tricyclic terpane maturity parameters response to heating by an igneous intrusion the effects of thermal maturation upon the abundance and composition of the tricyclic terpanes have been investigated within a single sedimentary horizon within the dun caan shale member isle of skye scotland intruded by a 0 9 m thick tertiary dolerite dyke heating by the igneous body caused the concentrations of all 13 h 14 h and 13 h 14 h tricyclic terpanes to increase towards or within the effective oil window recording generation from abundant non hydrocarbon precursors such as kerogen and asphaltene bound components progressive changes in tricyclic terpane composition accompany these concentration changes an increase in the c23 maturity parameter towards the dyke resulted from the relatively greater rate of generation of the isomer combined with the earlier decline in concentration by degradation or isomerisation of the component the tricyclics tricyclics hopanes parameter shows a dramatic increase within the latter part of the oil window due to the greater relative thermal stability of the tricyclic terpanes the quantitative data demonstrate the importance of the processes of generation from non hydrocarbon fractions of the sedimentary organic matter and thermal degradation in the operation of tricyclic terpane maturity parameters

identification of some bicyclic alkanes in petroleum abstract bicyclic alkanes containing the drimane skeleton have been identified in crude oils and sediments by synthesis of model compounds two other bicyclic alkanes whose skeletons do not conform to the isoprene rule have also been identified from mass spectral data a correlation between the relative concentration of c 29 steranes and diasteranes and drimane and two c 15 bicyclanes in an extensive sample suite suggests a similar mechanism is involved in the formation of diasteranes and the two c 15 bicyclanes in the subsurface the widespread occurrence of drimanes and drimane related compounds in crude oils and sediments indicates that these compounds are derived from a ubiquitous source the presence of methyldrimanes in a sample rich in methylhopanes suggests that drimanes and hopanes may be derived from similar precursors

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

electromagnetic induction in new zealand analogue model and field results abstract the behaviour of electric and magnetic variations over north island new zealand is studied with the aid of a laboratory analogue model the source field frequencies used in the analogue modelling simulate naturally occurring geomagnetic variations of 5 120 min periods in phase and quadrature magnetic and electric fields for a selection of traverses for the modelled region of north island are presented since north island is of a relatively narrow cross section the field responses even for inland locations are expected to show strongly the effects of the surrounding ocean the irregular coastlines as well as the strait between north and south islands lead to coastal and inland field anomalies due to induced currents being deflected and channelled to produce localized current densities the comparison of model results with field station measurements obtained earlier individually by ingham and by midha for sites in the northeastern central and southern near cook strait regions of north island demonstrates the large role the ocean has in the observed field responses differences in the model and field results at some sites are expected and should reflect the effects of the local geology and the conductive substructure related to the complex tectonics of the region not simulated in the model

em responses of an elongated conductor near an ocean analogue model studies abstract magnetic induction responses of an elongated conductor at sites near an ocean are studied with the aid of laboratory analogue model measurements the dependence on the period the distance from the ocean and the depth extent of the conductor is examined for the case of the elongated conductor parallel to the ocean coastline the parkinson induction arrows at sites near the conductor reverse directions from pointing towards the conductor at short periods to pointing towards the ocean at longer periods and the period at which the reversals occur differs for the in phase and the quadrature arrows it is also shown that except for sites right at the coastline and at the conductor the responses of the conductor alone and the ocean alone can be added to yield the responses of the model that includes both the ocean and the conductor these results have application to the subtraction of ocean induction effects from geomagnetic measurements in coastal regions to delineate the responses of any nearby elongated anomalous conductors

magnetovariational soundings across the south island of new zealand difference induction arrows and the southern alps conductor abstract magnetovariational mv data from 14 sites on a transect of the south island of new zealand are presented the induction arrows clearly show the effects of induction in the surrounding oceans comparison with the responses of an analogue model of the effect of the oceans on induction in the new zealand region indicates that the induction arrows also contain information on anomalous on land conductivity structure the use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two and three dimensional numerical models pertinent to the new zealand situation difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal in both real and quadrature arrows across the south island two dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the southern alps of an anomalously conducting region previously identified by magnetotelluric mt sounding use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation

the coast effect response in geomagnetic field measurements abstract two dimensional 2d numerical models of electromagnetic em induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean conductor separation distances that would permit to within an acceptable approximation a simple vector subtraction of the coast effect response the anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth the coast effect response range yr is arbitrarily taken to be the distance from the ocean at which the b z b y n coast effect response is reduced to a value of 0 2 the response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths island widths and ocean depths the response ranges are tested in 2d numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host earth the induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor as an application difference arrows obtained by subtracting three dimensional laboratory analogue model coast effect responses from field site responses are employed in the interpretation of induction arrow responses available for a number of sites in the bohai bay region of china and the kii peninsula region of japan

molecular marker and carbon isotope geochemistry of ordovician oils in the tuofutai bulge northern tarim basin implications for oil maturity source characteristics and ﬁlling directions abstract the tuofutai bulge in the tabei uplift of tarim basin western china has become a hot spot for petroleum exploration in recent years fifteen ordovician oil samples from 15 wells in the tuofutai bulge were analyzed for biomarkers and carbon isotopes using gas chromatography mass spectrometry and stable carbon isotope analysis to constrain its depositional environment thermal maturity source rocks and the possible petroleum charging directions the samples were characterized by abundant terpenoids relatively high contents of c29 regular steranes and dibenzothiophene relatively low ratios for pr ph and c22 c21 tricyclic terpanes and high ratios of c35 c34 homohopanes results suggest that the source rocks for the oils were deposited in a reducing marine carbonate and marl environment cambrian and ordovician source rocks contribute to the oils and the middle to upper ordovician source rocks exert a greater contribution n alkanes and abundant 25 norhopanes in the saturated fractions suggest that the oil is a mixture of early biodegraded oil and late normal crude oil charged at later time data from methyldiamantane parameters suggest that the crude oil is in a high maturity stage the tracing of oil charging suggests that preferential migration directions are primarily from south to north connected with oil migration along the shenbei no 1 fault from the shuntuoguole lower uplift to the south of tabei uplift

some aspects of the organic geochemistry of petroleum fluids the most important influences on the compositions of petroleum fluids are in many cases the composition of a petroleum sample may allow the depositional environment of its source rock and its temperature history before and after expulsion to be determined this information assists the search for more petroleum in a region we suggest that this approach should be used more frequently to validate models of petroleum generation and migration in the second part of the paper we discuss the origin and nature of the differences in petroleum composition which are frequently found within individual petroleum reservoirs petroleum accumulations in permeable reservoirs are normally found to be inmechanical equilibrium and verticalchemical equilibrium this is manifested by constant subsurface fluid density at any given depth across the accumulation and increases in subsurface density with depth known as gravitational segregation however even in permeable reservoirs lateral differences in composition inherited from the reservoir filling process are not totally eliminated by mixing processes such as diffusion and convection thus lateral chemical equilibrium is often not achieved and lateral compositional variations are observed in present day oil and gas reservoirs these observations are justified in terms of order of magnitude estimates for the rates at which reservoirwide mixing occurs a case history is presented

geochemistry and possible origin of petroleum in palaeozoic reservoirs from halahatang depression abstract the halahatang depression in the tabei uplift of the tarim basin is an active exploration area because it has substantial reservoir potential and contains or is near to many commercial oil fields geochemical analysis indicates that halahatang oils were derived from marine carbonate source rocks deposited under anoxic reducing conditions the maturities for halahatang oils are corresponding to the peak of the oil window and slightly higher than the neighboring tahe oils the halahatang oils feature low pr ph c 21 c 23 tricyclic terpane and c 28 c 29 sterane ratios high c 29 c 30 hopane and c 35 c 34 hopane ratios a v shape in the distribution of c 27 c 28 c 29 steranes and light carbon isotope ratios similar to the tahe oils and correlate well with the middle upper ordovician source rock however some source related biomarker parameters imply a more reducing source organofacies with more zooplanktonic contribution than that for the tahe oils

electromagnetic induction in the new zealand south island abstract laboratory magnetic field measurements for a scaled analogue model of new zealand and the surrounding oceans are used to provide induction arrows for simulated geomagnetic variations of 5 120 min periods for a series of traverses over the south island as the island is relatively narrow the field responses for all on land locations show significant effects of the surrounding oceans the model simulates a uniformly resistive island the oceans and a conductive asthenosphere at depth and thus discrepancies between the model and field results for sites on the island are attributed to anomalous geological structures not simulated in the model removing the coast effects before interpretation by subtracting the model induction arrows from the field site arrows at eyrewell permits an obvious reinterpretation that attributes a significant component of the field responses to the nearby alpine and marlborough faults rather than to the channelled current in cook strait

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

significance of coniferous rain forests and related organic matter in generating commercial quantities of oil gippsland basin australia contrary to the conventional belief that humic coal generates primarily gas 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic latrobe group upper cretaceous tertiary that serves as both the reservoir and the source for hydrocarbons in the offshore gippsland basin of southeastern australia evidence for generation of liquid hydrocarbons from the coaly succession includes 1 similarity of n alkane distribution in the oil and in the coal extracts 2 high wax content of oil up to 27 by weight 3 high ratio of pristane phytane in oil 5 6 and 4 dominance of c29 steranes in the oil in the gippsland basin coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting present temperate climate and kauri vegetation of new zealand are considered to be the modern analog to the gippsland basin the coniferous vegetation provided large quantities of hydrogen rich exinite macerals such as cutinite and resinite with potential to generate oil high rainfall raised ground water level low oxygen high acidity and low nutrient conditions of a raised bog setting were suitable for preserving organic matter a comparison of gas chromatograms of oils in the gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal and the naphthenic fraction was derived chiefly from resin

lower crustal and upper mantle electrical conductivity contrasts in the central north island of new zealand abstract magnetotelluric soundings made in the central north island of new zealand over the last 20 years have been reanalysed together with more recent soundings in an attempt to detect any electrical conductivity contrast associated with a known boundary in other geophysical properties invariant apparent resistivity curves from 11 sites in all have been compared and curves which appear to be representative of the different regions have been selected one dimensional modelling has led to the identification of a good conductor at lower crustal or upper mantle depths in the northwest of the north island no such conductor is identified in the south and east modelling of a single site in the taupo volcanic zone has led to the first indication yet obtained of a deeper conductor beneath the geothermal region

distinguishing cambrian from upper ordovician source rocks evidence from sulfur isotopes and biomarkers in the tarim basin abstract the reported source rocks for the abundant petroleum in the tarim basin china range from cambrian to lower ordovician and or upper ordovician in age however the difference between the two groups of source rocks is not well characterized in this study pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of crcl 2 reduction and grinding the kerogen and coexisting pyrite samples were then analyzed for 34 s values results show that the kerogen samples from the cambrian have 34 s values between 10 4 and 19 4 the values are significantly higher than those from the lower ordovician kerogen 34 s of between 6 7 and 8 7 which in turn are generally higher than from the upper ordovician kerogen samples 34 s of between 15 3 and 6 8 the associated pyrite shows a similar trend but with much lower 34 s values this stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil 34 s values from other basins suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite 34 s values the relatively high 34 s values in the cambrian to lower ordovician source rocks are associated with abundant aryl isoprenoids gammacerane and c 35 homohopanes in the extractable organic matter indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification compared with the upper ordovician the cambrian to lower ordovician source rocks show abundance in c 28 20r sterane c 23 tricyclic terpanes 4 23 24 trimethyl triaromatic dinosteroids and depletion in c 24 tetracyclic terpane c 29 hopane thus 34 s values and biomarkers of source rock organic matter can be used for distinguishing the cambrian and upper ordovician source rocks in the tarim basin

diamondoid hydrocarbon ratios novel maturity indices for highly mature crude oils abstract diamondoid hydrocarbons have been identified in oil and source rock samples from the tarim yinggehai qiongdongnan and other chinese basins two diamondoid hydrocarbon ratios have been used as novel high maturity indices to evaluate the maturation and evolution of crude oils and condensates in these chinese basins the two ratios 1 ma 1 ma 2 ma mai methyl adamantane index and 4 md 1 md 3 md 4 md mdi methyl diamantane index were found to increase with increasing vitrinite reflectance the initial values for the ratios are ca 50 and 30 respectively diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent based on this study crude oils from the tarim basin are generally believed to be very mature ro 1 1 the cambrian ordovician source beds are the major source rocks of crude oils and natural gases in this basin condensates from the tarim yinggehai and qiongdongnan basins have higher maturities equivalent to ro values of 1 6 2 0 gases and condensates from the yinggehai and qiongdongnan basins are generated mainly from tertiary source rocks below 4000 5000 m

preliminary result of magnetotelluric soundings in the fold thrust belt of taiwan and possible detection of dehydration abstract until the magnetotelluric mt soundings were conducted in the last two years there was no systematic view of the deep electric conductivity in the fold thrust belt of the taiwan orogen being a typical example of arc continental collision presented in this paper are the initial results and preliminary interpretation of the mt survey in taiwan the determinant response of the impedance tensor was calculated for inversion and occam s inversion which could generate smooth models was mainly used for interpreting mt data at each site the existence of a conductive zone with depths of 10 20 km beneath the island of taiwan is undoubtedly the most important feature found in this study the depths are well correlated with the inferred depth of dehydration reactions of suppe 1981 and the top of the aseismic lower crust observed by wang et al 1994 thus the authors believe that the cause of this conductive zone is probably the fluids released from dehydration reactions additionally close to the lishan fault the mt data show strong distortions which may be the result of induction in more complex three dimensional structures further multi dimensional analysis of the data is required in the future for delineating a more realistic image of the subsurface heterogeneous structure

origin and quantitative source assessment of deep oils in the tazhong uplift tarim basin abstract a large amount of deep oil has been discovered in the tazhong uplift tarim basin whereas the oil source is still controversial an integrated geochemical approach was utilized to unravel the characteristics origin and alteration of the deep oils this study showed that the lower cambrian oil from well zs1c 1x was featured by small or trace amounts of biomarkers unusually high concentration of dibenzothiophenes dbts high 34 s of dbts and high 13 c value of n alkanes these suggest a close genetic relationship with the cambrian source rocks and tsr alteration on the contrary the middle cambrian oils from well zs1 2a were characterized by low 13 c of n alkanes and relatively high 34 s of individual sulfur compounds and a general v shape of steranes indicating a good genetic affinity with the middle upper ordovician source rocks the middle cambrian salt rock separating the oils was suggested to be one of the factors responsible for the differentiation it was suggested that most of the deep oils in the tazhong uplift were mixed source based on biomarkers and carbon isotope which contain tsr altered oil in varied degree the percentage of the oils contributed by the cambrian lower ordovician was in the range of 19 100 average 57 controlled by several geological and geochemical events significant variations in the 34 s values for individual compounds in the oils were observed suggesting a combination of different extent of tsr and thermal maturation alterations the unusually high dbts concentrations in the tazhong 4 oilfield suggested as a result of mixing with the zs1c oil 1x and lower ordovician oils based on 34 s values of dbt this study will enhance our understanding of both deep and shallow oil sources in the tazhong uplift and clarify the formation mechanisms of the unusually high dbts oils in the region

magnetotelluric soundings across the southern alps orogen south island of new zealand data presentation and preliminary interpretation abstract as part of a major geophysical study of the orogen of the southern alps magnetotelluric soundings have been made on two transects across the south island of new zealand initial results are presented and indicate the existence of significant electrical conductivity variations across the island in particular one dimensional inversion of invariant responses from the central part of the southern transect in the southern alps shows the existence of a region of enhanced conductivity at a depth of around 10 km this depth correlates well with that to the top of the aseismic lower crust and can be related to enhanced crustal temperatures and the dehydration of greenschist zeolite facies rocks upthrust from depth the absence of a similar conductive structure beneath the equivalent area of the northern transect suggests lateral variation in structure along the southern alps one dimensional inversions of short period responses from sites on the canterbury plains and north west of the alpine fault show good correlation with depths to basement derived from other geophysical and geological data close to the alpine fault itself data show strong distortions these may result from induction in more complex three dimensional structures the deviation of telluric currents by electrically resistive intrusives or possibly from electrical anisotropy associated with the alpine fault shear zone

a novel microbial hydrocarbon degradation pathway revealed by hopane demethylation in a petroleum reservoir abstract the origin of 25 norhopanes is controversial despite their abundance in many biodegraded oils in a biodegraded oil reservoir we found concomitant and directionally opposite changes with depth in the concentrations of certain 17 hopanes and their 17 25 norhopane counterparts demonstrating microbially induced demethylation of hopanes these observations suggest a hitherto unknown biodegradation mechanism and provide insight into the poorly understood process of natural petroleum biodegradation

evolution of tricyclic alkanes in the espirito santo basin brazil abstract the distributions of tricyclic isoprenoid alkanes were characterized in the neocomian sequence from the espirito santo basin brazil by high resolution gas chromatography mass spectrometry and co injection with synthetic standards where available a suite of homologous components ranging from c 20 to c 28 were present throughout the sequence as a mixture of stereoisomers whose relative abundance changed with depth the epimerization rates were similar for the various tricyclic compounds irrespective of side chain length achieving equilibrium ratios before the main stage of oil generation this new maturity parameter should be useful as a supplement to sterane hopane measurements

alteration and multi stage accumulation of oil and gas in the ordovician of the tabei uplift tarim basin nw china implications for genetic origin of the diverse hydrocarbons abstract the ordovician is the most important exploration target in the tabei uplift of the tarim basin which contains a range of petroleum types including solid bitumen heavy oil light oil condensate wet gas and dry gas the density of the black oils ranges from 0 81 g cm 3 to 1 01 g cm 3 20 c and gas oil ratio gor ranges from 4 m 3 m 3 to 9300 m 3 m 3 oil source correlations established that most of the oils were derived from the mid upper ordovician marine shale and carbonate and that the difference in oil properties is mainly attributed to hydrocarbon alteration and multi stage accumulation in the tabei uplift there were three main periods of hydrocarbon accumulation in the late caledonian stage ca 450 430 ma late hercynian stage ca 293 255 ma and the late himalayan stage ca 12 2 ma the oil charging events mainly occurred in the late caledonian and late hercynian stage while gas charging occurred in the late hercynian stage during the late caledonian stage petroleum charged the reservoirs lying east of the uplift however due to a crustal uplifting episode in the early hercynian ca 386 372 ma most of the hydrocarbons were transformed by processes such as biodegradation resulting in residual solid bitumen in the fractures of the reservoirs during the late hercynian stage a major episode of oil charging into ordovician reservoirs took place subsequent crustal uplift and severe alteration by biodegradation in the west central basin resulted in heavy oil formation since the late himalayan stage when rapid subsidence of the crust occurred the oil residing in reservoirs was exposed to high temperature cracking conditions resulting in the production of gas and charged from the southeast further altering the pre existing oils in the eastern reservoirs a suite of representative samples of various crude oils including condensates lights oils and heavy oils have been collected for detailed analysis to investigate the mechanism of formation based on the research it was concluded that the diversity of hydrocarbon physical and chemical properties in the tabei uplift was mainly attributable to the processes of biodegradation and gas washing the understanding of the processes is very helpful to predict the spatial distribution of hydrocarbon in the tabei uplift and provides a reference case study for other areas

magnetovariational measurements in the cook strait region of new zealand abstract magnetovariational measurements have been made at 10 sites on the northern side of the cook strait new zealand single station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the cook strait is most important at around 1000 s period at longer periods the effect of induced currents in the pacific ocean predominates a two dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60 80 km distance from the strait closer to the strait the inductive process is strongly three dimensional a simple d c line current model of current flow has been shown to reproduce some of the features of the observed responses induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region

difference electromagnetic induction arrow responses in new zealand abstract geomagnetic observations in new zealand are expected to include ocean effects owing to geomagnetic induction in the surrounding oceans in this work these unwanted effects in the observations at each of 34 field sites are removed by subtracting laboratory model ocean effect induction arrows from geomagnetic induction arrows to yield difference arrows the difference arrows at sites in the central region of north island can be accounted for by induction primarily in conductive structure in the central volcanic region cvr whereas in the southern regions the arrows appear to include additional components that would be consistent with conductive structures at depth associated with the wellington fault system and the subducting pacific plate in the northern and southern regions of south island the arrows could be accounted for in part by conductive structures associated with the dun mountain ophiolites and haast schists and in part by the conductive structures in the alpine fault system whereas in the central regions conductive structures beneath the southern alps and the canterbury plains would appear to play the major role

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

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

oil filling history of the ordovician oil reservoir in the major part of the tahe oilfield tarim basin nw china abstract the ordovician oil reservoir in the tahe oilfield northern tarim basin is so far the largest marine oil reservoir in china ordovician oil was sampled from 73 exploration production wells in the oilfield according to oil oil correlation all the ordovician oil samples belong to the same oil population of mature to high mature phase based on the coexisting intact n alkane series evident ucm and 25 norhopanes in the oil as well as the bimodal distribution pattern of homogenization temperatures t h ranging from 80 to 100 c and from 115 to 135 c respectively in fluid inclusions within the reservoir rocks it is concluded that the oil reservoir has been twice charged during its oil filling history by means of 1d basin modeling the measured t h values can be related to relevant geological ages of 429 415 ma and of 8 2 ma respectively i e the middle to late silurian and the miocene to pliocene representing two phases of oil filling and entrapment using independent molecular parameters e g the 4 1 mdbt ratio and total concentrations of pyrrolic n compounds as indicators to reconstruct the oil filling process in the reservoir oil filling pathways can be traced and mapped the general orientation of oil filling is from south to north in the oil reservoir therefore it can be predicted that its source kitchen should be on the south side of the oilfield most probably at the shuntuoguole uplift between the awati and manjiaer depressions

biomarker distributions in crude oils and source rocks from different sedimentary environments abstract the saturated and aromatic hydrocarbons from a significant number of oils and source rocks derived from terrestrial and marine organic matter have been analysed by gc and gc ms results show that certain classes of biomarkers including bicyclic sesquiterpanes tricyclic terpanes pregnanes methylsteranes rearranged steranes hopane sulfides benzohopanes long chain alkylbenzenes and pentacyclic triterpanes can be used to distinguish differences in source input and or depositional environments variations in the distributions of the naphthalenes phenanthrenes fluorenes dibenzofurans dibenzothiophenes naphthobenzothiophenes and triaromatic steroid hydrocarbons may also reflect disparities in source input or sedimentary environments

evidence for mid cretaceous wrench faulting in the methow basin washington tectonostratigraphic setting of the virginian ridge formation the cretaceous methow basin of north central washington is the southernmost of a series of mesozoic successor basins in the canadian cordillera the albian campanian sedimentary sequence there documents a restricted asymmetric basin filled by a west derived laterally amalgamated fan delta system the virginian ridge formation and an east derived stable fluvial deltaic system the winthrop formation followed by a locally derived volcanic sequence the midnight peak formation this study focuses on the depositional and tectonic setting of the albian cenomanian virginian ridge formation the mid cretaceous methow basin is interpreted as a wrench fault basin generated in response to transpressive dextral offset and uplift of a highland to the west stratigraphy and sedimentology of the albian cenomanian methow sequence indicates that 1 the basin was asymmetric normal to its axis in that the western margin was a mobile zone generating considerable relief while the eastern margin was passive stable and of relatively low relief 2 the basin was asymmetric along its axis documenting transpression and dextral offset along the western margin 3 the relatively short and steep fan delta system deposited along the western margin was laterally amalgamated and possibly time transgressive to the north this basin and similar coeval successor basins to the north may record extensive transtensile transpressive mobility along the cordillera in middle to late cretaceous time

saturate biomarkers and aromatic sulfur compounds in oils and condensates from different source rock lithologies of kazakhstan japan and russia abstract representative petroleums from the seven basins of kazakhstan russia and japan were analyzed using gas chromatography with dual fid fpd and gc ms the present study has shown that the deviation in relative concentrations of aromatic sulfur compounds in petroleums could be related to the type of source rock all oil samples were classified into three groups on the basis of the relative abundance of aromatic sulfur compounds biomarker distribution and lithology of source rock the first group of oils derived from carbonate source rocks is rich in aromatic sulfur compounds such as methylbenzothiophenes mbt and methyldibenzothiophenes mdbt and poor in dibenzothiophene dbt compared to methylated homologs the second group derived from siliceous and siliceous carbonate source rocks contains various types of benzothiophenes and dibenzothiophenes including dbt the third group originating from argillaceous rocks has lower concentrations of aromatic sulfur compounds and especially of methylated benzothiophenes

an analogue model study of em induction in the japan korea china region abstract electromagnetic induction in the coastal region of japan korea china is studied using a scaled laboratory analogue model the model includes a simulation of the known ocean bathymetry the complex coastlines and the deep conductive pacific trench model measurements of the electromagnetic field components are carried out for simulated periods of 15 180 min for two mutually perpendicular polarizations of a uniform horizontal inducing source field in phase and quadrature bx by and bz model measurements are examined in detail for a series of traverses over japan the korean peninsula and the coastal region of china and the u s s r large anomalous in phase model magnetic fields are observed over the korea japan strait for e polarization and over the bohai tsugaru and la perouse straits for b polarization owing to current channelling the significant responses observed at short periods over the peninsulas in the shallow coastal areas decrease with increasing period large gradients in the in phase b2 field are observed over japan for e polarization for both short and long periods owing to the effects of induced currents in the surrounding oceans thus induction arrow responses over all regions of japan are expected to show the dominant effects of the ocean a comparison of analogue model and field site induction arrows over the kii peninsula of japan indicates that the ocean effects alone cannot fully account for the observed responses the differences in the model and field results can be attributed to lateral inhomogeneities in the conductivity structure under japan

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

magnetotelluric soundings across the south island of new zealand electrical structure associated with the orogen of the southern alps summary magnetotelluric soundings have been made at 31 locations on two transects across the central part of the south island of new zealand the calculated impedance tensor at each site has been decomposed using an assumed tectonic strike of n35 e to yield 2 d regional apparent resistivity and phase responses these have been inverted to produce 2 d models of electrical conductivity structure associated with the southern alps orogen the resulting structure is consistent with proposed models for the orogen and shows a zone of enhanced conductivity at a depth of only 10 km beneath the southern alps this correlates with suggestions of enhanced temperatures at shallow depths and can be interpreted as being due to the presence of wet greenschist facies rocks upthrust from lower crustal depth by the collision of the pacific and australian plates reduced resistivity also occurs at 25 km depth beneath the canterbury plains and can be tentatively associated with the existence of a lower crustal detachment surface the derived resistivity structure close to the alpine fault is highly complex and may be indicative of either 3 d induction in igneous intrusions or anisotropic structure associated with the fault zone