conodont palaeobiogeography and thermal maturation in the caledonides conodont elements are important in biostratigraphy in palaeoenvironmental and palaeobiogeographical analyses and in assessing the thermal history of lower palaeozoic strata throughout the caledonian belt the differentiation of ordovician conodont faunas into the north atlantic and north american midcontinent provinces is not closely comparable with the provincial patterns shown by other faunal groups and does not relate clearly to criteria for recognizing continental separation across the iapetus ocean some species associations adapted to cool normal marine waters were able to cross the ocean at the time of its presumed maximum extent the colours of conodont elements reflect the thermal history of strata and can be used in studies of low grade metamorphism in s wales for example metamorphic temperatures indicated by conodont colour are too high to he accounted for by simple burial beneath the known stratigraphical overburden under normal heat flow conditions elsewhere conodont colour trends also serve to constrain and test models of metamorphism that involve sedimentary burial in subsiding basins tectonic burial under nappes or the thermal effects of intrusions

4 d constrained depth conversion for reservoir compaction estimation application to ekofisk field ekofisk field in the norweigian sector of the north sea is a remarkable example of a strongly compacted chalk reservoir since the start of production in 1971 the water bottom has subsided by over 8 m as a result of reservoir compaction hermansen et al the field wide waterflood begun in 1989 was expected to halt pressure depletion and reservoir compaction after a few years of waterflooding ekofisk reservoir pressure began increasing but compaction and subsidence continued laboratory studies on chalk cores indicated that high porosity oil filled chalk compacts strongly when flushed with seawater these studies along with reservoir flow simulation and geomechanic modeling led to a model of a water weakening compaction sylte et al ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice a year compaction logging in a monitoring well daily gps measurements at the platforms and periodic bathymetry surveys nine surveys from 1970 to 1999

seismic versus sonic velocities a vertical seismic profiling study vertical seismic profiling vsp techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole the analysis of a vsp survey can also provide insight into seismic wave propagation especially when related to sonic measurements but vsp and sonic log velocities or traveltimes are often found to disagree recent field evidence of these differences suggests that the vsp traveltimes are delayed with respect to the integrated sonic times especially in the deep section 3000 ft by about 2 0 ms 1000 ft on the average the vsp has numerous applications in exploration geophysics such as calibrating the sonic log it is thus important to understand why the two measurements differ differences in the geometries source frequencies and instrumental errors of the two surveys are reviewed more detailed analysis of seismic wave propagation in the vsp shows that short path multiples and velocity dispersion can have a significant delaying effect on the seismic tr

fault whispers transmission distortions on prestack seismic reflection data transmission distortions are observed on prestack seismic data at two locations in the gulf of mexico these distortions produce anomalous amplitude versus offset avo signatures the locations of the distortion zones are determined using acquisition geometry and ray tracing no obvious reflection events such as shallow gas zones are observed at the predicted locations of the distortion zones instead the distortion zones correlate with buried faults and unconformities it is postulated that the distortions are produced by velocity changes across buried faults and unconformities the distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults a simple model is developed to explain many of the characteristics of the distortion pattern

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

uncertainties in quantitative time lapse seismic analysis most seismic time lapse studies so far have been of a qualitative nature identification of areas with minor or no seismic changes has been used to plan new infill drilling targets increased accuracy in seismic acquisition methods in both conventional streamer surveys and newer methods such as multicomponent sea bed seismic and permanent sensors opens possibilities for the next step quantitative time lapse analysis quantitative methods here mean the estimation of for instance a change in fluid saturation from 20 water to 90 water or the estimation of a pore pressure change of 5 mpa explicit expressions for the uncertainties associated with estimated changes in for instance reservoir pressure and fluid saturation are derived these formulae can be used to compare relative uncertainties between estimated parameters as well as to identify the critical factors in various estimation techniques the importance of accurate rock physics input as well as that of highly repeatable time lapse seismic data is emphasized furthermore uncertainty analysis can be used to find optimal weight factors when the same parameter e g saturation change is estimated by two or three different techniques

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

analysis of time lapse travel time and amplitude changes to assess reservoir compartmentalization fluid depletion within a compacting reservoir can lead to significant stress and strain changes and potentially severe geomechanical issues both inside and outside the reservoir we extend previous research of time lapse seismic interpretation by incorporating synthetic near offset and full offset common midpoint reflection data using anisotropic ray tracing to investigate uncertainties in time lapse seismic observations the time lapse seismic simulations use dynamic elasticity models built from hydro geomechanical simulation output and a stress dependent rock physics model the reservoir model is a conceptual two fault graben reservoir where we allow the fault fluid flow transmissibility to vary from high to low to simulate non compartmentalized and compartmentalized reservoirs respectively the results indicate time lapse seismic amplitude changes and travel time shifts can be used to qualitatively identify reservoir compartmentalization due to the high repeatability and good quality of the time lapse synthetic dataset the estimated travel time shifts and amplitude changes for near offset data match the true model subsurface changes with minimal errors a 1d velocity strain relation was used to estimate the vertical velocity change for the reservoir bottom interface by applying zero offset time shifts from both the near offset and full offset measurements for near offset data the estimated p wave velocity changes were within 10 of the true value however for full offset data time lapse attributes are quantitatively reliable using standard time lapse seismic methods when an updated velocity model is used rather than the baseline model

estimation of layer thickness and velocity changes using 4d prestack seismic data in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and is often associated with stretching and arching of the overburden rocks time lapse seismic data can be used to monitor these processes since compaction and stretching cause changes in layer thickness as well as seismic velocities it is crucial to develop methods to distinguish between the two effects we introduce a new method based on detailed analysis of time lapse prestack seismic data the equations are derived assuming that the entire model consists of only one single layer with no vertical velocity variations the method incorporates lateral variations in relative velocity changes by utilizing zero offset and offset dependent time shifts to test the method we design a 2d synthetic model that undergoes severe reservoir compaction as well as stretching of the overburden rocks finally we utilize the method to analyze a real 2d prestack time lapse seismic line from t

overburden 4d time shifts induced by reservoir compaction at snorre field abstract interpreting and understanding overburden seismic changes are important to avoid health safety and environmental hse accidents and well drilling problems furthermore overburden time shifts might indicate areas with reservoir compaction and depletion in the snorre sandstone field 4d seismic time shifts of as much as 3 ms are observed in the overburden the cumulative overburden time shifts captured at top reservoir correlate well with pressure depletion areas in the reservoir indicating that the time shifts might be related to changes in stress and strain because of reservoir compaction for further investigation a 2d geomechanical model is built in the area with the most prominent overburden time shifts based on the stiffness of the reservoir and surrounding rocks and simulated reservoir pore pressure changes the geomechanical model predicts a maximum downward displacement at the top reservoir of 0 4 m and maximum seabed subsidence of 0 25 m by defining an appropriate strain sensitiv

u pb ages and tectono magmatic evolution of middle ordovician volcanic rocks of the wild bight group newfoundland appalachians the wild bight group and spatially associated south lake igneous complex form one of the ordovician oceanic terranes of the central mobile belt of the newfoundland appalachians an integrated study of these rocks involving detailed mapping geochemistry sm nd isotopic analyses and u pb geochronology has shown that there are two temporally and genetically distinct volcanic sequences within the wild bight group the younger sequence comprises a lower volcanic succession associated with coarse volcaniclastic rocks and an upper volcanic succession interbedded with argillite chert and minor greywacke the lower volcanic succession has calc alkaline affinties and isotopic evidence for minor crustal contamination it is interpreted to represent a volcanic arc formed in proximity to the gondwanan margin above an east dipping subduction zone the age of this volcanic sequence is confined to 472 3 ma by felsic tuffs which occur stratigraphically above and below it the upper volcanic unit has predominantly enriched tholeiitic to alkaline geochemical characteristics with isotopic signatures indicative of little or no crustal contamination and is interpreted to represent arc rifting the age of this sequence was determined indirectly by dating two geochemically related gabbro sills 472 9 ma and 471 4 ma this work shows that despite different lithologies and stratigraphic and structural relationships between early and middle ordovician sequences in the northern and southern exploits subzone they have undergone essentially the same tectono magmatic events the age constraints on the magmatic events in the wild bight group provide evidence for the timing of obduction of early ordovician oceanic sequences and the reversal of subduction polarity along the gondwanan margin suggested by previous workers resume le groupe de wild bight et le complexe de south lake des terrains associes constituent un des terranes oceaniques ordoviciens de la ceinture mobile centrale des appalaches de terre neuve une etude pluridisciplinaire de ces roches incluant la cartographie detaillee la geochimie les analyses isotopiques sm nd et la geochronologie u pb a demontre que le groupe de wild bight est forme de deux sequences volcaniques qui sont genetiquement et

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

time lapse tomography in time lapse analysis we have to distinguish the seismic response changes due to oil and gas production at a reservoir over the years from several other causes such as the recording signature and random noise in this paper we focus our attention on the velocity macromodel provided by seismic tomography which is a basic tool for the data regularization its depth or time migration and a possible final subtraction among different vintages we show first that we cannot use just a single velocity model for all data sets because of seasonal variations of the overburden velocity which is mainly due to seawater temperature in marine cases and to the water table depth in land cases however we can exploit the basic assumption of time lapse analysis for constraining reflection refraction tomography i e by imposing the constraint that the layer structure and the local velocities do not change outside the reservoir and in the shallowest part over time we thus get coupled models that are physically consistent with a better spatial coverage and higher information redundancy the new method is illustrated by a marine case history from the north sea

tectonic controls of ordovician arc and marginal basin volcanism in wales ensialic destructive plate margin volcanism persisted in wales from late tremadoc to caradoc times late tremadoc uplift and erosion in nw wales is attributable to onset of subduction and development of a frontal ridge system fore arc south east of this in the vicinity of the harlech dome and in sw wales more localized uplift was associated with the development of arc volcanoes at a volcanic front in north wales the arc volcanism occurred in a regime of e w crustal extension and was centred on a deep crustal discontinuity during arenig times the arc terrane was transgressed subsequently marine conditions with local and temporary emergence persisted through arenig to caradoc episodes of marginal basin volcanism while there was widespread and persistent subsidence in wales during development of the marginal basin the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence the grabens mark the sites of deep and steep crustal discontinuities fractures one of which controlled the earlier arc volcanism changing sites of volcanism in wales can now be seen as due to transfer of active extension from one fracture to another there is evidence that the fractures were present by tremadoc times and they almost certainly formed before development of the cambrian marine rift basin they are most likely to have been strike slip faults or shear zones and their pattern in wales indicates that they may represent part of a complicated deep crustal strike slip duplex system the volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults flower structure which channelled magmas through the cover during extension across the fractures the fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during late caledonian compression and transpression in north wales the volcanically active grabens trended broadly n s oblique to the exposed ne sw basement tectonic grain and dominantly e w extension seems to have persisted throughout the ordovician volcanism while there is considerable uncertainty concerning ordovician plate tectonic configurations and displacements the extension is best explained by invoking a n s convergence vector between the oceanic and over riding microcontinental plates with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin the model highlights the suspect nature of the relationship between the welsh ordovician terrane and others to the north

ordovician faunas island arcs and ophiolites in the scandinavian caledonides ordovician faunal data from the scandinavian caledonides is tested with new geochemical information from zircons to give u pb ages and source origins of volcanic arc and ophiolite sequences early ordovician arenig llanvirn low latitude toquima table head faunas from the upper upper allochthon are associated with an island arc system formed adjacent to laurentia contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of archaean age the nearest source for such rocks is on the laurentian rather than the baltic side higher latitude celtic province faunas from the upper upper allochthon are from one insular site accessible to forms from both laurentia and baltica the late ordovician low latitude holorhynchus and subtropical hirnantia faunas occur in overstep sequences above deeply eroded early ordovician arc complexes the transgression appears to be coeval with a second generation of spreading related complexes single detrital zircons from sediments show sources from archaean proterozoic and early ordovician terranes this suggests that deposition was in a basin situated along the same continental margin laurentia to which the early ordovician ophiolite arc sequences had already become accreted the late ordovician faunas link both laurentia and baltica at a time of narrowing of lapetus the new geochemical data together with the faunal information is supported by recent palaeomagnetic studies

the subducted rivera cocos plate boundary where is it what is it and what is its relationship to the colima rift gravity data the geometry of the wadati benioff zone beneath western mexico and the seafloor morphology of the rivera cocos plate boundary west of the middle america trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the southern colima rift thus the southern colima rift likely formed in response to divergence between the subducting rivera and cocos plates due to direct coupling between these two plates and the overriding north american plate in contrast the subducted plate boundary lies east of and oblique to the northern colima and central colima grabens east of the central colima graben a low density zone overlies the boundary and underlies surface exposures of cretaceous granitoids and associated thermal springs and shallow focus earthquakes characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted rivera and cocos plates these characteristics along with the adjacent locations of the low density upper mantle and the central colima graben are consistent with crustal extension produced by the uniform sense normal simple shear mechanism

more gap than record qualitative and quantitative assessment of stratigraphic gaps in a field based study with examples from the lower silurian pentamerus beds of shropshire england and the lower ordovician ribband gp of county wexford ireland

fault creep growth model and its relationship with occurrence of earthquakes summary a fault model is used to analyse creep growth process of microcracks in damage zone near the tips of fault by means of the rheology fracture mechanics when microcracks in the zone link each other and their deformation quickly intensifies it means that an earthquake is taking place to a certain degree the relationships among some key parameters that are correlated to the growth of microcracks are analysed theoretically these parameters include failure time of fault number of microcracks in unit area stresses field yield strength and rheological properties of medium the results indicate that the model proposed here is reasonable to describe the pregnant process of an earthquake in the meantime this research shows that with the results of in situ surveys and lab experiments it is possible to estimate the time span for a fault expanding

cross matching with interpreted warping of 3d streamer and 3d ocean bottom cable data at valhall for time lapse assessment legacy streamer data and newer 3d ocean bottom cable data are cross matched and analysed for time lapse analysis of geomechanical changes due to production in the valhall field the issues relating to time lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross matching workflow that includes 3d warping additionally an assessment of the differences between the imaging using single azimuth streamer and multi azimuth ocean bottom cable data is provided the 3d warping utilized in the cross matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production as such this work also provides a demonstration of the use of high resolution 3d interpreted warping to resolve the 3d heterogeneity of the compaction and subsidence this is an important tool for valhall and possibly other fields where compaction and subsidence and monitoring thereof are key factors in the reservoir management since the predominant observed production induced changes are compaction of the soft high porosity chalk reservoir due to pore pressure reduction and the resultant overburden subsidence such reservoir compaction could have significant implications for production by changing permeabilities and production rates furthermore the subsidence effects could impact upon subsea installations and well bore stability geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes with the geological overburden being largely neglected the approaches suggested herein provide the potential for monitoring and assessment in three dimensions including the probable heterogeneity and shearing that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to for example mitigate well bore failures

global ordovician vertebrate biogeography abstract cambrian ordovician vertebrate and supposed vertebrate occurrences have been repeatedly claimed during recent decades with confirmed taxa bearing mineralized tissues with a vertebrate histomorphology still relatively rare the only biogeographic province that we can presently recognize is the gondwana endemic assemblage gea with possible late cambrian fragmentary remains from australia but more definite early ordovician arenigian to early late ordovician caradocian arandaspids i e sacabambaspis arandaspis and other taxa known from south america and australia certain chondrichthyans sharks at first without teeth and which might not constitute a monophyletic group might have originated in east gondwana province and then are found in the late ordovician and early silurian of mongolia tarim and south china the gea fauna proper disappears by middle late caradocian and vertebrates do not reappear in gondwana until mid late silurian late ordovician 455 myr or earlier vertebrates are also known with certainty from laurentia viz north america pteraspidomorphs astraspis eriptychius and various gnathostome like taxa including chondrichthyan placoderm and acanthodian like remains and siberia astraspid like microremains with an unusual histology which might correspond to a new group of lower vertebrates as well as scales from putative loganiid and thelodontidid thelodonts from north america and russia timan pechora the severnaya zemlya archipelago and siberia this is defined as the laurentia baltica siberia assemblage lbsa we also mention one enigmatic reference to a late ordovician anaspid in south africa there is no clear association of taxa between the gea and lbsa despite a small overlap in time various recent palaeogeographic models published for the ordovician are critically analyzed and considered within four groups the archetypal palaeogeographic reconstructions two alternative solutions and a compact version habitats of vertebrates in mostly ba1 marine intertidal to ba3 shallow subtidal environments and their dispersal capabilities are evaluated with regard to those models the main feature of ordovician vertebrate biogeography is endemism furthermore the present lack of complete descriptions of most taxa which are often represented only by isolated microremains and the need for a thorough phylogenetic analysis preclude any phylogenetic palaeobiogeographic study in such a framework we also evaluate possible links between external physical factors and the ordovician radiation of vertebrates the late proterozoic deposition of oceanic phosphate and the early cambrian increase in oxygen on earth might have been the spur for vertebrate evolution before the phase when hard tissues appeared the sharp decline of the marine strontium isotope ratio during the middle to late ordovician transition interpreted as having been controlled primarily by continental collisional tectonics and its associated erosion and weathering has been proposed as the consequence of a possible mantle superplume event which could have caused the prominent caradocian transgressive phase this might have been a factor in the changeover from a gondwanan to a laurentian focus for vertebrates

mineralogy bulk and rare earth element geochemistry of massive sulphide associated hydrothermal sediments of the brunswick horizon bathurst mining camp new brunswick massive sulphides are spatially and temporally associated with iron formation if and other hydrothermal sedimentary rocks in the vicinity of the brunswick no 12 brunswick no 6 and austin brook deposits bathurst mining camp sulphide carbonate oxide and silicate predominant if is present carbonate predominant if is best developed in and around the brunswick no 12 deposit whereas hematite bearing if is absent here but prominent in the austin brook brunswick no 6 area the if is composed dominantly of si co2 fe mn and ca minor constituents include mg p ti al and s statistically significant interelement correlations between eu fe mn pb zn cd au ca sr ba p co2 and s indicate that these elements were precipitated from hydrothermal fluids vented onto the seafloor positive interelement correlations between si ti al mg k zr rare earth elements ree s except eu se v y yb co ni and cr reflect the presence of detrital clastic mafic and aluminosilicate minerals and

evidence of regional subsidence and associated interior wetland loss induced by hydrocarbon production gulf coast region usa analysis of remote images elevation surveys stratigraphic cross sections and hydrocarbon production data demonstrates that extensive areas of wetland loss in the northern gulf coast region of the united states were associated with large volume fluid production from mature petroleum fields interior wetland losses at many sites in coastal louisiana and texas are attributed largely to accelerated land subsidence and fault reactivation induced by decreased reservoir pressures as a result of rapid or prolonged extraction of gas oil and associated brines evidence that moderately deep hydrocarbon production has induced land surface subsidence and reactivated faults that intersect the surface include 1 close temporal and spatial correlation of fluid production with surficial changes including rapid subsidence of wetland sediments near producing fields 2 measurable offsets of shallow strata across the zones of wetland loss 3 large reductions in subsurface pressures where subsidence rates are high 4 coincidence of orientation and direction of displacement between surface fault traces and faults that bound the reservoirs and 5 accelerated subsidence rates near producing fields compared to subsidence rates in surrounding areas or compared to geological rates of subsidence based on historical trends subsidence rates in the gulf coast region near producing fields most likely will decrease in the future because most petroleum fields are nearly depleted alternatively continued extraction of conventional energy resources as well as potential production of alternative energy resources geopressured geothermal fluids in the gulf coast region could increase subsidence and land losses and also contribute to inundation of areas of higher elevation

a new approach to separate seismic time lapse time shifts in the reservoir and overburden abstractfor a robust way of estimating time shifts near horizontal boreholes we have developed a method for separating the reflection responses above and below a horizontal borehole together with the surface reflection data the method uses the direct arrivals from borehole data in the marchenko method the first step is to retrieve the focusing functions and the up down wavefields at the borehole level using an iterative marchenko scheme the second step is to solve two linear equations using a least squares minimizing method for the two desired reflection responses then the time shifts that are directly linked to the changes on either side of the borehole are calculated using a standard crosscorrelation technique the method is applied with good results to synthetic 2d pressure data from the north sea one example uses purely artificial velocity changes negative above the borehole and positive below and the other example uses more realistic changes based on well logs in the 2d case with an adequ

quantitative estimation of compaction and velocity changes using 4d impedance and traveltime changes in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and it is often associated with changes in the overburden time lapse or 4d seismic data are used to monitor this compaction process since the compaction causes changes in both layer thickness and seismic velocities it is crucial to distinguish between the two effects two new seismic methods for monitoring compacting reservoirs are introduced one based on measured seismic prestack traveltime changes and the other based on poststack traveltime and amplitude changes in contrast to earlier methods these methods do not require additional empirical relationships such as for instance a velocity porosity relationship the uncertainties in estimates for compaction and velocity change are expressed in terms of errors in the traveltime and amplitude measurements these errors are directly related to the quality and repeatability of time lapse seismic data for a reservoir at 3000 m depth wit h9mo fcompaction and assuming a 4d timeshift error of 0 5 ms at near offset and 2 ms at far offset we find relative uncertainty in the compaction estimate of approximately 50 60 using traveltime information only

a case history of velocity problems in the shadow of a large growth fault in the frio formation texas gulf coast false structures can appear on seismic data in the shadow of normal growth faults with the upthrown time structure appearing to roll into the fault where no such roll exists in depth this phenomenon was encountered in a prospect in the frio formation of the texas gulf coast where a large growth fault expanded a thick low velocity shale above the frio two check shot velocity surveys one at either end of the seismic control demonstrated a severe average velocity gradient across the prospect at the level of the objective geologic formation assuming first one and then the other velocity survey was valid over the entire area for conversion of well data to time produced a drastic difference in the structural interpretation of the seismic data the more obvious interpretation required a down to the northwest fault with a downthrown rollover anticline at the objective formation this interpretation had been previously tested with a dry hole on the crest of the time structure this interpretation also cont

overburden 4d time shifts indicating undrained areas and fault transmissibility in the reservoir abstract production induced geomechanical stress changes cause velocity changes in the overburden that might be detected as 4d seismic time shifts the strength of the velocity changes depends on the degree of pressure changes and the elastic properties of the reservoir and overburden layers even small velocity changes less than 1 might accumulate into detectable seismic time shifts at the top reservoir since the overburden thickness typically ranges from one to several kilometers reservoir pressure changes inducing seismic time shifts are observed in the overburden of the snorre heidrun and statfjord fields all located on the norwegian continental shelf the strong correlation between overburden time shifts geomechanics and reservoir pressure changes is used to indicate undrained areas and transmissibility across faults which is useful information for increased oil recovery well planning and reservoir model updating 4d geomechanical models are built with input from simulated reservoir pres

time lapse tomographic inversion using a gaussian parameterization of the velocity changes most seismic studies of changes in traveltimes are of a qualitative nature and a major challenge in four dimensions is to use the information contained in time shifts to quantify the nature of velocity changes in the subsurface layers we propose a 4d tomographic inversion method that uses time shifts from prestack seismic data to estimate parameters describing the 2d velocity field after changes have occurred prestack data allow for the usage of many offsets thus increasing the information input for the inversion the velocity changes are parameterized by a chosen number of gaussian functions in two dimensions and weighted least squares inversion is used to estimate the parameters describing these functions we have found that the parameters describing the position and shape of the gaussian velocity anomalies can be estimated with this method for simple synthetic cases for more complex cases with overlapping gaussian functions resolution of the parameters can be difficult and in these cases our recom

early palaeozoic brachiopods and associated shelly faunas from western gondwana their bearing on the geodynamic history of the pre andean margin

the east carpathian crystalline mesozoic zone romania paleozoic amalgamation of gondwana and east european craton derived terranes abstract the alpine orogen of the eastern carpathians contains pre alpine terranes framed by younger rocks the geological data on these terranes are reviewed and interpreted in order to elucidate their geodynamic evolution during the paleozoic the rebra tulghes terrane shows a stratigraphic sequence similar to avalonia neoproterozoic platformal metasediments rebra group overlain by late neoproterozoic volcano sedimentary rocks pietrosu bistritei porphyroids and lower paleozoic cover tulghes group the bretila terrane made up of orthogneisses pelitic gneisses granites amphibolites and micaschists shows affinities with the basement of the east european craton the modern juxtaposition of the bretila terrane and the rebra tulghes terrane may be explained in terms of collision between the latter and the east european craton during the ordovician times as an effect of collision the bretila terrane was thrust over the rebra tulghes terrane and induced the occurrence of thrust faults within it a lower paleozoic emplacement of the east carpathian pre alpine thrust nappes is thus inferred

timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history van der plmjm b a 1987 timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tectonophysics 135 15 24 the newfoundland appalachians have been interpreted as an area where lower paleozoic plate convergence culminated in collision between an ordovician volcanic chain and the north american craton in middle ordovician times closure of the intervening proto atlantic iapetus ocean was considered incomplete subsequent deformation gave rise to regional folding and faulting recent studies in the newfoundland dunnage zone have revealed that the deformation history is far more complex than previously recognized large scale thrusting folding and faulting occurred in silurian devonian times furthermore it has been suggested that the dunnage zone is an allochthonous terrane underlain by dominantly continental crust rather than representing remnants of a rooted ocean basin in view of these results a revision of tectonic scenarios and zonal subdivision is warranted and a multi stage collision history will be discussed with emphasis on the spatial distribution and significance of silurian devonian deformation in central newfoundland subduction in lower paleozoic times gave rise to the formation of a volcanic terrane concurrently to the southeast a marginal sea was formed mariana type subduction in middle ordovician times the volcanic terrane collided with the north american craton first stage collision and back arc spreading terminated continued crustal shortening resulted in the formation of a silurian accretionary terrane telescoped marginal sea and its subsequent deformation second stage collision devonian carboniferous strike slip faulting represents the third stage in the collision history the model is applicable to large tracts of the caledonian appalachian chain its main characteristics are a the revised zonal subdivision of the area is based on characteristics of silurian and older rocks rather than middle ordovician and older rocks only b the central part of the orogen represents a telescoped marginal sea that formed to the southeast of the ordovician volcanic chain rather than a remnant of the incompletely closed iapetus ocean c the earliest deformation is progressively younger toward the southeast d the appalachian collision history is a result of the activity of a single deformation regime over a long period of at least 75 ma

relating 4d seismics to reservoir geomechanical changes using a discrete element approach a modified discrete element method is briefly introduced and used for modelling reservoir geomechanical response during fluid injection and depletion the modified approach works as a continuum method until some local failure is initiated after which it behaves like a discrete element method on a polygonal lattice the method is advantageous for modelling fracture developments in rocks it is applied here to synthetic models of two reservoirs taken from the north sea gullfaks and elginfranklin for gullfaks two cases of water injection were modelled one with low horizontal effective stress and the other with low vertical effective stress vertical fractures are developed in the first case whereas horizontal fractures are developed in the second case this would not have been seen using traditional methods based on 4d seismics data for the gullfaks field one may envision that horizontal fractures could have been formed the elgin franklin synthetic model is used to study various scenarios of changing stress field around the depleting reservoir based on 4d seismics data from this field one may see changes that could be interpreted in terms of possible fault reactivation

the fault shadow problem its nature and elimination the fault shadow problem is what explorationists call the zone of unreliable seismic imaging in the footwalls of faults although it can occur in all types of faults the term is usually applied to extensional faults these problems are common place but much current exploration still relies on images in which they remain in this article i present an example of the fault shadow problem from the wilcox trend eocene paleocene of south texas the stratigraphic velocity alternations in this area cause a distinctive suite of problems in conventional time imaging explorationists can cite numerous examples of wells drilled on time highs but significantly off structure in depth moreover imaging often degrades precipitously at levels below the middle wilcox even when wells indicate an uncomplicated structure i ll begin by demonstrating through schematic and synthetic examples the nature of these problems and their elimination through prestack depth imaging a real data example will then show identical ef

monitoring of subsidence and induced seismicity in the lacq gas field france the consequences on gas production and field operation abstract between 1957 when the lacq gas field was first put into production and 1967 2 5 cm of subsidence occurred this corresponded to a period in which the pressure had been depleted by 30 mpa further depletion from 1967 until 1989 of 25 mpa induced an additional subsidence of 3 0 cm which coincided with more than 1000 earthquakes 44 with magnitudes greater than 3 and 4 with magnitudes greater than 4 the strain energy release rate by seismic events increased for the period 1969 1979 and now decreases yearly the method used for processing and interpreting the seismological data takes into account the lithologies and the mechanical properties of the rocks and the source parameters the regional mechanism of the gas field deformation appears to be the lowering of a rigid block located at the top of the structure by a succession of movements along pre existing fractures and faults in the reverse direction to the original from spectrum analysis of seismic recordings the amplitude of the fault slips has been calulated to be of the order of millimetres this may be the reason why there has been no damage to the wells the radii of the sources may give an indication about the size of the blocks to be considered in the reservoir engineering models in spite of the obvious non elastic behaviour of the seismic events the regional response of the rock mass is an elastic one the rock mass can be modeled as a block assembly with localized movements between some blocks being responsible for the induced seismicity the size of the induced events is therefore limited by the pre existing fractures close monitoring of subsidence and seismic events is a new and useful aid to understanding the reservoir response to fluid withdrawal the practical consequences with regard to safety and gas production and a contribution to our knowledge of reservoir behaviour over the course of time

seismic time lapse effects and stress changes examples from a compacting reservoir compaction of unconsolidated reservoir rock represents a major drive mechanism in many oil and gas reservoirs significant volumes of produced hydrocarbons may be credited to this effect however compaction might also result in surface subsidence a dynamic overburden and local permeability losses in the reservoir

avalonian proximity of the ordovician miramichi terrane northern new brunswick northern appalachians paleomagnetic evidence for rifting and back arc basin formation at the southern margin of iapetus a paleomagnetic investigation of the middle ordovician tetagouche group in northern new brunswick was undertaken to determine the paleogeo aphi position of the miramichi terrane stepwise thermal demagnetization of pillow basahs reveals a high temperature characteristic gnet tion with a mean direction of b w z 69 k 22 aas 13 tih corrected n 7 sites 73 samples a positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is ordovician in age with a paleopole position of 52 n 352 e the corresponding pateolatitude of 535 places these vokanic rocks near the southern margin of the iapetus ocean at paleolatitudes similar to those revealed by avalon for the middle to late ordovician the mafic and felsic volcanic rocks and marine sedimentary rocks of the tetagouche and fournier groups have been interpreted to be remnants of a rifted continental margin and a middle ordovician back arc basin our results show that the process of rifting and back arc basin formation occurred at the avalonian margin of iapetus which implies that ordovician subduction was not restricted to the laurentian margin but also marks the southern margin of japetus

review paper a semi empirical model of strain sensitivity for 4d seismic interpretation we reformulate the original model of hatchell and bourne and roste stovas and landro that couples fractional velocity change to subsurface strain via a fundamental constant r the new model combines elastic compressibility of a dual porosity system for a sand shale mixture with horizontal planes of inter granular weakness the majority of observed r factor magnitudes from post stack 4d seismic data in both the reservoir and overburden can thus be explained r is predicted to depend strongly on lithology and also initial strain state the model is also extended to predict the observed angle dependence of time lapse time shifts from pre stack data an expression for the gradient of time shift with incidence angle is obtained in terms of the background vp vs and also the ratio of tangential to normal compliances bt bn representing loss or creation of inter granular coupling if accurately estimated from data this compliance ratio can be used as an additional parameter to assess the post production state of the overburden it is concluded that whilst r remains the over arching parameter controlling the magnitude of time shifts measured from 4d seismic data bt bn is a subtler parameter that may also prove of future value

paleomagnetism of the dunn point formation nova scotia high paleolatitudes for the avalon terrane in the late ordovician volcanic flows of the late ordovi clan dunn point formation contain a generally univectorial magnetization which passes the fold test its steep direction d 335 o i 61 o k 79 95 4 2o paleopole at 2os 136oe is un like any known for north america or for the ava lon terrane for post ordovician time so that a primary age of the magnetization appears to be very likely the paleoiatitude 42os for the avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic north american apparent polar wander path and a substantial post ordo vician displacement 3500 km of avalon with respect to the craton can be deduced in all likelihood the avalon terrane did not collide with north america until middle devonian time this collision produced the acadian orogeny high ordovician paleolatitudes have also been obtained for northern africa and for several localities in

geology and paleobiology of islands in the ordovician iapetus ocean review and implications a history of islands in the iapetus ocean and their destruction is recorded by widespread occurrences of lower to upper ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the appalachian caledonide orogen in north america and northwestern europe assemblages of arenigian and llanvirnian benthic shelly fossils chiefly brachiopods from these rocks constitute the celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces some that are younger than elsewhere and others that have not hitherto been found together evidence from these fossils and from the geology of the rocks pertaining to them suggests that north american terranes that included islands of arenigian llanvirnian age lay at middle to high latitudes both in mid ocean and within the armorican continental margin whereas coeval rocks in west central ireland and norway that have toquima table head shelly fossils were originally part of the laurentian continental margin eradication of the celtic assemblage and the reduction of ordovician faunal provinciality in the appalachian caledonide region coincide with llanvirnian obduction subduction of hundreds of kilometres of iapetus ocean crust late llanvirnian and younger ordovician rocks in the remnant basin including some of peri insular origin have brachiopods of scoto appalachian affinities no discrete islands of ashgillian age have been recognized although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places shelly fossils in these rocks and those in the siltstones and limestones from the gaspe peninsula are like those of ashgillian age elsewhere

ordovician provincial signals from appalachian caledonian terranes statistical analysis of mainly mid to deep shelf ordovician brachiopod faunas from localities within and marginal to the appalachian caledonian orogen reveals biogeographical patterns not entirely consistent with those presumed for the progressive orthogonal closure of the iapetus ocean correspondence analysis of faunas of late arenig early llanvirn late llandeilo earliest caradoc and late ashgill age emphasize the importance of marginal and island faunas the role of regressions and transgressions on faunal distributions and the effects of climatic change in the definition of biogeographical trends during the ordovician

time lapse full waveform inversion with ocean bottom cable data application on valhall field abstractknowledge of changes in reservoir properties resulting from extracting hydrocarbons or injecting fluid is critical to future production planning full waveform inversion fwi of time lapse seismic data provides a quantitative approach to characterize the changes by taking the difference of the inverted baseline and monitor models the baseline and monitor data sets can be inverted either independently or jointly time lapse seismic data collected by ocean bottom cables obcs in the valhall field in the north sea are suitable for such time lapse fwi practice because the acquisitions are of a long offset and the surveys are well repeated we have applied independent and joint fwi schemes to two time lapse valhall obc data sets which were acquired 28 months apart the joint fwi scheme is double difference waveform inversion ddwi which inverts differenced data the monitor survey subtracted by the baseline survey for model changes we have found that ddwi gave a cleaner and more easily interpr

monitoring overburden layer changes and fault movements from time lapse seismic data on the valhall field summary a method developed by roste et al which discriminates between layer thickness and velocity changes is tested on pre stack time lapse seismic ocean bottom cable obc data from the valhall field a key parameter in this discrimination process is the dilation factor which is the relative velocity change divided by the relative thickness change within a given layer the high quality and good repeatability of the obc data enables us to estimate with a reasonable accuracy for values between 0 and 5 for values below 5 complementary information like for instance geomechanics is required for the top reservoir horizon we estimate a maximum subsidence of 0 50 m 0 29 m and a corresponding velocity decrease for the sequence from the seabed to the top reservoir of 2 0 m s 1 0 40 m s 1 time lapse distortion patterns with characteristic time shift versus offset signatures are observed the positions and vertical extents of the distortion zones are determined from ray path considerations and modelling the distortion zones correlate with buried faults indicating that a time lapse distortion zone might be produced by a localized slip in a fault zone we present an extended method which allows for vertical in addition to lateral variations in the relative thickness and velocity changes this method can be viewed as a simplified version of time lapse tomography but involving fewer unknown parameters giving more stability to the estimated changes in thickness and velocity using this technique we are able to estimate for positions with localized time lapse distortions

silurian llandovery graptolites from the bay of exploits north central newfoundland and their geological significance only one specimen of a silurian graptolite has ever been recorded from newfoundland and it was not identifiable below generic level the graptolite assemblage discussed and figured here from a sequence of turbidites on upper black island north central newfoundland includes the first positively identified silurian taxa from the province and provides the first unequivocal evidence of silurian oceanic sedimentation in the dunnage zone the graptolite taxa include rastrites peregrinus barrande coronograptus sp cf c gregarius lapworth monograptus austerus sequens hutt orthograptus insectiformis nicholson monograptus spp indet climacograptus sp and glyptograptus sp this assemblage demonstrates that the strata previously assigned to the middle ordovician lawrence harbour formation are actually aeronian middle llandovery in age and that the turbidites considerably postdate both the lawrence harbour and point leamington formations of the exploits subzone furthermore the graptoli