87sr86sr in late proterozoic carbonates evidence for a mantle event at 900 ma ago abstract strontium isotopic measurements were made on late proterozoic carbonates from west african craton comparison of samples with acceptable trace element patterns with coeval data from southern africa and with the published australian results suggests that the 87 sr 86 sr ratio of the late proterozoic sea water evolved in the following manner about 0 7075 at 1000 50 ma 0 7056 to 0 7074 at 900 50 ma 0 7068 to 0 7091 0 7106 at 800 50 ma 0 7074 to 0 7077 at 700 50 ma and 0 7076 to 0 7089 0 7096 at 600 50 ma ago the variations are comparable in magnitude and frequency to those described previously for the phanerozoic strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of sr into late proterozoic oceans was of comparable isotopic composition to its present day counterpart 0 711 consequently the non radiogenic 87 sr 86 sr value at 900 50 ma ago signifies a large flux of mantle strontium into the ocean at this time because the present time resolution is only about 75 25ma additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed late proterozoic sea water curve

evidence for ophiolite obduction terrane accretion and polyorogenic evolution of the north scandinavian caledonides abstract recent work on the thrust sheets of the ofoten troms region north norwegian caledonides has resulted in revision of the tectonostratigraphic correlations providing new constraints on models for terrane accretion and poly orogenic cambro ordovician and siluro devonian development previous studies on late ordovician early silurian fossils in metasediments unconformably overlying the lyngen ophiolite complex within one of the troms nappes documented a period of ophiolite obduction prior to the major siluro devonian scandian orogenic event correlation of these units and underlying and overlying thrusts sheets southward into the ofoten area 150 km to the south is demonstrated this implies that an extensive completely transported upper ordovician lower silurian basin developed on top of the obducted ophiolite composes the bedrock of much of northern norway metamorphism and deformation within these basinal rocks resulted from the scandian phase of the caledonian orogeny the relatively weak metamorphic and deformational imprint on the ophiolitic basement provides a rare opportunity to evaluate the pre scandian tectonic evolution of northern scandinavia poly orogenic evolution a controversial subject in northern norway is documented the character and relative extent of cambro ordovician and siluro devonian orogenic zones in northern norway are elucidated the balsfjord ofoten unconformity formed approximately concomitant with other post ophiolite obduction unconformities in central and southern norway implying an orogen wide late cambrian early ordovician event

scandinavian caledonide metallogeny in a plate tectonic perspective caledonian metallogeny has endowed scandinavia with abundant metalliferous mineral resources of several genetical and compositional types many of which have been exploited at various scales from the seventeenth century onwards because of this long history of exploration exploitation coupled with excellent exposures the caledonian orogen in scandinavia can be taken as a model to illustrate the relationship between metallogenic evolution and plate tectonics its orogenic and metallogenic development can be related to a period of plate movements which started about 700 600 ma ago with rifting and break up of a proterozoic megacontinent rodinia followed by the opening up of a wide ocean iapetus and ending with collision between the laurentian and baltic continents the climactic scandian phase of orogeny in silurian times fragments of this sedimentary magmatic and tectonic history are recorded in various autochthonous and allochthonous units which were ultimately thrusted eastwards above the baltic plate margin

age and significance of sequence boundaries on jurassic and early cretaceous rifted continental margins the age and significance of sequence boundaries on jurassic to early cretaceous rifted continental margins in three ocean basins have been documented the margins are the santos basin in the south atlantic the grand banks in the north atlantic and the beaufort sea in the arctic ocean large industry data bases were used for the interpretation of each area megasequence boundaries separate the major phases of basin evolution for example syn rift and post rift boundaries developed with an average periodicity of 49 m y sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10 15 m y out of 27 total boundary ages most 16 are developed on just one margin only two possible age ranges overlap on all three margins ighty percent of the megasequence boundaries and 50 of the sequence boundaries show a direct causal connection with coeval faulting and or folding the rest of the boundaries appear as unstructured surfaces separating transgressive and or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence sediment input and long term eustatic sea level these data do not support theories advocating synchronous worldwide boundary development resulting from periodic short term falls in global eustatic sea level only in like basins of the same age with identical subsidence and sediment input rates are boundaries likely to develop synchronously hence the concept of global synchroneity of sequence boundary development may well be an illusion creat d by the similarity in age of the majority of basins studied as a result of this study it seems wise discard the global approach to basin analysis

laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure an interpretation of the electrical signature of molten rocks within the earth s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts this paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous 1 3 wt h2o metaluminous obsidians determined in an internally heated pressure vessel in the range of 50 400 mpa and 350 1325 c it is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an arrhenius law applying in the melt and the glass regions this suggests that a similar transport mechanism operates in both melt and glass the determined activation energies are 70 kj mol for the dry obsidian and 65 and 61 kj mol for the 1 and 3 wt h2o melts respectively the activation volume is 20 cm3 mol combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians the temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for na mobility in dry and hydrous rhyolites an increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium as a prospective it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization therefore its electrical signature must reveal its nature and maturity

new gas geothermometers for geothermal exploration calibration and application calibration of five gas geothermometers is presented three of which used co2 h2s and h2 concentrations in fumarole steam respectively the remaining two use co2h2 and h2sh2 ratios the calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures after establishing the gas content in the aquifer gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions it is shown that the concentrations of co2 h2s and h2 in geothermal reservoir waters are fixed through equilibria with mineral buffers at temperatures above 230 c epidote prehnite calcite quartz are considered to buffer co2 two buffers are involved for h2s and h2 and two functions are therefore presented for the geothermometers involving these gases for waters containing less than about 500 ppm chloride and in the range 230 300 c pyrite pyrrholite epidote prehnite seem to be involved but pyrite epidote prehnite magnetite or chlorite for waters above 300 c and waters in the range 230 300 c if containing over about 500 ppm the gas geothermometers are useful for predicting subsurface temperatures in high temperature geothermal systems they are applicable to systems in basaltic to acidic rocks and in sediments with similar composition but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones the geothermometry results may be used to obtain information on steam condensation in upflow zones or phase separation at elevated pressures measured aquifer temperatures in drillholes and gas geothermometry temperatures based on data from nearby fumaroles compare well in the five fields in iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected the results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled icelandic fields

resistivity structure of the upper crust in iceland the results of numerous resistivity soundings from all parts of iceland are analyzed the resistivity and the p wave velocity of the uppermost kilometer in the crust show a strong correlation with the geological structure of iceland by estimating porosity from p wave velocity archie s law is found not to be valid in general the temperature dependence of nine water saturated basaltic rock samples was investigated in a laboratory showing stronger temperature dependence than expected from pore fluid conduction these two facts suggest that interface conduction is the most important conduction mechanism a relation between bulk resistivity fracture porosity pore fluid resistivity and temperature is established comparison of regional bulk resistivity with different zeolite zones shows that the resistivity is mainly a function of the maximum depth of burial of the lava pile emphasizing the importance of clay minerals and zeolites for the measured bulk resistivity

tectonic history of northern alaska canadian arctic and spitsbergen regions since early cretaceous the correlation of interregional unconformities structures and magmatic activity suggests tectonic events in the arctic were controlled by the early cretaceous collision and combined interaction of the eurasian and north american plates in northeastern ussr the late mesozoic cenozoic accretion of exotic terranes in the north pacific and north atlantic arctic sea floor spreading contemporaneous with the accretion of exotic terranes in northeastern ussr and interior alaska the brooks range developed as a cordilleran mountain belt at the same time during the cretaceous between 125 and 85 ma the canada basin spread north of alaska about a pole of rotation located southeast of the mackenzie delta the alaskan plate was first a part of the northwestern nose of the north american plate when the canada basin spreading ceased the basin was severed by movement along the kaltag and associated strike slip faults and became a part of the northeasterly moving eurasian plate the kaltag fault system is inferred to extend from the western bering sea at the northern end of the kuril arc through alaska and along the outer margin of the arctic islands to the north of greenland it is postulated to have formed the transform boundary between the north american and eurasian plates with the alaskan plate bonded to the latter since the late cretaceous the late eocene change in the direction of pacific spreading terminated the subduction related laramide orogeny as well as the right lateral displacements of major cordilleran faults such as the tintina and denali faults changing eurasian plate movement may have caused most of the strong middle eocene right lateral displacement along the kaltag fault zone continuing displacement from the southwest caused the development of a late cenozoic foldbelt in coastal northeastern alaska herein named the romanzof disturbance coeval with balancing right lateral movement along the kaltag fault zone the huge undrilled marsh creek anticline is a part of this foldbelt seismic data show evidence of middle eocene compression followed by weak extension east of the kaltag fault zone in the beaufort sea the same activity is evident at the northeastern end of the north american plate at nares strait located between the arctic islands and greenland eocene fault movement along the kaltag nares strait and de geer separating greenland and spitsbergen transforms is regarded as complementary the presence of eocene thrust zones located on the western side of nares strait and in western spitsbergen suggests that northward movement of the greenland plate formed both the transforms and the thrust belts

forward seismic modeling as a guide improving detailed seismic interpretation of deltaic systems example of the eocene sobrarbe delta outcrop south pyrenean foreland basin spain as a reference to the analogous subsurface albian cenomanian torok nanushuk delta of the colville basin npra usa abstract a forward seismic model of the middle eocene sobrarbe delta outcrop south pyrenees spain has been made to investigate its seismic signature the sobrarbe delta complex corresponds to a syntectonic sedimentary wedge locally thicker than 1000 m this 15 km long highly progradational and moderately aggradational sedimentary system presents sigmoid and oblique clinoforms characterized by slopes ranging from 0 2 to 5 7 along the clinoforms the classical deltaic depositional environments grade progressively from one to the other alluvial plain delta plain delta front and prodelta locally specific carbonate deposits occur corresponding to transgressive pulses the transition from delta front to prodelta is locally marked by large slide scars affecting clinoform morphology a detailed outcrop transect of the sobrarbe delta was converted to an acoustic impedance model by assigning petrophysical properties to each geological facies this model was then used to create synthetic seismic lines characterized by various input wavelets the resulting various peak frequency synthetic seismic lines have been interpreted and the seismic interpretation compared to the outcrop description to calibrate the seismic interpretation and assess the limits of that interpretation this work shows that the seismic stratigraphy methodology is consistent and allows the correct interpretation of most seismic sequences however composite reflections from rapid lateral facies transitions and higher frequency sequence boundaries can clearly lead to seismic interpretation mistakes the seismic facies of seven facies associations have been described providing guidelines to recognize them on actual seismic lines the synthetic seismic image of four selected systems tracts comprising good reservoir bodies has been analyzed to highlight recognition criteria which can be used to interpret analog deltaic systems in the subsurface and therefore enhance the prediction and characterization of the reservoir bodies that they hold these guidelines were systematically applied to a direct subsurface analog the torok nanushuk delta colville basin npra usa

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

the hengill geothermal area iceland variation of temperature gradients deduced from the maximum depth of seismogenesis abstract given a uniform lithology and strain rate and a full seismic data set the maximum depth of earthquakes may be viewed to a first order as an isotherm these conditions are approached at the hengill geothermal area s iceland a dominantly basaltic area the likely strain rate calculated from thermal and tectonic considerations is 10 15 s 1 and temperature measurements from four drill sites within the area indicate average near surface geothermal gradients of up to 150 c km 1 throughout the upper 2 km the temperature at which seismic failure ceases for the strain rates likely at the hengill geothermal area is determined by analogy with oceanic crust and is about 650 50 c the topographies of the top and bottom of the seismogenic layer were mapped using 617 earthquakes located highly accurately by performing a simultaneous inversion for three dimensional structure and hypocentral parameters the thickness of the seismogenic layer is roughly constant and about 3 km a shallow aseismic low velocity volume within the spreading plate boundary that crosses the area occurs above the top of the seismogenic layer and is interpreted as an isolated body of partial melt the base of the seismogenic layer has a maximum depth of about 6 5 km beneath the spreading axis and deepens to about 7 km beneath a transform zone in the south of the area beneath the high temperature part of the geothermal area the maximum depth of earthquakes may be as shallow as 4 km the geothermal gradient below drilling depths in various parts of the area ranges from 84 9 ckm 1 within the low temperature geothermal area of the transform zone to 138 15 ckm 1 below the centre of the high temperature geothermal area shallow maximum depths of earthquakes and therefore high average geothermal gradients tend to correlate with the intensity of the geothermal area and not with the location of the currently active spreading axis

north slope alaska source rock distribution richness thermal maturity and petroleum charge four key marine petroleum source rock units were identified characterized and mapped in the subsurface to better understand the origin and distribution of petroleum on the north slope of alaska these marine source rocks from oldest to youngest include four intervals 1 middle upper triassic shublik formation 2 basal condensed section in the jurassic lower cretaceous kingak shale 3 cretaceous pebble shale unit and 4 cretaceous hue shale well logs for more than 60 wells and total organic carbon toc and rock eval pyrolysis analyses for 1183 samples in 125 well penetrations of the source rocks were used to map the present day thickness of each source rock and the quantity toc quality hydrogen index and thermal maturity tmax of the organic matter based on assumptions related to carbon mass balance and regional distributions of toc the present day source rock quantity and quality maps were used to determine the extent of fractional conversion of the kerogen to petroleum and to map the original toc toco and the original hydrogen index hio prior to thermal maturation the quantity and quality of oil prone organic matter in shublik formation source rock generally exceeded that of the other units prior to thermal maturation commonly toco 4 wt and hio 600 mg hydrocarbon g toc although all are likely sources for at least some petroleum on the north slope we used rock eval and hydrous pyrolysis methods to calculate expulsion factors and petroleum charge for each of the four source rocks in the study area without attempting to identify the correct methods we conclude that calculations based on rock eval pyrolysis overestimate expulsion factors and petroleum charge because low pressure and rapid removal of thermally cracked products by the carrier gas retards cross linking and pyrobitumen formation that is otherwise favored by natural burial maturation expulsion factors and petroleum charge based on hydrous pyrolysis may also be high compared to nature for a similar reason

evidence for biodegradation and evaporative fractionation in west sak kuparuk and prudhoe bay field areas north slope alaska abstract oils from the west sak kuparuk and prudhoe bay fields on the north slope of alaska display geochemical evidence for a complex petroleum filling history that includes multiple sources and alteration by evaporative fractionation and biodegradation source specific biomarkers in west sak oils indicate generation from the same source rocks in approximately the same proportions as prudhoe bay field oil consistent with the hypothesis that oil spilled from the deeper prudhoe accumulation and remigrated about 10 miles laterally and 5000 feet vertically into the west sak field west sak oils are moderately biodegraded but exhibit anomalously high concentrations of gasoline range saturate and aromatic compounds interpreted as a secondary gas condensate charge that arrived after biodegradation of the spilled prudhoe oil the shallowest west sak oils have less secondary gas condensate charge lower api gravities and reduced concentrations of methylcyclopentane benzene cyclohexane 3 methylhexane heptane methylcyclohexane and toluene relative to the deepest west sak zone a oils the carbon isotopic compositions of these c6 and c7 compounds in the shallowest west sak reservoirs are heavier than those of the deepest west sak oil reservoirs by up to 3 and the isotopic compositions of saturate compounds are altered more than those of aromatic compounds the heavier isotopic compositions of c6 and c7 compounds in the shallow west sak reservoirs are interpreted to result from a kinetic isotope shift caused by light biodegradation of the secondary gas condensate charge west sak field solution gas is also interpreted as biodegraded based upon its dryness 98 methane relatively high isobutane n butane ratio the presence of isotopically heavy carbon dioxide and isotopically light methane and propane that is isotopically heavier than either ethane or butane the isotopic compositions of methane and carbon dioxide in west sak gas suggest that biogenic methane was generated during co2 reduction under anaerobic conditions the isotopic compositions of c6 and c7 compounds in the deepest least biodegraded west sak oils are nearly identical to those of the underlying oils from the kuparuk field but differ from the prudhoe bay field oils the kuparuk field is therefore postulated as the source of the secondary gas condensate charge observed in oils from the west sak field ratios of toluene heptane and heptane methylcyclohexane in kuparuk oils provide independent evidence for loss of gas and condensate from the kuparuk reservoir by the process of evaporative fractionation and the oils subjected to the most evaporative fractionation are isotopically heavier in the c6 c7 range the isotopic compositions of west sak zone a c6 and c7 hydrocarbons suggest that a slight 0 8 isotopic fractionation resulted from evaporative fractionation of the gas condensate charge from the kuparuk field oil accumulation north northeast striking faults that connect the kuparuk and west sak reservoirs provide a potential migration pathway for the secondary gas condensate charge through 3000 feet of intervening shale this model of west sak oil biodegradation subsequent secondary gas condensate charge and further biodegradation has implications for development of north slope west sak oil reserves because heavily biodegraded west sak oils that lack a secondary gas condensate charge may be too viscous to develop by conventional waterflooding methods

the application of an indirect electromagnetic geothermometer to temperature extrapolation in depth we study the feasibility of the application of an indirect em geothermometer developed recently to the temperature extrapolation in depth using magnetotelluric data collected in the seismically active northern tien shan faulted area bishkek geodynamic test site kyrgyzstan and hengill geothermal zone iceland the approach used is based on the artificial neural network technique which does not imply the prior knowledge of the electrical conductivity mechanisms on the one hand and provides temperature estimates based on the analysis of the implicit conductivity temperature relations on the other the samples for neuronet teaching consisted in the well temperature records and electrical conductivity values determined for the same depths from the magnetotelluric data measured in the vicinities of eight boreholes in each testing area the testing of the taught neuronets was carried out using the temperature records not involved in the teaching process the results indicate that the temperature extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth in particular in extrapolation to a depth twice as large as the well depth the relative error is 5 6 and in case of threefold excess the error is around 20 this result makes it possible to increase significantly the depth of indirect temperature estimation in the earth s interior in particular for geothermal exploration based on the available temperature logs the practical application of an indirect electromagnetic geothermometer could provide the following facilities 1 more exact temperature estimation in the extrapolation mode 2 remote temperature estimates in the boreholes in areas characterized by extreme conditions for conventional geothermometers

a new conceptual model of the icelandic crust in the hengill geothermal area based on the indirect electromagnetic geothermometry abstract application of the indirect electromagnetic geothermometer calibrated by available temperature well logs has enabled the construction of the three dimensional temperature model of the hengill geothermal area iceland up to a depth of 20 km its analysis showed that the background temperature of the icelandic crust above 20 km does not exceed 400 it is overlapped by a network of interconnected high temperature low resistive channels which braid through the crust mainly at a level of 10 15 km and root to a depth greater than 20 km accordingly the probable heat sources feeding the geothermal system are supposed to be the intrusions of the hot partially molten magma upwelling from the mantle through faults and fractures the comparison between the vertical temperature cross sections and the projections of the earthquake hypocenters showed that they all are located in the areas where temperature does not exceed 400 which is a gabbro solidus in a silica rich icelandic crust joint analysis of the temperature model together with the resistivity and residual bouguer gravity anomalies enabled us to explain the distribution of the earthquake hypocenters by different geothermal regimes in adjacent parts of the area and cooling of large massifs of the partially molten solidified magma beneath seismically active areas basing on the above inferences we suggest a new self consistent conceptual model of the icelandic crust which agrees with most of the previous geophysical results and provides an explanation for the facts that the previous models failed to explain

pre sturtian 800 730ma depositional age of carbonates in sedimentary sequences hosting stratiform iron ores in the uppermost allochthon of the norwegian caledonides a chemostratigraphic approach abstract carbon and strontium isotope chemostratigraphy 52 13ccarb and 18o and 50 87sr 86sr analyses of carbonate components in whole rock samples was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite grade calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations the dunderlandsdalen type iron ores of previously unknown age in the rodingsfjallet nappe complex of the rana region nordland norway the least altered 87sr 86sr 0 70676 and 13c 2 5 to 5 6 values of the marbles dunderland marble 2a in the hanging wall sequence of the stensundtjern iron formation in the rana region are consistent with seawater composition in the time interval 800 730 ma hence the middle cryogenian pre sturtian the least altered sr and c isotopic values obtained from two other marbles units marble 3 and 4 of the rodningsfjallet nappe complex are consistent with the late cryogenian c 660 and 670 700 ma respectively the ages obtained provide the first insight into the depositional time of sediment hosted iron formations of the uppermost allochthon in the north central norwegian caledonides the middle cryogenian age of marbles associated with iron formations in the rana region and those located c 250 km to the north the hafjellet iron ore horizon share similar 87sr 86sr and 13c ratios and hence similar chemostratigraphic ages these iron formations were originally accumulated outside of baltica on a glacially influenced carbonate siliciclastic shelf apparently on a margin of an unknown microcontinent the scandinavian dunderlandsdalen and the hafjellet iron ores of middle cryogenian age 800 730 ma were accumulated in an open marine environment distant from volcanic centres and hence represent an outstanding exception to other reported neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings

the kingak shale of northern alaska regional variations in organic geochemical properties and petroleum source rock quality abstract the kingak shale a thick widespread rock unit in northern alaska that ranges in age from early jurassic through early cretaceous has adequate to good oil source rock potential this lenticular shaped rock unit is as much as 1200 m thick near the jurassic shelf edge where its present day burial depth is about 5000 m kingak sediment transported in a southerly direction was deposited on the then marine continental shelf the rock unit is predominantly dark gray shale with some interbeds of thick sandstone and siltstone the thermal maturity of organic matter in the kingak shale ranges from immature 2 0 r0 in the colville basin toward the south its organic carbon and hydrogen contents are highest in the eastern part of northern alaska south of and around the kuparuk and prudhoe bay oil fields carbon isotope data of oils and rock extracts indicate that the kingak shale is a source of some north slope oil but is probably not the major source

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

depositional setting sequence stratigraphy diagenesis and reservoir potential of a mixed lithology upwelling deposit upper triassic shublik formation prudhoe bay alaska the upper triassic shublik formation within the prudhoe bay field unit north slope alaska is a potentially economic hydrocarbon reservoir comprised of mixed lithology and mineralogy its composition includes limestone phosphate shale siltstone and sandstone as well as accessory amounts of siderite glauconite pyrite kaolinite and dolomite within the prudhoe bay field unit the shublik has been subdivided into four zones lettered from base to top d through a which become thinner and show evidence of deposition under higher energy conditions toward the northeast the formation is truncated to the east by the regional lower cretaceous unconformity zones within the shublik comprise a basal transgressive systems tract conglomerate lag at the shublik formation ivishak formation contact through basal zone c shales and two highstand shallowing upward parasequences zones c through b and zone a respectively the parasequences are bounded by shales interpreted to represent deposition during periods of marine flooding the contact between the shublik and the overlying sag river formation juxtaposes comparatively deeper marine shublik with shallower water glauconitic sandstones of the sag river formation the contact is unconformable and is interpreted to represent a regional sequence boundary lithofacies of the shublik are interpreted to have been coeval depositional facies of an upwelling system relative sea level changes durin shublik deposition are interpreted to have caused the observed vertical and lateral variability in lithofacies via systematic changes between anaerobic dysaerobic and aerobic upwelling conditions dissolution of carbonate allochems resulted in the creation of moldic porosity that positively affected reservoir quality i e permeability in the carbonate packstone grainstone facies areas of highest porosity are in the northern and northeastern parts of the field which correspond to a combination of facies controlled reservoir quality improvement toward the northeast and carbonate dissolution along the lower cretaceous unconformity and the north prudhoe bay fault zone oil in place for the shublik within the prudhoe bay unit is estimated to be between 250 and 500 million bbl although permeabilities are generally low throughout the field area the shublik formation has the potential to add significant reserves to the prudhoe bay field unit

geology and regional setting of kuparuk oil field alaska the kuparuk oil field is located on the alaskan arctic plain in the colville prudhoe basin 10 to 30 mi 16 to 48 km west of the prudhoe bay field the 24 api crude is similar in type to that in the permo triassic reservoirs in the prudhoe bay field however it is from the lower cretaceous kuparuk formation this reservoir is located in a basin between the colville and prudhoe highs the origin of the oil is believed to be predominantly lower sequence formations with migration occurring possibly via the prudhoe bay field the dominant trapping mechanism is stratigraphic pinch out and truncation of the reservoir at a local unconformity along the southern and western flanks of a southeast plunging antiform structural dip closure exists along the northern and eastern flanks the reservoir sandstones occur within sequences which become cleaner and coarser upward and are thought to be shallow marine in origin with a provenance to the northeast they are interpreted to be infrarift sediments on what is now a passive atlantic type continental margin two of the four major lithostratigraphic units mapped within the kuparuk formation exhibit good reservoir characteristics and extend over an area in excess of 200 mi2 518 km2 the cumulative net pay in the kuparuk field ranges up to 90 ft 27 m and the estimate of movable oil in place is 4 4 billion stock tank bbl there is no gas cap the field exhibits a variable oil water contact ranging from 6 530 ft 1 990 m in the southeast to 6 700 ft 2 042 m in the north after secondary waterflooding the potential recoverable reserves are estimated to be about 1 0 to 1 5 billion stock tank bbl kuparuk field therefore ranks as one of the largest oil fields in the united states

geologic methane seeps along boundaries of arctic permafrost thaw and melting glaciers in the arctic permafrost and glaciers form a cryosphere cap that traps methane leaking from hydrocarbon reservoirs restricting flow to the atmosphere aerial surveys and ground based measurements reveal the release of radiocarbon depleted methane along boundaries of permafrost thaw and retreating glaciers in alaska and greenland

magnetotelluric and geomagnetic modelling reveals zones of very high electrical conductivity in the upper crust of central java abstract we present results of two and three dimensional modelling of magnetotelluric and geomagnetic data from central java indonesia we adopt a hypothesis testing approach where we examine the influence of individual structural elements of competing models on the total model responses the models show that ocean anomalies do not overwhelm the signal due to strong electrical conductivity anomalies within java these highly conducting regions with resistivities of below 1 m are located in the central and northern parts of the island fitting of the geomagnetic data requires that the innermost parts of both regions consist of material with extremely low resistivity 0 2 m young sediments tephra and their alteration products can be responsible for some of the low resistivity regions at shallow depths the two zones of extremely high conductivity however are best explained as consequences of geothermal activity in the vicinity of active volcanism mt merapi in central java and by fluid or graphite enrichment along shear zones in northern java subduction zone related processes in a depth range of 70 400 km cannot be resolved because the highly conductive regions at shallow depths effectively screen deeper structures

global c cycle perturbations recorded in marbles a record of neoproterozoic earth history within the dalradian succession of the shetland islands scotland abstract the shetland islands in scotland contain a c 12 km thick mostly siliciclastic neoproterozoic succession that experienced deformation and greenschist to amphibolite facies metamorphism during the ordovician silurian caledonian orogeny correlating these rocks with neoproterozoic successions elsewhere has remained problematic however four marble units punctuate the succession these are in stratigraphic order the weisdale whiteness girlsta and laxfirth limestones two of which record hallmark c isotopic excursions diagnostic of neoproterozoic time the whiteness limestone is lithologically similar to the 635 ma marinoan post glacial cap carbonate sequence and displays a 13 c carbonate profile in which values decline from a range of 0 to 2 to a nadir of 5 and then rise towards and oscillate around 0 the girlsta limestone is a 700 900 m thick marble unit with values of 9 to 11 and is interpreted as recording the late neoproterozoic shuram wonoka event these correlations permit linking the shetland succession to neoproterozoic sections elsewhere and also show that rift related rocks in shetland post date the shuram wonoka event post c 600 ma pre c 550 ma thereby supporting the concept of a diachronous phase of extensional tectonism from c 700 ma in east central laurentia to near the close of the proterozoic in northeastern laurentia prior to the opening of the iapetus ocean

fault interactions and reactivation within a normal fault network at milne point alaska a normal fault network from milne point alaska is investigated focusing on characterizing geometry displacement strain and different fault interactions the network constrained from three dimensional seismic reflection data comprises two generations of faults cenozoic north northeast trending faults and jurassic west northwest trending faults which highly compartmentalize upper triassic to lower cretaceous reservoirs the west northwest trending faults are influenced by a similarly oriented underlying structural grain this influence is characterized by increases in throw on several faults strain localization reorientation of faults and an increase in linkage maturity reconstructing fault plane geometries and mapping spatial variations in throw identified key characteristic features in their interactions and reactivation of pre existing structures faults are divided into isolated abutting and splaying faults isolated faults exhibit a range of displacement profiles depending on the degree of restriction at fault tips fault splays accommodate step like decreases in throw along larger main faults with a throw maximum at the intersection with the main fault throw profiles of abutting faults are divided into two groups early stage abutting faults with throw minima at both the isolated and abutting tips and developed abutting faults with throw maxima near the abutting tip developed abutting faults accumulate throw after initial abutment locally reactivating and transferring throw onto the pre existing fault two abutting faults can link kinematically by reactivating a segment of the pre existing fault forming a trailing fault the motion sense of the trailing fault can be synthetic or antithetic to the reactivated pre existing fault producing increases or decreases in the throw of the pre existing fault respectively

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

isotopic evidence for a complex neoproterozoic to silurian rock assemblage in the north central norwegian caledonides abstract depositional ages of high grade marble sequences in the ofoten district of the north central norwegian caledonides have been constrained by the application of carbon and strontium isotope stratigraphy several marble units intercalated with various schists of the evenes group previously correlated over long distances farther to the north with a late ordovician early silurian low grade fossiliferous succession balsfjord group have been studied for carbon oxygen and strontium isotopes the least altered 87sr 86sr ratios 0 7066 and 0 7077 and the best preserved 13c values 5 0 and 8 0 obtained from the lowermost and uppermost marble formations of the evenes group are consistent with a seawater composition in the neoproterozoic 650 600 and 620 610 ma respectively the isotopic data indicate that only one formation 87sr 86sr 0 7083 13c 5 1 forming the middle evenes group is consistent with the previously suggested early silurian correlation this is supported by new isotopic data obtained from coral and brachiopod bearing llandovery 443 428 ma metalimestones 87sr 86sr 0 7083 13c 4 3 of the balsfjord group the remaining fourth formation of the evenes group shows isotope data 87sr 86sr 0 7088 13c 2 1 which are consistent with a cambrian seawater composition the results obtained have several implications i the late ordovician early silurian elvenes conglomerate ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650 600 ma ii the isotopic and geological data do not support the previously proposed correlation of the entire evenes group with a late ordovician early silurian lithostratigraphic succession farther north in the balsford area ii the evenes group is not a lithostratigraphic unit and should be abandoned as it is composed of a number of marble formations of different age tectonically emplaced in a non chronostratigraphic order the neoproterozoic depositional ages combined with the palaeogeographic position of baltica suggest that a large part of the carbonate rock succession forming the evenes group was initially accumulated in warm seas on a continental shelf probably laurentia and was tectonically transported onto baltica during the early silurian scandian collision at ca 425 ma the complex tectonic imbrication of the polymetamorphosed and polydeformed neoproterozoic cambrian and early silurian carbonate formations also suggests that more than one orogenic episode should be invoked to explain the tectonic juxtaposition of these assemblages some of the fault contacts juxtaposing rocks of neoproterozoic and cambrian age and the obduction of the ophiolite complex might have been associated with a mid to late ordovician taconian event

timing structural assembly metamorphism and cooling of caledonian nappes in the ofoten efjorden arca north norway tectonic insights from u pb and40ar 39ar geochronology u pb and 40ar 39ar geochronology from part of the northern norwegian caledonides indicates a cooling and unroofing history different from that inferred for the caledonides in southwest norway in the ofoten efjorden area of north norway the narvik nappe complex nnc represents the basal portion of the composite caledonian allochthon and consists of amphibolite facies oceanic affinity metasedimentary rocks intruded by numerous pretectonic felsic dikes two of these dikes yield indistinguishable u pb zircon ages of 437 1 ma providing a maximum age for tectonic assembly metamorphism and deformation at this structural level u pb ages of metamorphic monazite 432 2 ma and zircon 431 7 0 5 ma from pelitic schist and migmatite yield robust estimates of the age of peak metamorphism combined with structural evidence for transport parallel stretching and subvertical attenuation of the rock mass at the time of metamorphism these ages suggest that local and or episodic extensional modification of the

two oil types on north slope of alaska implications for exploration forty oil samples from across the north slope of alaska have been analyzed by the u s bureau of mines and the u s geological survey results of these analyses suggest two separate genetic oil types the first the simpson umiat oil type occurs in reservoir rocks of cretaceous and quaternary age and includes oil from seeps in the skull cliff cape simpson manning point and ungoon point areas and oils from wolf creek test 3 and the cape simpson and umiat oil fields these are higher gravity low sulfur oils with no or slight odd numbered n alkane predominance and pristane to phytane ratios greater than 1 5 also these oils have dgr13c values ranging from 29 1 to 27 8 parts per thousand ppt and dgr34s values from 10 3 to 4 9 ppt the s cond type the barrow prudhoe oil type occurs in reservoir rocks of carboniferous to cretaceous age and includes oils from south barrow gas field prudhoe bay oil field and the fish creek test well 1 physical properties of barrow prudhoe oils are variable but in general the oils are medium gravity high sulfur oils with slight even numbered n alkane predominance and pristane to phytane ratios less than 1 5 also these oils have dgr13c values of 30 3 to 29 8 ppt and dgr34s values from 30 0 to 2 1 ppt the two types are believed to originate from different source rocks the barrow prudhoe type may have originated from a carbonate or other iron deficient source rock and the simpson umiat type from a siliciclastic source rock occurrences of the two oil types when outlined on a map indicate at least two areas for additional exploration for the barrow prudhoe type in stratigraphic traps along and adjacent to the barrow arch and for the simpson umiat type in cretaceous rocks along the trend between the simpson and umiat oil fields and in cretaceous and tertiary rocks from prudhoe bay field to the william o douglas arctic wildlife range

sequence stratigraphy of the kingak shale jurassic lower cretaceous national petroleum reserve in alaska beaufortian strata jurassic lower cretaceous in the national petroleum reserve in alaska npra are a focus of exploration since the 1994 discovery of the nearby alpine oil field 400 mmbo these strata include the kingak shale a succession of depositional sequences influenced by rift opening of the arctic ocean basin interpretation of sequence stratigraphy and depositional facies from a regional two dimensional seismic grid and well data allows the definition of four sequence sets that each displays unique stratal geometries and thickness trends across npra a lower to middle jurassic sequence set includes numerous transgressive regressive sequences that collectively built a clastic shelf in north central npra along the south facing lobate shelf margin condensed shales in transgressive systems tracts downlap and coalesce into a basinal condensed section that is likely an important hydrocarbon source rock an oxfordian kimmeridgian sequence set deposited during pulses of uplift on the barrow arch includes multiple transgressive regressive sequences that locally contain well winnowed shoreface sandstones at the base of transgressive systems tracts these shoreface sandstones and overlying shales deposited during maximum flooding form stratigraphic traps that are the main objective of exploration in the alpine play in npra a valanginian sequence set includes at least two transgressive regressive sequences that display relatively distal characteristics suggesting high relative sea level an important exception is the presence of a basal transgressive systems tract that locally contains shoreface sandstones of reservoir quality a hauterivian sequence set includes two transgressive regressive sequences that constitute a shelf margin wedge developed as the result of tectonic uplift along the barrow arch during rift opening of the arctic ocean basin this sequence set displays stratal geometries suggesting incision and synsedimentary collapse of the shelf margin

temperature of the icelandic crust inferred from electrical conductivity temperature surface gradient and maximum depth of earthquakes abstract two different models of the structure of the icelandic crust have been presented one is the thin crust model with a 10 15 km thick crust beneath the axial rift zones with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up domed asthenosphere the thick crust model assumes a 40 km thick and relatively cold crust beneath central iceland the most important and crucial parameter to distinguish between these different models is the temperature distribution with depth three methods are used to estimate the temperature distribution with depth first the surface temperature gradient measured in shallow wells drilled outside geothermal areas second the thickness of the seismogenic zone which is associated with a 750 c isothermal surface third the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 c at the base of the crust combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust these results are strongly in favour of the thin crust model the scattered deep seismic reflectors interpreted as moho in the thick crust model could be caused by phase transitions or reflections from melt pockets in the mantle

formation history and physical properties of sediments from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract the synthesis of available geological information and surface temperature evolution in the alaska north slope region suggests that biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse grained layers that have relatively high hydraulic conductivity and low gas entry pressures hydrate started forming before the beginning of the permafrost eventually the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations the single tested specimen depth 620 47 620 62 m from the d unit consists of uncemented quartzitic fine sand with a high fraction of fines 56 by mass finer than sieve 200 the as received specimen shows no evidence of gas present the surface texture of sediment grains is compatible with a fluvial deltaic sedimentation environment and shows no signs of glacial entrainment tests conducted on sediments with and without thf hydrates show that effective stress porosity and hydrate saturation are the major controls on the mechanical and geophysical properties previously derived relationships between these variables and mechanical geophysical parameters properly fit the measurements gathered with mount elbert specimens at different hydrate saturations and effective stress levels we show that these measurements can be combined with index properties and empirical geomechanical relationships to estimate engineering design parameters volumetric strains measured during hydrate dissociation vanish at 2 4 mpa therefore minimal volumetric strains are anticipated during gas production at the mount elbert well however volume changes could increase if extensive grain crushing takes place during depressurization driven production strategies if the sediment has unexpectedly high in situ porosity associated to the formation history or if fines migration and clogging cause a situation of sustained sand production

the subsurface temperature assessment by means of an indirect electromagnetic geothermometer abstractadvances have been made in the subsurface temperature estimation basing on the indirect electromagnetic geothermometer the approach used was based on the artificial neural network technique which contrary to other available approaches does not imply the prior knowledge of the electrical conductivity mechanisms and rock properties application of the indirect em geothermometer to the interwell space interpolation in three areas tien shan kyrgyzstan soultz sous forets france and hengill iceland characterized by different geologic environments indicated that the temperature estimation errors are controlled by four factors faulting distance between the em site and the area where the temperature is estimated meteoric and groundwater flows and lateral geologic inhomogeneity although the latter factor is less restrictive if appropriate em inversion tools are used it was demonstrated that the extrapolation errors depend on two factors spacing between the em site and the borehole and ra

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

temperature profiles in the earth of importance to deep electrical conductivity models deep in the earth the electrical conductivity of geological material is extremely dependent on temperature the knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models the measured values of the terrestrial heat flow radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere various methods of deep temperature calculations are presented and discussed characteristic geotherms are proposed for major tectonic provinces of europe and it is shown that the existing temperatures on the crust upper mantle boundary may vary in a broad interval of 350 1 000 c the present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200 1 000 c it is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity versus temperature data into the conductivity versus depth data

temperature dependence of the electrical resistivity of water saturated rocks the electrical resistivities of several dacitic tuffs sandstone andesite granite and crystalline limestone samples saturated with a 0 001 m aqueous solution of kc x1d4c1 were measured in the range from room temperature to 250 c the experiments were made using a cell technique with platinum electrodes of particular interest are the data collected at temperatures above 200 c under high pressure a region scarcely documented in the literature basically the samples used in the present study show a quasi exponential decrease of resistivity with temperature up to 200 c the same temperature dependence is found for the resistivity of the saturating solution thus confirming that conduction in water saturated rocks is essentially electrolytic at temperatures above 200 c some specimens of highly porous dacitic tuff still closely follow the saturating solution in the pattern of resistivity variation with temperature exhibiting a minimum around 220 c however the behavior of low porosity crystalline rocks n

three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland