sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

diagenetic and catagenetic products of isorenieratene molecular indicators for photic zone anoxia a wide range of novel diagenetic and catagenetic products of the diaromatic carotenoid isorenieratene a pigment of the photosynthetic green sulphur bacteria chlorobiaceae has been identified in a number of sedimentary rocks ranging from ordovician to miocene compound identification is based on nmr mass spectrometry the presence of atropisomers and stable carbon isotopes atropisomers contain an axially chiral centre which in combination with other chiral centres results in two or more diastereomers that can be separated on a normal gc column chlorobiaceae use the reverse tca cycle to fix carbon so that their biomass is enriched in 13c high 13c contents of isorenieratene derivatives therefore support their inferred origins isorenieratene derivatives include c 40 c 33 and c 32 diaryl isoprenoids and short chain aryl isoprenoids with additional aromatic and or s containing rings c 33 and c 32 compounds are diagenetic products of c 33 and c 32 carotenoids formed from isorenieratene during early diagenesis through expulsion of toluene and m xylene respectively cyclisation of the polyene acyclic isoprenoid chain can proceed via an intramolecular diels alder reaction followed by aromatisation of the newly formed ring sulphurisation is also an important process during early diagenesis competing with expulsion and cyclisation sulphur bound isorenieratane is released during progressive diagenesis due to cleavage of relatively weak ss and cs bonds cleavage of c c bonds during aromatisation of newly formed rings and during catagenesis yields short chain compounds the inherent presence of a conjugated double bond system in carotenoids implies that similar diagenetic and catagenetic reactions can occur with all carotenoids chlorobiaceae live at or below the oxic anoxic boundary layer and require both light and h 2s the presence of isorenieratene or its diagenetic and catagenetic products in ancient sedimentary rocks and crude oils is therefore an excellent indication for photic zone anoxia in the depositional environment diagenetic and catagenetic products of isorenieratene are expected to find applications in reconstruction of palaeoenvironments and in oil oil and oil source rock correlation studies their presence in several petroleum source rocks suggests that anoxia is an important environmental parameter for the preservation of organic matter

early oligocene dinoflagellate cysts from the upper rhine graben sw germany paleoenvironmental and paleoclimatic implications using principal component analysis and cyst diversity and equity trends we can recognize four distinct dinoflagellate cyst dinocyst assemblages from four rupelian early oligocene cores in the mainz embayment of the northern rhine graben sw germany these assemblages are the spiniferites ramosus pc1 thalassiphora pelagica pc2 homotryblium tenuispinosum pc3 and vozzhennikovia spinula pc4 assemblages the four cores provide an onshore offshore transect in the mainz embayment the h tenuispinosum assemblage shows high factor loadings in proximal to intermediate cores which is interpreted to reflect temporary high salinity conditions mean dinocyst diversity and equity increase with distance from the mid rupelian shoreline indicating increasingly stable paleoenvironmental conditions towards the center of the mainz embayment within individual cores changes in dinocyst assemblages through time are related to paleoenvironmental and paleoclimatological changes the three proximal to intermediate cores show dominance of the h tenuispinosum assemblage repeatedly alternating with high factor loadings of the t pelagica assemblage in both cases dinocyst diversity and equity tend to be reduced highest factor loadings of the s ramosus assemblage occur in intervals where neither of the above assemblages is dominant and tend to coincide with dinocyst diversity and equity maxima we interpret this distribution pattern to denote different paleoceanographic conditions reflecting drier and more humid phases in the early oligocene of central europe during relatively dry periods increased salinity conditions prevailed in proximal to intermediate settings of the mainz embayment as reflected by the dominance of the h tenuispinosum assemblage during more humid periods increased runoff led to higher nutrient availability and the formation of a pycnocline separating slightly less saline surface waters from higher saline deeper waters thus impeding vertical circulation these environmental conditions are documented in high loadings of the t pelagica assemblage which is indicative of increased eutrophication and or oxygen depleted bottom waters transitions between drier and more humid periods i e episodes of normal marine conditions are characterized by high loadings predominantly of the s ramosus assemblage as well as increased dinocyst diversity and equity values we propose that the alternations between drier and more humid phases may be related to variations in the ocean atmosphere moisture flux from the north atlantic into central europe bearing a high latitude climate signal 2002 elsevier science b v all rights reserved

a spectral analysis of geoid undulation and gravity anomaly data computed with pratt s isostasy theory applied to moho depth variations in fennoscandia summary the analysis shows that the spectra of geoid undulation n and gravity anomaly g computed from the values implied by the osu89b harmonic coefficients degrees 4 100 rapp pavlis 1990 do not fit the spectra of n and g computed from values implied by a pratt type isostatic model applied to the moho depth variations in fennoscandia as suggested by anderson 1984 and marquart 1989 the magnitudes of the model spectra are that much larger than the osu89b spectra that the relevance of the isostasy model is questionable

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

methylhopanoids molecular indicators of ancient bacteria and a petroleum correlation tool abstract methylhopanoids are organic compounds synthesized by certain bacteria that when preserved in sediments act as molecular fossils or biomarkers for organic matter inputs from specific bacterial sources two series of methylhopanoids occur each mainly deriving from a distinct bacterial source cyanobacteria 2 methyl and methanotrophic bacteria 3 methyl the abundance and composition of methylhopanoids within sediments of modern depositional environments varies widely apparently due to different bacterial communities contributing to the sedimentary organic matter comparable molecular characteristics are found in oils and their source rocks consequently methylhopanoids are valuable in oil oil and oil source rock correlations distinguishing between samples related to different depositional environments in particular abundant 3 methylhopanoids from methanotrophic bacteria or an additional unknown bacterial source are characteristic of some modern alkaline saline lake environments comparable compositional features in the methylhopanes of oils allow the assignment of lacustrine oils offshore west africa to two distinct lacustrine source rock facies and to distinguish between different marine source facies thus refining oil source rock correlation

relationship between rank and composition of aromatic hydrocarbons for coals of different origins distributions of di and tricyclic aromatics were determined for upper carboniferous to oligocene coals of various origins in addition to upper carboniferous ruhr coals wealden lower creataceous coals from north west germany lower cretaceous coals from western canada and tertiary coals from japan and colombia were studied the coals ranged from sub bituminous to anthracite in rank and showed vitrinite reflectance from 0 4 to 4 9 rm the previously established relationship between the rank of coal in terms of vitrinite reflectance and the distribution patterns of individual aromatic hydrocarbon isomers has been verified and refined it is demonstrated how the above relationships can be used to calculate vitrinite reflectance from the methylphenanthrene index mpi 1 the chemical rank levels of coals can now be defined in terms of the calculated vitrinite reflectance rc

stratigraphy and geochemical characterization of the oligocene miocene maikop series implications for the paleogeography of eastern azerbaijan abstract the oligocene miocene maikop series is a world class source rock responsible for much of the oil and gas found in the south caspian basin it is composed of up to 3 km of marine mudstone and contains a nearly continuous record of deposition during progressive tectonic closure of the basin as the arabian peninsula converged northward into eurasia historically the stratigraphy of this interval has been difficult to define due to the homogenous nature of the fine grained clay dominated strata outcrop exposures in eastern azerbaijan allow direct observation and detailed sampling of the interval yielding a more comprehensive stratigraphic context and a more advanced understanding of syndepositional conditions in the eastern paratethys sea specifically the present investigation reveals that coupling field based stratigraphic characterization with geochemical analyses e g bulk elemental geochemistry rock eval pyrolysis bulk stable isotope geochemistry yields a more robust understanding of internal variations within the maikop series samples from seven sections located within the shemakha gobustan oil province reveal consistent stratigraphic and spatial geochemical trends it is proposed that the maikop series be divided into three members based on these data along with lithostratigraphic and biostratigraphic data reported herein when comparing rupelian early oligocene and chattian late oligocene strata the rupelian age strata commonly possess higher toc values more negative 15 n tot values more positive 13 c org values and higher radioactivity relative to chattian age rocks the trace metals mo and v normalized to al are positively correlated with toc with maximum values occurring at the rupelian chattian boundary and overall higher average values in the rupelian across the oligocene miocene boundary a slight drop in v al mo al ratios is observed along with drops in s and toc these results indicate that geochemical signatures of the maikop series are regional in nature and furthermore that analogous fine grained sections may be better characterized and subdivided using similar techniques in general geochemical indicators suggest that the basin was in limited communication with the tethys sea throughout the oligocene early miocene with suboxic to anoxic conditions present during the oligocene and to a lesser extent in the miocene this increased isolation was likely due to tectonic uplift to both the south and north of the study area and greatly enhanced by global sea level fluctuations these data serve as the basis for a more detailed understanding of the tectonic evolution of the region and support a standardized chemostratigraphic division of the important petroleum source interval

the oligocene maikop series of lahich eastern azerbaijan paleoenvironment and oil source rock correlation abstract the maikop formation deposited in eastern azerbaijan during oligocene and early miocene represents a prolific source unit for the oils found within the southern gobustan area and the kura depression based on biomarker characteristics the oils are believed to have originated from a clastic source rock deposited in an anoxic to suboxic transitional marine environment with low to moderate input of terrigenous organic matter however due to the thermal immaturity of accessible maikop rocks oil source rock correlation based on biomarker fingerprints is hampered the results from a sedimentary succession sampled in the shemakha gobustan area near the village of lahich revealed the presence of marginally mature rr 0 6 0 7 maikop rocks based on lithology geochemical data and paleontological analyses the sampled section contains upper eocene and lower oligocene sediments palynological analyses of dinoflagellate cysts indicate an early rupelian to early chattian oligocene age for the upper part of the investigated maikop succession however the stratigraphic position of the middle and lower part of the succession remains unclear maceral composition as well as biomarker and stable isotope c n data indicate a major contribution of aquatic organisms and variable inputs from macrophytes and land plants to organic matter deposition during the oligocene microbial communities most probably included heterotrophic bacteria and cyanobacteria as well as green sulfur bacteria higher inputs of terrigenous organic matter occurred during deposition of the lower part of the succession increased remineralisation of organic matter in the water column is suggested to result in low toc and hydrocarbon contents as well as 15n enriched total nitrogen of the eocene to earliest oligocene sediments organic carbon accumulation during the oligocene occurred in a stratified episodically euxinic environment this is indicated by low toc s ratios and higher contents of aryl isoprenoids in several samples from the upper units a moderate to good oil source rock correlation with oils from the kura depression is indicated by sterane and triterpane fingerprints as well as carbon isotopic composition of hydrocarbon fractions

lithostratigraphy of shell 272 1 and 273 1 wells implications as to depositional history of baltimore canyon trough mid atlantic ocs geologic note upper jurassic gray and red brown shale sandstone and coal are the oldest strata penetrated by the shell 272 1 and 273 1 wells located approximately 66 mi 106 km southeast of atlantic city new jersey in the northwest part of the baltimore canyon trough the lower cretaceous section consists of gray shale that is overlain by a sequence of thick bedded sandstones interbedded with thin bedded shales the upper cretaceous section is dominated by slightly calcareous mudstone but also contains a coniacian to campanian sandstone the cenozoic section is mainly mudstone and overlying unconsolidated sand seven major lithologic units were established and correlated to the cost b 2 and b 3 wells the depositional environments of the upper jurassic rocks range from an upper delta plain facies penetrated in the two shell wells and the b 2 well to a back reef facies penetrated in the b 3 well the lower cretaceous strata in a vertical sequence consist of upper delta plain lower delta plain interdistributary bay and or marginal marine and distributary mouth bar deposits a cenomanian sandstone penetrated in the b 3 well is the youngest indicator of deltaic deposition marine strata dominate the remainder of the upper cretaceous and cenozoic

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

identification of aryl isoprenoids in source rocks and crude oils biological markers for the green sulphur bacteria abstract a series of c13 to c31 aryl isoprenoids 1 alkyl 2 3 6 trimethylbenzenes have been identified in reef hosted oils and their source rocks from the middle and upper silurian of the michigan basin and middle devonian of the alberta basin canada their structure has been confirmed by unambiguous synthesis of the c14 member of the series their structure and isotopic composition indicate that they are derived from isorenieratene from the chlorobiaceae family of sulphur bacteria these results are consistent with geological and geochemical studies that show that the source rocks were deposited under metahaline to hypersaline sulphate and sulphide rich water columns the distribution of other biomarkers in these oils and source rocks indicates that a diverse biota contributed organic matter to the source environment in conjunction with the aryl isoprenoids they show that there is a remarkable similarity in composition between the two sets of oils and source rocks despite their great temporal and geographic separation this reflects the similarity of their environments and emphasizes the importance of sedimentary facies in controlling the composition of organic matter in source rocks and their derived oils

quantification of loss of calcite pyrite and organic matter due to weathering of toarcian black shales and effects on kerogen and bitumen characteristics abstract comparison of geochemical data on posidonia shale early toarcian from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering due to this process of weathering the bulk composition of organic matter especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles are changed estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution

isostatic topography and crustal depth corrections for the fennoscandian geoid abstract the medium wavelength geoid wavelengths the result of this study demonstrate that crustal thickness variations are important for the interpretation of the fennoscandian geoid or gravity however submoho deviatoric stresses might become very high about 200 mpa if the entire anomaly is assumed to be supported by lateral density variations at less than 80 km depth this implies that the pronounced negative anomaly is additionally maintained by dynamical flow inside the mantle

deep petroleum occurrences in the lower kura depression south caspian basin azerbaijan an organic geochemical and basin modeling study abstract high sedimentation rates as much as 2500 m ma during pliocene pleistocene with a resultant undercompacted section as thick as 10 000 m and lower than normal geothermal gradients are the main characteristics which have created all the means for generation and preservation of oil at deep layers in the lower kura depression oils collected from eight different oil fields for analyses seem to have originated from a common source rock which probably is clastic deposited in relatively subanoxic to suboxic transitional marine environment receiving low to moderate input of terrestrial organic matter oils from shallow a computer aided basin modeling study has been carried out to outline the spatial variation of the oil window and thus help in further identification of possible source rocks for the reservoired oil in the lower kura depression results suggest that the potential hydrocarbon source horizons of the miocene and pliocene red bed series of the so called productive succession are even at depocenter areas immature with respect to oil generation and thus are very unlikely to have been source rocks for the reservoired oils however the oligocene lower miocene maykop rocks are marginally mature to mature depending on locality and the eocene and older rocks are mature with respect to oil generation at all representative field locations oil generation commenced at the end of pliocene and continues at present at depths between 6000 and 12 000 m an unusually deep 10 000 m oil window in the depocenter areas has been caused by the depressed isotherms due to extremely high sedimentation rates up to 3000 m ma for the last two million years the main phase of oil generation is taking place at depths greater than what most of the wells in the study are have reached

perylene as an indicator of conifer fossil wood degradation by wood degrading fungi the occurrence of perylene in middle jurassic fossil wood and miocene xylites from poland is described along with its correlation with unsubstituted polycyclic aromatic hydrocarbons pahs as well as cellulose content both middle jurassic and miocene wood remnants were of relatively low maturity ca 0 2 0 3 vitrinite reflectance rr had excellent preservation of biomarkers and biomolecules and in the case of the middle jurassic fossil wood samples generally good preservation of anatomical structures due to early diagenetic mineralisation the results from 42 middle jurassic and 8 miocene most taxonomically defined fossil wood fragments demonstrated a negative correlation between the concentration of perylene and those of generally typical conifer biomarkers e g cadalene dehydroabietane simonellite and retene in addition good correlation r2 0 81 was observed between the ratio of perylene to the above conifer biomarkers and the ratio of pahs phenanthrene and fluoranthene and pyrene to the conifer biomarkers this implies that the high concentration of perylene in fossil wood indicate its extensive degradation during decay transport and early diagenesis we defined a conifer wood degradation index as cwdi perylene perylene cadalene retene simonellite dehydroabietane cwdi perylene perylene cadalene retene simonellite dehydroabietane and observed a wide range of values 0 001 for less degraded wood to 0 95 for highly degraded samples anatomical wood preservation was associated with cwdi values in most of the samples characterised by poorly preserved anatomy high cwdi values were observed while anatomically well preserved samples generally had lower cwdi values we determined similar 13c values for perylene from the fossil wood samples 26 4 to 27 8 whereas the values for the conifer biomarkers were slightly higher and varied from 25 6 to 26 6 in contrast pyrene was depleted in 13c 27 5 to 28 2 the carbon isotope values of perylene are consistent with an origin from wood degrading fungi

depositional environment and hydrocarbon source potential of the oligocene ruslar formation kamchia depression western black sea abstract the oligocene ruslar formation is a hydrocarbon source rock in the kamchia depression located in the western black sea area depositional environment and source potential of the predominantly pelitic rocks were investigated using core and cuttings samples from four offshore wells in these wells the ruslar formation is up to 500 m thick based on lithology and well logs the ruslar formation is subdivided from base to top into units i vi dysoxic to anoxic conditions and mesohaline to euhaline salinities prevailed during deposition of the ruslar formation relatively high oxygen contents occurred during early solenovian times lower part of unit ii when brackish surface water favoured nannoplankton blooms and the deposition of bright marls solenovian event anoxic conditions with photic zone anoxia were established during late oligocene times units iii and iv and probably reflect a basin wide anoxic event in the eastern paratethys during kalmykian times organic carbon content in the ruslar formation is up to 3 autochthonous aquatic and allochthonous terrigenous biomass contribute to the organic matter relatively high amounts of aquatic organic matter occur in the lower part of the ruslar formation units i and ii and in its upper part unit vi diatoms are especially abundant in the lower part of unit vi the kerogen is of type iii and ii with hi values ranging from 50 to 400 mghc gtoc units i and ii pshekian lower solenovian are characterized by a fair to good potential to produce gas and oil but potential sources for gas and oil also occur in the upper oligocene units iv vi

stable carbon isotopes of amorphous kerogens from phanerozoic sedimentary rocks amorphous kerogens from phanerozoic sedimentary rocks may be divided into two groups on the basis of their 13c values h amorphous kerogens have isotopic values in the range of 24 to 20 per mil and l amorphous kerogens have isotopic values in the range of 35 to 26 per mil the occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups phytoplankton and to a lesser extent zooplankton and bacteria are considered to be the precursors for both kerogen types a plausible explanation for this dichotomy may be found in the source of co2 utilized by phytoplankton which is considered to be controlled by environmental factors h amorphous kerogens are expected to occur in silled basins that are overlain by expansive well circulated deep 500 m water where the prevailing source of carbon in the photic zone is atmospheric derived co2 conversely l amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow 200 m water where the prevailing source of carbon in the photic zone may be organic derived co2

structural effects of the crust on the geoid modelled using deep seismic sounding interpretations summary according to the theory of isostasy the earth has a tendency to deform its surface in order to reach an equilibrium state the land uplift phenomenon in the area of the fennoscandian shield is thought to be a process of this kind the geoid as an equipotential surface of the earth s gravity field contains information on how much the earth s surface departs from the equilibrium state in order to study the isostatic process through geoidal undulations the structural evects of the crust on the geoid have to be investigated the structure of the crust of the fennoscandian shield has been extensively explored by means of deep seismic sounding dss the data obtained from dss are used to construct a 3 d seismic velocity structure model of the area s crust the velocity model is converted to a 3 d density model using the empirical relationship that holds between seismic velocities and crustal mass densities structural evects are then estimated from the 3 d density model the structural evects computed from the crustal model show that the mass deficiency of the crust in fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid it proves again that the crust has been isostatically compensated by the upper mantle in other words an anomalously high density upper mantle must exist beneath fennoscandia

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

facies evolution and stratigraphic correlation in the early oligocene tard clay of hungary as revealed by maceral biomarker and stable isotope composition abstract hydrocarbon source rocks i e tard clay formation containing type ii organic matter were deposited in the hungarian paleogene basin during lower oligocene a major contribution of aquatic organisms green algae dinoflagellates chrysophyte algae and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n alkane distribution patterns composition of steroids and 13c of hydrocarbon biomarkers microbial communities included heterotrophic bacteria cyanobacteria chemoautotrophic bacteria as well as green sulphur bacteria the presence of methanotrophic bacteria is indicated by 13c depleted hop 17 21 ene higher inputs of terrestrial organic matter occurred during deposition of the lower and uppermost units of the tard clay formation the terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the area of the hungarian paleogene basin diterpenoid hydrocarbons derived from the resins of conifers are about 2 3 enriched in 13c compared to the angiosperm biomarkers environmental conditions changed from marine to brackish accompanied by oxygen depletion in the lower parts of the water column organic carbon accumulation during this period was a consequence of stagnant bottom water conditions in the hungarian paleogene basin due to salinity stratification this is indicated by low pristane phytane ratios varying from 0 27 to 1 44 enhanced ratios of dimethylated versus trimethylated mttcs 0 14 0 59 and the presence of aryl isoprenoids from 0 4 to 14 1 g g toc a major marine incursion is evidenced by stable isotope ratios of organic matter and carbonates in the uppermost member of the tard clay a transition from brackish towards normal marine conditions is proposed up to 3 anoxic cycles are recognized in the drill core sections separated by minima in pristane phytane ratios and maxima in the depth trends of di tri methylated mttcs and aryl isoprenoid concentrations in combination with the position of maxima of 13c of carbonate and organic matter and an abrupt decrease in perylene concentrations the cycles can be used for intra formational correlation of the tard clay