mineralogic and isotopic constraints on the origin of strontium rich cap rock tatum dome mississippi u s a abstract the limestone portion of the salt dome cap rock at tatum dome mississippi is composed of an upper massive zone and a lower banded zone the upper zone consists of equigranular fine crystalline calcite with veinlets and disseminations of carbonaceous matter associated with minor amounts of detrital and authigenic quartz sulfides and sr minerals the lower zone is composed of alternating light and dark calcite bands the dark bands are composed of fine grained and peloidal calcite quartz bitumen and disseminated sulfide minerals the lighter bands consist of variable proportions of generally coarse crystalline euhedral calcite celestite and strontianite resulting in sr contents of up to 30 locally solubility data for celestite strontianite calcite and anhydrite suggest that a decrease in temperature favors the replacement of ca minerals by sr minerals which is consistent with the observed mineral textures and paragenesis however the source of cap rock sr is difficult to determine anhydrite at tatum dome contains 800 ppm sr but the abundance of sr minerals in the limestone cap rock and the 87 sr 86 sr ratios of limestone cap rock minerals require a sr source other than local anhydrite sr released by the replacement of anhydrite by calcite is capable of producing a molar ratio of celestite to calcite of only 0 001 yet locally this ratio is 3 a likely source of additional sr is oilfield brines such as those in central mississippi that contain up to 3000 ppm sr along with significant pb and zn episodic introduction of brines into the cap rock in combination with the action of sulfate reducing bacteria probably caused the sequential production of the dark bands the lighter bands probably reflect introduction of a later sr enriched fluid or evolution of the original fluid in combination with changes in the chemical parameters controlling mineral precipitation calcite was replaced by sr minerals during this later sr rich event

late ordovician turinian chatfieldian carbon isotope excursions and their stratigraphic and paleoceanographic significance five positive carbon isotope excursions are reported from platteville decorah strata in the upper mississippi valley all occur in subtidal carbonate strata and are recognized in the mifflin grand detour quimbys mill spechts ferry and guttenberg intervals the positive carbon isotope excursions are developed in a platteville decorah succession in which background 13c values increase upward from about 2 at the base to about 0 vienna pee dee belemnite vpdb at the top a regional north south 13c gradient with lighter values to the north and heavier values to the south is also noted peak excursion 13c values of up to 2 75 are reported from the quimbys mill excursion and up to 2 6 from the guttenberg excursion although there are considerable local changes in the magnitudes of these events the quimbys mill spechts ferry and guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities and completely starve out in deeper more offshore areas closely spaced chemostratigraphic profiles of these sculpted pyrite impregnated hardground surfaces show that they are associated with very abrupt centimeter scale negative 13c shifts of up to several per mil possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events

multiple fluid components of salt diapirs and salt dome cap rocks gulf coast u s a abstract salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions reported 87sr 86sr ratios of these salt hosted and cap rock anhydrites in the gulf coast u s a indicate their derivation from middle jurassic seawater however a much wider range of 87sr 86sr ratios incorporating a highly radiogenic component in addition to the middle jurassic component has been found in several gulf coast salt domes this wide range of 87sr 86sr ratios of anhydrite within the salt stocks records sr contributions from both marine water and formation water that has equilibrated with siliciclastics during cap rock formation this anhydrite either recrystallized in the presence of or was cemented by a low sr fluid with a late cretaceous seawter type sr isotope ratio or simply lost sr during recrystallization later the cap rock was invaded by warm saline brines with high sr isotope ratios from which barite and metal sulfides were precipitated subsequently low salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth the progenitor of these salt diapirs the louann formation is generally thought to have formed from marine water evaporated to halite and rarely higher evaporite facies salt domes in the east texas north louisiana and mississippi salt basins have 87sr 86sr and 34s values that corroborate a mid jurassic age for the mother salt however salt domes in the houston and rio grande embayments of the gulf coast basin have 87sr 86sr ration ranging to values higher than both middle jurassic seawater and all rb free marine phanerozoic rocks these anomalous 87sr 86sr ratios are probably derived from radiogenic sr bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to or during diapirism potential sources of the radiogenic 87sr component include clay and or feldspar located either in older units beneath the louann formation or younger units flanking the salt diapirs and k salts within the louann evaporites because partial sr exchange in anhydrite had to take place in a fluid medium admittance of radiogenic 87sr bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt the slight number of anomalous 87sr 86sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the gulf coast basin

hydrocarbons in mississippi fan and intraslope basin sediments abstract aliphatic and aromatic hydrocarbons in sediments of the middle and lower mississippi fan and two intraslope basins in the gulf of mexico are derived from terrestrial organic matter and thermogenic mature hydrocarbons the terrestrial hydrocarbon component consists primarily of terrigenous plant biowaxes n alkanes with 21 to 33 carbons the occurrence of thermogenic hydrocarbons in immature near surface sediments their molecular distributions and concentration variations with depth suggest that the majority of these mature hydrocarbons have migrated from a source much deeper in the sediment column a portion of the thermogenic hydrocarbons may be derived from recycled material and includes phenanthrene methyl phenanthrenes chrysene and benzopyrenes the migrated thermogenic hydrocarbons include normal and isoprenoid alkanes with less than 21 carbons naphthalene methyl naphthalenes ethyl naphthalenes and other aromatics of similar volatility i e biphenyl acenaphthene and fluorene triterpane sterane and aromatized sterane distributions suggest that the thermogenic hydrocarbons at both sites have a common source and are overprinted with immature sediment hydrocarbons the biomarker distributions and carbon isotopic compositions of the thermogenic hydrocarbons are atypical for petroleum produced in the gulf of mexico molecular distributions of the hydrocarbons are constant regardless of the present depth of occurrence suggesting that they have migrated in a separate phase the upward migration of hydrocarbons from deeper sources is a wide spread phenomenon in the gulf of mexico with several documented cases of massive seepage visible oil as well as the more diffuse permeation of pleistocene sediments of the mississippi fan and two intraslopes reported here

global upper ordovician correlation by means of δ13c chemostratigraphy implications of the discovery of the guttenberg δ13c excursion gice in malaysia apart from a single study of the early katian 13 c chemostratigraphy in two regions in china no investigations of the sandbian and katian chemostratigraphy have been published from anywhere in asia a recent study of the conodont biostratigraphy of the classical ordovician succession on langkawi islands peninsular malaysia showed the presence there of strata coeval with those having the guttenberg carbon excursion gice on the yangtze platform in an effort to establish for the first time the presence of this widespread 13 c excursion in southern asia a series of samples from the upper part of the kaki bukit formation was isotopically analysed this resulted in the discovery of a conspicuous 13 c excursion with peak values of 2 above the baseline values the excursion is located just above the baltoniodus alobatus subzone and near the level of the first appearance of hamarodus europaeus hence the same stratigraphic position as the gice on the yangtze platform using the gice the malaysian study interval is closely correlated with the gice intervals at three localities representing an approximately 23 000 km long transect from malaysia across baltoscandia to central north america this shows the usefulness of 13 c chemostratigraphy to clarify previously obscure stratigraphic relationships between geographically very widely separated localities

δ13c stratigraphy of a turinian chatfieldian upper ordovician foreland succession ottawa embayment central canada resolving local and inter regional isotope excursions in a tectonically active basin abstract positive 13ccarb excursions are correlated through an upper turinian to lower chatfieldian carbonate platform succession along the axis of the ottawa embayment and into outliers of the northern ottawa bonnechere graben in central canada successive turinian excursions e1 and e2 are lithostratigraphically constrained by erosional surfaces and hosted within the watertown and overlying l orignal formations respectively the latter coeval with the selby formation in the adjacent northern appalachian basin the excursions coincide with periods of regional transgression but geographic patterns of 13c depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions respectively differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas with preferential corg recycling with stratigraphic condensation by early chatfieldian time segmentation of the once regional carbonate platform l orignal formation produced a regional mosaic of low energy muddy carbonate banks rockland formation and a deeper water platform lower hull formation settings subject to fluctuating high to low energy current flow excursion e3 occurs in both successions but 13c enrichment is associated only with the bank top muddy facies this may identify preferential photosynthetic drawdown of 12c across the bank tops due to limited seawater exchange across the bank deeper platform boundaries excursions e1 to e3 and a younger excursion e4 in the hull formation are correlated with varying confidence with excursions across southern laurentia excursion e3 being the local expression of the guttenberg 13c excursion our study supports local modulation of regional if not global 13c excursions arising from structurally controlled changes in oceanography and productivity

metallic sulfide deposits in the winnfield salt dome louisiana evidence for episodic introduction of metalliferous brines during cap rock formation abstract winnfield dome is a shallow piercement salt structure which penetrates late jurassic through early tertiary siliciclastic and carbonate strata of the north louisiana basin quarrying operations in the calcite and anhydrite portions of the cap rock have exposed zones of metallic sulfides and barite within the anhydrite portion of the cap rock sulfides form laminar zones parallel to the tightly intergrown layered anhydrite sulfides occur either as cement between anhydrite grains or as coarse euhedral crystals in open space the sulfide layers are up to several centimeters thick and extend for tens of meters these layers locally thicken into laminated flat bottomed concave downward massive sulfide mounds monoclinic pyrrhotite is the dominant sulfide sphalerite galena or barite may also be present in weathered samples hydrating to gypsum marcasite is common and appears to be an alteration product of pyrrhotite a roughly laminated massive sulfide lens is exposed at the calcite anhydrite transition zone this sulfide body has a maximum thickness of about 5 m 16 ft and extends for about 45 m 150 ft the edges of the lens taper to a layer about 30 cm 1 ft thick which separates the overlying calcite from the anhydrite the massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite gypsum and anhydrite colloform sphalerite galena marcasite pyrite and barite line vugs and fractures pyrrhotite is found locally within the sulfide lens examination of rotary drill samples suggests that massive sulfide concentrations similar to the exposed example are common along the calcite anhydrite transition zone these sulfide concentrations are believed to have originated from the interaction of metalliferous basinal brines with reduced sulfur trapped within the cap rock textural relationships and variations in chemical compositions between the sulfide layers in the anhydrite portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations anhydrite grains that are completely surrounded by sulfides are euhedral and undeformed similar to the anhydrite disseminated throughout the salt mass anhydrite grains outside the mineralized areas are deformed and tightly intergrown these textures suggest that mineralizing fluids were introduced episodically along the salt anhydrite interface at the zone of salt dissolution before that portion of the anhydrite zone was compressed and accreted to overlying anhydrite cap rock therefore the earliest formed sulfides originating by this mechanism occur at the top of the anhydrite cap rock zone while the last sulfides to form are found at the base extensive sulfide concentrations along the anhydrite calcite contact suggest that this contact also acted as a permeable zone allowing metalliferous brines into the cap rock textural and compositional relationships suggest that sulfides that formed along the anhydrite calcite contact are locally superimposed on sulfides that formed at the salt anhydrite contact

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

modification of the petroleum system concept origins of alkanes and isoprenoids in crude oils discussion in their recent paper collister et al 2004 presented a statistical analysis of c10 c28 n alkane and c16 c21 regular isoprenoid distributions of oils and rock extracts from the timan pechora basin they found that all unaltered to moderately biodegraded samples could be described as mixtures of six end member compositions that were assigned to five unique biological sources and to one reservoir process biodegradation because the end members appeared to yield no indications of thermal maturation collister et al surmised that these hydrocarbons were generated not from the thermal cracking of kerogen but from the liquefaction of biopolymers if this theory is true our current paradigms of oil generation and source rock potential need to be changed drastically because it suggests that a significant quantity of liquid hydrocarbons can be generated directly from unaltered biological precursors instead of the thermal cracking of kerogen we respectfully disagree with the conclusions and hypotheses reached by collister et al we regard their interpretation of the statistical analysis of the hydrocarbon distribution data to be flawed and their hypotheses concerning the generation of c10 n alkanes and isoprenoids during diagenesis from biopolymers to be incorrect the mathematical procedure polytopic vector analysis used by collister et al has the ability to extract end member spectra from mixed systems they found that six end members were needed to explain most of the variance in the data this is not a unique solution however because the inclusion of additional end members will always improve the overall goodness of fit whether these six end member fluids truly are representative of specific biological precursors geologic processes or are merely mathematical constructs cannot be resolved from the statistical analysis alone although collister et al claim that five of the end members bear the signature of specific biological sources and

geochemical character and origin of oils in ordovician reservoir rock illinois and indiana usa twenty three oils produced from reservoirs within the ordovician galena group trenton equivalent and one oil from the mississippian ste genevieve limestone in the illinois and indiana portions of the illinois basin are characterized two end member oil groups 1 and 2 and one intermediate group 1a are identified using conventional carbon isotopic analysis of whole and fractionated oils gas chromatography gc of saturated hydrocarbon fractions isotope ratio monitoring gas chromatography mass spectrometry irm gc ms of n alkanes ranging from c15 to c25 and gas chromatography mass spectrometry gc ms of the aromatic hydrocarbon fractions group 1 is characterized by high odd carbon predominance in mid chain n alkanes c15 c19 low abundance of c20 n alkanes and an absence of pristane and phytane group 1a is characterized by slightly lower odd carbon predominance of mid chain n alkanes greater abundance of c20 n alkanes compared to group 1 and no pristane and phytane the carbon isotopic composition of n alkanes and the distribution of n alkylarenes for groups 1 and 1a indicate that these oils were generated from strata containing abundant gloeocapsomorpha prisca in contrast group 2 is characterized by low odd carbon predominance in mid chain n alkanes moderate abundance of c20 n alkanes and the presence of pristane and phytane the mississippia reservoired oil is characterized by low odd carbon predominance in mid chain n alkanes and greater abundances of c20 n alkanes pristane and phytane compared to group 2 distributions of isoprenoidal alkylarenes aryl isoprenoids for group 2 and the mississippian reservoired oil are similar suggesting that aryl isoprenoids fail to provide a diagnostic characteristic that can be used to precisely correlate oils and source rocks conventional correlations of oil to source rock based on carbon isotopic type curves and hopane m z 191 and sterane m z 217 distributions are of limited use in distinguishing ordovician reservoired oil groups and determining their origin oil to source rock correlations using the distribution and carbon isotopic composition of n alkanes and the m z 133 chromatograms of n alkylarenes show that groups 1 and 1a originated from strata of the upper ordovician galena group group 2 either originated solely from the upper ordovician maquoketa group or from a mixture of oils generated from the maquoketa group and the galena group the mississippian reservoired oil most likely originated from the devonian new albany group the use of gc irm gc ms and gc ms illustrates the value of integr ted molecular and isotopic approaches for correlating oil groups with source rocks

anatomy of an intruded coal ii effect of contact metamorphism on organic δ13c and implications for the release of thermogenic methane springfield no 5 coal illinois basin abstract concerns about the present day accelerated rate of carbon input to the atmosphere have prompted the search for comparable events throughout earth s history recent research has attempted to determine the cause of such events represented as negative 13 c excursions within the sedimentary record and their subsequent effect on the earth it has been suggested that such excursions could be the result of the large scale release of 13 c depleted thermogenic methane produced by the intrusion of large igneous provinces lips into organic rich rocks and or coal e g mcelwain et al 2005 svensen et al 2004 2007 our study asserts that if intrusions result in significant release of 13 c depleted thermogenic methane from intruded coals 13 c enriched coal should be observed adjacent to the intrusions in this study values of bulk organic 13 c 13 c org are reported for fifteen samples along a coal transect in the illinois basin where a pennsylvanian age coal the springfield or no 5 coal is intruded by a permian ultramafic dike the samples reveal significant bulk geochemical and petrographic changes within the alteration halo for example volatile matter vm decreases towards the intrusion whereas fixed carbon fc and vitrinite reflectance r r both increase accompanied by the development of natural coke both isotropic and anisotropic mosaic structure devolatilization vacuoles and carbonate mineralization reflectance levels and coke development are consistent with exposure to temperatures 450 500 c despite the high level of alteration of the organic matter no significant change is observed in 13 c org values across the transect vary by less than 0 5 along this transect 13 c org does not correlate with r r and does not appear to be influenced by maceral composition the intruded coals show no evidence of pyrolytic carbon that could influence 13 c org the relatively high vm contents of the intruded coals compared to coals that have undergone normal burial maturation and the lack of any trend in 13 c org approaching the intrusion raises questions about previously published models of the effects of large scale igneous intrusions on atmospheric composition and 13 c org excursions associated with oceanic anoxic events this study underscores the importance of the use of organic petrography in combination with isotope geochemistry to determine if contact metamorphosed coals and organic rich shales could have released a significant amount of 13 c org depleted volatiles leading to negative 13 c org excursions in the sedimentary record

a mid caradocian 453 ma drawdown in atmospheric pco2 without ice sheet development abstract previous workers have recognized a positive 13 c excursion during the mid caradocian in laurentian and baltic platform carbonates this study documents the mid caradocian guttenberg excursion gice in the kullsberg limestone in central sweden significantly we observed increases in both 13 c and 18 o unlike most previous studies of gice which only observed increased 13 c marine values brachiopods from the lower kullsberg limestone reflect pre excursion conditions having more negative 13 c 0 1 and 18 o 4 to 6 values compared with least altered marine cement from the upper kullsberg limestone which have more positive 13 c 2 1 3 6 and 18 o 1 8 to 3 6 values least altered marine cements from the kullsberg limestone include columnar calcite first generation equant calcite and translucent fibrous calcite from earliest to latest in timing of formation kullsberg seawater during the 13 c excursion event likely decreased in temperature by up to 15 c with an approximate 1 increase in 18 o h 2 o attributable to increased salinity during the mid caradocian the results of this study are consistent with multiple independent approaches to characterize the composition of the ordovician atmosphere hydrosphere systems specifically results of this study are consistent with formation of abiotic calcite at moderately low paleotemperatures during the gice excursion of 10 17 c and ordovician atmospheric pco 2 values of

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

strontium isotopes in brines and associated rocks from cretaceous strata in the mississippi salt dome basin southeastern mississippi u s a abstract sr concentrations and 87 sr 86 sr ratios have been determined for brines from formations spanning most of the cretaceous in two oilfields in the mississippi salt dome basin and from local salt domes chalk sands and shales the selma group is predominantly a fossiliferous chalk whereas the remainder of the stratigraphic sequence consists of sands and shales with only minor carbonate phases the sampled interval ranges from 2000 to just over 4000 m below land surface sr concentrations range from 443 to 1718 mg kg 1 for brines from baxterville field and from 528 to 1762 mg kg 1 for brines from soso field brine 87 sr 86 sr ratios range from 0 7071 to 0 7116 at baxterville field and from 0 7083 to 0 7119 at soso field with the exception of brines from the selma chalk all 87 sr 86 sr ratios are more radiogenic than cretaceous seawater on plots of 87 sr 86 sr vs sr concentration brines from individual formations within a producing field occupy distinct non overlapping areas with the exception of a single sample in some cases individual reservoirs within formations can be distinguished brines from formations that are present in both fields which are 85 km apart do not have the same 87 sr 86 sr ratio in each field brine sr concentration increases with depth 87 sr 86 sr ratios increase with depth to 3000 m and then appear to level off these trends are interpreted to be caused by dissolution or recrystallization of carbonate minerals which contain significant amounts of non radiogenic sr and decomposition of rb rich detrital micas and k feldspar which are sources of radiogenic 87 sr salt and cap rock from domes in southeastern mississippi have an average 87 sr 86 sr ratio of 0 70707 consistent with a middle jurassic age for the louann salt in baxterville field chalk from the upper cretaceous selma group has a cretaceous seawater 87 sr 86 sr ratio of 0 7076 87 sr 86 sr ratios range from 0 7091 to 0 7111 for sr leached from sands and shales from both baxterville and soso fields if these values reflect the 87 sr 86 sr values of the easily dissolved interstitial carbonate phases they indicate that the calcite cements precipitated from sedimentary fluids which were more radiogenic than the seawater of that time but which were not as evolved as the brines which are now present alternatively they may reflect leaching of radiogenic sources

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

origin of exotic minerals in mississippi salt dome cap rocks results of reaction path modeling abstract shallow salt domes in central mississippi pierce mesozoic formations hosting regionally extensive metal rich brines this association coupled with the multiple fluid migration pathways around salt domes suggests that these brines were the source of fe and base metal sulfide sr and ba minerals in salt dome cap rocks fluid inclusions in barite and celestite indicate that the brines had a relatively constant average composition of approximately 200 000 mg i total dissolved solids which is similar to present day oil field brines from central mississippi geochemical reaction path modeling indicates that the suite of observed cap rock minerals can be produced by alternating periods of biogenic so4 reduction with inorganic anhydrite dissolution or by mixing with seawater sulfide rich portions of cap rock at hazlehurst salt dome have molar fe pb and fe zn ratios similar to present day oil field brines indicating that mixing of brines with abundant amounts of h2s produced by biogenic so4 reduction led to rapid precipitation of virtually all dissolved metals as relatively insoluble sulfide minerals

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

petroleum geochemistry of texas and oklahoma oils along the marathon ouachita fold thrust belt south central u s a abstract the marathon uplifit of west texas and the ouachita mountains of oklahoma and arkansas comprise the surface expressions of a paleozoic orogenic belt extending across the south central u s a a century of petroleum exploration in the marathons and ouachitas has yielded several oil discoveries in this study detailed molecular elemental and isotopic data are presented for 9 texas oils 5 oklahoma oils and 4 oklahoma solid bitumens all associated with thrust belt rocks of west texas and southeast oklahoma oil oil correlations are proposed and the character of the organic matter in the source rock s is inferred from knowledge of the oil chemistry at least 16 of the 18 marathon ouachita fold thrust belt oils solid bitumens originate from the same or very similar organic facies despite a geographic separation along the fold thrust belt of 700 mi 1125 km many of the chemical differences in these samples derive from secondary effects including biodegradation e g solid bitumens and differing levels of thermal maturity the occurrence of unusual chemical compounds e g 25 28 30 trisnorhopane in some samples indicates the presence of anaerobic bacteria in isolated locales during deposition isotopic variability when interpreted in light of near constant molecular composition in the oils and an extensive period of early paleozoic starved trough source rock deposition implies that these oils originate from rocks of similar organic facies with source rock ages ranging from devonian through ordovician previous literature suggests either a devonian or an ordovician source for these oils molecular and isotopic data of the present study do not allow a distinction to be made these results when interpreted in terms of depositional patterns during early paleozoic time suggest that oil source rocks and the marathon ouachita fold thrust belt oil type may be present in the subsurface of central and east central texas

productivity control on oil shale formation mae sot basin thailand abstract neogene oil shales from the freshwater lacustrine intermontane mae sot basin western thailand are examined in order to define a oil source rock potential b variations in molecular and isotopic geochemical characteristics and c the influence on source potential of organic matter productivity and preservation twelve samples spanning 7 m of section were examined using conventional isotopic molecular and organic petrographic methods of geochemical analysis temporal resolution is 2700 yr per sample because thermal maturity differences and fluid migration within the sample set are absent and organic matter type input variation is either minor or absent based on visual kerogen analysis we propose that organic matter differences among the samples are caused by variability in surface water productivity all samples are excellent potential source rocks vr 0 4 0 5 ro containing types i and ii organic matter modified van krevelen plot determination consisting of almost 100 fluorescent amorphous kerogen visual kerogen determination numerous diagnostic molecular markers and characteristics are present including perhydro carotene dammaranes norsterane s and novel distributions of tricyclic terpanes stable carbon isotope ratios of kerogens and extract fractions range over 5 and co vary with hydrogen index this covariance is attributed to temporal increases in surface water productivity leading to co2 limiting conditions and consequent increase in 13c of the organic matter in the photic zone during times of elevated productivity increased amounts of 13c enriched organic matter reached the water sediment interface thus times of greater productivity are accompanied by higher 13c and hydrogen index values with 13c related to productivity under co2 limiting conditions and hi related to the relative amount of the water column that was anoxic during deposition our findings suggest that in petroleum geochemical studies involving lacustrine source rocks 13c values must be used with caution as oil oil and oil source rock correlation parameters

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

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

chemical and isotopic properties of kukersites from iowa and estonia abstract kukersite samples from estonia and iowa were analyzed for elemental composition functional group distribution and carbon and hydrogen stable isotope ratios the elemental and hydrogen isotope values together with other analytical data suggest a higher thermal maturity for the iowa kukersite the wide carbon isotopic range of 9 3 among kukersites with unusually negative 13c values reaching 33 2 indicates isotopically variable carbon sources for production of biomass and thus major paleoceanographic differences between the environments supporting biosynthesis

strangelove ocean before the cambrian explosion the palaeozoic and mesozoic eras were terminated by faunal changes involving mass extinction of the old and explosive evolution of the new fauna but the fossil record shows only a cambrian explosion at the end of the precambrian stanley speculated that the explosion was only possible after the ubiquitous algae community had been largely eliminated1 ecological niches were thus liberated for explosive evolution if the cambrian explosion were preceded by a mass mortality or by a mass extinction such an event should leave a record in the form of geochemical anomalies we have undertaken a search for geochemical anomalies at the precambrian cambrian contact we report here the discovery of a sharp negative carbon isotope shift in the carbonate of a clay immediately above a marker in the precambrian cambrian boundary the china c marker and interpret this signal as evidence of sudden decrease in fertility before the cambrian explosion of invertebrate evolution the discovery suggests that the precambrian cambrian boundary might be defined by an event marker at a palaeontologically correlative horizon

oil composition variation and reservoir continuity unity field sudan abstract a suite of oils from stacked reservoirs in the unity field in sudan has been analyzed by various geochemical techniques for molecular information to elucidate the geological processes which cause variations in oil composition and their resulting oil fingerprints in different reservoir units analyses of these highly paraffinic oils indicate that the chromatographic fingerprint variations are due to differences in the abundances of saturated compounds including branched and cyclic alkanes neither aromatics nor nso compounds have any significant effect on the observed fingerprint variations this association of saturates instead of aromatics and nso compounds with the fingerprint variations precludes rock fluid interactions as a cause of the variations biomarker analyses show that variations in thermal maturity and organic facies of the source rock are responsible for the fingerprint variations thermal maturity increases with the depth of the reservoir suggesting a multiple charge process for the oils to fill these reservoirs over an extended period of time apparently the source rock generated and expelled progressively more mature oils and little mixing occurred during migration thus knowledge of oil compositional variations from one reservoir to another organic facies variation and source rock maturity combined with tectonic history may help explain charging and timing of oil emplacement

upper ordovician carbon isotope chemostratigraphy on the yangtze platform southwestern china implications for the correlation of the guttenberg delta c 13 excursion gice and paleoceanic change abstract samples of upper ordovician marine carbonates from four sections on the yangtze platform 1 the liangcun section northeast of xishui county town guizhou province 2 the zhuazhuayan section southwest of leibo county town sichuan province 3 the xiaogangwan section east of wulong county town chongqing municipality 4 the shatan section north of nanjiang county town sichuan province have been analyzed for carbon isotope 13 c chemostratigraphy a distinct positive 13 c excursion 13 c max 2 is observed in all sections which ranges through the hamarodus brevirameus conodont zone and locally into slightly younger strata the base of this zone is the first appearance datum fad of h brevirameus and its top coincides with the fad of the conodont protopanderodus insculptus based on its position in the h brevirameus zone this excursion is identified as the well known guttenberg excursion gice confirming the global significance of this paleoceanographic event the fact that in the black knob ridge section in the usa the gssp of the base of the global katian stage the beginning of the gice is just below the base of the katian makes it possible to locate this key level in our study sections even in the absence of the diagnostic graptolite diplacanthograptus caudatus our carbon curves through the gice interval show three minor secondary peaks reflecting paleoceanographic changes or selective preservation of the isotope signal however because a global review of the gice shows that such minor peaks which are often based on a single elevated 13 c value are of irregular occurrence they are interpreted as local rather than global phenomena the suggested global cooling in the early katian might have favored the generation of methane hydrates and hence the reduction of the amount of 12 c in the marine carbonates

katian upper ordovician delta c 13 chemostratigraphy and sequence stratigraphy in the united states and baltoscandia a regional comparison delta c 13 values of numerous limestone samples from katian upper ordovician successions in oklahoma and the upper mississippi valley document the presence of at least three of the four positive excursions that have been recognized in the lower middle katian interval in the cincinnati region in ohio kentucky and indiana a comparison with estonia reveals that five out of the six positive excursions now recorded through the katian succession in the cincinnati region are present in the coeval stratigraphic intervals in estonia which suggests that these represent global perturbations in the carbon cycle a study of the relations between flooding events and delta c 13 excursions shows that only some of the excursions occur in transgressive intervals whereas other excursions are in regressive strata and hence this is in conflict with the idea that positive carbon isotope excursions are forced by rises in sea level part of a previously published attempt to use upper ordovician sequence stratigraphy for trans atlantic correlation is shown to be seriously out of phase with the biostratigraphy and chemostratigraphy several of the sequences recognized in the cincinnati region do not appear to have counterparts in estonia which suggests that they do not reflect eustatic but local conditions c 2010 elsevier b v all rights reserved less

fluid flow evolution in the albanide fold thrust belt insights from hydrogen and oxygen isotope ratios of fluid inclusions tectonic forces generated during thrust emplacement along active margins may drive complex fluid flow patterns in fold thrust belts and foreland basins isotope data 2h and 18o of fluid inclusions hosted in calcite vein cements are used to reconstruct regional fluid migration pathways in the albanide fold thrust system the calcite veins used in this study developed in a sequence of naturally fractured cretaceous eocene carbonate rocks as a result of episodic throughput of fluids from the early stages of burial onward the acquired fluid inclusion isotope data demonstrate that fluids circulating in the carbonates were derived from an underlying reservoir that consisted of a mixture of meteoric water and evolved marine fluids probably derived from deep seated evaporites the meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin and ascended as soon as fracturing induced a sufficient increase in permeability structural and petrographic observations provide time constraints for the various phases of fracture infilling and reveal an increasing dominance of meteoric water in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out similar fluid flow evolutions have previously been recorded in various fold thrust belt settings elsewhere in the world

low temperature sulphate reduction biological versus abiological sulphate is considered to have been a major source of sulphide in strata bound and stratiform base metal sulphide deposits many of these deposits however appear to have been formed at moderate temperatures 200 c which poses the question by what mechanism s was sulphate reduced to sulphide two modes of reduction have been established experimentally 1 catalysis by sulphate reducing bacteria which at present is only known to occur below ca 100 c and 2 abiological reduction by ferrous iron or organic matter which has only been clearly shown above ca 250 c several attempts have been made to demonstrate abiological reduction below 200 c and some new data are presented here although the results do not exclude the possibility that such a reaction may be geochemically significant there has been no unequivocal demonstration of nett sulphide formation from sulphate at these temperatures recent studies of the microbiology of hydrothermal regions have opened up the prospect of bacterial sulp

similar morphological and chemical variations of gloeocapsomorpha prisca in ordovician sediments and cultured botryococcus braunii as a response to changes in salinity abstract most ordovician source rocks consist of accumulation of a colonial marine microorganism gloeocapsomorpha prisca g prisca whose nature ecology and affinity with extant organisms have been in dispute for years furthermore recent studies have shown major differences in phenol moieties between two g prisca rich samples examination of five g prisca rich kerogens by electron microscopy and pyrolysis studies revealed i the occurrence of two markedly distinct morpho chemical types a closed phenol rich type baltic samples and an open phenol poor one north american samples and ii the selective preservation of the resistant micromolecular material building up the thick cell walls in the original organism comparison with extant botryococcus braunii a widespread green microalga grown on media of increasing salinity suggests that g prisca is likely to be a planktonic green microalga related to b braunii which can adapt to large salinity variations which in turn control its polymorphism the large differences in colony morphology and in the content of phenol moieties observed in fossil g prisca and the resulting occurrence of two morpho chemical types should therefore reflect depositional environments with different salinities the presence of thick highly aliphatic resistant walls in g prisca selectively preserved during fossilization accounts for the major contribution of this organism to ordovician organic rich sediments and for the resulting typical signature of ordovician oils

evidence for large scale vertical migration of dissolved fatty acids in louisiana oil field brines iberia field south central louisiana abstract abstract there is considerable interest at the present time in the origin and distribution of dissolved volatile fatty acids vfas in oil field brines this interest has developed as a result of the recognition of the role these compounds may play 1 in subsurface acid attack and porosity enhancement 2 as precursors of natural gas and 3 as possible proximity indicators of hydrocarbon accumulations the detailed study of the distribution of dissolved vfas on a field scale provides much useful information on processes of generation transport and degradation of these compounds at the iberia oil field one of several fields we have studied in the tertiary section of south central louisiana spatial variations in pore water compositions and temperatures indicate the presence of an on going dynamic subsurface circulation system deep brines with vfa levels in excess of 150 mg 1 are migrating up the south flank of the iberia salt dome a vertical distance of at least 2 km 6000 ft the vfas in these waters are dominated by acetate and propionate as these waters ascend they mix with an ambient mass of water having total vfa concentrations of 20 mg 1 or less and dominated by n butyrate preferential decarboxylations of acetate and propionate relative to iso and n butyrate and iso and n valerate are occurring in this system the byproducts of these decarboxylation reactions should be methane ethane and bicarbonate temperatures are cool enough 80 c in the shallower parts of the sequence to permit the existence of bacteria that coud break down acetate through fermentation the spatial distribution of individual dissolved vfa s is complex but systematic and must ultimately be related to the rates of advective transport dispersive mixing and chemical reaction we believe that a potential new application of the study of these dissolved organic compounds lies in helping to unravel the dynamics of some types of subsurface flow systems

sea level forcing of carbon isotope excursions in epeiric seas implications for chemostratigraphy five new 13c excursions are reported from the upper ordovician galena group of iowa the 13ccarb and 13corg trends are compared to sea level trends based on conventional sedimentological and novel chemostratigraphic end methods it is found that rising 13c values occur during rising sea level peak magnitude 13c values are coincident with inferred peak flooding and declining 13c values occur with declining sea levels the relationship to sea level is explained by a simple model of episodic flooding of the galena platform by cool oxygen depleted nutrient rich bottom waters originating from the sebree trough which was itself connected to the nearby iapetus ocean flooding of the inner galena shelf during sea level transgressions caused local changes in the carbon isotope balance of inner shelf waters due to 1 locally increased primary productivity and organic carbon burial 2 locally increased exchange of 13c enriched dissolved inorganic carbon dic with surrounding epeiric sea water masse

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

evolution of metal ratios and δ34s composition of sulfide mineralization during anhydrite cap rock formation hockley dome texas u s a metal zonation and sulfur isotope patterns of cap rock hosted sulfides reveal a complex evolution of the sulfide depositing system at the hockley salt dome the general metal trend observed is an overall increase of zn relative to pb downward in the anhydrite cap rock in the upper cap rock section the pb pb zn ratios commonly range from 0 25 to 0 40 ratios of 0 20 and commonly 0 10 are typical of the lower cap rock section the concentration of ag also appears to increase in the lower part of the anhydrite cap rock textural evidence indicates that some of the metal concentrations represent stratiform sulfides that were precipitated at the salt cap rock contact as the anhydrite zone accumulated by sequential underplating the metal ratio data suggest that the sulfide precipitating fluid became more zn and possibly ag rich during anhydrite cap rock accumulation 34s values of sulfide minerals within the hockley dome cap rock range from 30 to 4 cdt 59 analyses a profile of 34s values from a single drill hole indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend reverses the inflection point corresponds with a local highly mineralized biogenic calcite zone a mixing model involving variable contributions of isotopically heavy and light sulfur end members appears to best account for the observed isotopic trends fault leaked h2s from mature oil and gas reservoirs is a possible extrinsic source of heavy sulfur but local thermochemical sulfate reduction of cap rock sulfate cannot be eliminated by existing data bacterial reduction of cap rock sulfate is believed to be the source of light h2s the relative abundance of deep sourced metalliferous brines appears to have increased with time during the accretion of the upper part of the anhydrite cap rock biogenic h2s subsequently began to dominate the mixed sulfur reservoir resulting in a 15 shift and relatively light 34s values of pyrite to the present cap rock salt contact

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

local controls on carbon cycling in the ordovician midcontinent region of north america with implications for carbon isotope secular curves abstract the carbon isotope record from ancient epicontinental seas may contain much more of a local scale carbon cycling signal than is generally appreciated a unique opportunity exists to examine this issue in the case of the late ordovician mohawkian sea of eastern laurentia where the millbrig k bentonite stratigraphic framework has been used to delineate a time slice at 454 ma extending over 1 500 000 km2 of the eastern united states across the time slice carbonate and organic carbon 13c vary by 4 5 and 7 5 respectively a spatial variation that is as large as temporal secular changes in epeiric sea 13c that have been reported in the past these new data are considered in the context of geographic variations in lithological biological and other geochemical sediment characteristics collectively these sediment properties distinguish regions of the mohawkian sea which likely differed in terms of the nature and relative importance of carbon cycling processes water column depth and structure and barriers to free exchange of water across the mohawkian sea may have been overarching factors in the development of these regions raising the possibility that changes in circulation patterns such as those caused by sea level change played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses if the observed effects of local carbon cycling on the distribution of mohawkian sea 13c were commonplace in ancient epicontinental marine environments it would imply that local scale carbon cycling may have left a nontrivial imprint on epeiric sea records of secular variations in 13c in addition to the imprint left by changes in the global carbon cycle this may have contributed to the broad scatter in 13c values observed in the paleozoic portion of the global carbon isotope secular curve

petrography and geochemistry of water rock interactions in richton dome cap rock southeastern mississippi u s a abstract cap rock at richton dome consists of gypsum veined anhydrite overlain by dark and light banded calcite the anhydrite accumulated directly during dissolution of the salt stock without forming gypsum indicating temperatures 60 c and nacl salinity 4 m sr and 87 sr 86 sr were inherited directly from salt stock the anhydrite is a simple dissolution residue textures and 13c indicate that dark colored calcite formed as a replacement of precursor anhydrite cap rock in the presence of methane oxidizing bacteria coprecipitated light colored calcite is more enriched in 13c indicating influx of carbonate bearing groundwater as well during calcite cap rock formation all calcite appears to be enriched in trace metals relative to the anhydrite precursor sr is also enriched but 87 sr 86 sr is similar to jurassic seawater the calcite cap rock geochemistry is most easily explained if meteoric groundwaters at the top of the salt diapir were mixed with sr rich metal rich brines brought up the sides of the dome during calcite cap rock formation petrographic analysis suggests that gypsum veins in the anhydrite cap rock formed by simple hydration of the wall rock anhydrite along fractures d and 18o of the gypsum crystal water indicate that the incoming water was meteoric and that the water rock ratio was very low modern hydrologic communication through the cap rock is suggested by dissolution enhancement of fractures in both anhydrite and calcite cap rock particularly along the calcite anhydrite interface

isotope shifts in the late permian of the delaware basin texas precisely timed by varved sediments closely spaced samples 285 in number of varved sediments from the upper permian in delaware basin texas have been analyzed for 13ccarb 13corg 18ocarb corg ccarb and calcite dolomite 13c records a dramatic rise from 2 8 to 5 7 in only 4400 years detected in three sections across the basin extrapolating smoothly through a 600 year interruption by a local west side of the basin fresh water inflow evidenced by low 18o this continuity and low corg within the basin both indicate that the excess net deposition of corg necessary to generate the rise in 13c took place in the ocean external to the delaware basin correlation with similar records from the zechstein basin suggest that the event was world wide although this poses obvious difficulties for the carbon cycle the rate of rise of 13c and its sustained high level must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved

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

source rock characterization and oil to source rock correlation of a cambrian ordovician fold and thrust belt petroleum system western newfoundland abstract in this study we use biomarker and isotope data to address the complexities of petroleum system analysis in fold thrust systems specifically related to hydrocarbon generation and migration such as variable thermal maturities of source rocks complex migration pathways and mixing source rocks extracts and oil samples are taken from paleozoic rocks along the appalachian structural front in western newfoundland oil seeps along this fold and thrust belt have motivated episodic exploration efforts over the last 150 years however economic development has been unsuccessful to date in part because of the complex nature of oil to source relationships pyrolysis analyses identify promising source intervals in lower ordovician floian formations with an excellent source potential containing type i ii organic matter toc up to 9 35 hi up to 840 a second good source interval identified within the late cambrian furongian continental slope and rise sediments contains type ii iii organic matter toc up to 2 34 hi 380 source rock samples from outcrops are marginally mature to mature with tmax values between 436 and 447 geochemical analyses of source rock extract imply a clastic shale dominated source rock with a minor contribution from carbonate source rocks chemometric analyses yield 3 extract groups groups 1 and 2 contain lower ordovician samples with organic matter derived from oxidized micro plankton with varying amounts of gleocapsomorpha prisca extract group 3 originates from cambrian samples and shows evidence of a bacterial derived organic matter similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high resolution biomarker analyses seven oil samples originated from ordovician source rocks that produced high api oils with low bisnorhopane hopane tricyclic hopane ratios and high sterane concentrations supporting interpretation of a source containing algal derived organic matter three oils originated from cambrian shales these high api oils show high bisnorhopane hopane and tricyclic hopane ratios low sterane concentrations and low carbon isotope values all indicative of organic matter derived from bacteria generated over a wide range of maturities 0 98 1 26 ro thermal maturity sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack the cambrian source generated oils that locally experienced secondary cracking during acadian inversion whereas the ordovician source generated oil with low maturities after the acadian inversion oil operaters exploring fold and thrust belt petroleum systems may expect extreme changes of oil properties for example api gravity in spacially restricted areas

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

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

molecular organic geochemistry of the tarim basin northwest china we conducted organic geochemical analyses on the largest suite of oils and source rock extracts from the tarim basin northwest china currently available statistical cluster analysis of the entire suite of tarim oils distinguishes at least seven genetic groups of oils the largest group of oils was collected from the tazhong and tabei uplifts and originated from marine middle upper ordovician anoxic marls that mark slope facies at the margins of structural uplifts two other genetic groups most likely originated from marine middle upper ordovician source rocks but of distinct facies with one an oxic shale rich source west of the bachu uplift and the other an anoxic shale source at tazhong

possible late middle ordovician organic carbon isotope excursion evidence from ordovician oils and hydrocarbon source rocks mid continent and east central united states oils generated by middle ordovician rocks are found throughout the mid continent and east central regions of the united states gas chromatographic characteristics of these oils include a relatively high abundance of n alkanes with carbon numbers less than 20 a strong predominance of odd numbered n alkanes between c10 and c20 and relatively small amounts of branched and cyclic alkanes saturated and aromatic hydrocarbon fractions of 43 ordovician oils from the anadarko ardmore forest city illinois michigan salina sedgwick and williston basins and the iowa shelf demonstrate a wide range in carbon isotope composition dgr13csat 24 9 pmil to 33 9 pmil dgr13carom 24 3 pmil to 33 7 pmil saturated and aromatic hydrocarbons extracted from late middle ordovician shales 17 core samples show ranges in dgr13c similar to that of the oils the wide ranges in dgr13c for oils and rock extracts reflect a major positive excursion s 6 9 per pmil in organic matter dgr13c in late middle ordovician rocks this excursion has at least a regional significance in that it can be documented in sections 480 mi 770 km apart in south central kansas and eastern iowa the distance may be as much as 930 mi 1 500 km if the carbon isotope variations observed in michigan basin ordovician oils and in organic matter from late middle ordovician rocks in southwestern ontario are related to the same carbon isotope excursion organic matter dgr13c in core samples from south central kansas and eastern iowa is not directly related to variations in quantity or quality of organic matter or maceral compositi n the positive excursion in organic matter dgr13c is a possible result of increased organic matter productivity and or preservation the parallel shifts in organic and carbonate dgr13c in core samples from 1 e m greene well washington county iowa imply changes in the isotope composition of the ocean atmosphere carbon reservoir differences in the magnitude of the carbon isotope shifts between organic matter 8 8 pmil and carbonate 4 2 pmil in this core suggest a decrease either locally or regionally in available dissolved co2 possibly a result of high organic matter productivity and or limited circulation in the late middle ordovician seas

the marine photosynthetic carbon isotopic fractionation remained constant during the early oligocene abstract the oligocene has recently been suggested as a time of drastic decrease of marine photosynthetic carbon isotopic fractionation mphcif leading to a separation of 13 c depleted 13 c pre neogene and 13 c rich 13 c 24 neogene quaternary marine kerogen a more exact timing of this change in fractionation is hindered by the scarcity of well preserved marine kerogen in oligocene sedimentary records we have analyzed 13 c values of about 40 kerogen samples from the often anoxic early oligocene sediments of the hungarian paleogene basin and the carpathian flysch belt the bulk of the well preserved marine kerogen covering a time span between 35 0 and 31 5 m y has 13 c values more negative than 26 5 while marine kerogens from the boundary of early and late oligecene 30 5 m y representing a local ephemerous anoxic event have 13 c values between 23 1 and 23 3 the unambiguously low 13 c values of the early oligocene marine kerogens prove that the recently suggested global decrease of the mphcif could not have begun before the end of the early oligocene the high 13 c values of marine kerogens from the early late oligocene boundary may have been caused by a decrease in mphcif at the end of the early oligocene alternatively they may indicate a local depletion of hco 3 in the photic zone

a petrographic and geochemical model for the origin of calcite cap rock at damon mound salt dome texas u s a abstract calcite cap rock at damon mound texas consists of five calcite generations recognizable through standard petrography and cathodoluminescence stage ia calcite formed by dissolution of anhydrite followed by calcite precipitation and was partly replaced by stage ib calcite stage ii formed above stage ia and ib substrates either filling open voids or cementing breccia fragments stage iiia and iiib calcites fill late fractures that crosscut all previously formed generations microprobe analysis indicates that each calcite generation is geochemically distinct sr mg ratios indicate that calcite precipitated from variable proportions of formation water meteoric water and or seawater wide variation in calcite fluid inclusion compositions 1 0 9 4 eq wt nacl indicates mixing of high salinity water and low salinity meteoric water the single phase nature of most fluid inclusions and the homogenization temperatures indicate that calcite precipitated at temperatures of calcite 13 c values range from 31 1 to 14 4 pdb generally decrease with depth and are most similar to crude oil 13 c values calcite 18 o values range from 7 6 to 4 8 pdb generally increase with depth and are compatible with derivation from local meteoric water or formation water at high temperature late stage calcite can be either isotopically heavier or lighter than adjacent early stage calcite with regard to both carbon and oxygen and because isotopic values vary with depth it appears that fluid mixing is the mechanism most responsible for isotopic variations although 13 c and 18 o values of stage i calcite are similar to adjacent stage ii and iii calcite 87 sr 86 sr ratios of adjacent calcite pairs are generally different indicating that each formed from different fluids or a fluid whose 87 sr 86 sr composition varied through time due to mixing isotopic modeling indicates that calcite cap rock formed from either of two fluid mixtures 1 formation water and meteoric water between 40 and 80 c the source of light carbon being either thermogenic ch 4 or liquid hydrocarbons or 2 formation water meteoric water and seawater between 50 and 70 c with thermogenic ch 4 being the light carbon source sr was contributed by both anhydrite and fluid overall it appears that calcite cap rock formed from top to base from mixtures of formation water containing isotopically light carbon and heavy oxygen and seawater and or meteoric water containing isotopically heavy carbon and light oxygen during this progression the contributions of formation water increased at the expense of seawater and or meteoric water the correlation between 18 o and 13 c probably indicates that separate fluid pulses had different carbon abundances and or slightly different 13 c values

fracturing and calcite cementation controlling fluid flow in the shallow water carbonates of the jandaira formation brazil the shallow marine carbonate rocks of the jandaira formation have been subject to significant permeability variations through time due to various events of fracturing and calcite cementation as a consequence the jandaira formation accommodated fluid flow only during specific moments in time we reconstructed these episodes of fluid flow based on isotope characterizations and microscope characteristics of calcite veins and host rock cements the jandaira formation which belongs to the post rift sequence of the potiguar basin in northeast brazil was deposited from the turonian onward until a marine regression exposed it in the campanian due to the subaerial exposure meteoric waters flushed out marine connate waters leading to an event of early diagenesis and full cementation of the jandaira formation fluid flow through the resulting impermeable carbonate formation appears to be closely related to fracturing fracturing in the late cretaceous induced a drastic increase in permeability giving rise to extensive fluid circulation host rock dissolution associated to the circulating fluids led to calcite vein cementation within the fracture network causing it to regain an impermeable and sealing character in the research area fluid flow occurred during early burial of the jandaira formation at estimated depths of 400 900 m this study documents the first application of fluid inclusion isotope analysis on vein precipitates which allowed full isotopic characterization of the paleo fluids responsible for calcite vein cementation the fluid inclusion isotope data indicate that upwelling of groundwater from the underlying acu sandstones provided the fluids to the fracture network in miocene times renewed tectonic compression of a lower intensity created a secondary fracture network in the jandaira formation the density of this fracture network however was too low to induce a new episode of fluid circulation as a result this tectonic event is associated with the development of barren extensional fractures

carbon cycle changes of the zechstein sea isotopic transition zone in the marl slate the carbon isotope composition of carbonate rocks is one parameter used to study the carbon sulphur and oxygen cycles through geological time1 2 the understanding of these interrelated cycles is important in quantitative flux modelling of sedimentary rocks the carbon isotope composition in the large reservoir of dissolved carbonate in ocean water is a result of mass balance between the amount of carbon in the reduced organic reservoir and the oxidized carbonate reservoir at any given stage the reduced carbon reservoir preferentially sequesters 12c owing to biological fractionation processes any change in the removal ratio of oxidized reduced carbon will be reflected by the change of the 13c value of the oceanic carbonate system here we present a detailed study of the change in 13c which took place at the base of the upper permian zechstein sequence

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

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

fluid rock interactions in the salt dome environment an introduction and review abstract the salt dome environment in the gulf of mexico coast u s a provides one of the most diverse and long lived records of fluid rock interactions in an active sedimentary basin dominated by saline formation waters the geochemical record most of which is isotopic appears to span a temperature range between earth surface temperatures and lower greenschist facies most mineral forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters papers in this issue of chemical geology document some of the current research on fluid rock interactions in the salt dome environment these papers highlight the diversity of opinion about salt dome processes offering clues for further research the present paper reviews some of the key and more current literature on salt dome geochemistry fluid convection around salt domes and fluid rock interactions within salt and within the salt dome environment

modification of the petroleum system concept origins of alkanes and isoprenoids in crude oils kerogen the source material for petroleum can have a long history of alteration and diagenesis before crude oil forms a common assumption is that the bulk composition of the expressed oil reflects so much of these progressive alterations that most of the primary biological information of the original fixed carbon has been lost exceptions are trace constituents the biomarkers that comprise only a fraction of the organic material in most pristine crude oils analysis of the alkane acyclic isoprenoid fraction of a large number of crude oils and rock extracts from the timan pechora basin russia suggest that this fraction the main constituent of most crude oils is a direct product of liquefaction of biological debris that was preserved essentially unaltered to the point of oil generation therefore the primary biological provenance of this fraction is preserved in the oil fraction a set of gas chromatographic analyses of 242 crude oils as well as 83 solvent extracts from upper to middle paleozoic putative source rocks from the timan pechora basin russia were analyzed by a multivariate data analytical procedure new to organic geochemistry the distributions of n alkanes and acyclic isoprenoids 24 in all in the 325 samples could be reproduced by linear combinations of six end member compositions attributed to distinct biological inputs four of the six are assigned to primary producers waxes from higher plants cyanobacteria microalgae and the microorganism gloeocapsomorpha prisca these end members account for most of the n alkanes and acyclic isoprenoids in our samples the other two represent the products of secondary bacterial alteration of primary organics during sedimentation and low level bacterial alteration in the reservoir biodegradation each end member is composed of a spectrum of analytes whose abundances are related to one another by fixed ratios we surmise that each primary end member represents the breakdown of a resistant biopolymer that forms cell walls and partitions of a given biological group the n alkanes and acyclic isoprenoids in crude oils represent the weighted signatures of their various ancestors i e their primary organic inputs if the precursors of most oils are the products of a small set of chemically simple biopolymers then many of our assumptions concerning the importance of total organic carbon and the nature of the oil window must be reexamined

palaeoproductivity of the eastern arabian sea during the past 200 ka a multi proxy investigation abstract two sediment cores from the eastern arabian sea were investigated for reconstructing the late quaternary palaeoproductivity using a variety of proxies the relative abundance of florisphaera profunda in the total coccolithophore flora fp organic carbon c org and calcium carbonate contents the low abundance of fp during glacial periods suggests a significantly enhanced primary productivity during those periods the high fp abundance during terminations ii and i implies that the productivity was low during deglaciation the reduced productivity was probably a result of the intensification of the sw monsoon during the two terminations the high c org content documented for glacial periods marine isotopic stages 2 4 and 6 also suggests that productivity was high during glacial periods the consistent results obtained from two independent proxies fp and c org for palaeoproductivity confirm that productivity was indeed high during glacial periods and hence the high c org content is not just an artifact of better preservation of organic matter due to high sedimentation rate down core variations in the caco 3 content and 18 o show coherent fluctuations and typical glacial interglacial cyclicity glacial periods were characterised by low caco 3 and interglacials by high caco 3 content interestingly the low caco 3 content during glacials is in contrast to the high productivity suggested by the low relative abundance of fp and the high c org content therefore the low carbonate content during glacial periods was probably due to dilution by terrigenous material and or increased biogenic opal production spectral analysis of data on palaeoproductivity proxies shows a 23 ka periodicity indicating that the earth s precessional cycle exerts a significant control on the palaeoproductivity of the eastern arabian sea

a dynamic model for the permian panhandle and hugoton fields western anadarko basin panhandle hugoton the largest north american gas field has long been controversial because of extreme subnormal pressures variable gas composition and tilted fluid contacts commonly attributed to hydrodynamic flow despite the absence of an effective updip aquifer these anomalies are addressed in terms of a basin scale petroleum system history largely independent of the geographically underlying pre permian system hydrocarbons were already being generated in the deep anadarko basin during the early permian with efficient southward migration from all potential source rocks via bounding faults and pennsylvanian permian alluvial fans giant amarillo uplift drape structures trapped hydrocarbons immediately following permian evaporite deposition the pre laramide panhandle field at maximum burial depth and pressure contained most of the oil and gas now found in mid continent permian reservoirs the early tertiary laramide orogeny redistributed panhandle field fluid columns possibly spilling gas into the hugoton embayment subsequent erosion of permian reservoir facies in eastern kansas allowed water discharge to outcrops at elevations below the regional hydraulic head as regional pressure dropped in response the panhandle field gas cap expanded rapidly forcing a late tertiary quaternary mass movement of gas northward to fill hugoton and associated fields panhandle hugoton pressures upon discovery were subnormal relative to drilling depth but normal relative to reservoir outcrop elevations in eastern kansas indicating that pressures are controlled by aquifer communication with the surface instead of burial depth variations in fluid contacts pressure and gas composition suggest that reservoir fluids are still moving driven by decompression and the rapid volumetric expansion of a supergiant gas accumulation

integrated bio chemostratigraphical correlations and climatic evolution across the danian selandian boundary at low latitudes abstract the global boundary stratotype section and point for the base of the selandian stage is defined in the zumaia section spain at an abrupt change in lithology base of itzurun formation which coincides with the onset of a negative carbonate carbon isotope shift however this lithological change is not always very well expressed in other sections in order to document the stratigraphic position of the danian selandian boundary dsb on a more global scale we have investigated three sections across the dsb the zumaia reference section gssp the loubieng section auxiliary dsb reference section france and the sidi nasseur section tunisia the danian selandian boundary interval is subdivided and correlated throughout low latitudes from the altlantic bay of biscay to the southern tethys on the basis of seven calcareous nannofossil and planktonic foraminiferal events e events the base of the selandian is proved to coincide with the end of the braarudosphaera acme which correlates with the lowest consistent occurrence lcso of lithoptychius aff bitectus fasciculithus janii sensu steurbaut and sztrakos 2008 event e4 but which is slightly posterior to the second radiation of the fasciculiths up to now considered to represent the primary correlation tool of the dsb a short term 13 c org negative excursion associated with an increase in p co 2 is recorded at the very base of the selandian it is interpreted as a short period of global warming hyperthermal the duration of which is estimated at 30 kyr it is followed in all the three studied sections by a long term decoupled carbon isotope event marked by increasing 13 c org and decreasing 13 c carb values it may reflect a period of climatic cooling of a few 100 kyr interpreted as a possible precursor of the global cooling event marking the late paleocene in the north atlantic realm the integration of the biostratigraphic and the isotope data indicates major differences in sedimentation rates during the early selandian in the studied sections but there is no evidence of substantial breaks in sedimentation in any of the sections during this interval the lithological shift at the base of the selandian points to an abrupt palaeoenvironmental reorganisation although our integrated bio chemostratigraphical investigation does not allow for estimating its duration nor the presence of a hiatus at that time

geochemical study of potential source rocks and crude oils in the anadarko basin oklahoma paleozoic source rocks and crude oils from the anadarko basin oklahoma have been characterized by means of a variety of organic geochemical techniques the potential source rocks investigated include the viola group sylvan shale lower mississippian limestone chester group springer formation and morrow group the woodford shale which is an important source rock in the basin and has been discussed extensively in the literature was used as a laboratory standard in this study the crude oils examined in this study were obtained from reservoir formations of various ages from throughout the basin the source rock screening analysis indicates that the lower and middle subfacies of the viola especially from the southeastern part of the basin have substantial oil generation potential the sylvan is not a source rock although the black shales in the springer and morrow have substantial oil and especially gas generation potential the results of this study support the proposal for multiple sources of oil and gas generation in the anadarko basin selected source rock and oil samples were characterized in more detail and the distributions and concentrations both relative and absolute concentrations of biomarkers in different source rocks and oils were determined the biomarker characteristics were combined with other geochemical and geological data to interpret the sources depositional environments diagenesis and catagenesis processes migration and weathering oil source rock correlations were made between source rocks and a number of crude oils based on quantitative and qualitative biomarker distributions petrographic descriptions and graptolite reflectance measurements were used to evaluate copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received november 1 1995 revised manuscript received april 8 1996 final acceptance september 13 1996 2arco international oil and gas company 2300 w plano parkway plano texas 75057 3school of geology and geophysics university of oklahoma norman oklahoma 73019

stratigraphic correlations using trace elements in apatite from late ordovician sandbian katian k bentonites of eastern north america the early late ordovician sedimentary rocks of eastern north america contain a relatively large number 100 of widespread heavily altered tephra layers k bentonites these beds represent an intense period of subaerial volcanism that occurred from ca 455 to 449 ma the sedimentary rocks that contain these k bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies accurate correlation of these ancient ash fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono sedimentary provinces despite the relatively thorough study of a few of these k bentonites over the past several decades the full stratigraphic potential of these beds has yet to be realized to test the utility of the apatite trace element k bentonite correlation method on a larger scale we studied over 200 k bentonite samples from the mohawkian stage of eastern north america and statistically compared our results with previous studies on the same suites of k bentonites electron microprobe epma and laser ablation inductively coupled plasma mass spectrometry la icp ms results show that apatite trace element data provide unique bed discriminators each of the k bentonite layers exhibits unique and reproducible trends in mg cl mn fe ce y and other trace element concentrations in apatite statistical evaluation of results from our apatite analyses suggests correlations for 12 k bentonite beds providing a significant improvement in stratigraphic resolution the stratigraphic relations indicated by these new k bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic chemostratigraphic and sequence stratigraphic studies in eastern north america

reconstructing late ordovician carbon cycle variations abstract the role of carbon dioxide in regulating climate during the early paleozoic when severe glaciations occurred during a putative greenhouse world remains unclear here we present the first molecular carbon isotope proxy based estimates for late ordovician early katian pco2 levels and explore the limitations of applying this approach to the reconstruction of paleozoic pco2 carbon isotope profiles from three sites in laurentia iowa ontario and pennsylvania and one site in baltica estonia exhibit overall low isotope fractionation between organic and inorganic carbon during photosynthesis ep and these values declined during the early katian carbonate carbon isotope excursion or guttenberg carbon isotope excursion gice algal ep values are sensitive to changes in co2 concentrations algae cell morphologies and cell growth rates to constrain these factors we present molecular evidence that a decrease in the relative abundance of cyanobacteria and a change in the eukaryotic algae community co occurred with the gice regardless of local biotic or oceanographic influences a decline in ep values indicates photosynthesis was sensitive to carbon concentrations and via analogy with modern taxa constrains pco2 to below 8 pre industrial levels pil or about half of previous estimates in addition the global positive carbon isotope excursions expressed in a wide variety of sedimentary materials carbonate bulk organic matter n alkanes acyclic and cyclic isoprenoid hydrocarbons provide compelling evidence for perturbation of the global carbon cycle and this was likely associated with a decrease in pco2 approximately 10 million years prior to the hirnantian glaciations isotopic records from deeper water settings suggest a complex interplay of carbon sources and sinks with pco2 increasing prior to and during the early stages of the gice and then decreasing when organic carbon burial outpaced increased volcanic inputs