maceral composition and origin of organic matter input in neoproterozoic lower cambrian organic rich shales of salt range formation upper indus basin pakistan abstract the appropriate thermal maturity oil window or lower of the neoproterozoic rocks is a prerequisite for preserving robust and primary biological assemblages the neoproterozoic lower cambrian organic rich shales present in the upper part of salt range formation pakistan with abundant organic matter om total organic carbon toc up to 46 at relatively low thermal maturity protobitumen reflectance bro 0 2 0 4 represents by far the lowest thermal maturity neoproterozoic lower cambrian rocks analyzed to date the detailed petrographic description of neoproterozoic lower cambrian organic matter in the current study was lacking from previous source rock studies of coeval strata from oman eastern siberia australia and elsewhere eight organic rich shales of the salt range formation from three different outcrops have been analyzed for maceral composition and source rock characterization using organic petrography scanning electron microscopy and standard source rock assessment techniques the organic petrography reveals the organic rich shales are predominantly composed of alginites lamalginite and telalginite bituminite protobitumen and mineral bituminous groundmass overall the maceral assemblages suggest the deposition of these shales took place under a salinity stratified water column in a shallow restricted basin where both oxygenic and anoxygenic modes of primary production were operative the top oxygenated part of the water column was populated with the phototropic mat forming cyanobacterium whereas the lower low dissolved oxygen zone was dominated by algal mats and sulphate reduction bacteria the rare and scattered thucholites present within the salt range formation shales were derived from the radiation induced polymerization of adjacent om by the radioactive grains the heat from the radioactive grains however found to have a negligible effect on the overall thermal maturity of these shales

the development and evaluation of a constitutive model for the prediction of ground movements in overconsolidated clay the stress strain response of overconsolidated clay depends both on its current state and on the loading history followed to reach that state in particular the relative directions of the current and previous loading paths a constitu tive soil model is developed which predicts this behaviour by allowing elasto plastic deformations controlled by two nested kinematic hardening surfaces inside a conventional modified cam clay state boundary surface this relatively straightforward model requires only eight para meters each with a rational basis and which can be determined from a small number of well controlled stress path tests predictions of soil behaviour using this model are compared with data from triaxial stress path tests the close agreement confirms that the essential features of soil behaviour are predicted by the model the wider implications of the use of the model in geotechnical analysis are illustrated by compar ing predictions made using the model in conjunction with finite element analys

recognition of an infracambrian source rock based on biomarkers in the baghewala 1 oil india heavy sulfur rich oil produced from the infracambrian 540 640 ma jodhpur formation in the baghewala 1 well represents a new exploration play in the bikaner nagaur basin in india and the punjab basin in pakistan the baghewala 1 oil is nonbiodegraded and thermal maturation dependent biomarker ratios indicate generation from the source rock within the early oil window age diagnostic and source dependent biomarkers indicate that the oil originated from algal and bacterial organic matter with no higher plant input in an infracambrian carbonate rich source rock deposited under anoxic marine conditions these characteristics support a local origin of the baghewala 1 oil from organic rich laminated dolomites in the infracambrian bilara formation significant amounts of petroleum could originate from equivalents of the proposed bilara source rock in the punjab basin pakistan where the precambrian to lower paleozoic section is thicker and more deeply buried than in india deeper burial of the source rock in the punjab basin than in the bikaner nagaur basin could generate more mature equivalents of the baghewala 1 oil the baghewala 1 oil is geochemically similar to another heavy oil from the infracambrian salt range series in the nearby karampur 1 well in pakistan and to oils derived from carbonate evaporite facies of the infracambrian huqf group about 2000 km 1243 mi to the southwest in the eastern flank province of southern oman these findings are consistent with published evidence that subsiding rift basins in northwest india pakistan and southern oman were in close proximity during the infracambrian along the middle eastern edge of gondwanaland

petroleum systems of oman charge timing and risks after 35 years of exploration creaming of the conventional plays in oman is nearly complete and consequently the search has commenced for new less obvious plays many of the new opportunities occur beyond the known hydrocarbon provinces and are considered to have significant charge risks to define these risks extensive basin modeling studies have been conducted in recent years modeling and empirical data show that mesozoic and cenozoic kitchen areas are restricted to western north oman the only areas currently buried at their maximum temperature large parts of north and central oman depend on lateral migration from these kitchens for their charge progressive uplift of the east flank and basin inversion since the middle paleozoic provides favorable conditions for long distance migration in the post carboniferous interval in central oman geochemical tracer molecules benzocarbazoles suggest that a north south trending reactivated basement grain has funneled charge up to 300 km southeastward charge risks increase in the deeper sequence in which eastward migrating hydrocarbons have to traverse the ghaba salt basin a pronounced syncline at depths greater than 3 km the south oman salt basin is currently cool because of shallow depths and hydrodynamic fluid flow activity the shallow post cambrian reservoirs rely on storage of early cambrian ordovician charge by the ara salt cambrian sequence followed by release of hydrocarbons as the salt edge retreats through time basin modeling has outlined the extent of the different petroleum systems and provided us with risk maps to guide our next exploration phase it has revitalized some of the mature plays for instance the gharif formation where oil exploration is now focused along late cretaceous and tertiary migration paths deeper sections are envisaged to have significant scope for gas

modelling effects of recent suction history on small strain stiffness of unsaturated soil recent suction history has been found to affect shear modulus of unsaturated soil at small strains i e from 0 001 to 1 in this study a bubble model for unsaturated soil is developed within

facies stratigraphy and evolution of a middle ediacaran carbonate ramp khufai formation sultanate of oman the khufai formation is the oldest carbonate platform of the cryogenian to lowermost cambrian huqf supergroup a stratigraphic characterization of this unit includes detailed facies descriptions a sequence stratigraphic interpretation and evaluation of lateral heterogeneity and overall ramp evolution the khufai formation comprises one and one half depositional sequences with a maximum flooding interval near the base of the formation and a sequence boundary within the upper peritidal facies most of the deposition occurred during highstand progradation of a carbonate ramp facies tracts include outer ramp and midramp mudstones and wackestones ramp crest grainstone shoal deposits and extensive inner ramp microbially dominated peritidal deposits outcrops in the oman mountains are deep water deposits including turbiditic grainstone and wackestone interbedded with siliciclastic rich siltstone and crinkly laminite facies patterns and parasequence composition are variable both laterally across the outcrop area and vertically through time because of a combination of ramp morphology siliciclastic supply and possible syndepositional faulting the lithostratigraphic boundary between the khufai formation and the overlying shuram formation is gradational and represents significant flooding of the carbonate platform the stratigraphic characterization presented here along with the identification of key facies and diagenetic features will help further future exploration and production of hydrocarbons from the khufai formation

abstract new model for tectonic evolution of neoproterozoic cambrian huqf supergroup basins oman oman has prolific reservoirs within salt of precambrian cambrian age but exploration efforts have been restricted due to limited understanding of the tectonostratigraphic framework new ages constrain the rift related abu mahara group to have been deposited between 730 and 700 ma whereas shelf facies of the overlying nafun group were deposited between 600 and 550 ma this 100 my unconformity precludes a simple relationship between abu mahara crustal extension and nafun regional subsidence due to thermal decay the nafun group is temporally continuous into the ara group the onset of ara subsidence occurred at ca 550 548 ma and was accompanied by a shift to arid climate and uplift of basement blocks to form smaller basins that accumulated salt and carbonates the youngest ara strata are earliest cambrian in age ghadir manquil mirbat subsidence is related to extension at ca 725 ma of a composite basement terrain comprising older archean basement blocks stitched by older pan african ca 800 ma arcs which was sutured to the younger terrains of the arabian shield at ca 625 ma nafun regional subsidence started at this time and is considered here to relate to dynamic depression of the lithosphere associated with subduction of oceanic lithosphere beneath the arabian plate subduction is inferred to have occurred from the northeastern margin of gondwanaland eastern oman where an andean margin resulted in orographic retroarc desiccation in addition to uplift of basement blocks where subduction dip was gentle analogous to the bolivian segment of the andes volcanic ashes and calderas within the ara group record subduction related melting of the crust in a retroarc setting

determination of the anisotropic shear stiffness of an unsaturated decomposed soil the shear modulus at very small strains 0 001 or less g0 is one of the most important parameters for predicting ground movements and dynamic responses of many engineering earth structures wit

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

recognition of a novel precambrian petroleum system based on isotopic and biomarker evidence in yangtze platform south china abstract the precambrian doushantuo formation is regarded as a possible petroleum source rock horizon with toc values of 0 85 1 85 and type i kerogen in northwestern sichuan basin yangtze platform south china while showing high to post mature thermal maturity a set of outcropping bitumen and oil sands from various reservoirs display overall depleted values of 13 c from 36 to 32 average 35 typical of a precambrian source in addition the sterane distributions in these bitumens and oils are unusual compared to marine phanerozoic oils with a predominance of c 29 members as well as their triaromatic counterparts high amounts of 20 n alkylpregnane with very low diasteranes indicate restricted sulfur rich conditions typical of an anoxic carbonate source rock this interpretation is supported by some other specific biomarkers such as abundant 30 norhopane and c 24 tetracyclicterpane and elevated c 35 hopane furthermore the presence of gammacerane indicates a saline stratified water column hydrocarbons extracted from the solid bitumen and oil sands have many compositional characteristics in common with other oils derived from precambrian carbonate source rocks from elsewhere this implies the existence of a precambrian marine petroleum system in the yangtze platform with the doushantuo carbonate as the likely source it is supposed that the reduction of temperature and pressure following basin uplift resulted in petroleum fractionation deasphalting in lower paleozoic reservoirs with the fractionated oils having migrated along the faults becoming trapped in stratigraphically younger reservoirs where the petroleum underwent heavy biodegradation

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

a state dependent elasto plastic model for saturated and unsaturated soils a state dependent elasto plastic constitutive model for both saturated and unsaturated soils is presented in this paper the model which is developed under an extended critical state framework uses two independent stress state variables net normal stress and matric suction in addition the influence of a simplified non linear soil water characteristic curve on mechanical behaviour is incorporated in the model by using a bi linear function most importantly a state dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio internal state density and soil suction in this paper details of the model formulation and the determination of model parameters are described and reported by using the new model numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine grained sandy silt and coarse grained gravelly sand weathered soils the simulated triaxial tests include undrained shear tests on saturated specimens

biomarker signatures of the ediacaran early cambrian origin petroleum from the central sichuan basin south china implications for source rock characteristics abstract hydrocarbons with an ediacaran early cambrian origin from the central sichuan basin south china were investigated for biomarker compositions they were obtained by solvent extraction from the upper ediacaran reservoir core samples which contain abundant solid bitumen the solid bitumen is widely believed to be formed by thermal cracking of early charged oils in the reservoirs which have presumed source rocks of an ediacaran early cambrian age thermal maturity parameters based on phenanthrenes and aromatic steroids suggest a very high thermal maturity for the extracted hydrocarbons which is consistent with the severe maturation undergone by their host reservoirs age diagnostic biomarkers including c30 24 isopropylcholestanes and mid chain monomethyl alkanes support source rocks of an ediacaran early cambrian age the extracted hydrocarbons have very similar biomarker signatures which include high relative abundance of acyclic isoprenoids and pristane phytane 1 and the presence of c26 norcholestanes methyl and ethylsteranes the presence of hopanoids including extended series and methylhopanes indicates contributions from prokaryotic bacteria while the presence of various steroids including regular steranes and methylsteranes indicates contributions from the eukaryotic algae parameters based on tricyclic terpane and hopane dibenzothiophene phenanthrene ratios and c27 diasteranes diasteranes regular steranes ratios suggest a major shale source but a mixed shale carbonate source cannot be completely excluded presence of c30 demethylsteranes low pristane phytane ratios high gammacerane index and presence of 28 30 dinorhopane and 2 methyhopanes indicate that the source rocks were deposited in a stratified anoxic marine depositional environment with enhanced salinity conditions

extinction of cloudina and namacalathus at the precambrian cambrian boundary in oman biostratigraphic carbon isotope and u pb zircon geochronological data from the ara group of oman indicate an abrupt last appearance of cloudina and namacalathus coincident with a large magnitude but short lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the precambrian cambrian boundary u pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542 0 0 3 ma consistent with previous age constraints from siberia and namibia combined with the global biostratigraphic record these new data strengthen hypotheses invoking mass extinction within terminal proterozoic ecosystems at or near the precambrian cambrian boundary

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

saturated hydrocarbon biomarkers in oils of late precambrian age from eastern siberia abstract large quantities of petroleum derived from source rocks of late precambrian early cambrian age have been discovered in the lena tunguska region of eastern siberia over the last twenty five years we have examined three oils with presumed late precambrian vendian source rocks from this region for c 15 saturated hydrocarbon biomarkers using gas chromatography gc and gas chromatography mass spectrometry gc ms our results show these oils to have many characteristics in common with other oils of similar age from eastern siberia and oman amongst the compounds detected in the three oils analyzed were 1 n alkanes which show a marine derived distribution 2 centrally branched monomethyl alkanes 3 acyclic isoprenoids pr ph 1 4 tricyclic and tetracyclic terpanes 5 hopanes 6 c 29 unrearranged steranes and 7 c 30 nuclear steranes all of these compounds with the exception of the 4 methylsteranes which on current biochemical knowledge are unexpected in oils of this age can be assigned a prokaryotic source this and other evidence such as the lack of diasteranes suggest microorganism were the major contributors to organic matter deposited as part of a carbonate source rock

distinctive hydrocarbon biomarkers from fossiliferous sediment of the late proterozoic walcott member chuar group grand canyon arizona abundant extractable hydrocarbons in association with well preserved kerogen have been recognised in a dolostone from the chuar group approx 850 ma the biomarkers present include c15 c30 acyclic isoprenoids c26 c28 steranes c27 c35 hopanes and extended tricyclic terpanes among the striking characteristics of the distributions are the dominance of steranes and methyl steranes lacking side chain alkylation and the presence of putative c29 c35 neohopanes and gammacerane the high abundance of steranes is consistent with the presumed eukaryotic affinities of several types of microfossils including chuaria present in associated mudstones this stratum is the oldest in which gammacerane has been found and suggests contributions from protozoa to the depositional environment which appears to have been hypersaline there is a strong correlation between the carbon isotopic composition of the kerogen and that of the various polarity fractions of the bitumens chemical degradation and pyrolysis of the kerogens yielded n alkanes with similar characteristics to the bitumens but there are differences in the patterns of distribution of the acyclic isoprenoid sterane and triterpane biomarkers

petroleum inclusions atop unayzah gas condensate reservoir signpost for an undocumented chapter of the arabian basin filling history abstract the carboniferous permian unayzah gas condensate reservoir of the ghazal field in eastern saudi arabia contains abundant oil inclusions indicative of a palaeo oil column the molecular composition of a fluid inclusion rich zone at the top of the reservoir revealed a mid mature biomarker distribution 0 80 vitrinite reflectance equivalent vre that is considerably lower than that of the co occurring reservoired fluid 1 41 vre side chain cracking of the triaromatic steroids evident in the produced condensate is significantly less advanced in the inclusion oil as is also reflected in their alkylnaphthalene distributions the lower maturity signal in the inclusion oil is combined with calcareous related biomarkers that include relatively abundant 30 nor 17 hopane c24 tetracyclic terpane and alkyldibenzothiophenes in contrast to the reservoir hydrocarbon fluid derived from the lower silurian qusaiba shale inclusion gas is enriched in the higher hydrocarbons which might be attributed to a high proportion of oil or wet gas inclusions fluid inclusion ethane and propane 13c are very close to the mean of the produced gases but carbon dioxide 13c is about 5 and methane 13c 2 less negative than the average of the produced ghazal gases which may signify more than one hydrocarbon generation population within the fluid inclusions an earlier contribution into the unayzah reservoir from a non qusaiba source may therefore be inferred to have had preceded the volumetrically more significant and more mature qusaiba charges further research is needed to elucidate the source of this oil charge and its extent which could include deep cambrian or neoproterozoic sources that might be present in the region

understanding carbonate ramp dynamics using δ13c profiles examples from the neoproterozoic buah formation of oman the late neoproterozoic buah formation nafun group oman is a carbonate unit outcropping in the jabal akhdar and huqf areas it is composed mostly of shallow water carbonates deposited on a distally steepened carbonate ramp correlation of two 13c isotope shifts shows that in the jabal akhdar ramp differentiation into fast and slow subsiding areas was followed by lateral progradation in the huqf area however a uniform scenario of upward shallowing of the facies and lateral progradation is demonstrated by chemostratigraphic timelines cross cutting the facies belts the chemostratigraphic profiles show that the buah formation was deposited during sea level highstand conditions and that ramp differentiation was due to synsedimentary tectonics high resolution correlation of 13c profiles from the same lithostratigraphic unit whether precambrian or phanerozoic in age lacking biostratigraphic data can shed light on carbonate systems dynamics tectonic vs eustatic controls on depositional sequences and basin subsidence

origin of free and bound mid chain methyl alkanes in oils bitumens and kerogens of the marine infracambrian huqf formation oman the huqf oil from oman and other time equivalent oils are characterized by relatively large contents of mid chain branched monomethyl alkanes the huqf oil the saturated hydrocarbon fractions the desulfurized polar fractions of the bitumens and the kerogen pyrolysates of three potential source rocks from the huqf formation oman were analyzed in order to resolve the origin of these mid chain branched monomethyl alkanes using off line pyrolysis and chemical degradation in combination with compound specific carbon isotope analysis it was assessed that the mid chain branched monomethyl alkanes are not rearrangement products but are derived from a biological source probably from lipids with c28 carbon skeletons and methyl branching at the 12 or 13 position these compounds were incorporated in the kerogen through reactions of sulfur with functional groups at different positions in the precursor lipid s

neoproterozoic early cambrian biota and ancient niche a synthesis from molecular markers and palynomorphs from bikaner nagaur basin western india abstract the time frame of present study is neoproterozoic early cambrian an enigmatic and consequential time in the history of earth the present study elucidates the terminal proterozoic biota and palaeoenvironment by characterizing the soluble and insoluble fractions of organic matter extracted from sediment core samples recovered from marwar supergroup bikaner nagaur basin western india the normal alkanes range from c15 to c33 wherein c19 to c21n alkanes predominate and carbon preference index ranges from 1 12 to 1 19 pristane phytane ratio varies between 0 9 and 1 2 the kerogen matter occurs as homogenous laminae and discrete organic remains which are mostly derived from microbial mat triterpenoids include cheilanthanes and hopanes which are confirming evidences of microbial input algal presence has also been confirmed from palynological data green algae evolved to be a major biota during the neoproterozoic early cambrian time and this is validated by the appreciable abundance of c29 steranes in the bitumen fraction the amorphous pieces of kerogen residues lack any visibly distinct internal features however a very few simple ornamented acritarchs like micrhystridium sp lophosphaeridium sp and leiosphaeridia sp are observed in the organic residues micro ftir spectroscopy of the amorphous organic particles reveals the presence of long chain aliphatic components and unusually high c c aromatic ring stretching band the organic remains were deposited in restricted marine water and elevated saline condition as indicated by several salinity biomarkers the present study clearly demonstrates a microbial mat dominated niche with significant eukaryotic green algae that had been capable of surviving in the high salinity of the terminal proterozoic oceans

mean winds tides and quasi 2 day wave in the polar lower thermosphere observed in european incoherent scatter eiscat 8 day run data in november 2003 1 simultaneous european incoherent scatter eiscat radar observations using two eiscat radars the tromso 69 6 n 19 2 e uhf radar and the eiscat svalbard radar longyearbyen 78 2 n 16 0 e were conducted for 8 consecutive days from 11 to 19 november 2003 to study the lower thermospheric wind dynamics in the polar region altitude profiles of the amplitudes of the diurnal and semidiurnal components at tromso and longyearbyen are similar the semidiurnal amplitudes in the meridional and zonal components exhibit maxima at 105 107 km with values of 70 90 m s 1 the semidiurnal phases vary with roughly a 30 km vertical wavelength between 98 and 110 km the quasi 2 day wave q2dw was not detected in the lower thermosphere at either tromso or longyearbyen during the period while it was found between 70 and 82 km in the mesosphere with the colocated tromso mf radar this observational result suggests that in general the q2dw attenuates in the mesosphere and cannot penetrate into the lower thermosphere in winter ion drag acceleration of the wind is generally negligible below 107 km at tromso and below 118 km at longyearbyen but significant ion drag acceleration is found above these heights a comparison of mean wind and tidal amplitudes and phases with national center for atmospheric research thermosphere ionosphere mesosphere electrodynamics general circulation model predictions for the period shows some agreement

the dhahaban petroleum system of oman the dhahaban petroleum system from a source rock of cambrian age covers some 50 000 km2 and contains 1 6 x 109 m3 of oil and 1000 x 109 m3 of gas in place of which at least respectively 0 35 x 109 m3 and 700 x 109 m3 are recoverable for many years the origin of the so called q oil was enigmatic and defied typing to a source rock integration of recent advances in geochemistry and basin modeling has now permitted us to retrace its source areas model the generation and migration histories and outline the areal and stratigraphic extent of the petroleum system

biomarker signatures of sinian bitumens in the moxi gaoshiti bulge of sichuan basin china geological significance for paleo oil reservoirs abstract chen et al precambrian research 296 1 19 2017 presented a biomarker study on solid bitumens and associated source rock extracts from the moxi gaoshiti bulge of the sichuan basin sw china biomarker characteristics revealed that the solid bitumens and related paleo oils belong to the same group and are well correlated to source rocks of the neoproterozoic dengying and lower cambrian qiongzhusi formations the thermal maturity of solid bitumens and their migration direction have been assessed it appears that these interpretations are internally inconsistent and are also inconsistent with much published geochemical data and the biomarker data are over interpreted and flawed

methylhopane biomarker hydrocarbons in hamersley province sediments provide evidence for neoarchean aerobiosis abstract bacteriohopanepolyols with a ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa when preserved in ancient rocks the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies considering both their occurrences in modern organisms and their proterozoic and phanerozoic sedimentary distributions hopanes carrying 2 methyl or 3 methyl substituents are proposed to be derived from cyanobacteria and oxygen respiring methanotrophs respectively here we report the distribution of methylhopanes in 2 72 2 56 billion year old neoarchean rocks from the hamersley province on the pilbara craton the relative abundance of c 31 3 methylhopane but not that of c 31 2 methylhopane shows a strong correlation to the carbon isotopic composition of co occurring kerogen insoluble particulate organic matter the unanticipated nature of this relationship provides evidence for a shallow water locus of carbon cycling through aerobic oxidation of ch 4 and coincidentally a means to demonstrate biomarker syngenicity 2 methylhopanes are most abundant in both shale and carbonate from shallow water sediments and a positive correlation to carbonate abundance contrasts to variable but generally lower 2 methylhopane abundances of deeper water facies these observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow water ecosystems providing molecular oxygen fixed carbon and possibly fixed nitrogen

50 myr recovery from the largest negative δ13c excursion in the ediacaran ocean sedimentary rocks deposited during the ediacaran period 630 542 ma contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from 5 to 12 followed by a sub linear recovery to positive 13c values isotopic ages u pb and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at 600 ma and duration of recovery of approximately 50 myr the excursion is widely recognized in oman and has potential correlatives in ediacaran strata elsewhere and may thus represent a characteristic feature of the ediacaran period the amplitude of this carbon isotope excursion far exceeds those of other neoproterozoic anomalies the isotopic trend of negative excursion and long term recovery spanned at least one short lived glacial episode at 580 ma but appears unrelated to glaciation which indicates that negative anomalies in the neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages

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

discovery of the lower cambrian high quality source rocks and deep oil and gas exploration potential in the tarim basin china the global precambrian cambrian system includes an important series of hydrocarbon bearing strata however because rocks of this age are typically deeply buried few petroleum exploration breakthroughs have been made and the presence of source rocks remains somewhat controversial recently commercial condensate and gas were discovered from the deep 6900 m 22 600 ft zhongshen 1c zs1c exploratory well drilled in the tazhong uplift of the tarim basin china leading to renewed interest in the development of cambrian source rocks in the basin on the basis of outcrop reconnaissance and sample testing from around the tarim basin we show that a set of high quality source rocks were developed within the lower cambrian yuertusi formation j1y at the base of the lower cambrian these rocks are black shales and typically have a total organic carbon content between 2 and 6 but extending as high as 17 in selected regions this marine sequence is 10 15 m 33 49 ft thick in some outcrops along the margins of the basin seismic data indicate that these high quality source rocks may cover an area as large as 260 000 km2 100 000 mi2 their main organic parent material was benthic multicellular algae on the basis of high temperature thermal simulations conducted on these source rocks we show that the gas composition and carbon isotopes from the zs1c well are similar to the products generated at a thermal evolution stage corresponding to a vitrinite reflectance of between 2 2 and 2 5 late stage natural gas accumulated within these rocks over time the 34s correlation of organic sulfur compounds in the condensate with cambrian sulfates provides further evidence for a j1y source rock origin of the zs1c condensate and gas the cambrian dolomites in association with a salt seal exhibit favorable geological conditions for large scale hydrocarbon accumulation a new set of deep exploration strata can therefore be developed guiding future deep cambrian hydrocarbon exploration in the tarim basin

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

mapping of neoproterozoic source rocks of the huqf supergroup in the sultanate of oman using remote sensing abstract in recent years the neoproterozoic huqf supergroup formations of the oman salt basins have been the target for oil exploration the present study maps the surficial exposure of the huqf supergroup in and around khufai dome of the huqf area in the sultanate of oman using low cost multi spectral remotely sensed satellite data and image processing methods such as decorrelation stretching principal component analysis pca and spectral angle mapper sam as alternative to expensive and time consuming tools which have the capability and potential to be used by geoscientists for oil exploration in this research the study of advanced spaceborne thermal emission and reflection radiometer aster spectral bands 8 3 and 1 by decorrelation stretching well discriminated the masirah bay the khufai the shuram and the buah formations of the nafun group the source rocks of huqf supergroup with the quaternary deposits the analysis of visible and near infrared shortwave infrared spectral bands of aster by pca clearly showed the occurrence and spatial distribution of such formations in the rgb principal component images r pc1 g pc2 b pc3 the spatial distributions of such formations are assessed by confusion matrix after using maximum likelihood ml spectral angle mapper sam and spectral information divergence sid algorithms the matrix of ml algorithm has provided the best overall accuracy of 92 93 and kappa coefficient of 0 92 the minerals of the formations were detected by sam further the detection of such mineral groups was confirmed through the aster thermal infrared tir spectral indices image developed using the carbonate index ci quartz index qi and mafic index mi all results of image analyses are evaluated in the field and laboratory studies the study also evaluates the satellite data and image processing methods for the formations of jabal akhdar the equivalent formations of the khufai dome to show the sensor capability and the use of the image processing methods to study the source rocks the results of the study provided similar discriminations comparable to the khufai dome therefore the data and the techniques are recommended to the exploration geologists for use in similar regions of the world

geochemistry of the sinian cambrian gas system in the sichuan basin china abstract based on the outcrops core samples drilling cuttings seismic and geochemical data natural gases of the sinian cambrian gas system were derived from four sets of over mature high quality source rocks the cambrian qiongzhusi source rock is sapropelic with toc of 1 94 ro eq of 1 83 3 90 and a thickness of 140 m on average the sinian 620 541 ma doushantuo source rock is sapropelic with toc of 1 69 ro eq of 2 08 3 83 and a thickness of 11 m on average the 3rd member of the dengying fm is also sapropelic with toc of 1 43 ro eq of 3 16 3 21 and a thickness of 13 m on average the 2nd and 4th members of the dengying fm are dominated by algal dolomite with toc of exhibiting 0 39 ro eq of 1 97 3 46 and a thickness of 246 m on average most of the 13c2 values are lower than 28 and gas dryness is 99 indicating that the natural gas is oil derived with high maturity the high contents of isoparaffins and cycloparaffin suggest that natural gas in the sinian cambrian reservoir was originated from secondary oil cracking the cambrian longwangmiao gas was mainly derived from the cambrian qiongzhusi source rock and the gas within the sinian dengying formation mainly originated from the three sinian source rock layers the area with gas generation intensity superimposed the four sets of source rocks exceeding 6 109 m3 km2 covered 80 of the sichuan basin the proved reserves exceed 1 0 1012 m3 in the sinian cambrian gas system and the accumulated gas resource in the sinian cambrian gas system might exceed 5 1012 m3

geochemical peculiarities of pre cambrian source rocks in the east european platform abstract modern methods of organic geochemistry gc ms rock eval pyrolysis were used to investigate insoluble organic matter om and oils of upper pre cambrian riphean and vendian age from the east european platform mainly from the moscow syneclise ryasan saratov depression and the mesen depression riphean deposits up to 3 5 km thick accumulate in narrow graben structures while vendian sediments up to 1 3 km thick are more widely distributed against a background of low concentrations of om in the pre cambrian successions there are some bands of clayey rocks with om contents of significant percentage values the genetic potential of the upper riphean source rocks is 4 kg t 1 in the moscow graben the oil source potential of pre cambrian rocks has not been realized completely according to the sterane hydrocarbon hc distributions all the extract and oil samples studied can be divided into three groups the first group is well known with a strong prevalence of ethylcholestanes a second group of samples previously not known for pre cambrian om and oils has a relatively equal distribution of c 27 c 28 and c 29 steranes a third type of sterane distribution with distinct c 27 prevalence was observed in solid bitumen in riphean rocks the composition and distribution of biomarkers normal and isoprenoid alkanes triterpanes steranes in pre cambrian solid om and oils is not distinguished by specific features from om and oils of the phanerozoic the only biomarkers indicative of pre cambrian derived organic matter the 12 and 13 monomethyl substituted alkanes have not been observed in younger deposits

identification of aryl isoprenoids in source rocks and crude oils biological markers for the green sulphur bacteria abstract a series of c13 to c31 aryl isoprenoids 1 alkyl 2 3 6 trimethylbenzenes have been identified in reef hosted oils and their source rocks from the middle and upper silurian of the michigan basin and middle devonian of the alberta basin canada their structure has been confirmed by unambiguous synthesis of the c14 member of the series their structure and isotopic composition indicate that they are derived from isorenieratene from the chlorobiaceae family of sulphur bacteria these results are consistent with geological and geochemical studies that show that the source rocks were deposited under metahaline to hypersaline sulphate and sulphide rich water columns the distribution of other biomarkers in these oils and source rocks indicates that a diverse biota contributed organic matter to the source environment in conjunction with the aryl isoprenoids they show that there is a remarkable similarity in composition between the two sets of oils and source rocks despite their great temporal and geographic separation this reflects the similarity of their environments and emphasizes the importance of sedimentary facies in controlling the composition of organic matter in source rocks and their derived oils

properties and shale oil potential of saline lacustrine shales in the qianjiang depression jianghan basin china abstract the geochemical and petrographic characteristics of saline lacustrine shales from the qianjiang formation jianghan basin were investigated by organic geochemical analysis x ray diffraction xrd scanning electron microscopy sem and low pressure nitrogen adsorption analysis the results indicate that the saline lacustrine shales of eq3 member with high oil content are characterized by type i and type ii oil prone kerogen variable toc contents 1 0 10 0 wt and an early maturity stage ro ranges between 0 41 and 0 76 the mineral compositions of eq3 saline shale show strong heterogeneity brittle intervals with high contents of quartz and carbonate are frequently alternated with ductile intervals with high glauberite and clay contents this combination might be beneficial for oil accumulation but may cause significant challenges for the hydraulic stimulation strategy and long term production of shale oil the interparticle pores and intraparticle pores dominate the pore system of eq3 shale and organic matter hosted pores are absent widely distributed fractures especially tectonic fractures might play a key role in hydrocarbon migration and accumulation the pore network is contributed to by both large size inorganic pores and abundant micro factures leading to a relatively high porosity 2 8 30 6 and permeability 0 045 6 27 md within the saline shale reservoir which could enhance the flow ability and storage capacity of oil the oil content s1 100 toc mg hc g toc and s1 mg hc g rock and brittleness data demonstrate that the eq33x section has both great potential for being a producible oil resource and hydraulic fracturing considering the hydrocarbon generation efficiency and properties of oil the mature shale of eq3 in the subsidence center of the qianjiang depression would be the most favorable zone for shale oil exploitation

biomarker signatures from neoproterozoic early cambrian oil western india abstract the neoproterozoic early cambrian time is the cradle of multicellular eukaryotic life and thereafter metazoan life started populating the planet biomarkers which record the chronicles of biotic events on earth have been investigated using gas chromatography mass spectrometry from a suite of nine oil samples extracted from oil stained sediments and a crude oil of neoproterozoic early cambrian age from bikaner nagaur basin western india the biomarker distribution is characterized by high concentration of both tricyclic terpanes and pentacyclic hopanes the predominance of c 29 regular sterane over c 27 and c 28 steranes indicates green algal contribution and may imply the prominence and diversification of the same algal group the low pristane phytane ratio and occurrence of substantial quantity of gammacerane monomethyl 2 methyl 2 2 4 8 trimethyltridecyl chroman and 1 1 7 8 tetramethyl 1 2 3 4 tetrahydrophenanthrene are indicative of stratified water column as well as anoxic and enhanced salinity condition of the environment of deposition this paper reports for the first time the presence of methyltrimethyltridecyl chromans mttcs from neoproterozoic early cambrian time putative c 19 norsteranes probably indicative of sponge input are recorded in the studied samples and also reported from other infracambrian oils and sediments normal alkanes n c 17 and n c 18 and isoprenoids pristane and phytane are highly depleted in 13 c the overall biomarker distribution and carbon isotope data of oils from bikaner nagaur basin show similarities with those of other infracambrian oils like huqf oils from oman and baykit high oils from eastern siberia

secular episodic changes in stable carbon isotope composition of crude oils abstract stable carbon isotopic compositions of c15 saturate and aromatic hydrocarbon fractions from 514 crude oils from known source rocks were examined for secular episodic variations the oils have been correlated by means of biomarker analyses and or geological inferences to their respective source rocks and were further categorized into seven source rock types based on geochemical and petrologic indicators of depositional environments thus these oil samples represent a wide range of source paleoenvironments from thirteen age divisions riphean to neogene the stable carbon isotopic compositions of both fractions become enriched in 13 c with decreasing geologic age three major abrupt shifts toward 13 c enrichment occurred at the precambrian cambrian ordovician triassic jurassic and paleogene neogene boundaries analysis of smaller scale isotopic shifts reveals four distinct groups of oils generally independent of organic facies and secondary alteration processes comparison of evolutionary changes in the biosphere to episodic changes in stable carbon isotopic compositions throughout the phanerozoic indicates that isotopic shifts of crude oils may generally be related to the diversity of preserved phytoplankton

scarcity of the c30 sterane biomarker 24 n propylcholestane in lower paleozoic marine paleoenvironments abstract 24 n propylcholestane 24 npc a c30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae occurs in certain neoproterozoic rocks and oils and throughout the phanerozoic rock record this broad distribution leads 24 npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils here we report two significant hiatuses in the occurrences of 24 npc in the lower paleozoic marine rock record the first in the middle late cambrian and the second in the late ordovician early silurian transition for a range of lithofacies carbonates and siliciclastic rocks organic carbon contents both organic lean and organic rich and paleoceanographic environments shelf and deeper water marine settings and observed offshore of two paleocontinents laurentia and baltica the ordovician silurian gap is at least 9 million years and possibly up to 20 million years in duration robust older occurrences of 24 npc steranes in some neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of lower paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of early paleozoic time using lipid biomarker assemblages

an ion drag contribution to the lower thermospheric wind in the summer polar region 1 using the european incoherent scatter eiscat svalbard radar located in longyearbyen 78 2 n 16 0 e and with the eiscat uhf radar located in tromso 69 6 n 19 2 e we have analyzed the data obtained for six consecutive days from 1000 ut on 1 july 1999 to 1000 ut on 7 july 1999 to investigate the importance of ion drag on the lower thermospheric wind dynamics in the summer polar region the electric field observed at longyearbyen exhibited a stable diurnal variation primarily occurring throughout the first 3 days of observations while during the latter 3 days the diurnal variation was not clearly identified the electric field observed at tromso did not exhibit such a stable variation on the basis of the difference of the electric field variation observed at longyearbyen we divided the data sets into two intervals with a length of 3 days each we compared the horizontal velocities of the ions and the neutral gases at upper heights 110 120 km on the first 3 days the difference between the ion and neutral motion was clearly identified at longyearbyen while it was not at tromso we compared the diurnal tidal amplitudes and phases of the two intervals derived from the longyearbyen data the amplitude of the meridional diurnal tide for the first 3 days was larger than that of the latter 3 days over the height region between 100 and 118 km at longyearbyen furthermore we compared ion drag accelerations with total accelerations we concluded that the ion drag played an important role in the diurnal amplitude at longyearbyen on the first 3 days of the observations in addition we compared the national center for atmospheric research thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm predictions with the observations the time gcm predictions also indicated that the diurnal tide is influenced by the convection electric field

origin of petroleum in the neoproterozoic cambrian south oman salt basin abstract the south oman salt basin sosb is host to the world s oldest known commercial deposits most of the south oman oils have been proven to be associated with the source rocks of the neoproterozoic to cambrian huqf supergroup but the assignment of oils to specific huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the entire huqf sequence possibly as a consequence of limited change in the local palaeoenvironment and biota over the time of its deposition this study was conducted to establish improved correlations between organic rich rock units and reservoir fluids in the sosb through detailed molecular and isotopic analysis of the huqf supergroup with special emphasis directed towards understanding the ara carbonate stringer play unusual biomarkers tentatively identified as a norsteranes show distinctive patterns among carbonate stringer oils and rocks different from those observed in nafun sediments and ara rocks from the athel basin these putative a norsteranes form the basis for new oil source correlations in the sosb and provide for the first time geochemical evidence of a self charging mechanism for the carbonate stringer play the paucity of markers specific to the nafun group shuram buah and masirah bay formations confounds attempts to quantify their respective contributions to huqf oil accumulations nafun inputs can only be determined on the basis of subtle differences between nafun and ara biomarker ratios the most useful geochemical characteristics delineating nafun group organic matter from ara group intra salt source rocks included low relative abundance of mid chain monomethyl alkanes x compounds low relative abundance of gammacerane 28 30 dinorhopane 25 28 30 trinorhopane and 2 methylhopanes low c 22 t c 21 t and high c 23 t c 24 t cheilanthanes ratio values based on these parameters molecular evidence for major contributions of liquid hydrocarbons from nafun group sediments shuram buah and masirah bay formations is lacking our results suggest that the majority of sosb hydrocarbon accumulations originate from within the ara group either from the carbonate stringers or from the package of sediments that comprises the thuleilat athel silicilyte and u shale formations subtle aspects of the composition of some carbonate stringer and post salt huqf oils could suggest some degree of sourcing from the nafun rocks but stronger evidence is needed to confirm this

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

winds in the high latitude lower thermosphere dependence on the interplanetary magnetic field 1 wind observations in the summertime lower thermosphere at high southern latitudes measured by the wind imaging interferometer windii on the upper atmosphere research satellite are statistically analyzed in magnetic coordinates and correlated with the interplanetary magnetic field imf to determine influences of imf dependent ionospheric convection on the winds effects are clearly detectable down to 105 km altitude above 125 km the wind patterns show considerable similarity with ionospheric convection patterns and the speed of the averaged neutral wind in the polar cap often exceeds 300 m s the correlation between the imf bz component and the diurnal harmonic of the winds is generally best when the imf is averaged over the preceding 1 4 5 hours the magnetic zonal mean zonal wind below 120 km correlates best with the imf by component when the latter is averaged over approximately the preceding 20 hours the wind has a significantly stronger rotational than divergent component around and above 120 km a dusk side anticyclonic wind vortex is prominent consistent with earlier findings around 140 km and higher the dusk side vortex intensifies for negative bz but around 120 km it is the dawn side cyclonic vortex that responds more strongly to bz variations the dependence of the wind on the imf is nonlinear especially with respect to imf bz for positive bz the difference winds are largely confined to the polar cap while for negative bz the difference winds extend to subauroral latitudes a significant correlation between the diurnal bz dependent neutral and convection velocity components exists above 108 km when the convection velocity is suitably rotated in magnetic local time mlt with respect to the wind the rotation that maximizes the correlation ranges from 1 5 hours at 130 km wind preceding convection to nearly 6 hours at 108 km wind lagging convection the rotated diurnal bz dependent wind pattern projects onto the diurnal bz dependent ionospheric convection pattern with about 60 the amplitude of the latter above 125 km decreasing to about 17 at 108 km on timescales of 20 hours a by dependent magnetic zonal mean zonal wind generally exists with maximum wind speeds at 80 magnetic latitude typically 10 m s at 105 km increasing to about 60 m s at 123 km and 80 m s at 200 km in the southern hemisphere the wind is cyclonic when the time averaged by is positive and anticyclonic when by is negative the wind direction is expected to be opposite in the northern hemisphere

paleoenvironmental implications of novel c30 steranes in precambrian to cenozoic age petroleum and bitumen petroleums and bitumens from early proterozoic 1800 ma to miocene 15 ma age marine strata contain 24 isopropylcholestanes a novel group of c30 steroids the abundance of these compounds relative to 24 n propylcholestanes varies with source rock age late proterozoic vendian and early cambrian oils and or bitumens from siberia the urals oman australia and india have a high ratio of 24 isopropylcholestanes to 24 n propylcholestanes 1 while younger and older samples have a lower ratio 0 4 temporal changes in this parameter may reflect the relative abundance of certain porifera sponges and certain marine algae through time geochemical indicators such as this which can constrain the source rock age of a migrated oil are useful in source rock identification during petroleum exploration

mesoscale observations of joule heating near an auroral arc and ion neutral collision frequency in the polar cap e region 1 we report on the first mesoscale combined ionospheric and thermospheric observations partly in the vicinity of an auroral arc from svalbard in the polar cap on 2 february 2010 the eiscat svalbard radar employed a novel scanning mode in order to obtain f and e region ion flows over an annular region centered on the radar simultaneously a colocated scanning doppler imager observed the e region neutral winds and temperatures around 110 km altitude using the 557 7 nm auroral optical emission combining the ion and neutral data permits the e region joule heating to be estimated with an azimuthal spatial resolution of 64 km at a radius of 163 km from the radar the spatial distribution of joule heating shows significant mesoscale variation the ion neutral collision frequency is measured in the e region by combining all the data over the entire field of view with only weak aurora present the estimated ion neutral collision frequency at 113 km altitude is in good agreement with the msis atmospheric model

a 600 million year carbonate clumped isotope record from the sultanate of oman carbonate clumped isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks this study utilizes a 600 million year record of marine carbonate rocks from the subsurface and surface of the sultanate of oman to explore how burial and exhumation affected the carbonate clumped isotope thermometer samples span 6 km of burial depth and include calcite and dolomite mineralogies and a range of carbonate rock textures we find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial the first group of carbonates yield water 18 o vsmow compositions slightly enriched or equal to an expected ice free seawater composition of 1 2 and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment water interface temperatures from the second group sit on the present day geotherm yield highly enriched water 18 o vsmow compositions and display fair to poor textural preservation we find no evidence for solid state reordering in paired analyses of calcites and dolomites our results contribute to a growing body of work that indicates that the seawater 18 o vsmow composition has not changed significantly over 600 myr and was not 6 in the ediacaran

work input analysis for soils with double porosity and application to the hydromechanical modeling of unsaturated expansive clays a mechanical approach for unsaturated expansive soils considering double porosity has been developed based on the porous media theory in this approach the adsorbed and the capillary water as well as the micropores and macropores are two distinct phases an interaggregate stress considered as the work conjugate of the macrostructural strain increment has been defined both physicochemical and capillary effects of the pore water have been introduced at the macroscopic level other work conjugate variables relevant for the constitutive modeling of double porosity unsaturated media have also been identified consisting of the modified suction as conjugate of the increment of the macrostructural degree of saturation and the microstructural effective stress as conjugate of the microstructural volumetric strain increment a hydromechanical model for unsaturated expansive clays taking into account the interaction between the micro and the macrostructures in expansive clays can thus be built based on the bou

contrasting sterane signatures in neoproterozoic marine rocks of australia before and after the acraman asteroid impact abstract organic matter preserved in marine mudstones from three late neoproterozoic depocentres the officer and amadeus basins and the adelaide fold belt was examined using standard microscopic and geochemical methods including gas chromatography mass spectrometry gc ms total organic carbon toc contents of these sediments are typically low

tidal waves in the polar lower thermosphere observed using the eiscat long run data set obtained in september 2005 1 characteristics are presented of the lower thermospheric wind from a long run data set obtained by the eiscat uhf radar at tromso 69 6 n 19 2 e over 23 days from september 6 to 29 2005 the derived semidiurnal amplitude exhibited day to day variations 5 30 ms 1 at and above 109 km while the phase varied little with the day we have found a mode change of the semidiurnal tide occurring during september 17 22 2005 between september 6 and 16 the vertical wavelengths were estimated to be 58 km and 76 km for the meridional and zonal components respectively while between september 23 and 29 they became less than 24 km the day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide the diurnal amplitude of the meridional component increased with height except for 8 days between september 13 and 20 when the diurnal amplitudes were smaller values 40 ms 1 at and above 111 km than those for the other intervals furthermore the shapes of the altitude profiles of the meridional phase differ from those for the other intervals we have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes from the analysis of 22 5 days of wind data we found about 5 6 day oscillations in the lower thermosphere probably where there were planetary wave activities in the lower thermosphere

the occurrence of bisnorhopane trisnorhopane and 25 norhopanes as free hydrocarbons in some australian shales abstract the triterpenoid hydrocarbons of some west australian shales have been examined by gc ms in addition to the common 17 h 21 h hopanes 17 h 21 h hopanes and 17 h 21 h moretanes 28 30 bisnorhopane 25 28 30 trisnorhopane and 25 norhopanes were identified in the organic extracts in contrast pyrolysates of the solvent extracted sediments contained only the common hopane and moretane series indicating that 28 30 bisnorhopane 25 28 30 trisnorhopane and 25 norhopanes are not bonded to kerogen but rather are present in the sediments as free hydrocarbons

biomarker similarities between the saline lacustrine eocene green river and the paleoproterozoic barney creek formations abstract the paleoproterozoic barney creek formation which is currently interpreted as a restricted deep marine paleoenvironment plays a disproportionate role in our understanding of proterozoic ocean chemistry and the rise of complex life the barney creek formation hosts several unusual biomarker features specifically its methylhopane and carotenoid signatures herein we demonstrate that the saline lacustrine eocene green river formation shares a similar distribution of methylhopanes and carotenoids which is characteristic of saline lacustrine organic matter more generally these distinct methylhopane and carotenoid patterns are not observed together in marine organic matter of any geologic age these results imply a saline lacustrine depositional environment for the barney creek formation which agrees with earlier but now abandoned depositional models of this formation as a result models of proterozoic ocean chemistry and emergence of complex life that rely on a marine barney creek formation should be reexamined alternatively if paleoproterozoic marine biomarker signatures resemble those of younger saline lacustrine systems then this must be recognized to accurately interpret geologic biomarker and paleoenvironmental records

late precambrian cambrian sediments of huqf group sultanate of oman the huqf group is the oldest known sedimentary sequence overlying crystalline basement in the sultanate of oman it crops out on a broad regional high the huqf axis which forms a dominating structural element on the southeastern edge of the arabian peninsula subsurface and outcrop evidence within and outside of oman suggests that the sediments of the huqf group lie within the age span of late precambrian to early middle cambrian the huqf group is subdivided into five formations corresponding to an alternation of clastics abu mahara and shuram formations and carbonates khufai and buah formations deposited in essentially shallow marine to supratidal or fluviatile conditions and terminated by an evaporitic sequence ara formation evaporites are absent on the huqf axis but they are thickly developed to the west over a large part of southern and central oman where they acted as the major structure former of most of oman s oil fields and even locally pierced up to the surface in the subsurface the huqf group has been reached by the drill mainly in southern oman where its lithostratigraphy resembles that defined in the type area in central oman the oldest sediments exposed in the core of the oman mountains may be correlative with the huqf group regional correlations suggest that the predominantly carbonate evaporitic facies of the huqf group was widely distributed in late precambrian early cambrian time the huqf basin is tentatively considered part of a belt of evaporitic basins and intervening carbonate platforms which stretched across the pangea landmass from the indian subcontinent salt range of pakistan through south yemen oman and saudi arabia into the gulf states and iran hormuz series and carbonate platform north of the zagros

a summary of the geology and oil habitat of the eastern flank hydrocarbon province of south oman abstract the eastern flank hydrocarbon province borders the southeastern edge of the south oman salt basin in the southern part of oman an intensive exploration programme by pdo over the past seven years has led to the discovery of almost 2 10 9 m 3 of oil with current production of some 15000 m 3 d from six fields in stark contrast to other oil habitats of the arabian peninsula the main play involves rock units and geological events ranging in age from late precambrian to quaternary times while trap development and charging are closely related to syn tectonic salt dissolution the principal reservoirs and seals are poorly consolidated palaeozoic clastics which drape due to salt dissolution over residual cores principally composed of either lower palaeozoic sandstones haima group or late proterozoic carbonates huqf group or some combination of these oils have been geochemically correlated with algal source rocks of the huqf eastern flank structures are considered to have been charged from late mesozoic times onwards in conjunction with progressive subsurface salt removal and the onset of maturation of local source rocks the effectiveness of charging along the retreating salt edge is indicated by the small percentage of dry wells major oil zones are found in both anticlinal and truncation traps which are commonly developed around sandstone haima group cores oils show a wide range of characteristics but are typically heavy early expulsion crudes high sulphur with little associated gas bacterial transformation is not uncommon producing reservoirs mainly occur at relatively shallow depths 600 1200 m primary recovery factors of around 7 reflect the high density and viscosity of the eastern flank crudes and have initiated interest in enhanced oil recovery eor techniques two such pilot projects involving steam and polymer flooding are due to commence in 1984 and if successful will substantially increase the reserve base of these fields

chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in earth history the neoproterozoic shuram formation nafun group oman abstract oman records in its neoproterozoic nafun group huqf supergroup an essentially complete carbonate rich ediacaran succession the nafun group overlies the presumed marinoan rift related fiq member ca 635 ma and ends just below the precambrian cambrian boundary 542 ma tectonic subsidence caused by thermal contraction following fiq aged rifting allowed the preservation of about 1 km of postrift stratigraphy with no major stratigraphic breaks the nafun group above the marinoan cap carbonate hadash formation represents two siliciclastic to carbonate grand cycles both initiated by significant transgressions these cycles comprise the masirah bay khufai formations and the shuram buah formations the khufai shuram boundary is associated with the start of a major carbon isotope perturbation the uppermost ramp carbonates of the khufai formation record a smooth decrease in 13c from about 4 to values around 0 followed by two descending steps across which values plunge to a nadir of 12 in the overlying red siltstones and shales interbedded with thin limestones of the shuram formation this fall in isotopic values is temporally rapid and coincident in both shallow and deep water sections in the time span of a single parasequence the 13c nadir is then followed by 50 million years of monotonic recovery the shuram shift represents the largest 13c inorganic carbon negative excursion in earth history although the snowball earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation the non glaciated context of the shuram formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non unique although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic the long term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate

the significance of 24 norcholestanes 4 methylsteranes and dinosteranes in oils and source rocks from east sirte basin libya abstract the present paper involves a detailed evaluation of specific steroid biomarkers by gas chromatography mass spectrometry gc ms and gc metastable reaction monitoring mrm analyses of several crude oils and source rocks from the east sirte basin 24 norcholestanes dinosteranes 4 methyl 24 ethylcholestanes and triaromatic steroids have been identified in both source rocks and crude oils of the east sirte basin diatoms dinoflagellates including those potentially associated with corals and or their direct ancestors are amongst the proposed sources of these biomarkers these biomarker parameters have been used to establish a mesozoic oil source correlation of the east sirte basin hydropyrolysis of an extant coral extract revealed a similar distribution although immature of dinosteranes and 4 methyl 24 ethylcholestanes also observed in the sirte oils and source rocks this is consistent with the presence of dinoflagellates present during the deposition of the mesozoic aged east sirte basin formations a good data correlation for the rock extracts revealed a similar distribution of 3 24 dimethyl triaromatic steroids 3 methyl 24 ethylcholestanes 4 methyl 24 ethylcholestanes and 2 methyl 24 ethylcholestanes observed in one of the oil families and associated source rocks for the east sirte basin

origin and variability of a terminal proterozoic primary silica precipitate athel silicilyte south oman salt basin sultanate of oman the precambrian cambrian athel silicilyte is a 400 m thick salt encased siliceous succession in the south oman salt basin it is a self sourcing hydrocarbon reservoir and comprises up to 95 microcrystalline quartz and exhibits wavy discontinuous lamination comprising thin alternating organic rich and silica rich layers textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water within the water column at the sulphidic oxic interface the unique occurrence of the athel silicilyte in the terminal proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly basin anoxia coupled with the growth of microbial mats low ph and high silica pore water saturations created optimal chemical conditions for silica precipitation volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the athel basin at the basin margins and within the late highstand systems tract volumes of microcrystalline quartz decreased as the volume of detrital sediment increased mass balance calculations indicate that silica enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column in conclusion precipitation of the athel silicilyte was driven by the coincidence of basin restriction limited clastic input availability of organic matter and water column anoxia the observation that there are few documented examples of chert deposits younger than ca 700 ma prior to the cambrian explosion suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica secreting organisms direct precipitation from sea water was restricted this is tentatively related to the gradual increase in alkalinity of sea water through the palaeo proterozoic and meso proterozoic such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input

numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics a case study from the south oman salt basin large rock inclusions are embedded in many salt bodies and these respond to the move ments of the salt in a variety of ways including displacement folding and fracturing one mode of salt tectonics is downbuilding whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside we investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ ated deformation of brittle stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model in combination with adaptive remeshing the model set up is constrained by observations from the south oman salt basin where large carbonate bodies encased in salt form substantial hydrocarbon plays the model shows that depending on the displacement of the top salt the stringers can break very soon after the onset of salt tectonics and can deform in different ways if extension along the inclusion dom inates stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth spacingof the boudin bounding faults can be as close as 3 4 times the thickness of the stringer in contrast salt shortening along the inclusion may lead to folding or thrusting of stringers

the occurence of unusual c27 and c29 sterane predominances in two types of oman crude oil abstract two types of oman crude oils reveal unusual sterane distributions type a which is the more common 74 examples is characterised by a predominance of c 29 iso and normal steranes and generally none or only very low relative concentrations of rearranged steranes the triterpanes are characterised by the predominance of the c 29 17 h 21 h norhopane over the c 30 17 h 21 h hopane and non predominant c 20 c 30 tricyclic terpanes the c 29 steranes of this type of crude were not derived from the c 29 sterols of land plant origin frequently proposed as the source of c 29 steranes since there is good geological evidence that these crudes were generated from a pre cambrian source rock a geological period when land plants did not exist the type b crude oil 11 known examples is characterised by a strong predominance of c 27 iso normal and rearranged steranes relatively lower concentrations of 17 h 21 h hopanes and relatively high concentrations of c 20 c 30 tricyclic terpanes the remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different

biological marker distribution and significance in oils and rocks of the monterey formation california abstract the biological marker distributions of several oils core extracts and solid bitumens of the monterey formation of california have been studied sterane terpane and monoaromatic steroid hydrocarbons were analyzed in samples from the san joaquin los angeles ventura and santa maria basins the sterane patterns of both oils and extracts are characterized by 1 a low relative concentrations of diasteranes 2 b low 20s 20r 5 14 17 ethylcholestane ratios 3 c relatively high concentrations of cholestane vs methyl and ethylcholestane isomers san joaquin basin samples contain significant amounts of the 5 isomer which is generally absent in samples from other basins the carbon number distribution of 5 14 17 20r steranes is similar for all oils regardless of api gravity depth or basin location and is suggestive of open marine depositional conditions for the source material involved 17 h l8 h 21 h 28 30 bisnorhopane is present in almost all samples certain san joaquin basin oils and extracts contain 1 a a series of 25 nor hopanes including 25 28 30 trisnorhopane 2 b a distinctive monoaromatic steroid hydrocarbon distribution 3 c an aliphatic hydrocarbon fraction devoid of n paraffins biological marker characteristics suggest that the monterey oils examined originated early in the maturational sequence from elastics poor source material api gravities of the monterey formation oils examined vary monotonically with 1 a bisnorhopane hopane ratios 2 b aromatized regular sterane ratios and 3 c the concentration of monoaromatized steranes relative to terpanes and regular steranes these oil gravity correlations exist regardless of sample depth or basin location

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

hydrocarbon generation capability of sinian lower cambrian shale mudstone and carbonate rocks in the sichuan basin southwestern china implications for contributions to the giant sinian dengying natural gas accumulation the giant natural gas accumulation recently discovered within the neoproterozoic dengying formation in the sichuan basin of southwestern china represents a new and important supplement to the ancient neoproterozoic cambrian petroleum systems worldwide however the source of this gas is controversial because there are five possibilities based on geology from bottom to top including the lower sinian doushantuo mudstone the upper sinian dengying algal dolomite the mudstone of the third member of the dengying formation the lower cambrian maidiping argillaceous dolomite and the lower cambrian qiongzhusi shale to improve the understanding of this issue 33 samples from the five possible source rock units in the study area were investigated for their character and maturity via petrology total organic carbon toc pyrobitumen reflectance kerogen carbon isotope and biomarker analyses based on the results we address their hydrocarbon generation capabilities and contributions to the giant gas accumulation results show that present day toc values of the samples range from 0 03 to 2 36 and vary both regionally and stratigraphically with the qiongzhusi shales in the central basin and the weiyuan ziyang area ranking the highest in contrast the organic matter source and maturity within these rocks are relatively homogenous with slight variation throughout the study area the organic matter is predominantly derived from algae and bacteria with different compositions deposited in marine reducing environments the rocks range in thermal maturity from postmature to overmature these results imply that the generation capability of source rock had been high in the past and the gas in the study area is most likely originated from oil cracking and the late generation directly from the kerogen of source rocks with the qiongzhusi shales being the dominant source rock thus the hydrocarbon migration pattern in the study area is a lateral migration from deep seated and younger cambrian source rocks in sags to shallow seated and older sinian dolomite reservoirs in uplifts the migration process occurred in the oil phase with the oil being later cracked the differences in natural gas geochemistry may be controlled by complex factors e g thermal maturity the migration accumulation process and differing contributions from source rocks besides the dominant qiongzhusi regionally the central basin and the weiyuan ziyang area have had high hydrocarbon generation capability and thus should be considered as favorable targets for natural gas exploration in the future this study of china provides an additional example of neoproterozoic cambrian petroleum systems worldwide and thus can help to synthesize the systematics of such systems and understand the sedimentary transition across the precambrian cambrian boundary

micro and nano pores in intrasalt microbialite dominated carbonate reservoirs ara group south oman salt basin abstract dolomitic microbialites form important exploration targets for the oil and gas industry in many parts of the world it is known that a significant portion of their total porosity and potential storage capacity is formed by micropores despite their economic importance few studies attempted to characterize the micro to nanoporosity in dolomitic microbialites recent advances in the field of microscopy now allow to image and analyze their pore systems on a hitherto unprecedented scale this study evaluates pore systems in neoproterozoic ara group carbonates forming part of the deeply buried intra salt a2c hydrocarbon play of the south oman salt basin sosb micro and nanoporosity there is hosted by aphanocrystalline and to a minor extent microcrystalline dolomite image analysis derived from broad ion beam milling and scanning electron microscopy bib sem of representative samples display a contribution between 50 and 100 from micro and nanopores to total porosities which are up to 17 the main diagenetic controls on microporosity in the a2c are the formation and preservation of primary or early diagenetic aphanocrystalline dolomite and the presence of evaporites calcite and solid bitumen petrographic and petrophysical observations show good preservation of the original textural and geochemical attributes in most dolomite samples even after burial to 5 km depth for the past 500 million years this suggests a structurally and hydrodynamically isolated rockbuffered system with a limited extent of cementation confocal laser scanning microscopy clsm reveals significant amounts of hydrocarbons which manifest as degenerated organic material in the carbonates they originated from precursor bacterial organic matter in microbial laminite thrombolite and peloidal source rocks and might have prevented extensive recrystallization of the aphanocrystalline dolomite matrix and further cementation of the porespace primary dolomite might still be preserved in the a2c microbialites as petrographic observations reveal similarities in crystal sizes and the texture of euhedral aphanocrystalline dolomite as observed in comparable sedimentary systems

petroleum geology and geochemistry of the middle proterozoic mcarthur basin northern australia iii composition of extractable hydrocarbons abstract middle proterozoic organic rich sediments from the mcarthur basin in northern australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence are representative of the organic remains of microorganisms living at that time the major classes of hydrocarbons identified were n alkanes monomethyl branched alkanes cyclohexyl alkanes and acyclic isoprenoids there were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes low concentrations of steranes were present in most samples but like the triterpanes they were only easily detected in the least thermally altered sediments and hence were of limited use in detailed assessments of thermal maturity the presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 ma although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes