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

modelling phase assemblage diagrams for magnesian metapelites in the system k2o feo mgo al2o3 sio2 h2o geodynamic consequences for the monte rosa nappe western alps magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain p t conditions in terranes where metamorphism is not easily visible phase assemblage diagrams for natural and model magnesian metapelites in the system kfmash are presented to document how phase relationships respond to water activity bulk composition pressure and temperature the phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites it is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a h2o specifically the appearance of the characteristic hp assemblage chloritoid talc phengite quartz with excess h2o in the magnesian metapelites of the monte rosa nappe western alps is due to the reduction of a h2o furthermore the mineral assemblages are determined by the whole rock feo feo mgo ratio and effective al content x a as well as p and t the predicted mineral associations for the low and high x a model bulk compositions of magnesian metapelites at high pressure are not dependent on the x a variations as they show a similar sequence of mineral assemblages above 20 kbar the prograde sequence of assemblages associated with phengite with excess sio2 and h2o for low and high x a bulk compositions of magnesian metapelites is carpholite chlorite chlorite chloritoid chloritoid talc chloritoid talc kyanite talc garnet kyanite garnet kyanite biotite at low to medium p t conditions a low x a stabilises the phengite bearing assemblages associated with chlorite chlorite k feldspar and chlorite biotite while a high x a results in the chlorite phengite bearing assemblages associated with pyrophyllite andalusite kyanite and carpholite a high x a magnesian metapelite with nearly iron free content stabilises the talc kyanite phengite assemblage at moderate to high p t conditions taking into account the effective bulk composition and a h2o involved in the metamorphic history the phase assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system kfmash and therefore may contribute to gaining insights into the metamorphic evolution of terranes as an example the magnesian metapelites of the monte rosa nappe have been investigated and an exhumation path with p t conditions for the western roof of the monte rosa nappe has been derived for the first time the exhumation shows first a near isothermal decompression from the alpine eclogite peak conditions around 24 kbar and 505 c down to approximately 8 kbar and 475 c followed by a second decompression with concomitant cooling

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

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

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

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

geothermobarometry in albite garnet orthogneisses a case study from the gran paradiso nappe western alps abstract the aim of this paper is to estimate syntectonic p t conditions within albite and garnet bearing orthogneisses these rocks are generally characterized by the assemblage quartz albite biotite phengite cafe garnet epidote titanite garnet contains up to 55 mole per cent of grossular k feldspar is a relict magmatic phase p t conditions are estimated using several independent methods first it is shown that exchange reactions based on the fe mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks due to the high grossular content of garnet second maximum and minimum pressures are constrained respectively by the occurrence of albite instead of jadeite quartz and by the assemblage phengite biotite quartz third phase equilibria in albite and garnet bearing metagranites are modelled in the system k 2 o cao feo al 2 o 3 sio 2 h 2 o equilibrium curves are calculated for the observed phase compositions uncertainties in p t estimates mainly result from the choice of appropriate non ideal solution models for the garnet an application is developed for granites from the gran paradiso nappe western alps these granites show an heterogeneous deformation of alpine age expressed by mylonitic shear zones cutting across weakly deformed domains estimated p t conditions for the synkinematic assemblages are 10 16 kbar at 550 50 c

constraints on the late thermotectonic evolution of the western alps evidence for episodic rapid uplift forty two new apatite and zircon fission track ages are presented for samples from the western alps in southern switzerland northern italy and southeastern france measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling uplift and exhumation of the western alps similar fission track zircon ages in the penninic gran paradiso massif dent blanche nappe sesia lanzo zone and ivrea zone indicate cooling of all four units to 225 c by 33 ma differences in apatite ages reveal differential cooling of the four blocks between 33 ma and the present in the sesia lanzo zone similarity of apatite ages regardless of elevation together with near volcanic confined fission track length patterns suggest rapid cooling and uplift at 25 ma compared with slow cooling of other western alps units around 12 ma uplift is thus not continuous but episodic often over a short time interval beyond the resolution of other methods such episodes of uplift as revealed here in the sesia lanzo zone may be the rule rather than the exception

nucleation and growth of myrmekite during ductile shear deformation in metagranites myrmekite is extensively developed along strain gradients of continuous lower amphibolite facies shear zones in metagranites of the gran paradiso unit western alps to evaluate the role of stress strain energy and fluid phase in the formation of myrmekite we studied a sample suite consisting of weakly deformed porphyric granites wdgs foliated granites fgs representative of intermediate strains and mylonitic granites mgs in the protolith most k feldspar is microcline with different sets of perthite lamellae and fractures in the wdgs abundant quartz oligoclase myrmekite developed inside k feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction in the wdgs stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between k feldspar and albite in the fgs and mgs k feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine grained recrystallized myrmekite and perthite aggregates this was accompanied by a more pervasive fluid influx into the reaction surfaces and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone in the mgs myrmekite forms complete rims along the outer boundary of the small k feldspar porphyroclasts which are almost completely free of internal reaction interfaces therefore we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress mass balance calculations indicate that in al si conservative conditions myrmekite growth was associated with a volume loss of 8 5 this resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite

re os geochronology for the nist rm 8505 crude oil the importance of analytical protocol and uncertainty abstract the national institute of science and technology nist research material rm 8505 is a venezuelan crude oil certified for its vanadium concentration it has recently been proposed as a matrix matched standard for interlaboratory comparison of re os isotopic analyses while serving as a benchmark for petroleum based re os method development here the nist rm 8505 oil is characterized using a multi solvent asphaltene precipitation permitting interlaboratory re os data comparison and evaluation of techniques used to derive re os isochron ages we present a detailed discussion of analytical blank sources critical evaluation of analytical uncertainties method improvements and data corrections additionally we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages in addition to nist rm 8505 2 43 0 15 ng g re 30 0 0 7 pg g os we provide the first re os data for nist srm 1634c 1 73 0 01 ng g re 32 5 0 8 pg g os and enviromat scp science 39 1 pg g re 1 01 0 02 pg g os two other commercially available hydrocarbon trace metal standards the application of several asphaltene asph precipitations using different n alkane solvents pentane heptane and decane for the nist rm 8505 significantly improved the spread of data in the 187re 188os 187os 188os space multi solvent asphaltene precipitations yield an asphaltene only model 1 isochron 75 24 ma and the maltene crude oil asphaltene triplets yield a more precise model 1 isochron age of 78 5 2 3 ma with a 187os 188osinitial of 0 877 0 021 our age is considerably older compared to a previously reported model 1 age of 62 7 5 7 ma based on c7 asph crude oil malt triplets liu and selby 2017 and considerably younger than the model 1 age of 98 4 9 5 ma based on asph separates liu et al 2019 the analytical reasons leading to these age differences are discussed in this paper the geological context for the nist 8505 oil is unknown therefore we use trace metal data ni v fe mo etc to derive plausible constraints for the origin of this oil the nist 8505 oil shows trace metal data similar to published data for oils from the urdaneta district the la paz mara and lagunillas oil fields and source rocks of the la luna formation in the maracaibo basin venezuela in this geological context our 79 ma age for the nist 8505 oil implies a late cretaceous early hydrocarbon generation event from older upper cretaceous source rocks and overlaps with the youngest age of potential source rocks in the maracaibo basin however the age of large scale oil generation in the maracaibo basin is significantly younger lasting from the oligocene to the pliocene

retrograde formation of nacl scapolite in high pressure metaevaporites from the cordilleras beticas spain a permo triassic pelite carbonate rock series with interacalated metabasitic rocks in the cordilleras beticas spain was metamorphosed during the alpine metamorphism at high pressures pmin near 18 kbar the rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc of kyanite phengitetalc biotite and of quartz after sulfate minerals and relicts of baryte anhydrite nacl and kcl indicating a salt clay mixture of illite chlorite talc and halite as the original rock the evaporitic metapelites have a whole rock composition characterized by high mg mg ca ratios 0 7 variable alkaline and sr ba contents but are mostly k2o rich 8 8 wt the f 2600 ppm cl 3600 ppm and p2o5 0 24 wt contents are also high the pelitic member of this series is a fine grained biotite rock kyanite phengite talc biotite aggregates in pseudomorphs developed in the high pressure stage albite rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift scapolite rich in nacl ca ca na mol 24 40 and poor in so4 with cl cl co3 ratios between 0 6 and 0 8 formed as porphyroblasts sometimes replacing up to 60 of the rock in a late stage of metamorphism between 10 and 5 kbar near 600 c no reaction with albite is observed and the scapolite formed from biotite by begin gathered al biotite caco 3 nacl sio 2 hfill al poor biotite scapolite mgco 3 kcl hfill mgcl 2 h 2 o hfill end gathered

excess 40ar free phengite in ultrahigh pressure metamorphic rocks from the lago di cignana area western alps abstract ar ar analyses of phengites and paragonites from the ultrahigh pressure metamorphic rocks zoisite clinozoisite schist garnet phengite schist and piemontite schist in the lago di cignana area western alps were carried out with a laser probe step heating method using single crystals and a spot dating method on thin sections eight phengite and two paragonite crystals give the plateau ages of 37 42 ma with 96 100 of 39 ar released each rock type also contains mica crystals showing discordant age spectra with age fractions 20 35 ma significantly younger than the plateau ages phengite inclusions in garnet give ages of 43 2 1 1 ma and 44 4 1 5 ma which are significantly older than the spot age 36 4 1 4 ma from the matrix phengites and the plateau ages from the step heating analyses inclusion ages 43 and 44 ma are consistent with a zircon shrimp age 44 1 ma in this area these results suggest that the oceanic materials that underwent a simple subduction related uhpm form excess 40 ar free phengite and that the peak metamorphism is ca 44 ma or little older we suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40 ar from the crystals this has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age

alpine metamorphism of hercynian hornblende granodiorite beneath the blueschist faciesschistes lustresnappe of ne corsica the hercynian granitic basement which forms the tenda massif in ne corsica represents part of the leading edge of the european plate during middle to late cretaceous eoalpine high p metamorphism the metamorphism of this basement induced by the overthrusting of a blueschist facies schistes lustres nappe was confined to a major ductile shear zone c 1000m thick within which deformation increases upwards towards the overlying nappe metamorphism within the basement mostly records lower blueschist facies conditions crossite epidote except near the base of the shear zone where the greenschist facies assemblage albite actinolitic amphibole has developed instead of crossite study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation zone 1 biotite to chlorite towards zone 2 biotite to phengite zone 2 hornblende to actinolitic ca amphibole albite sphene and biotite to actinolitic ca amphibole albite phengite ti ore epidote zone 3 hornblende to crossite low ti biotite phengite sphene and biotite to crossite low ti biotite phengite ti ore sphene epidote p t conditions at the base of the shear zone are estimated to have been 390 490 c at 600 900 m pa 6 9kbar and the corsican basement is therefore deduced to have been buried to 20 30 km during metamorphism this relatively shallow metamorphism contrasts with some other areas in the western alps where the eoalpine event apparently buried the european continental crust to depths of 80 km or more as there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the european continent it is deduced that the eoalpine blueschists were produced during the collision of the insubric plate with europe rather than during tethyan intraoceanic subduction coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction

organic petrological and geochemical appraisal of the upper jurassic naokelekan formation kurdistan iraq abstract upper jurassic naokelekan formation being part of the intercepted strata of the well known miran oilfield in kurdistan region of northern iraq has been previously extensively evaluated as a potential effective source rock mainly by means of organic geochemistry in the present study a total of 60 samples were collected from four naokelekan fm outcrops those of banik galy derash barsarin and dwawa in order to conduct organic petrographical study and organic geochemical assessment including both biomarker and rock eval analyses the average recorded toc content for banik galy derash barsarin and dwawa section is considerable to very high corresponding to values of 27 5 9 0 3 5 and 5 2 wt respectively with the highest values representing the notably organic rich bottom part of the formation known as coal layer according to the petrographical analysis solid migrabitumens comprise the predominant component for all the analysed samples whereas the maceral groups of vitrinite liptinite and or inertinite showcase a minor to trace occurrence in terms of organic geochemistry each of the four sections of interest shows a different biomarker profile nevertheless a number of calculated parameters related to kerogen type and depositional conditions including the isoprenoids n alkanes ratios suggest a mixed terrestrial marine organic matter furthermore as indicated by the ro measurements along with rock eval parameters and the vitrinite reflectance equivalent based on aromatic compounds the maturity level is marginally mature in the cases of banik and galy derash whereas in the cases of dwawa and barsarin sections it is mature to post mature a synthesis of the organic petrography and geochemical appraisal applied in the present study suggests that the reported solid hydrocarbons in naokelekan fm particularly at its basal layers are non indigenous migrated and the precursor organic matter was deposited in a marine environment under oscillating conditions ranging from euxinic to well oxygenated accordingly the organic features of the notably bitumen rich coal bed fit the profile of a reservoir and or migration path

metamorphic petrology of a high pressure low temperature subduction complex in west central baja california mexico the western baja terrane wbt of west central baja california is an uplifted subduction complex that is divided into smaller subterranes on the basis of bounding faults and petrological differences each subterrane contains coherent early jurassic to early cretaceous sedimentary and mafic volcanic rocks not melange that have been metamorphosed under blueschist facies conditions key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene sodic amphibole lawsonite aragonite chlorite titanite and white mica pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite temperature indicators include the presence of lawsonite the absence of greenschist facies minerals and results from vitrinite reflectance studies conditions at the peak of metamorphism were 8 kbar 225 325 c for subterrane 1 7 8 kbar 170 220 c for subterrane 2 and 5 6 kbar 175 200 c for subterrane 3 these correspond to cold geothermal gradients 6 9 km vein assemblages that include aegerine jadeite and aegerine albite aragonite lawsonite and sodic amphibole indicate uplift during continued cold conditions probably during steady state subduction

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

the upper thermal stability of chlorite quartz an experimental study in the system mgo al2o3 sio2 h2o experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite quartz talc kyanite h2o chlorite quartz talc cordierite h2o and talc kyanite quartz cordierite h2o by reversed runs in the system mgo al2o3 sio2 h2o mash these reaction curves intersect at an invariant point ip1 at ph2o 6 4 0 2 kbar and a temperature of 624 4 c the curve of the chlorite quartz breakdown to talc kyanite h2o at water pressures above 6 4 kbar shows a negative dp dt with the slope decreasing with rising pressure whereas the slope of the breakdown curve to talc cordierite h2o at water pressures is clearly positive the composition of the chlorite solid solution reacting with quartz has been estimated to be approximately mg4 85al1 15 al1 15si2 85o10 oh 8 over the entire pressure range investigated the composition of the talc solid solution forming by the breakdown of chlorite quartz appears to be mg2 94al0 06 al0 06si3 94o10 oh 2 at ph2o 2kbar with increasing pressure the al content of talc decreases reaching a value of about 0 06 atoms per formula unit at p h2o 21 kbar as a consequence of the new experimental data the existing phase topologies of the mash system and k2o mash system have been revised for example the invariant point ip1 and the univariant reaction curve kyanite talc h2o chlorite cordierite are stable for this reason the development of medium to high temperature metamorphic rocks compositionally approximating the mash system must be reconsidered the whiteschists from sar e sang afghanistan are treated as an example the application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components this leads to the conclusion that the introduction of fe2 into magnesian chlorite extends its stability field in the presence of quartz by 10 15 c in comparison with pure mg chlorite

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

high frequency analysis of seismic background noise as a function of wind speed and shallow depth we used a deep 1500 m cased borehole near the town of datil in west central new mexico to study high frequency 1 hz seismic noise characteristics the remote site had very low levels of cultural noise but strong winds winter and spring made the site an excellent candidate to study the effects of wind noise on seismograms along with a three component set of surface sensors teledyne geotech gs 13 a vertical borehole seismometer gs 28 was deployed at a variety of depths 5 43 and 85 m to investigate signal and noise variations wind speed was measured with an anemometer event triggered and time triggered data streams were recorded on a reftek 72 02 data acquisition system located at the site our data show little cultural noise and a strong correlation between wind speed and seismic background noise the minimum wind speed at which the seismic background noise appears to be influenced varies with depth 3 m sec at the surface 3 5 m sec at 43 m in depth and 4 m sec at 85 m in depth for wind speed below 3 to 4 m sec we observe omni directional background noise that is coherent at frequencies below 15 hz this coherence is destroyed when wind speeds exceed 3 to 4 m sec we use a test event md 1 6 and superimposed noise to investigate signal to noise ratio snr improvement with sensor depth for the low q valley fill of the datil borehole dbh site we have found that snr can be improved by as much as 20 to 40 db between 23 and 55 hz and 10 to 20 db between 10 and 20 hz by deploying at a 43 m depth rather than at the surface at the surface there is little signal above noise in the 23 to 55 hz frequency band for wind speeds greater than 8 m sec thus high frequency signal information that is lost at the surface can be recorded by deploying at the relatively shallow depth of 40 m because we observe only minor further reductions in seismic background noise sbn at deeper depths 40 m is likely to be a reasonable deployment depth for other high frequency monitoring sites in similar environmental and geologic conditions

wind induced ground motion wind noise is a problem in seismic surveys and can mask the seismic signals at low frequency this research investigates ground motions caused by wind pressure and shear stress perturbations on the ground surface a prediction of the ground displacement spectra using the measured ground properties and predicted pressure and shear stress at the ground surface is developed field measurements are conducted at a site having a flat terrain and low ambient seismic noise triaxial geophones are deployed at different depths to study the wind induced ground vibrations as a function of depth and wind velocity comparison of the predicted to the measured wind induced ground displacement spectra shows good agreement for the vertical component but significant underprediction for the horizontal components to validate the theoretical model a test experiment is designed to exert controlled normal pressure and shear stress on the ground using a vertical and a horizontal mass spring apparatus this experiment verifies the linear elastic rheology and the quasi static displacements assumptions of the model the results indicate that the existing surface shear stress models significantly underestimate the wind shear stress at the ground surface and the amplitude of the fluctuation shear stress must be of the same order of magnitude as the normal pressure measurement results show that mounting the geophones flush with the ground provides a significant reduction in wind noise on all three components of the geophone further reduction in wind noise with depth of burial is small for depths up to 40 cm

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

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

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

high pressure low temperature metamorphism in the gran paradiso basement western alps the gran paradiso basement complex of the french and italian alps is composed of metasediments termed the gneiss minuti and metabasic rocks both of which are intruded by a late hercynian granite the bonneval gneiss which crops out at the western edge of the complex is composed of highly deformed metasediments volcanics and volcaniclastic rocks eclogites now highly altered occur in the metabasic rocks kyanite and blue green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the bonneval gneiss a moderately high pressure low temperature metamorphic event of probable alpine age occurred in the basement complex this metamorphic event differs from that in the overlying sesia unit and ophiolites of the schistes lustres nappe in being at lower pressures below the ab jd100 qz transition and post dating the major d2a deformation the origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the d2a event subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift

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

a preliminary investigation into the relationship between long period seismic noise and local fluctuations in the atmospheric pressure field summary the displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long period seismic noise field the study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space the numerical studies indicate that pressure waves with amplitudes of 100 pbar or more can contribute significantly to the long period vertical background noise observed at the surface provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre these same waves can also create significant tilt noise on long period horizontal seismographs located at or near the surface regardless of the rock type the seismic disturbances created by pressure waves decay rapidly away from the surface therefore it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths

a boom in boomless seismology taking advantage of cheap long lasting sensors developed by the oil and gas industry seismologists are increasingly exploring the top 10 kilometers of the crust with dense surveys of hundreds or even thousands of seismometers what s more the surveys are being operated passively which means that ambient noise from earth is used to illuminate the subsurface passive surveys are cheaper and easier to permit than active surveys which rely on explosives or vibrations from special thumper trucks seismologists predict that the dense surveys will yield precision imaging of magma chambers underneath volcanos and earthquake rupture zones along faults

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

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

miocene to present differential exhumation in the western alps insights from fission track thermochronology 1 analysis of cooling age patterns yielded by low temperature thermochronometers provides key information about the role played by tectonic discontinuities during the late stages of exhumation of metamorphic belts in the western alps fission track data published so far are heterogeneously scattered and concentrated in few structural domains preventing analyses at the scale of the whole belt the new apatite fission track data reported in this work obtained with the external detector method as well as the population method in very low u content samples fill this gap they constrain the postmetamorphic evolution of the western alps along two transects from the foreland to the retroforeland unraveling the effective role played by some major faults during the exhumation of the belt at shallow crustal levels a clear regional pattern characterized by decreasing ages moving from the axial sector to the european external sector of the belt and by an along strike gradient with increasing ages from north to south has been unraveled evident breaks in this age pattern have been detected in correspondence of faults that are near parallel to the trend of the belt pointing to the occurrence of active tectonics during and after exhumation the most apparent breaks have been observed in the axial sector of the belt where the postmetamorphic deformation would have been negligible according to classic tectonic models faults located in the axial sector split the belt into two major blocks eastern and western since the miocene the western block experienced higher exhumation rates than the eastern one such differential exhumation was accommodated in the northern portion of the belt thanks to reverse motion along the internal houiller fault which occurred in a convergent transcurrent framework to the south it was accommodated instead by normal reactivation of the brianconnais front and by activity of the longitudinal fault system which occurred in a divergent transcurrent framework the tectonic activity affecting the axial sector of the belt in a context of regional dextral strike slip is coeval with the forward propagation of the external thrusts and of similar magnitude we suggest that the contrasting kinematic regimes i e convergent versus divergent transcurrence observed in the western alps moving along strike were responsible of the increasing exhumation rates toward the north revealed in both blocks by the along strike age gradient the higher exhumation rates recognized northward would be related to an increasing importance of crustal shortening that promoted erosion during the late stages of exhumation of the belt

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

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

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

tectonic interpretation of an inverse gradient of zircon fission track ages with respect to altitude alpine thermal history of the gran paradiso basement extensive sampling along vertical and north south cross sections in the gran paradiso basement internal crystalline massif western alps reveals an inverse gradient of zircon fission track ages with respect to altitude which is explained by an inverted metamorphism during eocene time subsequent to the obduction of the schistes lustres nappe high pressure eoalpine phase the gran paradiso massif cooled down 90 80 myrs ago the austro alpine nappe of monte emilius dent blanche overthrust the gran paradiso massif 38 40 myrs ago inducing in its basement an inverted metamorphism or more precisely a nappe emplacement metamorphism greenschist facies the last cooling below 100 c occurred as early as 35 40 myrs ago in the southern gran paradiso massif in the northern part this last cooling occurred 20 myrs ago accompanied by an important uplift phase 2 mm year

water level and redox fluctuations in a sichuan basin lacustrine system coincident with the toarcian oae abstract increasing cases have indicated that oceanic anoxic events oaes appear to have records in terrestrial sediments which provide valuably high resolution materials for the debated and challenging issues of oaes however the current works are preliminary and need more geological analyses to test here we present a case study in the lower jurassic da anzhai member of the toarcian ca 180 ma sichuan basin sw china the petrology and sedimentary facies of the da anzhai member were used to reconstruct the relative water level fluctuations in the sichuan basin which are revealed to be consistent with coeval changes in global sea levels the da anzhai member was deposited during a period of lake shallowing with enhanced salinities corroborated by the sr ba and s toc ratios when sea level was generally high after global transgression during the lake shallowing there were four complete secondary lacustrine deepening shallowing cycles over shorter time scales the total organic carbon contents and organic carbon isotopic stratigraphy show there was a basin wide terrestrial response to the t oae as recorded by the da anzhai member the enrichment factors of mo relative to paas moef and corg p values imply a dysoxic redox condition the terrestrial response of oaes is likely caused by changes in hydrological conditions which were related to the global warming triggered by the karoo ferrar large igneous province the associated organic carbon accumulation mechanism is a combined effect of elevated primary productivity and oxygen depleted preservation conditions our data suggest that oaes do have terrestrial response and induce organic carbon accumulation but the mechanisms are still not much clear and need further studies

tectono metamorphic evolution of the siliciclastic units in the middle tuscan range inner northern apennines mg carpholite bearing quartz veins related to syn metamorphic syn orogenic foliation abstract this paper accounts for a new finding of high pressure low temperature hp lt metamorphism documented in continental derived rocks exposed in the central part of the middle tuscan range representing the easternmost location in the tyrrhenian region where blueschist facies metamorphism has been documented it describes the development mechanism of mg carpholite bearing quartz veins in the framework of the deformational events described for the inner northern apennines the hp lt peak metamorphism can be estimated in p 1 1 gpa and t 370 420 c as supported by the mineralogical assemblage consisting of quartz muscovite chloritoid pyrophylite hematite chlorite paragonite ti oxides documented in the aluminous metasediments pelites and quartzose sandstones of the triassic verrucano succession this paragenesis defines the s 1 schistosity an axial planar tectonic foliation associated to map and mesoscale isoclinals folds f 1 s 1 schistosity is a composite foliation resulted from a non coaxial progressive deformation associated to the de 1 collisional event late oligocene early miocene mg carpholite bearing quartz veins developed as tension gashes in en echelon arrays within brittle ductile shear zones the development of quartz veins was guaranteed by the high volumes of si rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks

watching the wind seismic data contamination at long periods due to atmospheric pressure field induced tilting abstract following the installation of a new broadband seismic station near the pacific coast of the olympic peninsula washington the pacific northwest seismic network recorded elevated background seismic noise at periods exceeding 30 s we observed power spectral density variations of about 15 20 db in the horizontal component seismograms between times of high and low noise and differences of up to about 25 db between the horizontal and vertical components during noisy intervals the amplitude of the signal at long periods exhibited clear correlations with cyclic daytime fluctuations of air temperature and wind speed our observations provided measurable evidence that the variations of the local atmospheric pressure field associated with local wind systems represents an effective source of ground displacements and rotations the rotational component of the pressure field induced motion is recorded by horizontal component seismometers that below their low frequency corner are sensitive to surface tilts the response of seismometers to ground rotations is characterized by a transfer function which we use to estimate the amplitude of tilt we measured a pressure field induced surface tilt of the order of several tenths of rad based on simple modeling tilt contamination is expected to attenuate rapidly with depth and to become negligible just a few hundreds of meters below the surface removal of this unwanted effect from seismograms remains a challenging task as the details of tilting depend on the nonstationary character of the pressure field the compliance of the near surface to pressure variations and the design and construction of the seismic station online material animated time sequenced plots of daily seismic noise pdfs

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