude the manco biodegradation scale proposed here is based on integrating the extent of degradation of various members of compound classes not included in previous biodegradation scales they include alkyl aromatic and alkyl thiophenic compounds that show variable extent of alteration in samples degraded to uniform levels on standard scales but which may show variation in local degradation systematics related to biodegradation mechanisms and extent of oil mixing the manco scale uses a combination of a consideration of the extent of alteration within a compound class together with a consideration of biodegradation across a range of compound classes it can be reliably used as a basis for interpreting geochemical changes in heavily biodegraded oil suites and can also be used to differentiate biodegraded oil samples likely to be amenable to cold production from those requiring production strategies such as steam or chemical flooding as with other biodegradation scales the scale may also provide evidence for the influx of later higher quality oil into a reservoir fluid that had been previously biodegraded

origin and migration pathway of biodegraded oils pooled in multiple reservoirs of the chepaizi uplift junggar basin nw china insights from geochemical characterization and chemometrics methods abstract the chepaizi uplift possesses three sets of oil bearing reservoirs six sets of source intervals multiple episodes of hydrocarbon charging and variable physical properties of crude oils in this study based on geochemical characterization source intervals and the biodegradation rank of the crude oils four oil populations named as group a b c and d d1 d2 were defined the combined results of q cluster analysis qca and discriminant analysis da indicate that group a occurred mainly in lower neogene shawan formation n1s in the western chepaizi uplift and were primarily derived from the middle lower jurassic xishanyao badaowan sangonghe formations j2x j1b j1s and a little of paleogene anjihaihe formation e2 3a of the sikeshu sag group b occurred essentially in upper carboniferous xibeikulasi formation c2x of the western chepaizi and originated mainly from the j1b mudstone of the shawan sag in addition the crude oils of group c occurred mainly in the lower cretaceous tugulu group k1tg of the eastern chepaizi and were primarily derived from carboniferous sourced oil from the chepaizi uplift the crude oils of groups d d1 and d2 are mixed with permian p1f and p2w of shawan sag and carboniferous sourced oil from the chepaizi uplift based on the reconstruction of the migration history the oils of group a in the n1s formation of western chepaizi migrated predominantly from the south sikeshu sag to the north and northeast with wells su1 2 and c27 as the initial filling points the oils of group b in the n1s formation of the western chepaizi travelled predominantly from the southeast shawan sag to the north and northwest the oils of group d1 in the c2x formation of eastern chepaizi migrated mainly from the northeast and southeast shawan sag to the west with well p661 as the initial filling point wells sm011 and c64 are the initial filling points of the k1tg group in the hongche fault belt crude oils of group c in the k1tg group travelled predominantly from the southeast shawan sag and northeast to the west into the eastern chepaizi and from well p68 to the southwest according to the paleo oil preferential filling orientations and paleogeomorphology some favourable targets for petroleum exploration of the chepaizi uplift are forecast in this study

carbon isotope geochemistry of paleozoic oils from big horn basin abstract twenty crude oils from paleozoic reservoirs in the big horn basin wyoming were fractionated into light hydrocarbons saturates monoaromatics diaromatics polyaromatics and polars and asphaltenes amounts and isotopic composition of each fraction were found to be internally consistent with the degree of maturation of the oil confirming the established single source origin for these oils a variation of approximately three per mil in the carbon isotopic composition of the whole oil and individual fraction was explained as being caused by maturation the isotope type curves for these oils the variations in isotopic composition as a function of compound group type were not found to be as simple as commonly believed other alteration processes such as migration and deasphalting might have influenced the pattern of the isotope distributions among fractions the conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil showed that the maturation of crude oil consists of consecutive processes leading from larger and more polar components into smaller and less polar components culminating in natural gas calculations were made to project the amount of condensates associated with the crude oils and the amount of natural gas which had been generated from crude oils of a given maturity

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

charging of oil fields surrounding the shaleitian uplift from multiple source rock intervals and generative kitchens bohai bay basin china abstract this paper discusses origin and charging directions of oil fields on the shaleitian uplift bohai bay basin the shaleitian uplift is a footwall uplift surrounded by three sags containing mature source rocks the origins of the four oil fields on the shaleitian uplift both in terms of source rock intervals and in terms of generative kitchens were studied using biomarker distributions for 61 source rock samples and 27 oil samples hierarchical cluster analysis using 12 parameters known to be effective indicators of organic matter input and or depositional conditions allowed the identification of six oil types or classes these six oil classes could then be linked to three distinct source rock intervals ranging in age from 43 0 ma to 30 3 ma the third member 43 0 38 0 ma in age and first member 35 8 32 8 ma of the eocene shahejie formation and the third member of the oligocene dongying formation 32 8 30 3 ma each sourced one class of oil the other three classes represent mixtures of oil generated from multiple source rock intervals traps on the shaleitian uplift were charged in the east by oil generated from the eocene shahejie formation in the bozhong sag in the southeast by oil generated from the eocene shahejie and then oligocene dongying formations in the southwestern part of the bozhong sag and or in the eastern part of the shanan sag and in the southwest by oil generated from the eocene shahejie formation in the western part of the shanan sag the estimated migration distances range from less than 5 km to about 20 km the compositional heterogeneity within fields and multiple parameter comparisons between oils from nearby wells in different fields have proven to be a powerful tool to determine the in filling histories of oil fields in cases where multiple source rock intervals and multiple generative kitchens exist

dibenzothiophenes and benzo b naphthothiophenes molecular markers for tracing oil filling pathways in the carbonate reservoir of the tarim basin nw china abstract dibenzothiophenes dbts and benzo b naphthothiophenes bnts are the main sulfur containing heterocyclic aromatic compounds in crude oils and sedimentary rock extracts because of the relatively higher electronegativity and an unshared pair of electrons in the outer shell orbital of the sulfur atom hydrogen bonds can be formed between the sulfur and hydrogen atoms of the surrounding medium in carrier beds therefore dbts and bnts are potential geochemical markers for tracing oil filling pathways a correlation of oils in the halahatang oilfield of the tabei uplift in the tarim basin nw china shows that the molecular marker compositions of all oils are quite similar implying that these oils have been sourced from the same source kitchen and belong to a same oil family using molecular indicators relative to dbts and bnts namely 4 1 mdbt 4 1 methyldibenzothiophene 4 6 1 4 1 6 dmdbt 4 6 1 4 1 6 dimethyldibenzothiophene 2 6 3 6 1 4 1 6 dmdbt 2 6 3 6 1 4 1 6 dimethyldibenzothiophene and 2 1 bnt 2 1 bnt 1 2 bnt benzo b naphtho 2 1 d thiophene benzo b naphtho 2 1 d thiophene benzo b naphtho 1 2 d thiophene this paper traces the oil migration orientation and filling pathways in this oilfield the isopleth maps of these molecular indicators unequivocally reveal an overall oil migration filling points and preferred filling pathways orientation from south to north based on this a potential source kitchen is predicted in the southern part of the halahatang depression most probably at the manxi shuntuoguole low uplift just between the awati and manjiaer depressions the southern part of the halahatang depression along the oil charging pathways is predicted to be the most promising region for oil exploration

biodegradation of 25 norhopanes in a liaohe basin ne china oil reservoir a suite of reservoir cores in an oil column from the shuguang oil field liaohe basin ne china was characterized geochemically to investigate the behaviour of 25 norhopanes nhs during biodegradation the extracted bitumens show similar organofacies type and maturity levels with biodegradation as the primary control on compositional variation all bitumens suffered severe biodegradation influence which removed all regular steranes and most of the pentacyclic terpanes leaving nhs as dominant components in total ion chromatograms of the saturated hydrocarbon fraction however the nhs are not the endpoint of hopane destruction they are biodegraded as well biodegradation of nhs can be illustrated by significant depletion in their concentrations and systematic variation in ratios of resistant to vulnerable components throughout the oil column the biodegradation sequence is somewhat similar to normal pentacyclic terpanes among the nh series 18 25 30 bisnorneohopane is the most recalcitrant to biodegradation the 18 22 25 29 30 tetranorneohopane is more resistant to biodegradation than 17 22 25 29 30 tetranorhopane and 34nhs are more resistant than other nhs degradation of nhs provides a new angle to understanding the complicated inter relationships in subsurface biodegraded reservoirs