least squares reverse time migration with a wavefield separation imaging condition and updated source wavefields abstractdirectly imaging steeply dipping fault zones is difficult for conventional migration including reverse time migration rtm we developed a new least squares rtm lsrtm method to directly image steeply dipping fault zones the method uses a wavefield separation imaging condition and updated source wavefields during each iteration our new imaging method produces horizontal looking images that show mostly steeply dipping fault zones conventional least squares rtm does not update source wavefields and cannot directly image vertical fault zones we numerically determined that it is crucial to update source wavefields to image steeply dipping fault zones using synthetic seismic data we proved that our new lsrtm method can directly image steeply dipping fault zones with dipping angles up to 90 compared with conventional lsrtm our lsrtm method was less sensitive to the smoothness and the velocity error of the initial migration velocity model

biomarker maturity parameters for high maturities calibration of the working range up to the oil condensate threshold abstract in a study of oils and source rocks from the kimmeridge clay united kingdom and the hanifa qatar formations several biomarker maturity parameters with working ranges up to vr e vitrinite reflectance equivalent 1 0 oil condensate threshold have been identified and calibrated this represents a significant improvement compared to commonly used biomarker maturity tools sterane and hopane isomerisation with their maturity maximum of approximately vr e 0 7 early part of oil window these new parameters are 1 the absolute concentrations of steranes and hopanes decreasing gradually with increasing maturity 2 the ratio of tricyclic terpanes tricyclic terpanes hopanes increasing with maturity 3 the ratio of diasteranes 14 h 17 h steranes depending both on maturity and on source rock lithology and 4 the ratio of t s t m 22 29 30 trisnorneohopane 22 29 30 trisnorhopane increasing rapidly at very high maturities during artificial maturation experiments the biomarker maturities described in the natural samples were also known to be time temperature dependent up to rather severe laboratory heating conditions this corresponds with the working range up to high maturity levels as observed in the natural samples

the analysis of oil trapped during secondary migration abstract during secondary migration there is an opportunity for oil to be trapped as fluid inclusions fis within framework grains such as quartz and within diagenetic cements that have a crystalline structure oil saturation on migration pathways remains relatively low so typically fewer oil inclusions get trapped compared with samples from an oil column geochemical analysis of the much smaller amounts of inclusion oil present in samples from interpreted oil migration pathways has been attempted for two samples from the champagny 1 and delamere 1 wells in the vulcan sub basin northern offshore australia a combination of petrographic analysis bulk geochemical inclusion analysis and log evaluation confirmed that both samples were from oil migration pathways despite the small number of oil inclusions reliable geochemical data were acquired from both samples that were significantly above the levels detected for the system and outside rinse blanks the fi oil trapped on the interpreted oil migration pathway in champagny 1 was generated from clay rich marine source rock with little terrigenous organic matter input it was generated at peak oil window maturity and correlates best with oils derived from the late jurassic lower vulcan formation in contrast the delamere 1 fi oil contains evidence of greater input of terrigenous organic matter and was generated at early oil window maturity this fi oil also contains a signature of a biodegraded component which could have been generated either from the middle jurassic plover formation or from an older source rock these data indicate that it is feasible to geochemically map migration pathways across prospects or basins and to analyse palaeo oil compositions in oil zones where the few inclusions get trapped this also suggests that the few oil inclusions that sometimes occur in proterozoic or archaean rocks may be analysable in the future which would provide relatively pristine and robust data on the composition and diversity of earth s early biosphere

linearized inversion of seismic reflection data this is the first of a series of papers giving the solution of the inverse problem in seismic exploration the acoustic approximation is used together with the assumption that the velocity field has the form the forward problem is then linearized thus neglecting multiple reflected waves and the inverse problem of estimating is set up its rigorous solution can be obtained using an iterative algorithm each step consisting of a classical kirchhoff migration hyperbola summation plus a classical forward modeling step circle summation

carbonate source rock with low total organic carbon content and high maturity as effective source rock in china a review abstract carbonate rocks with high maturity and low total organic carbon content are widely distributed in china whether they can be effective source rocks remains controversial but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks dataset reveals that as the depth or maturity increases total organic carbon toc content atomic h c ratio and hydrocarbon generation potential of carbonate source rocks with low total organic carbon csrltoc all decrease because of hydrocarbon expulsion moreover some pyrolysis experiments also indicate that the csrltoc are able to generate and expel a quantity of hydrocarbons the above studies indicate that csrltoc can be effective source rocks for oil gas pools the unique features of carbonate rocks and csrltoc demonstrate that they are easier to expel hydrocarbons and contribute to oil gas reservoirs than shale and thus the lower limit of toc lltoc of effective carbonate source rocks could be smaller than shale 0 5 the lltocs of effective carbonate source rocks with type i ii1 kerogen mainly sapropelic group at the levels of immaturity and low maturity maturity high maturity and over maturity are toc 0 5 0 5 0 3 0 3 0 2 and 0 2 0 1 respectively if the carbonate source rocks are at the high over mature stage we could take 0 2 as the lltoc the present organic poor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods especially carbonate source rocks if type i ii1 kerogen some typical oil and gas fields in china derive partly or mostly from the csrltoc proving that csrltoc could be effective source rock in china

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

petroleum charge history in the lunnan low uplift tarim basin china evidence from oil bearing fluid inclusions abstract the petroleum charge history in the lunnan low uplift tarim basin china was investigated by comparing the geochemical characteristics of fluid inclusion fi oils from ordovician and triassic reservoirs with crude oils from stratigraphically different reservoirs quantitative grain fluorescence qgf and total scanning fluorescence tsf were used to screen the reservoir samples so as to identify suitable samples for molecular composition of inclusion mci analysis all of the samples contain c 30 n propylcholestanes diagnostic of marine source rocks the ordovician hosted fi oils differ from the triassic hosted fi oils having higher relative abundances of c 19 tricyclic terpane c 24 tetracyclic terpane and rearranged hopanes ts and c 29 ts and higher pr ph ratios while the triassic hosted fi oils have lower pr ph ratios and higher relative abundances of c 23 tricyclic terpane arylisoprenoids 25 norhopanes and c 35 homohopane several of the crude oils are similar to the triassic hosted fi oils based on the geochemical characteristics of the fi and crude oils combined with information on the burial history of the area at least two petroleum charge episodes can be distinguished in the lunnan low uplift an early charge represented by the ordovician hosted fi oils was biodegraded as evidenced by the presence of 25 norhopanes in the triassic hosted fi oils and crude oils but not in the ordovician hosted fi oils ordovician hosted fi oils provide the only evidence for oil generated from cambrian lower ordovician source rocks triassic hosted fi oils and crude oils in the ordovician carboniferous and triassic reservoirs are mixtures of biodegraded oil from an early charge episode and more mature oil from later charge

catalytic hydropyrolysis of asphaltenes in marine oil from the tarim basin nw china implications to complicated oil charging histories in an old composite basin abstract oil asphaltenes have been used to characterize the source s and timing of expulsion of early charged oils however the application on asphaltene in a complex petroleum basin have been limited marine oils from the tarim basin nw china were selected to release asphaltene bound molecules by the catalytic hydropyrolysis technique hypy the analyzed samples were 10 heavy oils from two major oil production regions the tabei and tazhong uplifts in the basin the asphaltenes may be mixed products from multiple hydrocarbon charging stages if we consider the main oil filling events and the regional geothermal histories of corresponding reservoirs however the similar molecular and isotopic characteristics of the hydropyrolyzates indicate contributions from one major source rock possibly lower cambrian mudstone during relatively early periods based on the presently known source characteristics the preservation of asphaltenes probably reflects a low reservoir temperature after charging the asphaltenes were derived from marine algae deposited in a strongly reducing environment given the biomarker data e g relatively abundant aryl isoprenoids and low pr ph and carbon isotopic data 35 5 to 34 4 asphaltenes were expelled around the oil generation peak moreover molecular ratios indicative of maturity e g tricyclic terpanes hopanes derived by hypy of asphaltenes suggest that the asphaltenes of the tazhong uplift are generally more mature than those of the tabei uplift moreover this is mainly related to different maturities when the oil was expelled the free hydrocarbons in the tazhong uplift show greater molecular and isotopic variations than those in the tabei uplift this might be attributed to more complex accumulation processes in the former late oils with a range of maturities mixing proportions with early biodegraded oils and degrees of alteration by thermochemical sulfate reduction could all have played a role in generating this variability the results indicate that the hydrocarbons released from oil asphaltenes by hypy can provide insights into multiple charging processes in an old composite petroleum basin

tracing early charged oils and exploration directions for the wenchang a sag western pearl river mouth basin offshore south china sea the wenchang a sag is the largest petroleum producing sub depression in the western pearl river mouth basin offshore south china sea the eocene wenchang formation and the early oligocene enping formation are the two major source rocks in the sag the oil discoveries in this region mainly originated from the enping fm how to objectively estimate the petroleum potential of the wenchang fm and trace the wenchang oils has long been the focus of research and exploration in this region in this paper inclusion oils obtained from reservoir sandstones as well as typical reservoir oils and potential source rocks were investigated by systematic geochemical methods oil oil and oil source rock correlations indicate that inclusion oils in the central and marginal areas of the wenchang a sag had similar geochemical characteristics and mainly originated from the wenchang source rocks the reservoir oils in the central area are derived mainly from the enping source rocks while in the marginal areas they have mixed characteristics of both the enping and wenchang formations combining the tectonic sedimentary evolution and petroleum generation histories it is concluded that the two source rocks in the wenchang a sag have generated oils but in the central areas the main stage of oil generation of the wenchang fm was much earlier than the formation time of the traps resulting in the dissipation of most of the early generated wenchang sourced oils in the marginal areas the main stage of oil generation of the wenchang fm was later and the wenchang oils not only could be captured as fluid inclusions in the reservoir quartz grains but also migrated and accumulated into oil pools in which they mixed with the enping sourced oils the early lithological and structural traps in the central and marginal areas of the wenchang a sag especially in the northern part of the shenhu uplift are the most prospective areas for further exploration of the sag

molecular memory effects recording the accumulation history of petroleum reservoirs a case study of the heidrun field offshore norway abstract the immobile portion of the oil occupying the pore space of reservoir sandstones which is mostly represented by visible impregnations is shown by the present study to contain geochemical signals that record their filling history the clue to unravelling these signals is sequential extraction of such impregnations of residual oil by a high pressure solvent flow through cell in which the solvent percolates through the intact and primary pore system of the rock sample gas chromatography mass spectroscopy gc ms analysis of residual oils extracted from samples of certain reservoir sandstones within the heidrun field offshore norway reveal significant contrasts in the chemical maturity of the first and the last fractions recovered as high as 0 69 versus 0 50 for the mpi 1 ratio corresponding to 0 79 versus 0 67 rc the hypothesis is proposed that by using this extraction technique oil impregnations from different portions of the pore system i e from large versus small pores are obtained separately this interpretation is especially supported by the regular occurrence of reservoir sandstones in which the residual oil exhibits maturity contrasts between those fractions extracted from large and small pores of the same rock within the heidrun field the distribution of reservoir sandstones with such maturity contrasts is strictly controlled by their position in the oil column and the location of their fault compartment these contrasts are interpreted to reflect the maturity levels of end members of the petroleum charges which have filled this trap during an extended period of geological time reservoir sandstone samples containing such end member charges are limited to the uppermost portion of the oil column lower in the oil column the pore system of all reservoirs contains oil impregnations of a uniform maturity level regardless of their pore size origin this distribution is believed to indicate an early stage of the accumulation history of the heidrun field characterised by the formation of a palaeo oil water contact well above the present one moreover the four fault compartments studied each controlled by bordering faults with different sealing capacities were not filled during the same stages of the accumulation history in summary the present study has shown that oil impregnations depending on their location within the pore system of individual reservoir sandstones can retain molecular memory effects which in turn allow the accumulation history of petroleum fields to be reconstructed

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

molecular correlation of crude oils and oil components from reservoir rocks in the tazhong and tabei uplifts of the tarim basin china molecular data from a large set of source rock crude oil and oil containing reservoir rock samples from the tarim basin demonstrate multiple sources for the marine oils in the studied areas of this basin based on gammacerane c31 hopane and c28 c27 c 28 c 29 sterane ratios three of the fifteen crude oils from the tazhong uplift correlate with cambrian lower ordovician source rocks while the other crude oils from the tazhong uplift and all 39 crude oils from the tahe oilfield in the tabei uplift correlate with middle upper ordovician source rocks these two ratios further demonstrate that most of the free oils and nearly all of the adsorbed and inclusion oils in oil containing reservoir rocks from the tazhong uplift correlate with cambrian lower ordovician source rocks while the free and inclusion oils in oil containing carbonates from the tahe oilfield correlate mainly with middle upper ordovician source rocks this result suggests that crude oils in the tazhong uplift are partly derived from the cambrian lower ordovician source rocks while those in the ordovician carbonate reservoirs of tahe oilfield are overwhelmingly derived from the middle upper ordovician source rocks the scatter of c23 tricyclic terpane c23 tricyclic terpane c30 17a 21b h hopane and c21 c21 rc29 sterane ratios for the free and inclusion oils from oil containing carbonates in the tahe oilfield possibly reflects the subtle organofacies variations in the source rocks implying that the ordovician reservoirs in this oilfield are near the major source kitchen in contrast the close and positive relationship between these two ratios for oil components in the oil containing reservoir rocks from the tazhong uplift implies that they are far from the major source kitchen

geological conditions and geochemical effects of secondary petroleum migration and accumulation the processes of secondary petroleum migration and reservoir filling remain among the least understood steps with respect to the characterisation and economic assessment of a petroleum system spatial heterogeneities of compositions of unaltered petroleum within individual fields have repeatedly been recognised and interpreted to result from compositional changes of multiple petroleum charges during the accumulation history in combination with limited degree of in reservoir mixing of petroleum fluids england w a mackenzie a s mann d u quigley t m 1987 the movement and entrapment of petroleum fluids in the subsurface j geol soc london 144 327 347 leythaeuser d ruckheim j 1989 heterogeneity of oil composition within a reservoir as a reflection of accumulation history geochim cosmochim acta 53 8 2119 2123 most previous attempts to improve this situation have aimed at an understanding of the conditions and effects of the flow of petroleum fluids by studying case histories on regional or field scales in the study reported here we have in addition attempted to reconstruct the mode of filling of sandstone reservoir rocks on pore scales

a novel sequential extraction system for whole core plug extraction in a solvent flow through cell application to extraction of residual petroleum from an intact pore system in secondary migration studies abstract the rapidly developing field of reservoir geochemistry and studies into secondary migration require new methods and techniques to characterise the composition of organic fluids obtained from reservoir rock samples ideally the composition of residual petroleums should be compared with the mineralogical and petrophysical properties from the same sample this requires a non destructive extraction procedure that keeps the rock in its natural state we have developed a flow through extraction system capable of efficiently extracting core plugs while preserving the pore system of the sample extractions can be performed sequentially to obtain various oil phases present in the pore system i e free or mobile oil can be obtained separately from residual oils adsorbed onto mineral or organic matter surfaces the free and adsorbed residual oils exhibit marked compositional differences due to fractional adsorption effects and maturity variations monitoring the pressures at the inlet and outlet positions of the extraction cell allows the measurement of pressure gradients across the core enabling the calculation of the relative permeability of the sample according to darcy s law application of the sequential flow through extraction method gives a substantial increase in information on the rock sample allowing a better understanding of fluid flow and fluid rock interaction in the subsurface

a review of the occurrences and causes of migration contamination in crude oil abstract migration contamination occurs when a migrating or trapped crude oil functions as a solvent dissolving molecular components from the syndepositional organic matter of the migration conduit and or the reservoir rock this review documents the causes and worldwide occurrences of migration contamination and suggests methods of identifying the signatures of this phenomenon within the molecular and isotopic composition of crude oils instances of migration contamination have been identified in oils of australia asia africa and north america and are best identified by the presence of molecular and or isotopic imbalances in the oil such imbalances are most apparent when co occurring molecular suites are incompatible with one another particularly in terms of thermal maturity occurrences of migration contamination in oils of the us gulf of mexico documented by molecular maturity imbalances such as the presence of unsaturated hydrocarbons known to be unstable at petroleum generation temperatures provide the best examples of this phenomenon an attempt is made to quantify the extent and impact of this process in the vermilion 14 field offshore louisiana in the us gulf of mexico here molecular maturity markers in the migrated fluid and in the syndepositional organic matter of the reservoir rocks are compared and the proportion of contamination in each reservoired fluid is estimated examples such as this may assist in explaining maturity anomalies observed in several oils worldwide particularly in tertiary reservoired oils that have migrated through and are trapped in thermally immature sediments it is suggested that the low molecular maturity ratios often observed in tertiary reservoired oils and attributed to unusually high heating rates may be caused at least in part by migration contamination

mixing and biodegradation of hydrocarbons in the daerqi oilfield baiyinchagan depression northern china abstract crude oils in the daerqi oilfield vary greatly in viscosities including normal viscous and highly viscous oils they are also dramatically different in thermal maturity from immature to mature based on biomarker parameters according to bulk and molecular geochemical characteristics crude oils can be classified into three groups in this oilfield group i oils are mature and derived from the deeply buried and relatively old source rocks within the tengger formation and the second member of the arshan formation of early cretaceous age group ii oils are immature to marginally mature and derived from the relatively young source rocks that experienced shallow burial within the first member of the duhongmu formation of earlier cretaceous age group iii oils are mixtures of group i and group ii oils with dominance of the group i oils 70 the viscosity is low for the non biodegraded group i and group iii oils 7 1 24 mpa s but very high for the non biodegraded group ii oils e g 433 104 mpa s for the representative oil da12o1 the group iii oils are similar to the group i oils in gross properties e g gross composition and viscosity but are similar to group ii oils in terms of their molecular maturity parameters the oil reservoirs are currently at a shallow depth

origins of palaeozoic oils in the tarim basin evidence from sulfur isotopes and biomarkers abstract crude oil h 2 s and pyrite samples from palaeozoic formations in the tarim basin have been analyzed for 34 s values together with oil samples for biomarkers and fluid inclusions for chemical composition to elucidate the origin of sulfur in the oil and to address potential applications in oil source rock correlation the results show that crude oil samples have sulfur contents from 0 02 to 2 47 and 34 s values from 11 9 to 28 2 non degraded oils without associated h 2 s gas have 34 s values from 11 9 to 20 5 and are relatively rich in c 28 20r sterane aryl isoprenoids and or gammacerane the features are well correlated with the cambrian and lower ordovician source rocks with 34 s values from 10 4 to 19 4 non degraded oils with associated h 2 s gas have 34 s values from 15 1 to 19 1 close to the h 2 s from 15 0 to 18 5 suggesting that the h 2 s was incorporated into the oils leading to the generation of 2 thiaadamantanes and likely alkyl thiolanes in the oil with a 34 s value of 18 5 heavily biodegraded oils have the highest sulfur contents and show 34 s values from 20 0 to 28 2 significantly heavier than those of any of the potential source rocks 15 3 to 19 4 thus it is likely that isotopically heavy sulfides have been incorporated into the biodegraded oils resulting in 34 s values of the oils becoming closer to cambrian and ordovician age seawater sulfates the sulfides may have originated from thermochemical sulfate reduction tsr as evidenced by the occurrence of h 2 s rich fluid inclusions and late stage and fracture filling pyrite with 34 s values mainly from 25 to 34 in the palaeozoic reservoirs

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

topographic elastic least squares reverse time migration based on vector p and s wave equations in the curvilinear coordinates elastic least squares reverse time migration has been applied to multi component seismic data to obtain high quality images however the final images may suffer from artefacts caused by p and s wave crosstalk and severe spurious diffractions caused by complex topographic surface conditions to suppress these crosstalk artefacts and spurious diffractions we have developed a topographic separated wavefield elastic least squares reverse time migration algorithm in this method we apply p and s wave separated elastic velocity stress wave equations in the curvilinear coordinates to derive demigration equations and gradient formulas with respect to p and s velocity for the implementation of topographic separated wavefield elastic least squares reverse time migration the wavefields gradient directions and step lengths are all calculated in the curvilinear coordinates numerical experiments conducted with the two component data synthetized by a three topographic layer with anomalies model and the canadian foothills model are considered to verify our method the results reveal that compared with the conventional method our method promises imaging results with higher resolution and has a faster residual convergence speed finally we carry out numerical examples on noisy data imperfect migration velocity and inaccurate surface elevation to analyse its sensitivity to noise migration velocity and surface elevation error the results prove that our method is less sensitive to noise compared with the conventional elastic least squares reverse time migration and needs good migration velocities as other least squares reverse time migration methods in addition when implementing the proposed method an accurate surface elevation should be obtained by global positioning system to yield high quality images

molecular and isotopic compositions of bitumens in silurian tar sands from the tarim basin nw china characterizing biodegradation and hydrocarbon charging in an old composite basin abstract biodegradation and oil mixing in silurian sandstone reservoirs of the tarim basin one of the largest composite basins in china were investigated by analyzing the molecular characteristics and stable carbon isotopic signatures of low molecular weight lmw saturated hydrocarbons and high molecular weight hmw asphaltenes detection of 25 norhopanes and 17 nortricyclic terpanes in most silurian tar sands from the tabei uplift in the tarim basin suggests a much greater degree of biodegradation here than in the tazhong uplift this explains the relatively more abundant tricyclic terpanes gammacerane pregnane and diasteranes in tar sands from the tabei uplift than in those from the tazhong uplift hence care must be taken when assigning oil source correlations using biomarkers in tar sands because of the biodegradation and mixing of oils derived from multiple sources in such an old composite basin asphaltenes in the tar sands seem to be part of the oil charge before biodegradation depending on the relative anti biodegradation characteristics of asphaltenes the similarity in carbon isotopic signatures for asphaltenes and their pyrolysates and the consistent product distribution for flash pyrolysis and for regular steranes in asphaltene pyrolysates regardless of whether the tar sands were charged with fresh oil according to the relative distributions of regular steranes and the relatively abundant 1 2 3 4 tetramethylbenzene significantly enriched in 13 c the oil sources for asphaltenes in the tar sands might be related to lower paleozoic marine source rocks formed in euxinic conditions nevertheless the relatively low abundance of gammacerane and c 28 regular steranes observed in asphaltene pyrolysates and residual hydrocarbons within limited samples investigated in this work made a direct correlation of oils originally charged into silurian tar sands with those cambrian source rocks reported so far seem not to be possible comparison of carbon isotopic signatures of n alkanes in asphaltene pyrolysates with those of lmw saturated hydrocarbons is helpful in determining if the abundant n alkanes in tar sands are derived from fresh oil charges after biodegradation the limited carbon isotopic data for n alkanes in lmw saturated hydrocarbons from the tar sands can be used to classify oils charged after biodegradation in the composite basin into four distinct groups

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

molecular analysis of petroleum in fluid inclusions a practical methodology a method is described for extracting the petroleum from petroleum bearing fluid inclusions hosted in the diagenetic cements of sedimentary rocks whilst minimising contamination from petroleum in the pore space a clean up technique involving the addition of extraction standards to the rock prior to analysis has been developed that allows increased confidence that the hydrocarbons extracted are only from included petroleum and are not mixtures of included petroleum and petroleum or other organic material adhering to the surface of the host minerals within the sample replicate quantitative analyses indicate that the technique produces highly reproducible results the problems of analysing included petroleum and the clean up method development are discussed and examples of analyses of included petroleums from carbonate reservoir and carrier units are given

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 1 oil family classification abstract a series of palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the tarim basin nw china have been examined to delineate oil families crude oils from the mesozoic foreland kuqa depression were also analyzed for comparison the genetic family was assessed using bulk hydrocarbon isotopic composition alkylation pattern of polycyclic aromatic hydrocarbons pahs and biomarker distributions detailed oil oil correlation suggests two distinct families occur in the cratonic region despite the diversity of the reservoir rocks and some variability in oil physical properties the tabei and tazhong oils are classified in general into a single family with a possible indication for secondary subdivision these oils are characterized by low pristane phytane pr ph ratio high dibenzothiophene phenanthrene dbt p ratio light bulk isotopic composition high degree of pah alkylation high proportion of c23 and c24 tricyclic terpanes and low abundance of c24 tetracyclic terpane and triaromatic dinosteroids this family is inferred to have a middle upper ordovician source rock the tadong family distinctly different from the tabei and tazhong families is characterized by relatively high pr ph ratio low dbt p ratio slightly heavier bulk isotopic compositions and low degree of pah alkylation it also has a unique biomarker signature with a high proportion of triaromatic dinosteroids possibly indicating cambrian lower ordovician source rocks the oils from the kuqa depression are easily distinguished from the other two families due to their terrigenous origin the major characteristics of the kuqa oils are high pr ph ratio very low dbt p ratio heavy bulk isotopic composition low pah alkylation degree high proportion of c20 and c21 tricyclic terpanes and c24 tetracylic terpane and abundance of triaromatic dinosteranes and 4 methyl 24 ethyltriaromatic cholestanes these characteristics are consistent with terrestrial organic source rocks deposited under sub oxic conditions

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 2 maturity assessment abstract the thermal maturity of forty three crude oil samples collected from the cratonic portion of the tarim basin nw china has been assessed using a variety of parameters while compound ratios of diamondoids alkylated naphthalenes methyl phenanthrenes and methyl dibenzothiophenes showed consistent trends with increasing maturity a large discrepancy was observed for the maturity levels of the oils determined from these parameters equivalent vitrinite reflectance values calculated from methylphenanthrene index 1 mpi1 methyldibenzothiophene ratio mdr and methyldiamontane index mdi were in the range of 0 66 0 97 1 11 1 64 and 0 89 1 74 ro respectively while we caution that some of the discrepancy may be related to calibration problems the difference observed from different compound classes could be real reflecting the multiple charge to petroleum accumulations as the diamondoid hydrocarbons were possibly generated at later maturation stages than most aromatic hydrocarbons as diamondoids with methyl substitution on the bridgehead positions are more stable than those with substitution on other positions several compound ratios based on ethyl admantanes dimethyl admantanes trimethyl admantanes and dimethyl diamantanes also correlated with the established maturity indicators such as mdr and mdi while condensates with high maturity levels represent the late charge hydrocarbons some of them with relatively low maturity levels may be affected by evaporative fractionation processes the normal gravity oils cover a wide maturity range consistent with multiple charges and in reservoir mixing while the waxy oils with high apparent maturity levels may be the products of wax precipitation due to a later gas injection into an oil column heavy oils with low apparent maturity levels could be the residue of oil that has lost its light end

sequential extraction a useful tool for reservoir geochemistry abstract little is known about the microscale distribution and potential molecular fractionation of bitumen residing in reservoir rocks few data exist on the comparative organic geochemistry of hydrocarbons in authigenic inclusions formed in reservoir rocks and the bitumen in the pore space a three step sequential extraction technique has been applied to clastic petroleum reservoir samples using a suite of solvents with increasing polarity in order to address these questions in the analyzed samples the following main trends were observed i the first extract represents 95 of the total extract ii the relative polar compound content increases going from the first to the last extract iii the last extract has generally a slightly lower apparent maturity as assessed using biomarker parameters additionally a reduction in the degree of biodegradation from the first to the last extracts was observed implying tentatively that biodegradation takes place dominantly in the center of the pores fluid inclusion extracts show distinct source characteristics however the last extract appears to approach the fluid inclusion extract signature in some aspects the use of sequential extraction as a routine tool in the reservoir geochemical context is judged to be limited however it may help to solve specific problems

organic geochemistry of carboniferous source rocks and their generated oils from the eastern junggar basin nw china abstract carboniferous source rocks have been gaining increasing attention after the discovery of the kelameili gasfield in the eastern junggar basin in 2005 two sets of source rocks within the lower carboniferous dishuiquan formation c 1d and upper carboniferous batamayineishan formation c 2b respectively have been identified in this area in this paper clear differences between these two source rocks are demonstrated in molecular and carbon isotopic compositions the c 1d source rocks in comparison with the c 2b source rocks have remarkably lower pr n c 17 ratio higher ts ts tm and c 30 diahopane c 30 diahopane c 30 hopane ratios lower concentration of c 24 tetracyclic terpane relative to c 23 and c 26 tricyclic terpanes and higher gammacerane c 31 hopane ratio 13 c values of individual n alkanes decrease with carbon number for c 1d source rocks while the opposite is true for c 2b source rocks among the 10 oils collected from the studied area four were generated exclusively from c 1d source rocks while the others were mainly derived from c 1d source rocks but contaminated by a small or trace amount of oil components which were derived from the c 2b middle permian pingdiquan formation p 2p and lower jurassic source rocks the free adsorbed and inclusion oils from three oil containing volcanic reservoir rocks within the c 2b formation generally correlate with c 1d source rocks based on molecular and carbon isotopic data however the three types of oil from the same reservoir rock vary significantly in molecular and carbon isotopic compositions reflecting facies and maturity changes of charging oil during the reservoir filling

geochemistry of alkylbenzenes in the paleozoic oils from the tarim basin nw china a b s t r a c t monoaromatic hydrocarbons alkylbenzenes in paleozoic oils condensates from the tarim basin were quantitatively analyzed using gas chromatography mass spectrometry their relative abundances and distributions show large variation which are most likely controlled by the temperature and duration of the catagenesis stage of petroleum maturation relatively high abundance of long chain linear substituted alkylbenzenes and alkyltoluenes in the studied samples implies that source rocks of these oils condensates contain high contents of algal material the dominance of dodecylbenzene in these n alkylbenzenes suggests the formation of alkylbenzenes from fatty acids via lactone formation dehydration the 1 2 3 4 tetramethylbenzene temb 1 methyl 3 phytanylbenzene 1 m 3 pyb and aryl isoprenoids are mainly derived from carotenoids produced by chlorobiaceae and their occurrence may indicate a highly reducing depositional environment however the concentration of these compounds diminishes significantly at high maturity with increasing thermal maturity the prominent trend of demethylation of alkylbenzenes is accompanied by shortening of alkyl side chains large variation in the c4 alkylbenzene isomer distribution is also controlled by thermal maturity the 1 2 3 5 temb and 1 2 4 5 temb isomers are more stable than other temb isomers and the overall thermodynamic stability of tembs is lower than that of dimethylethylbenzenes therefore several maturity parameters can be derived based on alkylbenzene stability absolute concentrations of alkylbenzenes generally increase with increasing thermal maturity but concentrations of 1 m 3 pyb and aryl isoprenoids decrease significantly at high maturity maturity parameters obtained from alkylbenzenes are especially useful for highly mature oils and condensates when most biomarkers are depleted caution is advised when interpreting the paleoenvironment and thermal maturity of an oil from the alkylbenzene signatures because they are affected by various other processes such as evaporative fractionation thermal cracking and biodegradation as well

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

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

comparative studies on compounds occluded inside asphaltenes hierarchically released by increasing amounts of h2o2 ch3cooh being the heaviest fraction of crude oils asphaltenes are liable to aggregate and other molecules in the oils can be steadily adsorbed onto and even occluded inside the macromolecular structures of the asphaltenes these occluded compounds inside the asphaltenes can survive over geological time in oil reservoirs owing to effective protection by the macromolecular structures of the asphaltenes the asphaltenes of a crude oil zg31 from the central tarim basin nw china were hierarchically degraded by increasing the amount of h2o2 ch3cooh to release the occluded compounds besides the common components series of even numbered n alk 1 enes and 3 ethylalkanes were detected among the occluded compounds comparison of the biomarker distributions and the compound specific c isotopic results between the compounds from the maltenes and those from the occluded fraction the zg31 reservoir was suggested to have been charged multiple times with different charges being derived from different strata of source rocks

molecular organic geochemistry of the tarim basin northwest china we conducted organic geochemical analyses on the largest suite of oils and source rock extracts from the tarim basin northwest china currently available statistical cluster analysis of the entire suite of tarim oils distinguishes at least seven genetic groups of oils the largest group of oils was collected from the tazhong and tabei uplifts and originated from marine middle upper ordovician anoxic marls that mark slope facies at the margins of structural uplifts two other genetic groups most likely originated from marine middle upper ordovician source rocks but of distinct facies with one an oxic shale rich source west of the bachu uplift and the other an anoxic shale source at tazhong

isotopic compositions and biomarkers in crude oils from the tarim basin oil maturity and oil mixing abstract hydrogen and carbon isotopic compositions of n alkanes as well as biomarkers in various types of oil from the tarim basin were analyzed data were used to discuss the occurrence and formation of marine oils from this old composite basin of which we have limited understanding first hydrogen and carbon compositions of n alkanes clearly classify the collected oils into two distinct groups supporting the reported oil classifications based on biomarkers group i the majority of oil samples is characterized by relative enrichment in d and n alkane d values vary from 110 to 75 group ii includes two heavy oils and d values of n alkanes range from 145 to 142 second variations in the 13c value of n alkanes 2 3 show good correlations with the maturity indices indicating that kinetic isotopic fractionation is one control on the 13c value of n alkanes in the maturation process d values of n alkanes in most oils are also evidently influenced by the oil maturity finally by combining the isotopic compositions with biomarkers of oils two distinct oil mixing processes were recognized in the oils from the tabei and tazhong uplifts in this old composite basin the former process is the mixing of oils from the same source at different maturity stages while the latter involves the mixing of oils from distinct sources at different maturity stages biodegraded oils may contribute to the total oils but their proportions are minor

migrated hydrocarbons in outcrop samples revised petroleum exploration directions in the tarim basin abstract the application of age specific biomarker distributions established from mature exploration areas of the tarim basin northwestern china indicates that most carboniferous permian outcrop samples in the eastern segment of the southwest depression previously believed to have significant petroleum source potential in fact contain migrated hydrocarbons derived from cambrian lower ordovician strata new geochemical results have led to a major revision of petroleum exploration directions in this area

recent debris flows on mars seasonal observations of the russell crater dune field 1 debris flows occur on slopes in the russell crater dune field the morphology of the erosion features resembles terrestrial viscous slurry flows mudflows and suggests that a flow of fine grained material mixed with liquid water might have been responsible for their formation seasonal mgs tes and moc imagery based observations of the dune field show 1 an annual frosting and defrosting cycle and 2 that liquid h2o could be stable within a limited time period in the summer of the southern hemisphere these observations lead to the conclusion that debris flows in the russell crater dune field may form under current climatic conditions by episodic or seasonal melting of small amounts of autumn winter condensed water ice

molecular correlation of free oil and inclusion oil of reservoir rocks in the junggar basin china free oils and inclusion oils oil bearing fluid inclusions of 12 samples collected from the sandstone reservoir formations in the central eastern and northern areas of the junggar basin northwest china were analyzed by gc and gc ms analytical results indicate very similar biomarker distributions within each of the free oils and their associated inclusion oils in the two samples collected respectively from the northwestern and eastern border of the junggar basin the free oil and inclusion oil of sample md1 1 collected from the northwestern border correlate well with the permian source rock of the fencheng formation while those of sample dn1 1 from the eastern border correlate with the permian source rock of the pingdiquan formation in contrast in the central area the free and inclusion oils vary significantly in most cases which suggests variations of sources for oil charges during the filling process these data and the correlation between the free and inclusion oils are consistent with the field and seismic data which show that in areas where samples md1 1 and dn1 1 are located only one available source rock exists while in the central area multiple source rocks are present

geochemical characteristics and implications of hydrocarbons in reservoir rocks of junggar basin china abstract a practical sequential extraction approach has been applied to two core samples collected from oil zones of triassic sandstones in junggar basin northwest china five bitumen fractions were obtained from each sample using this approach these fractions are free oil components in inter granular pores bitumen 1 adsorbed oil components on grain surfaces and liberated by hcl treatment bitumen 2 oil bearing fluid inclusions hosted by grains bitumen 3 and adsorbed oil components on clay mineral surfaces and liberated by hcl and hcl hf treatments bitumens 4 and 5 striking differences have been observed both in bulk compositions and in aliphatic biomarker distributions e g terpanes and steranes among the five bitumen fractions this phenomenon is ascribed to the indigenous compositional variations and differential maturation behaviors between adsorbed oil components in a network of polar compounds resins and asphaltenes and free oil components in even a same oil charge in addition to fractional adsorption effects and compositional changes of the oil charges in reservoir rocks during filling process the molecular compositions and homogenization temperatures of oil bearing fluid inclusions in these two samples indicate that trapping of these types of inclusions apparently ceased in sample a while continued in sample b after the initial oil charge suggesting that the relative content of polar components in the initial oil charge compared to the later oil charges plays a dominant role controlling the trapping of oil bearing fluid inclusions in an oil saturated zone sequential extraction studies are important and useful for reservoir geochemistry especially in the cases where only limited samples of oil and reservoir rocks are available and or where compositional heterogeneity of free oils has disappeared in a reservoir karlsen d a nedkvitne t larter s r bjorlykke k 1993 hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history geochim cosmochim acta 57 3641 3659

molecular correlation of free oil adsorbed oil and inclusion oil of reservoir rocks in the tazhong uplift of the tarim basin china the free adsorbed and inclusion oils were recovered by sequential extraction from eleven oil and tar containing reservoir rocks in the tazhong uplift of tarim basin the results of gas chromatography gc and gc mass spectrometry analyses of these oil components and seven crude oils collected from this region reveal multiple oil charges derived from different source rocks for these oil reservoirs the initially charged oils show strong predominance of even over odd n alkanes in the range n c12 to n c20 and have ordinary maturities while the later charged oils do not exhibit any predominance of n alkanes and have high maturities the adsorbed and inclusion oils of the reservoir rocks generally have high relative concentrations of gammacerane and c28 steranes similar to the cambrian lower ordovician source rocks in contrast the free oils of these reservoir rocks generally have low relative concentrations of gammacerane and c28 steranes similar to the middle upper ordovician source rocks there are two interpretations of this result 1 the initially charged oils are derived from the cambrian lower ordovician source rocks while the later charged oils are derived from the middle upper ordovician source rocks and 2 both the initially and later charged oils are mainly derived from the cambrian lower ordovician source rocks but the later charged oils are contaminated by the oil components from the silurian tar sandstones and the middle upper ordovician source rocks

source facies of the paleozoic petroleum systems in the tabei uplift tarim basin nw china implications from aryl isoprenoids in crude oils abstract aryl isoprenoids have been detected for the first time in crude oils from paleozoic petroleum systems in the tabei uplift tarim basin northwestern china the principal compounds possess the 1 alkyl 2 3 6 trimethyl substitution pattern characteristic of diaromatic carotenoids found in the chlorobiaceae family of photosynthetic sulfur bacteria with a predominance of c13 c23 homologues in these samples flash pyrolysates of the asphaltene fractions isolated from crude oils show an unusual abundance of 1 2 3 4 tetramethylbenzene an indicator for a significant contribution of diaromatic carotenoids to the source kerogen of these oils the wide distribution of these compounds in crude oils reveals that penetration of the photic zone by sulfidic waters during deposition enabled prolific growth of photosynthetic green sulphur bacteria chlorobiaceae this suggests that the source rocks for the paleozoic petroleum systems in the tabei uplift were likely deposited under euxinic conditions with sulfate and sulfide rich water bodies which doesn t support previously published conclusions of a middle upper ordovician source that marks the slope facies at the margins of structural uplifts

constraining the oil charge history of the south pepper oilfield from the analysis of oil bearing fluid inclusions abstract the south pepper oilfield located in the barrow sub basin on the nw margin of the australian continent has experienced a multi phase charge history abundant oil bearing fluid inclusions are present in samples from within the current gas cap suggesting that an oil column existed prior to gas this palaeo oil gas leg fi oil has ts tm and c29 c30 hopane ratios of 1 and the c35 homohopanes are a significant proportion of the extended homohopanes it has lower pr ph and diasterane sterane ratios than the currently reservoired live oil and contains gammacerane a series of peaks tentatively identified as c30 to c34 17 h 30 norhopanes and a large amount of 2 methylhopanes collectively geochemical analysis of the gas leg fi oil suggests that it was generated from a less mature more calcareous source rock deposited under more reducing conditions than the upper jurassic dingo claystone the main source of the live oil in addition the presence of c30 dinosteranes in the gas leg fi oil provides a triassic or younger age constraint this makes palaeozoic carbonates an unlikely source possible source intervals for the gas leg fi oil are thin lower jurassic limestones and marls which occur at the base of the lower dingo claystone or a thin limestone unit the cunaloo member at the base of the triassic locker shale samples from within the present oil leg also contain abundant oil inclusions consistent with high oil saturations at the present day however these oil inclusions exhibit different fluorescence colours suggesting they represent a second oil charge geochemically the oil leg fi oil has an intermediate composition between the live oil and the gas leg fi oil suggesting that gas charge displaced the first oil charge samples of which were preserved as fluid inclusions in the oil leg sample biodegradation of the first oil charge indicated by the presence of 17 h 25 norhopanes in the currently reservoired live oils can be attributed to the ingress of meteoric waters probably during sub aerial exposure of the basin margin during miocene wrenching changing environmental conditions curtailed bacterial activity and allowed unaltered oil sourced from the dingo claystone to accumulate below the gas cap and mix with the biodegraded residues of the first oil charge to achieve the live oil composition the biodegradation event must have preceded the second charge as the live oil contains compounds such as n alkanes which would have been removed had alteration occurred after the second charge complex charge histories are common and the analysis of palaeo oils trapped within fluid inclusions provides the opportunity to achieve a more comprehensive assessment of hydrocarbon charge

least squares data to data migration an approach for migrating free surface related multiples free surface related multiples can provide extra illumination of the subsurface and thus can be usefully included in migration procedures however most multiple migration approaches require separation of primaries and free surface related multiples or at least prediction of multiples in advance which is time consuming and prone to errors the data to data migration ddm method migrates free surface related multiples by forward and backward propagating the recorded full data containing primaries and free surface related multiples for ddm there is no need to predict or separate multiples but the migration results suffer from the crosstalk generated by crosscorrelations of undesired seismic events e g primaries and second order free surfacerelated multiples we have developed least squares ddm lsddm for marine data to eliminate the crosstalk generated by ddm in each iteration the forward propagated primaries and free surface related multiples are crosscorrelated with the backward propagated primary and free surface related multiple residuals to form the reflectivity gradient we use a three layer model and the marmousi model for numerical tests the results validate that lsddm can provide a migrated image with higher signal to noise ratio and more balanced amplitudes than ddm the lsddm approach might be valuable for general subsurface imaging for marine seismic data when the migration velocity is accurate and the acquired data have sufficient recording time

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

geochemical comparison of oil bearing fluid inclusions and produced oil from the toro sandstone papua new guinea abstract a detailed study was carried out to compare the geochemistry of the crude oil in the lower cretaceous toro sandstone reservoir in the iagifu 7x well with oil trapped in fluid inclusions in this sandstone in the iagifu 7x and p nyang 2x wells in the papuan fold belt the drill stem test dst oils and the oils trapped in fluid inclusions show different source and maturity dependent geochemical parameters biomarkers suggest that the dst oils at iagifu 7x were derived from clay rich mixed marine terrestrial source rocks deposited under oxic conditions probably middle to late jurassic mudstones the fluid inclusion oils were derived from a less terrestrially influenced marine source rock deposited under less oxic conditions the fluid inclusion oils contain 1 2 7 trimethylnaphthalene and oleanane biomarkers indicative of angiosperm input and a cretaceous or younger source rock these oils are more mature calculated reflectance rc 1 02 than the reservoired oil at iagifu 7x rc 0 84 and condensate at p nyang 2x rc 0 84 source and thermal maturity parameters for all three fluid inclusion samples are similar suggesting that they were derived from similar source rock facies furthermore samples with predominantly blue vs white fluorescing oil inclusions have no discernible geochemical maturity difference suggesting that the use of fluorescence colours as a qualitative guide to the maturity of the trapped oil is not valid in this case fluid inclusion oils were trapped prior to charge of the reservoir with oil at the iagifu 7x and gas condensate at the p nyang 2x wells the fluid inclusion oils therefore represent an early oil charge from a deeply buried probably cretaceous source rock which started migrating into the toro sandstone during the miocene at iagifu this early oil was probably diluted by a larger volume of jurassic oil generated at the end of the miocene the absence of fluid inclusions with compositions like the currently reservoired oil is interpreted to be due to oil charging an already oil saturated zone when diagenesis is likely to have been inhibited at the p nyang well the transition from oil as represented by the oil inclusions to gas condensate was most likely caused by gas displacement