the effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs abstract thermochemical sulfate reduction tsr is a well known process that can lead to sour h2s rich petroleum accumulations most studies of tsr have concentrated upon gas chemistry in this study we have investigated palaeoformation water characteristics in a deep anhydrite bearing dolomite sour gas reservoir of permian age in abu dhabi using fluid inclusion stable isotope petrographic and gas chemical data the data show that low salinity isotopically distinct water was generated within the reservoir by reaction between anhydrite and methane the amount of water added to the reservoir from tsr indicated by reduced fluid inclusion salinity and water 18o values varied systematically with the extent of anhydrite reaction with methane water salinity and isotope data show that the original formation water was diluted by between four and five times by water from tsr thus we have shown that large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement the salinity of formation water in evaporite lithologies is therefore not necessarily high modelling based upon a typical khuff gas reservoir rock volume suggests that initial formation water volumes can only be increased by about three times as a result of tsr the extreme local dilution shown by the water salinity and 18o data must therefore reflect transiently imperfect mixing between tsr water and original formation water the creation of large volumes of water has important implications for the mechanism and rate of thermochemical sulphate reduction and the interpretation of gas volumes using petrophysical logging tools

gas condensate migration and oil fractionation in deltaic systems abstract the term evaporative fractionation is proposed to describe several phenomena involved in the secondary alteration of reservoired oil firstly it is suggested that oil is frequently partially vaporized in the reservoir secondly that gas bearing substantial portions of the oil in solution is conducted along faults to form independent gas condensate accumulations thirdly that residual oils formed in this fashion bear internal evidence of fractionation in addition to the conspicuous loss of light ends there is an increase in the content of light aromatic and naphthenic hydrocarbons relative to paraffins in the residual oil evaporative fractionation effects are recognized for the first time in the distillation and physical property data of us bureau of mines routine method oil analyses fractionation effects are postulated to be the cause of sigmoid as opposed to normal curvilinear aromatic profiles the effects of evaporative fractionation are widespread in oils throughout the united states they are evident in approximately 75 of oils in the gulf coast region and in a large proportion of the gas condensate accumulations the process is a major and apparently unrecognized means of petroleum alteration and of generation of gas condensates experimental evidence shows that partial vaporization results in enhanced levels of light aromatic hydrocarbons in residual oils oil samples were partially vaporized in excess methane at elevated pressure and temperature followed by removal of the vapour the gas chromatographic features of many gulf coast oils were reproduced various levels of light end depletion and enhancement of aromaticity were brought about

distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the jinxian sag north china abstract an unusual series of c 22 c 27 monounsaturated sterenes and c 24 c 30 tetracyclic terpanes 17 21 seco hopanes were detected in relatively high concentrations in an immature evaporitic marl sediment of the jinxian sag bohai bay basin north china the site of unsaturation in these novel sterenes is assigned tentatively to the d ring on the basis of mass spectral interpretation which also distinguishes them from reported unsaturated sterenes other hydrocarbon biomarker or stable isotope characteristics are indicative of microbial e g methyl hopanes phytoplankton or higher plant depleted 13 c values of isoprenoids and hopanes inputs and an anoxic carbonate depositional environment hexacyclic hopanes tetracyclic terpanes the hydrocarbon composition showed no obvious biodegradation and the relatively high concentration of unsaturated terpenoids e g gammacerene and low values of other established maturity parameters t s t m 0 23 r o 0 44 t max 417 c are consistent with sediments of low maturity the novel low molecular weight sterenes and the tetracyclic terpanes may be early diagenetic products of microbial sources in a carbonate environment

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

early cretaceous sediment failure in the southwestern sable subbasin offshore nova scotia conventional cores from lower cretaceous outer shelf and upper slope prodelta facies in the alma and glenelg fields of the scotian basin show a wide range of synsedimentary deformation storm dominated prodelta sandstone and mudstone beds have common load casts and structureless sandstone beds overlain by deformed sediment and sandstone dykes that are caused by storm or earthquake induced local liquefaction blocks of sediment 5 15 m 15 50 ft thick principally on the outer shelf have foliated mudstone at their base common internal shear zones in mudstone shear deformation of sandstone and are capped by intraclast conglomerates that are interpreted as debris flow deposits these are all features observed in shallow slides on the modern mississippi prodelta at the paleoshelf edge and upper slope larger slide blocks are recognized with zones of intense internal deformation and high angle thrusts near their base the lack of a basal foliated mudstone may be the result of more rapid slide motion than on the shelf resulting in hydroplaning in addition there is early postdepositional interstratal deformation as a result of loading by sandy delta distributaries and by slide blocks on the upper slope the scale of many deformation structures is below the limits of three dimensional seismic vertical resolution yet they are likely to substantially influence reservoir properties

early generation of 20s 5α h 14α h 17α h and 5α h 14β h 17β h steranes in low salinity lacustrine shales abstract a quantitative sterane biomarker study was conducted on a series of paralic freshwater lacustrine shale samples ranging in maturity from immature to near oil window maturity taken from section 3 of the shahejie formation es3 in the liaohe basin n e china concentrations of 5 h 14 h 17 h 20s and 5 h 14 h 17 h steranes remain nearly constant throughout the sample suite however the decrease in the absolute concentrations of the 20r 5 h 14 h 17 h c 29 steranes with increasing maturity results in an increase in the conventionally defined maturity parameters 20s 20 s 20r and sterane ratios in addition the data suggest that relatively early generation of 5 h 14 h 17 h 20s and 5 h 14 h 17 h steranes has occurred in lacustrine sediments with a vitrinate reflectance 0 3 r o the data provide strong support for the major importance of relative thermal stability of epimers but do not exclude the possibility of isomerization as a viable mechanism for production

mesozoic and cenozoic stratigraphy eastern scotian shelf new interpretations a 320 km long deep seismic reflection line across the central part of the scotian basin provides new insight on the deep stratigraphy of the basin confirms the existence of up to 18 km of mesozoic and cenozoic sedimentary strata and provides additional control on the relationship of proximal and distal stratigraphic units within the basin detailed biostratigraphic and seismic correlation from three outer shelf upper slope wells yields important new information on late cretaceous and tertiary unconformities and identifies a number of third order sequence boundaries the dating of prominent unconformities and correlation with the oceanic seismic markers ac and au results in the reidentification of these horizons in the sohm abyssal plain

a statistic for estimating strain with confidence intervals from deformed line distributions with an application to schists and gneisses of the western gneiss region west central norway abstract it is shown how to calculate the axial ratio of the strain ellipse r s from a deformed set of lines assuming that they were initially uniformly distributed and deformed passively using an exact but simple relationship between r s and the mean resultant length r of the distribution the method applies to distributions of lines modified under a general 2d deformation w k w k is the kinematical vorticity number by using the circular variance for circular data expressions for confidence intervals and confidence levels are calculated for strain modified distributions of lines these expressions indicate that confidence interval widths decrease with increasing r s and more importantly that the method is incapable of accurately estimating low strains the method was applied to 18 gneiss and schist samples from joa west central norway and found to produce realistic strain estimates which are internally consistent and concur with field relationships

tsr origin of sulfur in permian and triassic reservoir bitumen east sichuan basin china solid bitumen h2s pyrite and anhydrite samples from the lower triassic feixianguan formation and upper permian changxing formation and source rock kerogen samples from the cambrian ordovician silurian and permian in the east sichuan basin were analyzed for their sulfur content and d 34 s values to determine if the measurements were consistent with a thermochemical sulfate reduction tsr origin of the produced h2s and solid bitumen the results show that the solid bitumen samples have sulfur contents from 2 6 to 12 5 wt and d 34 s values from 13 4 to 22 1 the d 34 s values are significantly heavier than those of potential upper permian source rock kerogens that average 1 3 n 3 suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy h2s of tsr origin from 12 0 to 13 7 interestingly two gases produced from gas to water transition zones during drill stem testing dst show significantly lighter d

evaluating the sulfur isotopic composition of biodegraded petroleum the case of the western canada sedimentary basin abstract the sulfur isotopic composition 34s of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis however secondary processes could affect the original 34s of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs in this study the different processes that may affect the 34s of in reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the lower cretaceous mannville group western canada sedimentary basin wcsb based on the molecular study these samples appear to have reached various levels of maturity and biodegradation ranging from 0 to 6 5 on the biodegradation scale of peters and moldowan in addition mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the wcsb investigation of the 34s shows a trend that seems a priori correlated to the level of biodegradation however a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed 34s variations have rather to be ascribed to contributions of oils generated by various source rocks alternatively variations of 34s could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation in the case of some specific core samples showing a common origin based on biomarker study 34s variations might not be related to different sources but to secondary sulfur incorporation exchange processes occurring within the reservoir these processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into mannville reservoirs this hypothesis is supported by laboratory experiments showing sulfur exchange incorporation under plausible conditions for shallow reservoirs

gas genetic type and origin of hydrogen sulfide in the zhongba gas field of the western sichuan basin china abstract natural gases and associated condensate oils from the zhongba gas field in the western sichuan basin china were investigated for gas genetic types and origin of h 2 s by integrating gaseous and light hydrocarbon geochemistry formation water compositions s isotopes 34 s and geological data there are two types of natural gas accumulations in the studied area gases from the third member of the middle triassic leikoupo formation t 2 l 3 are reservoired in a marine carbonate sequence and are characterized by high gas dryness high h 2 s and co 2 contents slightly heavy c isotopic values of ch 4 and widely variable c isotopic values of wet gases they are highly mature thermogenic gases mainly derived from the permian type ii kerogens mixed with a small proportion of the triassic coal type gases gases from the second member of the upper triassic xujiahe formation t 3 x 2 are reservoired in continental sandstones and characterized by low gas dryness free of h 2 s slightly light c isotopic values of ch 4 and heavy and less variable c isotopic values of wet gases they are coal type gases derived from coal in the triassic xujiahe formation the h 2 s from the leikoupo formation is most likely formed by thermochemical so 4 reduction tsr even though other possibilities cannot be fully ruled out the proposed tsr origin of h 2 s is supported by geochemical compositions and geological interpretations the reservoir in the leikoupo formation is dolomite dominated carbonate that contains gypsum and anhydrite petroleum compounds dissolved in water react with aqueous so 4 species which are derived from the dissolution of anhydrite burial history analysis reveals that from the temperature at which tsr occurred it was in the late jurassic to early cretaceous and tsr ceased due to uplift and cooling thereafter tsr alteration is incomplete and mainly occurs in wet gas components as indicated by near constant ch 4 13 c values wide range variations of ethane propane and butane 13 c values and moderately high gas dryness the 34 s values in so 4 elemental s and h 2 s fall within the fractionation scope of tsr derived h 2 s high organo s compound concentrations together with the occurrence of 2 thiaadamantanes in the t 2 l reservoir provide supplementary evidence for tsr related alteration

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

comments by on organic geochemistry 36 1717 1730 abstract in this short communication we reply to the comments of worden and cai worden r h cai c f 2006 geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jinxian sag of bohai bay basin by zhang et al organic geochemistry 36 1717 1730 organic geochemistry 37 on the data and interpretation of zhang et al zhang s c zhu g y liang y b dai j x liang h b li m w 2005 geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin organic geochemistry 36 1717 1730 concerning the origin of an eocene sour gas and high sulfur oil accumulation from the jinxian sag bohai bay basin china we provide clarifications on the current temperature range of sour gas reservoirs and comment on the reliability of 34 s data of various sulfur species used by zhang et al 2005 and cai c f worden r h wolff g a bottrell s wang d l li x 2005 origin of sulfur rich oils and h 2 s in tertiary lacustrine sections of the jinxian sag bohai bay basin china applied geochemistry 20 1427 1444 we conclude that there is no verified evidence in support of bacteria sulfate reduction as the dominant geochemical process that contributed to the sour gases in the eocene reservoirs of the jinxian sag but more geological and sulfur isotope data are still needed to confirm the thermochemical sulfate reduction occurring in deeper parts of the petroleum reservoirs

the west indian road pit central nova scotia key to the early cretaceous chaswood formation the west indian road pit is the only large outcrop in nova scotia of the chaswood formation the terrestrial equivalent of the offshore missisauga and logan canyon formations it provides outcrop information on sedimentology gravel petrology and structures for a formation that is otherwise known from a few small overgrown pits and from boreholes the chaswood formation in the pit is 60 m thick and consists principally of sorted sand and gravel with three thinner clay units successions of sedimentary structures indicate deposition from a coarse bedload river flowing to the east southeast gravel consists principally of vein quartz quartz arenite and subarkose together with minor igneous lithologies that can be matched to sources in the cobequid highlands to the north quartz arenite and subarkose appear derived from carboniferous horton group single crystal 40 ar 39 ar dates of detrital muscovite are a little older than the muscovite ages for the south mountain batholith interpreted to mean that the muscovite is second cycle from the horton group which records the earliest unroofing of the batholith the chaswood formation accumulated during progressive tectonic deformation along nne trending strike slip faults in basement rocks resulting in syn sedimentary faulting and local unconformities sedimentation kept pace with the creation of accommodation unrelated younger deformation folded the chaswood formation at the pit into an east west trending syncline the early cretaceous paleogeography of the maritime provinces is interpreted to have consisted of fault bound horsts shedding coarse detritus surrounded by an interconnected series of basins that accumulated fluvial sands and gravels and overbank muds with well developed paleosols

a new conceptual model for the structural evolution of a regional salt detachment on the northeast scotian margin offshore eastern canada in this study we examine using seismic data in conjunction with numerical modeling a regional scale salt detachment and associated synkinematic sediments from the scotian margin offshore eastern canada this part of the scotian margin is characterized by an up to 4 5 km 2 8 mi thick and approximately 175 km 108 mi long synkinematic wedge of jurassic sediments with internal sigmoidal landward dipping reflectors the synkinematic wedge is laterally extensive encompassing an area of approximately 30 000 km2 11 600 mi2 and soles into an interpreted salt detachment the jurassic synkinematic wedge which is interpreted to have formed as an open ended allochthonous salt nappe was loaded by prograding sediments during the jurassic this loading squeezed the salt seaward and caused the overlying sediments to undergo extension and gravity spreading and gliding detaching on the salt sheet the open ended nappe model provides a mechanism for producing a large amount of extension with very little compensating contraction numerical model results indicate that high rates of extension detaching on even a thin salt layer can result in similar sigmoidal landward dipping strata based on the numerical modeling and seismic interpretation results we propose a new conceptual model for the jurassic paleogene structural evolution of the study area this model may also have implications for other passive margin salt basins with regional salt detachments

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

shape preferred orientation of object populations automatic analysis of digitized images abstract an automatic technique is presented for systematic shape preferred orientation spo analysis of crystal fabrics in rocks this technique is based on digitization of an image with a ccd camera and separation of an object population from its matrix the image is analysed globally using a rotating grid that is superimposed on the square pixel grid of the computer screen the parametric method is based on counting intercepts in all directions the interceps rose provides information on both the orientation and strength of the spo problems of hardware related counting noise anisotropy are avoided using an original method of filtering the intercept function with a linear sliding mask on the counting grid the technique was tested both on single objects of different shapes and on different populations of identical objects a very high degree of accuracy was obtained for orientation measurements fabric shape parameters are introduced which are specific to this intercept technique they compare favourably with shape parameters used in axial fabric analysis

unique chemical and isotopic characteristics and origins of natural gases in the paleozoic marine formations in the sichuan basin sw china isotope fractionation of deep and high mature carbonate reservoir gases abstract large quantities of natural gases have recently been discovered in the marine carbonate formations in the sichuan basin southern china the origins and isotopic compositions of the gases are poorly understood and highly controversial we examined the chemical and isotopic compositions of the carbonate reservoir gases in marine formations from the sinian to lower triassic as well as shale gas from the silurian and cambrian formation in the sichuan basin it is found that the carbon isotopes of the carbonate reservoir gases had been evolved through three stages corresponding to pre rollover rollover and post rollover in the plot of 13c2 versus wetness which is consistent with that found in shale gas by tilley and muehlenbachs 2013 the unique carbon isotopic compositions indicate that both kerogen and oil cracking gas have contributed to the accumulation of the carbonate reservoir gases in the sichuan basin a mixing of two thermogenic gases with different maturities can cause the 13c reversal between methane and ethane which is commonly observed in the carbonate reservoir gases in the carboniferous permian and triassic formations and the shale gas in the silurian and cambrian formations in the sichuan basin we established two mixing models for quantitatively determining the contribution of each type gas for the natural gases in different formations kinetic extrapolation based on thermal history of the anyue gas field demonstrates that ethane cracking indeed occurred in the deep reservoir formations which may explain the disappearance of the 13c reversal for methane and ethane in the carbonate reservoir gases in the post rollover zone it is proposed that organic inorganic interactions may be responsible for the presence of d isotope rollover for methane in carbonate reservoir gases with ultra high maturity the organic inorganic interactions include thermal reactions between water and residual organic matters or carbons and hydrogen isotope exchange between water and methane and their mechanism needs further investigations in future

the effect of sample size on geological strain estimation from passively deformed clastic sedimentary rocks abstract the role of sample size in the estimation of geological strain both finite strain rs and that of the orientation of the finite strain ellipse s is investigated for clastic sedimentary rocks this study looks at four strain methods the robin method the linearization method the mulchrone and meere method and the mean radial length method that are initially tested using simulated strained data sets and subsequently by applying the methods to real data it is found that the optimum strain analysis sample size for a clastic sedimentary rock is primarily dependant on the intensity of strain suffered by that rock because of the error behavior associated with rs estimates an iterative process is therefore recommended starting with a minimum sample size of 150 which can be maintained or reduced based on the initial rs estimates

the complexity secondary geochemical process genetic mechanism and distribution prediction of deep marine oil and gas in the tarim basin china abstract most paleozoic marine craton basins in china are rich in natural gas except the tarim basin where both oil and gas are prolific various hydrocarbon types have been found in this basin including deep ultra heavy oil heavy oil normal oil volatile oil condensate and natural gas petroleums with diverse types even coexist within a single reservoir generally the ordovician and cambrian reservoirs are deeply buried 5500 8000 m and intensively altered after initial oil and gas accumulations bringing challenges to physical property prediction before drilling and evaluation of exploration targets based on recent exploration discoveries and experimental data we systematically analyzed the origin of oil and gas demonstrated the mechanisms and processes of secondary geochemical alteration e g biodegradation gas washing fractionation tsr and high temperature cracking moreover the preservation mechanism of ultra deep normal oil was also clarified and the distribution patterns of oil and gas were predicted tarim basin has been uplifted and shallowly buried in the early accumulation stage then rapidly and deeply buried in the late stage resulting in various geochemical alteration processes at different burial stages through the reconstruction of complex deep strata oil and gas accumulation histories the spatial distribution of deep petroleum types were predicted this review has provided detailed assessment with case studies in the tarim basin and shed light on the preservation mechanisms and the alteration processes of deep liquid petroleums in addition to massive gas accumulations liquid oils still have great exploration potential in strata deeper than 8000 m in the tarim basin

origin of natural gas from the ordovician paleo weathering crust and gas filling model in jingbian gas field ordos basin china abstract the ordos basin the second largest natural gas producing basin in china is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation the origin and potential source of the jingbian gas field one of the five giant gas fields in the basin containing the ordovician paleo weathering crust as reservoir strata remains unclear the question is whether gas is generated from carboniferous permian coal measures or ordovician carbonates this study provides abundant gas geochemical evidence which clarifies its origin and source among 82 natural gas samples collected from the ordovician paleo weathering crust in the jingbian gas field c1 c4 gaseous hydrocarbons are dominant with low amounts of non hydrocarbons co2 h2s and n2 the content of c1 c4 ranges from 79 26 to 99 93 with a gas dryness c1 c1 4 of 0 981 0 999 suggesting that natural gases are dry and possibly originate from a high to over mature source the carbon isotopic compositions of methane and its homologues in natural gases from the ordovician paleo weathering crust vary widely with 13c1 13c2 and 13c3 values of 38 9 to 29 1 37 5 to 22 2 and 30 0 to 20 9 respectively the 13c2 and 13c3 values in the jingbian gas field are much more negative than those of typical coal type gas in sulige wushenqi yulin daniudi and kela2 gas fields the carbon isotopic signature suggests that the natural gas in the ordovician paleo weathering crust in the jingbian gas field most likely originates from ordovician carbonates containing sapropelic organic matter the measured d values of methane range from 177 to 155 and support our hypothesis of the natural gases originating from marine sapropelic organic matter the thermal maturity of the natural gases in the ordovician paleo weathering crust is at a high to over mature stage suggesting that the gases originated from secondary cracking of oil and gas at high temperature and pressure thermochemical sulfate reduction tsr might partially play a role in the alteration of gas dryness and 13c1 in those areas where gypsum is present as an effective cap rock mixing of high to over mature secondary cracking gas from the ordovician source and coal type gas from carboniferous permian coal in the jingbian gas field is minor

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

geochemical identification of clastic sediment provenance from known sources of similar geology the cretaceous scotian basin canada abstract this study tests the effectiveness of a geochemical approach in identifying provenance in a basin where different sources do not show strongly contrasting geology petrological studies indicate that at least three distinct rivers draining reactivated horsts of the appalachian orogen supplied sediment to the lower cretaceous deltaic sandstones and mudrocks in the offshore scotian basin ninety five samples mostly from conventional core were analyzed for 44 major and trace elements the data were first screened for variability unrelated to provenance including changes in elemental abundance due to weathering and diagenesis and the effects of grain size and sorting on element variation the effect of hydraulic sorting was distinguished from the effects of concentration of ultrastable heavy minerals from polycyclic sources multivariate statistical analysis by principal component analysis pca was used to test the hypothesis of three discrete sources suggested by the petrologic model major element pca on sandstones discriminated the sources but results from trace element pca required further investigation using element biplots to understand their significance only a few elements were found to be diagnostic of different sources namely k rb sr u th nb and ti with the latter two of value only for sandstones all of these are elements that are abundant in the granites of the appalachians most published geochemical discrimination diagrams did a poor job of distinguishing the three petrographically recognized sources in other basins with quite different hinterland geology other elements are known to be of value in discriminating provenance thus it is unlikely that a globally applicable set of elemental discriminants can identify terrigenous sediment sources rather systematic investigation is needed that evaluates processes such as diagenesis and sorting and then tests geographic and stratigraphic variability in bulk geochemistry informed by at least semiquantitative petrographic data

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

sulfur redox reactions and formation of native sulfur veins during low grade metamorphism of gypsum evaporites cameros basin ne spain vein deposits containing native sulfur gypsum quartz and rare sphalerite are described from cervera del rio alhama in the very low grade metasediments of the mesozoic cameros basin of ne spain the veins are hosted by lacustrine evaporites which comprise alternations of dolomite and gypsum anhydrite during metamorphism layers fluid inclusion homogenization temperatures and quartz sulfate oxygen isotope geothermometry indicate formation of the veins at 225 c fluid inclusions contain s along with a gas phase comprising h2s n2 co2 and minor ch4 these are all likely reactants and products of s generation by thermochemical sulfate reduction tsr by organic matter followed by partial re oxidation of some h2s by so42 to produce s tsr type reactions during low grade metamorphism are thus concluded to be the origin of the s veins the tsr reactions we described differ from those observed in most petroleum related sour gas settings firstly there is no evidence for secondary carbonate precipitation secondly significant s isotopic fractionations exist between sulfate around 20 and reduced products s is around 11 this is attributed to relatively increased rates of isotopic equilibration compared to tsr that may be related to low availability of organic matter during the formation

the effects of strike slip motion along the cobequid chedabucto southwest grand banks fault system on the cretaceous tertiary evolution of atlantic canada the newfoundland fracture zone the southwest grand banks transform and the cobequid chedabucto fault zone form a linked strike slip fault system from the atlantic ocean to southeastern canada this paper suggests that several large scale geological features in southeastern canada are the result of a small amount of strike slip motion on the system during the mid cretaceous and oligocene regional extension features developed in the releasing bend in the laurentian sub basin during the mid cretaceous but the same area experienced oligocene compression this tectonic model accounts for the distribution of mid cretaceous volcanism fault bounded basins and regional unconformities as well as mid to late cretaceous subsidence of the scotian basin and oligocene uplift of the eastern scotian shelf

tsr altered oil with high abundance thiaadamantanes of a deep buried cambrian gas condensate reservoir in tarim basin abstract the first exploratory well the zs1c well with 158 545 m 3 daily gas production was discovered in 6861 6944 m deep strata of the cambrian gypsolyte layer of the tarim basin china in 2014 the discovery opens a new target for the cambrian reservoired oil and gas exploration and directly leads to large scale oil and gas exploration of the deep reservoired cambrian oil and gas fields in the basin comprehensive two dimensional gas chromatography time of flight mass spectrometry and a comprehensive two dimensional gas chromatography flame ionization detector revealed the presence of abundant adamantane compounds 2 thiaadamantanes and 2 thiadiamantanes and a large amount of sulfur containing compounds in the condensate oil the formation of organic sulfur containing compounds such as 2 thiaadamantanes is an indication of sulfur incorporation from the gypsum in the stratum into oil and gas in the course of tsr this reservoir has apparently suffered severe tsr alteration because 1 high content of h 2 s 2 h 2 s sulfur isotopes 3 co 2 carbon isotopes and others abundant data to support this findings similar sulfur isotopic composition of h 2 s oil condensate and the gypsum in the cambrian strata indicate that the produced condensate is experienced tsr alteration therefore the deep accumulated cambrian oil reservoir has experienced severe tsr alteration and accumulated natural gas and condensate contains high sulfur content

microfabric reconstruction via quantitative digital petrographic image analysis for weakly foliated gneisses abstract detection and documentation of petro structural features such as alignment features of minerals grains and extraction of such spatial property data are two fundamental steps for structural geology such tasks were mostly carried out manually however manual analysis is laborious and potentially biased these drawbacks are less obvious when the foliation is well developed and the amount of platy mineral is higher for samples with weakly developed foliation and low platy mineral content automatic method is required for subjective interpretation a semi automatic computerized method of 3d foliation orientation reconstruction via two dimensional petrographic shape fabric analysis from serial oriented digital microphotograph has been developed and demonstrated in this study the foliation is reconstructed by fitting a best fitting plane to the maximum modal peaks of micro textural parameters spo for different mineral groups of platy and granular minerals from each thin section on a stereonet for four coarse grained biotite gneiss samples collected along the jialie fault se tibet china regardless of platy or granular mineral aggregates the reconstructed foliations showed similar orientation within 10 angular variation to the field measurement the 10 angular variation can be maintained if the foliations are reconstructed by consecutive thin section groupings 50 angular intervals and a horizontal thin section the angular spread increased to 30 for thin section groupings with 50 to 100 angular intervals with a horizontal thin section major advantages of the computerized photometric methods demonstrated by this study are the reduction of human prejudice and obtaining quantified and repeatable data sets

non cracked oil in ultra deep high temperature reservoirs in the tarim basin china abstract thermal stability of liquid petroleum in the subsurface is closely linked to reservoir temperature most oil accumulations occur at temperatures 150 160 c leading to the dominance of gas condensate and free gas accumulations in ultra deep high temperature reservoirs the recently drilled fuyuan 1 exploration well northern tarim basin produced high quality non cracked single phase black oil from a carbonate reservoir located at maximum depth 7711 m and temperature 172 c this is the deepest oil discovery in china to date and among the deepest in the world the oil density 0 825 g cm3 at 20 c or api gravity 40 relatively low gas oil ratio 135 m3 m3 or 758 scf bbl low variety and abundance of adamantanes as well as lack of thiaadamantanes and dibenzothiophenes indicate that the oil was expelled from a source rock at moderate thermal maturity and has not been cracked the molecular and isotopic composition of oil associated gases are consistent with this interpretation we suggest that the oil remained uncracked because the residence time at temperatures 150 160 c was relatively short

controls on regional variability in marine pore water diagenesis below the seafloor in upper jurassic lower cretaceous prodeltaic sandstone and shales scotian basin eastern canada abstract diagenesis in the uppermost jurassic to lower cretaceous deltaic sandstones and shales of the scotian basin is an important control on reservoir quality ferruginous zone sub oxic marine pore water diagenesis controls the initial formation of fe2 silicates that are the precursors of grain rimming chlorite that preserves porosity this study assesses the regional controls on the type of marine pore water diagenesis by studying the sedimentology mineralogy and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin coated grains preserve a record of whether marine pore water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone four types of coated grain were distinguished each with a different mineral paragenesis mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite and the correlation of p with fe or ti pyrite and fe calcite are found where the sulphidic zone is more significant than the ferruginous zone ferruginous zone diagenesis was common in low sedimentation rate retrogradational sediments with low organic carbon and in delta front turbidites and river mouth sandstones estuarine tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower fe ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products the relative contribution of colloidal iron hydr oxides appears greater in distal low sedimentation rate environments where large changes in sedimentation rate occurred at ravinement surfaces the underlying progradational rocks have evidence of ferruginous zone diagenesis whatever their facies rapid upward migration of the pore water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore water zones this study has shown that the availability of fe and organic carbon varying in a complex manner in marine deltaic sediments but that the resulting diagenesis by marine pore water can be predicted from facies and paleogeographic setting

non rotation of garnet porphyroblasts and 3 d inclusion trail data an example from the imjingang belt south korea abstract we describe non rotation of garnet porphyroblasts in non coaxially deformed metapelites of the imjingang orogenic belt in the middle korean peninsula the three dimensional geometry of inclusion trails within 1 2 mm garnet porphyroblasts was analyzed by serial polishing and reflected light microscopy most of the garnet porphyroblasts have straight inclusion trails of quartz and some plagioclase s i oblique to an external foliation s e serial polishing of two samples normal to mineral elongation lineation a plane and normal to s e and parallel to the lineation b plane with twenty intervals of about 0 1 mm reveals that the straight s i is a planar structure measurement of the angles between s i and s e on both planes from samples of sixteen outcrops shows that s i orientation of individual garnets is fairly constant in each sample the attitude of s i in geographic space determined by using its pitch angles on the a and b planes is more or less constant throughout the area over 6 5 km traversing normal to the strike of regional foliation the sense of asymmetry of strain shadows at the margin of garnet porphyroblasts and the s i s e relationship indicate rotation of s e rather than garnet porphyroblasts with respect to the geographic reference frame rotation of s e is also supported by a sense of shear inferred from other shear criteria all indicating a top up to the south shearing

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

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

formation mechanisms of secondary hydrocarbon pools in the triassic reservoirs in the northern tarim basin abstract the northern tarim basin tabei is a rich composite hydrocarbon accumulation region with multi layers of oil and gas reservoirs in the ordovician carboniferous and triassic strata in the triassic the tabei region was dominated by non marine sedimentation that developed deltaic sandstone reservoirs with favourable petrophysical properties for hydrocarbon accumulations hydrocarbons in the triassic hydrocarbon pools are dominated by oil and condensate gas analysis of the hydrocarbons in combination with well testing and drilling data reveals that oils in the triassic hydrocarbon pools are commonly biodegraded and originated mainly from the middle upper ordovician source rocks the predominant dry gases in the reservoirs are cracked from oils derived from the cambrian source rocks the origin of hydrocarbons in the triassic pools is similar to those in the underlying reservoirs fluid inclusions and k ar dating confirm that the hydrocarbons were accumulated during the himalayan orogenic movement because the middle upper ordovician source rocks already experienced peak hydrocarbon generation in the late hercynian orogeny late permian the triassic hydrocarbon pools are from secondary adjustment of earlier oil and gas pools from the underlying ordovician and carboniferous reservoirs due to the rapid subsidence of the kuqa foreland basin to the north of the tabei region since the beginning of the himalayan orogenic movement the triassic reservoir strata was changed from south dipping to north dipping resulting in the formation of plunging anticlines and hydrocarbon migration toward the south at the same time fault reactivation occurred in the area causing the hydrocarbons from the deeper reservoirs to migrate into the triassic structural traps through faults and unconformities and thus formed the widespread secondary hydrocarbon pools the structure inversion during the triassic and the formation of nose structures since the himalayan orogeny 250 290 ma permian controlled the secondary accumulation in the triassic reservoirs the reformation process of the hydrocarbon pools continued until the late himalayan orogeny when the gas was charged and developed condensate pools in the eastern area

detrital quartz and quartz cement in upper triassic reservoir sandstones of the sichuan basin characteristics and mechanisms of formation based on cathodoluminescence and electron backscatter diffraction analysis abstract the relationship between detrital quartz grains and silica cement is significant because cement is often a major cause of porosity reduction in oil and gas reservoirs thus degrading hydrocarbon reservoir quality on the other hand the attributes of grain surfaces may inhibit cementation thus preserving porosity in the reservoirs in this study we focus on the microstructures in quartz grains and secondary overgrowths quartz cement in the heavily cemented sandstones from uplifted upper triassic siliciclastic rocks in the southern sichuan basin china electron backscatter diffraction ebsd optical and cathodoluminescence cl imaging methods have been used to address the microstructural and crystallographic attributes of the detrital grains and their overgrowths this study confirms that 1 the overgrowth zones of quartz are clearly visible in optical and cl images 2 all silica overgrowths are perfectly crystalline quartz 3 quartz cements have a syntaxial relationship with the host detrital grain indicating crystallographic continuity between the detrital quartz and its overgrowth and 4 crystallographic maps confirm that dauphine twins commonly occur both in the detrital grains and in the quartz cement most dauphine twins are developed at grain grain contacts including the margins of detrital grains where they are in contact with quartz cement some twins are limited to a single grain whereas others cross grain cement boundaries based on these twin distributions we can distinguish inherited twins from twins that formed during after burial compaction this study documents the grain deformation structures induced by sedimentary compaction and these structures can be accurately illustrated by ebsd data

applications of petroleum geochemistry to exploration and reservoir management abstract petroleum geochemistry improves exploration efficiency by accounting for many of the variables that control the volumes of crude oil and natural gas available for entrapment including source rock distribution richness and quality thermal maturity and the timing of generation migration accumulation relative to trap formation it is most powerful when used with other disciplines such as seismic sequence stratigraphy and reservoir characterization four key technology milestones form the basis for most modern applications of geochemistry to exploration these are the concepts and applications of 1 petroleum systems and exploration risk 2 biomarkers stable isotopes and multivariate statistics for genetic oil oil and oil source rock correlation 3 calibrated three dimensional thermal and fluid flow modeling and 4 controls on petroleum composition by secondary processes petroleum geochemistry offers rapid low cost evaluation tools to aid in understanding development and production problems some technology milestones in reservoir geochemistry include 1 assessment of vertical and lateral fluid continuity 2 determination of proportions of commingled production from multiple zones and leaky casing 3 prediction of oil quality in reservoir zones and 4 prediction of gas oil and oil water contact locations as described in the conclusions future research will continue a trend toward predictive geochemistry examples of predictive tools that draw major research support include piston core surveys to assess deepwater petroleum systems prior to drilling and three dimensional basin modeling to predict the regional timing of generation migration and accumulation of petroleum among other research objectives models are needed to better predict the distribution and quality of petroleum in reservoirs

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

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

formation and preservation of a giant petroleum accumulation in superdeep carbonate reservoirs in the southern halahatang oil field area tarim basin china giant petroleum accumulations worldwide with burial depths more than 7000 m 23 000 ft occur mostly in mesozoic and cenozoic reservoirs and yield predominantly natural gas recently however a giant oil accumulation with reservoir depths between 7000 m 23 000 ft and 8000 m 26 000 ft was discovered in the lower paleozoic section in the southern part of the halahatang region in the tarim basin china petroleum sourced from lower paleozoic rocks is contained in ordovician karst fracture cave reservoirs and sealed by middle upper ordovician limestones and mudstones the newly discovered superdeep accumulation is among the deepest black single phase oil accumulations worldwide and opens up new avenues for petroleum exploration in deep marine carbonate reservoirs reservoir pressures are between 75 mpa 10 878 psi and 85 mpa 12 328 psi with pressure coefficients between 1 2 and 1 7 and temperatures ranging between 140 c 284 f and 172 c 342 f charging and accumulation of petroleum occurred during the late hercynian orogeny followed by subsequent gradual deep burial which took place before rapid subsidence beginning circa 5 ma following subsidence the thickness of overlying strata increased by more than 2000 m 6600 ft before finally attaining current depth therefore this oil accumulation represents a well preserved ancient petroleum system based on the geochemical features of oils and gases the crude oils can be classified as mature sourced from mixed marine organofacies of shale marl and carbonate whereas the gases were cogenerated with oils despite very high present day reservoir temperatures no oil cracking has occurred because of the relatively short exposure of oils to high temperatures in a low geothermal gradient regime thus there is significant exploration potential under similar conditions for liquid petroleum in superdeep strata faults and reservoirs are major factors controlling petroleum accumulation interlayer karsts with excellent fracture cavity connectivity developed adjacent to faults generally resulting in the enrichment of oil and gas along fault zones high quality reservoirs in this area are easy to identify because they exhibit strong bead like amplitude features in seismic sections wells located near faults produce relatively large amounts of oil and gas effective karst fracture cave reservoirs with noncracked oil may exist below 8000 m 26 000 ft in the tarim basin and represent a significant exploration target in china

significance of dextral reactivation of an e w transfer fault in the formation of the pennsylvania orocline central appalachians 1 three hypotheses for orocline development including 1 primary plate boundary shape 2 arc rotation and 3 arc overprinting were evaluated for the formation of the pennsylvania orocline maryland usa using foliation intersection inflection axes preserved within porphyroblasts fia the distribution and the timing of sequences of fia data measured from garnet porphyroblasts within the loch raven formation are statistically the same across the pennsylvania orocline suggesting that all subregions of the arc have experienced the same deformation history this may suggest that the geometry of the paleocontinental margin controlled the basic shape of this orocline however if the orocline is treated as three separate regions defined by the sw we ne trending portions of its slightly staircase shaped outline a much higher proportion of the nne ssw trending fias set ii are preserved in the w e trending region than those to either side this geometric relationship could have resulted from wnw ese directed bulk stress causing a zone of dextral shear along what is now the w e trending portion of this orogen it appears that what could have been an early formed w e trending sinistral transform shear zone was preferentially dextrally reactivated during the taconian orogeny

geochemistry and origin of natural gas in the eastern junggar basin nw china abstract the eastern junggar basin in nw china is an important region for natural gas exploration with the giant kelameili gas field having been discovered however the origin of gas in the region is unclear which limits the understanding of gas formation and accumulation as well as the planning of exploration strategies here we address the origin of gas in this region through a comprehensive investigation of gas geochemistry in the context of geological setting including gas chemical compositions carbon isotopes and light hydrocarbons and biomarkers of retrograde condensates results show that four main types of natural gases were identified 1 highly mature to post mature humic type gas sourced from carboniferous rocks with type iii kerogen 2 mature to highly mature humic type gas sourced from permian rocks with type iii kerogen 3 mature to highly mature sapropelic type gas sourced from permian rocks with type i ii kerogen and 4 microbial gas sourced from permian rocks with type ii iii kerogen these gases vary significantly in geochemistry and thus can be easily identified the distribution of the gases follows the source controlled principle in that it is controlled by the distribution and maturity of hydrocarbon source rocks as such the gases sourced from the carboniferous and permian rocks are the most viable exploration targets with the areas in and surrounding the hydrocarbon generation centers being favorable for exploration these results also provide guidance to natural gas exploration and research in other basins with similar geological settings

tidal signatures in a shelf margin delta based on its anomalous thickness 150 m and stratigraphic position above continental slope mudstone an upward coarsening succession consisting in part of tidal rhythmites in the glenelg field offshore nova scotia canada is interpreted to be a strongly tide influenced shelf margin delta deposit a large funnel shaped erosional shelf edge invagination is observed where the paleoshelf edge is resolved in three dimensional seismic data adjacent to glenelg we propose that the delta at glenelg prograded into a similar shelf edge invagination within which tidal currents were amplified and wave energy was attenuated given that funnel shaped invaginations e g slope canyon heads slump scars fluvially incised knickmarks are relatively common along modern shelf edges and that fluvio deltaic systems should be focused into these topographic lows during regression across the shelf it seems likely that shelf edge invaginations play an important but underappreciated role in mediating terrigenous clastic sedimentation during sea level lowstands

dating and characterizing primary gas accumulation in precambrian dolomite reservoirs central sichuan basin china insights from pyrobitumen re os and dolomite u pb geochronology abstract primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the precambrian dengying formation of the gaoshiti moxi block central sichuan basin china in this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the dengying dolomite reservoirs using laser ablation inductively coupled plasma mass spectrometry la icp ms in situ u pb dating we obtained a high precision absolute age of 259 4 3 0 ma for the saddle dolomite cementation consistent in timing with the eruption of emeishan flood basalts 262 251 ma combined with high homogenization temperatures th and salinities of primary fluid inclusion as well as rare earth element ree compositions we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the emeishan mantle plume the identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo oil reservoirs these bitumens are characterized by high equivalent vitrinite reflectance requ no fluorescence and deformed fibrous textures typical of highly mature pyrobitumens gas geochemistry data indicates that only minor thermochemical sulfate reduction tsr has occurred thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo oils and its associated re os isochron age of 154 21 ma representing the timing of dry gas formation petrographical evidence suggests that the paleo oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation this combined with coeval aqueous inclusion th data and published thermal burial models constrains the time of paleo oil emplacement to ca 222 205 ma suggesting an oil filling event related to the indosinian orogeny this study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world

giant gas discovery in the precambrian deeply buried reservoirs in the sichuan basin china implications for gas exploration in old cratonic basins abstract an estimated trillions of cubic meters of natural gas have recently been discovered in the sinian dolomite reservoirs in the sichuan basin china the 20 26 m thick reservoirs were found in deep structural lithologic traps of 5000 5500 m deep the sinian gas is thought to have been derived mainly from the sinian source rocks and partly from the cambrian source rocks the gas is thermally matured with a dryness ratio c1 c1 4 of 0 997 0 9998 c2h6 content

origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in dodan field se turkey abstract gas occurrences consisting of carbon dioxide co 2 hydrogen sulfide h 2 s and hydrocarbon hc gases and oil within the dodan field in southeastern turkey are located in cretaceous carbonate reservoir rocks in the garzan and mardin formations the aim of this study was to determine gas composition and to define the origin of gases in dodan field for this purpose gas samples were analyzed for their molecular and isotopic composition the isotopic composition of co 2 with values of 1 5 and 2 8 suggested abiogenic origin from limestone 34 s values of h 2 s ranged from 11 9 to 13 4 h 2 s is most likely formed from thermochemical sulfate reduction tsr and bacterial sulfate reduction bsr within the bakuk formation the bakuk formation is composed of a dolomite dominated carbonate succession also containing anhydrite tsr may occur within an evaporitic environment at temperatures of approximately 120 145 c basin modeling revealed that these temperatures were reached within the bakuk formation at 10 ma furthermore sulfate reducing bacteria were found in oil water phase samples from dodan field as a result the h 2 s in dodan field can be considered to have formed by bsr and tsr as indicated by their isotopic composition hc gases are of thermogenic origin and were generated within the upper permian kas and gomaniibrik formations as indicated by the heavier isotopic composition of methane and ethane hc gases were later altered by tsr based on our results the dodan gas field may have formed as a result of the interaction of the following processes during the last 7 8 ma 1 thermogenic gas generation in permian source rocks 2 the formation of thrust faults 3 the lateral up dip migration of hc gases due to thrust faults from the kas formation into the bakuk formation 4 the formation of h 2 s and co 2 by tsr within the bakuk formation 5 the vertical migration of gases into reservoirs through the thrust fault and 6 lateral up dip migration within reservoir rocks toward the dodan structure

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

identification of c24 and c25 lanostanes in tertiary sulfur rich crude oils from the jinxian sag bohai bay basin northern china abstract unusual short chain lanostanes c 24 and c 25 and c 30 lanostane were identified in sulfur rich crude oils from the jinxian sag bohai bay basin northern china besides the regular steranes c 27 30 a series of 4 methyl steranes c 22 23 c 27 30 4 4 dimethyl steranes c 22 24 c 28 30 short chain steranes c 23 26 abundant pregnanes c 21 22 and androstanes c 19 20 together with sulfur containing steroids 20 thienylpregnanes and thienylandrostanes were detected in the aliphatic and branched cyclic hydrocarbon fraction of these crude oils a literature survey of some long chain sterane analogues e g a nor steranes norcholestanes c 30 steranes lanostanes and pregnanes seems to point to a sponge and or dinoflagellate source 4 methyl 4 4 dimethyl steroids and lanosterols 4 4 14 trimethyl steroids as the basic skeleton of lanostanes can be derived from methanotrophic bacteria thus a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes c 23 26 4 methyl c 22 23 and 4 4 dimethyl steranes c 22 24 as well as lanostanes c 24 25 and c 30 analogues in our oil samples generally the steroids appear to have been extensively sulfurized with sulfur substitution at the c 22 position in the side chain during the early stage of diagenesis which was readily subject to attack by bacterial degradation enzymatic cleavage and or abiotic oxidation as a consequence short chain sterane analogues e g abundant pregnanes and androstanes in this study and short chain lanostanes c 24 c 25 might later be released through cleavage of weak c s bonds at the c 22 carbon in the sulfurized steroids and lanostane sulfides finally the formation of the short chain c 24 c 25 lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir

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

statistical analysis of forward stratigraphic models in complex salt provinces the central scotian basin case study predicting vertical and lateral facies variations in various depositional environments remains a major challenge in the oil and gas industry because it impacts petroleum system assessments and the associated exploration risking phase the use of multidisciplinary constraints geomorphology geology geophysics in forward stratigraphic models sheds light on the complex interaction of local regional and global driving mechanisms that influence sediment transport and deposition along continuously evolving landscapes in this paper we develop an integrated statistical approach to examine the sensitivity of forward stratigraphic models in complex salt provinces to several parameters including water discharge sedimentary load grain size and associated diffusion coefficients and slope this statistical analysis was applied to the barremian albian sequence of the central scotian basin canada and highlights the influence of complex salt kinematics on sediment pathway diversion and accumulation around salt domes and canopies forward stratigraphic modeling results point to regions of higher probability of lower cretaceous sandy reservoirs automating simulation runs significantly reduced the time required to achieve a statistically valid number of simulations and allowed the sensitivity of the model to be evaluated

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

mixed siliciclastic carbonate systems and their impact for the development of deep water turbidites in continental margins a case study from the late jurassic to early cretaceous shelburne subbasin in offshore nova scotia progradation of deltaic systems that reach the shelf edge is considered a primary mechanism to deliver sand to continental margins sediment bypass processes can dominate the outer shelf to upper slope transition causing poor preservation of reservoir quality sandstones turbidites can carry the bulk of the coarse fraction downdip where a thicker sand pile can be deposited once the channel to lobe transition is reached predicting this lateral and vertical variability along the slope is challenging this configuration can be further complicated when mixed siliciclastic carbonate systems are present the roseway missisauga case study from nova scotia is used here to explore potential implications associated with the development of deep water turbidites that are time equivalent to outer shelf mixed siliciclastic carbonate units two scenarios are possible 1 the carbonate factory is dominant and the development of carbonate reefs and pinnacles on the outer shelf prevents the passage of siliciclastic systems beyond the shelf break in this case the siliciclastic component is sequestered within the inner outer shelf 2 favorable conditions for carbonate production gradually deteriorate by the activation of fluviodeltaic systems that prograde outboard reaching the slope region in this last scenario low relief and lateral discontinuous carbonate shoals are ubiquitous in the outer shelf representing the last outboard remnants of the carbonate factory shelf edge deltas circumvent or breach these carbonate shoals establishing sedimentary pathways on the shelf break region that connect with deep water turbidites observations suggest that this last scenario is the most likely in this part of the scotian margin during the late jurassic to early cretaceous

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

rock structure characterization of a magnetite gneiss with foliation parallel discontinuities for footwall slope design abstract the warramboo magnetite iron ore deposit in south australia is predominantly hosted by gneiss design of the footwall slope requires an assessment of the role of foliation parallel discontinuities as planes of weakness parallel to the slope a review of published studies of joint spacing and joint persistence in gneiss shows spacing and persistence of foliation parallel discontinuities relative to non foliation parallel discontinuities varies greatly foliation parallel discontinuities form a spectrum between simply being one joint set in a blocky rock mass through to being a significant anisotropic weakness based on detailed analysis of the characteristics and frequency of foliation in drill core in the warramboo magnetite deposit it is inferred based on analysis of spacing that foliation parallel discontinuities are no more frequent or persistent than other joints therefore the rock mass is blocky rather than anisotropic and footwall slopes steeper than the foliation dip are viable

stable carbon isotopes of alkane gases from the xujiahe coal measures and implication for gas source correlation in the sichuan basin sw china abstract the upper triassic xujiahe formation in the sichuan basin sw china consists of a series of coal measures the first third and fifth members of this formation are dominated by gas prone dark mudstones and coals the mudstones contain type ii and iii kerogens with average organic carbon contents around 1 96 these source rocks are mature in the central sichuan and highly mature in the western sichuan basin characterized by gas generation with subordinate amounts of light oil or condensate oils the source rocks are intercalated with the sandstone dominated second fourth and sixth members of the xujiahe formation thus leading to three separate self contained petroleum systems in the region the proven gas reserves in the xujiahe formation are only less than that of the triassic feixianguan formation and the xujiahe formation has the second largest gas field guang an gas field in the basin gases derived from the xujiahe formation coals generally show a normal stable carbon isotopic trend for c1 c4 n alkanes with the highest 13c2 values among the nine gas pay zones in the basin 20 7 to 28 3 and 13c1 values as low as 43 0 in the central sichuan gas accumulations with an oil leg have also been found in the eastern and southern sichuan where the thickness of the xujiahe formation is significantly reduced gases in these accumulations tend to show low 13c2 values 30 0 to 36 3 characteristic of oil prone source rocks

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

natural gas constituent and carbon isotopic composition in petroliferous basins china abstract there are abundant gas resources in petroliferous basins of china large to midsize gas fields are found in eastern central and western of china however origin constituents and isotopic composition of natural gas in different gas fields are varied distinctly and some present strong chemical secondary alteration and show variation both in age and space based on the systematic analysis of constituents and carbon isotope of a large number of gas samples combined with the geological characteristics this paper classifies the origins of the gases explores the gas isotope characteristics and evolutionary regulation with the variation time and space and further discusses the distinctive geochemistry of the gases in china these gases are dominated by dry gas its methane carbon isotope values range from 10 to 70 ethane from 16 to 52 propane from 13 to 43 and butane from 18 to 34 the carbon isotopes of most gases show the characteristics of humic derived gas and crude oil cracked gas in addition large primary biogenic gas fields have been discovered in the qaidam basin inorganic derived alkane gases have been discovered in deep of the songliao basin half of these gas fields are characterized by the alkane carbon isotope reversal in different degrees research indicates there are several reasons can result in carbon isotope reversal firstly gas charge of different genetic types or different source in one gas reservoir may cause carbon isotope reversal besides high over mature evolution of gas can also lead to the carbon isotopic reversal of alkanes thirdly secondary alteration of hydrocarbons may also result in abnormal distribution of carbon isotope isotope transforms to unusual light and heavy

quantitative analysis of crystal grain sizes and their distributions in 2d and 3d abstract we review methods to estimate the average crystal grain size and the crystal grain size distribution in solid rocks average grain sizes often provide the base for stress estimates or rheological calculations requiring the quantification of grain sizes in a rock s microstructure the primary data for grain size data are either 1d i e line intercept methods 2d area analysis or 3d e g computed tomography serial sectioning these data have been used for different data treatments over the years whereas several studies assume a certain probability function e g logarithm square root to calculate statistical parameters as the mean median mode or the skewness of a crystal size distribution the finally calculated average grain sizes have to be compatible between the different grain size estimation approaches in order to be properly applied for example in paleo piezometers or grain size sensitive flow laws such compatibility is tested for different data treatments using one and two dimensional measurements we propose an empirical conversion matrix for different datasets these conversion factors provide the option to make different datasets compatible with each other although the primary calculations were obtained in different ways in order to present an average grain size we propose to use the area weighted and volume weighted mean in the case of unimodal grain size distributions respectively for 2d and 3d measurements the shape of the crystal size distribution is important for studies of nucleation and growth of minerals the shape of the crystal size distribution of garnet populations is compared between different 2d and 3d measurements which are serial sectioning and computed tomography the comparison of different direct measured 3d data stereological data and direct presented 2d data show the problems of the quality of the smallest grain sizes and the overestimation of small grain sizes in stereological tools depending on the type of csd

geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin abstract natural gas from the zhaolanzhuang field of the jizhong superdepression bohai bay basin contains the highest proportions of h 2 s 40 92 among the sour gases encountered in china the gas payzones include the eocene oligocene kongdian formation ek and the es4 member of the shahejie formation the sedimentary sequence consists of halite anhydrite carbonate sandstone and shale interbeds deposited in the evaporative brackish water lacustrine salt lake setting in the deepest part of the jinxian sag the total thickness of evaporites is more than 1000 m of which halite accounts for over 40 various organic rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window with a depth of 2500 3500 m and likely the source for the oil and sour gas in the zhaolanzhuang field the temperatures of the gas reservoirs range from 75 to 100 c too low for significant thermochemical sulfate reduction the co occurrence of abundant elemental sulfur with the sour gas and the 34 s values of the various sulfur containing compounds indicate that the h 2 s gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred

preferred orientation of planar microstructures determined via statistical best fit of measured intersection lines the fitpitch computer program the fitpitch computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection lines on different sections measured pitch or strike angles of such intersection lines are fitted to the theoretical intersection lines of a single model plane or a combination of two or three model planes the degree of fit is quantified in terms of the deviation between natural and theoretical intersection lines normalised by uniform data a first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the variscan iberian massif was measured analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes fia in porphyroblasts a second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a variscan granite in the rhine graben qualitatively established best fit planes derived originally from this data are compared with the numerically established planes using the fitpitch program and discussed in terms of the advantages and potential limitations of the method q 2002 published by elsevier science ltd

sulfur isotope analysis of bitumen and pyrite associated with thermal sulfate reduction in reservoir carbonates at the big piney la barge production complex sulfur isotopes of solid bitumen and associated pyrite from the madison limestone in the big piney la barge production complex were measured using a secondary ion mass spectrometry sims method the solid bitumens a product of thermochemical sulfate reduction yielded 34s values of 18 9 3 9 that are consistent with inferred values for native mississippian sulfate in contrast coarse and fine grain pyrite grains were found to be 34s depleted with values similar to that of the produced h2s 34s 10 we interpret these results to indicate that two different sources of sulfate were involved with tsr within the madison limestone autochthonous anhydrite which is now completely replaced with calcite and permian age sulfate dissolved in the aquifer while checking for inclusions within the bitumen that could lead to erroneous measurement we found the bitumen possesses a 5 m rim and internal worm like features enriched in organic sulfur we hypothesize that the rim is the result of back reaction of the late forming h2s with the solid bitumen and that the 1 m diameter wormy features may result from liquid liquid immiscibility occurring at the high temperatures of formation

origin of deep strata gas of tazhong in tarim basin china abstract tazhong is located in the interior of the taklamakan desert in the tarim basin deep carbonates are targeted exploration areas with abundant gas and the largest ordovician gas condensate field has been discovered the hydrocarbon is mainly distributed in the reef shoal complex of the upper ordovician lianglitage formation and in the interbedded weathered reservoirs of the lower middle ordovician yingshan formation despite the deep burial depth of the reservoirs and strong heterogeneity the reservoir bodies are large multiple layered and enrichment in hydrocarbons indicates a great exploration potential the ordovician gas condensate reservoir in the tazhong consists mainly of methane with a gas dryness coefficient of 0 91 0 98 the ordovician gas in the tazhong belongs to secondary oil cracking gas the comprehensive analysis of natural gas composition stable carbon isotopes and adamantane content showed that the natural gas was from paleo reservoir cracking at depth basin geothermal history burial history fluid inclusion data and geological analysis show that the gas accumulation occurred 5 ma ago the sulfur isotope of sulfides the presence of 2 thia adamantane and existence of light co 2 carbon isotopes indicate that the h 2 s was formed via tsr the natural gas accumulated through migration along faults

a revised biostratigraphic and well log sequence stratigraphic framework for the scotian margin offshore eastern canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120137 in this study we build on a foundation of previous biostratigraphic studies for the scotian basin offshore eastern canada by conducting new quantitative multidisciplinary biostratigraphic studies of the mesozoic sections in eight wells bonnet p 23 chebucto k 90 cohasset l 97 glenelg j 48 glooscap c 63 mohican i 100 south desbarres o 76 and south griffin j 13 these wells were chosen to provide good spatial coverage stratigraphic penetration and correlation with the seismic grid we have also evaluated pre existing biostratigraphic data and undertaken the well log sequence stratigraphic interpretation of upper triassic to cenozoic sediments in 16 additional wells using a consistent multidisciplinary event scheme derived from the new wells key to the dating of some horizons has been integration of the palynology and micropaleontology disciplines mostly used previously on the scotian margin with new nannofossil data simultaneously we have closely integrated the biostratigraphic results with

unprecedented occurrence of novel c26 c28 21 norcholestanes and related triaromatic series in evaporitic lacustrine sediments a novel series of c26 c28 21 norcholestanes and their triaromatic analogs have been tentatively identified as major components in eogene evaporitic sediments in jinxian sag of the bohai bay basin northern china these compounds may be viewed as representatives of a new class of molecules whose biological or diagenetic origin remains to be determined their unprecedented high abundance in the evaporitic lacustrine sediments and related oils show important source and or depositional environment controls in addition to thermal maturity

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

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

new directions in an established gas play promising dolomite reservoirs in the middle triassic leikoupo formation of the sichuan basin china the discovery of carbonate gas fields in the middle triassic leikoupo formation of the sichuan basin has a complex history in recent years a series of structural fields have been discovered in the western sichuan basin their discovery confirms the immense exploration potential of the leikoupo formation in this study we analyze the characteristics of leikoupo formation exploration plays using exploration wells and test data aiming to provide a reference for further discoveries the leikoupo formation represents the uppermost unit in the sichuan marine carbonate platform succession during its deposition the whole basin was characterized by a restricted and evaporitic platform two classes of reservoirs developed one is pore fracture reservoirs in marginal platform and intraplatform shoals and another is fracture vug reservoirs in the karstic weathering crust of the formation capping unconformity three hydrocarbon accumulation models were established for the leikoupo formation based on the spatial and temporal relationship among the source reservoir and cap rocks two types of exploration plays are present in the leikoupo formation that is shoal including intraplatform shoal and marginal platform shoal dolomite plays and karstic dolomite weathering crust plays including intraplatform shoal karst and marginal platform shoal karst the western sichuan depression in the karstic slope belt presents immense exploration potential because of a proximal hydrocarbon supply charging via an extensive fracture network shoals and karstic reservoir a good seal rock of terrestrial mudstone and potential composite hydrocarbon accumulations in stratigraphic traps making it a promising area for future exploration

paleohydrogeological and thermal events recorded by fluid inclusions and stable isotopes of diagenetic minerals in lower cretaceous sandstones offshore nova scotia canada upper jurassic lower cretaceous sandstones of the scotian basin offshore eastern canada are important gas reservoirs previous studies of fluid inclusions in jurassic limestones and apatite thermochronology of cretaceous sandstones have suggested a late mesozoic thermal event fluid inclusions in different authigenic cements were analyzed to determine the temperature and composition of basinal fluid at the time of precipitation and the timing of hydrocarbon migration and entrapment fifty one sandstone samples were analyzed for stable isotope composition 18o and 13c of carbonate cements trapping temperatures for primary aqueous inclusions hosted in quartz overgrowths 89 175c and in late carbonate cements 138 173c are higher than predicted by two dimensional modeling from burial alone based on postrift geothermal gradients these inclusions contain high salinity fluids mostly 19 22 wt nacl equivalent second predominantly aqueous inclusions have much lower salinities 5 2 6 7 wt nacl equivalent and some contain liquid hydrocarbons late fe calcite cement in hauterivian sandstone shows negative values of 13 17 to 9 2 whereas cements in deeper and shallower sandstones have higher 13c values these data indicate that high temperature high salinity fluids preceded hydrocarbon migration the oldest and youngest rocks studied show less effect of high temperatures than do rocks that were buried to depths of at least 2 km 1 2 mi after 135 ma and before 100 ma both fluid inclusions and 13c of carbonate cement suggest that highest temperatures were achieved during burial at 115 to 105 ma this early cretaceous thermal event had a geothermal gradient of at least 55c km it is earlier than the previously reported event based on apatite fission tracks but is consistent with the fission track data the event is synchronous with regional evidence of volcanism and its peak coincides with rapid salt tectonic deformation in the deep basin hydrocarbon charge to the outer shelf wells occurred after this thermal event and thus not before the late cretaceous

allochthonous prodeltaic sediment facies in the lower cretaceous at the tantallon m 41 well implications for the deep water scotian basin lower cretaceous shelf margin deltas in the scotian basin prograded across a deep outer shelf platform and may have supplied sand to the deep water scotian margin conventional core from the tantallon m 41 well was used to evaluate sediment facies and evidence for the transfer of deltaic sand to deep water two cores with shallow water lithofacies show sediment deformation structures resulting from surficial debris flow and deeper seated landsliding of deltaic sediment interpreted by a comparison with modern sea floor piston cores both mass movement types produced a basal strongly foliated mudstone such allochthonous blocks create potential pitfalls in interpretation of paleowater depth from sedimentary structures in conventional core and microfossils in ditch cuttings autochthonous sediment on the deep outer shelf platform is a distinctive red brown mudstone with rare turbidite sandstone beds the deepest core consists of thin bedded sandstone turbidites showing bouma tb c d intervals and fading ripples characteristic of overbank settings deposited on a high adjacent to 50 150 m 160 500 ft deep channels these sandstones provide direct evidence that sand was transferred through channels into deep water in the early cretaceous sandy turbidity currents were sourced both by prodeltaic failure and transformation and by hyperpycnal flow from the mountainous rivers that supplied sediment to the scotian basin

geochemistry origin and accumulation of continental condensate in the ultra deep buried cretaceous sandstone reservoir kuqa depression tarim basin china abstract a continental condensate field with the deepest burial depth 7084 m so far in china was recently discovered from the cretaceous sandstone reservoir in the bozi area of the kuqa depression tarim basin in this paper we report general features of this field including geochemistry of the condensates their origin and migration and accumulation general geochemistry of the condensate reflects that it is an over pressured condensate reservoir analytical results by newly developed high resolution gc gc tofms method indicate that the condensate is rich in n alkanes from nc 3 to nc 34 and in diamondoid hydrocarbons with a high content of adamantanes the condensate has a high abundance of aromatic hydrocarbons the gas 13 c 2 values in these are 23 3 which is characteristic of coal derived gases compared with the contents of conventional condensate oils the bz1 well has a high content of aromatic hydrocarbons suggesting that the condensate is in a stage of highly mature cracking analyses of oil biomarkers carbon isotopes gas compositions etc revealed that the condensate is dominantly sourced from the highly mature coals of the jurassic qiakemake formation in the western kuqa depression this set of coal source sequences has generated large quantities of condensate since 3 ma when the study area was subjected to a rapid subsidence with sediments over 2000 m and consequently attained at the condensate generating stage such condensates then migrated vertically into the cretaceous traps along faults therefore the bozi condensate field is a typical case of large scale oil gas field characterized by late stage accumulation multiple elements favor the formation of this large scale condensate field the timing of trap formation accords with that of the condensate generation since 3 ma the traps and source rocks are stacked vertically faults connect the source rock sequences and the reservoirs the hydrocarbon charge is strong and efficient thick paleogene gypsum beds act as an excellent cap rock favoring the accumulation and preservation of the condensate

sulfur isotope ratios in petroleum research and exploration williston basin the three major types of crude oil in the williston basin the type i oils of the winnipeg red river system the type ii oils of the bakken madison system and the type iii oils of the tyler pennsylvanian system can be distinguished by their sulfur isotope compositions they have characteristic dgr34s values of 5 8 1 2 parts per thousand ppt 2 8 0 8 ppt and 4 0 0 7 ppt respectively highly mature oils have less typical values type ii oils which have migrated over a distance of some 150 km beyond the region of generation have maintained their characteristic dgr34s values even though sulfur may have been lost this indicates little or no interaction with reservoir sulfates under normal circumstances on the periphery of the basin type ii oils altered by water washing and biodegradation have altered dgr34s values which increase from 2 9 to 9 4 ppt with the increasing degree of crude oil degradation the bakken shales source of the type ii oils have dgr34s distribution patterns in the reduced sulfur typical of marine sediments the dgr34s values for the type ii oils match most closely the dgr34s value of organic sulfur in the black bituminous shales of the lower bakken

predicting co2 occurrence on a regional scale southeast asia example abstract prediction of co 2 occurrence in natural gases is difficult owing to the interplay among basinal sources e g volcanic emanations carbonate hydrolysis and metamorphism organic diagenesis and maturation reservoir processes e g mineralogical transformations and poorly understood physicochemical phenomena e g differential solubility of gas components carbonate buffering phase segregation published approaches to assessing co 2 risk are based either on circumstantial evidence of basinal influences or statistical analysis of reservoir attributes the present study presents the results of empirical statistical and neural network analysis of co 2 abundance in natural gas and discusses potentially influential basinal and reservoir attributes to quantify risk factors for anomalous co 2 occurrences in southeast asia the results of this study indicate that tectonic setting including basement crustal age basement fault density reservoir temperature and reservoir pressure are key elements controlling co 2 abundance assignment of co 2 risk at the basin level can be roughly approximated using the results of this study more accurate assessments of co 2 risk at the basin play field and reservoir levels require more comprehensive geological and geochemical data as well as consideration of potentially influential physico chemical processes

alteration of solid bitumen by hydrothermal heating and thermochemical sulfate reduction in the ediacaran and cambrian dolomite reservoirs in the central sichuan basin sw china abstract solid bitumen in reservoir rocks is an allochthonous organic matter formed from bitumen or liquid hydrocarbons by various alteration processes in this study two types of altered solid bitumen have been identified in both the ediacaran dengying and cambrian longwangmiao formations in the central sichuan basin using petrographic microthermometric chemical and isotopic data the hydrothermally altered bitumen type 1 is associated with the pore filling saddle dolomite and quartz in which homogenization temperatures of some fluid inclusions are more than 10 c higher than the maximum burial temperature of the dengying formation it has various anisotropic textures with significantly high bireflectance values rbmax rbmin between 3 89 and 9 21 moreover the hydrothermal alteration led to 13c enrichment in the type 1 bitumen compared to the normally matured bitumen derived from the same source on the other hand the thermochemical sulfate reduction tsr altered bitumen type 2 has s c atomic ratios of up to approximately 0 06 the 34s values of type 2 bitumen are close to those of the carbonate associated sulfate and similar to the tsr derived h2s in the same reservoirs a strong negative correlation exists between s c atomic ratios and 13c values in the type 2 bitumen in the dengying formation which is interpreted as a result of an increasing incorporation of the 13c depleted ethanethiol into the bitumen during tsr this interpretation is consistent with the fact that the remaining ethane became isotopically heavier as tsr proceeded in addition the reactive sulfate for tsr was primarily derived from the hydrothermal sulfate such as barite this is evidenced by the replacement of hydrothermal barite by the 34s enriched pyrite and the small differences in 34s value between the barite and the type 2 bitumen or the tsr derived h2s in the same strata we suggest that tsr in the ediacaran and cambrian hydrocarbon reservoirs was probably initiated by hydrothermal activity

biogenic carbonates are they a criterion for underlying hydrocarbon accumulations an example from the gulf of suez region egypt the gulf of suez petroliferous basin is characterized by the occurrence of a middle upper miocene evaporite cap field and subsurface investigations in the southwest gulf of suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur the first type of carbonate is biogenic syngenetic in origin and was formed in low temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate reducing bacteria the resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum calcite sulfur or as sulfur ooids sulfur globules and stromatolitic sulfur that are enclosed in gypsum or calcite the biogenic syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings the second type of carbonate is a biogenic epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying miocene evaporites the carbonate is vuggy is included within or caps the evaporites and usually shows hydrocarbon stains or emits a hydrocarbon smell the sulfur is usually yellow but may be brown to black due to bitumen staining carbonates replace gypsum or anhydrite and occur as aragonite calcite or dolomite the d13c mean values for carbonate minerals range from 7 8 to 27 2 pdb being relatively heavier than the d13c values of the nearby crude oil 28 1 to 29 3 pdb carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions where the lighter methane is stable the occurrence of biogenic epigenetic carbonates and sulfur in the evaporite deposits of the gulf of suez petroliferous basin is similar to that recorded in other basins such as the gulf coast basins of texas louisiana and southern mexico the fergana and amudarya depression of central asia and the mesopotamian depression in iraq the biogenic epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon bearing reservoir at depth

constraints of molecular and stable isotopic compositions on the origin of natural gas from middle triassic reservoirs in the chuanxi large gas field sichuan basin sw china abstract the middle triassic leikoupo formation in the sichuan basin is a gypsum bearing stratum the chuanxi cx large gas field has been recently discovered in middle triassic reservoirs with a controversial gas source the gas geochemistry is analyzed in this study to investigate the gas origin and source the h2s bearing dry gas in the cx gas field has an h2s content ranging from 1 07 to 4 09 with a gas souring index gsi of 0 0120 0 0438 the 13c1 13c2 and 2hch4 values range from 31 8 to 30 4 from 32 9 to 29 6 and from 153 to 136 respectively genetic identification based on the geochemical characteristics suggests that the gas is an oil type and derives from the secondary cracking of oil according to the gsi and alkane carbon isotopes the gas experienced weak tsr alteration without ethane potential high quality source rocks include the upper permian mudstones and marlstones as well as the lower cambrian mudstones with a total organic carbon content generally higher than 2 0 a comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the upper permian rather than the lower cambrian source rocks the extremely low development of solid bitumen in the reservoirs suggests the inexistence of large scale paleo oil pools in the leikoupo formation the middle triassic natural gas in the cx gas field originates from the upper permian longtan formation where the gas pools are dominated by direct filling of secondary cracking gas