coupled re os and u pb geochronology of the tonian chuar group grand canyon well preserved strata of the late tonian chuar group exposed in the grand canyon host fossil evidence for the development of eukaryotic predation the presence of unique biomarkers and large changes in c s and mo isotope chemostratigraphy despite the importance of this critical succession few radioisotopic age constraints are available to place these records into a global context here we couple high precision u pb chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims on zircon crystals with the rhenium osmium re os sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of earth history zircons recovered from a tuff within the uppermost walcott member of the kwagunt formation yield a weighted mean 206pb 238u age of 729 0 0 9 ma mean square of weighted deviates mswd 0 86 differing significantly from the previous air abrasion upper intercept age of 742 6 ma on zircons from this same horizon organic rich carbonates from the carbon canyon member of the galeros formation yield a model 1 re os age of 757 0 6 8 ma mswd 0 47 n 8 and an initial os isotope 187os 188os osi composition of 1 13 0 02 the radiogenic osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the carbon canyon member in agreement with sedimentological and stratigraphic data the re os geochronology of marcasite fes2 nodules from the awatubi member of the kwagunt formation yield a model 1 age of 751 0 7 6 ma mswd 0 37 n 5 with an osi of 0 44 0 01 this re os date is interpreted to constrain the growth of the marcasite nodules in the awatubi member during deposition the formation of sulfides and the less radiogenic osi value are consistent with an influx of sulfate laden seawater to the basin during deposition of the kwagunt formation attempts to apply the re os geochronometer to the walcott and tanner members of the chuar group failed to yield meaningful ages despite elevated re enrichments 20 ng g the re os data from these units yielded negative osi values which suggest disturbance to the re os system the low os abundances typically 100 pg g relative to the amount expected based on the elevated re abundances suggest leaching of os due to oxidative weathering on geologically recent time scales finally the carbon canyon member provides a useful case study for quantifying how input uncertainties in the re os geochronometer propagate into the resulting age uncertainty and we discuss the protocols that will yield the best improvement in age precision for future studies the u pb and re os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods our analysis suggests that for our data the most efficient way of reducing uncertainties in the presented re os dates is through improved precision of measured os values

building the biomarker tree of life it is the great challenge of geomicrobiology to study microorganisms in the context of their environments both in earth s distant past and in the present planet earth and its biosphere have evolved together and a chronicle of earth s ecosystems and their geochemical cycles is recorded in sedimentary rocks spanning billions of years a relatively new and very powerful approach to read these subtle microbial and environmental signatures in ancient rocks is the study of molecular fossils or biomarkers within the context of the biochemistry and phylogeny of their origins biomarkers are organic compounds primarily lipids that have particular biosynthetic origins and can be preserved in sediments and sedimentary rocks the most valuable biomarkers are taxonomically specific i e they can be assigned to a defined group of organisms and are resistant to degradation reading the biomarker signatures in rocks can give information about the ancient record of anoxic conditions in the water column e g summons and powell 1986 the intensity of uv radiation penetrating lakes leavitt et al 1997 hypersalinity in evaporitic environments grice et al 1998 and the function of microbial communities at methane seeps hinrichs et al 1999 biomarkers have helped to reconstruct the first appearance of major groups of organisms e g mccaffrey et al 1994 moldowan et al 1994 moldowan and talyzina 1998 brocks et al 2005 elucidate events of global climate change e g brassell et al 1986 record major perturbations and reorganization of geochemical cycles e g logan et al 1995 kuypers et al 1999 and document catastrophic losses in biodiversity e g grice et al 2005 they are even used as tools to help in the discovery of major new petroleum reservoirs for a review see peters et al 2004 the field of biomarker research is young and many new applications wait to be

loess and life out of earth abstract loess is a homogeneous typically nonstratified porous friable slightly coherent often calcareous fine grained silty pale yellow wind blown aeolian and often by water reworked sediment that is distributed on continents world wide recent studies have highlighted the importance of biological loess crusts in trapping and transforming aeolian material into loess and also in prevention from wind and water erosion of deposited material some planets and satellites in our solar system are characterized by conditions comparable to those related to loess formation in terrestrial environments given the requirements for loess deposition on earth this study aims to ask the following questions 1 if loess like sediment formation is possible outside earth what are the mechanisms of sediment deposition and preservation 2 could the possible loess like sediments be stabilized through the activity of extraterrestrial equivalents of biological loess crusts found on earth as loess formation on earth is related to biological activity we propose a new direction in the search for extraterrestrial life evidence about extraterrestrial loess might indicate extraterrestrial life connected to loess formation on the basis of comparative analysis of some astrophysical conditions of seven celestial bodies we propose mars and titan and possibly venus and io to be assessed for evidence of loess deposition and the possible presence of related life forms and their biomarkers following recent advances in our understanding of terrestrial loess deposition we propose new indicators of extraterrestrial life research biomarkers in loess cyanobacterial and other microbial

petroleum systems of oman charge timing and risks after 35 years of exploration creaming of the conventional plays in oman is nearly complete and consequently the search has commenced for new less obvious plays many of the new opportunities occur beyond the known hydrocarbon provinces and are considered to have significant charge risks to define these risks extensive basin modeling studies have been conducted in recent years modeling and empirical data show that mesozoic and cenozoic kitchen areas are restricted to western north oman the only areas currently buried at their maximum temperature large parts of north and central oman depend on lateral migration from these kitchens for their charge progressive uplift of the east flank and basin inversion since the middle paleozoic provides favorable conditions for long distance migration in the post carboniferous interval in central oman geochemical tracer molecules benzocarbazoles suggest that a north south trending reactivated basement grain has funneled charge up to 300 km southeastward charge risks increase in the deeper sequence in which eastward migrating hydrocarbons have to traverse the ghaba salt basin a pronounced syncline at depths greater than 3 km the south oman salt basin is currently cool because of shallow depths and hydrodynamic fluid flow activity the shallow post cambrian reservoirs rely on storage of early cambrian ordovician charge by the ara salt cambrian sequence followed by release of hydrocarbons as the salt edge retreats through time basin modeling has outlined the extent of the different petroleum systems and provided us with risk maps to guide our next exploration phase it has revitalized some of the mature plays for instance the gharif formation where oil exploration is now focused along late cretaceous and tertiary migration paths deeper sections are envisaged to have significant scope for gas

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

sterane isomerisation and moretane hopane ratios in crude oils derived from tertiary source rocks the extent of sterane isomerisation reactions and the moretane hopane ratios of 234 crude oils taken world wide from a wide variety of source rocks of differing geological ages have been measured this data indicates that in 78 crude oils derived from tertiary source rocks sterane isomerisation reactions as determined by the 20s 20s 20r ration of the c29 5 h 14 h 17 h normal steranes and the c29 iso iso normal ratio iso 5 h 14 h 17 h are mainly incomplete and sometimes considerably so in addition the same crude oils have 17 h 21 h moretane 17 h 21 h hopane ratios which are significantly greater predominantly in the range 0 10 0 30 than those of crude oils derived from older mature source rocks mainly less than 0 1 this data for crude oils lends support to the hypothesis proposed by mackenzie and mckenzie 1983 for source rock extracts that the time temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in tertiary sediments is generally insufficient despite generation of crude oil at relatively high temperatures an alternative hypothesis is that the incomplete sterane isomerisation of tertiary crude oils may be due to generation of these crude oils from their deltaic land plant containing source rocks under low heating conditions a third hypothesis proposes that the tertiary crude oils may have picked up the incompletely isomerised steranes from immature sediments during migration although possible in particular instances such a mechanism does not appear to be generally applicable since in that case the phenomenon would then appear to be restricted to the tertiary the higher moretane hopane ratios of the tertiary crude oils could suggest that constraints similar to those applying in sterane isomerisation also operate in the conversion of moretane to 17 h hopane

triterpenols from sediments of santa monica basin southern california bight u s a the triterpenol geochemistry of the santa monica basin from the southern california borderland off the u s a is described from the study of two sets of trap deployments five box cores 30 cm and a hydroplastic core 1 m the biogenic sources and diagenetic stability of the triterpenols are discussed the 17 h 21 h hopanols 22r isomer occur in the carbon number range from 30 to 32 and their abundance is nearly uniform in the shallow sediment sections however the three hopanols follow the order of abundance c32 c31 c30 in deeper sections their concentrations spans from trace levels to 156 g g organic carbon 15 ng to 7 g g dry sediment tetrahymanol gammaceran 3 ol has been identified in all the samples except in one set of trap particles collected at 100 m water depth from trace level 1 g to 215 g g organic carbon 20 ng to 9 g g dry sediment diplopterol is also detected in trace amounts in some samples the triterpenols in the trap material generally increase with the water column depth and decrease with the subbottom depth in the sediment cores the extended hopanols are either degradation products of polyhydroxybacteriohopanes or are biosynthesized by bacteria tetrahymanol is probably the only suggested biological precursor of gammacerane the reduced counterpart of tetrahymanol which has been recognized in numerous crude oils and lithified sediments although it has been reported earlier from green river shale and from a residual pleistocene lake sediment tetrahymanol has so far been positively identified from recent marine sediments only in two recent studies the decreasing content of tetrahymanol in sedimentary depth profiles in the santa monica basin would favor an origin for this compound in the water column or at the sediment surface the ubiquitous occurrence of this compound throughout the study area suggests that this triterpenol most probably originates from primitive organisms protozoa bacteria hitherto not identified or more likely not yet analyzed for their lipid composition

biochemical evolutionary significance of ordovician oils and their sources abstract geochemical data from several ordovician oils and kerogens from distinct basins in canada united states estonia and australia show some unique compositional characteristics the oils analyzed by gas chromatography and gas chromatography mass spectrometry are characterized by an odd dominance in the c9 c19 molecular weight range of normal paraffins and an odd dominated homologous series of cyclohexyl substituted normal paraffins the oils contain an unusually small amount of c20 components and pristane and phytane are conspicuous by their virtual absence the compositions of the oils and kerogens correlate closely and display characteristics that have not been observed in younger sediments the predominant organism found in the source kerogens for these unique oils is gloeocapsamorpha prisca a primitive prokaryotic organism data from the analysis of gloeocapsamorpha rich kerogens and the association of this organism with ordovician source rocks indicate a genetic relationship exists between the organism and the unusual oil chemistry the chemistry of the oils and kerogens suggests that gloeocapsamorpha was either non photosynthetic or that it fixed light using a chlorophyll that did not contain a phytyl side chain e g a bacteriochlorophyll the widespread occurrence of oils and kerogens possessing such a uniform composition appears unique to the ordovician certainly to the early paleozoic this suggests that paleocological conditions of ordovician anoxic environments were very similar and g prisca was particularly well adapted to these conditions

comparative microbial diversity and redox environments of black shale and stromatolite facies in the mesoproterozoic xiamaling formation abstract the composition of microbial communities and their relationship to ocean redox structure in the precambrian are topics of continuing interest in geobiology our knowledge of organismic diversity and environmental conditions during this time are mostly based on fragmentary paleontological and geochemical records and might be skewed accordingly in north china the xiamaling formation 1 37 ga is characterized by black shales of relatively low thermal maturity t max is 445 c and has been identified as a potential petroleum source rock to date however the biological sources of the organic matter and the environmental conditions prevalent during the deposition of these sediments remain unclear in this study we analyzed the hydrocarbon biomarker compositions of the xiamaling formation shales and a superjacent stromatolitic carbonate in order to shed light on the microbial diversity in the sedimentary environments they represent the hydrocarbons extracted from both sediments are dominated by low molecular weight n alkanes with a maximum at c 15 18 suggesting that bacteria and or algae were primary biotic precursors our inability to detect steranes in bitumen i and only traces of rearranged steranes in bitumen ii of black shales indicates that modern eukaryotic algae were either ecologically insignificant or not preserved due to a taphonomic bias the high relative concentration of hopanes and diahopanes ranging from c 27 to c 35 as well as monomethylalkanes suggests that cyanobacteria may have been the dominant primary producers and could have contributed to the biologically available nitrogen pool through n 2 fixation this observation is supported by the low nitrogen isotopic composition of the kerogens even though all facies zones appear to have been anoxic but not sulfidic on the basis of biomarker ratios and trace metals subtle but distinct molecular differences are observed between the stromatolite and the black shales which can be attributed to both lithologically controlled diagenetic rearrangements and differential biotic input the discrepancy between the presence of a large ucm and high abundances of alkyl lipids on one hand yet the absence of a stable carbon isotopic offset between lipids and kerogen on the other suggests that strong heterotrophic reworking might not be the sole source of the biodegraded fingerprint that is so typical for proterozoic bitumens and demands alternative explanations

the origin distribution and hydrocarbon generation potential of organic rich facies in the nonesuch formation central north american rift system a regional study abstract precambrian unmetamorphosed organic rich clastic rocks whilst uncommon in the geologic record are of interest with respect to their potential for hydrocarbon generation the 1 1 billion year old nonesuch formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central north america previous organic geochemical investigations of the nonesuch formation have been limited to isolated samples from the basal portion of the nonesuch formation at a copper mine white pine in northern michigan in the present study 183 outcrop and core samples of the nonesuch formation from northern wisconsin and michigan upper peninsula were collected for sedimentologic and organic geochemical analyses total organic carbon toc values for the samples ranged from 0 0 to 2 5 and showed a strong correlation between organic richness and depositional environment detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies 1 and 2 which could be further distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms whereas the composition of organic petrography 1 kerogens is primarily aliphatic the composition of organic petrography 2 kerogens is dominated by aromatic and or phenolic constituents the geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source input or preservation superimposed over maturity differences is responsible for both organic petrographic and kerogen pyrolysate compositions

saturated hydrocarbon biomarkers in oils of late precambrian age from eastern siberia abstract large quantities of petroleum derived from source rocks of late precambrian early cambrian age have been discovered in the lena tunguska region of eastern siberia over the last twenty five years we have examined three oils with presumed late precambrian vendian source rocks from this region for c 15 saturated hydrocarbon biomarkers using gas chromatography gc and gas chromatography mass spectrometry gc ms our results show these oils to have many characteristics in common with other oils of similar age from eastern siberia and oman amongst the compounds detected in the three oils analyzed were 1 n alkanes which show a marine derived distribution 2 centrally branched monomethyl alkanes 3 acyclic isoprenoids pr ph 1 4 tricyclic and tetracyclic terpanes 5 hopanes 6 c 29 unrearranged steranes and 7 c 30 nuclear steranes all of these compounds with the exception of the 4 methylsteranes which on current biochemical knowledge are unexpected in oils of this age can be assigned a prokaryotic source this and other evidence such as the lack of diasteranes suggest microorganism were the major contributors to organic matter deposited as part of a carbonate source rock

distinctive hydrocarbon biomarkers from fossiliferous sediment of the late proterozoic walcott member chuar group grand canyon arizona abundant extractable hydrocarbons in association with well preserved kerogen have been recognised in a dolostone from the chuar group approx 850 ma the biomarkers present include c15 c30 acyclic isoprenoids c26 c28 steranes c27 c35 hopanes and extended tricyclic terpanes among the striking characteristics of the distributions are the dominance of steranes and methyl steranes lacking side chain alkylation and the presence of putative c29 c35 neohopanes and gammacerane the high abundance of steranes is consistent with the presumed eukaryotic affinities of several types of microfossils including chuaria present in associated mudstones this stratum is the oldest in which gammacerane has been found and suggests contributions from protozoa to the depositional environment which appears to have been hypersaline there is a strong correlation between the carbon isotopic composition of the kerogen and that of the various polarity fractions of the bitumens chemical degradation and pyrolysis of the kerogens yielded n alkanes with similar characteristics to the bitumens but there are differences in the patterns of distribution of the acyclic isoprenoid sterane and triterpane biomarkers

occurrence and origin of mono di and trimethylalkanes in modern and holocene cyanobacterial mats from abu dhabi united arab emirates n alkanes highly branched isoprenoids monomethylalkanes mmas dimethylalkanes dmas and trimethylalkanes tmas are the most abundant components in the hydrocarbon fractions of extracts of four modern and two holocene cyanobacterial mats t1500 and 5110 170 y bp collected in abu dhabi united arab emirates the homologous families of mmas dmas and tmas were identified by comparison of mass spectral and relative retention time data with published data dmas were also identified by synthesis of authentic standards 3 9 dimethyltricosane 5 9 dimethyltricosane and 11 15 dimethylheptacosane mmas dmas and tmas of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures mmas and dmas in the c16 c22 range are characterized by methyl substituents mainly located at c 6 or 6 and c 7 or 7 and are derived from cyanobacteria they are relatively abundant components in the modern cyanobacterial mats but with increasing age of the mats they become much less abundant on the contrary mmas dmas and tmas in the c24 c45 range are exclusively found in the holocene cyanobacterial mats their longest chains mainly contain an odd number of carbon atoms and they always carry the methyl substituents at odd numbered carbon atoms the similarity in composition of this very specific group of branched alkanes with that encountered in insect epicuticular waxes suggests that these sedimentary hydrocarbons originate from insects which probably grazed on the cyanobacterial mats

petroleum geology and geochemistry of middle proterozoic mcarthur basin northern australia ii assessment of source rock potential five potential source rocks have been discovered in the middle proterozoic of the mcarthur basin the lacustrine barney creek formation mcarthur group and the marine velkerri formation roper group compare favorably in thickness and potential with demonstrated petroleum source rocks in the phanerozoic the former contains up to 7 total organic carbon toc and kerogen types i and ii toc values in the latter range up to 6 5 and the kerogen is type ii petrographic examination shows the organic matter is mostly lamalginite which on maturation yields nonfluorescent lamalginite and bitumen extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred tmax values derived from rock eval analysis may be used to define maturation levels in much the same way as in phanerozoic sediments at low maturation levels the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices but converge in the mature zone this convergence reflects both the inherent lower reflectivity of hydrogen rich macerals and the source control on methylphenanthrene indices at low maturity maturation levels in mcarthur group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults hydrocarbon generation occurred prior to the deposition of the younger proterozoic sediments of the roper group in contrast the source rocks in the roper group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the roper group although an early paleozoic timing cannot be excluded

geochemical characterization of source rocks and oils from northern iraq insights from biomarker and stable carbon isotope investigations abstract the origin of oil in the cretaceous tertiary reservoirs of northern iraq remains controversial consequently a detailed geochemical study including rock eval pyrolysis gas chromatography mass spectrometry and carbon isotope analysis was conducted on 65 source rock core and cuttings samples from the najmah chia gara and balambo formations northern iraq twenty four crude oils were also analyzed to provide new insights into source organic matter depositional environment and correlations between crude oils and their probable source rocks bulk geochemical analysis of candidate source rock samples indicates that the najmah formation has more petroleum potential than the chia gara and balambo formations these three candidate source rocks were deposited in slightly different environments and have different organic matter types as determined by screening analysis and biomarker data the najmah formation contains organic rich toc up to 4 2 wt oil prone type i ii kerogen deposited under anoxic conditions the chia gara and balambo formations contain mixed type ii iii and type iii kerogens oil and gas prone deposited under less reducing conditions with more terrigenous organic matter input unfortunately most of the rock samples contain less than 2 5 wt toc which likely excludes them as source rocks but more deeply buried equivalents may contain more organic matter biomarker and stable carbon isotope compositions suggest that cretaceous and miocene reservoired oils from the northern iraq basins although predominantly derived from marine carbonate organic matter constitute five genetically distinct oil families based on age specific biomarkers the oils were probably derived from three active petroleum systems in northern iraq jurassic jurassic early cretaceous and early to mid cretaceous biomarker maturity parameters indicate that most of the oils were generated in the early oil window similar to the extracts

biogeochemical formation of calyx shaped carbonate crystal fans in the subsurface of the early triassic seafloor abstract the kuh e dena sedimentary section in iran developed on the southwestern margin of the neotethys ocean the fossil and bioturbation lean early triassic section late dienerian to early smithian is typified by finely laminated calcareous shales that are interbedded with laterally extending carbonate beds which are mostly composed of calyx shaped carbonate crystal fans the calyxes consist of fibrous crystals that apparently formed within the subseafloor in soft water saturated sediment by displacive growth rare earth element and yttrium patterns indicate that calyx shaped crystals precipitated from anoxic pore waters biomarker patterns of the carbonate beds reveal major input of lipids from prokaryotes that typically occur within layered benthic microbial mats i e cyanobacteria anoxygenic phototrophic bacteria sulphate reducing bacteria and methanogenic archaea along their length the fibrous crystals of the calyxes reveal a trend of increasing 13 c carb values from 2 7 to 3 3 suggesting that archaeal methanogenesis affected the carbonate pool apart from the putative benthic prokaryotes molecular fossils of halophilic most likely planktic archaea were detected in the carbonate beds low pristane to phytane ratios 0 8 and the presence of halophilic archaea are in accord with the lack of bioclasts and bioturbation which likely reflects increased salinities the former presence of a benthic microbial mat with cyanobacteria underlain by anoxygenic phototrophic bacteria sulphate reducing bacteria and methanogenic archaea is in accordance with studies on living microbial mats in hypersaline low oxygen marine environments the early triassic subseafloor seems to have been a sink for carbon driven by biologically induced formation of secondary diagenetic carbonate

hydrocarbon biomarkers from ordovician sediments and the fossil alga gloeocapsomorpha prisca zalessky 1917 numerous ordovician oils worldwide are known to show unusual and distinctive distributions of hydrocarbons which it has been suggested are derived from a unique benthonic mat forming non photosynthetic prokaryotic organism gloeocapsomorpha prisca zalessky 1917 which is the major contributor of organic matter organic matter rich sediments from the canning and amadeus basins of australia known to contain g prisca fossils and to have the characteristic predominance of odd carbon number c13 c19 n alkanes were investigated to determine other hydrocarbon distributions sediments from both basins contained abundant n alkylcyclohexanes with odd carbon number predominance c13 c19 and methyl n alkylcyclohexanes c14 c20 which in immature sediments showed an even carbon number predominance the isomer distribution of these latter compounds was determined by direct comparison with synthetic standards the sediments from both basins also contained very similar distributions of steranes and pentacyclic triterpanes and the derived kerogens had a characteristically light carbon isotope signature pyrolysis of a g prisca rich kerogen yielded a hydrocarbon mixture with a similar composition to the sediment extract except that there was a marked increase in the relative abundance of pristane phytane alkylcyclohexanes steranes and hopanes relative to n alkanes we argue on the basis of the geological geochemical and palynological data that g prisca was probably planktonic photosynthetic and very possibly eukaryotic and that the striking character of ordovician oils and sediments derive from bacterial and other diagenetic imprints superimposed on the primary signature of this organism

application of biological markers in the recognition of palaeohypersaline environments the hydrocarbon fractions of a marl sample from a messinian basin in the northern appenines and four oils rozel point oil utah and three seep oils from sicily were studied

biological markers in bitumens and pyrolyzates of upper cretaceous bituminous chalks from the ghareb formation israel abstract the sterane and triterpane distributions of three bituminous chalks from the upper cretaceous ghareb formation israel were investigated both in the original extractable bitumens and in extracts obtained after pyrolysis of whole rock and isolated kerogen samples at 450 c pyrolysis was performed in a closed system under hydrous whole rock and anhydrous conditions isolated kerogens the carbon number distributions of steranes and triterpanes differ significantly between original bitumen and pyrolyzates unlike the bitumens in which diasteranes were not detected the anhydrous pyrolyzates contain small amounts of diasteranes the presence of water during pyrolysis leads to an increase of sterane isomerization the abundant formation of diasteranes and an increase of the 18 h trisnorneohopane 17 h trisnorhopane ratio sterane isomerization maturation parameters show a closer match between original bitumen and pyrolyzates after pyrolysis in a closed system when compared with an open system

dinoflagellate origin for sedimentary 4α methylsteroids and 5α h stanols biological marker compounds provide useful tools for evaluating the depositional environments of recent and ancient sediments and petroleums1 largely through established relationships between the organic matter of sediments and source organisms1 2 such relationships are best tested and explored by investigating the lipid components of natural populations of autochthonous biota that can be clearly recognized as contributors to underlying bottom sediments in this context we have studied the lipids of the freshwater dinoflagellate peridinlum lomnickii woloszynska order peridiniales class dinophyceae collected from the waters of priest pot3 a eutrophic lake in the english lake district its distributions of both 4 methylsterols and 4 methylstanones closely resemble those of the underlying bottom sediments demonstrating that dinoflagellates are important contributors of these sedimentary compounds the 4 methylsteroidal hydrocarbons found in ancient sediments and petroleums4 6 are presumably diagenetic products of such 4 methylsteroids and therefore reflect dino flagellate inputs to the original depositional environments furthermore the prominence of 5 h cholestan 3 ol in p lomnickii suggests that dinoflagellates may be the long sought direct biological source of sedimentary 5 h stanols

hydrocarbon shows and petroleum source rocks in sediments as old as 1 7 109 years we report the discovery of indigenous live oil in 1 4 gyr old rocks in the mcarthur basin of northern australia previously reported occurrences of indigenous precambrian oil are 1 gyr old1 5 potential petroleum source rocks in the mcarthur basin range in age from 1 4 to 1 7 gyr and were deposited in marine and lacustrine environments in parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons they span the period corresponding to the appearance of eukaryotic organisms and because of their low degree of thermal alteration they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers the hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin these results show that exploration of previously ignored mid proterozoic sediments may lead to the discovery of new reserves of oil

organic geochemical characteristics of highly mature late neoproterozoic black shales from south china reappraisal of syngeneity and indigeneity of hydrocarbon biomarkers abstract during the late neoproterozoic to early cambrian period the transition from mainly microbial ecosystems to eukaryotic marine primary productivity was one of the most profound ecological revolutions in earth s history abundant biomarkers have been reported from high maturity late neoproterozoic sedimentary rocks in south china including the cryogenian datangpo and ediacaran doushantuo formations these biomarkers were suggested to be organic molecular evidence of the survival of photosynthetic eukaryotes in palaeo oceanic environments during the snowball earth era to advance the understanding of late neoproterozoic ecosystems and to re evaluate the provenance and validity of the biomarkers fresh drill core was collected from black shales of the datangpo and doushantuo formations in south china in order to ensure the indigeneity of the solvent extracted hydrocarbons their composition was determined by conducting experiments under strict laboratory procedures including using slice experiments on a precision saw that removed external surfaces the distribution of the recovered aliphatic and aromatic hydrocarbons indicates very a high thermal maturity for the analysed black shales which is consistent with the equivalent vitrinite reflectance 2 5 and the lack of specific biomarkers both prokaryotic bacterial and eukaryotic algal organisms are likely producers during the neoproterozoic however no long chain steranes c26 steranes were detected in the interior of any of the samples and only trace amounts of hopanes and pregnanes in just one sample from the doushantuo formation xs 199 dst on the contrary solvent extraction of millimetre thick slices cut from the external surfaces of the black shales reveals strikingly high level of contamination from organic residues on the sample surfaces these contaminants include n alkanes c10 c33 isoprenoids monomethylalkanes aromatic hydrocarbons naphthalene phenanthrene and their alkyl isomers and abundant biomarkers such as tricyclic terpanes hopanes steranes and diasteranes the concentrations of these biomarkers in the exterior sub samples exceed blank concentrations by more than three orders of magnitude due to surficial contamination these results indicate that previously studied late neoproterozoic samples from south china likely contain mixtures of abundant biomarker contaminants and some indigenous over mature hydrocarbons therefore special care need to be taken with regard the existing biomarker evidence from south china that has been used to illustrate the contribution of eukaryotic photosynthesis to marine primary productivity during the snowball earth period no late neoproterozoic rocks from south china within the appropriate thermal maturity window for survival of indigenous biomarkers are currently known but these may be found during future exploration

the biomarker composition of some crude oils from sumatra abstract crude oils from the north central and south sumatra basins have been analysed using gc and gc ms techniques and the biomarker compositions of the crude oils have been used to classify the samples into three types one oil from the north sumatra basin contained 3 methylhexadecane 2 methylhexadecane pristane and phytane in similar abundances regular steranes and 24 n propylcholestanes indicative of an algal contribution a series of 30 norhopanes and an absence of plant derived pentacyclic triterpanes these features have been interpreted as indicating a marine carbonate depositional setting two crude oils from the central sumatra basin are waxy crudes with abundant botryococcane pristane phytane and abundant 4 methyl 24 ethylcholestanes relative to dinosteranes these features suggest a brackish lacustrine depositional setting one light oil from the north sumatra basin and two crude oils from the south sumatra basin have abundant pristane compared with n c 17 and a high pristane phytane value the saturated polycyclic compounds are dominated by bicadinanes and other pentacyclic triterpanes of land plant origin the presence of abundant oleananes in these samples has been interpreted as evidence for deltaic near shore depositional setting these results show that crude oils from the sumatran region have source rocks with a range of depositional environments however the major oil fields are associated with lacustrine source rocks

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

molecular and stable carbon isotopic composition of monomethylalkanes from one oil sand sample source implications abstract unusually abundant long chain monomethylalkanes mmas c 25 c 36 and their short chain homologues c 14 c 22 have been tentatively identified in a late triassic oil sand sample from the lunnan oilfield tarim basin nw china molecular distributions and stable carbon isotopic compositions of these compounds have been determined to investigate their bio and or geosynthetic sources the results show that the most abundant isomers of short chain mmas were 4 methyl m z 70 5 methyl m z 84 6 methyl m z 98 and 7 methyl m z 112 alkanes whereas in long chain mmas there is a clear predominance of the 9 methyl and 10 methyl isomers combined with the compound specific stable carbon isotopic analysis it is concluded that short chain mmas and long chain mmas must have different sources the similar isotopic compositions of mmas and n alkanes in this sample suggest that they may share the same sources the short chain mmas as previously suggested are probably related to a cyanobacterial origin while the long chain mmas may be mainly associated with microorganisms or heterotrophic bacteria rather than cyanobacteria

carbon isotope excursions in precambrian cambrian boundary beds morocco the precambrian cambrian boundary is one of the most significant points on the geological timescale because it marks the change in the fossil record from soft bodied organisms to skeletonized faunas the cambrian explosion or radiation event1 saw the evolution of many new life forms especially various molluscs and archaeocyathids from the base of the cambrian tommotian stage others including the trilobites and bivalves evolved a little later in the succeeding atdabanian stage2 3 a massive increase in the biomass of the photic zone through phytoplankton blooms has also been postulated for this time4 particularly in the late riphean early vendian and in the early cambrian5 there has been much discussion concerning where precisely to place the boundary6 7 and here are several contenders in the world for the type section8 i present here a carbon isotope profile from the precambrian cambrian boundary beds in morocco where two distinctive positive 13c excursions are revealed these are interpreted as records of increased organic productivity as part of the precambrian cambrian explosion of life 13c excursions could be useful points for correlation between boundary sections

an isotopic biogeochemical study of neoproterozoic and early cambrian sediments from the centralian superbasin australia abstract organic matter from neoproterozoic and early cambrian sediments of the amadeus and officer basins of the centralian superbasin australia has been studied for biomarker distributions and the carbon isotopic compositions of kerogen and individual hydrocarbons these sediments represent both shallow and deep water marine facies in the older sections and marine and saline lacustrine carbonate deposits in the cambrian hydrocarbon biomarker patterns were found to be quite consistent with the known sedimentary environments and provide valuable insights into the biogeochemical changes which accompanied the transition from a microbially dominated ocean to the early stages of metazoan radiation in particular carbon isotopic data for n alkyl and isoprenoid lipids presented here and in earlier studies showed a reversal in carbon isotopic ordering between the proterozoic and phanerozoic by comparison with the 13c of kerogen n alkyl lipids from deep water proterozoic sediments were enriched in 13c and appear to be derived mainly from heterotrophs whilst open marine phanerozoic counterparts are 13c depleted and evidently derived mainly from autotrophs data from the samples studied here are consistent with a model invoking a change in the redox structure of the ocean possibly aided by the innovation of faecal pellets

post last glacial maximum evolution of a fjord type lake based on high resolution seismic data the lago roca acigami southern tierra del fuego argentina chile

organic geochemistry of impactites from the haughton impact structure devon island nunavut canada organic matter in impactites from the 24 km wide and 39 ma old haughton impact structure canadian high arctic is a mixture of fossil and modern biological components the fossil component represents a conventional oil that was generated from lower palaeozoic marine source material before impact and permeates bedrock dolomites biomarker maturity parameters record the thermal effect of the mid tertiary impact maturity influenced sterane rearranged hopanoid and triaromatic steroid ratios all increase towards the centre of the impact structure where thermal alteration was greatest the heating was probably dominated by an impact related hydrothermal system as such systems last long enough for kinetically based thermal alteration to occur kinetically related biomarker data suggest that the hydrothermal heating lasted for c 5000 years biomarkers are also preserved in dolomite clasts within impact melt breccia and indicate strong thermal alteration modern biological contamination of the rocks is responsible for the superposition of two geochemical signatures which could be cyanobacteria non marine algae or higher plant matter onto the fossil component but they can be recognized and distinguished the data show that the impact structure system holds a record of both the pre impact organic signature and the thermal signature of the impact and thereby indicates that organic geochemistry is a valuable tool in documenting the response of rocks to impacts

origin of petroleum in the neoproterozoic cambrian south oman salt basin abstract the south oman salt basin sosb is host to the world s oldest known commercial deposits most of the south oman oils have been proven to be associated with the source rocks of the neoproterozoic to cambrian huqf supergroup but the assignment of oils to specific huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire huqf sequence possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition this study was conducted to establish improved correlations between organic rich rock units and reservoir fluids in the sosb through detailed molecular and isotopic analysis of the huqf supergroup with special emphasis directed towards understanding the ara carbonate stringer play unusual biomarkers tentatively identified as a norsteranes show distinctive patterns among carbonate stringer oils and rocks different from those observed in nafun sediments and ara rocks from the athel basin these putative a norsteranes form the basis for new oil source correlations in the sosb and provide for the first time geochemical evidence of a self charging mechanism for the carbonate stringer play the paucity of markers specific to the nafun group shuram buah and masirah bay formations confounds attempts to quantify their respective contributions to huqf oil accumulations nafun inputs can only be determined on the basis of subtle differences between nafun and ara biomarker ratios the most useful geochemical characteristics delineating nafun group organic matter from ara group intra salt source rocks included low relative abundance of mid chain monomethyl alkanes x compounds low relative abundance of gammacerane 28 30 dinorhopane 25 28 30 trinorhopane and 2 methylhopanes low c 22 t c 21 t and high c 23 t c 24 t cheilanthanes ratio values based on these parameters molecular evidence for major contributions of liquid hydrocarbons from nafun group sediments shuram buah and masirah bay formations is lacking our results suggest that the majority of sosb hydrocarbon accumulations originate from within the ara group either from the carbonate stringers or from the package of sediments that comprises the thuleilat athel silicilyte and u shale formations subtle aspects of the composition of some carbonate stringer and post salt huqf oils could suggest some degree of sourcing from the nafun rocks but stronger evidence is needed to confirm this

tetrahymanol its widespread occurrence and geochemical significance abstract the occurrence of tetrahymanol gammaceran 3 ol in sediments from various marine depositional environments such as the santa monica basin and palos verdes shelf southern california bight the santa barbara basin off the northern california borderland the atlantic shelf slope and rise and the antarctic region as well as in bacterial algal mats from the santa barbara basin and baja california together with its detection in sediments from the peru upwelling region odp leg 112 and baffin bay odp leg 105 suggests that tetrahymanol occurs ubiquitously in marine samples tetrahymanol is the only known likely biological precursor of gammacerane which is found in many petroleums and shales the common occurrence of tetrahymanol in marine environments implies that primitive organisms similar to tetrahymena or organisms other than tetrahymena other protozoa bacteria are also likely to contain this compound its isomer diplopterol has also been detected in several sediment sections

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

geochemical significance of 17α h diahopane and its application in oil source correlation of yanchang formation in longdong area ordos basin china abstract relatively high abundance of 17 h c30 rearranged hopane has been detected in some oils and rock extracts from yanchang formation of longdong area southwestern ordos basin oil shale silty mudstone and carbonaceous mudstone extracts from yanchang formation all have low abundance of c30 rearranged hopane while high concentrations of c30 rearranged hopane were observed in the dark mudstone extracts analysis on the geochemical parameters of the organic matter origin thermal maturity and sedimentary organic facies were carried out the result showed that the depositional redox environment and lithology may be the critical factors for the rearrangement of hopanes which usually occurred in suboxic condition with clay catalyzed reactions as is consistent with the viewpoint of peters and moldowan 1993 according to the relative abundance of diahopane three oil classes can be identified class 1 oil with low abundance of diahopanes was originated from oil shale of chang 7 member while class 2 oil and class 3 oil with middle to high abundance of diahopanes were derived from dark mudstones of chang 4 5 to chang 9 members of yanchang formation of the 151 analyzed oil and extracts samples from different stratigraphic intervals 105 samples belong to class 1 oil coming from oil shale while 46 samples belong to class 2 and class 3 oil derived from mudstones so oil shale of chang 7 member is the main effective source rock of longdong area and dark mudstones of chang 4 5 to chang 9 member are also important source rocks

branched alkanes from ancient and modern sediments isomer discrimination by gc ms with multiple reaction monitoring abstract linked scanning of a tandem sector mass spectrometer has been used to identify abundant first field free region ffr1 unimolecular fragmentations in branched and isoprenoid hydrocarbons the most intense structure specific reactions were selected to establish multiple reaction monitoring mrm parameters for gc ms analysis this methodology has been used to study the identity of co eluting and closely eluting hydrocarbon biomarkers from modern and ancient sediments anand from extant micro organisms some sediments of cambrian and proterozoic age have been found to contain suites of monomethylalkanes with all possible isomers present and with little apparent preference for the site of branching

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

the chemical structure of a pronounced series of iso alkanes in south oman crudes gas chromatograms of the saturated hydrocarbon fractions of the south oman crudes are characterised by a striking series of peaks called x peaks throughout this paper eluting between the n alkanes mainly in the c20 c26 range a gs ms study combined with optimised gc resolution shows that each peak is a superposition of subpeaks which correspond to a complex series of monomethylalkanes the x20 peak in the chromatogram for instance is found to be composed of virtually co eluting peaks corresponding to 5 6 7 8 9 and 10 methylnonadecane the larger x22 x23 and x24 peaks are superpositions of a similar set of monomethylalkanes and in addition a series of presumably dimethylalkanes of which the structures are only partly solved the large abundance of these iso alkanes is very specific for south oman crudes and may be caused by their pre cambrian origin or a specific source material

alternative calibration of the methylphenanthrene index against vitrinite reflectance application to maturity measurements on oils and sediments abstract the methylphenanthrene index mpi 1 radke and welte advances in organic geochemistry 1981 pp 504 512 wiley chichester 1983 has been calibrated against vitrinite reflectance using australian carboniferous permian jurassic and tertiary sediments and coals containing terrestrial organic matter the correlation obtained was slightly different from that found previously and has resulted in an extension of the relationship to lower maturation levels vitrinite reflectance 0 5 ro in this sample set the methylphenanthrene distribution factor kvaldheim et al geochim cosmochim acta51 1883 1888 1987 showed a poorer correlation to vitrinite reflectance than mpi 1 hydrogen rich organic matter from an immature marine sequence gave anomalously high methylphenanthrene indices and confirms a source control on the index at low maturation levels the index has been used to confirm the low maturity of some australian terrestrial oils and to determine the degree of maturation of organic matter and oils in proterozoic sediments

gc ms characterisation of c27 and c28 triterpanes in sediments and petroleum abstract earlier studies have shown that an unusual c 27 triterpane is abundant in sediments from the norwegian continental shelf and the north sea this compound was assigned the tentative structures 24 28 30 trisnormoretane or 25 28 30 trisnormoretane but we have now shown from detailed retention time measurements and a reinterpretation of the mass spectral data that its correct structure is 17 h 18 h 21 h 25 28 30 trisnorhopane two other triterpanes 25 28 30 trisnormoretane and 28 30 bisnormoretane have also been identified as minor constituents of extracts of sediments from the north sea possible origins for these compounds are discussed

precambrian to jurassic rocks of arabian gulf and adjacent areas their facies depositional setting and hydrocarbon habitat the first sediments to onlap the metamorphosed precambrian arabian shield were infracambrian proterozoic to middle cambrian carbonates clastics and evaporites the oldest arabian reservoir rocks occur in the precambrian to lower paleozoic ara salt of the huqf group which forms the birba field of oman the middle cambrian sequence was followed by late cambrian through early permian marine sandstones and continental to littoral siltstones and variegated shales the first commercial oil discovered in the arabian gulf region occurs in fluvial sands of the ordovician to permian haima and haushi groups of the marmul field in south oman these strata are also productive in other fields and are sealed by unconformable contact with the al khlata formation or beneath shale of he albian nahr umr formation the deeply buried kerogen sediments of the huqf group to the southeast are believed to be the source rocks for these fields of south oman the late permian to triassic deposits of the arabian peninsula are mainly widespread carbonates and evaporites that were deposited during a period of relative tectonic stability their deposition on an epeiric shelf was punctuated by a series of transgressions and regressions significant gas reserves have been proven in deep wells in the arabian gulf these wells penetrate large deep structures in the permian khuff shelf carbonates these carbonates have developed secondary porosity and lie beneath interbedded shale and dolomites of the sudair or suwei formation the source of gas in the khuff is unknown but could lie in more deeply buried formations the large deep structures of the khuff are considered to be among the most attractive for gas potential in the region today the stable shelf depositional environment established during the permian and triassic continued through the mesozoic jurassic rock units of the arabian gulf formed a sedimentary megacycle of epeiric shelf carbonates these include relatively deep water mudstones and wackestones to shallow water grainstones and packstones with the greatest facies variations occurring in the post dhruma to arab d rocks some of the best and most extensive reservoirs of the area occur in the primary and secondary porosity of the upper jurassic arab formation and in the middle jurassic araej and dhruma formations of saudi arabia qatar and abu dhabi the sources for hydrocarbons in these jurassic sediments are thought to be in the dhruma and lower hanifa formation diyab equivalent of the gulf area n general the jubailah is a source in qatar and some of the offshore gulf the most common seals are the argillaceous carbonate at the top of the dhruma and the hith anhydrite the latter also marking the termination of jurassic deposition across the arabian platform the upper jurassic cycles of marine limestones and anhydrites of the arab and hith formations accumulated in response to changes of sea level caused either by regional epeirogenic movements or by eustasy

a summary of the geology and oil habitat of the eastern flank hydrocarbon province of south oman abstract the eastern flank hydrocarbon province borders the southeastern edge of the south oman salt basin in the southern part of oman an intensive exploration programme by pdo over the past seven years has led to the discovery of almost 2 10 9 m 3 of oil with current production of some 15000 m 3 d from six fields in stark contrast to other oil habitats of the arabian peninsula the main play involves rock units and geological events ranging in age from late precambrian to quaternary times while trap development and charging are closely related to syn tectonic salt dissolution the principal reservoirs and seals are poorly consolidated palaeozoic clastics which drape due to salt dissolution over residual cores principally composed of either lower palaeozoic sandstones haima group or late proterozoic carbonates huqf group or some combination of these oils have been geochemically correlated with algal source rocks of the huqf eastern flank structures are considered to have been charged from late mesozoic times onwards in conjunction with progressive subsurface salt removal and the onset of maturation of local source rocks the effectiveness of charging along the retreating salt edge is indicated by the small percentage of dry wells major oil zones are found in both anticlinal and truncation traps which are commonly developed around sandstone haima group cores oils show a wide range of characteristics but are typically heavy early expulsion crudes high sulphur with little associated gas bacterial transformation is not uncommon producing reservoirs mainly occur at relatively shallow depths 600 1200 m primary recovery factors of around 7 reflect the high density and viscosity of the eastern flank crudes and have initiated interest in enhanced oil recovery eor techniques two such pilot projects involving steam and polymer flooding are due to commence in 1984 and if successful will substantially increase the reserve base of these fields

structure and distribution of branched aliphatic alkanes with quaternary carbon atoms in cenomanian and turonian black shales of pasquia hills saskatchewan canada abstract twelve pseudohomologous series of branched alkanes with one or two quaternary carbon atoms baqcs c14 c33 range are present in the extractable organic matter of cenomanian and turonian black shales of pasquia hills saskatchewan canada five of these pseudohomologous series are also observed in a holocene sample from the russian arctic three of the pseudohomologous series were identified by kenig et al pnas 100 2003 12554 12558 as 3 3 and 5 5 diethylalkanes deas and 2 2 dimethylalkanes dmas the nine other pseudohomologous series of baqcs are tentatively identified on the basis of their characteristic mass spectral fragmentation and retention indices as 3 3 dimethylalkanes dmas 3 ethyl 3 methyl and 5 ethyl 5 methylalkanes 9 9 7 7 and 6 6 diethylalkanes deas 3 3 3 3 and 5 5 5 5 tetraethylalkanes and 5 butyl 5 ethylalkanes pseudohomologous series of branched alkanes with tertiary carbon atoms such as monomethyl and monoethylalkanes are also present in the same carbon number range all these pseudohomologous families share the particularity of including isomers with exclusively odd or exclusively even carbon numbers clearly indicating a biosynthetic origin baqcs such as 5 5 deas 3 3 deas and 3 ethyl 3 methylalkanes seem to be widespread in sediments and sedimentary rocks ranging from modern to 2 1 billion years a xenobiotic source for baqcs can be excluded at this point baqcs seem to have a very low biodegradability in part explaining their widespread occurrence in ancient sediments

biomarkers of black shales formed by microbial mats late mesoproterozoic 1 1 ga taoudeni basin mauritania a b s t r a c t hydrocarbon biomarkers in late mesoproterozoic black shales from the taoudeni basin mauritania northwestern africa were analysed for palaeoenvironmental reconstruction within the atar group touirist formation shales showed the highest content of organic carbon 20 at very low maturity rc less than 0 6 microfacies and biomarker data indicate a shallow marine high productivity low oxygen setting with benthic mats as key players for the accumulation of organic matter both high c s ratios and the occurrence of rearranged hopanes indicate that euxinic conditions were not prevalent likely reflecting the shallow depositional environment steranes were not observed indicating only a minor importance of modern eukaryotic algae although low in abundance a palynological survey however reveals the presence of simple ornamented acritarchs input of highly aromatic biopolymers typical of mesoproterozoic acritarchs or microbial exopolymeric substances may be the origin of an unusually high aromaticity observed for the biomarker extracts high amounts of hopanes suggest that the benthic mats were dominated by cyano bacteria furthermore the presence of 2 3 6 trimethyl aryl isoprenoids points at contributions of organic matter from anoxygenic phototrophic bacteria however low concentrations of these compounds argue against major photic zone anoxia in the overlying water column and rather suggest a role of anoxygenic phototrophs as part of the benthic microbial mat community the high abundances of hopanes in these samples suggest that nitrogen limited conditions may have been common in the taoudeni basin at 1 1 ga these results are consistent with ideas of widespread nitrogen deficiency that emerged in mesoproterozoic oceans due to high denitrification rates in anoxic deep waters cyano and other phototrophic bacteria as well as a limited number of acritarch species were able to cope with such conditions much better than modern algae with their higher nitrogen and trace metal demands 2011 elsevier b v all rights reserved

the origin and fate of 4 methyl steroid hydrocarbons i diagenesis of 4 methyl sterenes abstract treatment of 4 methylcholest 4 ene under mild acid conditions at low temperatures gives chemical evidence for certain features seen in the distributions of sedimentary 4 methyl steroid hydrocarbons and further indicates that many low temperature diagenetic reactions of steroids are explicable in terms of acid catalysed rearrangements specifically the results provide i indirect evidence that the 4 ene skeleton is a key intermediate in the dehydration of 4 methyl stanols in sediments ii an explanation for the distribution of 4 methyl sterenes and a nor sterenes in the lacustrine messel shale eocene iii an explanation for the presence of 4 methyl steranes in relatively immature sedimentary rocks despite the precursor stanols having the 4 methyl configuration with increasing maturity in the paris basin shales lower toarcian the less stable 4 methyl steranes decrease gradually in abundance relative to their 4 methyl counterparts at a rate fairly similar to the change in pristane stereochemistry

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

unusual distributions of long chain alkenones and tetrahymanol from the highly alkaline lake van turkey long chain c37 to c40 alkenones with di tri and tetra unsaturation are very abundant in sediment trap material and holocene to late pleistocene core samples from the earth s largest soda lake lake van turkey thus the known distribution range of these typical biomarkers for haptophyte microalgae is extended to highly alkaline environments the observed unsaturation patterns differ strikingly from those found in open marine haptophytes and sediments by an enhanced relative abundance of the tetra unsaturated compounds especially the c37 4 methyl ketone their preponderance is suggested to be a facies marker pattern for lacustrine and marginal marine areas of sedimentation using published u37k calibrations no reliable absolute temperatures were obtained for the lake van samples accordingly marine sea surface temperature determinations based on long chain alkenones should be applied with caution when a contribution of these compounds from coastal or nonmarine sources can not be excluded the presence of tetrahymanol and gammacer 3 one in the lake van materials is attributed to organic matter contributions of ciliates the relative abundance of long chain alkenones and of tetrahymanol gammacer 3 one is considered to reflect changes in the environmental conditions in particular in the hydrological setting we suggest that times of pronounced stagnation are recognised by very high tetrahymanol gammacer 3 one concentrations together with drastically increased stanol stenol ratios and intervals of enhanced convection or of high freshwater input are characterised by high alkenone contributions

monocyclic alkanes in ordovician organic matter abstract the major compounds in the c 15 branched cyclic alkane fractions of two ordovician oils pine unit and midland farms oils and an immature ordovician rock guttenberg oil rock are monocyclics one series of these compounds was identified as n alkylcyclohexanes and another tentatively identified as methyl n alkylcyclohexanes the carbon number distribution of these compounds resembled those of the n alkanes found in these samples with an odd carbon number predominance it is suggested that the monocyclic alkanes in these ordovician samples may be principally derived from the cyclisation of straight chain algal fatty acids by mechanisms that involve decarboxylation however there is evidence from the m z 97 fragmentograms of these samples to indicate that methyl n alkylcyclohexanes can also be derived from fatty acids by a less preferred mechanism that does not involve decarboxylation or from other precursors in addition a sample of kerogen from the guttenberg oil rock was hydrously pyrolysed the saturated hydrocarbon products of this experiment showed very similar distributions including monocyclics to those observed in the ordovician oils this suggests that although the oils and the guttenberg oil rock come from widely differing geographical locations their precursor algal derived organic matter was very similar

petroleum geology and geochemistry of the middle proterozoic mcarthur basin northern australia iii composition of extractable hydrocarbons abstract middle proterozoic organic rich sediments from the mcarthur basin in northern australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence are representative of the organic remains of microorganisms living at that time the major classes of hydrocarbons identified were n alkanes monomethyl branched alkanes cyclohexyl alkanes and acyclic isoprenoids there were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes low concentrations of steranes were present in most samples but like the triterpanes they were only easily detected in the least thermally altered sediments and hence were of limited use in detailed assessments of thermal maturity the presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 ma although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes