depositional environment and oil generation in ordovician source rocks from southwestern ontario canada organic geochemical and petrological approach the ordovician trenton group sherman fall and cobourg formations and the lindsay collingwood member and blue mountain formations of southwestern ontario were examined using rock eval pyrolysis gas chromatography gas chromatography mass spectrometry and incident light microscopy to evaluate their paleodepositional environments thermal maturities and source rock potential all units contain sufficient amount of oil prone type ii predominantly marine organic matter to be considered as petroleum source rocks unstructured bituminite with varying proportions of unicellular alginite are the dominant dispersed organic matter macerals the bituminite typically occurs in massive to laminated granular or patchy populations that commonly show microtextural relationships persistent inclusions of leiosphaeridia telalganite demonstrate that planktonic algal debris was a primary organic substrate for blooming microbes disseminated coccoidal gloeocapsomorpha prisca is found in minor amounts usually in association with common to abundant acritarchs zooclasts chitinozoa graptolites scolecodonts and solid bitumen also are present as maceral inclusions within the bituminite network the biomarker distributions for all of the studied units are those expected for marine organic matter deposited in a clastic dominated environment the extracts are characterized by a smooth n alkane profile with low abundance of c20 members typical for marine derived organic matter pristane phytane ratios range from 0 97 to 1 72 indicating dysoxic conditions during deposition smooth c31 c35 homohopane profiles ts tm ratios typically above 1 0 and a higher concentration of diasteranes relative to regular steranes all appear to indicate the clay bearing character of these rocks the predominance of c30 hopane over c29 regular sterane is interpreted to reflect a primary microbial input and extensive reworking of the organic matter optical reflectance fluorescence and geochemical tmax biomarker data thermal maturity parameters indicate that the trenton and blue mountain strata are within the zone of prolific oil generation throughout the whole area of study the collingwood shales are mature with respect to petroleum generation in the eastern part toronto area and only marginally mature in the northern part georgian bay area of the study area in general the biomarker composition of the extracts from all examined units is compatible with that of the oils found in the trenton reservoirs of southwestern ontario however geochemical and geological evidence suggests that the organic rich shaly laminae within the trenton group are the principal source of these oils accumulation of organic carbon in the ordovician sediments of southern ontario is suggested to derive from low energy normal marine environments grading from shallow shelf into deep shelf and open basinal settings the nutrient availability and consequently higher bioproductivity more intense consumption of oxygen and progressing anoxia controlled by a low latitude location diminished water circulation stratification of the water column and a depressed pycnocline resulted in high preservation rates the amorphous nature of kerogen reflects significant microbial interaction at the water sediment interface and within the sediments where reducing conditions must have periodically predominated

evidence for gloeocapsomorpha prisca in late devonian source rocks from southern alberta canada abstract organic rich intervals within the late devonian camrose member of southern alberta canada show optical and chemical characteristics very similar to those of ordovician samples whose organic matter is dominated by gloeocapsomorpha prisca derived alginite this includes saturate fraction gas chromatograms dominated by n alkanes with a pronounced odd carbon number up to c19 and much lower abundances of c19 n alkanes acyclic isoprenoids and biomarkers microscopy reveals these samples contain g prisca like alginite with a morphology that previously has been suggested for the growth of this organism under higher salinity conditions which is in agreement with the depositional environment of the camrose member one unusual feature of the g prisca rich samples is the very high concentration of c21 and c23n alkylnaphthalenes and n alkylbenzenes in the aromatic fractions of their extracts the origin of these compounds is not known but their occurrence could be related to the high concentrations of c21 and c23n alkyl phenols that have been noted in pyrolysates of estonian kukersites by previous workers other camrose member extracts show very different distributions that are more typical for carbonates deposited under anoxic higher salinity conditions many oils that occur in nisku formation reservoirs in the enchant hays field in southern alberta show saturate fraction gas chromatograms and other evidence that indicate an important contribution to these oils from the g prisca rich intervals these are thought to have a contribution from camrose member source rocks hence caution should be used in only attributing g prisca geochemical characteristics to cambro ordovician oils as on rare occasions they can also occur in younger source rocks and their derived oils

the upper hance coal bed in southeastern kentucky palynologic geochemical and petrographic evidence for environmental succession the upper hance coal bed in bell county eastern kentucky exhibits considerable variation in depositional setting on a mine scale palynology and geochemistry of the coal are examined in order to supplement previous petrographic examinations the various depositional settings of the upper hance mire progress from the pioneer possibly partially allochthonous settings at the mire margins and base to the well developed planar to domed main body of the mire and finally to the terminal events of flooding clastic influx and temporary reestablishment of mires now seen as the thin rider coals palynologic diversity generally decreases upwards from the basal lithotype through the main body of the coal bed the cannel and rider lithologies represent distinct depositional events

geochemical characterization of solid bitumen in the chesterian mississippian sandstone reservoir of the hitch field southwest kansas solid bitumen was identified within the mississippian chester sandstone reservoir in the hitch field southwest kansas the adjacent etzold field has similar reservoir properties but lacks solid bitumen although the hitch and etzold fields were thought to be in pressure communication and have common source rocks hitch and etzold crude oils and core extracts were characterized geochemically to gain a better understanding of the reservoir filling history and geological and geochemical controls on solid bitumen formation in the hitch reservoir based on a variety of geochemical characteristics we propose that the hitch and etzold oils are mixtures derived from ordovician and late devonian early mississippian woodford shale source rocks the solid bitumen in the hitch reservoir probably results from mixing of oils having different geochemical compositions which filled the reservoir over an extended period no evidence of severe biodegradation or thermal alteration is observed a reservoir filling scenario is proposed in an effort to explain why the hitch oils are geochemically more heterogeneous than etzold oils furthermore gas de asphalting and regional pressure drops as a result of post laramide orogeny may have contributed to a phase change in the reservoir fluid and deposition of solid materials by disturbance of thermodynamic equilibrium the major significance of this study is related to improving our knowledge on the occurrence of the presence of solid bitumen in reservoirs that can introduce barriers into the reservoir complicating waterflood operations and leading to erroneous oil in place calculations and subsequently lower than expected oil recovery

barriers and bridges early human dispersals in equatorial se asia past environments of equatorial se asia must have played a critical role in determining the timing and trajectory of early human dispersal into and through the region however very few reliable terrestrial records are available with which to contextualize human dispersal events this circumstance coupled with a sparse archaeological record and the likelihood that much of the archaeological record is now submerged means we have an incomplete understanding of the role that geography climate and environment played in shaping human pre history in this region from a review of the literature we conclude that there must have been a substantial environmental barrier resulting in a genetic separation between east and west sundaland that persisted even though a terrestrial connection was present for most of the pleistocene this barrier is likely to be a north south corridor of open non forest vegetation and its existence may have encouraged the rapid dispersal of early humans through the interior of sundaland and on to sahul we conclude that more reliable terrestrial palaeoenvironmental records are required to better understand the links between past environments and dispersal events we highlight avenues of particular research value such as focusing on eastern sumatra western southern borneo and the islands in the java sea where the purported savanna corridor most probably existed and including edaphic factors in palaeovegetation modelling

discrimination of bitumen sources in precambrian and lower palaeozoic rocks southern u k by gas chromatography mass spectrometry abstract two distinct sources for bitumen shows in lower palaeozoic and precambrian rocks in the welsh borderland can be successfully discriminated using gas chromatography mass spectrometry bitumens are widespread in the region in rocks from late precambrian longmyndian to silurian age previous geological studies have suggested that some bitumens originated from pre devonian source rocks while others migrated downwards laterally from carboniferous rocks this possibility has now been confirmed by organic geochemical studies which have also clarified the origin of some bitumens whose affinities had not been clear those bitumens concluded to have migrated from the carboniferous have also been correlated with bitumens within carboniferous rocks further east they are characterized by relatively high levels of hopanes c 27 c 29 to c 35 max c 29 or c 30 the bitumens with an assumed pre devonian source contain n alkylcyclohexanes range c 15 to c 32 max c 18 or c 19 which are absent in the other bitumens and they can be matched with a longmyndian black shale although detailed correlations are limited by the very high maturity of the samples the pre devonian bitumens also show several affinities with the distinctive ordovician oils

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

comparison of michigan basin crude oils abstract michigan basin oils from the ordovician trenton silurian niagaran and devonian dundee formations have been geochemically compared by gc gc ms and carbon isotope mass spectrometry one oil from each formation was selected for detailed analysis which included measurement of individual n alkane 13 c values the ordovician and devonian oils are strikingly similar to one another yet clearly different from the silurian oil this pattern is unexpected because ordovician and devonian reservoirs are physically separated by the silurian strata from time temperature considerations the devonian oil probably was formed in older strata and has migrated to its present location our analyses suggest a common source for the devonian and ordovician oils

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

pristane phytane and lower molecular weight isoprenoid distributions in oils phytane in oils is assumed to be mainly a diagenetic product of the phytol side chain of the ubiquitous plant pigment chlorophyll pristane may be derived from several sources in addition to phytol isoprenoids containing fewer than 19 carbons are rare in the biomass and consequently are derivatives of pristane phytane or higher molecular weight compounds the interdependence of phytane and isoprenoids containing less than 20 carbons is considered in this study the relationships of pristane to phytane and pristane and phytane to lower molecular weight isoprenoids are linear the lines of best fit commonly possess a positive intercept on the dependent variable axes pristane and lower molecular weight isoprenoids the positive y axis intercept suggests that a fraction of pristane and a fraction of lower molecular weight isoprenoids cannot be attributed to chlorophyll phytol diagenesis this observation supports the conclusion that some amount of the nonphytane isoprenoid assemblage must be attributed to a higher molecular weight parent compound s whose identity can only be suggested isoprenoid ratios and distributions have been useful in geochemical correlations oil oil however the correlation value of these hydrocarbons should be viewed critically because of the partial interdependence of phytane and lower molecular weight isoprenoids

identification of a novel c33 n alkylcyclohexane biomarker in permian triassic sediments abstract a c 33 n alkylcyclohexane n heptacosylcyclohexane has been used as a biomarker for oil correlation studies in the perth basin basal triassic western australia for almost two decades but the compound has never been formally identified we now confirm the identification using two perth basin source rock samples by comparison of mass spectra and gas chromatographic retention behaviour on three gc stationary phases with an authentic reference compound recently the c 33 n alkylcyclohexane has been reported to occur in several samples from a key permian triassic section of eastern greenland

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

basin evolution in western newfoundland new insights from hydrocarbon exploration the humber zone is the most external zone of the appalachian orogen in western newfoundland it records multiphase deformation of the cambrian ordovician passive margin and of the ordovician to devonian foreland basins by the taconian salinian and acadian orogenic events the recent phase of exploration drilling has provided new evidence for structural stratigraphic reservoir and source rock maturation models of western newfoundland the first well port au port 1 supported the hypothesis that the round head thrust had an earlier extensional history prior to the acadian compressional inversion that created the present day structural high of the port au port peninsula the well tested a small anticline formed in a footwall shortcut fault of the round head thrust the second well long point m 16 was drilled at the northern tip of long point to test a triangle zone identified by previous workers this well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the taconian allochthon and compressional basement involved faults that have uplifted the cambrian ordovician carbonate platform the structural model developed in the port au port area with the aid of these wells has been extended throughout the humber zone in western newfoundland changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region the reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a middle ordovician hiatus the st george unconformity structural relief became more pronounced as extensional collapse continued through the middle ordovician begin page 394 these structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the devonian fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the st george group and the table point formation the reservoir model maturity and source rock data and the structural models have been combined with seismic and onshore surface geology this enables the prospectivity of the western newfoundland cambrian ordovician play trend to be evaluated for further exploration

source rock evaluation by pyrolysis gas chromatography a pyrolysis gas chromatography system has been developed for the rapid evaluation of potential source rocks for the determination of organic richness and maturation this system uses the pyrolysis methods previously described however the determination of hydrocarbon production type and the identification of contamination by migrated hydrocarbons or drilling additives are accomplished by gas chromatographic gc analysis of the thermal extracts and pyrolysis products of rock samples or isolated kerogens the production type is recognized either qualitatively by gc fingerprint traces or quantitatively by hydrocarbon composition c1 to c6 c6 to c11 c11 from the kerogen peak ii pyrolysate oil prone erogens are recognized by gc traces with a full spectrum of c1 to c28 hydrocarbons or by high concentrations of c11 compounds in contrast gas prone kerogens are characterized by the predominance of light hydrocarbons from c1 to c4 and higher contributions of aromatic compounds mixed type production is intermediate in character between the two contaminants are identified from the gc analysis of the thermally extractable material in peak i possible mineral organic matter reactions during sample heating make interpreting data from whole rock samples more difficult

geochemical significance of seep oil from the subandino norte northwest bolivia abstract this report presents an interpretation of the geochemical significance of a surface seep oil sample from the subandino norte of bolivia the oil from this seep is unusual because it contains a limited range of low molecular weight normal paraffins normal paraffins containing more than 12 carbons are absent from the oil while normal paraffins having fewer than 12 carbons are conspicuous components of the oil the compositional peculiarities of the oil are attributed to transport of hydrocarbons in water with water playing an active role in the final composition of the seep the composition of the seep oil is qualitatively consistent with that anticipated if the hydrocarbons were moved in molecular solution in water

mixing of thermogenic natural gases in northern appalachian basin stable isotopic studies were undertaken to investigate the origin of 20 oil associated and non associated gases from western and central new york state in the northern appalachian basin gas wetness c2 c5 c1 c5 methane dgr13c and methane dgrd values range from 0 8 to 26 9 47 9 to 30 7 pmil and 305 to 143 pmil respectively corresponding ethane and propane dgr13c values range from 40 9 to 32 0 pmil and 34 9 to 27 9 pmil these data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature equivalent vitrinite reflectance ro approximately 0 5 to post mature equivalent ro 2 0 inferred gas maturities generally in rease with increasing reservoir age from late to middle devonian marcellus and overlying rocks middle devonian to latest silurian oriskany onondaga akron silurian lockport herkimer medina to ordovician queenston and black river oil and gas in many late to middle devonian reservoirs may have migrated from interbedded organic rich shales in other reservoirs particularly those of ordovician and silurian age unusual isotopic reversals methane dgr13c ethane dgr13c suggest that gas generated locally from equivalent age rocks within the oil window mixed with post mature gas migrated from deeply buried sources to the south and southeast the post mature gas was probably sourced from middle to late ordovician shales regional migration of middle ordovician gas may have occurred along the middle ordovician knox unconformity gas was presumably channeled into overlying silurian and devonian reservoirs along faults and fractures

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

biomarker and isotopic signatures of an early cambrian lagerstatte in the stansbury basin south australia abstract the lower cambrian series 2 emu bay shale biota found at big gully on the north coast of kangaroo island south australia is by far the richest burgess shale type bst fauna in the southern hemisphere while the trace element geochemistry of the host shale shows it accumulated beneath an oxic water column this appears difficult to reconcile with the exceptional preservation of organs such as gut glands muscles and eyes in some of its fauna micro scale sealed vessel mssv pyrolysis of kerogen isolated from a solvent extracted sample of fossiliferous emu bay shale provided independent confirmation of its redox status and also implicated cyanobacteria in the preservation of its fauna thermal extraction and pyrolysis each yielded alkanes displaying a low ratio of pristane to phytane pr ph 1 and n alkanes with a marked odd even predominance in the 20 range the latter molecular signature is diagnostic of gloeocapsomorpha prisca and is the first indication that mats of this or a similar coccoid cyanobacterium were involved in the taphonomy of a bst deposit hopanes including 2 methylhopanes are far more abundant than steranes and the shale s bulk organic matter has 13 c values in the range 32 to 28 providing further evidence of the cyanobacterial affinity of its microbial biomass thus mat forming cyanophytes rather than sulfur oxidising bacteria may have played a key role in maintaining a sharp redox boundary at the sediment water interface during deposition of the emu bay shale lagerstatte thereby enabling the survival of soft body parts within its fossil fauna for long enough to permit inorganic replication of their fine detail during late diagenesis

thermal subsidence and generation of hydrocarbons in michigan basin temperature histories for selected stratigraphic horizons in the michigan basin are computed from three dimensional continually filled subsidence models mechanical evolution of the michigan basin is modeled as flexure of the lithosphere caused by thermal contraction results are compatible with the subsidence record of the sediments and free air gravity anomalies paleotemperature is determined from excess temperature due to the thermal anomaly plus burial temperature predicted from subsidence curves for an equilibrium temperature gradient of 22 c km 1 2 f 100 ft surface temperature of 20 c 68 f and an equilibrium surface heat flow of 1 1 hfu excess temperature paleotemperature and surface heat flow do not exceed 15 c 27 f 110 76 c 230 f and 2 5 hfu respectively these estimates are consistent with upper limits set by paleomagnetic studies the low value for excess temperature is caused by concentration of the thermal anomaly below 15 km 9 mi in agreement with gravity results the great depth of the thermal anomaly can explain the lack of evidence for an initial heating event prior to subsidence once the thermal history of the sediments is specified the oil potential of the basin can be determined from laboratory derived kinetic equations for degradation of kerogen to petroleum for the michigan basin predicted temperature conditions are sufficient for source rock 25 conversion of type i and type ii kerogen only in ordovician and older rocks in the southern peninsula of michigan by this model petroleum found in rocks younger than ordovician would have had to migrate upward from the older rocks geochemical studies of dundee devonian and trenton ordovician crude oils in michigan are compatible with this interpretation niagaran silurian crude oils appear to come from a different source than the dundee and trenton oils if their source is silurian rocks on the flanks of the basin either the scheme used to calculate maturity is not applicable to these oils or the temperature of the source rocks was significantly higher in the past than today more likely the model underestimates the depth of burial of silurian source rocks near the center of the basin especially in local regions of downfaulting

an overview of pre devonian petroleum systems unique characteristics and elevated risks abstract the radiation of land plants at the onset of the devonian resulted in a change in the character of available biomass which in turn introduced changes to the nature of petroleum systems this study examines the nature of pre devonian petroleum systems prior to this significant evolutionary event conventional hydrocarbon resources have been associated with pre devonian petroleum systems across the globe including the neoproterozoic infracambrian huqf supergroup of the oman basin the riphean vendian sequences of east siberia the cambrian of the murzuq basin the ordovician of the tarim basin and the silurian qusaiba of saudi arabia although individual accumulations can be quite significant e g hassi messaoud has ultimate recoverable reserves of 10 billion barrels the relative importance of pre devonian derived oils is thought to be limited compared to the global conventional resource base with growing interest in unconventional resources the relative importance of these systems is expected to increase as resource plays such as the ordovician utica shale of the appalachian basin the cambrian of china and possibly the silurian of central and eastern europe develop the advanced ages of these systems result in some unique properties and amplify risks that exist in younger petroleum systems differences in the nature of the biomass contributing to pre devonian source rocks may give rise to oils that display unique geochemical characteristics for example there are some precambrian oils where c 29 steranes dominate even though land plants were absent there are also ordovician source rocks dominated by gloeocapsamorpha prisca a primitive prokaryote which yield oils containing limited amounts of c 20 components and nearly lack pristane and phytane the potential for unconventional reservoirs in pre devonian systems may also be highly dependent upon the age of the system as biological evolution influences the availability and nature of biogenic silica an important factor controlling brittleness and fracability literature has shown that biogenic silica from different sources displays varying degrees of resistance to diagenesis which allows the formation of a silica network and influences brittleness for example radiolaria present during the cambrian are more resistant to diagenesis than diatoms which did not evolve until the jurassic risks associated with preservation of hydrocarbons are also amplified in pre devonian petroleum systems many such systems have been exposed to significant thermal stresses resulting in the cracking of oil and wet gas others have had complex tectonic histories potentially resulting in the breaching of seals or changes in pvt conditions that can result in gas loss gas loss may also occur through diffusion from these older reservoirs as a result of long residence times

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

geochemical indicators of depositional environment and source rock potential for the upper ordovician maquoketa group illinois basin two depositional cycles are recognized within the upper ordovician maquoketa group of the illinois basin in a core from the new jersey zinc 1 parrish well of fulton county illinois organic carbon corg total sulfur 13c content of the organic carbon dgr13corg hydrogen and oxygen indices hi and oi from rock eval pyrolysis and yields of extractable organic matter eom vary through the cycles dark brown to black laminated shales are present in the lower portion of each cycle and have high values of corg 1 0 3 0 hi 500 1000 mg hydrocarbon hc g total organic carbon toc and eom 500 2500 ppm and more negative dgr13corg values dgr13corg 30 to 30 5 mil gray to greenish gray bioturbated shales are present in the upper portion of each cycle and have low values of corg 1 0 hi 500 mg hc g toc and eom 500 ppm and more positive dgr13corg values 28 5 to 29 5 pmil compared to the laminated shales although thermally immature to marginally mature in this core the laminated shales represent potential source rocks for petroleum because they contain good to excellent quantities of oil prone organic matter and are more deeply buried in other areas of the basin kerogen elemental data and rock eval data suggest that the source of organic matter in the maquoketa was uniform with the notable exception of graptolite rich layers distributions of saturated hydrocarbons for maquoketa samples resemble those derived from amorphous organic matter the organic walled microfossil gloeocapsomorpha prisca was not identified in kerogens of the maquoketa shale and its absence is most likely due to an unfavorable depositional environment and or climate variations in bulk geochemical data and carbon isotopic compositions within the maquoketa group indicate substantial reworking and degradation of organic matter associated with bioturbation and oxygenated depositional conditions organic rich shale was deposited in an environment where circulation of the water column was sluggish and dissolved oxygen concentrations were low resulting in the preservation of organic matter organic poor shale was deposited in an environment where circulation of the water column was strong dissolved oxygen concentrations were high and burrowing organisms thoroughly reworked the sediment resulting in the degradation of organic matter a maximum difference in carbon isotopic ratios of 2 pmil between organic rich and organic poor units of the maquoketa roup suggests that heterotrophic reworking of organic matter was important in the depositional environment the carbon isotopic composition and h c ratio of kerogen for graptolite rich shale suggests that organic productivity may have played an intermittent role in the accumulation of organic matter in the maquoketa shale

black shale source rocks and oil generation in the cambrian and ordovician of the central appalachian basin usa nearly 600 million bbl of oil mmbo and 1 to 1 5 trillion ft3 tcf of gas have been produced from cambrian and ordovician reservoirs carbonate and sandstone in the ohio part of the appalachian basin and on adjoining arches in ohio indiana and ontario canada most of the oil and gas is concentrated in the giant lima indiana field on the findlay and kankakee arches and in small fields distributed along the knox unconformity based on new geochemical analyses of oils potential source rocks bitumen extracts and previously published geochemical data we conclude that the oils in both groups of fields originated from middle and upper ordovician black shale utica and antes shales in the appalachian basin moreover we suggest that approximately 300 mmbo and many trillions of cubic feet of gas in the lower silurian clinton sands of eastern ohio originated in these same source rocks oils from the cambrian and ordovician reservoirs have similar saturated hydrocarbon compositions biomarker distributions and carbon isotope signatures regional variations in the oils are attributed to differences in thermal maturation rather than to differences in source total organic carbon content genetic potential regional extent and bitumen extract geochemistry identify the black shale of the utica and antes shales as the most plausible source of the oils other cambrian and ordovician shale and carbonate units such as the wells creek formation which rests on the knox unconformity and the rome formation and conasauga group in the rome trough are considered to be only local petroleum sources tmax cai and pyrolysis yields from drill hole cuttings and core indicate that the utica shale in eastern and central ohio is mature with respect to oil generation burial thermal and hydrocarbon generation history models suggest that much of the oil was generated from the utica antes source in the late paleozoic during the alleghanian orogeny a pervasive fracture network controlled by basement tectonics aided in the distribution of oil from the source to the trap this fracture network permitted oil to move laterally and stratigraphically downsection through eastward dipping impermeable carbonate sequences to carrier zones such as the middle ordovician knox unconformity and to reservoirs such as porous dolomite in the middle ordovician trenton limestone in the lima indiana field some of the oil and gas from the utica antes source escaped vertically through a partially fractured leaky upper ordovician shale seal into widespread lower silurian sandstone reservoirs

light hydrocarbon gasoline range parameter refinement of biomarker based oil oil correlation studies an example from williston basin abstract we evaluated geochemical compositions of 189 crude oils produced from paleozoic reservoirs across the williston basin emphasis is placed on compositional variations in the gasoline range i c 5 h 12 n c 8 h 18 to verify the biomarker based classification of oil families the oils belong to four distinct compositional oil families a b c and d broadly confined to specific stratigraphic intervals the unique character of each oil family evident from their n alkane and biomarker signatures is supported by distinctive gasoline range characteristics in general and c 7 mango parameters in particular an invariance in the k1 parameter among oils from a single compositional group is observed for most of the oils the k1 ratio although relatively constant within each suite of oils is different for each oil family clearly indicating their compositional distinction other mango parameters n2 p2 p3 show a similar reflection of the oil families however while c 7 parameters provide excellent evidence for distinct familial association of oils from families a b and d family c often overlaps with the latter two families perhaps indicating greater genetic and source heterogeneity in the family c oils nevertheless differences in the gasoline range composition suggest that the existing biomarker based classification of oil families can be more universally applied throughout the entire williston basin moreover because the light hydrocarbon parameters prove very useful in refining oil oil correlations routine gasoline range analysis shows good potential as a supplementary component in geochemical correlation of crude oils especially when high levels of thermal maturity decrease the usefulness of biomarker compounds

comment on comparison of michigan basin crude oils by vogler et al abstract data is presented for michigan basin oils that indicate in addition to the three distinct oil types oils of mixed composition intermediate between the silurian and ordovician types also exist these mixed oils which occur in devonian detroit river reservoirs are identified primarily by their isoprenoid and normal paraffin compositions

the ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks abstract the ratio of dibenzothiophene to phenanthrene and the ratio of pristane to phytane when coupled together provide a novel and convenient way to infer crude oil source rock depositional environments and lithologies such knowledge can significantly assist in identifying the source formation s in a basin thereby providing valuable guidance for further exploration the ability to infer this information from analysis of a crude oil is especially valuable as frequently the earliest samples in a new area may be shows and or drill stem test samples from exploratory wells which are characteristically drilled on structural highs stratigraphically remote from the source formation s a cross plot of dibenzothiophene phenanthrene versus the pristane phytane ratios measured on seventy five crude oils from forty one known source rocks ranging in age from ordovician to miocene consistently classified the oils into the following environment lithology groups marine carbonate marine carbonate mixed and lacustrine sulfate rich lacustrine sulfate poor marine and lacustrine shale and fluvial deltaic carbonaceous shale and coal the dibenzothiophene phenanthrene ratio alone is an excellent indicator of source rock lithology with carbonates having ratios 1 and shales having ratios the dibenzothiophene to phenanthrene and the pristane to phytane ratios can also be used to classify source rock paleodepositional environments the classification scheme is based on the premise that these ratios reflect the different eh ph regimes resulting from the significant microbiological and chemical processes occurring during deposition and early diagenesis of sediments the dibenzothiophene phenanthrene ratio assesses the availability of reduced sulfur for incorporation into organic matter and the pristane phytane ratio assesses the redox conditions within the depositional environment interpretation of these ratios has been aided by quantitative biomarker analysis and by carbon isotope data for pristane and phytane obtained by gas chromatography isotope ratio mass spectrometry

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

molecular indicators of redox and marine photoautotroph composition in the late middle ordovician of iowa u s a abstract saturated and aromatic hydrocarbons were used to evaluate depositional redox conditions and marine photoautotroph contributions to middle caradocian strata of the central united states ia at the base of the spechts ferry member of the decorah formation 13c enriched aryl isoprenoids derivatives of green sulfur bacteria become abundant indicating the development of photic zone anoxia this is coincident with the disappearance of the organic walled microfossil gloeocapsomorpha prisca and a marked decrease in the relative abundances of cyanobacterial biomarkers the development of dysoxic to anoxic conditions and or associated changes in basin circulation potentially affected the distributions and abundances of these organisms in the overlying guttenberg member g prisca derived organic matter becomes dominant but relative cyanobacteria abundances remain low in addition the percentage total organic carbon is greater than 20 even though selected biomarker ratios pristane phytane ratios greater than 3 and homohopane indices less than 0 5 and the presence of bioturbation indicate that bottom waters were oxygenated it is suggested that deposition of g prisca affected both organic matter preservation and depositional redox conditions observed variations in redox indicators and marine photoautotroph contributions are associated with changes in siliciclastic deposition reported macrofauna turnover and with evidence for oceanic cooling and a change in circulation patterns documented in the eastern united states

geochemistry and possible origin of crude oils from bashituo oil field tarim basin the entire range of hydrocarbons in bashituo oils from devonian and carboniferous reservoirs were investigated to determine their origin as well as their alteration mixing and maturity using gas chromatography gas chromatography mass spectrometry and stable carbon isotope analysis these geochemical studies indicate that the crude oils in the devonian and carboniferous reservoirs share similar c15 molecular marker compositions of marine organic matter origin whereas their light hydrocarbon compositions show distinct differences the carboniferous reservoired oils are characterized by a relatively greater abundance of cycloalkanes and methylcyclohexane which indicate a terrestrial organic matter om contribution based on biomarker and carbon isotope analysis cambrian ordovician j o marine rocks are assumed to be the main source for both the devonian and carboniferous reservoired oils whereas carboniferous rocks with terrestrial om input also contributed to the carboniferous reservoired oils the coexistence of 25 norhopanes evident humps from unresolved complex mixture and intact n alkanes in devonian reservoired oils indicate a mixture of early charged biodegraded oils with late fresh oils corresponding to at least two oil generation episodes by the j o rocks light hydrocarbon indicators suggest a relatively high maturity beyond peak oil generation for the j o sourced late fresh oil whereas c15 molecular marker parameters indicate a maturity equivalent to early peak oil generation for the j o sourced early charged biodegraded oil the maturity of the carboniferous sourced oil is equivalent to peak oil generation the application of the entire range of hydrocarbons is essential when assessing a mixed or altered oil system because light hydrocarbons and biomarkers may yield different source oil correlations and maturities

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