geochemical and geological characteristics of the es3l lacustrine shale in the bonan sag bohai bay basin china abstract although extensive studies have been conducted on unconventional shale oil resources in recent years little work has been performed on the resource potential of lacustrine shales the es3l lower sub member of the third member of the eocene shahejie formation shale of the bonan sag a set of thick lacustrine deposited sediments offers a good example for furthering the understanding of these rocks geochemical and mineralogical investigations were carried out on a large sample set from the 220 m thick sequence of well luo69 in total 200 core samples were analyzed for total organic carbon toc and rock eval pyrolysis and 421 core samples were analyzed for mineral compositions the es3l shales have a very high toc content with type i lacustrine oil prone kerogen in the early oil generation stage rock eval tmax suppression is evident in the lower part of the es3l member associated with a rise in pi production index with respect to fracability or brittleness the content of brittle minerals in the es3l shale is generally higher than that of marine shales in the usa with higher calcite mineral averaging 51 82 wt and lower clay content averaging 18 86 wt interparticle pores in minerals grains or clay aggregates intraparticle pores intercrystalline dolomite and pyrite framboid and four types of fractures tectonic fractures abnormal pressure fractures clay shrinkage fractures and inter laminar fractures are developed in the es3l shale organic matter pores are not present in the studied samples probably due to lower maturity tectonic fracture may play an important role in hydrocarbon migration and accumulation the results of this study suggest that the es3l shale in the bonan sag has very good shale oil potential especially in the lower section

petrographic and geochemical composition of kerogen in the furongian u cambrian alum shale central sweden reflections on the petroleum generation potential abstract this paper presents an integrated geochemical and petrological study of the hallekis 1 core from the furongian upper cambrian alum shales in central sweden to characterise organic matter composition depositional environment and potential hydrocarbon generation capability the results show that organic rich alum shale toc 8 9 28 0 wt contains mainly immature predominantly algal derived kerogen with unusually reduced hydrocarbon generation potential as suggested by relatively low hydrogen index hi values hi 251 471 mg hc g toc and high degree of aromaticity in the absence of thermal generation of hydrocarbons in these immature shales the recorded low hi values are explained by an unusual chemistry of the biota during the deposition of the alum shale accentuated by a high degree of sustained bacterially mediated degradation e g sulphate reduction and possibly intense nuclear degradation of labile kerogen by radiation from uranium rich minerals the higher degree of aromaticity in alum shale suggests lower than expected oil proneness the results of this study suggest that the alum shale is a gas prone source producing aromatic hydrocarbon mixtures with an unexpectedly low concentration of n alkanes

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

gravity anomalies and flexure of the brazilian shield beneath the bolivian andes abstract seismological and geological data suggest that the brazilian shield is now underthrusting beneath the sub andes in order to constrain the minimum amount of shortening that results from this underthrusting we analyzed bouguer gravity anomalies over the shield and the bolivian andes assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate brazilian shield loaded by the weight of the andean mountain range gravity anomalies show that airy type isostatic equilibrium does not exist the sub andes are overcompensated and the chaco plain to the east is undercompensated the flexure of the plate can help explain both the overcompensation and the undercompensation if the brazilian shield behaves as an essentially elastic plate that extends at least 150 km and probably 200 km beneath the sub andes and the cordillera oriental and if the flexural rigidity of the plate is between 0 1 and 2 10 24 n m the fit of observed and calculated anomalies however is far from perfect and additional sources of short wavelength anomalies appear to be present assuming that the sediments and sedimentary rocks in the sub andes and cordillera oriental were deposited on the brazilian shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate we computed a family of estimates for the minimum amount of crustal shortening these estimates depend strongly on the amount of sedimentary rock deposited before deformation which does not seem to be well constrained by published geologic mapping nevertheless the calculations suggest that at least 100 km of crustal shortening have occurred and probably more implying that the andes may have been built mostly by crustal shortening

new paleomagnetic data from the northern argentine puna central andes rotation pattern reanalyzed 1 along the central andes a pattern of vertical axis rotations carp has been paleomagnetically identified these rotations which are counterclockwise north of arica and clockwise to the south have been detected in mesozoic and cenozoic rocks cropping out in the andean forearc and the altiplano puna different models have been proposed to explain the geodynamic evolution of the central andes but the origin of these rotations remains controversial such models involve oroclinal bending and or local block rotations this lack of agreement is partly due to the scarce paleomagnetic database particularly for the argentine puna here we report new paleomagnetic data obtained in the northern argentine puna from 57 sites drilled from miocene volcaniclastic rocks and 12 sites from the cretaceous eocene salta group it was determined that 1 the morro blanco area did not undergo significant vertical axis rotation since at least the middle miocene 2 the loma blanca mine area underwent a clockwise vertical axis rotation of 10 later than the late miocene and 3 the coranzuli area underwent a clockwise vertical axis rotation of 40 later than the cretaceous eocene the central andes database was analyzed and some features of the carp were determined these features were explained considering domino style models for the forearc and a local block rotation model in response to penetrative shear for the altiplano puna penetrative shear would occur along a high electrical conductivity zone however the possibility of the existence of an oroclinal bending of 5 cannot be discarded

an accreted continental terrane in northwestern peru abstract a paleomagnetic study of over 250 cores from 26 sites sampled in early to late cretaceous and paleogene volcanic plutonic and sedimentary formations of the lancones basin in the piura province of northern peru indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable south america a clockwise rotation ranging from 90 for the lowermost units to 35 for the uppermost ones has been documented although the lack of precise chronology has not allowed a detailed temporal description four sites from late carboniferous pennsylvanian formations in the amotape tahuin range also show a 110 clockwise rotation and yield evidence for a northward displacement when considered together with previous geological studies these data are consistent with the hypothesis of the accretion of an amotape tahuin continental terrane to the peruvian margin in neocomian times the accretion was followed by in situ rotation suggesting a dextral shear regime these results indicate that the geodynamical evolution of northern peru is more closely related to the processes observed in ecuador than to those classically assumed for the central andes of peru

formation of nanoporous pyrobitumen residues during maturation of the barnett shale fort worth basin abstract hydrocarbon generation reactions occur within organic rich shales as a response to thermal maturation here we report observations on samples from the organic rich mississippian barnett shale gas system fort worth basin texas usa at varying stages of thermal maturation a multiscale characterization was achieved using a combination of compositional organic geochemistry and spectromicroscopy techniques including synchrotron based scanning transmission x ray microscopy stxm and transmission electron microscopy tem we document the chemical evolution of the macromolecular structure of barnett shale kerogen with increasing maturity from an immature kerogen containing a significant aliphatic component and an important concentration of oxygen and sulfur containing functional groups to an overmature kerogen dominated by poorly condensed aromatic structures in addition to the presence of bitumen in samples of oil window maturity very likely genetically derived from thermally degraded kerogen the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity likely resulting from the formation of gaseous hydrocarbons by secondary cracking of bitumen compounds

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

the importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs abstract the effect of shale composition and fabric upon pore structure and ch 4 sorption is investigated for potential shale gas reservoirs in the western canadian sedimentary basin wcsb devonian mississippian d m and jurassic shales have complex heterogeneous pore volume distributions as identified by low pressure co 2 and n 2 sorption and high pressure hg porosimetry thermally mature d m shales 1 6 2 5 vro have dubinin radushkevich d r co 2 micropore volumes ranging between 0 3 and 1 2 cc 100 g and n 2 bet surface areas of 5 31 m 2 g jurassic shales which are invariably of lower thermal maturity ranging from 0 9 to 1 3 vro than d m shales have smaller d r co 2 micropore volumes and n 2 bet surface areas typically in the range of 0 23 0 63 cc 100 g co 2 and 1 9 m 2 g n 2 high pressure ch 4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon toc content methane sorption in d m shales increases with increasing toc and micropore volume indicating that microporosity associated with the organic fraction is a primary control upon ch 4 sorption sorption capacities for jurassic shales however can be in part unrelated to micropore volume the large sorbed gas capacities of organic rich jurassic shales independent of surface area imply a portion of ch 4 is stored by solution in matrix bituminite solute ch 4 is not an important contributor to gas storage in d m shales structural transformation of d m organic matter has occurred during thermal diagenesis creating and or opening up microporosity onto which gas can sorb as such d m shales sorb more ch 4 per weight percent wt toc than jurassic shales inorganic material influences modal pore size total porosity and sorption characteristics of shales clay minerals are capable of sorbing gas to their internal structure the amount of which is dependent on clay type illite and montmorillonite have co 2 micropore volumes of 0 78 and 0 79 cc 100 g n 2 bet surface areas of 25 and 30 m 2 g and sorb 2 9 and 2 1 cc g of ch 4 respectively dry basis a reflection of microporosity between irregular surfaces of clay platelets and possibly related to the size of the clay crystals themselves mercury porosimetry analyses show that total porosities are larger in clay rich shales compared to silica rich shales due to open porosity associated with the aluminosilicate fraction clay rich sediments low si al ratios have unimodal pore size distributions

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

assessing the age of relief growth in the andes of northern chile magneto polarity chronologies from neogene continental sections the andes of northern chile currently experience a phase of relief rejuvenation as indicated by valleys that are 1000 m dissected into pediplains however it has been unclear when this phase of relief enhancement started this paper discusses the use of palaeomagnetic chronologies from four sections in the taracapa region northern chile to assess this age the sections are located in distal and proximal positions sediment accumulation occurred between c 22 5 and 8 7 5 ma with a hiatus that possibly spans a time interval between c 19 5 and 11 ma the magnetic polarity chronologies suggest a preferred age between 8 0 and 7 5 ma for the time when relief growth started in proximal positions however alternative correlations suggest an age of 8 5 ma in addition the palaeomagnetic data reveal no rotation of the analysed strata suggesting a minimum age of c 22 5 ma for the tectonic block rotation south of the arica deflection

palaeomagnetism of the cretaceous el way and coloso formations from the northern chilean andes summary a palaeomagnetic study mainly of limestones of the el way formation and fine sandstone of the coloso formation near antofagasta northern chile 23 83 s 70 38 w gives an early cretaceous palaeomagnetic pole 76 4 s 150 7 e a 6 4 for the chilean andes comparison of this pole with the saki pole of valencio ef al 1983 for cratonic south america suggests a relative clockwise rotation of 12 f 9 considered in isolation little confidence can be placed in this possible rotation because of its large error and non small flattening of 10 f 10 however if the pole is combined with all available late jurassic and cretaceous palaeomagnetic poles from the andes it is very clear that the poles group according to the geographical location of their sampling areas peruvian andes hinge part of the andes and chilean andes the rotation angle between the two blocks calculated from the peruvian andes mean pole and the chilean andes mean pole amounts to a significant 41 f 18 assuming the rotation centre is located at the hinge of the coast a jurassic sandstone of the arica group from cuya in northernmost chile seems to have been completely thermally remagnetized by cretaceous dike swarms which intrude the sandstone the site mean direction of the sandstone is indistinguishable from that of the dike swarms whose palaeomagnetism has been studied by nomura et al 1983

organic geochemistry and petrography of lower cretaceous wealden black shales of the lower saxony basin the transition from lacustrine oil shales to gas shales abstract ninety seven wealden black shale samples from three wells in the lower saxony basin have been studied by organic geochemical and organic petrographical methods to determine their maturity organic facies and depositional environment the maturities of the three wells range from early mature ex a late to postmature ex c to overmature ex b as determined by vitrinite reflectance measurements diamondoid ratios and other geochemical maturity parameters ex c and ex b show distinct petrographic features related to oil generation and migration in particular the occurrence of dispersed solid bitumen replacing initial type i kerogen suggests a formerly active petroleum system structural and textural differences between early mature alginites and solid bitumen in postmature to overmature samples show an alteration of the pore system with increasing maturity a freshwater depositional environment is indicated by widespread occurrence of botryococcus algae and other small alginite particles predominating in the immature well these alginites are absent in the more mature gas shales of wells ex c and ex b geochemical evidence of algae and phytoplankton in general is provided by numerous biomarker parameters while the occurrence of carotane in some samples indicates events of increased salinity although no hypersaline conditions are inferred due to very low gammacerane indices increased amounts of vitrinite and inertinite in samples of ex b suggest locally significant terrigenous input of organic matter for some periods during wealden shale deposition high sulfur organic carbon ratios provide evidence for sulfate rich waters and partly anoxic bottom water conditions while the lower mature lacustrine source rocks generate paraffinic waxy oils gas and condensates are produced at post mature stages furthermore maturity distribution maps from 3d numerical petroleum systems modeling reveal substantial differences in respect to petroleum generation

mineralogy and geochemistry of mississippian and lower pennsylvanian black shales at the northern margin of the variscan mountain belt germany and belgium abstract exploration of unconventional gas resources from paleozoic formations in northwestern germany is just getting started large potential gas reservoirs are presumed to be present north of the rhenish massif where mississippian and pennsylvanian marine black shales occur this paper comprises geochemical and mineralogical data and other important aspects of potentially economic black shale formations of the carboniferous additionally the burial and thermal history was reconstructed using 1d modeling software schlumberger these 1d models were calibrated with vitrinite reflectance data from outcrops and shallow wells in general all paleozoic black shales are at present highly mature between about 1 5 and 3 vitrinite reflectance the shales of the uppermost mississippian upper alum shale chokier formation have high contents of organic carbon are tens of meters thick and can be regarded as potential gas shale targets most other mississippian and pennsylvanian black shales are relatively thin adjacent carbonates are often stained black and rich in solid bitumen indicating former oil impregnation of these reservoirs

palaeomagnetism of neogene ocros dyke swarm the peruvian andes implication for the bolivian orocline summary remanent magnetization directions of 32 dykes and lava flows sampled near ayacucho the peruvian andes revealed 14 2 5 5 counterclockwise rotation after the neogene intrusion of this dyke swarm palaeo magnetic results of these rocks and other palaeomagnetic evidences from the central andean mesozoic rocks suggest relatively recent occurrence of the andean oroclinal bending around the axis at the peru chile border

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

geochemical truths in large data sets i geochemical screening data abstract organic geochemistry especially when undertaken on a well by well or outcrop by outcrop basis arguably lacks statistically significant numbers of analyses to establish reliable trends can a reliable vitrinite reflectance vs depth trend be established from say 12 analysed samples when the number of analysed samples are in the hundreds or even thousands much clearer general trends are established and anomalous measurements more readily recognised interpretation of anomalies is itself highly instructive but forms another paper in this paper a global data set of some 5000 source rock samples is assembled from the open literature and released data e g open file reports and the results discussed to produce practical oil industry interpretations from basic screening data a complex conclusion can be extracted from simple data sets and are supported by mass balance and kinetic theory it is concluded that interpretation always falls back on a comparison of new data with pre existing knowledge in the form of tables and plots total organic carbon toc values are generally log normally distributed so that using mean standard deviation values will under estimate the upside potential when fed into risked probabilistic hydrocarbon charge prediction models kerogen type is initially best determined using plots of rock eval s 2 vs toc rather than the hydrogen index hi s 2 toc vs oxygen index oi s 3 toc or pseudo van krevelen plot the intercept of the s 2 vs toc plot predicts the average dead carbon inertinite content of the source rock set with the slope corresponding to the hydrogen index of the labile component hi l maturity trends of the hydrogen index vs t max or r o show reversals at high maturities as supported by the kinetics of the thermal breakdown of the three standard kerogen types at elevated maturities type iii kerogens have higher hydrogen indices than types i and ii kerogens a feature that must be taken into account when correcting mature kerogens to their original hi values hi o the reduction in hydrogen index is a measure of generation with the corresponding increase in production index pi s 1 s 1 s 2 being effectively a measure of hydrocarbon retention a cross plot of hi vs pi can be interpreted in terms of expulsion efficiencies expelled generated minus retained hydrocarbon measured data support a pore saturation and or generation pressure driven process for expulsion and indicate a limiting pore saturation equivalent to a production index of 0 6 for rich 2 toc source rocks

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

rotation of the peruvian block from palaeomagnetic studies of the central andes palaeomagnetic studies were performed on the rocks of the central andes to test the hypothesis of oroclinal bending of the andes around the peru chile border natural remanent magnetization of mesozoic volcanic and sedimentary rocks of peru shows a counterclockwise shift of declination by several tens of degrees relative to the field directions of the same ages in the stable south american craton two mesozoic dyke swarms from arica region northernmost chile also indicate about 20 of declination shift in a counterclockwise sense these results suggest a considerable amount of counterclockwise rotation of large area north of arica that is oroclinal bending of the andes around some hinge near the peru chile border one neogene dyke swarm sampled in central peru also gave declination which deviates counterclockwisely by 15 if this declination shift is a result of the wide area tectonic rotation as suggested by the mesozoic rocks oroclinal bending appears to have continued until relatively recent times

new data on the geodynamics of southern peru from computerized analysis of spot and sar ers 1 images abstract several geological structures are extracted from spot panchromatic and ers 1 radar images of the central andes arequipa area southern peru our new methodology of numerical image processing is an aid for identification of the structures this numerical approach helps to locate characterize and determine the amplitude of the vertical offset of structures the results show that the nw se trending major faults are principally normal with a small sinistral strike slip component the open ne sw faults are tension gashes extension in the studied area trends ne sw and the major nw se striking faults form an extensional en echelon pattern including an active volcanic zone these new kinematic results lead to a regional geodynamic model which takes into account oblique convergence between pacific and south america plates within the wadati benioff zone of the peru trench

porosity of coal and shale insights from gas adsorption and sans usans techniques two pennsylvanian coal samples spr326 and spr879 in1 and two upper devonian mississippian shale samples mm1 and mm3 from the illinois basin were studied with regard to their porosity and pore accessibility shale samples are early mature stage as indicated by vitrinite reflectance ro values of 0 55 for mm1 and 0 62 for mm3 the coal samples studied are of comparable maturity to the shale samples having vitrinite reflectance of 0 52 spr326 and 0 62 spr879 in1 gas n2 and co2 adsorption and small angle and ultrasmall angle neutron scattering techniques sans usans were used to understand differences in the porosity characteristics of the samples the results demonstrate that there is a major difference in mesopore 2 50 nm size distribution between the coal and shale samples while there was a close similarity in micropore 2 nm size distribution micropore and mesopore volumes correlate with organic matter content in the samples accessibility of pores in coal is pore size specific and

fault fracture mesh petroleum plays in the zhanhua depression bohai bay basin part 2 oil source correlation and secondary migration mechanisms abstract the chemical compositions of crude oils from over 20 oilfields in the zhanhua depression bohai bay basin were studied in order to understand their genetic relationships thermal maturity and biodegradation ranking molecular geochemical parameters indicate the presence of three possible oil families in reservoirs ranging from paleozoic basement to neogene a number of biomarker features support deposition of the source rocks for these oils under a wide range of depositional environments the majority of the oils discovered in the depression belong to the fl family and display characteristics consistent with a dominantly freshwater to slightly brackish water lacustrine source rock in the es 3 member of the shahejie formation the somewhat geographically restricted sl family oils are apparently linked to saline lacustrine carbonate evaporite source rocks distributed mostly in the upper es 4 member of the shahejie formation from the bonan gubei sag areas the thermally less mature bl family oils occur mostly within the es 1 es 2 or dongying formation reservoirs near the central sag areas and show close affinities to the interbedded or intercalating es 1 calcareous mudstones and oil shales deposited in a slightly saline to dominantly brackish water lacustrine setting the sl and fl family oils are generally mature whereas most of the bl family oils usually show low maturity oil source correlation demonstrates strong genetic relationships between the mature es 4 and es 3 source rocks of the eocene oligocene shahejie formation in the sag areas and the oils accumulated in the shallow neogene petroleum plays the geochemical data provide strong support for two sharply contrasting hydrocarbon migration scenarios 1 short distance lateral migration is dominant in the central sag areas and 2 the fault fracture mesh facilitated vertical migration and coupled with the regional porous guantao formation sand sheets makes the structural stratigraphic and lithological features over basement high blocks the focal points for petroleum accumulation therefore further exploration in this highly mature petroleum province should focus on heavy oils in the shallow neogene strata and light oils in the tight sandstones of the deep sag areas

andean age structure of eastern cordillera province of la paz bolivia a moho root beneath the bolivian andes 40 km deep is consistent with 230 km of overlap of neogene age on a single trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff main andean thrust mat only 10 of the andean crustal volume is ascribable to magmatic addition the mat is located within south american crust of full thickness it intersects the basement top 450 km inland from the neogene crustal margin it is not a collision suture as shown by persistent pre neogene facies continuity thrusting is not accompanied by terrane accretion the present bilaterally symmetrical thrust belt responds to elastic line loading and to coulomb rheology in the hanging wall of the mat a deep high stress wedge base builds a steep critical slope in the footwall the foredeep response is fast subcritical growth by progradation and blind thrusting on a low stress decollement interaction is maintained by out of sequence renewal of movement on the mat

paleomagnetism of upper miocene ignimbrites at the puna an analysis of vertical axis rotations in the central andes the origin of the rotations detected paleomagnetically in the central andes is controversial tectonic models proposed to explain it involve late cenozoic oroclinal bending the bolivian orocline and or small block rotations driven by oblique subduction in this paper we report paleomagnetic data from upper miocene ignimbrites from the puna in the presumed southern limb of the orocline these rocks showed high unblocking temperature and high coercivity magnetization the resulting pole position latitude 85 7 s longitude 80 5 e a95 7 9 k 22 n 17 indicates that the study area has not been significantly affected by vertical axis rotation since late miocene onward this implies that the whole puna has not undergone significant regional rigid body rotation during the same time span a farther analysis based on neogene paleomagnetic data from 31 andean localities distributed from 11 s to 31 s shows the presence of rotated and unrotated areas systematic counterclockwise rotations in peru and northern bolivia are observed whereas clockwise rotations are present in southern bolivia northern chile and perhaps northwestern argentina no systematic rotations are found farther south the overall time spatial distributions of these neogene paleomagnetic data suggest that if orogenic bending occurred it must have taken place before the middle miocene thus the oroclinal hypothesis cannot explain the rotations observed in middle miocene rocks or younger ones which also show orocline like declination anomalies we suggest that small block rotations driven by distributed shear may be a single process that can account for the whole rotations in the central andes it is hypothesized that horizontal shear in the andean crust could be controlled by ancient structures

late cenozoic paleomagnetism and chronology of andean basins of bolivia evidence for possible oroclinal bending new paleomagnetic and radioisotopic data are reported from two late tertiary high elevation non marine basins in the eastern cordillera of bolivia 1 quebrada honda located at 22 s lat consists of a 300 m thick section containing abundant santacrucian or friasian middle miocene fossil mammals this locality is constrained by mean 40 k 40 ar ages of 12 83 0 11 ma and 11 96 0 11 ma and the local magnetostratigraphy is correlated to chrons c5aa through c5a on the magnetic polarity time scale mpts the fossil mammals from quebrada honda have an extrapolated age of about 13 0 to 12 7 ma 2 micana located at 17 s lat consists of a 205 m thick section containing late miocene fossil mammals including a megatheriid sloth and the tiny mesothere microtypotherium cf m choquecotense this locality is constrained by a fission track age determination of 6 9 1 1 ma and the local magnetostratigraphy is correlated to chron 7 on the mpts the fossil mammals from this section have an extrapo

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

organic facies and maturation of jurassic cretaceous rocks and possible oil source rock correlation based on pyrolysis of asphaltenes scotian basin canada abstract this paper establishes source rock characterization and oil oil and oil source rock correlations of selected organic rich shale source rocks and selected light oils and condensates from jurassic and cretaceous formations of the scotian basin multiple source rocks of kerogen types iia oil prone iia iib oil and condensate prone iib condensate and gas prone iii gas prone and iv nonsource are identified in various stratigraphic horizons vitrinite reflectance and t inmax data indicate variable maturation histories in different parts of the basin which are related to differences in lithology related heat conductivity and the presence of older sediments closer to areas of rifting multiple linear regression and cluster analyses of aromatic hydrocarbon ratios reveal two oil families some kerogen type iib rock extracts match many of the light oil and condensates in terms of the aromatic hydrocarbon distributions a second family of oils could not be correlated with any of the analyzed source rock extracts pyrolysis gas chromatography of asphaltenes from the cohasset a 52 and sable island 3h 58 wells are significantly different in n alkane aromatics and phenol content the asphaltene pyrolyzates of samples from venture b 52 and s desbarres o 76 have a high aromatic content low content of n alkanes and high phenol values s desbarres which is typical of a terrestial source they correlate well with one another and also with sable island light oil the n triumph g 43 whole rock and extract asphaltene pyrolyzates are dominated by normal hydrocarbons with aromatic phenolic and thiophenic compounds in low concentrations suggesting an algal source algal type organic matter may have made a contribution to the cohasset petroleum as its asphaltene pyrolyzate is also dominated by normal hydrocarbons

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

thermal maturity of woodford shale gas and oil plays oklahoma usa abstract being a hydrocarbon source rock and having a brittle silica rich lithologic character makes the woodford shale late devonian to early mississippian an important oil and gas shale in oklahoma since 2004 woodford shale plays have expanded from producing primarily thermogenic methane in one geologic province to producing thermogenic methane condensate oil and biogenic methane in four geologic provinces at thermal maturities from mature 0 5 vitrinite reflectance ro to post mature 2 to 3 ro condensate is produced at a thermal maturity up to 1 67 ro oil is produced from naturally fractured silica rich shale biogenic methane is produced in shallow

miocene volcanism and deformation in the western cordillera and high plateaus of south central peru new radiometric ages on tuffs from south central peru support the postulated flare up of volcanic activity during early miocene time in the region of huancavelica julcani and lircay lower miocene rocks lie on folded strata of pre cenozoic age the absence of units of eocene and early oligocene age indicates that this area remained positive after incaic deformation in eocene time conglomerate beds reflecting erosion attendant on the first pulse of late cenozoic quechuan compressive deformation do not appear in the stratigraphic record until after 21 5 m y b p in one section beds of coarse conglomerate are underlain by tuff dated at 18 3 0 6 m y and overlain by tuff dated at 17 3 0 2 m y at another locality beds of conglomerate conformably overlie tuff dated at 19 6 0 8 m y if we incorporate published data from other areas in central and southern peru it appears that quechuan deformation began in early miocene time between 19 5 and 17 m y ago a firm age for the end of the first pulse of quechuan deformation in this region is provided by an ash flow sheet dated at 12 to 12 5 m y that unconformably overlies strata of eocene to early miocene age

the porphyry cu mo au deposit at altar argentina tracing gold distribution by vein mapping and la icp ms mineral analysis the altar porphyry cu mo au deposit occurs in the miocene to early pliocene copper belt of the high andes of western argentina close to the chilean border a cluster of late miocene porphyries intruded a more extensive complex of early miocene rhyolitic to andesitic volcanic and subvolcanic rocks potassic alteration in the central porphyry was partially overprinted by feldspar destructive alteration and finally by argillic to advanced argillic alteration early stockwork veins have been cut successively by quartz pyrite veins tourmaline veins and enargite veins based on correlation analysis of assay results and mapped abundances of these four vein types we estimate that approximately 11 to 26 of the copper in the altar orebody is associated with enargite veins whereas the remainder is associated with the early stockwork veining and potassic alteration gold concentration is low compared with typical cu au porphyries of the andean back arc region but higher than in the giant miocene deposits of chile at an average au cu ratio of 0 14 10 4 by weight across the central porphyry orebody at altar gold is dominantly associated with chalcopyrite in the domain of stockwork veining and potassic alteration small scale assays in combination with petrographic observations and laser ablation inductively coupled plasma mass spectrometry microanalyses show that a significant fraction of the gold in the stockwork veins is enclosed in pyrite as submicroscopic particles attached to inclusions of a bi te pb ag enriched cu fe sulfide phase microscopic grains of native gold also occur along grain boundaries between pyrite and chalcopyrite and rarely as larger particles in enargite veins

source rocks in mesozoic cenozoic continental rift basins east china a case from dongying depression bohai bay basin abstract lake basin evolution microscopic and organic geochemistry characteristics are investigated for the paleocene hydrocarbon source rocks of dongying depression bohai bay basin the upper e 2 3 s 4 and the lower e 2 3 s 3 are balanced filled lake basin sediments which show strong climate driven rhythmicity source rocks of these two units have relatively high organic matter content and are dominated by type i kerogen sedimentary and geochemical characteristics indicate there exist persistent lake water stratification and bottom anoxicity during the deposition process by contrast the middle e 2 3 s 3 is an over filled lake deposit dominated by uniform mudstones lacking obvious transgressive regressive cycles it indicates that the lake water lacks stratification and the bottom water is ventilated source rocks of this unit have relatively low organic abundance and are dominated by types ii and iii kerogen thermal evolution studies show that the upper e 2 3 s 4 has a relatively early and multi stage of hydrocarbon generation process and can generate immature oils while the lower e 2 3 s 3 and the middle e 2 3 s 3 have a relatively late and simple hydrocarbon generation process and mainly generate mature oils in addition the persistent increase of eom toc and total hydrocarbons toc indicates that the expelled hydrocarbons from the middle e 2 3 s 3 are in relatively small amounts compared with the other two units the middle e 2 3 s 3 acts as regional caprocks instead of significant source rocks as previously thought the viewpoint will be of great significance in future exploration

undulatory silver rich polymetallic veins of the castrovirreyna district central peru fault growth and mineralization in a perturbed local stress field strongly undulatory silver veins of the castrovirreyna district central peru formed in a dilatant anastamosing low displacement left lateral fault system during late miocene east west directed compression the steeply dipping veins have multiple orders of undulations as seen in map view and underground mines expose strong dip reversals kinematic indicators from the east northeast to west northwest striking veins record mainly left lateral slip and lesser oblique reverse slip these striation data considered along with vein intersection angles and map patterns indicate that the veins did not form as a simple conjugate fracture set the preferred interpretation is that the curved fracture growth and complex slip distributions developed because of local stress field modifications by the propagating faults and dilating vein segments during regional east west directed compression variably oriented fault striations compound displacement profiles and multiple scales of vein curvature are all characteristics of stress interaction between subparallel veins veins possessing these features may be expected to host oreshoots with a complex geometry that depends not only on local accommodation space produced at dilant jogs but also on the proximity to other veins and changing slip directions

characterization of pore systems in seal rocks using nitrogen gas adsorption combined with mercury injection capillary pressure techniques abstract porous microstructure parameters of seal rock samples originating from different depths within brazilian geological formations were correlated to empirical models which predict the intrinsic permeability mercury injection capillary pressure micp and nitrogen gas adsorption n 2 ga were applied in combination as complementary techniques micp to obtain the porosity values and the size distribution of meso and macropores and n 2 ga associated with the brunauer emmett and teller bet theory to determine the specific surface area s o the barret joyner and hallenda bjh theory was applied to find the size distribution of the micro and mesopores the combination of the micp and n 2 ga curves showed that the samples analyzed present a polymodal pore size distribution psd and a total porosity ranging from 0 33 to 10 44 the s o values measured by n 2 ga were higher than those calculated by micp due to the majority of the samples having a mean pore size of 20 1000 a the intrinsic permeability could also be predicted applying the measured parameters s o psd curves and total porosity in the carman kozeny and series parallel models the ranges of permeability values obtained were 4 09 10 24 4 96 10 21 m 2 and 9 48 10 27 9 14 10 22 m 2 respectively these results were compared with values reported in the related literature and those obtained for four samples submitted to pressure pulse decay permeability pdp tests

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

gravity anomaly across the peruvian andes we carried out gravity surveys along four routes across the peruvian andes one in northern peru one in central peru and two in southern peru two of the routes extend from the coast to the flat lands of the amazon basin across the high andes while the other two routes extend only from the coast to the central high plateau or the altiplano one route through nazca contains 250 data points over a length of 800 km and thus offers one of the best gravity profiles across the andes this profile shows quite asymmetric gravity anomalies associated with the western cordillera and the eastern cordillera in marked contrast with the symmetric pattern of topographic profile using these profiles we constructed crustal models varying layer thicknesses rather than layer densities the crustal thickness has a maximum beneath the western cordillera although the maximum thickness varies from 45 km in northern peru to 55 km in central peru and 65 km in southern peru in contrast to the assumed thickness of 35 km beneath the stable brazilian shield possible effects of lateral variations of crustal and mantle densities tend to reduce the above estimates of maximum thickness the very shallow moho beneath the peruvian coast steeply deepens eastward in correspondence to the steep western slope of the andes the crust is thicker in the western cordillera than in the eastern cordillera the moho beneath the eastern cordillera tends to be relatively flat and shallows abruptly eastward across the sub andes the wadati benioff zone lies in general well below the model south american continental crust leaving a mantle wedge in between a notable exception is the nazca profile where the base of the model south american continental crust is in direct contact with the top of the wadati benioff zone this provides a direct evidence for uplifting of the buoyant slab with an aseismic ridge the nazca ridge at its top which tends to attach itself to the bottom of the continental crust we examined whether or not the load due to mountain topography is balanced with the buoyancy load due to excess mass of the crust in the andes the western cordillera is found to be approximately in isostatic equilibrium but the eastern cordillera is not this contrast in mechanical state and the difference in recent geology suggest that the western cordillera and the eastern cordillera have not uplifted through entirely the same tectonic process

sedimentary evolution of an active margin during middle and upper cretaceous times the north peruvian margin from late aptian up to senonian the sedimentary evolution of the north peruvian margin during upper cretaceous can be summarized as follows

bib sem characterization of pore space morphology and distribution in postmature to overmature samples from the haynesville and bossier shales abstract four haynesville shale and four bossier shale samples were investigated using a combination of scanning electron microscopy sem and broad ion beam bib polishing this approach enables the microstructure and porosity to be studied down to the mesopore size r in the haynesville shale to 1 79 2 26 vr r in the bossier shale this variety within the samples enabled us to study controls on the porosity distribution in these shales visible pores exist as intraparticle pores mainly in carbonate grains and pyrite framboids and as interparticle pores mainly in the clay rich matrix pores in organic matter show a characteristic porosity with respect to the type of organic matter which mainly consists of mixtures of amorphous organic matter and minerals organic laminae and discrete macerals a clear positive trend of organic matter porosity with maturity was found pore sizes are power law distributed in the range of 4 4 m to at least 36 nm in equivalent diameter the differences in power law exponents suggest that a more grain supported coarse grained matrix may prevent pores from mechanical compaction porosities measured in the bib cross sections were significantly lower in comparison to porosities obtained by mercury intrusion porosimetry mip this difference is mainly attributed to the different resolution achieved with bib sem and mip and type of pore network extrapolation of pore size distributions psds enables the bib sem porosity to be estimated down to the resolution of the mip and thus to upscale microstructural observation at the confined space of the bib sem method to bulk porosity measurement these inferred porosities are in good agreement with the mip determined porosities which underpins the assumption that pores segmented in bib sem mosaics are representative of the mip methodology

post oligocene rotations in southern ecuador and northern peru and the formation of the huancabamba deflection in the andean cordillera abstract a paleomagnetic study of more than 410 samples from 41 sites from paleocene to early oligocene volcanic and intrusive formations in southern ecuador and northern peru reveals a pattern of rotations in opposite senses on both sides of the huancabamba deflection the amplitude of the rotations is 25 clockwise north of the deflection and 20 anticlockwise south of it north of the deflection the results do not allow us to ascertain whether the rotation arises from oroclinal bending or from block rotation in a distributed shear south of it the absence of geological evidence for widespread strike slip faulting suggests that the results reflect a rotation of the peruvian margin which could be related to the shortening documented in southern peru and to the uplift of the central andes

tectonic rotations and transcurrent deformation south of the abancay deflection in the andes of southern peru we report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from nazca to cuzco where the general structures of the peruvian andes are strongly offset across the abancay deflection nine new 39ar 40ar ages better constrain the timing of volcanism along the western edge of the western cordillera at the latitude of nazca a mean paleomagnetic result from 22 sites in the lower miocene volcanics does not show significant rotation r 2 3 7 7 of the western margin of the central andean plateau since the early miocene within the western cordillera we sampled three structural blocks bounded to the north by the abancay fault system in the westernmost block a large counterclockwise rotation r 65 0 11 1 is found in mesozoic limestones and paleocene eocene red beds magnitude of rotation decreases toward the east from r 35 6 12 8 in the central block to r 4 5 8 4 south of the town of cuzco the anisotropy of magnetic susceptibility ams recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation we show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations the present study confirms results from the peruvian fore arc showing that rotations are not older than circa 40 ma and likely not younger than circa 20 ma the spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the abancay deflection concomitant with a broad late eocene oligocene oroclinal deformation in southern peru

shale characteristics in the southeastern ordos basin china implications for hydrocarbon accumulation conditions and the potential of continental shales abstract continental shales in china are generally characterised by low thermal maturity and high clay content characteristics that are significantly different from those of marine shales documented in the usa and elsewhere whether such continental shales have commercial hydrocarbon potential as marine shale in the usa is a question for the china petroleum industry recently vertical drills in the lacustrine shales in the yanchang formation produced an average of two to five tons of oil and 1000 3000 m3 of gas per day by reservoir hydraulic fracturing which demonstrates that the continental shales can also achieve viable hydrocarbon production shale and gas samples collected from the chang 7 c7 and chang 9 c9 members yanchang formation in the southeastern ordos basin were examined for geochemical petrologic and gas content analysis the results show that the c7 and c9 members developed a huge volume of organic rich shales with 2 4 wt toc in the deep or semideep lacustrine characterised primarily by type ii kerogen with a relatively low thermal maturity and a vitrinite reflectance ranging from 0 5 to 1 5 ro which decreases from west to east the hydrocarbon product varies with the thermal maturity in the southwestern corner the gas content measured by canister desorption equals 1 15 3 49 m3 t rock and the gas absorption capacity ranges from 3 to 5 m3 t rock whereas in the eastern part the gas content is low and oil production is 1 4 m3 day on average nanometre scale pores and micro fractures are well developed all of this indicates that the continental shales in the southeastern ordos basin might have huge shale hydrocarbon resources however the clay content of the continental shales of c7 and c9 ranging from 40 to 60 of the bulk mineral content are much higher than for gas produced in marine shales which might lead to significant challenges for successful development

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

new temporal constraints on the rotation of the peruvian central andes obtained from paleomagnetism new paleomagnetic data from middle eocene to middle miocene volcanic and plutonic formations of the western cordillera of central peru document a counterclockwise rotation of 11 7 4 this rotation similar to previous results reported for the northern peruvian margin is post middle miocene in age it is thus coeval with the acceleration of the andean uplift and supports isacks model

palaeomagnetic studies of a cretaceous molasse sequence in the central andes coloso formation northern chile the coloso formation is an early cretaceous conglomerate dominated molasse sequence in northern chile the sequence was deposited rapidly and records the unroofing of granodioritic and andesitic source rocks thin sand units throughout the sequence have been studied palaeomagnetically the results are complex but can be related to the diagenesis of arid zone alluvium natural remanent magnetization nrm may be normal or reversed but is usually a combination of the two isolation of magnetic components by vector analysis of thermal and af demagnetization data yields both normal and reversed components which are approximately antiparallel and yield an average pole position at 70 s 192 e this is consistent with late cretaceous data from northern chile but not with the established apparent polar wander apw path for south america it may indicate late cretaceous remagnetization but is considered to be more likely an indication that the present apw path assumes no tectonic rotation and that important relative movements have taken place between northern chile and argentina

lacustrine source rock deposition in response to co evolution of environments and organisms controlled by tectonic subsidence and climate bohai bay basin china three paleogene syn rift intervals from the bohai bay basin the most petroliferous basin in china were analyzed with sedimentological and geochemical techniques to characterize the lateral source rock heterogeneities to reveal the environmental and ecological changes through geologic time and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions the third es3 and first es1 members of the eocene shahejie formation and the oligocene dongying formation ed display widely variable total organic carbon contents hydrogen indices and visual kerogen compositions suggesting changes in organic facies from deep to marginal sediments carefully selected deep lake facies samples from any interval however display fairly uniform biomarker composition these three intervals have distinctly different biomarker assemblages which indicate weakly alkaline freshwater lakes with a moderately deep thermocline during es3 deposition alkaline saline lakes with shallow chemocline during es1 deposition and acidic freshwater lakes with deep unstable thermocline during the deposition of the dongying formation such environmental changes corresponded to changes in subsidence rate and paleoclimate from rapid subsidence and wet climate during es3 deposition through slow subsidence and arid climate during es1 deposition to rapid subsidence and wet climate during ed deposition and resulted in synchronous changes in terrigenous organic matter input phytoplankton community and primary productivity the co evolution of environments and organisms controlled by tectonic subsidence and climate accounted for the deposition and distribution of high quality lacustrine source rocks with distinctly different geochemical characteristics most rift basins experienced changes in subsidence rates and possibly changes in climates during their syn rift evolutions the models constructed in this paper may have important implications for source rock prediction in other lacustrine rift basins

sulfur rich petroleum derived from lacustrine carbonate source rocks in bohai bay basin east china abstract crude oils produced from the zhanhua depression bohai bay basin are high in sulfur and differ from most lacustrine oils in this basin and elsewhere physical properties biomarker ratios and aromatic hydrocarbon assemblages reveal that they are derived from carbonate source rock horizons representative source rock samples with abundant organic matter toc 1 and s 2 5 mg hc g rock were selected for further correlation and their high bitumen contents indicated their indigenousness to saline or carbonate depositional environments the results of oil source rock correlation suggest that these sulfur rich heavy oils originate from es 3 4 carbonates and marls limited to the bonan sag maturity assessments with burial history showed that they were expelled at the early oil generation window high levels of 20s 5 14 17 and 5 14 17 steranes together with low values for the ts ts tm ratio and two methylated aromatic ratios mdr and mpi 1 were observed in these oils sulfur associated with evaporite and carbonate environments is a significant factor not only in controlling the occurrence of biomarker isomers but also for petroleum formation at abnormally low thermal stress levels

the nature of porosity in organic rich mudstones of the upper jurassic kimmeridge clay formation north sea offshore united kingdom abstract analyses of organic rich mudstones from wells that penetrated the upper jurassic kimmeridge clay formation offshore united kingdom were performed to evaluate the nature of both organic and inorganic rock constituents and their relation to porosity in this world class source rock the formation is at varying levels of thermal maturity ranging from immature in the shallowest core samples to mature in the deepest core samples the intent of this study was to evaluate porosity as a function of both organic macerals and thermal maturity at least four distinct types of organic macerals were observed in petrographic and sem analyses and they all were present across the study area the macerals include in decreasing abundance 1 bituminite admixed with clays 2 elongate lamellar masses alginite or bituminite with small quartz feldspar and clay entrained within it 3 terrestrial vitrinite fusinite semifusinite grains and 4 tasmanites microfossils although pores in all maceral types were observed on ion milled surfaces of all samples the pores largely nanopores with some micropores vary as a function of maceral type importantly pores in the macerals do not vary systematically as a function of thermal maturity insofar as organic pores are of similar size and shape in both the immature and mature kimmeridge rocks if any organic pores developed during the generation of hydrocarbons they were apparently not preserved possibly because of the highly ductile nature of much of the rock constituents of kimmeridge mudstones clays and organic material inorganic pores largely micropores with some nanopores have been observed in all kimmeridge mudstones these pores particularly interparticle i e between clay platelets and intraparticle i e in framboidal pyrite in partially dissolved detrital k feldspar and in both detrital and authigenic dolomite are noteworthy because they compose much of the observable porosity in the shales in both immature and mature samples the absence of a systematic increase in organic porosity as a function of either maceral type or thermal maturity indicates that such porosity was probably unrelated to hydrocarbon generation instead much of the porosity within mudstones of the kimmeridge appears to be largely intraparticle and interparticle adjacent to inorganic constituents so the petroleum storage potential in these organic rich mudstones largely resides in inorganic pores

magnetic polarity stratigraphy and mammalian fauna of the deseadan late oligocene early miocene salla beds of northern bolivia the salla beds contain a rich assemblage of deseadan mammals that traditionally has been considered of early oligocene age these deposits located 90 100 km se of la paz bolivia consist of more than 540 m of principally fluviatile clays and silts with numerous interbedded tuffs paieomagnetic samples were collected from 104 sites spaced at stratigraphic intervals of about 6 m as a result of thermal demagnetization at 400 c or greater unambiguous polarities were determined for 76 of the 104 originally sampled sites the pattern of reversals stage of evolution of the fossils and preliminary isotopic dates including a k ar age of 26 4 pm 1 0 ma and zircon fission track ages of ca 22 ma from interbedded tuffs suggest that the salla beds correlate to chrons c10 through c6 of the magnetic polarity time scale or an absolute time interval from about 28 5 to 24 ma pyrotherium and caviomorph rodents occur throughout the section the primate branisella is known to occur at about 27 ma the salla f

paleomagnetic study of cretaceous rocks of peru south america evidence for rotation of the andes abstract paleomagnetic data for the cretaceous volcanic and sedimentary rocks in the andean region of peru are given reliable paleomagnetic field directions were obtained for three cretaceous albian to cenomanian formations from calcareous sediments in northern peru stable remanent magnetization directions were also derived from twelve cretaceous lava flows and dikes in coastal peru paleomagnetic data of the same age from the stable areas of south america such as brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole but peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts this suggests counterclockwise tectonic rotation of an extensive block which includes the whole of andean peru

fractures of lacustrine shale reservoirs the zhanhua depression in the bohai bay basin eastern china abstract fractures play a vital role in the exploration and development of shale oil and gas by providing effective space for shale reservoirs and significantly improving the fluid flow capability core observations microscopic analyses of thin sections scanning electron microscopy and formation microscanner imaging fmi were used to determine the types causes of formation and development characteristics of the fractures in lacustrine shale reservoirs in the lower part of the paleogene shahejie formation es 3 in the zhanhua depression bohai bay basin eastern china x ray diffraction xrd analysis total organic carbon toc measurements and porosity and permeability measurements were used to study the controlling factors of the fractures in the shale reservoirs and to analyze the impact of the fractures on the shale reservoirs properties and subsequent exploration and development the studied shale reservoir mainly displays tectonic fractures as well as various types of non tectonic fractures the non tectonic fractures mainly include over pressure fractures diagenetic fractures inter layer bedding fractures and fractures of mixed origins in the study area the tectonic fractures which were formed under the combined action of tensile and shear stress display the following characteristics the dip angle of the tectonic fractures varies significantly unfilled or half filled effective fractures have a high proportion these fractures are mainly oriented in the ne sw nne ssw and wnw ese directions with fractures in the ne sw direction accounting for the highest proportion the tectonic and non tectonic fracture development is affected by multiple types of factors such as the presence of faults mineral composition lithology abnormal pressure and organic matter content abnormally high pore pressure is a very important factor in the development of non tectonic fracture it is inferred that the over pressure is mainly related to hydrocarbon generation during thermal evolution fractures effectively improve the porosity and permeability of the shale reservoirs and the enhancement of permeability is particularly significant the current stress field affects the fluid flow capability of the fracture reservoirs and the present maximum principal stress in zhanhua depression is oriented in the nee sww direction which has a small angle with fractures in ne sw direction we propose that the fractures in this direction have the greatest connectivity and thus are a high priority target for petroleum exploration and development

geological and geochemical characterization of the lower cretaceous pearsall formation maverick basin south texas a future shale gas resource as part of an assessment of undiscovered hydrocarbon resources in the northern gulf of mexico onshore mesozoic section the u s geological survey usgs evaluated the lower cretaceous pearsall formation of the maverick basin south texas as a potential shale gas resource wireline logs were used to determine the stratigraphic distribution of the pearsall formation and to select available core and cuttings samples for analytical investigation samples used for this study spanned updip to downdip environments in the maverick basin including several from the current shale gas producing area of the pearsall formation the term shale does not adequately describe any of the pearsall samples evaluated for this study which included argillaceous lime wackestones from more proximal marine depositional environments in maverick county and argillaceous lime mudstones from the distal lower cretaceous shelf edge in western bee county most facies in the pearsall formation were deposited in oxygenated environments as evidenced by the presence of biota preserved as shell fragments and the near absence of sediment laminae which is probably caused by bioturbation organic material is poorly preserved and primarily consists of type iii kerogen terrestrial and type iv kerogen inert solid bitumen with a minor contribution from type ii kerogen marine based on petrographic analysis and pyrolysis carbonate dominates the mineralogy followed by clays and quartz the low abundance and broad size distribution of pyrite are consistent with the presence of oxic conditions during sediment deposition the pearsall formation is in the dry gas window of hydrocarbon generation mean random vitrinite reflectance values ro 1 2 2 2 and contains moderate levels of total organic carbon average 0 86 wt which primarily resides in the inert solid bitumen solid bitumen is interpreted to result from in situ thermal cracking of liquid hydrocarbon generated from original type ii kerogen that was prevented from expulsion and migration by low permeability the temperature of maximum pyrolysis output tmax is a poor predictor of thermal maturity because the pyrolysis s2 peaks from rock eval analysis are ill defined vitrinite reflectance values are consistent with the dry gas window and are the preferred thermal maturity parameter a maverick basin pearsall shale gas assessment unit was defined using political and geologic boundaries to denote its spatial extent and was evaluated following established usgs hydrocarbon assessment methodology the assessment estimated a mean undiscovered technically recoverable natural gas resource of 8 8 tcf of gas and 3 4 and 17 8 tcf of gas at the f95 and f5 fractile confidence levels respectively significant engineering challenges will likely need to be met in determining the correct stimulation and completion combination for the successful future development of undiscovered natural gas resources in the pearsall formation

paleomagnetism of the puente piedra formation central peru abstract paleomagnetic samples were collected from 15 sites in the early cretaceous puente piedra formation near lima peru this formation consists of interbedded volcanic flows and marine sediments and represents the oldest known rocks of the andean coastal province in this region the puente piedra formation is interpreted as a submarine volcanic arc assemblage which along with an overlying sequence of early cretaceous clastic and carbonate rocks represents a terrane whose paleogeographic relationship with respect to the peruvian miogeocline in pre albian time is unknown moderate to high coercivities blocking temperatures below 320 c and diagnostic strong field thermomagnetic behavior indicate that pyrrhotite is the dominant magnetic phase in the puente piedra formation this pyrrhotite carries a stable crm acquired during an event of copper mineralization associated with the intrusion of the santa rosa super unit of the coastal batholith at about 90 5 m y b p the tectonically uncorrected formation mean direction of d 343 2 i 28 6 95 3 4 is statistically concordant in inclination but discordant in declination with respect to the expected direction calculated from the 90 m y reference pole for cratonic south america the observed declination indicates approximately 20 of counterclockwise rotation of the puente piedra rocks since about 90 m y this is consistent with other paleomagnetic data from a larger crustal block which may indicate modest counterclockwise rotation during the cenozoic associated with crustal shortening and thickening in the region of the peru chile deflection

basement control on oblique thrust sheet evolution seismic imaging of the active deformation front of the central andes in bolivia abstract in the area of the bolivian orocline we examine the deformation pattern associated with the active development of a new thrust sheet a dense grid of reprocessed 2 d seismic lines from hydrocarbon exploration industry is interpreted and a 3 d simplified structural and kinematic model is deduced in the boomerang hills onlapping paleozoic and foredeep sediments are detached from the underlying s dipping basement they are thrust northeastwards by less than 2 km two zones can be differentiated along the andean deformation front 1 a w e to nw se striking frontal segment of predominantly orthogonal shortening comprising a thrust and anticline system 2 a wsw ene striking lateral zone of oblique shortening within a complex system of thin skinned strike slip faults and minor folds the deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the paleozoic basin the observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography which provides a near oblique ramp for the onlapping wedge of sediments the sw ne thrusting direction is divided into orthogonal and tangential components these are accommodated by convergent and strike slip structures respectively which sole into a common detachment horizon the structural evolution of the new thrust sheet in the bolivian orocline is primarily controlled by the paleorelief of the brazilian shield because 1 the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and 2 small asperities in close to the top of the basement promote fault localization the coincidence of a relatively high basement position and a structural high of the eastern cordillera leads to the conclusion that the shape of the brazilian shield also controls the structural evolution of the pronounced eastern border of the bolivian orocline

late cretaceous late turonian coniacian and santonian petroleum source rocks as part of an oae tarfaya basin morocco abstract late turonian coniacian and santonian source rock samples from a recently drilled well tafaya sondage no 2 2010 in the tarfaya basin were analyzed for quantity quality maturity and depositional environment of the organic matter om to our knowledge such a thick sequence of organic matter rich turonian to santonian source rocks was investigated in that great detail for the first time organic geochemical and organic petrological investigations were carried out on a large sample set from the 200 m thick sequence in total 195 core samples were analyzed for total organic carbon corg total inorganic carbon contents and total sulfur ts contents rock eval pyrolysis and vitrinite reflectance measurements were performed on 28 samples chosen on the basis of their corg content non aromatic hydrocarbons were analyzed on selected samples by way of gas chromatography flame ionization detection gc fid and gc mass spectrometry gc ms the organic matter rich carbonates revealed a high source rock potential representing type i kerogen and a good preservation of the organic matter which is mainly of marine phytoplankton origin hi values are high 400 900 mg g corg and in a similar range as those described for more recent upwelling sediments along the continental slope of north africa ts corg ratios as well as pristane over phytane ratios indicate variable oxygen content during sediment deposition all samples are clearly immature with respect to petroleum generation which is supported by maturity parameters such as vitrinite reflectance 0 3 0 4 tmax values 401 423 c production indices s1 s1 s2 0 1 as well as maturity parameters based on ratios of specific steranes and hopanes

vertical axis rotations across the puna plateau northwestern argentina from paleomagnetic analysis of cretaceous and cenozoic rocks between 10 s and 30 s the central andes are marked by both a major topographic anomaly the altiplano puna plateau and a westward concave geometry whose origin remains controversial the arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes indeed in the central andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc throughout peru northernmost chile and northern bolivia and clockwise rotations on the southern limb throughout southern bolivia northwestern argentina and northern chile to fill a gap in data from northern argentina and to contribute to the ongoing debate on the origin of rotations in the central andes we have undertaken a paleomagnetic study of 373 cores taken at 29 sites grouped into seven localities the samples are from sediments and lava flows of cretaceous to tertiary age located in intermontane basins of the puna plateau in northwestern argentina vertical axis rotations calculated from paleomagnetic declinations are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central andes however significant variations in the amount of rotation occur from one locality to another suggesting that they are at least in part influenced by local tectonics as most faults in the puna plateau have reverse dip slip components we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults whether or not this mechanism has operated across the entire area of thickened crust in the central andes remains to be demontrated even if such faulting has locally influenced rotations cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central andes

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

a tale of two continents some tectonic contrasts between the central andes and the north american cordillera as illustrated by their paleomagnetic signatures comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large scale tectonic processes comparison of the paleomagnetic signatures of the western cordillera of north america and the central andes shows that the western edges of these belts have had profoundly different mesozoic and cenozoic histories specifically the north american cordilleran pattern shows strong evidence of post middle cretaceous relative northward displacement of outboard crustal blocks but there is almost no comparable evidence of margin parallel displacement in the andes we speculate that this may largely be a consequence of a simple difference in shape the convex westward western margin of north america facilitates margin parallel displacement as a response to oblique subduction whereas the concave westward margin of south america inhibits it the patterns of block rotations found along the western edge of the two orogens also are quite different nearly everywhere within the western north american cordillera crustal blocks have rotated clockwise since mid cretaceous time reflecting a pervading state of dextral shear within the western andes of peru and northern chile however mesozoic and cenozoic rocks in peru and northernmost chile are rotated strongly counterclockwise whereas rocks of the same age in the remainder of chile to about latitude 48 s are rotated clockwise a model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations

natural sealed fractures in mudrocks a case study tied to burial history from the barnett shale fort worth basin texas usa abstract the mississippian barnett shale texas usa consisting of organic rich shales and limestones hosts the largest gas fields of north america this study examines sealed fractures from core and outcrop samples of the barnett shale of the fort worth basin and aims to 1 characterize the phases occurring in the fractures from samples having experienced different burial histories 2 establish a paragenetic sequence to relate the timing of fracture origin and sealing with the burial history of the basin and 3 contribute to the understanding of the mechanisms of fracture formation in shales including overpressure origin four fracture generations were distinguished in the most deeply buried core samples by characterizing the sealing minerals petrographically and geochemically the generations were inserted into the framework of a reconstructed burial history for the fort worth basin which allowed a time sequence for fracture development to be established this in turn allowed inference of conditions of fracture development and consideration of fracture mechanisms as well as the origin of the parent fluids of sealing minerals type 1 fractures formed during early mechanical compaction at a few 10 s to 100 m of depth of still not fully cemented sediments type 2 fractures formed during moderate burial 2 km from slightly modified seawater their timing is consistent with overpressure generated during rapid deposition and differential compaction of pennsylvanian lithologies during the onset of the ouachita compressional event type 3 fractures formed during deep burial 3 km from silica rich basinal brines possibly derived from clay diagenesis type 4 fractures formed at very deep burial 4 km from hot and 18 o rich fluids carrying light oil 20 30 api and record the opening of the fluid system after hydrocarbon migration differences are highlighted between the timing and thermal regimes under which fractures formed in barnett lithologies from different areas of the basin this suggesting that extrapolation of outcrop observations to subsurface must be used with due care

lower bound on the amount of crustal shortening in the central bolivian andes balanced cross sections across the cordillera oriental and subandean zone of the central bolivian andes indicate that crustal shortening probably has played the dominant role in orogeny in this convergent margin setting a minimum amount of shortening 210 km is documented which can account for two thirds of the present day crustal cross sectional area along a transect spanning the entire mountain range substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere a large minimum amount of crustal shortening in the bolivian andes shows contrary to common assumptions about orogeny that 1 magmatic addition may be volumetrically less important in orogeny in andean type margins and 2 crustal shortening is not uniquely associated with continental or island arc collision

tectonic rotations on the leading edge of south america the bolivian orocline revisited cretaceous paleomagnetic poles for south america form three distinct groups one each for the andes north and south of arica northernmost chile and one for the craton relative to the cratonal group the northern andean group is displaced in a counterclockwise sense and the southern andean group in a clockwise sense these data have been used to support a model of orodinal bending of the andes but distributed shear caused by oblique subduction is a more likely mechanism

magnetic characterization and mossbauer spectroscopy of magnetic concentrates from greek lake sediments abstract bulk magnetic properties of three greek lake sediments indicated that the main magnetic constituent was predominantly pseudo single domain magnetite this was confirmed by successfully extracting and directly identifying the magnetite the majority of the magnetite in the magnetic concentrates is present as grains of about 2 3 m in size the overall size ranging from with occasional grains up to 20 m the grains are roughly equidimensional and angular in form indicating that they are of primarily detrital origin although an authigenic origin for the very fine grains must be a possibility mossbauer spectra of the magnetic concentrates suggest that either the magnetite is slightly non stoichiometric to a similar degree in the three lakes or contains impurity ions haematite was also found in the concentrates but its abundance is only 10 15 that of magnetite and its is not detectable in any of the magnetic properties of the sediments

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

correction criteria for the suppression of vitrinite reflectance in hydrogen rich kerogens preliminary guidelines abstract thermal maturity is an important factor when assessing the petroleum potential of a basin or particular source rocks although the vitrinite reflectance vr technique provides excellent results from coal samples it has limitations when source rocks are used one problem is the anomalously low vrs the suppression of vr vr suppression in hydrogen rich kerogens the anomalously low suppressed vrs discussed in this article are mainly caused by the problems that organic petrographers encounter in finding and measuring appropriate vitrinite particles in hydrogen rich kerogens vr suppression is recognized when lower than normal vrs are measured on exinite rich or amorphous rich intervals in a vr well profile vr vs depth it can also be recognized when anomalously low vrs relative to the regional coalification maturation pattern or to regional oil gas occurrences are encountered in an interval although vr suppression in organic rich rocks coals and oil shales is well documented guidelines for its correction are lacking a preliminary model fig 1 is presented to help correct the vr suppression in hydrogen rich kerogens measured suppressed vr in r o along with original when immature hydrogen index hio estimated via rock eval pyrolysis and or organic petrography data of a sample can be used to estimate the maximum true unsuppressed vr although the vr and hio data for an individual sample is sufficient to estimate the maximum true vr we recommend the use of data from several closely spaced samples the model was tested with examples from published data and found to reasonably correct vr suppression in hydrogen rich kerogens we plan to refine the model using data from rock samples suppressed unsuppressed pairs and pyrolysis data

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we first re examined the apparent polar wander curves for stable eurasia and africa since the triassic these curves were then combined together with curves of north and south america according to the kinematics of the atlantic ocean and a synthetic polar wander curve was given then most of the paleomagnetic results from the tethys mobile belt from the atlantic to the pamirs were analysed several groups of plates microplates and blocks can be seen first relatively stable regions like maghreb and sicily which have not moved much then we have a group formed by iberia sardinia italy and to a lesser extent corsica and the western and central alps for these blocks movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of africa but they are sometimes stronger to the east a major change takes place the north of the aegean sea and the ionian zone are clockwise rotated and these rotations are recent oligocene miocene for the first part pliocene to the present for the second part a major problem arises in turkey caucasus and iran paleomagnetic results indicate a position far to the south of eurasia and at the same time geological evidence is in favour of a position close to eurasia we discuss these discrepancies

paleomagnetic evidence for rapid vertical axis rotation in the peruvian cordillera ca 8 ma paleomagnetic results from 31 neogene sites in the peruvian andes yield primary magnetizations as demonstrated by positive fold and reversal tests strata dated as 18 9 ma record a significant counterclockwise rotation 11 5 whereas unconformably overlying younger strata 7 6 ma are not rotated the age of rotation thus is between 9 and 7 ma a period that coincides with the widespread quechua 2 deformation phase moreover eight independent studies on 107 9 ma rocks from peru between 9 s and 15 s reveal similar and significant rotations 15 6 this suggests that the region rotated during a 2 m y period of deformation ca 8 ma when the andes underwent rapid uplift and important deformation commenced in the subandean zone

fault fracture mesh petroleum plays in the zhanhua depression bohai bay basin part 1 source rock characterization and quantitative assessment abstract in the prolific zhanhua depression of the bohai bay basin eastern china over 76 of the proven petroleum reserves occur in the neogene guantao and minhuazhen formations detailed geological and geochemical mapping of the phanerozoic strata in this area reveals that oil shales and dark mudstones in the es4 es3 and es1 members of the eocene oligocene shahejie formation have excellent potential as petroleum source rocks regional distributions of total organic carbon content kerogen type and thermal maturation indicate that abundant mature source rocks are present in several discrete sag areas bounded by extensive basement related fault networks in contrast source rocks along the paleo topographic highs are currently immature with respect to the generation of conventional oils this study examined the key molecular characteristics for the common isoprenoid hopanoid and steroid biomarkers in relation to source facies variations in lacustrine sediments the hydrocarbon source potential and expulsion efficiency for each of the source rock units were evaluated which integrated the source rock characteristics with regional thermal subsidence and sediment burial an important observation made during this study is the striking contrast between the source potential and proven oil reserves within each stratigraphic section illustrating how significant the migration of hydrocarbon fluids derived from deep mature source kitchens through over 2000 m of fine grained formations must have been to form several giant oil accumulations in the shallow strata these results support the concept of the neogene fault fracture mesh petroleum plays proposed by zhang et al 2004 marine petroleum geology 21 651 668 and the vital role that the faults and fracture systems have in controlling hydrocarbon migration conduits and accumulation habitat in this rift basin

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

rotated oceanic blocks in western ecuador a paleomagnetic study of over 250 cores from 21 sites sampled over a distance exceeding 250 km in the early cretaceous pinon formation in the costa of western ecuador indicates that the sampled lithologies essentially morb type basalts and dolerites carry a stable remanent magnetization whose direction significantly deviates from that of coeval formations of stable south america although both normal and reverse directions have been recorded the two polarities cannot easily be distinguished because of the low latitude of the ocean floor at the time of formation the most likely interpretation is that the entire area has undergone 70 clockwise rotation since its genesis these results thus yield new evidence for an allochtonous origin of this suspect terrane

source rocks and petroleum systems in the hungarian part of the pannonian basin the potential for shale gas and shale oil plays abstract the objectives of our study were to assess the thickness lateral extent organic richness and maturity of the potential source rocks in hungary and to estimate the volumes of hydrocarbons generated in order that potential shale gas and shale oil plays could be identified and characterised the upper triassic kossen marl in south west hungary could represent the best potential shale gas shale oil play due to its high organic richness high maturity and the presence of fracture barriers the area of gas and oil generative maturity is around 720 km 2 with the unexpelled petroleum estimated to be up to 9 billion barrel oil equivalent the lower jurassic sediments of the mecsek mountains and under the great plain contain fair quality gas prone source rocks with low shale gas potential except for a thin toarcian shale unit which is richer in organic matter the latter could form a potential shale gas play under the great hungarian plain if it is thicker locally the lower oligocene tard clay in north east hungary could represent the second best potential shale oil play due to its organic richness favourable maturity and large areal extent 4500 km 2 with around 7 billion barrel oil equivalent estimated in place volume of petroleum middle miocene marine formations could represent locally developed shale gas plays they have fair amounts of organic matter and a mixture of type ii iii kerogen but their vertical and lateral variability is high the upper miocene lacustrine endr d marl contains less organic matter and the kerogen is mainly type iii which is not favourable for shale gas generation the high carbonate and clay content plus the lack of upper and lower fracture barriers would represent additional production challenges

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

paleomagnetisme dans les hellenides internes analyse des aimantations superposees par la methode des cercles de reaimantation there are very few reliable and well dated paleomagnetic data on the internal hellenides which were strongly affected by successive tectonic events we have analysed 268 samples from two separate sedimentary formations of this domain these correspond to two important moments in the geotectonic evolution of the hellenides the toarcian and the jurassic cretaceous boundary we show that all the analysed samples probably contain both detrital magnetite and secondary haematite and generally carry at least two components of superimposed magnetization the diversity and convergence of the remagnetization circles allow us to show that these two components are antiparallel and to determine their directions because of tectonic disruption the calculated declinations are only of local interest on the other hand inclinations remain significant for calculating paleolatitudes these constitute important constraints on reconstructions of the jurassic paleogeography of the hellenic domain in light of our results

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

bib sem study of the pore space morphology in early mature posidonia shale from the hils area germany abstract this contribution reports on the study of the pore space morphology in two early mature vr r 0 59 and 0 61 samples of posidonia shale from the hils syncline in germany using broad ion beam bib polishing and high resolution scanning electron microscopy sem this allows imaging pores with resolution down to 10 nm in equivalent diameter d eq and quantitative estimation of porosity using a combination of bse and se detectors and semi automatic segmentation of the gigapixel images the representative elementary area of the samples on the scale of a few mm is inferred to be about 140 140 m 2 pore morphologies and pore sizes are clearly related to the mineral fabric with large differences very large typically several microns pores with internal faceted crystal morphology in recrystallized calcite clasts and smaller pores d eq cf schizosphaerella nanofossils typically 200 nm pores are less common in organic material and in pyrite aggregates pore characteristics are very similar for both samples and porosity resolvable by bib sem is 2 75 and 2 74 pore size distribution can be described by a power law with an exponent about 2 0 and 2 2 respectively for the pore population excluding the fossils pores in the carbonate fossils show dual power law distribution with power law exponents of about 1 and 3 by extrapolating the power law distribution for each sample total porosity is estimated to be 4 8 0 9 1 7 and 6 5 2 7 7 2 respectively this estimate can be compared with 3 4 3 7 and 3 3 3 6 as measured by mercury injection porosimetry we interpret this difference to reflect the unconnected for mercury part of the porosity comparison of imaged pores and mercury injection porosimetry suggest a very high pore body to pore throat ratio this results in a pore model where large pores represented mainly by pores in fossils and calcite grains are connected via a low porous and low permeable clay rich matrix with pore throats below 10 nm in both samples