sedimentology of a marine intermontane pleistocene gilbert type fan delta complex in the crati basin calabria southern italy the crati basin is a pliocene holocene extensional basin filled by the progradation of different types of marine fan delta systems coarse grained gilbert type fan deltas developed during the pleistocene they represent the sedimentary response to a strong differential uplift involving the basin margins and the basin itself the differential uplift was responsible for the fragmentation of the basin into several sub basins into which these fan deltas prograded the protection and lateral confinement by structural highs steep coastlines and the absence of strong tidal action in the adjacent ionian sea allowed the regular progradation of these fan deltas in the restricted gulfs and narrow embayments of the crati basin for the classical gilbert type fan deltas in lacustrine settings homopycnal inflow favours a rapid mixing of water masses and deposition of sediment close to the river mouth in the case of the example described here the density contrast between the sea water and the inflowing river water caused the separation of the muddy fraction from the coarse sandy and conglomeratic part of the sediment this allowed the development of steep mud poor coarse grained delta foresets slope instability features slump scars conglomeratic flow slides are fairly scarce in the proximal parts of the san lorenzo del vallo system towards the north where protection from the ionian sea was less they increase in importance

tectonic quiescence punctuated by strike slip movement influences on late jurassic sedimentation in the moray firth and the north sea region

contrasting styles of late jurassic syn rift turbidite sedimentation a comparative study of the magnus and oseberg areas northern north sea abstract contrasting types of sediment gravity flow and hemipelagite deposition are recorded by the late jurassic syn rift successions in the magnus and oseberg areas in the northern north sea in both areas the late jurassic rift interval was characterized by recurrent tectonic phases during the late oxfordian to early volgian the penguin halfgraben in the magnus area received a significant amount of coarse clastics and can be regarded as an example of a well fed deeper marine riftbasin periods with high rates of extensional faulting are characterized by isolated sediment gravity flow lobes which stack in an uneven albeit downdip shifting or backstepping manner the stratigraphic motif of these intervals is overall mud prone with coarse clastics represented by thick conglomerate sandstone mudstone aprons along the basin bounding masterfaults in contrast periods with low rates of extensional faulting are characterized by basinwide sheet like parallel layered sediment gravity flow packages which infill the riftbasin in a progradational manner from the middle volgian the penguin halfgraben became sediment starved and shows a similar development to the oseberg area in the oseberg area the late jurassic syn rift succession provides a typical example of a sediment starved deeper marine riftbasin sediment gravity flow deposition in this area is limited to periods of high rates of extensional faulting these coarser clastic depositional episodes mimic their counterparts in the penguin halfgraben being characterized by mud prone strata with localized isolated sediment gravity flow lobes and footwall derived clastic wedges along the basin bounding masterfaults in contrast to the well fed basins the intervening stages of tectonic quiescence in the sediment starved basins are characterized by basinwide blanketing of clay commonly of high source rock potential in both basins the rotational tilt packages can be subdivided into an early a climax and a late syn rotational interval based on the architecture of their contained facies tracts these depositional episodes reflect the temporal evolution of the riftbasins catchment areas and their linked depositional systems it is mainly the nature and amount of sediment supply during the following tectonic quiescence or post rift stages that determine whether the riftbasin remained sediment starved or became infilled simplified sequence stratigraphic models are of limited use in the analyses and lithology prediction of riftbasin infills as individual halfgrabens often have their own specific infill history and hence characteristic syn rift stratigraphic signature the main controls on the variability of these deeper marine syn rift sediment gravity flow hemipelagite packages are the style and rate of extensional faulting sediment supply the source area s sediment yield potential and calibre and the halfgraben configuration and location with respect to the main hinterland areas

role of clastic sedimentology in the exploration and production of oil and gas in the north sea summary the nature and quality of clastic reservoir rocks is the result of a complex interplay of tectonic setting provenance depositional environment and diagenesis in particular it is now well established that the reconstruction of depositional environments in clastic successions provides the optimum framework for describing and predicting reservoir development and reservoir quality distribution on both regional exploration and field production scales this review in the context of the 21st anniversary of the british sedimentological research group bsrg expands the above mentioned theme in relation to the development of depositional models of each of the major oil and or gas bearing clastic reservoirs in the u k sector of the north sea which has been the subject of a surge in geological interest following the first gas discoveries approximately 21 years ago each major reservoir interval is used to illustrate specific themes with emphasis placed on how the application of depositional models has contributed to a better understanding of the exploration for and the production from the following reservoirs i permian aeolian and fluvial sands rotliegendes sandstones southern north sea ii jurassic fluvial deltaic and marginal marine sands lower jurassic statfjord formation and middle jurassic brent group northern north sea iii jurassic shallow marine and deeper marine mass flow sands and conglomerates upper jurassic fulmar piper type sands and brae magnus type deposits central and northern north sea and iv tertiary deep marine turbidite fan deposits palaeocene and eocene sands central and northern north sea

changes in physical properties of a reservoir sandstone as a function of burial depth the etive formation northern north sea abstract rock physical properties like velocity and bulk density change as a response to compaction processes in sedimentary basins in this study it is shown that the velocity and density in a well defined lithology the shallow marine etive formation from the northern north sea increase with depth as a function of mechanical compaction and quartz cementation physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial at shallow depths

an application of heavy mineral analysis to lithostratigraphy and reservoir modelling in the oseberg field northern north sea abstract three distinctive detrital garnet associations have been recognized in brent group sandstones of the oseberg area northern north sea by means of electron microprobe analysis these have lithostratigraphical significance with one association occurring in the etive and oseberg formations one in the ness formation and one in the tarbert formation the stratigraphic variation in garnet population reflects changes in provenance and in the pattern of distribution of clastic detritus although a detailed reconstruction of source areas is not possible on the basis of the available information differentiation of ness and etive oseberg formations by means of detrital garnet geochemistry has enabled the recognition of fluvial chanbel sandstones belonging to the ness formation which have cut down into the etive formation shoreline complex this provides direct evidence of communication between ness and etive oseberg adding an extra degree of sophistication to the geological input for reservoir simulation a strong permeability anisotropy can be modelled around the fluvial channels and a net sand volume can be modelled in agreement with channel orientation

episodic river flooding events revealed by palynological assemblages in jurassic deposits of the brent group north sea spore and pollen sporomorph assemblages from middle jurassic marine deposits of the brent group in the northern north sea are investigated to assess temporal and spatial variations in vegetation and depositional processes four wells were sampled for palynology from the penguins cluster and the don north east fields through the rannoch formation shoreface succession hyperpycnite deposits occur throughout but are concentrated within the lower part of the section these are expressed by sand prone beds displaying waxing and waning current motifs normally graded muddy beds and structureless mudstones hyperpycnal hypopycnal deposits resulting from episodic river flooding represent important sedimentary features as they may be preserved below fair weather wave base in more offshore settings and potentially be the only record of the former presence of a nearby river mouth the hyperpycnites typically contain abundant botryoccocus spp amorphous organic matter aom and hinterland sporomorph taxa with relatively few marine components compared to associated marine shoreface facies variations in palynofacies assemblages and botryococcus spp abundances indicate frequent river mouth avulsion ordination of samples using non metric multidimensional scaling nmds indicates that shoreface samples of the sampled wells are relatively distinct but hyperpycnite samples are highly similar regardless of their sampled well this suggests that depositional processes and spore pollen sources i e catchment zones were similar among hyperpycnite events across different wells abundant bisaccate pollen botryococcus spp and aom within interpreted hyperpycnites suggest sediment mixing along the fluvial drainage path during flooding events the terrestrial signature of hyperpycnite sporomorph assemblages demonstrates that underflows remained coherent as they descended the shoreface profile with little turbulent mixing with ambient marine waters sporomorph assemblages display few large changes through time suggesting vegetation on the adjacent coastal plain was relatively static through the studied interval

the petroleum geology of the unst basin north sea abstract the unst basin is situated in the northern north sea between the east shetland basin and the shetland isles the basin is essentially a three armed permo triassic fault controlled basin containing up to 3600 m of red beds this is overlain by a westerly thickening jurassic and early cretaceous sequence the stratigraphy of which is very similar to that of the east shetland basin in particular the brent group 140 m humber group 685 m and cromer knoll group 300 m are well represented as a result of laramide uplift of the area the thick upper cretaceous and palaeocene strata of the east shetland basin are absent from the unst basin this uplift resulted in substantial erosion within the unst basin providing the major source for palaeocene sands in the viking graben and the faeroes basin late palaeocene and younger tertiary strata transgress westwards across this erosion surface petroleum exploration within the basin culminated in the drilling of two exploration wells these wells encountered potential reservoir and source rocks in the jurassic section however geochemical analyses indicate these source rocks are immature for hydrocarbon generation within the unst basin it is concluded that the unst basin has a low petroleum potential

footwall uplift associated with late jurassic normal faulting in the northern north sea tilted fault blocks in the north sea show varying degrees of erosion such footwall erosion is predicted by the domino model of extension in which rigid fault blocks rotate as extension proceeds profiles of three north sea fields snorre brent and heather are analysed to estimate the amount of uplift that accompanied late jurassic normal faulting and these estimates are compared with the footwall uplift predicted by the domino model agreement between observed and predicted uplift is excellent varying from 1 km at snorre to c 100 m at heather for a given amount of extension large fault blocks tend to be bounded by large faults and to show a greater degree of erosion the brent province of the northern north sea is an area of prolific hydrocarbon discoveries almost all of the major oil fields occur in tilted fault blocks which were formed by late jurassic extension and subsequently buried by cretaceous tertiary thermal subsidence badley et al 1988 marsden et al 1990 the crests of the tilted blocks are typically eroded or degraded with varying amounts of middle lower jurassic and upper triassic section removed this footwall erosion is commonly ascribed in the literature to pre rifting doming ziegler 1982 heterogeneous stretching coward 1986 local transpression beach 1985 or even regional compression frost 1987 the purpose of this paper is to show that the uplift and degradation is a direct consequence of the late jurassic normal faulting and fault block rotation the degree of uplift is related in a simple way to the size across strike of

normal fault growth and fault related folding in a salt influenced rift basin south viking graben offshore norway abstract three dimensional seismic data were analysed to reconstruct the structural and stratigraphical development of a salt influenced rift basin and thus gain an understanding of the relationships between normal fault growth salt tectonics and the evolution of syn rift depocentres the sleipner basin south viking graben northern north sea is ca 30 km long by 8 km wide and is bound to the east by a major extensional fault zone sleipner fault zone two types of fault related fold are identified within the basin 1 a fault parallel monocline interpreted as an extensional forced fold which formed through the upward propagation of the sleipner fault zone through ductile evaporites of the zechstein supergroup and 2 three fault perpendicular salt cored anticlines that compartmentalise the basin into four sub basins and are related to displacement gradients along strike of the sleipner fault zone detailed seismic stratigraphic analysis of pre and syn rift stratal units reveals a complex interplay between fault growth and salt movement which strongly controlled the evolution of syn rift depocentres during the early syn rift a series of depocentres separated along strike by the fault perpendicular folds were offset into the axis of the basin ca 3 4 5 km to the west of the sleipner fault zone by the fault propagation fold later in the rift event the influence of the fault perpendicular folds depleted resulting in a larger interconnected depocentre that shifted into the immediate hangingwall of the fault as the surface of the fault propagation fold was breached the results of this study have implications for normal fault growth and sedimentary depocentre development in salt influenced rift basins and contribute to the general understanding of the controls on salt migration

dunlin group sequence stratigraphy in the northern north sea a model for cook sandstone deposition the dunlin group in the northern north sea consisting of the johansen amundsen burton cook and drake formations of late sinemurian toarcian age hosts important hydrocarbon reservoirs in the cook formation sandstones the johansen formation is associated with a relative fall of sea level and is interpreted to be a large sandstone delta confined within a broad incised valley at the base of the group during a later stage of relative sea level rise the finer grained amundsen and burton formations were deposited the overlying cook formation consists of four sandstone tongues each of which is characterized by a lower zone of sharp based upward coarsening thinly bedded shoreface sandstones and siltstones reflecting forced regression during falling relative sea level and an erosively based upper zone of thin tidal flat and thick deltaic estuarine sandstones reflecting lowstand incision as well as initial progradation and subsequent transgressive backfill of estuaries during relative sea level rise the drake formation shales were deposited during continued relative sea level rise several types of erosional surfaces are recognized within the studied succession 1 sequence boundaries occur at the base of the johansen formation and within the cook formation and represent the bottoms of incised valleys that truncate the underlying copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received may 4 1995 revised manuscript received march 27 1996 final acceptance september 12 1996 2department of geology paleontology university of zagreb 10000 zagreb croatia 3department of geology geophysics university of wyoming laramie wyoming 82071 this paper was part of the sequence stratigraphy research project at the university of bergen norway and is a product of studies on the architecture of lower jurassic shelf sandstones of the northern north sea the work was sponsored by statoil through a vista fellowship awarded to t marjanac and this support is gratefully acknowledged we are grateful to d mellere and l m falt for discussion and encouragement during the work we also thank else lien for drafting the sections and jane ellingsen for reprographs constructive criticism from d nummedal j webb and k t biddle on the final version of the manuscript is much appreciated

contrasting behavior between dispersive seismic velocity and attenuation advantages in subsoil characterization a careful look into the pertinent models of poroelasticity reveals that in water saturated sediments or soils the seismic p and s wave velocity dispersion and attenuation in the low field seismic frequency band 20 200 hz have a contrasting behavior in the porosity permeability domain taking advantage of this nearly orthogonal behavior a new approach has been proposed which leads to unique estimates of both porosity and permeability simultaneously through realistic numerical tests the effect of maximum frequency content in data and the integration of p and s waves on the accuracy and robustness of the estimates are demonstrated

seismicity of the cyprus region we have used a combination of macroseismic and instrumental techniques to re evaluate the seismicity of cyprus and the surrounding region for the period 1890 1900 we identified 56 events which we believe to have a surface wave magnitude ms of 4 5 or greater early events are located by felt effects alone and macroseismic information is used to improve the location of many later events we find the broad pattern of seismicity follows that given by standard listings an area of weak seismicity to the north west of cyprus in the gulf of antalia includes intermediate depth events as are found in the hellenic arc to the west a gap of reduced activity separates this activity from that of the main region of cyprus where we could find no evidence for sub crustal foci there appears to be no present seismological connection along the presumed plate boundary to the north east to the junction of the east anatolian fault and the extension of the dead sea rift system we find a much greater level of activity in the first part of this century than is given in the standard listings such as that of the isc up to 1963 we identified 47 events of ms 4 5 compared with only 10 given in the isc files recent improved station coverage locates more events in the region but these have not been of large magnitude and the moment release in recent decades has been appreciably less than earlier in the century for the events in our list the total moment release is 2 7x 1019 nm with 75 of this originating in the main cyprus region we reaffirm the danger of uncritical use of standard earthquake listings for tectonic interpretation and in particular for hazard evaluation in areas such as this where neither the historical nor the instrumental seismicity record is complete

analysis of floodplain sedimentation avulsion style and channelized fluvial sandbody distribution in an upper coastal plain reservoir middle jurassic ness formation brent field uk north sea

properties of aftershock sequences in southern california the temporal behavior of 39 aftershock sequences in southern california 1933 1988 was modeled by the modified omori relation minimum magnitudes for completeness of each sequence catalog were determined and the maximum likelihood estimates of the parameters k p and c with the standard errors on each were determined by the ogata algorithm the b value of each sequence was also calculated many of the active faults in the region both strike slip and thrust were sampled the p values were graded in terms of the size of the standard error relative to the p value itself most of the sequences were modeled well by the omori relation many of the sequences had p values close to the mean of the whole data set 1 11 0 25 but values significantly different from the mean as low as 0 7 and as high as 1 8 exist no correlation of p with either the b value of the sequence or the mainshock magnitude was found the results suggest a direct correlation of p values is with surface heat flow with high values in the salton trough high heat flow and one low value in the san bernardino mountains and on the edge of the ventura basin both low heat flow the large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean 74 mw m2 if the hypothesis that aftershock decay rate is controlled by temperature at depth is valid the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature surface heat flow is taken as an indicator of crustal temperature at hypocentral depths but the effects on heat flow of convective heat transport and variations in near surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution the interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials leading to a faster decay rate higher p value in 1967 mogi proposed such a relation on the basis of japanese data seismic velocity distributions published for southern california generally support the hypothesis with low velocities higher temperatures corresponding to high p values and vice versa

slide block of jurassic sandstone and submarine channels in the basal upper cretaceous of the viking graben norwegian north sea abstract seismic facies analysis of a seismically anomalous sequence of mid cretaceous cenomanian turonian basin fill in lease block 34 8 viking graben norwegian north sea reveals a possible slide block composed of jurassic age brent group strata that apparently detached from the adjacent structural high 34 8 structure and slid downslope onto lower cretaceous sediments in the viking graben floor in addition this mid cretaceous sequence contains two broad submarine channels that trend southward into the graben from the southern tip of the slide block and the adjacent structural high the channels appear to be erosional features formed by turbidity currents and mass wasting processes the brent slide block may represent an additional hydrocarbon prospect within the lease block 34 8

magnetostratigraphy of the middle jurassic brent group in the oseberg oil field northern north sea abstract magnetopolarity stratigraphy of the brent group and the uppermost part of the dunlin group has been established in two well cores from the oseberg field well 30 6 7 and well 30 9 2 northern north sea palaeomagnetic analysis of thermal demagnetization results defines a series of normal and reverse magnetozones in both wells remanent coercivity analysis of these middle jurassic clastic sediments indicates a magnetomineralogy independent of grain size interpreted to be of diagenetic origin thus suggesting that the magnetization is of early post depositional origin integration with the existing palynostratigraphic framework results in the establishment of several isochronous horizons especially in the ness formation which suggests lateral continuity of magnetostratigraphic datum planes between the two wells at the oseberg field although the exact dating of the magnetozones is not established the combined application of magnetopolarity and biostratigraphy provides a stratigraphic framework of improved resolution for these rock formations

seismic behaviour of the dead sea fault along araba valley jordan the dead sea fault zone is a major left lateral strike slip fault south of the dead sea basin the wadi araba fault extends over 160 km to the gulf of aqaba the dead sea fault zone is known to have produced several relatively large historical earthquakes however the historical events are unequally distributed along the fault and only four events have been reported in the araba valley over the last few thousands of years magnitudes estimated from the historical record are probably slightly smaller than that of the m w 7 3 earthquake that struck the gulf of aqaba in 1995 the fault cuts straight across pleistocene to holocene alluvium and shows morphologic evidence for essentially pure strike slip motion regional seismic monitoring reveals little microseismicity along the fault except around the dead sea and gulf of aqaba where the fault splays into complex pull apart basin fault systems we have investigated the fault zone at several sites selected from spot images and the study of aerial photography at the site of the now destroyed tilah castle a well preserved wall dated to be about 1200 yr bp 14 c age on charcoal is cut by the fault and offset by 2 2 m comparison with offset gullies at a nearby site 3 km to the north and at three other sites respectively 25 50 and 65 km to the south reveals that this specific fault displacement is probably related to the last seismic event that ruptured that fault segment possibly in ad 1458 moreover the offset gullies suggest a characteristic slip behaviour with recurring slip of about 1 5 m on average given the 4 2 mm yr 1 slip rate derived for this fault segment we infer that the fault should produce m w 7 earthquakes along some segment in the araba valley about every 200 years the historical period with only four well documented large earthquakes in ad 1068 ad 1212 ad 1293 and ad 1458 thus appears to have been relatively quiescent with a 20 per cent deficit of m w 7 earthquakes however our data do not exclude the possibility of larger m w 7 6 earthquakes or time clustering of earthquakes over longer timespans an alternative seismic behaviour involves m w 7 6 earthquakes about every 6000 years and m w 7 earthquakes about every 250 years the historical catalogue would then appear to be complete for m w 7 earthquakes

a new palaeogeographic reconstruction for the middle jurassic of the northern north sea an estuarine sequence equivalent in age to the brent group deltaic deposits further north is recognized in the central viking graben it indicates a persistent marine connection southwards from the boreal ocean during deltaic accumulation and suggests that the brent delta did not prograde from the south along the graben as previously suggested evidence from the uk north viking graben indicates the brent delta was probably derived transversely rather than axially as were the other jurassic clastic systems infilling the graben

the middle jurassic oseberg delta northern north sea a sedimentological and sequence stratigraphic interpretation the aalenian oseberg formation 0 80 m thick is an important reservoir unit in the middle jurassic brent group in the northern north sea consisting of multiple sets of sandy gilbert type deltas small scale 1 5 10 m fining upward units seen in the gamma ray log correspond with individual delta sets as independently confirmed by steepening upward trends seen in the dip log within each set the steep foreset slopes typically show thinly bedded sandstone facies avalanche grain flows whereas the lower foreset slopes toesets and bottomsets are formed largely by massive sandstone facies sandy debris flows on an intermediate scale up to 40 m the gamma ray logs show both fining upward and coarsening upward trends through stacked delta sets and these trends traceable between wells are interpreted in terms of decelerating and accelerating rates of relative sea level rise respectively the relative abundance of the sandy debris flow deposits reflects a periodic and significant instability of the delta s upper foreset slope probably during times of increased water depth in front of the delta the normal progradation of individual gilbert type sets however is likely to have been along a subhorizontal topography during periods of little or no change in water depth the long term change to produce the observed vertical stacking of deltaic sets was one of a generally rising relative sea level modeling of the sea level rise in a steplike manner can account for the rarity of topsets the progressive eastward truncation of the underlying drake formation on the horda platform by the oseberg formation suggests tectonic uplift and a lowstand of sea level immediately prior to the development of the oseberg deltas the oseberg formation is thus seen as a lowstand prograding wedge that developed during an interval of relatively high sediment supply and a variably rising sea level culminating in the latest aalenian flooding seen in the base of the overlying rannoch formation the dip log data integrated with cores and other logs are critical to correctly interpreting the reservoir s internal architecture the slope readjustment model predicts that the development of stacked massive reservoir sandstones of oseberg type will occur preferentially in local areas of high subsidence or syndepositional faulting or where there is deeper water in the lee of already existing fault scarps

in situ ground freezing to obtain undisturbed samples of loose sand in situ ground freezing with liquid nitrogen can be an excellent technique for obtaining high quality undisturbed samples from a wide range of loose sand deposits with their in situ conditions preserved this paper describes the techniques used during the canlex project to carry out in situ ground freezing and undisturbed sampling of the loose sand deposits encountered at six test sites specific details with respect to the effects of difficult subsoil or ground water conditions encountered at the sites are provided modifications made to the ground freezing and sampling system to accommodate difficult ground conditions are described the actual advance of the freezing front and the consumption of liquid nitrogen are compared with theoretical predictions the dry coring technique utilized to recover undisturbed frozen sand samples is described the void ratios measured for the frozen core samples are compared with void ratio measurements obtained using geophysical logging in boreholes located within a 5 m radius of the freeze pipe the costs associated with in situ ground freezing utilizing liquid nitrogen and coring of the frozen sand at six test sites are presented

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

a sedimentological approach to refining reservoir architecture in a mature hydrocarbon province the brent province uk north sea improved reservoir characterisation in the mature brent province of the north sea aimed at maximising both in field and near field hydrocarbon potential requires a clearer understanding of sub seismic stratigraphy and facies distributions in this context we present a regional high resolution sequence stratigraphic framework for the brent group uk north sea based on extensive sedimentological re interpretation of core and wireline log data combined with palynostratigraphy and published literature this framework is used to place individual reservoirs in an appropriate regional context thus resulting in the identification of subtle sedimentological and tectono stratigraphic features of reservoir architecture that have been previously overlooked we emphasise the following insights gained from our regional high resolution sequence stratigraphic synthesis 1 improved definition of temporal and spatial trends in deposition both within and between individual reservoirs 2 development of regionally consistent predictive sedimentological models for two enigmatic reservoir intervals the broom and tarbert formations and 3 recognition of subtle local tectono stratigraphic controls on reservoir architecture and their links to the regional middle jurassic structural evolution of the northern north sea we discuss the potential applications of these insights to the identification of additional exploration potential and to improved ultimate recovery

systematic destruction of k feldspar in deeply buried rift and passive margin sandstones systematic patterns of detrital k feldspar decline with increasing burial depth are revealed during deep burial 1 5 4 5 km of sandstones in diverse sedimentary basins from three rift and two passive margin settings in the north sea and usa gulf coast k feldspar destruction is predominantly by dissolution though in situ alteration to albite also occurs dissolution can remove up to 15 solid volume of the rock by 4 5 km so that primary arkosic sandstones become diagenetic quartzarenites at depth with increased secondary primary porosity ratios the quantity of k feldspar destroyed during temperature increase is very variable 0 05 1 0 solid volume c compaction continues during deep burial and petrographic textures diagnostic of secondary porosity are destroyed so that many sandstones preserve exceptions to this scenario of detrital k feldspar dissolution are noted proximity of k rich evaporites high sio 2 and k activity due to the breakdown of volcanic clasts or very low p co 2 can stabilize both k feldspar and a suite of k rich cements

constrained 1d joint inversion of seismic surface waves and p refraction traveltimes we present a joint inversion scheme that couples p wave refraction and seismic surface wave data for a layered subsurface an algorithm is implemented with a damped least squares approach the estimated parameters are s and p wave velocities and layer thicknesses while densities are assumed constant during inversion the coupling is both geometric and physical layer thicknesses are the same for sand p wave velocity profiles and p wave velocities enter in both forward algorithms sensitivity analysis performed on synthetic data reveals that surface wave dispersion curves can be sensitive also to p wave velocity of some layers especially for poisson s ratio values smaller than about 0 35 allowing synergic resolution of this parameter applications on both synthetic and field data show that the proposed approach mitigates the hidden layer problem of seismic refraction and leads to more accurate results than individual inversions also for surface waves additional constraints on the objective function on a priori poisson s ratio values allow unrealistic and not admissible vp and vs values to be avoided such constraints were applied in one field case considering the a priori information available about water table depth it is also shown that estimation of porosity can help the selection of the proper constraint on a priori poisson s ratio

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

modelling of bed contours and cross sections adjacent to planar normal faults the geometry of beds in the volume surrounding a single blind planar fault depends on i the axial ratio of the fault surface ellipse ii the ratio between the maximum displacement and the maximum dimension of the fault surface iii the displacement distribution on the fault surface from the point of maximum displacement to zero displacement at the tip line loop fault surface displacement profile and iv the rate at which the displacement decreases to zero along a normal to the fault surface reverse drag displacement profile these relationships which are either known or can be estimated are incorporated in a computer model which generates structural contours on horizons intersected by a planar fault structural contour patterns also vary with the pre faulting dip and strike of beds the dip and strike of the fault and the level at which a bed intersects the fault surface when the boundary conditions are relaxed to include synsedimentary faults the ratio of hangingwall to footwall displacement varies systematically with fault dip a synthetic structural contour map is matched with that of an actual fault model isopachs based on simple depositional models are generated for synsedimentary units cross sections are generated for synsedimentary faults by sequential fault growth and sedimentary deposition and show features similar to actual cross sections the soft domino model is applicable primarily to dip slip tectonic faults with maximum displacements of up to about 5 km listric faults are not considered

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

jurassic extension estimates for the north sea triple junction from flexural backstripping implications for decompression melting models abstract the rattray volcanics lie at the triple junction between the three arms of the north sea rift system the mildly undersaturated volcanics in this province were erupted during the early syn rift phase of a middle late jurassic rifting event and are thought to be the products of decompression induced partial melting of the mantle resulting from lithospheric attenuation during rifting situated as they are in one of the most intensively studied rift basins in the world the rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by mckenzie and bickle 1 this study uses a modified version of the backstripping technique which takes into account the flexural strength of the lithosphere to constrain the stretching factor for the jurassic rift phase two dimensional interpreted sections are flexurally backstripped to the base post rift to give an initial estimate of is then further constrained by forward modelling to the base post rift basin geometry generated by backstripping forward modelling uses the flexural cantilever model a combined simple shear pure shear model for continental extension which takes into account the thermal rheological and flexural isostatic consequences of extension this is an approach that is different from that of previous studies 2 5 which assume local rather than regional isostatic compensation to allow the backstripping of individual wells the results obtained in this new study give maximum jurassic factors on each of the three rift arms close to the triple junction of 1 2 1 35 this suggests a maximum jurassic factor in the triple junction itself of 1 35 1 75 this is considerably lower than previous estimates and is due to the breakdown of the assumption used in these previous studies that airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis whilst such a factor is insufficient to generate melt by decompression alone an anomalously high mantle potential temperature and the presence of volatiles as well as a complex mesozoic and late palaeozoic rifting history are likely to have played a role in the generation of melt

regressive and transgressive cycles in a rift basin depositional model and sedimentary partitioning of the middle jurassic hugin formation southern viking graben north sea abstract the jurassic hugin formation consists of shallow marine sandstones that belong to a significant hydrocarbon reservoir in the sleipner area in the norwegian north sea the formation encompasses coarsening upward units of mouth bar and shoreface facies interpreted to record delta outbuilding during regression and fining upward units with tidal channel dune and tidal flat facies interpreted as part of an estuary environment during transgression the correlations reveal that the studied part of the hugin formation consists of 8 sequences each with a transgressive and a regressive unit representing the transgressive systems tract and the highstand systems tract respectively the sequences are stacked retrogradationally landward as a result of rapid tectonic subsidence and rifting of the viking graben rifting led to the development of an elongate graben where tidal currents were amplified wave action damped and longshore drift as sediments supply reduced or absent lowstand and forced regressive systems tracts are not identified and their absence is interpreted to reflect suppression of relative sea level falls in a rapidly subsiding basin where the basin subsidence rate outpaced any potential fall in eustatic sea level through facies interpretation and sequence stratigraphic correlations between wells these regressive and transgressive units are shown to exhibit characteristic thickness trends in the form of sigmoidal shaped wedges stacked in an offset manner in a landward to basinward orientation these thickness trends illustrate sediment partitioning within the sequences and are explained by the relationship between accommodation versus sediment supply in terms of mass balance during regression the focus of sedimentation was pushed basinward and during transgression it was pushed landward as sediments were trapped there the mapping of these sequence stratigraphic units serves as input to reservoir models and to help increase recovery and identify new exploration targets

aftershock decay of three recent strong earthquakes in the levant aftershock sequences are studied from three recent strong earthquakes in the levant the mw 7 1 earthquake of 22 november 1995 and the ml 5 8 earthquake of 3 august 1993 both in the gulf of aqaba and the mw 6 8 earthquake of 9 october 1996 off the southwest coast of cyprus the modified omori s law provides a good fit to the decay of aftershock activity for both events the two sequences from the gulf of aqaba have rather low p values 0 90 in 1993 and 0 75 in 1995 reflecting slow decay of the aftershock activity this may be attributed to low heat flow close to the epicenters for the cyprus sequence the p value is 1 09 similar to that of many other sequences around the globe

preservation and grain size trends of holocene wave dominated facies successions in eastern japan implications for high resolution sequence stratigraphic analysis vertical grain size trends provide significant information for the sequence stratigraphic analysis of siliciclastic wave dominated facies successions this paper discusses the relationships between the preservation timing and the grain size trends of holocene wave dom inated successions in two contrasting coastal plains the kujukuri and sendai using high resolution ams 14 c dating depositional facies and grain size modal grain size is proposed as a good criterion for eval uating qualitative changes of depositional depth the kujukuri and sendai successions are both fining upward to coarsening upward wave dominated successions that have developed upon ravinement surfaces the fining upward part of the kujukuri succession formed in regressive deposits during a highstand period whereas that of the sendai formed during transgression a difference in the timing of the generation of upward fining is due to the varying preservation potential of transgressive shoreface deposits on the ravi nement surface in the sendai area the potential was much higher than in the kujukuri area because the sendai ravinement surface had re ceived greater sediment supply from the adjacent rivers during trans gression a differing rate of tectonic uplift between the areas was not critical to the preservation potential because of the short time scale the successions show bimodal grain size distributions in the basal parts and unimodal distributions elsewhere the bimodality is consid ered to be the result of mixing of a coarse transgressive lag with finer sediment derived from external sources the finer mode forms a con sistent trend with unimodal grain sizes in overlying deposits the trend is of a simple upward coarsening in the kujukuri and an upward fining to coarsening in the sendai showing distinctive correlation with chang es of depositional depth thus the vertical trend of the modal grain size and not mean or median is best for evaluating changes of de positional depth in ancient wave dominated facies successions

crustal thinning and nature of extension in the northern north sea from deep seismic reflection profiling a regional network of deep seismic reflection profiles recorded in the northern north sea has been used to map mesozoic and cenozoic basin thickness and crustal thickness in the viking graben and adjacent platform areas depth to the reflection moho varies from about 20 km in parts of the viking graben to about 32 km beneath the shetlands and the norwegian margin the shallowing of the reflection moho beneath the viking graben implies crustal stretching factors for mesozoic extension greater than 2 in the center of the north sea basin the axis of crustal thinning is located directly beneath the viking graben the axis of the sedimentary basin steeply dipping basin bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle but no continuous reflective feature extending from the sedimentary basin to beneath the moho is observed on any of the deep profiles thus these data do not support the existence of lithosphere penetrating low angle detachments zones of simple shear as the cause of extension in the northern north sea some of the mantle reflectors dip to the west and some to the east suggesting that simple shear if it occurs in the upper mantle is not of uniform sense rather these data suggest a complex depth dependent pattern of brittle extensional deformation in the upper crust pervasive ductile extension bulk pure shear in the lower crust which is decoupled from deformation in the mantle and extension accommodated by discrete dipping shear zones in the lithospheric mantle

regional seismicity of the mid atlantic ridge observations from autonomous hydrophone arrays summary autonomous hydrophones arrays are an excellent tool for monitoring mid ocean ridge seismic activity the major advantage of using arrays of autonomous hydrophones for recording deep ocean ridge earthquakes is its low magnitude detection thresholds achievable using hydroacoustic techniques regional analysis of the detection thresholds of the different autonomous hydrophones arrays deployed along the mid atlantic ridge reveals the strong influence of the detection threshold in the number of recorded events and it must be taken into account in any further analysis in this study the analysis of both autonomous hydrophones and teleseismically detected mid atlantic ridge seismicity reveals that the background seismicity from the relatively short recording periods of the autonomous hydrophones mimic the results of the much longer teleseismic recording it also reveals that seismicity generally cluster at both the segment scale and on mantle bouguer anomaly maxima the big majority of these clusters seem to be related to dyke intrusions and propagation along the mid atlantic ridge these dyke intrusions interact with the mainshock aftershock sequences the seismic sequences mainshock aftershock analysis reveals that the strength of the faults is highly influenced by the mode or style of faulting detachment faults which are ubiquitous along the mid atlantic ridge can produce more prolific shorter duration seismic sequences revealing faster and reduced strain releases in comparison to higher angle normal faults this reduced strain release is most likely to occur due to the presence of higher levels of serpentinization on detachment faults higher levels of serpentenisation can also promote an aseismic transient slip on the mainshock aftershock sequences

lower cretaceous tides revealed by cross bedding with mud drapes the sand dominated folkestone beds widely exposed around the weald of south east england are of upper aptian to lower albian lower cretaceous age and range in thickness up to 84 m refs 1 3 their content of ammonites bivalves and sponges indicates a marine origin while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land one striking sedimentary facies is formed by medium to coarse grained cross bedded sands in which several foreset and or bottomset surfaces support mud drapes although it was suggested4 that this facies may have been formed by the storm induced movement of sand waves i now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides and that the mud drapes are formed at slack water a model of this process has been described elsewhere5 and i now describe measurements of a section which supports my earlier conjecture6that the separation of the mud drapes is systematically related to the phase of the spring neap tidal cycle at the time of deposition although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime

heat flow and continental breakup the gulf of elat aqaba heat flow measurements were made at five sites in the major basins of the gulf of elat aqaba northern red sea the gulf is located at the southern portion of the dead sea rift which is a transform plate boundary vertical temperature gradients were measured with a probe which allows multiple penetration of the bottom during a single deployment and thermal conductivities were determined by needle probe measurements on sediment cores a mean heat flux of about 80 mwm 2 was found and the values tend to increase from north to south this latitudinal gradient corresponds to the general trend of gradual thinning of the continental crust of the gulf toward the red sea where oceanic crust exists the heat flow data together with other geophysical data are consistent with a propagation of rifting from the red sea northward along the dead sea rift where the fractures are primarily shear the gulf forms a transition between these two tectonic regimes extensional red sea to the south and transform dead sea to the north

controls on stratigraphic architecture in contemporaneous delta systems from the eocene roda sandstone tremp graus basin northern spain abstract the eocene roda sandstone of the spanish pyrenees is comprised of two coeval coarse grained delta systems which share correlatable surfaces but show a markedly different internal facies architecture the two deltas developed contemporaneously with the differences in internal architecture attributed to basin morphology and the role of two distinct sediment sources while the correlatability of the larger scale packages indicates an external allocyclic control as such the roda sandstone provides the possibility to address the relative roles of different parameters known to interact and control stratal architecture the roda sandstone crops out at the ne margin of the tremp graus basin a piggyback basin initiated in palaeogene times in the foreland of the southern pyrenees the study area 30 km2 is composed of the well understood and well exposed parts of the roda sandstone cropping out along the isabena valley and the less worked and less exposed outcrops to the north along barranco de ricans and barranco de codoneras data for this study include a combination of conventional sedimentology data coupled with analyses of 3d virtual outcrop models based on these methods 8 different facies associations have been distinguished their distribution suggests that the roda sandstone is composed of two different but coeval delta systems and their linked subenvironments a gilbert type delta outcropping along isabena valley and a shoal water fan delta along barranco de ricans and barranco de codoneras along the isabena valley the outcrops of the gilbert type deltaic system are composed of at least 6 distinct packages roda u z characterized by texturally mature delta front sandstones separated by offshore mudstones the stacked basinward stepping architecture of the roda u y units here represents a minimum of 3 major phases of sw directed progradation capped by hardground horizons formed during the subsequent flooding phases tidally modified delta front deposits found in the distal exposures of roda y represent the redistribution of the delta toeset by w nw directed ebb tidal currents active during advanced stages of progradation of the gilbert type delta the more gentle sloping shoal water fan delta system found in barranco de ricans and barranco de codoneras is characterized by progradational to aggradational stacking of texturally immature wave modified mouth bar sandstone packages separated by offshore mudstones the zone of interference between the gilbert type and shoal water delta is dominated by wave reworked sediments deposited as shoreface barrier and spit deposits biostratigraphic dating suggests that the roda sandstone was deposited during a 3rd order relative sea level cycle as a shelf margin systems tract however the stacking of the individual roda u y units demarcated by offshore mudstones both within the gilbert type and the shoal water deltaic systems suggest that 4th order fluctuations occurred these 4th order units are physically correlatable between the two deltaic systems which implies that both deltas responded synchronously to a combination of regional subsidence and eustatic sea level fluctuations the differing internal architecture such as the steeply dipping textural mature foreset beds of the gilbert type delta versus the more gentle sloping textural immature mouth bars of the shoal water fan delta are controlled by different hinterland characteristics and intrabasinal topography

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

the use of field analogues in the correlation and static reservoir methodology used in the tern field northern north sea uk three dimensional modelling of reservoirs deposited in different depositional environments suggests that they require different correlation methodologies an example drawn from the tern field located in the brent province in the northern north sea offshore uk shows that the rannoch formation interpreted from core as a shoreface deposit can successfully be correlated using a sequence stratigraphic framework constrained by flooding surfaces in contrast the younger lower ness formation interpreted as dominantly coastal plain in character also required a sequence stratigraphic approach but one based upon broader depositional cycles in the latter correlation it was important not to correlate isolated sandbodies within the channel belts which would have led to an overestimation of reservoir volumes field analogues from the book cliffs of utah and breathitt group of kentucky support the concepts behind the contrasting approaches to correlation while not necessarily providing precise depositional analogues the interpolation of reservoir properties in the tern field reservoir model demonstrates that the correlation strategy is robust and provides evidence of a northwesterly progradation direction within the rannoch formation of the tern field

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

apatite fission track results from ireland and the porcupine basin and their significance for the evolution of the north atlantic abstract apatite fission track analysis was applied to samples from the irish mainland and exploration boreholes 13 3 1 26 26 1 and 26 28 1 in the northern porcupine and donegal basins to study the regional thermal and tectonic history of the irish continental margin samples from the irish mainland have apatite ages ranging from 301 25 to 56 5 ma with mean track lengths in the range 13 89 0 25 to 11 19 0 22 m and indicate higher temperatures during the middle jurassic about 160 180 ma and during the late cretaceous to early tertiary about 50 80 ma these two periods of increased palaeotemperatures correlate with major reorganizations in plate movements and suggest that uplift and erosion after the maximum palaeotemperatures was achieved as a consequence of a variation in the regional pattern of crustal stress the results from borehole 26 28 1 indicate that within the resolution of the data all samples are presently at their maximum post depositional burial temperatures to produce the mean track length results in the range 11 24 0 49 to 11 81 0 20 m from borehole 26 26 1 it is necessary to invoke post palaeocene geothermal gradients in excess of 50 c km increased relative to the present day gradient 28 c km similar increased geothermal gradients are recognized during the early tertiary in adjacent sedimentary basins and may be explained as a result of heat advection by gravity driven fluid flow established in response to tectonic uplift of the basin margins

seismic observations of the 22 11 1995 gulf of aqaba earthquake sequence abstract we analyzed the aftershock sequence of the gulf of aqaba earthquake recorded by short period and broad band stations between november 1995 and december 1996 the seismicity pattern reveals significant activity often confined to known surface traces of active faults or within local grabens most of the aftershock activity is concentrated north of the main shock in the aragonese and eilat basins while the activity is diffused south of the main shock in the dakar and tiran basins most of the moderate to strong aftershocks of the whole aftershock sequence occurred in the aragonese basin mainly in the first 100 days following the main shock the eilat basin is characterized by a larger number of aftershocks composed mainly of small magnitude implying a slower decay rate assuming a detection level of ml 3 2 we obtain b values of 1 01 1 15 and 0 89 for the whole gulf of aqaba and eilat and aragonese basins respectively in good agreement with recent studies of the dead sea fault following omori s law we obtained a decay parameter of 0 94 0 88 and 0 96 for the aftershock sequences in the whole gulf of aqaba and the eilat and aragonese basins respectively correlative with regions of low heat flow for 1630 events in the local magnitude range of 0 6 ml 6 2 we found seismic moment estimates m0 of 5 1018 m0 7 7 1026 dyn cm and brune stress drop estimates between 1 and 200 bars with a characteristic value of 90 bars in the eilat and aragonese basins for m0 5 1021 dyn cm these characteristics are comparable to those for events occurring on the main dead sea jordan fault most of the moment release in the basins occurred shortly after the occurrence of the main shock the total moment release in the eilat and aragonese basins is only a fraction of the moment release of the main shock suggesting an almost complete stress release

application of high resolution sequence stratigraphy to northwest hutton field northern north sea implications for management of a mature brent group field the middle jurassic brent group reservoirs in northwest hutton comprise shallow marine marginal marine and coastal plain reservoir sandstones of both channelized and sheet geometries the connectivity and production characteristics of these sandstones can be predicted through the application of high resolution sequence stratigraphic correlation techniques the broom formation overlies a basal brent sequence boundary and is a lowstand to transgressive systems tract tidal estuarine complex the overlying rannoch formation comprises shoreface sands that represent a highstand systems tract the overlying etive formation is a multilayer fluvial to estuarine channelized system of near fieldwide development interpreted as incised valley fills the superjacent ness formation is genetically related to the etive and is the main oil bearing interval over much of the field the lower ness comprises mouth bar parasequences and is interpreted to be a transgressive systems tract to the etive lowstand the upper ness is a nonmarine delta plain succession interpreted as a highstand systems tract upper ness channel sands typically correlate between several wells and are oriented west east an interval of widespread stacked channels indicates a change in drainage orientation and is related to a high frequency relative sea level fall the uppermost reservoir unit is the tarbert formation related to a further major sequence boundary original repeat formation tester data support a stratigraphically well connected reservoir model compartmentalization exists and is due to stratigraphic layering laterally persistent mudstones creating low vertical permeability and variable dimensions of channels and valley fills

a quantitative simulation study of asymmetrical tectonic subsidence control on non synchronous sequence stacking patterns of eocene lacustrine sediments in bohai bay basin china abstract compared with large scale passive margin marine basins continental basins are susceptive to local or regional tectonic geomorphologic and climatic influences and can form complicated sequence stratigraphic architectures configurations and sedimentary facies we report herein a quantitative simulation study on the influences of asymmetrical tectonic subsidence on sequence stratigraphic stacking patterns in rift basins by varying tectonic subsidence sediment supply and intra rift lake levels it has been demonstrated that isochronous sequences within a rift basin can exhibit both synchronous and non synchronous stacking patterns as a result of differential asymmetric tectonic subsidence in sequence stratigraphy synchronous stacking patterns refer to isochronous sequences at any parts of a basin which are either all retrogradational or all progradational we propose the concept of non synchronous stacking patterns of isochronous sequence stratigraphy for rift basins non synchronous stacking patterns refer to the fact that within the same basin some parts may concomitantly exhibit stacking patterns that are exactly opposite to that of other parts non synchronous stacking patterns can form when the accommodation space increment a is less than that of sedimentary supply s in one area but larger than or equal to that of s in another moreover the duration of non synchronous stacking patterns is controlled by both a and s recognition of non synchronous stacking patterns within isochronous sequence stratigraphy provides a conceptual model for correlation of sequence stratigraphic units in tectonically asymmetric rift basins in eastern china and similar basins around the world

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

sea level fluctuations and the geometric variability of tide dominated sandbodies abstract this paper presents examples of various large tidal sandbodies from the eocene roda sandstone in the southern pyrenees and the late pleistocene and early holocene in the east china sea an attempt is made to summarize the geometric variability of these large tidal sandbodies in relation to the sediment supply and tidal discharge of the depositional system transverse sand bars were developed in low sinuosity high gradient channels with high influxes of coarse sediments and water from fluvial systems tidal point bars were formed in meandering low gradient estuarine channel where tidal influence was stronger and sediment was finer than those of the transverse sand bar a tidal delta complex was built up at the estuary mouth with an abundant sediment supply and an increased tidal discharge tidal sand ridges were formed when relict fluvial or deltaic sands were eroded and reworked by strong tidal currents during subsequent sea level rise since the sediment supply and the tidal discharge of the depositional system were closely related to the eustatic sea level change and basin subsidence i e the relative sea level change special attention will be given to the relationship between geometric variability of tidal sandbodies and the sequence stratigraphic framework in which various sandbodies occurred three orders of eustatic sea level fluctuations can be recognized the third order eustatic sea level cycle together with basin subsidence controlled the development of systems tracts and the occurrence of different tidal sandbodies such as estuary and tidal flat facies during the late stage of a lsw systems tract type 1 sequence or a sm systems tract type 2 sequence tidal point bar facies tidal delta facies or tidal sand ridge facies during a tr systems tract estuary facies during an early hs systems tract and fluvial sand bar facies in a late hs systems tract and the early stage of a sm or lsw systems tract there are also the fourth order and fifth order eustatic fluctuations which are superimposed on the third order eustatic changes and have important control on the build up abandonment and preservation of composite and single tidal sandbodies respectively since the deposition of tidal sandbodies is very sensitive to eustatic sea level changes recognition of various tidal sandbodies is important in sequence stratigraphy analyses of sedimentary basins and in the facies prediction of clastic sediments in basin modelling

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

porosity of fluid saturated porous media from measured seismic wave velocities this paper presents a novel procedure for determining porosity in fluid saturated porous media from measured shear and dilatational wave velocities the method is developed using the theory of linear poroelasticity in the low frequency limit which is equivalent to considering the fluid saturated porous medium as a closed un drained system with the pore fluid moving in phase with the soil skeleton the new procedure for the in situ measurement of porosity has been applied at two sites in italy using cross hole seismic data the results obtained are in excellent agreement with independent laboratory data gathered from undisturbed soil specimens