geochemical modelling of the principal source rocks of the barinas and maracaibo basins western venezuela this study presents a geochemical modelling of the principal source rocks in the western venezuelan basins the area covers more than 100 000 km sup 2 and includes lake maracaibo and barinas basins the geochemical modelling recognizes three source rocks 1 a principal k3 k4 k5 cretaceous sequences represented by la luna capacho and navay formations 2 a secondary corresponding to the t4 oligocene sequence represent by the carbonera formation and 3 an accessory source rock k7 k8 paleocene sequences represented by the carbonaceous shales and coals of the orocue group and marcelina formation three periods of hydrocarbon expulsion were defined for la luna formation early eocene late eocene middle miocene early miocene and early miocene holocene and a principal period of hydrocarbon expulsion for orocue group and carbonera formation plio pleistocene and middle miocene plio pleistocene the 90 of hydrocarbons generated correspond to the principal source rock la luna formation and the 10 to tertiary source rocks carbonera formation and orocue group five petroleum systems were identified lake maracaibo southwest of the lake maracaibo basin the lara nappes the extensive basins of eastern zulia and the barinas subbasin

the microbial input in carbonate anhydrite facies of a sabkha palaeoenvironment from guatemala a molecular approach abstract nine representative rock samplesincluding four anhydrites an oil stained dolomite and a laminated mudstone source rock from a sabkha type palaeoenvironment la felicidad 1 well guatemala have been compared in order to tentatively characterize each facies by a geochemical signature each sample has been studied by using basic parameters toc rock eval pyrolysis yields and analyses of alkanes and aromatics by computerized gc ms the following families of compounds have been reviewed n alkanes branched and cyclic alkanes n alkylbenzenes benzohopanes monoaromatic steroids d ring aromatic 8 14 secohopanoids alkylnaphthalenes alkylphenanthrenes alkyldibenzothiophenes and triaromatic steroids despite close geochemical relationships of all the sample a more in depth investigation taking into account molecular patterns and semi quantitative data reveals significant differences among the samples anhydrites may be differentiated from carbonates which illustrates the efficiency of organic geochemistry in environmental palaeoreconstruction even in organiccarbon lean sediments toc 0 04 0 08 in anhydrites in these anoxic marginally mature sediments the strong predominance of triterpanes over steranes tt to st 20 associated with other molecular features leads to the conclusion that the main contribution to the organic matter of all facies is by bacteria several prominent components including novel families iso c23 and iso c27 alkanes tetracyclic terpanes d ring aromatic 8 14 secohopanoids 1 phenylpentadecane undecylnaphathalene and their mono and dimethylated counterparts have been interpreted as derived from halophilic bacteria these conclusions may be extended to all crude oils from the same area which may be considered as true bacterially derived petroleums the la felicidad 1 well data are discussed with reference to other palaeosabkha type sequences from the sw aquitaine basin france in which the bacterial input is less important however in these sabkha sequences 1 phenylpentadecane undecylnaphthalene and related structures have also been identified

evidence for mixed and biodegraded crude oils in the socororo field eastern venezuela basin this work presents a geochemical study of crude oils from the socororo field socororo major area in the maturin sub basin of the eastern venezuelan basin the v ni total concentrations and v v ni ratios suggest the presence of two oil groups these oils are heavy aromatic asphaltic and according to their sulfur content api gravity and hydrocarbon compositions fall into the same two groups established from the v and ni proxies although the analyzed crude oils show evidence of biodegradation it was possible to determine based on various biomarkers parameters that the oils originated from marine organic matter in a marine carbonate or marine marl source rock deposited under anoxic to suboxic conditions the oils were generated near the peak of the oil window distributions of n alkanes isoprenoids hopanes and steranes combined with a prominent unresolved complex mixture ucm indicate that the oils are biodegraded moreover the presence of pristane and phytane along with alteration of hopanes and steranes and the presence of 25 norhopanes reveals that these oils are mixtures showing different levels of biodegradation sulfur content v and ni concentration and saturate aromatic resin and asphaltene sara composition provide additional evidence for oil mixing such mixed oils originate from several episodes of charging and biodegradation in the reservoirs the first charge was severely biodegraded and many biomarkers susceptible to degradation n alkanes acyclic isoprenoids terpanes methyl hopanes methyl steranes and steranes were extensively altered or completely removed the second oil charge was subjected to a lower degree of biodegradation and only the n alkanes show significant biodegradation both oil charges seem to come from the same source rock which was deposited under variable redox conditions moreover due to their similar maturity the generation and migration of these oils to reservoirs likely occurred close together in time

organic geochemistry of the carapita formation and terrestrial crude oils in the maturin subbasin eastern venezuelan basin abstract geochemical characteristics of the organic matter in the thick shaly sequence of the carapita formation lower miocene were analyzed in five exploratory wells drilled in the maturin subbasin eastern venezuela the study also includes crude oils from well one analyzed to establish their origins and possible sources the quantity and quality of organic matter in the carapita formation toc h c hi oi and kerogen type indicate vertical variation shales of the carapita formation increase in source rock potential from top to bottom this suggests two facies with different hydrocarbon potential maturity parameters t max 428 446 c r o 0 35 0 73 indicate that the upper section is immature and the lower section is mature thus the lower section is sufficiently mature to generate oil matoil modeling in well one shows that the lower section of the carapita formation began to generate oil in late miocene time 6 4 m a at a depth of about 15 900 ft 4846 m geochemical characteristics of oils from well one indicate generation from a source with abundant higher land plant input these are the first terrestrial crude oils of this type found in the fold thrust belts of the maturin subbassin gc ms analyses of saturated hydrocarbon fractions suggest a possible correlation with the terrestrial organic matter in the carapita formation

an investigation of seawater and sediment depth using a prototype airborne electromagnetic instrumentation system a case study in broken bay australia previous studies using commercial airborne electromagnetic equipment that is not optimized for marine surveying have demonstrated the use of airborne electromagnetic methods for measuring water depth and estimating sediment thickness a new prototype helicopter time domain airborne electromagnetic system seatem 0 is now under development for bathymetric surveying the first sea trial of the seatem 0 system took place over broken bay new south wales australia in shallow water up to 30 m in depth broken bay was chosen because the separate paleodrainage systems for the hawkesbury river brisbane water and pittwater which join in broken bay give rise to paleovalleys infilled with unconsolidated sediments ranging in thickness between 0 m bedrock outcrop and 200 m the survey area also included a tombolo with a beach either side which provided the opportunity to measure water depth through a surf zone sediment thickness and water depth is predicted from stitched layered earth inversion of data based on a simplified two layer model that represents seawater and sediment overlying a resistive half space basement bedrock the resulting bathymetric profiles show agreement typically to within 1 m and 0 5 m with known water depths in areas less than 20 and 6 m deep respectively the inverted depth profile of the second sediment layer is noisy however the profiles reveal coarse topographic features of paleovalleys to depth limits of 60 to 80 m below sea level in 20 to 30 m water depth as well as resolving bedrock ridges and exposed reefs in shallow waters

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

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

on mapping seafloor mineral deposits with central loop transient electromagnetics electromagnetic methods are commonly employed in exploration for land based mineral deposits a suite of airborne land and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose in contrast although the commercial value of marine mineral deposits has been recognized for decades the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy one particularly interesting electromagnetic method which could be used to image a mineral deposit on the ocean floor is the central loop configuration central loop systems consist of concentric transmitting and receiving loops of wire while these types of systems are frequently used in land based or airborne surveys to our knowledge neither system has been used for marine mineral exploration the advantages of using central loop systems at sea are twofold 1 simplified navigation because the transmitter and receiver are concentric and 2 simplified operation because only one compact unit must be deployed we produced layered seafloor type curves for two particular types of central loop methods the in loop and coincident loop configurations in particular we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick modeling and resolution analyses showed that using a 50 m 2 transmitting loop with 20 a of current these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness in the most extreme case absolute voltage errors on the order of 10 nv are required to resolve the base of a 30 m thick ore deposit whether such noise floors can be achieved in real marine environments remains to be seen

distribution of trace elements in crude oils from southern germany the concns of s v cr fe co ni zn se br i ag and hg were detd in 45 crude oil samples of 17 oilfields originating from 9 different mesozoic or tertiary reservoir rock formations in the southern german alpine foreland furthermore the concns of na mg s cl ca fe co cu zn br sr and i were detd in 15 oilfield water samples the main anal technique was instrumental neutron activation anal to a minor degree x ray fluorescence spectrometry at absorption and emission spectrometry and titrimetric methods were applied the amts of s v ni and se are relatively const in the different oilfields therefore they are suitable for distinguishing between different classes of oil the crudes can be divided into 3 groups by their concns of s v and ni triassic and lower jurassic crudes of the western molasse basin crudes from the bausteinschichten lower chattian in the western molasse except the one from aitingen and crudes from priabonian sands malm and lower cretaceous in the eastern molasse and secondary altered crudes of the ampfinger schichten in the eastern molasse basin the crudes of aitingen are exceptional with their high content of s and v being enhanced 3 and 20 times resp relations between trace element distributions in crudes and in accompanying oilfield waters could not be found only the amts of i in crudes are significantly correlated with those from the corresponding oil field waters

origin of beatrice oil by co sourcing from devonian and middle jurassic source rocks inner moray firth united kingdom commercial hydrocarbons in the moray firth area of the north sea occur in reservoirs ranging in age from devonian to tertiary with the bulk of the reserves located in upper jurassic and lower cretaceous clastic rocks most hydrocarbon discoveries in this area have been made in the outer moray firth primarily in the witch ground graben a single oil field beatrice has been developed in the inner moray firth area the unusual location and chemical composition of beatrice compared to piper and other north sea oils generated from the kimmeridge clay have contributed to the longstanding controversy on its origin oil source potential in the inner moray firth has been recognized in lacustrine devonian dolomitic siltstones middle jurassic paralic coals upper jurassic marine shales and other rocks however prior to this study only the upper jurassic kimmeridge clay was recognized as a significant effective source rock in the main graben area of the north sea biomarker and stable carbon isotope analyses conclusively show that beatrice oil could not have been generated from the classic kimmeridge clay source rock but is a mixture of products derived from effective devonian and middle jurassic source rocks this work demonstrates the power of a multiparameter geochemical approach in solving this difficult oil to source rock correlation problem use of this approach can lead to the recognition of new exploration plays in previously well explored basins

major elements trace hydrocarbon sources in over mature petroleum systems insights from the sinian sichuan basin china abstract identifying hydrocarbon sources in over mature petroleum systems is important and challenging here we present a pilot study using inorganic geochemistry to trace hydrocarbon sources in the giant natural gas accumulations within the sinian neoproterozoic dengying formation in the sichuan basin sw china we investigated the major elements ti al fe mn mg ca na k and p as tracers of hydrocarbon sources for the first time our results show that the over mature solid bitumen in the reservoirs contains parts per million amounts of major elements in addition to carbon i e organic matter na and k in the bitumen occur mainly in the v ni porphyrin form al fe and mn are present in the fe porphyrin form the behavior of p is relatively complex and it may reside mainly in a non porphyrin organic form the other elements ti mg and ca may be present as emulsified invisible nano minerals the mn fe ratio and ternary 10mn fe p diagram can be used for hydrocarbon source rock correlations based on the proxies it is suggested that the lower cambrian qiongzhusi formation was the main source of hydrocarbons in the dengying formation our data suggest that inorganic geochemistry is an effective method for the study of over mature hydrocarbons and complements traditional organic geochemical approaches

testing the limits of aem bathymetry with a floating tem system a floating transient electromagnetic tem system sea ring simulating a low altitude helicopter airborne electromagnetic aem system was constructed to test the accuracy of the aem method for measuring water depth and estimating sediment thickness in shallow coastal waters a square transmitter loop 10 10 m plus concentric inner and outer receiver loops was strung from masts supported by the circular sea ring base data were stacked over periods from 1 to approximately 60 s and with loop heights ranging between approximately 5 5 and 12 m above sea level the towed sea ring provides a stable platform at a known fixed altitude in calm waters we have undertaken modeling to investigate the effect of vertical and horizontal displacements of the loops and to compare circular and square loopgeometries in proximity to the sea surface with relatively long stacking times as long as approximately 60 s the uncertainty in altitude can be reduced to very low levels the sea ring has been deployed near

biological marker characteristics of oils and asphalts from carbonate source rocks in a rapidly subsiding graben dead sea israel abstract a detailed gc ms study of biological marker compounds in the saturated and aromatic hydrocarbon fractions of oils and asphalts from the dead sea area israel provided decisive information to the solution of a long lasting controversy by showing that the asphalts are products of early generation in an immature stage from the same type of carbonate source rock which generated more mature oils the asphalts are not biodegraded residues of the oils oils from six different wells and asphalts from wells outcrops and a floating block from the dead sea all have very similar sterane and triterpane patterns they all lack rearranged steranes diasteranes indicating a carbonate source matrix and compare reasonably well with a sample of upper cretaceous bituminous chalk from nebi musa the main difference between the oils and the asphalts is a significantly higher triaromatic to mono plus triaromatic steroid hydrocarbon ratio in the former this is explained as a result of rapid subsidence and heating of their source rock close to the deep parts of the dead sea graben the oils thus were generated in the more deeply buried source rock blocks under the graben fill whereas the asphalts either originate from an immature source rock section closer to the graben rims or represent an earlier phase of generation and expulsion this study also provides general information on the evolution of biological markers in carbonate source rocks low activation energy processes like isomerisation of steranes appear to occur much faster at low temperatures than in shales the high sulfur content and less cross linking of the biogenic organic matter into a complex kerogen structure are suggested to be responsible for this care should be taken when using only sterane isomerisation to assess the maturity of hydrocarbons from carbonate rocks and of carbonate derived oils

iron availability as a dominant control on the primary composition and diagenetic overprint of organic matter rich rocks iron is known to stimulate surface ocean productivity as well as intervene with bacterially mediated processes of organic matter remineralization during early diagenesis in this paper we examine the influence of iron supply on the geochemistry trace metals 34s organic matter of sedimentary rocks deposited in a clastic dominated marine ramp environment to this end we studied two late jurassic formations of the boulonnais area north france both formations were deposited under quite similar conditions but they differ in the reactive iron supply they received only one of the two formations was affected by the particulate iron shuttle process our results indicate that 1 the iron shuttle may be recorded through concomitant enrichments in p mo as and sb 2 a limited reactive iron supply will allow the sulfurization of organic matter even in a context of moderate productivity thus sulfurization can be a factor favoring a noticeable accumulation of organic matter iron may thus be an important agent in the c cycle

improved seawater depth determination using corrected helicopter time domain electromagnetic data abstract two areas within sydney harbour were surveyed in 2002 with a helicopter borne time domain electromagnetic system to test its potential for bathymetric mapping in shallow seawater as delivered the data were improperly calibrated therefore a re calibration was performed to reconcile the measured data with ground truth synthetic electromagnetic transients were computed for two layer models representing the seawater and the sediment overlying bedrock at a number of locations within the survey area the seawater depth in the models varied between 3 m and 70 m the measured and calculated data were compared at each delay time and were found to be linearly related the slope and intercept of the line of best fit were used to correct all the measured data inversion of the corrected time domain electromagnetic data generally resolved the bathymetry to submetre accuracy down to depths of about 55 m

geochemical compositional gradients in a mixed oil reservoir indicative of ongoing biodegradation abstract this study reports the results of organic geochemical characterisation of oil extracts from cores through an oil reservoir column within the grane field north sea the results show small gradients in composition attributed to ongoing biodegradation occurring at the oil water contact biodegradation is affecting the labile constituents that have been replenished by recent charging of this mixed oil and there is no apparent ongoing biodegradation effect upon the resistant compounds associated with earlier oil charge the use of principal component analysis has allowed compositional changes due to evaporation during storage to be distinguished from the effects of biodegradation and has highlighted the c19 n alkanes hopanes steranes ratio as a biodegradation sensitive parameter that shows a gradient in composition through the oil column the concentration of the c19 n alkanes though the oil column confirms that the n alkanes are depleted towards the oil water contact this approach has shown that biodegradation related trends indicative of ongoing biodegradation can be identified in reservoirs that contain multiple altered oil charges even when affected by subsequent compositional changes during sample storage

organic geochemistry of venezuelan extra heavy crude oils 2 molecular assessment of biodegradation abstract the extent of biodegradation of extra heavy crude oils from the eastern venezuelan basin orinoco tar belt has been assessed using biological marker compounds which include steranes triterpanes aromatic steroids and phenanthrenes analyzed by gc ms and petroporphrins analyzed by hplc the analyses indicate that biodegradation takes place in two regions namely surface or close to surface and subsurface with a different pattern of biodegradation taking place at each type of location some oils show patterns of biomarkers which may be the combined result of degradation at both main sites of biodegradation however two empirical biodegradation parameters are proposed to achieve this classification the classification of crude oils in terms of degree of biodegradation is further complicated by the identification of mixed crudes

the controls on the composition of biodegraded oils in the deep subsurface part 1 biodegradation rates in petroleum reservoirs abstract biodegradation rates in oilfields have been assessed conservatively using whole oil column minimum rate estimates diffusion controlled oil column compositional gradient modelling and mixed oil kinetic models biodegradation rate constants first order are around 10 6 10 7 yr 1 for hydrocarbons in the degradation zones these corresponding well with zero order field wide minimum rate estimates of about 10 8 kg hydrocarbons kg oil year for the whole oil column with biodegradation induction times of around 1 2 ma to perturb an entire oil column for light oil reservoirs and 10 20 ma for heavy oil reservoir degradation the results indicate that where we see continuous gradients in the oil columns degradation must have been occurring episodically for many millions of years to remove the n alkanes from an oil i e about 10 of an oil around ca 15 ma is needed for a heavy oil ca 5 ma for a light n sea oil the timescales of oilfield degradation and filling are thus very similar and consequently the degree of biodegradation will be substantially controlled by oilfield charge history assessment of mixed degraded non degraded oil occurrence provides an independent confirmation that these rates are realistic and that timescales of degradation and field charging are similar the maximum effective rate constant of degradation ultimately controlled by the limiting effect of diffusion of alkanes to the oil water contact owc ca 10 4 yr 1 for a 130 m thick oil column first order rate constant is well above the estimated rate constants indicating oil biodegradation rate is not limited by electron donor supply i e hydrocarbons but by supply of nutrients or oxidants this suggests that diffusive transport of nutrients and electron acceptors in the aquifer to the site of biodegradation may be adequate to maintain the low rate biosphere

trace metals and organic geochemistry of the machiques member aptian albian and la luna formation cenomanian campanian venezuela abstract two proposed source rock sequences in the lower aptian albian and the upper cenomanian campanian cretaceous sediments of the perija range maracaibo basin were geochemically analyzed in order to find environmentally controlled characteristic fingerprints trace metals and sulfur concentrations were found to be closely related with the physicochemical conditions during deposition of the rocks based on trace metal analyses and sulfur relationships with toc the aptian albian sequence machiques member shows alternate oxic and anoxic episodes close to the bottom water surface where mixed marine and terrestrial organic matter were deposited the cenomanian campanian sequence la luna formation persistently shows anoxic euxinic conditions that allowed the preservation of marine organic matter ratios of trace metals v mo and zn characterize both cretaceous sedimentary columns however some limitations are visualized in using trace metals as correlation tools in further geochemical studies biomarker ratios are related to the organic matter type and lithology some specific ratios and individual compounds allow a clear differentiation between these two sequences and offer in the context of 13 c an excellent tool for further oil source rock correlation studies

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

airborne electromagnetic bathymetry and estimation of bedrock topography in broken bay australia abstracta helicopter transient electromagnetic tem survey was flown over shallow coastal waters in broken bay australia overlying several palaeovalleys and exposed reef sections the infilled palaeovalleys contain unconsolidated sediments with variable thicknesses exceeding 100 m previous marine seismic reflection and vibrocore studies provide an estimate of sediment thickness and surficial sediment conductivity respectively which combined with known bathymetry sonar soundings and measured seawater conductivity can be used to generate a crude 1d geoelectric ground truth model consisting of two layers seawater sediment overlying a relatively resistive basement the primary focus was to examine the accuracy of interpreted water depths obtained by 1d inversion of airborne tem data assuming a two layer over resistive basement model by comparing these depths with known water depths the secondary focus was to compare the interpreted sediment thickness i e the thickness of the second geoelectr

use of molecular parameters and trace elements in oil oil correlation studies barinas sub basin venezuela abstract this is a geochemical study of oils from seven fields la victoria guafita caipe silvestre sinco silvan and palmita in the barinas sub basin venezuela the study focused on the type and quality of the crude oils their relative correlation degree of maturation alteration and the depositional paleoenvironment of the source rock the crude oils of the barinas sub basin can be classified in the range of paraffinic naphthenic la victoria and guafita to aromatic naphthenic caipe silvestre sinco silvan and palmita these crude oils were probably all generated from a similar source rock interval but with lateral variations in lithofacies and redox potential across the basin we interpret all of the crude oils to have been sourced from marine algal and bacterial organic matter with a systematic variation in terrigenous organic content of the source rock bulk elemental and biomarker geochemistry indicators suggest that la victoria and guafita crude oils were generated from organic matter deposited in a more siliciclastic dominated lithofacies crude oils from the caipe silvestre sinco silvan and palmita fields seem to have been generated from a more calcareous lithofacies based on interpretation of thermal indicators from saturate and aromatic biomarkers all of the oils studied were generated during peak levels of maturation of the source rock la victoria and guafita crude oils seem to have been generated at slightly lower thermal maturities than the other crude oils studied the oils in some of these fields have undergone extensive biodegradation and water washing hydrocarbon recharge and later incipient biodegradation of the recharged crude this explains much of the chemical and physical properties variability observed across individual fields and between accumulations

distribution of major and trace elements in la luna formation southwestern venezuelan basin the la luna formation maraca section maracaibo basin was studied by means of v and ni analysis of the bitumen total organic carbon toc total sulfur st major elements si al fe mg mn ca ti na k p trace elements v ni co cr cu and zn and electron microprobe analysis epma of the whole rock and st major elements si al fe mg mn ca ti na k p trace elements v ni co cr cu zn mo ba u th and rare earth elements la ce nd sn eu th yb lu of the carbonate free fraction the results are discussed based on the organic and inorganic association of trace elements and their use as paleoenvironmental indicators of sedimentation an association between v and organic matter is suggested by means of correlation between v and ni vs toc the use of epma whole rock and v and ni concentrations carbonate free fraction whereas ni is found in the organic matter and the sulfide phase fe is present as massive and framboidal pyrite whereas zn precipitates into a separate phase sphalerite and ni cu and in some cases zn can be found as sulfides associated with pyrite concentrations of v and ni bitumen toc st v ni cr cu and zn whole rock u th mo carbonate free fraction are indicative of changes in the dysoxic sedimentation conditions in the chert layers toc st v ni cu and low zn and v cr 4 in the sequence corresponding to the argillaceous limestone variations in the concentrations of toc st v ni zn cu and cr whole rock can be observed also suggesting variable sedimentation conditions the following is proposed i sedimentation intervals under euxinic conditions associated with high contribution and or preservation of organic matter allowing a high concentration level of v and ni in the organic phase and the accumulation of cu zn and ni in a smaller proportion in the sulfide phase ii sedimentation intervals under anoxic conditions and in the presence of relatively lower h2s which allowed lower concentrations of v and ni in the organic phase and higher concentrations of cu zn and ni in the sulfide phase rare earth elements ree concentrations exhibit a marked increase in ce nd sm eu y and lu for the qm 3 interval relative to post archean average shale paas ree enrichment in shales has been related to the presence of phosphate minerals such as monazite or apatite however these minerals were not detected through xrd or epma in the whole rock or in the carbonate free fraction the association of ree with organic matter is suggested due to the absence of phosphate minerals although assessment of these elements require further analysis 2002 elsevier science ltd all rights reserved

geochemistry of paleozoic marine oils from the tarim basin nw china part 4 paleobiodegradation and oil charge mixing abstract fifty three oil samples from paleozoic marine reservoirs of the tarim basin were analyzed by gc fid and gc ms to elucidate the influence of biodegradation and mixing on molecular compositions biodegradation has a major impact on many classes of compounds but the ultimate compositions and physical properties of the oil are influenced strongly by mixing with late oil charge subsequent to the process of biodegradation almost all samples display biodegradation signatures characterized by variable size of the unresolved complex mixture ucm and variable amounts of 25 norhopanes 25 nhs and 17 nortricyclic terpanes 17 nts the ubiquitous occurrence of n alkanes suggests that the petroleum accumulated in these fields is mainly a mixture of previously biodegraded oils with later migrated fresh oils the unaltered late charge of oil which has commingled with the initial biodegraded charge gives the wide range of oil gravities and asphaltene contents observed most commonly used maturity and depositional environment diagnostic parameters in the saturated hydrocarbon fraction are affected by biodegradation based on the intensity of 25 nhs and 17 nts oils from the tabei uplift appear to have experienced a much greater degree of biodegradation than those from the tazhong uplift however we caution against the simplistic use of molecular indicators to define a specific level of biodegradation in this case study as it may be somewhat misleading for physical property prediction a wide range of estimated maturity values from different components provides an important clue for multiple charge and mixing interpretations the relationship between the absolute concentrations of compound class and peak hump ratios of the samples can potentially qualitatively differentiate the pulse of inputs to the mixed oils the positive correlation between the concentrations of a given compound class and peak hump ratios may suggest that that class of compounds is most likely derived from the late charge after biodegradation while the negative correlation may indicate compounds largely inherited from the early charge before biodegradation some compounds show almost no correlation between their concentrations and peak hump ratios implying more similar intensity of charge inherited from early pulse and amalgamated from later pulses in some extremely biodegraded cases where ucm hump can also be diminished not in present case study the correlation between compound class concentrations and peak hump ratios will be more complicated

geochemistry and origin of heavy oils in the south part of zhanhua depression east china abstract the sanhecun sag located in the south of the zhanhua depression has yielded a significant amount of heavy oil in recent years and is becoming a new exploration focus in the jiyang sub basin surprisingly oils in the deep shahejie formation have higher densities and viscosities than those in the shallow guantao formation in this study geochemical and fluid inclusion analyses of oils from different depths were carried out to clarify their source and the cause of the high densities oils in the shahejie formation with high contents of resins and asphaltenes 47 56 are generated from the es4 carbonate source rocks that were deposited in a hypersaline environment with a relatively high terrigenous organic matter contribution maturity parameters and fluid inclusions show that the oils with low saturated hydrocarbon contents and high resin and asphaltene contents were charged to the shahejie formation from 29 ma to 27 ma when the source rocks were in the early oil generation window although subsequent biodegradation further increased oil density the main control on the high densities of oils in the shahejie formation is the nature of the source kerogen oils in the shallow guantao formation can be divided into two groups a group from es3 source rocks in the gunan sag and a second from es3 and es4 source rocks in the bonan sag they were expelled within the oil generation window and charged the reservoirs between 3 ma and the present day oils in the guantao formation have undergone light to heavy biodegradation ranging from pm levels 2 to 5 with the increase of biodegradation level saturated hydrocarbon contents gradually decrease from 50 to 22 while the contents of resins and asphaltenes increase from 28 to 43 biodegradation is the primary cause of oil densification in the guantao formation although oils in the guantao formation suffered more severe biodegradation they still have higher contents of saturated hydrocarbons than those in the shahejie formation source type maturity and biodegradation thus combine in different ways to control the density of heavy oils in the sanhecun sag this study also indicates that low mature oils from es4 carbonate source rocks can charge reservoirs in the southern part of zhanhua depression before late oligocene uplift and low mature oil may be a potential future target in this area

organically bound metals and biomarkers in the monterey formation of the santa maria basin california abstract systematic variations in metal concentration ratios and biomarker ratios are observed with depth in cores of the monterey formation of the union leroy 51 18 well in the santa maria basin california specific metal ratios v v ni and mo mo cr correlate between kerogen and extractable organic matter further the organic matter in the two predominant lithofacies siliceous shales vs phosphatic shales can be distinguished based upon metal concentration ratios e g v v ni and certain biomarker characteristics e g sterane terpane mono triaromatic steroid hydrocarbon ratios maximum 28 30 bisnorhopane hopane ratios occur over a narrow range of v v ni ratios in kerogen concentrates and extractable organic matter this suggests that metal ratios in kerogen concentrates and eom may be useful for oil source rock correlation studies in the monterey further while kerogen parameters metals are discontinuous at the lithofacies boundary extract parameters biomarkers are gradational at this boundary suggesting about 30 ft 9 m of vertical hydrocarbon migration across the lithofacies contact

origin of granite at cabo de santo agostinho northeast brazil a 4 km2 exposure of shallowly emplaced leucogranite on the atlantic coast at cabo de santo agostinho 30 km south of recife brazil has been extensively studied chemically and isotopically twenty three major element analyses indicate that the cabo granite ranges from peralkaline to peraluminous na2o k2o is very high 7 4 to 10 4 wt with cao low 0 3 and mgo vanishing 0 06 microprobe analyses confirm the presence of arfvedsonite biotite absent and nearly total absence of plagioclase the rocks are moderately to highly enriched in lree la 45 to 350 times chondritic with extremely pronounced negative eu anomalies eu eu 0 02 to 0 07 whole rock 18o is consistent at 8 5 0 3 osmow a rb sr isochron age of isotopically slightly disturbed samples is 104 8 1 8 ma with initial87sr 86sr 0 7084 0 0011 sr is depleted 2 20 ppm but ba is 200 750 ppm

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

vanadium nickel and sulfur in crude oils and source rocks and their relationship with biomarkers implications for the origin of crude oils in venezuelan basins abstract this work presents a study of vanadium nickel and sulfur concentrations and biomarkers in a suite of crude oils and source rocks from three venezuelan basins maracaibo and eastern basins and barinas sub basin crude oils range from unaltered to altered by biodegradation and source rocks are characterized by having different kerogen types type ii iii or iv and maturity levels early mature to post mature vanadium nickel and sulfur concentrations v ni or v v ni and saturate biomarkers were used to classify the oils and source rocks based on inferred paleo redox environment lithology and maturity of the source rock oils are classified into five groups based on v ni ratios they appear to be related to variations in the paleo redox environment from suboxic dysoxic to anoxic suboxic of source rocks with variable lithological composition and organic matter input regardless of their maturity or biodegradation level these five groups are also classified based on biomarkers related to maturity organic matter type paleo redox environment and source rock lithology in source rocks vanadium nickel and sulfur concentrations together with v ni or v v ni ratios provide information about the paleo redox environment and are related to lithology regardless of their maturity these results also indicate variable paleo redox conditions during the sedimentation of venezuelan source rocks all this suggests that while the main source rocks in venezuelan basins are well established there are still uncertainties regarding their lateral and vertical variations in organic and inorganic facies and paleo redox conditions the presence of other source rocks contributing to the accumulation of hydrocarbons in the venezuela basins is also a possibility

the controls on the composition of biodegraded oils in the deep subsurface part ii geological controls on subsurface biodegradation fluxes and constraints on reservoir fluid property prediction the principal controls on the fluid properties of biodegraded oil systems have been determined by a combination of petroleum geochemistry numerical modeling of oil biodegradation in reservoirs and analysis of oil property data sets from a variety of geological settings petroleum biodegradation proceeds under anaerobic conditions in any reservoir that has a water leg and has not been heated to temperatures more than 80c in most reservoirs with low concentrations of aqueous sulfate methanogenic degradation is a primary mechanism of petroleum degradation whereas in waters containing abundant sulfate sulfate reduction and sulfide production may dominate net degradation of petroleum fractions in reservoirs is primarily controlled by the reservoir temperature the chemical compounds being degraded and relationships between the oil water contact owc area and oil volume the relative volumes of water leg to oil leg prior level of oil biodegradation and reservoir water salinity act as second order controls on the process typically degradation fluxes kilograms of petroleum destroyed per square meter of oil water contact area per year or kg petroleum m2 owc yr1 for fresh petroleum in clastic reservoirs are in the range of 103 104 kg petroleum m2 owc yr1 and increase with decreasing reservoir temperature from zero near 80c to a maximum flux at the owc of less than 103 kg petroleum m2 owc yr1 at a temperature less than 40c at very low reservoir temperatures and with severely degraded oils such as are seen in the near surface canadian tar sands at the present day the net degradation fluxes are much less than maximum values nutrient supply from the aquifer and adjacent shales mostly buffered by mineral dissolution probably provides the ultimate control on the range of degradation flux values oil compositional gradients and resulting oil viscosity variations are common on both reservoir thickness and field scales in biodegraded oil reservoirs and are a defining characteristic of heavy oil fields produced by crude oil biodegradation continuous vertical gradients in the oil columns document episodic degradation for many millions of years suggesting that the time scales of oil field degradation and petroleum charging are similar the flux temperature relationship we have derived coupled with typical reservoir charge histories defines the range of variation of fluid properties seen in many biodegraded oil provinces and identifies oil charge mixing of biodegraded and fresh oils and reservoir temperature history as the primary controls on fluid properties these flux temperature relationships are easily integrated into prospect charge modeling procedures sensitivity analyses show that the limiting factor in fluid property predictions using even this first level approach are ultimately constrained by the accuracy of current oil charge modeling estimates the absence today of any functional geochemical proxies for assessing oil residence time in oil fields and the substantial uncertainty in petroleum charging times estimated by forward basin modeling is a major obstacle to more accurate fluid property predictions that needs to be addressed