sulfide formation in reservoir carbonates of the devonian nisku formation alberta canada an ion microprobe study the processes affecting sulfur during diagenesis in carbonates have been investigated by ion microprobe analysis of 34s values of pyrite marcasite and anhydrite from the devonian nisku formation in the western canada sedimentary basin pyrite and marcasite from three nisku wells have 34s ranging between 35 and 20 cdt most lie between 35 and 5 cdt the 34s values of sulfides are heterogeneous within individual thin section sized samples varying by as much as 25 the 34s values increase by up to 50 with increasing well depth in two different nisku wells and 34s also increases by up to 15 as grain size increases from 40 to over 100 m in individual samples the appearance of native sulfur is accompanied by an increase of up to 30 in pyrite 34s values bulk and ion probe analyses of anhydrite are relatively uniform 34s 22 to 30 cdt the predominately low 34s values of nisku sulfides indicate bacterial sulfate reduction textural relations indicate that fe sulfide formation in nisku carbonates may have occurred by two different mechanisms much of the sulfide has 34s values that suggest that it was associated with bacterial sulfate reduction although most fe sulfides did not form until after pervasive matrix dolomitization depths of 300 1000 m other sulfide may have formed later during deep 4 km burial via thermochemical sulfate reduction the range in 34s values in a single thin section and correlations between pyrite morphology and isotopic values suggest that sulfate reduction was a very localized process and that the sulfate reduction environment varied considerably on a small scale

evidence for episodic cementation and diagenetic recording of seismic pumping events north coles levee california u s a abstract bands of calcite and dolomite cements alternating with zones of nearly carbonate free sand occur in the stevens sandston aat north coles levee san joaquin valley california temperatures calculated from o isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section burial analysis suggests that these cements precipitated from 7 ma to the present over the temperature range of 45 to 95 c carbon isotope data suggest that the c in the cements is a mixture derived from two sources detrital shell material 13c pdb and co2 liberated from maturing kerogen 13c 24 plots of 13c vs time and depth of crystallization show that the cementation sequence was 1 dolomite cements possibly concretionary precipitated at depths a log of 13c vs calculated depth of crystallization correlates with the stratigraphic column at north coles levee if the correlation is valid the light 13 in each cement sample can be tied to its source a model based on this interpretation suggests that the early light c was derived from maturing kerogen in the kreyenhagen formation eocene as it passed through the oil window between 4 and 5 ma the subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light co2 which was reflected in the relatively heavier c isotopes in the later cements it is suggested that the epidsodic injections of hot water carried dissolved gases and minerals principally calcite upward from rocks as deep as 2 3 km below the stevens sandstone and reprecipitated the calcite in more permeable zones in the rock degassing of co2 from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement the sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation

geochemistry characteristics and genetic types of natural gas in central part of the tarim basin nw china abstract according to the chemical carbon isotopic and hydrogen isotopic compositions and 3 he 4 he ratios of 127 natural gas samples the natural gas in the tarim basin is mainly composed of gaseous alkanes dominated by methane the contents of the gaseous alkanes decrease with the increase of their carbon numbers the 13 c values of ch 4 c 2 h 6 and c 3 h 8 in the platform area of the tarim basin vary in the range of 54 4 24 4 an average value of 39 2 43 1 26 7 an average value of 36 1 and 37 9 21 6 an average value of 31 8 respectively the overall carbon isotopic composition of the alkanes follows the trend of 13 c 1 13 c 2 13 c 3 13 c 4 and in some cases varies with 13 c 1 13 c 2 or 13 c 3 13 c 4 the lower carbon isotope values of methane 13 c 1 13 c 2 may be attributed to thermal sulfate reduction tsr and the higher propane value 13 c 3 13 c 4 was caused by the mixing of the natural gases of the same genetic type formed at different stages the methane hydrogen isotopes 2 h c 1 of the natural gases in the tarim basin are in the range of 195 122 with an average value of 156 the 3 he 4 he ratios of the natural gases are in the range of 2 3 10 8 66 8 10 8 suggesting that there is no deep mantle derived gas in the platform area of the tarim basin the carbon isotope fractionation of alkane gases 13 c 2 13 c 3 1 c 2 vs lnc 2 c 3 and c 2 c 3 vs 13 c 2 13 c 3 of the natural gases indicate that the oil type gas in the tarim basin has different predominant formation processes including kerogen cracking oil cracking oil and gas cracking secondary gas cracking and mixing of the kerogen cracking and oil cracking gases the gas evolved from gas cracking gas to oil and gas cracking gas and oil cracking gas along the direction from the west side of the manjar sag to the central tarim basin the natural gases in some areas of tazhong are the mixtures of the kerogen cracking and oil cracking gases

some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins large volumes of carbon dioxide i e up to 90 v v have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the co2 including mantle degassing metamorphic reactions or magmatic processes the formation of large accumulations of large volumes of co2 in sedimentary basins is discussed with specific reference to published data from the pannonian basin hungary the cooper eromanga basin australia and the south viking graben north sea these basins have occurrences of large volumes i e over 15 v v of isotopically heavy carbon dioxide thought to be of inorganic origin and although they differ somewhat in terms of sedimentary fill all have major structural features close to the gas accumulations the available geochemical data are currently neither sufficient to confirm the specific origin s of the co2 e g mantle derived carbonate metamorphism nor unscramble co2 mixed from two or more sources ongoing studies especially those involving noble gas analyses should provide fuller answers

petroleum source in the tazhong uplift tarim basin new insights from geochemical and fluid inclusion data abstract a total of 108 drill stem test dst crude oil samples and 10 reservoir fluid inclusion samples were investigated geochemically to identify the characteristics and origin of the crude oil in the tazhong uplift tarim basin nw china results show that the majority of oils share typical biomarker characteristics from the middle upper ordovician o2 3 source rock and related crude oil features these features include a distinct v shape in the relative abundance of c27 c28 and c29 regular steranes and low abundances of dinosterane 24 norcholestanes triaromatic dinosteroids and gammacerane only a few oils display typical biomarker features indicating a cambrian lower ordovician o1 genetic affinity such as linear or anti l shape c27 c28 and c29 regular sterane distributions and relatively high concentrations of dinosterane 24 norcholestanes triaromatic dinosteroids and gammacerane it appears that most of the oils studied were derived from the o2 3 intervals as suggested by previous studies however the 13c values of individual n alkanes indicate that most of the crude oils in the tazhong uplift represent a mixture of two end member oils an o1 derived oil such as from well td2 or tz62 s and an o2 3 derived oil such as from well ym2 the data suggest that most of the oils in the uplift have a mixed origin and do not originated from the middle upper ordovician strata alone this conclusion is supported by data on the molecular composition of petroleum inclusions this new oil mixing model is critical for reconstructing the hydrocarbon formation and accumulation history for the region and may have important implications for further petroleum exploration in the tarim basin

light hydrocarbons and diamondoids of light oils in deep reservoirs of shuntuoguole low uplift tarim basin implication for the evaluation on thermal maturity secondary alteration and source characteristics abstract the shuntuoguole low uplift in tarim basin is particular because it provides insights into geochemistry of light oils at ultra deep ordovician oil reservoirs 7200 8200 m the light hydrocarbons and diamondoids of 16 oil samples from shunbei no i fault zone f1 and v fault zone f5 were analyzed to investigate their thermal maturity source characteristics and secondary alteration the oils show high similarity in the characteristics of light hydrocarbons and diamondoids the light hydrocarbons are characterized by high contents of alkanes and low contents of cycloalkanes and aromatics and the diamondoids show a domination of adamantanes and their alkylated isomers compared diamantanes thermal maturity indicators derived from diamondoids and c5 c7 light hydrocarbons indicate that the oils are highly mature and the corresponding equivalent vitrinite reflectance rc determined by light hydrocarbons and methyldiadamantane index are in the ranges of 1 2 1 5 signatures of biodegradation and thermochemical sulfate reduction were not observed in the oils the geochemical information from the light hydrocarbons and distribution patterns of molar concentration of n alkanes indicate that the oils from f1 experienced noticeable evaporative fractionation and the oils from f5 did not this was supposed to be the primarily reason for the physical difference between oils from the both fault zones the parameters derived from the light hydrocarbons and diamondoids indicate the oils are from same source rocks which are associated with marine materials with type ii kerogen such as algae and bacteria and both of cambrian lower ordovician o1 and middle upper ordovician o2 3 source rocks in the deep lower palaeozoic strata contributed to the oils

chemical and isotopic evidence for secondary alteration of natural gases in the hetianhe field bachu uplift of the tarim basin h2s and co2 are found in elevated concentrations in the reservoirs near the carboniferous ordovician unconformity in the hetianhe field of the tarim basin nw china chemical and isotopic analyses have been performed on produced gases formation waters and reservoir rocks to determine the origin of co2 and h2s and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids it is unlikely that h2s and co2 had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source instead h2s and co2 are probably the result of sulphate reduction of the light hydrocarbon gases lhg increasing h2s concentrations and co2 co2 c1 4 values to the west of the hetianhe field occur commensurately with increasingly heavy hydrocarbon gas 13c values however thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low no h2s or rare pyrite was detected in deeper reservoirs where more tsr should have occurred and inferred 34s values of h2s from late stage pyrite in the carboniferous and ordovician reservoirs are as low as 24 9 such low 34s values discount the decomposition of organic matter as a major source of h2s and co2 bacterial sulphate reduction of the light hydrocarbon gases in the reservoir possibly coupled indirectly with the consumption of organic acids and anions is most likely the result is the preferential oxidation of 12c rich alkanes due to the kinetic isotope effect and decreasing concentration of organic acids and anions modern formation water stable isotope data reveal that it is possible that sulphate reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion related unconformity this may account for the apparently stronger influence of bacterial sulphate reduction to the west of the hetianhe field and the consequent greatest decrease of the 13c co2 values and the greatest increase in 13c values of the alkane gases

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

late paleozoic depositional history of the tarim basin northwest china an integration of biostratigraphic and lithostratigraphic constraints this study provides the first detailed lithostratigraphic and biostratigraphic constraints for improving stratigraphic resolution for hydrocarbon prospecting and exploration in the tarim basin a total of 49 stratigraphic units 38 formations and 11 members ranging in age from the latest devonian to permian are reviewed or redefined in terms of nomenclatures lithology age constraints and lateral distributions based on the detailed field works or newly published data of these the piqiang formation new formation is proposed to include the reefal carbonates of asselian sakmarian age from the northern tarim the subsurface upper paleozoic stratigraphic framework of the desert areas of the basin is also established for the first time the high resolution basinwide stratigraphic correlations reveal that the sedimentation of the basin in the late paleozoic was extremely uneven of these the famennian to changhsingian successions are completely recorded in the southwestern margin areas of the basin here five eustatic sedimentary cycles are well recognizable suggesting the sedimentation was more eustatically controlled and little affected by local tectonism the late paleozoic successions of both kalpin and taklimakan regions are commonly interrupted by major hiatuses at various horizons suggesting that the sedimentation was apparently modified by local tectonism of these the northward movement of the tarim block and its subsequent collision with the yili microcontinent part of the kazakhstan plate may be principally accountable for the discrepancy in the sedimentation of the various regions in the basin in the late paleozoic

sulfur and carbon isotopic compositions of the permian to triassic tsr and non tsr altered solid bitumen and its parent source rock in ne sichuan basin abstract reservoir solid bitumens from the permian to triassic and potential source rocks in the northeastern sichuan basin were measured for elemental and stable carbon 13c and sulfur 34s isotopic compositions to determine the chemical and isotopic composition of non tsr and tsr altered solid bitumen and to develop proxies to reflect the extent of bitumen alteration by tsr non tsr altered reservoir bitumen samples showed s c atomic ratios 0 03 and had 13c values closer to longtan formation p3l kerogen than to the lower silurian and cambrian kerogens and 34s values slightly more enriched than p3l kerogens the tsr altered solid bitumens have incorporated significant amounts of tsr derived h2s resulting in s c atomic ratios 0 047 0 074 and 34s values 13 4 24 0 being significantly higher than those of the solid bitumen not altered by tsr s c 0 013 0 030 34s 5 8 9 6 and the potential source rock kerogen the incorporation has produced a decrease in n c ratio and negative shift of 2 7 in 13c values that resulted from increasing tsr h2s back reactions with 12c rich n alkanes from the gas phase soluble solid bitumen or paleo oil to generate organic sulfur compounds such as ethanethiol that were incorporated into the solid bitumens which is followed by further aromatization and polymerization the solid bitumens show positive relationships of s c ratios and 34s values to the associated gas h2s h2s c1 6 or tsr extents and thus the s c ratios and 34s values can be used as proxies to reflect tsr extents

sulfur cycle in buried evaporites sulfur isotopes are potent indicators of the way in which sulfur behaves chemically during diagenesis we have studied sulfur isotope ratios 34s 32s from a number of minerals and compounds across the permian triassic boundary in the khuff formation abu dhabi the 34s in dissolved marine sulfate increased by 10 from the late permian to the early triassic despite precipitation of gypsum from permian and triassic seawater and its subsequent dehydration to anhydrite at depths greater than about 1000 m the primary marine stratigraphic sulfur isotope variation has been preserved in anhydrite in the khuff formation a combination of biostratigraphic and absolute age data show that this 10 shift occurred over 2 m y gypsum dehydration to anhydrite has not involved significant isotopic fractionation or diagenetic redistribution of material in the subsurface the sulfur isotope variation across the permian triassic boundary is also present in elemental sulfur and h2s at depths greater than 4300 m formed by reaction of anhydrite with hydrocarbons via thermochemical sulfate reduction this demonstrates that sulfate reduction has not led to isotope fractionation it also demonstrates that significant mass transfer has not occurred at least in the vicinity of the permian triassic boundary even though elemental sulfur and h2s are both fluid phases at depths greater than 4300 m thus despite two major diagenetic processes that converted the sulfur in gypsum into elemental sulfur and h2s by 4300 m burial and the potentially mobile nature of some of the reaction products the primary differences in sulfur isotopes have been preserved in the rocks and fluids all reactions occurred in situ there was no significant sulfur isotope fractionation and only negligible sulfur was added subtracted or moved internally within the system

the fate of mantle derived carbon in a continental sedimentary basin integration of c he relationships and stable isotope signatures abstract the isotopic composition and abundances of the rare gases he ne and ar and active gases co2 ch4 have been determined in a series of commercial gas reservoirs in the pannonian and vienna basins of hungary and austria respectively in these zones of continental extension significant components of mantle derived 4he up to 39 8 and 21ne up to 58 are identified the results of this study indicate a major component of mantle derived carbon in these systems as well ranging in composition from close to 100 co2 to ch4 dominated reservoirs with trace concentrations ppm of co2 these gas reservoirs provide a unique opportunity to examine the relationship of the conservative rare gases to the active gas components and to examine the sources and sinks for mantle and crustal derived carbon phases with the exception of kismarja gas field which shows evidence of addition of 3he depleted crustal co2 all gas fields exhibit a trend whereby the most co2 rich samples approach co 2 3 he mntl values similar to morb 2 109 to 7 109 while co2 depleted samples extend to co 2 3 he mntl values as low as 105 the lack of any correlation of co 2 3 he mntl ratios with r ra values indicates the observed trends are not a function of mixing between crustal and mantle derived endmembers but instead reflect progressive loss of the mantle derived co2 carrier phase during volatile transport and emplacement in the continental crust based on stable isotopic signatures from the kismarja field a crustal co2 endmember with an isotopic composition of 6 8 and a mantle co2 endmember with an isotopic composition no more depleted in 13c than 5 0 can be identified one of the few instances where the isotopic signatures of mantle and crustal derived co2 can be reliably distinguished in a continental setting covariation in 13cco2 and 13cch4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent co2 and co 2 3 he mntl ratios can be accounted for by 1 loss of the mantle derived co2 carrier phase 2 addition of crustal derived co2 3 addition of crustal derived thermogenic ch4 and potentially 4 conversion of the co2mntl carrier phase to mantle derived ch4 based on an estimated total mantle 4he flux for the pannonian basin of 4 2 108 atoms m 2 s 1 mantle carbon flux estimates for this basin alone range from 3 108 g c yr to 1 109 g c yr which over the lifetime of the basin is only 4 5 orders of magnitude less than flux estimates based on the total area of the spreading ridges clearly the addition of mantle derived carbon to the crust in areas of continental extension is significant and may have been previously underestimated we demonstrate here that integration of 13c data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings

origin and migration of brines from paleozoic strata in central tarim china constraints from 87sr 86sr δd δ18o and water chemistry chemistry of major and minor elements 87sr 86sr d and 18o of oilfield waters and 87sr 86sr of whole rock were measured from paleozoic strata in the central tarim basin nw china the aim is to elucidate the origin and migration of formation water and its relation to petroleum migration high salinity oilfield waters in carboniferous silurian and ordovician reservoirs have maintained the same na cl ratio as seawater indicative of subaerially evaporated seawater two possible sources of evaporitic water are carboniferous cii and cambrian both of which contain evaporitic sediments geographic and stratigraphic trends in water chemistry suggest that most of the high salinity water is from the cambrian strontium h and o isotopes as well as ion chemistry indicate at least 3 end member waters in the basin high salinity cambrian evaporitic water was expelled upward into ordovician silurian and carboniferous reservoirs along faults and fractures during compaction and burial meteoric water has likely invaded the section throughout its history as uplift created subaerial unconformities meteoric water certainly infiltrated silurian and older strata during development of the ciii unconformity and again in recent times modern meteoric water enters carboniferous strata from the west and flows eastward mixing with the high salinity cambrian water and to a lesser degree with paleometeoric water the third end member is highly radiogenic shale derived water which has migrated eastward from the awati depression to the west enrichment of ca and sr and depletion of k mg and so4 relative to the seawater evaporation trajectory suggest waters were affected by albitization of feldspars dolomitization illitization of smectite and so4 reduction the mixing of meteoric water occurred subsequently to seawater evaporation main water rock interactions and brine migration the direction of brine migration is consistent with that of petroleum migration suggesting water and petroleum have followed the same migration pathways

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

strontium boron oxygen and hydrogen isotope geochemistry of brines from basal strata of the gulf coast sedimentary basin usa abstract significant spatial heterogeneities exist in the stable isotopic composition of saline formation waters from reservoirs of the smackover formation upper jurassic we focused on the southwest arkansas shelf a structurally simple portion of one of the interior basins of the northern gulf coast sedimentary basin here faulting and facies changes juxtapose dominantly oolitic carbonate strata against basal evaporites red beds and siliciclastics as well as metamorphosed basement rocks brines from this area have exceptionally high br and alkali element concentrations and have spatially heterogeneous hydrogen sulfide concentrations strontium boron oxygen and hydrogen isotope compositions exhibit coherent relations with other aspects of brine geochemistry sr isotope compositions range from those expected for carbonates and evaporites deposited from jurassic seawater 0 7071 to radiogenic ratios as high as 0 7107 generally most radiogenic sr isotope values are associated with h 2 s rich waters which also have elevated alkali element li b k rb concentrations these alkali element rich waters are associated with portions of the south arkansas fault system which reach basement boron isotope compositions are similarly heterogeneous ranging from values of 26 to 50 brines with highest b contents are most depleted in 11 b consistent with boron input from brines generated from high temperature siliciclastic diagenetic reactions normalizing b contents to br in the brines reveals a reasonable mixing trend between a dead sea type composition and texas gulf coast type shale sand reservoir waters oxygen and hydrogen isotope data exhibit regional variations which are controlled by meteoric water invasion along the northern limb of the southwest arkansas fault which has surface expression although oxygen isotope compositions are often near equilibrium with respect to reservoir carbonate it is more difficult to ascribe trends in d values to local water rock interaction the stable isotope trends can be explained either in terms of an evolved marine evaporite brine or a water evolved via water rock interaction and mixing with meteoric water in the vicinity of the southwest arkansas fault spatial heterogeneity in isotopic composition of formation waters from this relatively restricted study area suggests that assumptions of a homogeneous sr reservoir in dating regional mineralization events require careful assessment in ancient systems

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

genetic origin of sour gas condensates in the paleozoic dolomite reservoirs of the tazhong uplift tarim basin abstract twenty gas condensates six oils and their associated formation water from the tazhong uplift in the tarim basin nw china were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates thermochemical sulfate reduction tsr is the main secondary alteration process relevant to the generation of sour gas condensates those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations the hydrogen sulfide h 2 s concentrations in gas phase show a strong positive correlation to gas oil ratio organosulfur compound osc concentrations in the liquid phase while methane isotopic values are mainly controlled by thermal maturity wet gas components show unusual 13 c enrichment at a low degree of thermal cracking geochemical anomalies occur mainly in the lower ordovician yingshan fm where more h 2 s has been discovered while the upper ordovician lianglitage fm shows less irregularity and h 2 s concentration is much lower generation of sour gas condensate is most likely caused by tsr alteration which could oxidize liquid hydrocarbons and form a large quantity of oscs isotopic enrichment of 34 s in formation water sulfate and elevated osc concentrations provide supplemental evidence of tsr alteration both burial history reconstructions based on fluid inclusion analysis and pvt simulation indicate that in situ tsr reactions mainly occurred around last 10 ma during the late himalayan orogeny and overall tsr extent is relatively low in the tazhong uplift

integrated geological geophysical characterizations of deeply buried fractured vuggy carbonate reservoirs in ordovician strata tarim basin abstract the central uplift region of the tarim basin contains a series of thick ordovician carbonates that form fractured vuggy hydrocarbon rich reservoirs along an unconformity these fractured vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale geometry and spatial distribution in this paper a series of deeply buried fractured vuggy carbonate reservoirs are characterized on micro macro and mega scales these characterizations are based on previous studies detailing the carbonate sedimentation diagenesis and tectonic processes in the study area and abundant geological and geophysical data including core samples thin sections and well logging and seismic datasets which have been obtained and upscaled using the choosing classification evaluation generalizing upscaling ccegu method in this area different facies can be identified at different scales 1 detailed core and thin section observations performed based on the dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro scale 0 01 m 2 logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro scale 0 1 m 1 m and the presence of electro facies in individual wells can be recognized and 3 these 9 different electro facies comprise 6 distinct seismic facies at the mega scale 10 m and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties the targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near ne trending strike slip faults this study combines geological and geophysical response data from fractured vuggy carbonate reservoirs to achieve seamless connections among micro scale macro scale and mega scale observations to effectively characterize a series of strongly heterogeneous fractured vuggy reservoirs this method can also be applied to characterize other deeply buried paleokarst reservoirs in the tarim basin as well as other strongly heterogeneous reservoirs

fluid rock reactions in an evaporitic melange permian haselgebirge austrian alps tectonically isolated blocks of carbonate rocks present within the anhydritic haselgebirge melange of the northern calcareous alps record a complex history of deformation and associated deep burial diagenetic to very low grade metamorphic reactions fluids were hot up to 250 c and reducing brines charged with carbon dioxide individual carbonate outcrops within the melange record different regimes of brine rock reactions ranging from pervasive dolomite recrystallization to dedolomitization early diagenetic features in these carbonates were almost entirely obliterated matrix dolomite alteration was related to thermochemical sulphate reduction tsr recognized by the replacement of anhydrite by calcite pyrite native sulphur pyrite associated with tsr is coarsely crystalline and characterized by a small sulphur isotope fractionation relative to the precursor permian anhydrite carbonates associated with tsr show low fe mn ratios reflecting rapid reaction of ferrous iron during sulphide precipitation as a result tsr related dolomite and calcite typically show bright mn ii activated cathodoluminescence in contrast to the dull cathodoluminescence of many ferroan carbonate cements in other deep burial settings in addition to carbonates and sulphides silicates formed closely related to tsr including quartz k feldspar albite and k mica 40ar 39ar analysis of authigenic k feldspar yielded mostly disturbed step heating spectra which suggest variable cooling through the argon retention interval for microcline during the late jurassic this timing coincides with the recently recognized subduction and closure of the meliata hallstatt ocean to the south of the northern calcareous alps and strongly suggests that the observed deep burial fluid rock reactions were related to jurassic deformation and melange formation of these permian evaporites

carbon dioxide in clastic rocks and silicate hydrolysis the amount of carbon dioxide in clastic rocks in sedimentary basins increases with depth organic matter dissolved organic species and dissolution of carbonate minerals have been suggested as sources of co sub 2 which increases in abundance with depth and temperature isotopic compositions of aqueous hco sub 3 sup minus gaseous co sub 2 and calcite from conventional hydrocarbon wells and steam assisted recovery of heavy oil suggest that calcite or other carbonate minerals are the source of co sub 2 particularly at higher temperatures detailed examination of the stability of diagenetic minerals such as kaolinite smectite and analcime among others relative to the coexisting waters indicates that silicate hydrolysis is the driving force for dissolution of carbonate minerals and the ultimate source of abundant co sub 2 in clastic rocks in diagenetic environments at temperatures over approximately 100 c

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

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

plutonium 238 observations as a test of modeled transport and surface deposition of meteoric smoke particles 1 there are large uncertainties in the transport and surface deposition of upper atmospheric particles used to construct climate proxies here we use a 3 d chemistry climate model ccm to simulate the transport and deposition of plutonium 238 oxide nanoparticles formed after the ablation of a power unit in the upper stratosphere 11 s in 1964 the model reproduces both the observed hemispheric asymmetry and time scale of pu 238 deposition we then use the ccm to investigate the transport of meteoric smoke particles msps from the upper mesosphere the strongest msp deposition is predicted to occur at middle latitudes providing a significant source of fe fertilization to the southern ocean the model also predicts substantially more deposition in greenland than in antarctica by a factor of 15 in agreement with ice core measurements showing that climate proxy measurements from a limited number of sites must be interpreted with care

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried

geochemistry and accumulation of carbon dioxide gases in china we propose the following identification indicators for co2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples the co2 carbon isotope values of 390 samples and the helium isotope composition of 256 samples collected from wells of 16 oil and gas bearing basins and gas seepages in china and reviewing literature on organic and inorganic co2 throughout the world co2 is of organic origin if co2 content 13cco22 is of inorganic origin if dgr13cco2 8 pmil or co2 60 and co2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if 10 pmil 13cco213cco2 values of organic co2 mainly range from 10 to 22 pmil in tectonically stable oil and gas bearing basins where deep faults and magmatism are undeveloped organic co2 occurs almost exclusively in tectonically active oil and gas bearing basins where active faults and magmatism are present or in areas of modern volcanic activity the dgr13cco2 values of inorganic co2 range from 2 to 8 pmil and are distributed along faults the inorganic co2 with a dgr13cco2 value of 0 3 pmil is of metamorphic origin and that with dgr13cco2 values of 6 2 pmil is mantle derived and of magmatic origin in china the dgr13cco2 values of inorganic co2 gas pools range from 3 41 to 8 44 pmil and the helium ratio r ra is 1 which indicates that these gases are typically derived from the mantle

biogenic and petroleum related ore forming processes in dongsheng uranium deposit nw china abstract sandstone hosted roll type u deposits has recently been found in the middle jurassic zhiluo formation in the dongsheng area north of the ordos basin in order to assess the origin of the uranium mineralization the host sandstone has been investigated with emphasis on 34s values of ore stage pyrite 13c of calcite fluid inclusion oil biomarkers x ray diffraction icp ms as well as studies by electron microprobe and scanning electron microscope the mineralization consists mainly of coffinite some of the coffinite is intimately intergrown with ore stage pyrite indicating co precipitation ore stage pyrite has 34s values from 34 to 17 suggesting the pyrite sulfur originated from bacterial sulfate reduction in the host sandstone calcite cement exhibits 13c values from 27 6 to 2 1 and fluid inclusion oils show evidence of biodegradation these lines of evidence indicate petroleum was likely oxidized by sulfate reducing bacteria srb and thus supplied a source of some of the carbon in the calcite microbial degradation of petroleum is partially supported by the low temperature diagenetic environment

biomarker geochemistry of crude oils and lower paleozoic source rocks in the tarim basin western china an oil source rock correlation study abstract in order to select biomarker parameters that have potential to quantitatively distinguish between the cambrian lower ordovician and upper ordovician source rocks in the tarim basin with a total organic carbon content 0 5 42 lower paleozoic shale samples were collected from a number of wells and outcrops in the tarim basin northwestern china the biomarkers in the extracts of the source rocks were analyzed by gas chromatography mass spectrometry the results show that c23 and c30 were dominant in the tricyclic terpanes and pentacyclic triterpanes respectively and c27 and c29 were both dominant in c27 29 steranes of the source rock samples the distribution of n alkanes isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded and they originated from a reducing depositional environment with considerable marine algal input in comparatively upper ordovician source rock samples presented lower values of c28 c29 regular steranes and higher values of c29 20s 20s 20r and c29 abnormally lower values of c29 20s 20s 20r and c29 were observed in the cambrian lower ordovician strata the set of data in this study show that homohopane index including c31homohopane c30hopane c31 35homohopanes c30hopane and the ratios related to the triaromatic steroids including c26 20s c28 20s tas c27 20r c28 20r c26 20r c27 20s c28 20s and c28 c26 c27 tas could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the tarim basin the values of those parameters in above order in cambrian lower ordovician source rocks were 0 3 0 5 0 85 and 0 8 1 9 1 2 respectively the correlation between the source rock samples and 19 oils obtained from the tadong tabei and tazhong uplifts in the tarim basin show that most of the oils in tabei uplift are probably from upper ordovician source rocks the oils in tadong uplift are more likely from cambrian lower ordovician source rocks and the tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

diagenetic alteration of permian strata at oil fields of south central oklahoma usa abstract many oil fields in south central oklahoma including cement velma carter knox and eola have a chimney of altered rock within slightly deformed permian red beds the chimneys occur above and within the oil reservoirs where the chimneys crop out the altered rock is commonly well exposed the alteration consists chiefly of removal of haematite from sand grains cementation of sandstone by pyrite marcasite calcite and dolomite and at the cement field replacement of a bed of gypsum by calcite the total volume of these late diagenetic minerals is vast many hundreds of millions of tons the diagenetic alteration of red beds occurred when crude oil and abundant natural gas migrated vertically from major anticlines in pennsylvanian and older strata into the gently folded gypsum bearing permian red beds during cementation the rush springs sandstone in the cement field had a high porosity 40 50 which suggests that here alteration occurred before substantial burial in the red beds anaerobic bacteria mediated the oxidation of ch 4 and possibly alkanes within crude oil to co 2 and simultaneously reduction of so 4 4 anions to h 2 s the cementation resulted when carbonate species reacted with ca 2 and mg 2 and the bleaching resulted when sulphide species reacted with fe 3 and fe 2 the evidence for the bacterially mediated alteration is the anomalously low isotopic values for carbon and sulphur carbonate minerals have 13 c values as low as 39 2 0 and most values are less than 15 0 o sulphide minerals have 32 s values as low as 37 9 and most values are less than 8 5 0

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

the influence of rock fabric and mineralogy on thermochemical sulfate reduction khuff formation abu dhabi abstract thermochemical sulfate reduction tsr is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce h2s and calcite in this study of the dolomite hosted hydrocarbon gas reservoirs in the permo triassic khuff formation abu dhabi a geochemically well constrained rock gas system we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of tsr reactions and thus on h2s concentration in the gas phase the controls on the rate on h2s accumulation were tsr reaction kinetics tsr became significant as temperature exceeded 140 c a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane anhydrite dissolution rate once initiated the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide i e suggesting total reaction for each unit of sulfate that dissolves also finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite likely a result of relative surface area suggesting that anhydrite dissolution was the rate limiting step transport rates are unlikely to have been rate limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals transport rate later as tsr proceeded it became transport controlled as calcite growing on the surface of anhydrite crystals began to isolate them from dissolved methane tsr ceased once calcite had effectively armor plated totally isolated the remaining anhydrite finely crystalline anhydrite underwent more extensive and more rapid tsr than coarser anhydrite crystals because these had a greater ratio of surface area to volume allowing more and faster dissolution and requiring more calcite to isolate them from methane localized loss of h2s occurred in the reservoir by reaction with indigenous fe bearing clays consequently reservoirs with a relatively high siliciclastic content have less h2s than would be expected from the advanced state of anhydrite replacement by calcite in order to predict tsr related h2s concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum emplacement history

the isotopic compositions of molecular nitrogen implications on their origins in natural gas accumulations abstract the isotopic compositions of n2 15 n atm imply the geochemical origins of molecular nitrogen in natural gas accumulations but the irregular variation of 15 n n 2 for most petroliferous basins puzzles scientists we believe that this is due to multiple origins of n2 which are often mixed together and leads to the irregularities in gas pools the four petroliferous basins the west siberian basins in russia the yinggehai basin of china the californian great valley basin of usa and the mid european basin have significantly different isotope characteristics of n2 which may provide insight into the isotope geochemical implications of n2 in natural gas accumulations n2 with 19 15 n n 2 10 may be from sedimentary organic matter at the immature and the early mature stage ro 0 6 values of 10 15 n n 2 2 may indicate that n2 originates from mature including high mature sedimentary organic matter ro 0 6 2 0 values of 2 15 n n 2 15 n n 2 0 and n2 ar 38 84 suggest that the atmosphere is the origin values of 1 15 n n 2 15 n n 2 4 is the feature of n2 from saltpeter in evaporites values of 4 15 n n 2 18 indicate that n2 maybe derived from post mature sedimentary organic matter ro 2 0

methane dominated thermochemical sulphate reduction in the triassic feixianguan formation east sichuan basin china towards prediction of fatal h2s concentrations abstract new sour pools have recently found in the lower triassic feixianguan fm carbonate reservoirs in the east sichuan basin in china with h 2 s up to 17 4 by volume a recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of h 2 s in order to assess the origin of fatal h 2 s as well as the cause of petroleum alteration h 2 s concentrations and the isotopes 34 s and 13 c have been collected and measured in gas samples from reservoirs anhydrite pyrite and elemental sulphur 34 s values have been measured for comparison the high concentrations of h 2 s gas are found to occur at depths 3000 m temperature now at 100 c in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs where h 2 s concentrations are greater than 10 its 34 s values lie between 12 0 and 13 2 cdt this is within the range of anhydrite 34 s values found within the feixianguan fm 11 0 to 21 7 average 15 5 3 5 cdt thus h 2 s must have been generated by thermochemical sulphate reduction tsr locally within the reservoirs burial history analysis and fluid inclusion data reveal that the temperature at which tsr occurred was greater than about 130 140 c suggesting that the present depth temperature minimum is an artifact of post tsr uplift both methane and ethane were actively involved in tsr since the petroleum became almost totally dry no alkanes except methane and methane 13 c values become significantly heavier as tsr proceeded methane 13 c difference thus reflects the extent of tsr while it is tempting to use a present day depth control 3000 m to predict the distribution of h 2 s in the feixianguan fm this is an invalid approach since tsr occurred when the formation was buried some 1000 2000 m deeper than it is at present the likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the feixianguan fm so that it is possible to determine which parts of the basin have been hotter than 130 140 c

a benthic foraminiferal proxy of pulsed organic matter paleofluxes in the temperate open ocean of the modern northeast atlantic the spring bloom of phytoplankton leads to a seasonal pulse of detrital organic material phytodetritus to the ocean floor opportunistic benthic foraminifera rapidly colonise this food resource producing large numbers of individuals whose tests are ultimately added to the sediment one of these taxa epistominella exigua shows periodic peaks in abundance in the fossil record at many open ocean sites previously such peaks have been commonly interpreted to result from changes in physicochemical properties of bottom water mass and thus deep sea circulation the main purpose of this paper is to propose thate exigua may be used as a proxy of pulsed organic matter inputs to the deep ocean and therefore as an indicator of relative changes in productivity

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

burial diagenesis and thermochemical sulfate reduction smackover formation southeastern mississippi salt basin the smackover formation in the southeastern mississippi salt basin has been buried from 3 03 to 6 06 km and subjected to temperatures of 100 to 200c pre hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite these phases formed at temperatures of less than 100c the oxygen isotopic composition of calcite ranges from minus 5 to minus 8 4 pdb and carbon isotopic composition ranges from 3 5 to 6 0 pdb the sulfur isotopic composition of late anhydrites ranges from 18 1 to 19 1 cdt post hydrocarbon migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur and precipitation of post bitumen calcite cement these diagenetic phases formed at temperatures above 150c and are present only in the deep part of the basin the oxygen isotopic compositions of calcites that replace anhydrite range from minus 3 0 to minus 4 0 pdb and carbon isotopic compositions range from minus 2 0 to 4 2 pdb the oxygen isotopic compositions of post bitumen calcites range from minus 3 4 to minus 5 6 pdb and carbon isotopic compositions range from minus 1 6 to minus 16 3 pdb two samples of elemental sulfur have sulfur isotopic compositions of 15 8 and 16 1 cdt the high temperature replacement of anhydrite by calcite more the presence of post bitumen calcite cement the sulfur isotopic composition of sulfates and elemental sulfur and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the mississippi salt basin at temperatures as low as 150c less

applications of mango s light hydrocarbon parameters to petroleum from tarim basin nw china abstract light hydrocarbons in oils from the tarim basin nw china were analyzed by gc the light hydrocarbon parameters proposed by mango revealed the distributions of the oils as two main types marine and terrigenous sources the mango parameter k1 not only displays a remarkable invariance 1 in most of the oils but also shows significant variations ranging from 1 20 to 1 54 in the oils occurring in the eastern part of the tazhong fault uplift eptfu located in the center of the tarim basin this variation of the k1 value may indicate different petroleum systems in the areas which are superimposed in the eptfu parameter k2 proposed by mango shows a significant variation between the two main oil types the marine oils are characterized by relatively low values of k2 average 0 23 and the terrigenous oils by relatively high k2 values average 0 35 with general invariance within the same oil set a plot of p3 p2 n2 vs n 1 6 based on the model proposed by mango can be used not only to discriminate between the two main genetic oils from the tarim basin but also to classify the marine oils from the lunnan area into two sub types which may indicate two sub petroleum systems existing in the area

the tazhong hybrid petroleum system tarim basin china abstract the tazhong petroleum system is geographically located in the central tarim basin including the tazhong uplift the north slope and the southern area of the manjiaer depression the total thickness of early palaeozoic to cenozoic sediments reaches 6500 9500 m the manjiaer depression and the north slope are the two major petroleum source areas with three sets of source rocks cambrian black shales and ordovician dark carbonates of the manjiaer depression and ordovician argillaceous limestones of the north slope there are three sets of reservoir cap rock combinations in this petroleum system the main periods for oil generation from these source rocks occurred during the middle silurian to early devonian late carboniferous to early triassic and tertiary there were three corresponding phases of petroleum migration well developed thrust faults and unconformity surfaces in this area provided the main pathways for petroleum migration as some of the faults cut through all the lower palaeozoic strata the oil in any given reservoir rock may be actually a mixture of oils from different sources complicating the origin and properties of the hydrocarbons in the reservoir and forming a hybrid petroleum system the formation of a typical oil and gas pool in this petroleum system may be attributed to one or all of three petroleum events which were closely associated with the regional tectonics the first event occurred in the early palaeozoic tectonic cycle and the generated oil accumulated in lower silurian sandstone the second event occurred in the late palaeozoic tectonic cycle and formed light oil pools mainly in lower silurian and ordovician reservoirs the third event took place during the cenozoic tectonic cycle and formed normal crude oils the oil and gas generated from different sources in different phases may co exist with one another in a reservoir rock however the commercial oil and gas pools are related to the cenozoic and late palaeozoic events