recognition of 18β h oleanane in several crudes and tertiary upper cretaceous sediments definition of a new maturity parameter abstract ekweozor et al chem geol 27 11 28 and 29 37 first discovered an unknown compound with a gc retention time very close to that of the 18 h olenane peak in oils and tertiary sediments of the nigerian delta this compound was called j several authors wang et al advances in organic geochemistry 1981 pp 108 113 wiley chichester 1983 schoelle et al ibid pp 156 163 reported the occurrence of 18 h oleanane without underlying the presence of j because of their coelution in this study the presence of j has been confirmed in more than 150 rock extracts and 19 oils from different countries and is identified as the 18 h oleanane by comparison to retention time and mass spectrum and by its coelution with an authentic standard apart from tertiary sediments the 18 h and 18 h oleananes have also been found in upper cretaceous sediments which extends the occurrence of these compounds back in age a maturity correlation has been found between the relative concentration of 18 h oleanane and 18 h oleanane in fact an increase in the 18 h 18 h oleanane ratio has been observed with increasing depth in several wells the 18 h 18 h oleanane ratio has been correlated with other classical maturity parameters i e ts tm ro and tmax

a geochemical reconstruction of oil generation in the barrow sub basin of western australia abstract a suite of crude oils and petroleum source rock extracts from the barrow sub basin of western australia have been analysed for biological marker compounds by capillary gc ms and for volatile hydrocarbons by whole oil capillary gc these analyses were used to calculate values for twenty three biomarker parameters in order to assess aspects of source type maturity migration and biodegradation of the hydrocarbons the crude oils had a source in the upper jurassic dingo claystone formation these hydrocarbons accumulated in the reservoir sands and in some cases were biodegraded several accumulation and biodegradation episodes have been recognised while the basin continued to subside which resulted in a suite of oils showing marked differences in composition

hydrocarbon reserves of gachsaran oilfield sw iran geochemical characteristics and origin abstract biomarkers and carbon isotope data were used to depict geochemical properties and to delineate source of the crude oils from asmari and bangestan reservoirs of the giant gachsaran oilfield sw iran despite the distinct horizons of the reservoirs and severe petrophysical heterogeneity inside each reservoir all oil samples show an identical level of thermal maturity furthermore similar source related parameters reflect identical source rock for both reservoirs they indicate carbonate marl source deposited in a marine environment under anoxic suboxic conditions strong marine organic matter signatures of predominantly algal origin along with evidence of a terrigenous angiosperm plant contribution were detected geochemical evidence suggests that the gachsaran field asmari and bangestan oils being sourced from a mix of organic rich kazhdumi and pabdeh source rocks of albian and middle eocene early oligocene ages respectively carbon isotope composition and v ni ratio signatures indicate kazhdumi formation as the main source rock meanwhile significant presence of oleanane and less than unity values of c 29 c 30 hopanes ratio demonstrate a partial contribution of pabdeh formation this contradiction could be explained by two scenarios 1 incorporating very immature non expelled pabdeh oils into the mature kazhdumi oils during migration or 2 partial charge form low maturity pabdeh oils that is insufficient to impact a typical mixed signature over the geochemical fingerprint of the reservoired oils additionally geochemical similarities give rise to an assumption of reservoir and trap inter connection for this giant field

the albian oceanic anoxic events record in central and northern tunisia geochemical data and paleotectonic controls the albian organic rich successions of the lower part of the fahdene formation albian to cenomanian tunisia were studied using sedimentology analysis of carbonate contents and observation of thin sections bulk organic geochemistry rock eval pyrolysis and molecular biomarker distributions the selected outcrops cover different structural domains from western central tunisia jebel hamra to the diapir zone or the tunisian trough koudiat berkouchia jebel ghazouan and ain asker from the southwest to the northeast respectively total organic carbon values between 0 17 and 3 43 evidenced the record of the albian oceanic anoxic events oae1b and oae1c in this southern tethys margin rock eval data and molecular biomarkers n alkanes steranes and terpanes revealed thermally mature organic matter om consisting of a mixed type ii iii om marine planktonic and continental origins the abundant tricyclic terpanes c30 17 h diahopanes and the regular decrease of homohopanes c31 to c35 in the terpane series were also indicative of a high contribution of bacterial om that had been developed under stagnant dysoxic anoxic environments

the oleananes origin maturation and limits of occurence in southern nigeria sedimentary basins oleananes occur within well defined age and paleogeographic boundaries in the sedimentary province of southern nigeria thus signifying that the biomarkers may be analogous to index fossils in anambra syncline lower benue trough the biomarkers are found in rocks which are as old as late upper cretaceous in tertiary niger delta the concentration composition and distribution of the oleananes in rocks and oils are determined mainly by palaeodepositional environment and thermal history and are independent of age and geographical location of specific samples oleanane content varies with increasing burial depth with peak concentration coinciding within rm range 0 4 0 6 implying early generation of hydrocarbons from the basin s typically resin rich kerogens it is evident that the oleanane content parameter is superior to the hopane and sterane isomerisation indices for monitoring thermal maturation in the delta and for differentiating paralic from paralic marine source facies as well as early from late generated oils

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time abstract the analysis of the sterane data of a large set of crude oils 414 derivedfrom marine carbonate 27 and siliciclastic source rocks 14 where influences of terrestrial or lacustrine derived organic matter can reasonably be excluded shows that there are increases in the relative content of c28 steranes and decreases in the relative content of c29 steranes through geological time there are no consistent variations in the relative content of c27 steranes through time with one major exception a proterozoic oil from oman palaeozolic and older crude oils are thus generally characterised by strong predominances of c29 steranes and low relative concentrations of c28 steranes significantly higher proportions of c28 steranes and lower proportions of c29 steranes occur in oils derived from jurassic and upper cretaceous source rocks these changes through time do not appear to reflect the chemical evolution of the sterols of one particular variety of marine organism the increase in c28 steranes may be related to the increased diversification of phytoplantonic assemblages in the jurassic and cretaceous possible sources of the c28 sterols necessary for the observed changes in crude oil steranes includes diatoms coccolithophores and dinoflagellates although the technique does not give an accurate means of determining the age of the source rock of a crude oil it is possible to distinguish younger crudes derived from the upper cretaceous and tertiary from palaeozoic and older crudes

quantifying the hierarchical organization of compensation in submarine fans using surface statistics abstract stratigraphy is often interpreted within hierarchical or scale dependent frameworks that subdivide deposits based on distinct jumps in characteristics such as duration of deposition or scale while the interpretation is logically valid few studies quantitatively demonstrate that the jumps exist rather recent work has quantitatively shown some characteristics of stratigraphy to be fractal or scale invariant compensational stacking the tendency for sediment transport systems to preferentially fill topographic lows is a concept widely used in stratigraphic interpretation here we use the compensation index a metric that quantifies the strength of compensational stacking in sedimentary deposits to describe the architecture of stratigraphy exposed in outcrops of submarine fan strata in the carboniferous ross sandstone representing contrasting architectural styles 1 predominantly lobe elements and 2 predominantly channel elements in both datasets the stratigraphic architecture is classified into hierarchical classes of beds stories and elements results are the following first at both sites we document statistically significant increases in the strength of compensation across larger hierarchical levels supporting the use of hierarchical interpretations of stratigraphy it is therefore plausible for some characteristics of sedimentary systems to be hierarchical and others to be fractal second we document that lobe elements stack more compensationally than channel elements we interpret this pattern to document that compensation increases along a longitudinal transect through this distributive submarine fan

magnetostratigraphy of miocene pliocene zagros foreland deposits in the front of the push e kush arc lurestan province iran abstract the timing of the deformation in the zagros simply folded belt is poorly constrained because of the lack of an accurate absolute chronology of the syntectonic sedimentary sequences the foreland basin infill at the front of the push e kush arc is composed of fine grained fluvial plain deposits agha jari fm and coarse conglomerates at the top of the section bakhtyari fm a magnetostratigraphic study was carried out in a composite section spanning about 2800 m in order to date growth strata and to constrain the timing of the deformation in the mountain front flexure mff magnetostratigraphic correlation of the base of the agha jari fm with chron c5a yields an age of 12 8 to 12 3 ma for this base the transition to the conglomerates of the bakhtyari fm correlates with the upper gauss chron c2an at approximately 3 ma the deposition age of the top of the preserved bakhtyari fm is extrapolated around the pliocene pleistocene boundary the base of the agha jari fm growth strata and thus the beginning of the deformation in the front of the push e kush arc is dated at 8 1 7 2 ma the topmost preserved bakhtyari is folded in the ne flank of the changuleh anticline and is unconformably overlying the sw flank of the anaran anticline this indicates that the tectonic deformation in the front of the push e kush arc was active at least during 5 my the mff is a relatively long lived structure active from 8 1 to 7 2 ma to about the pliocene pleistocene boundary partly synchronous with the changuleh anticline to the foreland after mff tectonic cessation only the changuleh anticline remained active

source rock distribution and maturation in the zagros orogenic belt provenance of the asmari and bangestan reservoir oil accumulations abstract the zagros orogenic belt of iran is one of the most prolific petroliferous areas accounting for approx 12 of proven global reserves correlation techniques reveal that a majority of the oils variously reservoired in asmari oligo miocene and bangestan group upper cretaceous anticlinal structures share a common provenance oils of the n e khuzestan fields are an exception showing a conspicuously different origin candidate source potential for the crucial central zagros embayment and contiguous areas was assessed on field survey and from well materials collected during the decade from 1968 onwards although varying locally in vertical and lateral distribution attractive sources occur throughout the section included were silurian shales the sargelu middle jurassic garau gadvan lower cretaceous kazhdumi albian gurpi upper cretaceous and pabdeh paleogene fms maturation of these source units was essentially controlled by the foredeep subsidence related to the zagros orogeny being a neogene event hydrocarbon generation and migration are very recent and essentially synorogenic consolidation of this information with the real distribution of candidate sources application of carbon isotope based source oil assignments and biomarker results successfully differentiated the provenance of the generic oil groupings over much of khuzestan the kazhdumi was recognised as the major contributor elsewhere the pabdeh fm yields isotopically distinctive oils in n e khuzestan evaluation of this basin allowed identification of prolific source rocks in low energy euxinic argillaceous lime mudstones similarly it provided an early application of pyrolysis techniques carbon isotopic correlations and subsidence history maturation migration modelling approaches now routinely employed in basin evaluation

organic geochemistry and petroleum potential of early cretaceous garau formation in central part of lurestan zone northwest of zagros iran abstract deposition of organic rich black shales and dark gray limestones in the berriasian turonian interval has been documented in many parts of the world the early cretaceous garau formation is well exposed in lurestan zone in iran and is composed of organic rich shales and argillaceous limestones the present study focuses on organic matter characterization and source rock potential of the garau formations in central part of lurestan zone a total of 81 core samples from 12 exploratory wells were subjected to detailed geochemical analyses these samples have been investigated to determine the type and origin of the organic matter as well as their petroleum generation potential by using rock eval toc pyrolysis gc and gcms techniques the results showed that toc content ranges from 0 5 to 4 95 percent pi and tmax values are in the range of 0 2 and 0 6 and 437 and 502 c most organic matter is marine in origin with sub ordinary amounts of terrestrial input suggesting kerogen types ii iii and iii measured vitrinite reflectance rrandom values varying between 0 78 and 1 21 indicating that the garau sediments are thermally mature and represent peak to late stage of hydrocarbon generation window hydrocarbon potentiality of this formation is assessed fair to very good capable of generating chiefly gas and some oil biomarker characteristics are used to provide information about source and maturity of organic matter input and depositional environment the relevant data include normal alkane and acyclic isoprenoids distribution of the terpane and sterane aliphatic biomarkers the garau formation is characterized by low pr ph ratio

comment on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty journal of structural geology 27 2005 1599 1609

influence of a permanent low friction boundary on rotation and flow in rigid inclusion viscous matrix systems from an analogue perspective abstract natural occurrences and recent experimental work show that a low friction inclusion matrix boundary can be responsible for antithetical rotation and development of stable shape preferred orientations in simple shear the flow of a viscous matrix around a rigid inclusion to which it may or may not be adherent is still not well studied but it is relevant to the understanding of the behaviour of structural elements in mylonite zones we used two dimensional 2 d analogue experiments to address these issues the experimental results with a permanent low friction inclusion matrix boundary nonadherent mode show the following 1 the rotation behaviour of inclusions in this mode is markedly different from the theoretical predictions and experimental results for the adherent mode 2 inclusions with aspect ratio equal to 1 rotate synthetically at a rate that depends upon inclusion shape and orientation 3 ellipse rectangle and lozenge shaped inclusions rotate antithetically when starting with their greatest axis parallel to the shear direction 4 back rotation is limited in all cases studied and the angle between the inclusion greatest axis and the shear direction represents a stable orientation which depends on inclusion aspect ratio and shape 5 a metastable orientation exists which is strongly dependent upon inclusion shape and aspect ratio and separates fields of antithetic and synthetic rotation our experimental results show that the overall flow pattern is bow tie shaped in adherent and nonadherent modes however there are major differences in the way the matrix flows near the inclusion i in the nonadherent mode the nearby matrix flows past the inclusion whereas in the adherent mode the nearby matrix flows with and follows the inclusion therefore in the adherent mode passive marker lines parallel to the shear direction and streamlines show considerable deflections at the inclusion crests in marked contrast with their straight character in the nonadherent mode ii stagnation points or closed flow lines near the inclusion were not observed in the nonadherent mode which means that there is no closed separatrix around the inclusion in this mode despite the fact that the overall flow shape is bow tie iii in the adherent mode the line of flow reversal is stable throughout deformation but in the nonadherent mode it changes position and orientation with progressive shearing this shifting of the line of flow reversal can be an important factor controlling rotation behaviour in the nonadherent mode iv in the nonadherent mode the inclusion behaviour is similar to that observed in confined flow v the flow pattern in the nonadherent mode provides an explanation for the observed lack of drag folds associated with small scale rigid inclusions in mylonites

structural framework of the zagros fold thrust belt iran abstract data gleaned from the literature on the zagros have been compiled and used in conjunction with new interpretations to provide a better picture of the structures sedimentation history and deformation of this hydrocarbon rich mountain belt the belt parallel mountain front the n s trending kazerun izeh and e w trending bala rud fault zones are defined as the master structural elements of the belt these fault zones divide the zagros basin into zones with different stratigraphic successions and different rheological profiles this resulted in different structural styles developing along the belt during the subsequent collision the kazerun and izeh fault zones acted as the depositional system transition zone between the lurestan and fars regions in jurassic cretaceous time and partly controlled the distribution of the kazhdumi formation one of the major source rocks by the end of the cretaceous the nw se trending mountain front fault divided the present folded belt of the zagros into a major foreland basin to the southwest and a piggyback basin to the northeast activity along other major transfer fault zones including the izeh kazerun and bala rud fault zones occurred at this time and controlled the sedimentation and subsidence of the dezful embayment the main target region for hydrocarbon exploration these fault zones controlled the thickness of the asmari formation one of the main reservoirs and also the considerable thickness and facies changes of the gachsaran formation the major seal to the asmari formation the present morphology of the mountain belt is interpreted as being mainly related to the pinning of the mountain front fault to the northwest of the dezful embayment and to the north of the strait of hormuz

recurrent depletion of benthic oxygen with 4th order transgressive maxima in the cretaceous western interior seaway abstract a study was designed to investigate the relationship between depletion of benthic oxygen and 4th order sea level change within the cretaceous western interior seaway of north america three 4th order sea level change cycles were studied in strata belonging to the tropic shale and the tununk shale member of the mancos shale in utah these sedimentary deposits accumulated within prodeltaic environments in utah strata belonging to all three 4th order cycles exhibit evidence for depletion of benthic oxygen at times of peak 4th order transgression and development of condensed sections the evidence includes decreased intensities of bioturbation increased cr al ni al and zn al ratios and decreased mn fe ratios the data also suggest that depletion of benthic oxygen in the distal section preceeded that in the more proximal one and thus may reflect episodic expansion of an oxygen depleted water mass es from the pelagic to the prodeltaic environment of the seaway evidence for subsequent increases in benthic oxygen level with the onset of 4th order regressions is interpreted to reflect either aggradation of the seafloor above the oxygen poor water due to sediment accumulation or contraction of the oxygen depleted water mass from the prodeltaic part of the seaway or a combination of the two s organic c ratios of strata interpreted to have been deposited under conditions of depleted benthic oxygen are distinctly below 0 4 and thus were not useful for recognition of paleo oxygen levels in the prodeltaic environment this study highlights the importance of depletion of benthic oxygen as a mechanism for enhancing preservation of organic carbon in condensed sections on a hundred thousand year time scale our results underline the potential for source rock formation in similar prodeltaic deposits of other epicontinental seaways during periods of relatively high sea level and global depletion of benthic oxygen

bitumen molecular maturity parameters in the ikpikpuk well alaskan north slope abstract several bitumen molecular maturity parameters alkylnaphthalene ratios mnr enr dnr methylphenanthrene indices mpi 1 mpi 2 dpi n c 17 pristane pristane phytane c 15 alkane distributions and gc ms analyses of triterpanes steranes diasteranes triaromatic steroids and monoaromatic steroids for samples of the ikpikpuk well alaskan north slope are reported the molecular maturity parameters are compared and contrasted with previous data for lower molecular weight c 1 c 7 hydrocarbons production index vitrinite reflectance and thermal alteration index bitumen maturation indices c 15 alkane distributions and distributions of steranes diasteranes triterpanes triaromatic and monoaromatic steroids agree with kerogen maturation indices in the kingak section of ikpikpuk well bitumen maturation indices and distributions of steranes diasteranes triterpanes triaromatic and monoaromatic steroids diverge from vitrinite maturation profiles for the shublik section of ikpikpuk indicating either a major influence of changes in kerogen type or the presence of residues of expulsion migration in the shublik the data are consistent with the shublik being a source and or conduit of migration of oil

variation of structural style and basin evolution in the central zagros izeh zone and dezful embayment iran abstract structural style and its relationship to sedimentary facies and the evolution of sedimentary depocenters since late cretaceous times have been studied on the basis of one regional balanced transect and several updated isopach maps in the izeh zone and the dezful embayment central zagros this study relies on fieldwork data existing geological maps seismic data and well information a new structural classification for part of the zagros sedimentary cover is presented to highlight the different mechanical behavior of the formations in the stratigraphic column it shows the existence of several local decollement levels activated during folding these decollement levels separate lithotectonic units which accommodate shortening in different ways the lower paleozoic is the basal decollement level throughout the studied area triassic evaporites albian shales eocene marls and miocene evaporites can act as intermediate decollement levels and present variable facies in the central zagros lateral facies and thickness variations the sedimentary overburden and the close relationship with inherited fault patterns influenced the wavelength amplitude and style of folding in the study area furthermore surface structures do not necessarily coincide with deeper objectives where these disharmonic levels are involved in folding the evolution of sedimentary depocenters from the late cretaceous obduction episode until early miocene collision is described based on updated isopach maps and tectonic subsidence curves this analysis shows continuous compression and movements along n s and nw se trending faults in the zagros basement which predate the neogene zagros folding and influence sedimentation history it also indicates a southwestward migration of the depocenters through time in addition to the basement involvement below some folds during the zagros orogeny

wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison panfish j t and peterson f 1988 wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 261 282 wind directions for middle pennsylvanian through jurassic time are predicted from global circulation models for the western united states these predictions are compared with paleowind directions interpreted from eofian sandstones of middle permsylvanian through jurassic age predicted regional wind directions correspond with at least three quarters of the paleowind data from the sandstones the rest of the data may indicate problems with correlation local effects of paleogeography on winds and lack of resolution of the circulation models the data and predictions suggest the following paleoclimatic developments through the time interval studied predominance of winter subtropical high pressure circulation in the late pennsylvanian predominance of summer subtropical high pressure circulation in the permian predominance of summer monsoonal circulation in the triassic and earliest jurassic and during the remainder of the jurassic influence of both summer subtropical and summer monsoonal circulation with the boundary between the two systems over the western united states this sequence of climatic changes is largely owing to paleogeographic changes which influenced the buildup and breakdown of the monsoonal circulation and possibly owing partly to a decrease in the global temperature gradient which might have lessened the influence of the subtropical high pressure circulation the atypical humidity of triassic time probably resulted from the monsoonal circulation created by the geography of pangaea this circulation is predicted to have been at a maximum in the triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west

stratigraphic hierarchy of organic carbon rich siltstones in deep water facies brushy canyon formation guadalupian delaware basin west texas the first systematic test for a predictive relationship between organic carbon content and stratigraphic hierarchy in a deep water slope to basin floor deposit was performed the studied section includes the pipeline shale the brushy canyon formation and the lower part of the cherry canyon formation of the delaware mountain group west texas this interval represents one large scale 3rd order genetic sequence within which 4th and 5th order stratigraphic cycles are recognized samples of fine grained facies throughout the section were collected from outcrop and analyzed for organic carbon content and hydrogen index degree of pyritization was also determined for a subset of the samples the results indicate that organic enrichment is closely correlated to the stratigraphic hierarchy at the 3rd 4th and 5th order levels the data suggest that quantity and quality of preserved organic matter are controlled by changes in bulk sedimentation rate dilution vs condensation which affect organic matter inputs to the sediment as well as the balance between 1 burial and preservation of organic matter and 2 its degradation on the sea floor during times of sediment starvation

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

organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in brazilian marginal basins abstract an investigation of bulk isotopic and molecular features of a wide selection of source rocks and oils from brazilian marginal basins ranging from lower cretaceous to tertiary in age provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo environments the methods employed include evaluation of organic carbon contents rock eval pyrolysis vitrinite reflectance measurements determination of carbon isotope ratios elemental and visual kerogen analysis assessment of oils and solvent extracts by liquid and gas chromatography and quantitative biological marker investigations using gc ms for aliphatic hydrocarbons and uv vis spectrophotometry for metalloporphyrins selected data were evaluated using principal component analysis the results enable the recognition and differentiation of seven depositional regimes lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate marine deltaic marine highly anoxic with a predominance of calcareous mudstone lithology and marine anoxic with predominance of siliciclastic lithology diagnostic features in the biological markers from these depositional environments include the carbon number distributions of n alkanes pristane phytane ratios absolute concentrations of carotane higher acyclic isoprenoids c 30 hopane and steranes and the abundance of the latter relative to 4 methylsteranes the occurrence and abundance of particular biological markers such as gammacerane tricyclic and tetracyclic terpanes 18 h oleanane 28 30 bisnorhopane and 25 28 30 trisnorhopane and the relative abundances of nickel and vanadyl porphyrins

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

reply to the comment by marques et al on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

a model for iron deposition to euxinic black sea sediments sulfur isotope and c s fe elemental data from the lower devonian siegenian emsian hunsruck slate of germany allow the conditions that led to the pyritization of soft bodied fossils to be determined the slates preserving pyritized soft tissues generally contain low concentrations of organic matter 0 3 0 4 percent c and pyrite sulfur 0 0 2 percent s respectively but are unusually rich in total and hcl extractable iron concentrations of iron are significantly lower in samples of the hunsruck slate from areas where fossils lack pyritized soft tissues the fossil pyrite is very enriched in s 34 compared to fine grained disseminated pyrite in the adjacent slates delta fossil sediment 10 to 50 permil suggesting that pyritization of the fossils persisted into later stages of authigenic mineralization pyrite formation in the sediments was limited by the low concentrations of metabolisable organic matter leaving residual sulfate and iron available for continued fossil pyritization in organic rich microenvironments a diffusion with precipitation model indicates that a critical control on the initial preservation of readily metabolisable soft tissue in this way is the presence of high concentrations of sediment iron capable of being solubilized during shallow burial diagenesis in association with low concentrations of metabolisable organic matter in the matrix pore waters rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

degree of pyritization of iron as a paleoenvironmental indicator of bottom water oxygenation abstract a new method is presented for recognizing the degree of bottom water oxygenation in organic carbon bearing rock the method uses the parameter degree of pyritization dop which is the ratio of pyritic fe to pyritic fe acid soluble fe sediments with ages ranging from devonian to cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions deposited in fully oxygenated bottom waters restricted marine conditions deposited in waters with low oxygen concentrations or inhospitable bottom conditions where little or no oxygen is present and h2s may be continually or intermittently present aerobic dop

possible forced folding in the taurus zagros belt of northern iraq the folds in the taurus zagros belt of northern iraq have generally been considered to be decollement buckle folds this implies the presence of a decollement horizon at or near the base of the sedimentary cover the infra cambrian hormuz salt and a passive role of the precambrian basement in the tectonic evolution of the folded belt structural stratigraphic geophysical and remote sensing evidence suggests that forced folding due to faulting in the basement has played a significant role in the development of many of the folds in this region this is clear from the substantial evidence of basement faulting the lack of any convincing evidence for the presence of an extensive and regional decollement horizon i e the hormuz salt between the basement and the cover rocks and the geometries of the folds and related mesostructures the study shows that in the taurus foothills zone the folds are short oval in plan shape and arranged in an en echelon pattern along two sets of dislocation zones in the basement however the folds in the zagros foothills zone are longer linear and arcuate in plan shape many of the studied folds show similar features to those observed in the rocky mountains u s a

hydrocarbon potential and palynological study of the latest ordovician earliest silurian source rock sarchahan formation in the zagros mountains southern iran abstract source rock studies are one of the key issues of petroleum exploration activities in the supercontinent of gondwana ice ages related to the upper ordovician hirnantian and rising sea levels caused by glacial melting at the end of the ordovician and early silurian llandoverian created excellent source rocks along the margin of gondwana investigations conducted in the arabian peninsula have been indicated indicating that the lower qalibah formation the so called qusaiba member or hot shale is a good source rock for the paleozoic petroleum system in this area likewise the sarchahan formation was recently introduced as a source rock in the zagros basin of iran which is probably equivalent to the qalibah formation in the arabian peninsula in this study samples were prepared from surface and subsurface paleozoic rock units in iran s zagros basin the emphasis of the paper was on the sarchahan formation in kuh e faraghan ranging in age from the late ordovician hirnantian to lower silurian llandoverian to determine whether the high richness of organic matter in the sarchahan formation is related to the late ordovician or lower silurian the basal part of the sarchahan formation belongs to the late ordovician hirnantian because of the presence of the persculptus graptolite biozone while the remainder belongs to the lower silurian the ordovician and early llandoverian parts of the sarchahan formation contain type ii and iii kerogen with toc ranging from 2 94 to 7 19 but the rest of the sarchahan formation late llandoverian has toc ranging from 0 1 to 0 58 therefore the hot shale in iran falls within the hirnantian and early llandoverian rhuddanian and not the latest llandoverian aeronian and telychian utilizing organic petrography kerogen type was found ii iii the carbon stable isotope studies revealed that the source rock of hydrocarbons in dalan and kangan reservoirs has been the sarchahan formation based on analytical data the kerogenous shales in the lower part of the sarchahan formation are at end of gas window and the gamma ray amount is approximately 180 api this research indicates the differences between the source rocks in the southern and northern persian gulf and suggesting the hot shale should be considered in different views and used in modeling studies of sedimentary basins for future exploration targets

anatomy of the organic carbon related to the miocene syn rift dysoxia of the rudeis formation based on foraminiferal indicators and palynofacies analysis in the gulf of suez egypt abstract the syn rift deposits from the gulf of suez witness different episodes of anoxia and dysoxia conditions in many areas the sediments of rudeis formation that represent the deposition in the time of maximum rift subsidence are considered as the principal organically rich and oil prone sources while in other areas they are not these differences are attributed to relative positions to the location of the syn rift dysoxia and amounts of the carbon flux during active paleoproductivity time in the present study an extensive analysis of the syn rift dysoxia and paleoproductivity during the miocene is conducted in burdigalian rudeis formation in ramadan 1 well to determine their influence on the accumulation of organic carbon the analysis depends on the statistical investigation of the benthic foraminiferal assemblages the foraminiferal assemblages are discriminated into two biofacies in r mode suboxic and dysoxic and three biotopes in q mode suboxic s dysoxic da and db sample groups the sediments are divided into three stratigraphic sequences five oxygen minimum zones are determined at the extreme oxygen depletion intervals and they are defined by suboxic s and dysoxic da sample groups in descendent order of oxygen depletion intensity and mostly coincide with the high sea levels the distribution of the organic matter showed large enrichment with partial fluorescence amorphous organic matter indicating kerogen type ii in the three biotopes as clarified from palynofacies analysis the organic carbon enrichment from the studied samples ranged from fair to very good 0 51 and 2 38 wt and the relatively larger enrichments are found to be correlated with dysoxic sample group da of intermediate oxygen depletion the pyrolysis gas chromatography pgc specifies the algal rich type i kerogen that deposited in oxygen depleted marine environment the mathematical calculation of the paleoproductivity oligotrophic to eutrophic range 106 352 gcm 2yr 1 carbon flux 13 77 and 36 19 gcm 2yr 1 indicates that higher levels of organic productivity are correlated with dysoxic sample group da the larger organic carbon values are found in the rudeis formation when the dysoxic conditions from the deeper topographic lows extend to the higher tectonic position of ramadan 1 well during the sea level rise that furthermore enhances the paleoproductivity

geochemical study on a well in the western canada basin relation of the aromatic distribution pattern to maturity of organic matter abstract thirty one rock samples from a western canada basin well have been analysed for aromatics using glass capillary gas chromatography with simultaneous flame ionization and sulfur selective flame photometric detection a uniform aromatic distribution pattern with a marked predominance of di and tricyclic aromatic hydrocarbons was observed throughout a depth interval of 3000 meters comprising cretaceous and jurassic sediments the very high relative abundance of sulfur aromatics at two narrow intervals in the triassic and mississippian is attributed to a major change in facies gradual changes with increasing depth have been observed for a series of compound ratios which had been calculated from the isomers of methyl and dimethylphenanthrene and their parent compounds these changes reflect the increasing thermal maturation of the sedimentary organic matter a methylphenanthrene index mpi has been introduced which exhibited a very good correlation with vitrinite reflectance data the mpi is the first maturity parameter which is based on a series of aromatic hydrocarbons this hydrocarbon internal maturity parameter permits the recognition and definition of maturity levels in extracts which can be compared to the maturity e g vitrinite reflectance of source rocks

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

relationship between sedimentation rate and total organic carbon content in ancient marine sediments an analysis of deep sea drilling project dsdp cores indicates quantitative relations between sedimentation rate and total organic carbon content in fine grained ancient marine sediments of jurassic cretaceous and cenozoic ages these relations are largely independent of geographic setting geologic age and differential compaction but are strongly affected by lithology for any given sedimentation rate the total organic carbon content toc by weight percent increases from calcareous sediments to calcareous siliceous sediments to siliceous sediments to black shale the total organic carbon content also varies with the sedimentation rate for each lithology starting at low rates of sedimentation toc first increases with sedimentation rate owing to a more rapid passage of the sediment through the near surface zone of intense organic degradation above a critical sedimentation rate toc typically decreases with increasing sedimentation rate owing to clastic dilution of the organic input near surface organic degradation however continues to be less efficient at higher burial rates therefore the oil generation potential of oil prone organic constit ents should continue to increase with increasing sedimentation rate similar relations were also established between total organic carbon accumulation rate and grain accumulation rate and support both reduced near surface organic degradation and the clastic dilution effect by using the toc cutoff for a petroleum source rock these relations can be used to identify the presence of petroleum source rock formations in marine sedimentary basins the results of this study may have important implications for petroleum exploration in frontier regions sedimentation rate can be determined from seismic reflection records or seismic isopachs in basins where only limited well data are generally available seismic records can thus be used to identify potential petroleum source rocks and to estimate the relative oil generation potential of source rocks in various frontier sedimentary basins prior to and during the early phases of exploratory drilling

geochemical investigation oil oil and oil source rock correlation in the dezful embayment marun oilfield zagros iran the authors wish to thank m l bordenave for many fruitful discussions critical and helpful comments and reviewing an earlier draft of manuscript we also wish to thank hormoz ghalavand national iranian oil company for providing the oil samples and support provided by shahid beheshti university tehran chamran university ahwaz the work depended on support provided by technical staff at the university of aberdeen u k and kharazmi university the work was much improved by reviews and comments provided by anonymous reviewers

organic geochemistry and petrography of kazhdumi albian cenomanian and pabdeh paleogene potential source rocks in southern part of the dezful embayment iran there are several source rock units in the zagros basin but the cretaceous kazhdumi and paleogene pabdeh formations probably have produced the majority of the commercial hydrocarbons in this area among the hydrocarbon provinces of iran the dezful embayment which is located southwest of zagros mountains is one of the most prolific regions in the middle east numerous studies have been made in the northern part of the dezful embayment but relatively few have been done in its southern part the present study focuses on organic matter characterization of two potential source rocks kazhdumi and pabdeh formations in southern part of the dezful embayment cuttings samples 114 were collected from 10 wells and evaluated using rock eval pyrolysis and organic petrography in order to characterize the content and type of organic matter and thermal maturity the results showed that the average total organic carbon toc content of kazhdumi and pabdeh formations are 2 48 and 1 62 wt respectively the highest toc contents for both formations are found in the northern compartment and decreased gradually toward the south pyrolysis data reveal that organic matter has a fair to very good hydrocarbon generation potential and are classified as type ii iii and type iii rock eval t max and vitrinite reflectance show that the majority of samples are in the early mature to mature stage of the oil generation window c 2012 elsevier ltd all rights reserved

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

hot shale in an ice world paleoceanographic evolution of the northern gondwana margin during the early paleozoic tanezzuft formation tunisia abstract the tanezzuft formation deposited in marine periglacial conditions on the northern gondwana margin during the end of the ice house climate that characterized the lowermost silurian the basal part of this sedimentary sequence is characterized by organic rich facies with locally very high measured total organic carbon toc content up to values greater than 20 while deposition of organic rich sediments during greenhouse time interval is well known deposition of black shales during ice house conditions is poorly documented the extraordinary paleoceanographic conditions that led to the accumulation and preservation of enormous amounts of organic matter in periglacial settings makes this formation an atypical example of black shales deposition the study area is situated on the north african platform in southern tunisia on the northern flank of the ghadames basin petrophysical logs biostratigraphic organic and inorganic geochemical data from nine wells are here integrated with the aim of reconstructing the depositional history of the tanezzuft formation and the role played by organic matter production preservation and dilution through time data indicate that dilution that is depositional style and framework had an important control on lateral and vertical distribution of the c organic rich facies toc and hydrogen index hi maxima are found in correspondence of the main transgressive phases with repetitive stacking patterns strongly associated with source rock properties organic matter production as observed by detailed palynofacies analysis was mainly marine in nature with important contribution by amorphous organic matter leiosphaeridia tasmanaceae and minor amounts of graptolites fragments geochemical data indicate that the organic richest interval hot shale of the tanezzuft formation deposited under severe anoxic conditions that resulted in enhanced organic matter preservation at the sea floor of mod 3d an organic facies modelling software tool by sintef is used in order to reconstruct and quantify the peculiar processes that controlled the exceptional accumulation of organic matter at the time of deposition modelling results indicate that water stratification was the most plausible process that drove organic matter sedimentation under mesotrophic conditions density stratification was probably controlled by the deglaciation phase after the hirnantian glaciation and or by the flooded complex basin physiography that induced an inefficient circulation of the water masses the complete recovery from these extreme paleoceanographic conditions implied progressive processes that took several hundred thousand years

sensitivity of biomarker properties to depositional environment and or source input in the lower toarcian of sw germany abstract a sequence of lower toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil to source correlation lower liassic marls are representative of a well aerated shallow water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions various properties have been found to vary considerably within these two units amongst the most important are carbon isotopic composition of the kerogen the pristane phytane ratios nickel as opposed to vanadyl porphyrins and the c27 dia regular steranes although maturation within the profile does not change some of the maturation dependent biomarker properties such as the monoaromatic steroid side chain cracking and the tm ts ratio exhibit large changes which can be assigned to diagenetic processes another maturation dependent property the 20s 20r epimerization of c29 steranes exhibits smaller changes which could also be due to early diagenetic processes the study suggests that reducing and oxidizing conditions i e eh and ph in the sediment exert an influence on several biomarker precursor product pathways in maturation studies initial variations due to depositional conditions have therefore to be taken into account

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

sheath folds formed by drag induced by rotation of rigid inclusions in viscous simple shear flow nature and experiment abstract natural examples of fold patterns associated with rotated boudins occur in the ossa morena zone s portugal and triggered the present study in ductile marbles with embedded competent mafic boudins the folds seem to have originated by rotation of these bodies and show differential development from gentle deflections to sheath folds this suggests a dependence of the structure type on the distance of the metamorphic layering to the rigid body the experimental work presented here was performed with rotating inclusions in bulk simple shear and introduces a new variable the distance of the marker layer from the rigid inclusion d expressed in terms of e the ratio between the spacing between marker layers d and the greater principal dimension of the inclusion a contrary to previous experimental work in the literature we used planar marker layers without concomitant boudinage our results show new processes of development of sheath folds from planar layers and also that some non cylindrical layer deflections are transient and thus that sheath folds do not originate from them when d is small and the inclusion starts with its longest axis normal to the shear plane sheath folds develop by drag pull processes on originally planar marker layers transient non cylindrical deflections develop on originally planar marker layers when the distance between the marker layers is smaller than the longest principal dimension of the rigid particle the monoclinic symmetry of the drag sheath fold patterns associated with rotating rigid inclusions indicate non coaxial flow and can be used as a shear sense criterion quarter structure

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

comparison of linear regression and a probabilistic neural network to predict porosity from 3 d seismic attributes in lower brushy canyon channeled sandstones southeast new mexico the lower brushy canyon formation of the delaware basin new mexico consists of a series of overlying sand filled channels and associated fans separated by laterally extensive organic siltstone and carbonate interbeds this laterally and vertically complex geology creates the need for precise interwell estimation of reservoir properties in this paper we integrate wireline log and 3 d seismic data to directly predict porosity in the area of an existing oil field in southeast new mexico the 3 d seismic data were used to interpret the location of major stratigraphic markers between wells and these seismic horizons were used to constrain a time window for a volume based attribute analysis stepwise regression and crossvalidation were used to combine seismic attributes to predict porosity in wells where the porosity was known from the well logs the results of a linear regression porosity model showed good correlation r2 0 74 between seven seismic attributes and the observed porosity logs at 11 wells in

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

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

stratal order in pennsylvanian cyclothems interpretation of continental to shallow marine sedimentary sequences is commonly based on a perception of the presence of order in the upsection recurrence and or arrangement of one or more rock types these patterns of stratal organization span a spectrum of scales ranging from thick transgressive to regressive sequences within passive continental margins to thin upward shallowing parasequences reported from many peritidal carbonate successions however in spite of the widespread acceptance of this idea little quantitative assessment has been conducted to ascertain the relative dominance of stratal order and disorder in sedimentary sections because perception of high frequency stratigraphic repetition is especially prevalent in pennsylvanian cyclothemic terrestrial to marine successions generally coeval with gondwanan glaciation hypotheses of stratal order have also been widely linked to possible periodic changes in global sea level because of the general importance of implied relations between stratal order and glacioeustasy lithofacies thicknesses recurrences and transition frequencies were examined in outcrops of west central illinois which comprise the type region of cyclothem deposition in order to evaluate the incidence of depositional cyclicity at this rather intermediate scale of consideration data on compositions thicknesses and vertical stratigraphic arrangement of 1031 units were derived from measured sections of wanless these comprise 11 distinct lithofacies types that span 868 m of section exposed at 44 localities philosophically stratigraphic cyclicity could be represented as expectation in lithofacies thickness lithofacies recurrence or as some regular pattern in lithofacies ordering frequency distributions of lithofacies thicknesses and stratigraphic separations between recurrent lithofacies are exponential in form these distributions of stratigraphic intervals would result if horizons of lithologic change or recurrence occurred independently randomly in sedimentary successions stratigraphic cyclicity manifested as lithofacies ordering is evaluated by considering frequencies with different rock types succeed each other in stratigraphic sections presumption of cyclothemic order requires that lithofacies9 transitions exhibit some manifestation of statistical predictability markov chain analyses indicate that most high frequency transitional order exists at underclay to coal horizons because underclay development may in part reflect a post depositional diagenetic process that was associated with plant growth and coal formation underclay to coal couplets may exist as one lithofacies association as opposed to two genetically unrelated superjacent units few other transitions exhibit statistically significant predictability that cannot be ascribed to linkages between gradational rock types or to low frequency formation scale variation in the bulk rock composition of the aggregate illinois succession in addition to these statistical attributes cyclothemic associations designated in numerically generated random sequences that have the same lithofacies attributes as do the illinois sections exhibit virtually identical thicknesses and abundances of lithofacies elements as those recognized by wanless thus we conclude that the classic pennsylvanian successions in west central illinois exhibit no compelling evidence of high frequency cyclic order either as regularity in the recurrence or in the compositional ordering of lithofacies elements on the basis of these considerations we suggest that thicknesses recurrences and degrees of stratal order among pennsylvanian lithofacies reflect largely uncorrelated durations intensities and styles of sediment deposition along the western margin of the illinois basin moreover portrayal of cyclothemic order in the illinois sections was found to be distinct from the rather obvious alternation of marine and terrestrial units that characterize these and many other carboniferous successions presumption of recurrent change in sea level is an assumption that is largely unrelated to the presence or absence of cyclothemic scales of stratal ordering if these illinois sections are at all typical of other carboniferous successions then cyclothemic order manifested as recurrent and potentially periodic accumulation may be much less evident than is generally perceived to be the case during this important interval of earth history

geophysical constraints on the crustal structure of the southern rio grande rift abstract evidence of geologic activity still occurring in the rio grande rift rgr includes quaternary faulting seismicity and widening at a small rate we map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern rgr srgr specifically we computed receiver functions and receiver function stacks for 147 stations from earthscope usarray transportable array and previously collected data in the southwestern united states and interpolated the crustal and velocity results using a kriging interpolation scheme we include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles we produce constrained crustal models that characterize the srgr as showing a shallower moho 30 km in the region of the srgr which is thicker than previously suggested the crust appears to be delaminated west of the rgr in the basin and range province and thicker east of the rgr toward the great plains with a denser lower crustal layer we also find that the delaware basin shows a significant geophysical anomaly likely due to thick saturated sediments given that there is no deep mantle anomaly determined from other studies in this region we suggest that rifting could be the result of small scale convection of the uppermost mantle and we conclude that the rgr is at a minimum the boundary between the basin and range province and the great plains perhaps reflecting the terminus of flat slab subduction

30 norhopanes their occurrence in sediments and crude oils abstract sediments and crude oils have been analysed using gc ms techniques and shown to contain two homologous series of hopanes namely 30 nor 17 h hopanes and 25 30 bisnor 17 h hopanes as well as the common 17 h hopanes homologues c28 to c34 of the 30 nor 17 h hopane series have been identified in crude oils associated with carbonate source rocks the abundances of these hopanes relative to the 17 h hopanes in the soluble organic matter from carbonate sediments has been shown to depend upon sample maturity the more mature samples having higher relative abundances of these norhopanes a comparison of the relative abundances of the norhopanes in the soluble organic matter of sediments and in the hydrocarbon fraction released when the extracted sediments were pyrolysed showed that the 30 nor 17 h hopanes were more abundant in the pyrolysis product these observations indicate that the proportion of norhopanes relative to normal 17 h hopanes in bound form in the kerogen is greater than that present in the soluble form and so kerogen cracking proceeds with increased maturity of the sediments and a relative enrichment of these norhopanes occurs in the soluble fraction the series of 25 30 bisnor 17 h hopanes has been identified in a severely biodegraded crude oil from a putative carbonate source this observation together with the reported occurrence of enriched 25 norhopanes in severely biodegraded crude oils suggests that 25 30 bisnor 17 h hopanes may be biomarkers for severely biodegraded crude oils from carbonate rich source rocks

controls on the geometry of tails around rigid circular inclusions insights from analogue modelling in simple shear abstract we used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems results show that 1 inclusions can develop in both slipping and non slipping modes but inclusions only form in the latter 2 inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix inclusions form when the mantle is initially outside the separatrix and later comes inside this line 3 in the slipping mode the wedge shaped tail of inclusions always has a straight external surface parallel to the shear plane whereas in the non slipping mode the external surface is curved inwards external embayments 4 together with earlier theoretical results and inclusions always show stair stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation 5 maximum stair stepping occurs in the slipping mode and is at least equal to the inclusion diameter if our models bear significant similarity to nature then i the straight or curved character of inclusions could mean that they had respectively a slipping or non slipping interface with the surrounding recrystallized matrix and ii inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems

progressive unconformities within an evolving foreland fold thrust belt zagros mountains a major angular unconformity between the bakhtyari conglomerates and the underlying agha jari formation has long been interpreted as indicating that orogeny in the zagros simply folded zone took place in plio pleistocene times this study uses field evidence of unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the zagros simply folded zone has propagated in time and space these unconformities indicate that deformation started as early as end eocene in the northeast of the simply folded zone and propagated progressively to the southwest where unconformable contacts are only seen between younger units as shortening continued the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the persian gulf and mesopotamia the climax of orogeny took place at end pliocene time when the most extensive unconformity in the zagros simply folded zone developed between the upper bakhtyari formation and older units active seismicity and documented present uplift imply that the simply folded zone is still propagating southwestward

detachment folding in the central and eastern zagros fold belt iran salt mobility multiple detachments and late basement control abstract detachment folding has been studied on the basis of field and subsurface examples from the central and eastern zagros iran we discuss different aspects of detachment folding well illustrated in the studied area in particular we focus on salt mobility multiple decollements and late basement control salt mobility concerns the hormuz basal detachment and the gachsaran upper detachment for the latter it is shown that mobility results not only from folding related diapirism but also from early gravity driven migration from growing anticlines towards intervening synclines concentric folding between two detachment levels is directly observed in the izeh zone central zagros where it is shown that the wave length depends on the distance between the two active decollements and that the fold shape is a function of the level of erosion throughout the zagros detachment folds mainly developed during an initial thin skinned phase of deformation which was followed by the current thick skinned stage this succession is particularly well expressed in the eastern zagros where basement faults cut early detachment folds obliquely

pebble shale early cretaceous depositional environments in national petroleum reserve in alaska npra abstract a pebble shale of lower cretaceous age occurs across the north slope and continues into northwestern canada this organic shale 1 to 5 organic carbon is possibly the source for the prudhoe bay hydrocarbons and includes localized well developed sand bodies such as those in the giant kuparuk oil field the inferred rifting of the arctic basin subsequent subsidence of a northern source area and the southern orogeny during the late jurassic and early cretaceous contributed to the unique lithology and regional setting of the pebble shale the pebble shale in npra consists of black anaerobic dysaerobic shales silty aerobic shales pebbly mudstones and sandstones the shales contain matrix supported very well rounded aeolian derived fine to very coarse floating quartz grains deposition of these diagnostic floating quartz grains occurs rarely in the late oxfordian late jurassic time by are abundant during the neocomian early cretaceous in the south and central area of npra on the southern flank of the barrow arch an almost continuous sedimentation record of pebble shale deposition exists as penetrated by the tunalik 1 well to the north the formation thins in a series of intraformational unconformities converging on the barrow arch overlying the uppermost unconformity on the barrow arch a pebbly mudstone 3 o 8 m 10 to 26 ft thick of hauterivian barremian age contains well rounded sand grains pebbles and cobbles pelletal glaunconite shell fragments wood chips and burrows chert pebbles from this pebbly mudstone were processed for radiolarians and recovered spumellarians of probable pre late devonian age a thin zone basal to an intra pebble shale sand at walakpa no 2 contains siderite and appreciable phosphate in the form of carbonate fluorapatite a zone of intense gamma radiation grz which is an easily traced seismic and gamma ray log horizon across npra is a black carbonaceous papery shale with a end page 424 relatively high above background uranium thorium concentration 11 versus 3 ppm u 26 versus 8 ppm th previously this zone marked the upper boundary of the pebble shale however two horizons are recognizable within the gamma zone the lower one containing rounded quartz grains and hence pebble shale and the upper one devoid of floating quartz grains clay mineralogy organic geochemistry micropaleontology and sedimentary structures from core and cuttings indicates a stratigraphic sequence through the neocomian of upper to lower slope facies followed by inner and outer shelf facies and finally deposition of euxunic sediments the pebbly mudstone facies derived from a northerly source is interpreted as a lowstand and subaqueous delta environment formed as the northern provenance was uplifted during valanginian time rapid subsidence of this basin margin was related to the inferred atlantic margin type downfaulting north of the barrow arch the subsequent deposits produced a blanket of sediment 300 000 mi2 482 700 km2 in areal extent thin 2 to 9 ft 0 8 m fine grained sands occur within the pebble shal of the barremian silty shale facies and are restricted to the barrow area these sands have an average combined thickness of 45 ft 15 m and contain traces of oil paleogeographic considerations imply better reservoir and coalescing of these sands to the north toward the paleoshoreline and suggest potential for discovery of hydrocarbon reserves from drilling locations on the northern barrier island systems and offshore end of article last page 425

burial history reconstruction and thermal maturity modeling for the middle cretaceous early miocene petroleum system southern dezful embayment sw iran abstract the middle cretaceous early miocene petroleum system is the most important hydrocarbon system in the zagros basin rock eval pyrolysis organic petrography and thermal 1d modeling were performed on wells located in the southern dezful embayment the results provide information on organic matter quantity and quality as well as burial and thermal histories of source rocks in the framework of the basin evolution a total of 98 cutting samples from bk 4 kk 1 si 4 and rs 2 wells were pyrolyzed by rock eval 6 also thermal maturity of organic matter derived from tmax of rock eval was interpreted in conjunction with vitrinite reflectance data 50 samples the toc values of the sequences mainly range from 0 5 to 2 5 wt the average values of tmax and vitrinite reflectance indicate that samples from the wells have reached maturities corresponding to early mid oil generation reconstruction of thermal history suggests that various steady heat flow values 62 54 50 and 56 mw m2 for bk 4 kk 1 si 4 and rs 2 wells respectively resulted in the best fit between the calculated and the observed vitrinite reflectance and bottom hole temperatures in our modeling thus values of heat flow decrease from 60 mw m2 in the southern basin margin to about 50 mw m2 along the ne of the dezful embayment and the zagros basin according to our modeling results the onset of oil generation from source rocks in the studied area occurs well after deposition of the seal rock and formation of traps ensuring entrapment and preservation of migrated hydrocarbons