compositional fractionation of petroleum from reservoir to wellhead in the niobrara shale oil play abstract hydraulic fracturing is routinely used to stimulate unconventional reservoirs in the niobrara shale oil play there are several fracking targets but the actual stimulated rock volume is not well constrained this means that allocating produced oil to specific reservoirs in essence oil reservoir correlation is not straightforward the data presented here support that the chalk units are the principal production zones in the niobrara shale oil play several lines of evidence such as the presence of heavy end n alkanes n c31 consistence in composition of gross fractions the distribution of c27 28 29 diasteranes as well as the pristane to phytane ratio all suggest that oils were produced from the a b chalks particularly from the a chalk during production a selective partitioning of lighter hydrocarbons into the migrating fluids could nevertheless occur in particular as regards the n alkanes domain and their carbon isotopic compositions regarding the latter a depletion of 13c in the produced oil s n alkanes by more than three per mil relative to all of those core extracts was observed this finding suggests that compound specific isotope analysis in both oil reservoir and oil source rock correlations cannot be taken at face value i e by simple matching of values besides that fractionation appears to result in the enrichment of rather aliphatic nitrogen containing compounds in crude oils

de convoluting crude oil mixtures from palaeozoic reservoirs in the tabei uplift tarim basin china abstract geochemical characteristics of 61 crude oil samples collected from palaeozoic reservoirs in the tarim basin 60 from the tabei uplift and one from the tazhong uplift were analyzed the samples proved to be mixed oils of different maturity from diverse source rocks concentrations of 40 biomarkers and carbon isotopic compositions for the whole oils were analyzed by alternating least squares als regression to de convolute the mixtures three endmember em oils were identified em1 is the minimum contributor to the mixed oils accounting for less than 10 of most oils em1 originated from cambrian lower ordovician source rocks in the early to peak oil window stage and experienced two phases of mixing and biodegradation em2 is the secondary contributor with proportions ranging from 10 to 40 in most oil samples em2 originated from middle upper ordovician source rocks at the early oil generation stage and underwent two phases of mixing and one stage biodegradation in the reservoirs em3 is the major contributor to most samples with proportions ranging from 13 to 95 em3 was generated from middle upper ordovician source rocks at the late oil generation stage and mixed with earlier emplaced mixtures in the reservoirs the final mixtures that were not biodegraded are currently exploited from palaeozoic reservoirs in the tabei uplift biomarkers in the crude oils reveal mixed characteristics including evidence for two phases of oil charge and severe biodegradation

structure of the middle america trench in oaxaca mexico abstract deep seismic profiling was carried out in south and central oaxaca as a multinational mexico spain and switzerland project sixteen sea bottom explosions ranging from 20 to 1000 kg were recorded by thirty portable instruments along three profiles two along the coast and one going inland in an approximate n s direction gravity surveys were carried out over the seismic profile lines and the resulting bouguer anomalies are interpreted together with the seismic data preliminary results indicate changes in the crustal thickness along the coast near the town of pinotepa nacional from 23 5 km in the northwest to 19 km in the southeast reaching a minimum of some 15 km near the middle of the profile about 140 km northwest of puerto angel the coastal structure section consists roughly of two layers an upper one with p wave velocities that range from 5 1 5 4 to 5 8 6 0 km s and a lower one where the p velocity range is from 6 0 6 2 to 6 3 6 4 km s overlying material with p wave velocities of 7 45 km s along the coast from puerto angel to salina cruz the dip of the cocos plate appears to be much less than it is to the northwest a low velocity zone which corresponds to the top of the subducted oceanic crust with p wave velocities of 6 5 6 9 km s is found beneath the 2 3 km thick 7 45 km s layer the possible presence of an intrusive body is suggested by anomalous seismic arrivals and by a large gravimetric anomaly near puerto angel close to the southern tip of mexico

the guerrero suspect terrane western mexico and coeval arc terranes the greater antilles and the western cordillera of colombia a late mesozoic intra oceanic arc accreted to cratonal america during the cretaceous the guerrero suspect terrane composed of late jurassic early cretaceous sequences extends from baja california to acapulco and is considered to be coeval with the late mesozoic igneous and sedimentary arc sequences of the greater antilles the west indies venezuela and the western cordillera of colombia these sequences represent the remnants of an arc which accreted to the north american and northern south american cratons at the end of the cretaceous in western mexico the arc sequences built on continental crust consist of high k calc alkaline basalts andesites and rhyolites enriched in lree with abundant siliceous pyroclastic rocks interbedded either with aptian albian reefal limestones or red beds they do not show magmatic changes during the arc development in contrast the arc sequences built on oceanic crust show an evolution with time arc activity began with the development of depleted low k tholeiitic mafic suite guanajuato igneous sequence followed first by mature tholeiitic basalts and then by calc alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of early cretaceous age at the end of the arc growth during aptian albian times calc alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc the tholeiitic and shoshonitic mafic rocks as well as the calc alkaline lavas are mildly enriched in lree y and nb and show high nd ratios typical of oceanic arcs in contrast the calc alkaline mafic suite enriched in lree y and nb exhibits lower nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by paleozoic subducted sediments or old source enrichments oib the cretaceous arc rocks of the greater antilles interbedded with and or capped by aptian albian limestones the cretaceous andesites of northern colombia the cretaceous tholeiitic and calc alkaline volcanic rocks of venezuela and the cretaceous volcano plutonic arc assemblage of tobago share a similar magmatic evolution with the western mexican oceanic arc the tholeiitic plutono volcanic assemblage of tobago depleted in lree and characterized by high nd values is similar to the guanajuato volcano plutonic sequence of mexico considered to represent the pristine stage of the arc the mature tholeiitic sequences exposed in the proto caribbean arc show flat to moderately enriched lree patterns like those of the guerrero terrane however felsic plutonic and volcanic rocks prevail in the caribbean calc alkaline suites accompanied locally by shoshonitic lavas characterize the end of arc magmatic activity in both places thus the geochemical features of the late jurassic cretaceous arc series of the guerrero terrane and the proto caribbean are consistent with the following plate tectonic model the guerrero terrane and the proto caribbean probably belonged to the same intra paleo pacific arc system the development of which was related to the subduction of oceanic basins fringing the north and northern south american cratons this subduction zone was wsw dipping while subduction was going on these magmatic arcs drifted moved closer to the north and south american cratons and finally collided with the american borderlands at different periods during the cretaceous the late mesozoic guerrero and proto caribbean arc sequences show striking similarities with the miocene calc alkaline lavas dredged from the banda ridges the north marianas seamount province and the halmahera and philippine arcs we suggest that the diverse but mostly submarine segments of this late mesozoic intra pacific arc rimmed the north and south american cratons as much as these tertiary arcs rim southeast asia

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

chemometric identification of crude oil families and deconvolution of mixtures in the surrounding niuzhuang sag bohai bay basin china a total of 46 crude oil samples recovered from the surrounding niuzhuang sag were investigated to gain insight into the petroleum systems in the dongying depression chemometrics identified four genetically distinct oil families family 1 fails to match any of the source rocks that have been discovered thus far the candidate source rock must contain algal organic matter that could produce more negative carbon isotopic composition large quantities of wax c29 steranes and tricyclic terpenoids compared to the eocene shahejie es formation intervals es3 and es4 and deposited in a reducing stratified saline water environment family 2 is featured by high ratios of sterane hopane gammacerane hopane and c35 c34 homohopane and low pristane phytane ratios the source rock is the upper es4 interval family 3 is characterized by distinctive or even opposite features to family 2 the source rock is the lower es3 interval family 4 is a mixed oil derived from the lower es3 and upper es4 source rocks at different thermal maturation three end member oil samples em1 em2 and em3 were identified by alternating least squares as a minor contributor em1 has an average value of 10 as the main contributors em2 and em3 average 53 and 37 respectively oil samples from em1 and em2 are derived from the upper es4 source rock but em1 was generated at a lower thermal maturity than em2 oil from em3 was derived from the lower es3 source rock at peak hydrocarbon generation

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

origin charging and mixing of crude oils in the tahe oilfield tarim basin china abstract forty eight crude oil samples collected from different reservoirs in the tahe oilfield of the tarim basin were investigated based on geochemical characteristics it is concluded that the oil samples originated from multiple marine source rocks deposited in various sedimentary environments and subsequently altered by different levels of thermal maturation these mixtures were de convoluted to three endmember oils em1 em2 and em3 by alternating least squares regression using 38 concentration parameters em1 is the minimum contributor with an average of 13 while em2 and em3 are the main contributors to the mixtures with averages of 52 and 35 respectively em1 oil originated from cambrian lower ordovician source rocks in the early to peak oil window and subsequently experienced two phases of mixing and biodegradation em2 and em3 oils originated from middle upper ordovician source rocks but em2 was generated at lower thermal maturity than em3 the em2 oil underwent two phases of mixing and one stage biodegradation while the em3 oil mixed with previous existing mixtures in the reservoirs the final mixtures that might be affected by secondary processes such as evaporative fractionation are currently produced from the tahe oilfield the general orientation of oil filling was from south to north and east to west based on variations in the relative contributions of ems and the total concentrations of dibenzothiophenes and dibenzofurans in the oils considering the histories of sedimentary tectonic evolution hydrocarbon generation and expulsion and the de convolution results a model of three stages of oil charge and two phases of mixing and biodegradation was established for the tahe oilfield oils

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

early human occupation of iberia the chronological and palaeoclimatic inferences from vallparadis barcelona spain abstract vallparadis is one of the best calibrated and most accurately dated archaeological sites from the european early pleistocene chronological analyses combined with palaeomagnetism esr u series and osl and the biochronology of macro and micromammals are fully consistent and situate the site just above the upper limit of the jaramillo subchron in this article we compare the mandibular first molar m1 of individual adult specimens of mimomys savini recovered from level 10 evt7 at vallparadis with specimens from the stratigraphic sequence at gran dolina atapuerca fuente nueva 3 and barranco leon d orce this comparison allows us to chronostratigraphically relate level 10 at vallparadis with level td5 at gran dolina and to fix the former s chronology to around 0 98 0 95 ma mis 27 and therefore prior to level td6 in which fossil remains of homo antecessor were recovered the chronology of vallparadis and the set of contemporary palaeoclimatic proxies regarding the iberian peninsula strengthen the hypothesis that hominins continuously populated europe at least in iberia throughout the late early pleistocene between the jaramillo subchron and the matuyama brunhes boundary by overcoming the climatic fluctuations and changes to the landscape that occurred during this period

second look at suspect terranes in southern mexico the boundary between the xolapa and the guerrero mixteca and juarez or oaxaca terranes is a zone of normal faulting indicating north south subhorizontal extension stratigraphic and geochronometric evidence dates tectonic uplift of the xolapa terrane as late cretaceous and tertiary we propose that the xolapa terrane represents a late mesozoic early tertiary magmatic arc built near or on north american continental crust and we discuss as possible tectonic uplift mechanisms 1 extension associated with back arc rifting 2 extension during gravitational spreading of the upper and middle crust and 3 transtension within a strike slip regime established during formation of the caribbean both far and near field deformations indicate distributed transtension therefore a single regional tectonic framework can account for the mesozoic and cenozoic geologic history of these terranes

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

multiple sourced features of marine oils in the tarim basin nw china geochemical evidence from occluded hydrocarbons inside asphaltenes abstract the tarim basin nw china is a large composite basin with multiple sets of petroleum source rocks the basin has undergone numerous episodes of hydrocarbon generation migration and accumulation making it difficult to assess the source rocks and oil source correlation for the widespread marine oils in this area protected by the molecular structure of asphaltenes occluded hydrocarbons can provide information about the early source rocks in this work the occluded hydrocarbons released from the asphaltenes by a mild chemical degradation method were compared with the crude oil maltenes and the adsorbed compounds from asphaltenes analysis of biomarker distribution and the carbon isotope composition of individual n alkanes suggests that the widespread marine oils in the tazhong uplift tabei lunnan uplift and halahatang depression were contributed by cambrian lower ordovician source rocks at an early stage and later mixed with hydrocarbons derived from middle upper ordovician source rocks the marine oils in the tarim basin demonstrate extensive characteristics of having been derived from multiple source rocks

depth and age in the cayman trough abstract caribbean plate boundaries are almost everywhere either strike slip or convergent so oceanic crust accreted at the spreading axis within the cayman trough provides information critical to determining the relative motions and tectonic interactions among the caribbean and surrounding plates the tectonic evolution of the trough as inferred from magnetic anomaly identifications is not entirely compatible with either its crustal subsidence or the geological evidence for time of opening and rate of offset movement along the north american caribbean plate boundary a close examination of basement topography and depth within the trough suggests that the total opening of the cayman trough is about 1100 km and that this opening occurred in two stages crust generated by the present ongoing stage of very slow spreading has extensive relief and shows a distinct increase of crustal depth with increasing distance from the spreading center crust generated by an earlier episode of faster spreading is characterized by reduced topographic relief and no appreciable differential subsidence and is separated from the younger crust by distinct topographic breaks the rate of spreading of the current slow spreading episode is15 5mm yr based upon a new crustal subsidence curve which accommodates lateral as well as vertical cooling of the accreting slab the trough has opened at this rate for about 30 ma the rate of opening prior to 30 ma cannot be determined with any reliability from crustal subsidence a reexamination of magnetic anomalies in the central part of the trough shows that these anomalies can be generated at a much slower spreading rate than previously thought this revised interpretation of cayman trough evolution brings magnetic subsidence and geological evidence into much better agreement

paleo heat flow evolution of the tabei uplift in tarim basin northwest china abstract the paleo heat flow evolution of the tabei uplift in the tarim basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling numerous geological parameters such as temperature data and missing sediment thickness by erosion were used in the modeling the basin model was calibrated using 460 measured vitrinite reflectance vr o and vitrinite like maceral reflectance vlmr o values to constrain the validity of the maturity model the heat flow history of the tabei uplift tarim basin shows the following characteristics 1 the highest paleo heat flow was predicted to have occurred in the early ordovician as 65 5 mw m 2 and gradually decreased to 55 5 mw m 2 during the late carboniferous 2 a thermal kick was modeled to have occurred in the permian as suggested by an abrupt rise in the heat flow 3 the heat flow gradually decreased since the triassic 4 the present day heat flow was predicted to be as low as 38 mw m 2 this heat flow history honors the geologic and tectonic evolution history of the tabei uplift and is suggested as the best case heat flow model

biochronology of spanish quaternary small vertebrate faunas abstract thousands of fossils of small vertebrates are recorded in one of the most complete stratigraphic sequences of the continental european pleistocene in the sierra de atapuerca this sequence with a few interruptions spans the last 1 5 million years the stratigraphic series is recorded in six karstic cavities in the sierra de atapuerca from the oldest record to the most modern one these are the sima del elefante gran dolina galeria zarpazos sima de los huesos el portalon and el mirador the faunal successions of continental microvertebrates have made it possible to divide the atapuerca sequence into 7 faunal units atapuerca faunal units ata fus which are based on the first and last local appearances of taxa lo ho the spanish quaternary small vertebrate faunas are grouped into 5 distinct biozones from the oldest to the most recent allophaiomys pliocaenicus allophaiomys lavocati allophaiomys chalinei iberomys brecciensis and iberomys cabrerae these are calibrated with radiometric and palaeomagnetic dating allowing the proposal of a continental biochronological scale for the pleistocene of the iberian peninsula

tectonic interpretation of the trans mexican volcanic belt discussion discussion sur l article de shurbet d h et cebull s e paru dans tectonophysics nld 1984 101 159 165

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

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

crustal structure of oaxaca mexico from seismic refraction measurements seismic refraction and gravity data have been analyzed to obtain a model of the compressional wave structure of the ocean to continent transition in the state of oaxaca in southwestern mexico crustal thickness on the continent at the latitude 18 n is 45 4 km based on reflected phases from the moho discontinuity the crust has been modeled with three layers with velocities of 4 3 to 4 6 5 0 to 5 7 and 6 85 to 7 0 km sec each with positive velocity gradient the crust thins to 10 km at the coast near pinotepa nacional where precambrian metamorphic rocks are exposed 45 km from the mid america trench offshore the oceanic structure consists of an 8 km thick crust with a normal crustal velocity structure spudich and orcutt 1980 the apparent dip of the subducting plate beneath western mexico is 10 on the oceanic side strong reflections suggest a minimum depth of 35 km for the lithosphere asthenosphere boundary the asthenosphere has a seismic velocity of 7 6 km sec and a thin lid in which the velocity is 8 6 km sec

chemometric differentiation of crude oil families in the san joaquin basin california chemometric analyses of geochemical data for 165 crude oil samples from the san joaquin basin identify genetically distinct oil families and their inferred source rocks and provide insight into migration pathways reservoir compartments and filling histories in the first part of the study 17 source related biomarker and stable carbon isotope ratios were evaluated using a chemometric decision tree cdt to identify families in the second part ascendant hierarchical clustering was applied to terpane mass chromatograms for the samples to compare with the cdt results the results from the two methods are remarkably similar despite differing data input and assumptions recognized source rocks for the oil families include the 1 eocene kreyenhagen formation 2 eocene tumey formation 3 4 upper and lower parts of the miocene monterey formation buttonwillow depocenter and 5 6 upper and lower parts of the miocene monterey formation tejon depocenter ascendant hierarchical clustering identifies 22 oil families in the basin as corroborated by independent data such as carbon isotope ratios sample location reservoir unit and thermal maturity maps from a three dimensional basin and petroleum system model five families originated from the eocene kreyenhagen formation source rock and three families came from the overlying eocene tumey formation fourteen families migrated from the upper and lower parts of the miocene monterey formation source rocks within the buttonwillow and tejon depocenters north and south of the bakersfield arch the eocene and miocene families show little cross stratigraphic migration because of seals within and between the source rocks the data do not exclude the possibility that some families described as originating from the monterey formation actually came from source rock in the temblor formation

data review on the small mammals from the late early pleistocene of vallparadis estacio layer evt7 valles penedes basin ne iberian peninsula biochronological and palaeoenvironmental implications abstract the chronology of layer evt7 of the vallparadis estacio section has been a hotly debated topic during recent years with some authors supporting an age close to the early middle pleistocene boundary 0 83 ma others favouring an age older than the jaramillo magnetostratigraphic subchron ca 1 2 ma and others proposing a chronology between 0 98 and 0 95 ma for layer evt7 here we review the biochronological data from evt7 and review the biochronology based on the study of mimomys savini using all the available remains of this species which is the most abundant rodent species from this layer based on the morphologic and biometric comparison with remains of the same species from other similarly aged iberian localities gran dolina td4 6 from atapuerca and barranco leon d and fuente nueva 3 from the guadix baza basin we propose an estimated age for evt7 in the chronologic range from 0 99 to 0 94 ma we further perform a paleoenvironmental study based on the whole small mammal assemblage from evt7 which allows us to infer an open and humid environment in the vicinity of a lacustrine area or fluvial course recently published work also studied the rodent assemblage from evt7 however in this study only a part of the small mammals recovered from evt7 was studied and therefore its results can be partially biased

marine oil source of the yingmaili oilfield in the tarim basin abstract although a large amount of petroleum has been discovered in the tarim basin the origin of the oils is still disputed yingmaili oilfield was selected as a case study to further examine hydrocarbon generation and accumulation in the basin detailed geochemical studies were conducted to unravel the formation mechanisms the oils in the field correlate well with middle upper ordovician o 2 3 rocks or similar rocks oil oil and oil source rock correlation by maturity parameters of saturate and aromatic hydrocarbons suggest that the yingmaili and adjacent oils are primarily comparable to source rocks deeper than 6000 m the results also showed that carbonates might be important potential source rocks as indicated by a relatively high abundance of aromatic sulfur compounds and relatively high c 29 c 30 hopane and c 35 c 34 hopane ratios of the oils which are typical features of carbonate source rocks we suggest the oil from wells yg2 and yg2 1c are more suitable to be end members from the o 2 3 source rocks or similar rocks than the oil from well ym2 as previously suggested we also suggest that the previous model of hydrocarbon migration along the ordovician unconformity should be revised

palaeozoic oil source correlation in the tarim basin nw china a review abstract oil source correlation in the cratonic region of the tarim basin nw china has long been controversial current knowledge of the potential source rock distribution end member selection oil source correlation and impacts of secondary alteration processes have been reviewed two source rock systems from the cambrian lower ordovician o 1 and middle upper ordovician o 2 3 potentially contributed to oil accumulations in the cratonic region geochemical correlations suggest that oils are dominantly derived from the o 2 3 source rocks while geological evidence and sulfur isotopic compositions supports the o 1 source rocks as a main contributor such inconsistency is rooted in the selection of end members and their characterization changes in some biomarker parameters such as sterane distribution gammacerane index and ratio of tricyclic terpanes to pentacyclic terpanes are mainly controlled by thermal maturation and biodegradation rather than source input and depositional environment variation no clear boundary between the two source systems can be established using biomarkers except for a few diagnostic components some o 1 source signatures such as even odd predominance of n alkanes unusual tricyclic terpane distribution unusually enriched 13 c carbon isotopic compositions of oils and kerogens and the occurrence of combustion related polycyclic aromatic hydrocarbons largely result from an abnormal heating influence either from igneous intrusion or hydrothermal fluid activity local thermochemical sulfate reduction can also remove most source related signatures these so called markers in currently selected end members result from extensive secondary alteration processes and do not reflect source input differences while more research is needed for full reconstruction of oil source correlation in the cratonic region of the tarim basin the main source rocks most likely reside in the o 1 succession rather than o 2 3

correlation of crude oils and oil components from reservoirs and source rocks using carbon isotopic compositions of individual n alkanes in the tazhong and tabei uplift of the tarim basin china abstract carbon isotopic compositions were determined by gc irms for individual n alkanes in crude oils and the free adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the tazhong uplift and tahe oilfield in the tabei uplift of tarim basin as well as extracts of the cambrian ordovician source rocks in the basin the variations of the 13 c values of individual n alkanes among the 15 oils from the tazhong uplift and among the 15 oils from the triassic and carboniferous sandstone reservoirs and the 21 oils from the ordovician carbonate reservoirs in the tahe oilfield demonstrate that these marine oils are derived from two end member source rocks the major proportion of these marine oils is derived from the type a source rocks with low 13 c values while a minor proportion is derived from the type b source rocks with high 13 c values type a source rocks are within either the cambrian lower ordovician or the middle upper ordovician strata not drilled so far while type b source rocks are within the cambrian lower ordovician strata as found in boreholes td2 and fang 1 in addition the three oils from the cretaceous sandstone reservoirs in the tahe oilfield with exceptionally high pr ph ratio and 13 c values of individual n alkanes are derived or mainly derived from the triassic jurassic terrigenous source rocks located in quka depression the difference of the 13 c values of individual n alkanes among the free adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process in general the initial oil charge is derived from the type b source rocks with high 13 c values while the later oil charge is derived from the type a source rocks with low 13 c values the 13 c values of individual n alkanes do not simply correlate with the biomarker parameters for the marine oils in the tazhong uplift and tahe oilfield suggesting that molecular parameters alone are not adequate for reliable oil source correlation for high maturity oils with complex mixing

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

multiscale geological geophysical characterization of the epigenic origin and deeply buried paleokarst system in tahe oilfield tarim basin abstract the irregular structures of paleokarst systems are among the major factors resulting in carbonate reservoir heterogeneity which restricts the further exploration of paleokarst reservoirs using multiscale geological geophysical g g data including thin section core sample and well log data the caves cave fillings and fractures in individual wells in the northern tahe oilfield were characterized guided by seismic datasets the interwell paleokarst structures were interpreted in profile and plane view the paleokarst system in the study area was divided into four vertical zones and the relative genetic structures were classified in the upper three zones the epikarst zone includes paleo canyons karst towers dolines paleo soils and fractures and the water flows horizontally the vadose zone includes vadose caves and fractures and the water flows vertically the epiphreatic zone includes chambers main channels branch channels and fractures and the water flows primarily horizontally along the water table for each genetic type the total hydrocarbon production and monthly production were analyzed furthermore a genetic classification model was built for the paleokarst reservoirs of the study area which increased our understanding of the structure of the paleokarst system the results of the structure classification and production analysis can effectively guide hydrocarbon exploration and this approach can be applied to similar paleokarst reservoirs

uplift and subduction erosion in southwestern mexico since the oligocene pluton geobarometry constraints details of the late oligocene to middle miocene uplift and tectonic erosion episodes of the southwestern continental margin of mexico can be inferred using altot geobarometry of igneous hornblendes geochronology and field relations on the basis of such analyses carried out between acapulco and huatulco we find the following 1 calc alkaline batholiths exposed along the coast from acapulco to huatulco mostly in the 35 25 ma age range were emplaced at depths between 13 and 20 km 2 the contact relationships between these plutons and their host rocks and the exposure of volcanic counterparts 70 km from the coastline indicate a landward decrease in the amount of uplift 3 a comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were in general higher between acapulco and huatulco than those along the margin between puerto vallarta and manzanillo 700 km northwest of acapulco 4 the uplift of this coastal belt occurred during the late stages of magmatism and after its cessation triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the xolapa complex these findings in conjunction with the geometry of the present continental margin as well as the offshore tectonic and stratigraphic features support previous interpretations of very active late oligocene to middle miocene subduction erosion after the onset of strike slip tectonics related to the detachment and subsequent eastward displacement of the chortis block subduction erosion involved both trench sediments and crystalline continental framework rocks different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework comparing the calculated depths of pluton crystallization with the present depth of the continental crust subducted slab boundary interpreted using previously published seismic refraction and gravity models we conclude that onshore basal erosion played a subordinate role during miocene episodes of subduction erosion major removal of lower crustal sections was probably restricted to offshore regions plate reconstructions of the cocos plate and its predecessors with respect to north america indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the guadalupe plate beneath southwestern mexico

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

circum arctic petroleum systems identified using decision tree chemometrics source and age related biomarker and isotopic data were measured for more than 1000 crude oil samples from wells and seeps collected above approximately 55n latitude a unique multitiered chemometric multivariate statistical decision tree was created that allowed automated classification of 31 genetically distinct circum arctic oil families based on a training set of 622 oil samples the method which we call decision tree chemometrics uses principal components analysis and multiple tiers of k nearest neighbor and simca soft independent modeling of class analogy models to classify and assign confidence limits for newly acquired oil samples and source rock extracts geochemical data for each oil sample were also used to infer the age lithology organic matter input depositional environment and identity of its source rock these results demonstrate the value of large petroleum databases where all samples were analyzed using the same procedures and instrumentation

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

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

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

oil source correlation in the slope of the qikou depression in the bohai bay basin with discriminant analysis abstract the commonly used tools for oil source correlation such as mass chromatograms of biomarkers and bivariate cross plots of geochemical parameters cannot deal with multiple geochemical parameters and plenty of samples simultaneously leading to uncertainties in the results and even failures sometimes in this paper the discriminant analysis da is selected from supervised machine learning algorithms as it is superior to the commonly used tools as well as other multivariate statistical methods with the accumulation of geochemical data of source rocks and oils in the slope of the qikou depression the main source rocks in the third es3 and first members es1 of the paleogene shahejie formation were deposited in similar depositional environments the source rocks cannot be distinguished with the commonly used tools we firstly extended geochemical parameters and then used stepwise da to select informative parameters and to develop a discriminant model for oil source correlation the 22 selected parameters are supported by geochemical characteristics of the source rocks in the study area the da of these parameters for oil source correlation achieved a high correct rate of original validation 96 8 and leave one out cross validation 89 4 indicating a sufficient discriminatory power the oil source correlation results with high posterior probabilities showing strong similarity between the sources and oils coincide with geological conditions and illustrate that there is still much exploration potential in the study area especially for the es3 petroleum system all these illustrate that da is one of the most useful tools for oil source correlation with the accumulation of geochemical data of source rocks and oils

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

the gran dolina site lower to middle pleistocene atapuerca burgos spain new palaeoenvironmental data based on the distribution of small mammals new palaeoenvironmental data are reported after analysing the distribution of micromammals orders rodentia insectivora chiroptera and lagomorpha in the 11 stratigraphical levels at the gran dolina site td1 td11 early to middle pleistocene atapuerca spain a continental dry and cold climate is inferred at the beginning of the succession td3 to lower td5 the fauna in the upper td5 and in td6 reflects a complex interglacial period with fluctuations in the degree of relative humidity in the lower part of td8 the presence of microtus aff ratticepoides indicates a relatively cold period finally data from upper levels td8b td10 td11 reflect another interglacial period with slight oscillations in relative moisture the different levels at gran dolina are also correlated with oxygen isotope stages ois td3 td4 and td5 may be correlated with ois 22 td6 is correlated with ois 21 lower td8 td8a is correlated with ois 18 upper td8 td8b is correlated with ois 13 or 15 td10 and td11 are correlated with ois 9 or 11

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

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

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

low angle contact between the oaxaca and juarez terranes deduced from magnetotelluric data we present the electrical resistivity model along a profile perpendicular to the middle america trench in southern mexico that reveals previously unrecognized tectonic features at upper to mid crustal depths our results support the hypotheses that the upper crust of the oaxaca terrane is a residual 20 km thick crust composed by an 10 km thick faulted crustal upper layer and an 10 km thick hydrated and or mineralized layer oaxaca basement overthrust the younger juarez or cuicateco terrane the electrical resistivity model supports the interpretation of a slab subducting at a low angle below oaxaca uplift in the oaxaca region appears to be related to fault reactivation induced by low angle subduction in the juarez terrane isostatic forces may contribute to uplift because it is largely uncompensated in the sierra madre del sur closer to the coast uplift is facilitated by slab dehydration driven buoyancy both gravity and resistivity models are consistent with a thinned upper crust in the northeast end of the profile

environmental and climatic reconstruction of mis 3 in northwestern europe using the small mammal assemblage from caverne marie jeanne hastiere lavaux belgium abstract marine isotope stage 3 mis 3 ca 60 30 ka is characterized by dynamic alternations of forest expansion with semi arid area expansion in accordance with the warming and cooling respectively of the sea surface temperatures in northern europe it was in this context of rapid fluctuations that the terrestrial sequence of caverne marie jeanne hastiere lavaux belgium in northwestern europe was formed the habitat weighting method and the bioclimatic model as well as the simpson diversity index are applied to the small mammal assemblage of caverne marie jeanne in order to reconstruct the environmental and climatic fluctuations that are reflected in the mis 3 sequence of the cave revision of the small mammal fossil material deposited in the collections of the royal belgian institute of natural sciences rbins brussels belgium shows that the lower layers 6 to 4 of the cave pertaining to mis 3 ca 50 40 ka underwent cold dry environmental and climatic conditions for these layers this is indicated by temperatures lower than at present and precipitation slightly higher than at present together with an environment dominated by open woodland formations and open dry meadows our results are consistent with the available chronological large mammal herpetofaunal and mollusc datasets for this lower part of the sequence they are also consistent with regional loess studies in belgium and with previous work performed on small mammals from mis 3 in belgium and elsewhere in europe

towards a middle pleistocene terrestrial climate reconstruction based on herpetofaunal assemblages from the iberian peninsula state of the art and perspectives abstract the pattern of the varying climatic conditions in southern europe over the last million years is well known from isotope studies on deep ocean sediment cores and the long pollen records that have been produced for lacustrine and marine sedimentary sequences from greece italy and the iberian margin however although relative glacial and interglacial intensities are well studied there are still few proxies that permit quantitative terrestrial temperature and precipitation reconstruction in this context fauna based climate reconstructions based on evidence preserved in archaeological or palaeontological sites are of great interest even if they only document short windows of that climate variability because a they provide a range of temperature and precipitation estimates that are understandable in comparison with present climate b they may allow the testing of predicted temperature changes under scenarios of future climate change and c quantitative temperature and precipitation estimates for past glacials and interglacials for specific regions latitudes can help to understand their effects on flora fauna and hominids as they are directly associated with those cultural and or biological events moreover such reconstructions can bring further arguments to the discussion about important climatic events like the mid bruhnes event a climatic transition between moderate warmths and greater warmths during interglacials in this paper we review a decade of amphibian and reptile based climate reconstructions carried out for the iberian peninsula using the mutual ecogeographic range method in order to present a regional synthesis from mis 22 to mis 6 discuss the climate pattern in relation to the mid bruhnes event and the thermal amplitude suggested by these estimates and finally to identify the chronological gaps that have still to be investigated

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

the origin of a terrane u pb zircon geochronology and tectonic evolution of the xolapa complex southern mexico terrane analysis has established differences in lithology style of deformation metamorphism and radiometric ages among several basement complexes in southern mexico in this paper we identify similarities in the geologic history of the adjacent acatlan oaxaca and xolapa complexes through a u pb zircon study on crustal sources and timing of magmatic metamorphic and tectonic activity in the xolapa complex our regional tectonic scenario is based on both new chronologic and previously established structural data inherited zircon in the xolapa complex indicates the presence of a proterozoic 1 0 1 3 ga crustal component with an age range that overlaps grenville crystallization dates 1 0 1 2 ga from the oaxaca basement and an inherited grenville crustal component 1 0 1 1 ga identified in zircons from the acatlan complex the proterozoic component in the xolapa complex indicates either that it received sediments from a continental region of grenvillean age for example the oaxaca basement which is the closest exposed grenville crust in southern mexico or that the xolapa complex has a grenville basement in the latter case the xolapa complex has been modified by widespread high grade metamorphism and large scale migmatization metamorphism and migmatization occurred from 66 to 46 ma and locally continued into oligocene time magmatism in the xolapa complex terminated with crustal growth by plutonism which is characterized by a systematic pattern of eastward younging crystallization ages from 35 ma in the west west of acapulco to 27 ma in the east east of puerto angel metamorphism and migmatization in the xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the farallon and north american plates e g rate and direction of subduction and the early evolution of the caribbean this plate reorganization triggered sinistral transtension distributed across the southern mexican continental margin and the eastward translation of the chortis block the eastward younging of the xolapa plutons also is consistent with the motion of the chortis block during eastward displacement of the caribbean plate this induced a shift of the magmatic arc from its cretaceous to early tertiary position along the xolapa and chortis blocks to its present mid mexican position we conclude that a single tectonic framework accounts for the mesozoic and cenozoic geologic history of the southern mexican terranes

the evolution of middle america and the gulf of mexico caribbean sea region during mesozoic time a plate tectonic model for the evolution of middle america and the gulf of mexico caribbean sea region is presented the model which is based upon the existence of the mojave sonora megashear incorporates into the triassic pangea reconstruction three microplates between north and south america thus avoiding the overlap of the bullard fit these plates are the yaqui bounded on the north by the mojave sonora megashear the east and west maya plates bounded on the north by the mexican volcanic zone and on the south by a predecessor of the motagua fault zone and the chortis plate parts of guatemala and honduras during late jurassic time as north america split away from europe africa and south america shear with left lateral sense of displacement occurred along the transform faults that bounded the micro plates if 800 km of left lateral displacement along the mojave sonora megashear 300 km along the mexican volcanic belt and 1 300 km along a proto motagua megashear are restored and if yucatan and cuba are rotated to fit against northern south america then 1 a curvilinear belt of late paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in texas eastern mexico nuclear central america and colombia 2 a mediterranean like sea is delineated that was a precursor of most of the present gulf of mexico 3 correlation is implied between the distinctive quartzose san cayetano formation of cuba and the caracas and juan griego groups of venezuela geometric constraints suggest that probably shear initially occurred along the mexican volcanic zone near the end of the middle jurassic subsequently probably about 160 m y ago displacements that total 800 km began along the mojave sonora megashear contemporaneously yucatan and fragments of pre cretaceous rocks that compose parts of central and western cuba migrated northward toward their present positions rotation of yucatan was facilitated by considerable displacement along the proto motagua zone and along a zone that is probably coincident with the modern salina cruz fault accumulation of widespread major salt units of late jurassic callovian to early oxfordian age in the gulf basin probably occurred contemporaneously with the arrival of these blocks at their present positions clastic units that interfinger with some of the youngest salt units and rim the gulf of mexico have not recorded major recognized translations since their accumulation clockwise rotation of south america and the chortis plate occurred during early cretaceous time this movement which was manifested by subduction of jurassic ocean floor against the previously rifted precursor of the island of cuba and under parts of hispaniola and puerto rico is recorded by circum caribbean orogeny abrupt changes in the relative motions between north and south america during late cretaceous time may have resulted in extension and outpourings of basalt upon the jurassic rocks of the ocean floor of the venezuelan basin west of beata ridge sea floor spreading formed the colombian basin related subduction occurred as the chortis plate including part of central america the nicaraguan rise and southeastern cuba was sutured against the maya east plate along the present motagua fault and cayman trench our model is constrained by published geologic data the relative positions of north and south america from atlantic sea floor magnetic anomalies and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between north and south america paleomagnetic data from middle america are sparse but do not conflict with the predicted motions of some of the microplates especially chortis

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

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

chemometric recognition of genetically distinct oil families in the los angeles basin california the prolific los angeles basin in california may be the most petroliferous province on earth per volume of sedimentary fill however because most exploration in the basin occurred prior to the advent of modern geochemical methods genetic relationships among the various petroleum accumulations and their source rocks have remained speculative a training set of 24 source related biomarker and stable carbon isotope ratios for 111 non or mildly biodegraded oil samples from the basin was used to construct a chemometric multivariate statistics decision tree the decision tree allows genetic classification of additional oil or source rock extract samples that might be collected the decision tree identifies 6 tribes and a total of 12 genetically distinct oil families the families have different bulk properties such as api gravity and sulfur content which were previously explained as resulting from secondary processes including thermal maturity or biodegradation however the chemometric assignments are based on genetic properties that reflect distinct organofacies the oil families occur in different locations and reservoir intervals in the basin consistent with their origins from different organofacies of active source rock the source rock depositional environment for each oil family can be inferred using biomarker and isotope ratios the samples show stable carbon isotope ratios for saturate and aromatic hydrocarbons that indicate different organofacies of miocene marine source rocks tribes 1 and 2 straddle the central trough mainly occur east of the newport inglewood fault zone nifz and show evidence of proximal clay rich source rock deposited under suboxic conditions with elevated angiosperm input tribes 3 6 occur west of the nifz and show evidence of more distal clay poor source rock deposited under anoxic conditions geochemistry and stratigraphy of the oil tribes 1 6 below suggest the following source rock organofacies

de convoluting mixed crude oil in prudhoe bay field north slope alaska abstract seventy four crude oil samples from the barrow arch on the north slope of alaska were studied to assess the relative volumetric contributions from different source rocks to the giant prudhoe bay field we applied alternating least squares to concentration data als c for 46 biomarkers in the range c19 c35 to de convolute mixtures of oil generated from carbonate rich triassic shublik formation and clay rich jurassic kingak shale and cretaceous hue shale gamma ray zone hue grz source rocks als c results for 23 oil samples from the prolific ivishak formation reservoir of the prudhoe bay field indicate approximately equal contributions from shublik formation and hue grz source rocks 37 each less from the kingak shale 26 and little or no contribution from other source rocks these results differ from published interpretations that most oil in the prudhoe bay field originated from the shublik formation source rock with few exceptions the relative contribution of oil from the shublik formation decreases while that from the hue grz increases in reservoirs along the barrow arch from point barrow in the northwest to point thomson in the southeast 250 miles or 400 km the shublik contribution also decreases to a lesser degree between fault blocks within the ivishak pool from west to east across the prudhoe bay field als c provides a robust means to calculate the relative amounts of two or more oil types in a mixture furthermore als c does not require that pure end member oils be identified prior to analysis or that laboratory mixtures of these oils be prepared to evaluate mixing als c of biomarkers reliably de convolutes mixtures because the concentrations of compounds in mixtures vary as linear functions of the amount of each oil type als of biomarker ratios als r cannot be used to de convolute mixtures because compound ratios vary as nonlinear functions of the amount of each oil type

micromammals from the 1995 mammoth excavation at west runton norfolk uk morphometric data biostratigraphy and taxonomic reappraisal abstract in this paper the small mammals recovered from sediments associated with the west runton elephant have been analysed and compared with sites in other parts of europe major taxonomical problems are indicated and we suggest ways of utilising such morphological complexity to refine biostratigraphical and chronostratigraphic attributions the micromammal assemblage from the west runton elephant site so far totals 16 species from the west runton freshwater bed wrfb there are strong similarities with the arvicolid spectrum of basal layer h8 in the kon prusy c 718 profile czech republic which accumulated under cooler conditions preceding the thermal maximum of an early middle pleistocene interglacial rare steppic indicators e g cricetus at west runton also imply a somewhat cooler and more continental climate than the present day but overall the fauna is fully temperate in character west runton shares several stratigraphically significant small mammal taxa with voigtstedt central germany and morphometric comparisons suggest that voigtstedt may be slightly younger than west runton the presence of mimomys savini with a latest occurrence in the early part of marine isotope stage mis 15 together with the palaeomagnetic evidence suggests the wrfb may have been emplaced during the early part of mis 17 however given the complexity of the marine isotope curve during the early part of the brunhes chron and differences between global and regional climatic evolution it is difficult to assign the normally magnetised wrfb to a particular marine isotope stage mis

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

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

the middle pleistocene site of la cansaladeta tarragona spain stratigraphic and archaeological succession abstract la cansaladeta is a site located in a narrow passage dug out by the francoli river in the pre littoral catalan range ne iberia the archaeological deposit lies on top of a 45 50 m fluvial terrace and contains a rich middle pleistocene stratigraphic succession in which 10 archaeological layers have been identified here we present the description of the sedimentary succession of the site the available chronological framework including tl tt osl and esr u series ages as well as biostratigraphical information the characteristics of the lithotechnical and faunal assemblages as well as the first interpretation of the human occupations at the site three main occupation phases have been differentiated the oldest archaeological layers are embedded in a palaeosol formed on a clayey layer that contains limestone fragments detached from the shelter s wall and roof during the second phase low energy fluvial sediments coming from lateral river floods were deposited and sealed the archaeological layers after a transitional phase the upper layers appear to be derived from slope sediments numerical dating yielded internally consistent dates of 380 400 ka for the sequence however the study of the fossil assemblages potentially suggest a tentative early middle pleistocene age for the lowermost horizons the rich technological record is made up of several local raw materials especially chert but also schist quartzite quartz and other minor rocks knapping sequences seem to be autochthonous as all the elements and size categories of the reduction sequence are present and refits are common techno typological features observed along the sequence are not very diagnostic however the lower layers include some acheulean forms among the large shaped tools restricted variability among the small retouched tools mostly denticulates and a virtually absence of prepared cores the faunal assemblage is scarce and appears strongly altered only the lower layers have provided fruitful information although neither combustion areas nor structures related to fire use have been documented to date heat damage has been repeatedly recorded on both lithic and faunal remains this may point to systematic fire use within the archaeological sequence the data provided by the ongoing excavations at la cansaladeta provide new insight into the early human settlement history of northeast iberia to be reconstructed combining these data with those coming from the close site of barranc de la boella enable us to better reconstruct the timing nature and context of early human occupation in the region