mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

detecting the thermal aureole of a magmatic intrusion in immature to mature sediments a case study in the east greenland basin 73 n the cretaceous and triassic argillaceous rocks from the passive margin of greenland have been investigated in order to detect the thermal aureole of magmatic intrusions ranging from metric dyke to kilometric syenite pluton rock eval data tmax generally 0 3 all indicate a maximum of 5km burial for the argillaceous rocks whatever the distance to an intrusion the k ar dating of the clays 2 m fraction suggests that illites are mostly detrital except near magmatic intrusions where younger ages are recorded to get more information about the extent of the thermal aureole rock magnetism data were determined at distance away from the thermal aureole of the syenite intrusion triassic argillaceous rocks reveal a standard magnetic assemblage compatible with their burial r 0 4 percent it is constituted essentially by neoformed stoichiometric magnetite fe o in contrast within the thermal aureole of the magmatic intrusions the cretaceous argillaceous rocks contain micron sized pyrrhotite fe s firmly identified through the recognition of besnus transition at 35k the thermal demagnetization of natural remanence carried by this pyrrhotite shows a diagnostic square shouldered pattern indicating a narrow grain size distribution of pyrrhotite the extension of this diagnostic pyrrhotite maps a 10 km thick aureole around the syenitic pluton away from this aureole the magnetic assemblage is diagnostic of those found in argillaceous rocks where organic matter is mature

palaeomagnetism of the ordovician dolerites of the crozon peninsula france summary in order to obtain a lower palaeozoic pole for the armorican massif and to test the origin of the ibero armorican arc the ordovician dolerites of the crozon peninsula have been palaeomagnetically studied the samples show a multicomponent magnetization which has been revealed by af and thermal demagnetization and thoroughly investigated with rock magnetic experiments polished section examinations and k ar dating four groups of directions have been recognized often superimposed on each other in an individual sample one component d has always the lowest blocking temperatures and coercivities and is considered to be of viscous origin acquired recently in situ or in the laboratory during storage two components a and b are interpreted to be of secondary origin and to correspond to the observed k ar age distribution between 300 and 190myr these ages represent the time interval between two regional thermo tectonic events associated with the hercynian orogeny and the intrusion of dykes related to the early opening of the central atlantic ocean and the bay of biscay a fourth component c could be of ordovician or younger palaeozoic age it is not clear whether the age of the magnetization is pre or post folding but a prefolding age would yield a direction of magnetization similar to ordovician results from the iberian peninsula the latter interpretation suggests a fairly high palaeolatitude which is in agreement with a glacio marine postulated for sediments overlying the dolerite sills

intracontinental movements in western gondwanaland a palaeomagnetic test abstract early cretaceous intracontinental movements within africa and or south america to account for misfit problem in the pre drift reconstruction of western gondwana have been tested using palaeomagnetic poles from both continents each continent has been considered as comprising separate subplates according to the boundaries proposed by pindell and dewey 1982 and curie 1983 i e northern africa naf southern africa saf northern south america nsa and southern south america ssa visual and statistical distribution of late permian to jurassic poles from the african subplates indicate two distinct groups before rotating saf relative to naf while after the rotation they become indistinguishable the distribution and sparsity of data from south america limit the study in this continent as the data are statistically indistinguishable before and after rotation of ssa with respect to nsa when the test is extended to the reconstruction of western gondwana the tightest grouping of the poles occurs when an intracontinental movement is assumed within africa this suggests that the misfit between the two continents is probably due to intracontinental deformation within africa and not in south america if confirmed this may have profound implications in the kinematics of the processes related to the opening of the south and central atlantic oceans and also the origin of the west african rift system

preliminary experiments to evaluate the gassmann equation in carbonate rocks calcite and dolomite fluid substitution is used to predict potential changes in seismic data due to production in oil reservoirs the fluid changes in clastic rocks are often predicted by the gassmann equation however the applicability of gassmann s equation in carbonates is not well understood apparently part of this failure is due to the violation of the assumption of a constant shear modulus for different fluids baechle et al 2005 adam et al 2006 but it is not clear yet

gassmann s fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld someareundergoing enhanced oil recovery most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones but applying relations developed for sandstones to carbonates is problematic at best we measure in the laboratory nine carbonate samples from the same reservoir at seismic 3 3000 hz and ultrasonic 0 8 mhz frequencies samples are measured dry humidified and saturated with liquid butane and brine our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation however we explore the applicability of gassmann s theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion andgassmann stheoryassumptions forourcarbonatesetat high differential pressures and seismic frequencies the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by gassmann s relation we also explore in detail some of the assumptions of gassmann s relation especiallyrock framesensitivitytofluidsaturation our carbonate samples show rock shear modulus change from dry to brine saturation conditions and we investigate several rock fluid mechanisms responsible for this change to our knowledge these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange

laboratory measurements of elastic properties of carbonate rocks during injection of reactive co2 saturated water 1 to investigate the coupled physical and chemical effects of injecting co2 rich water into carbonate rock samples we monitor the elastic and transport properties of six carbonate rocks along with the calcium content and ph of the pore fluid under constant confining pressure carbonate samples range from calcite limestones containing dolomite to pure calcite mudstones which allow us to study how mineral composition and microstructure affect the magnitude of the observed changes the elastic properties of both the saturated and dry rock i e rock frame show a gradual loss of sstrength upon injection as testified by the continuous decrease in the dry p and s wave velocity and by proxy the bulk and shear moduli the magnitude of the observed changes in the elastic properties varies among the samples with a maximum relative change of 24 7 871 m s and 21 2 443 m s in p and s wave velocities respectively the decrease is also accompanied by a relative increase in permeability up to 495 and porosity up to 19 the observed changes likely derive from a change in microstructure which is monitored over time via scanning electron microscopy the variation in porosity results from two competing and interdependent processes the chemical dissolution of calcite and the resulting mechanical compaction under pressure the results of this study show that upon injection of fluids that are in chemical disequilibrium with the hosting rock the variation of the elastic properties cannot be described by existing rock physics models

critical porosity and elastic properties of microporous mixed carbonate siliciclastic rocks by integrating elastic property measurements and quantitative mineralogic and petrographic analyses of 45 mixed carbonate siliciclastic samples from two wells drilled in late cretaceous rock of the south provence basin southeast france we can 1 identify and quantify the parameters controlling elastic properties 2 demonstrate that micrite can be considered as a porous medium with a low critical porosity averaging 18 and 3 relate diagenetic transformations pore structure modifications and elastic property changes microporous carbonates with compact anhedral and euhedral microrhombic micrites display a steeper decrease in compressional and shear velocities with increasing porosity than do carbonate rocks with moldic intergranular or intercrystalline macroporosity the low value of critical porosity estimated in micrites 18 as well as the steep slopes of velocity porosity relationships at low porosity is believed to result from a pore network geometry characterized by very flat thin pores bounded by planar faces of micrite crystals cementation of microrhombic micrite steeply increases elastic moduli whereas dissolution processes significantly increase porosity with low variations of elastic moduli thus critical porosity concepts can help describe and model elastic properties of micritic microporous carbonate

the continental margin of central east greenland in relation to north atlantic plate tectonic evolution the east greenland continental margin is inferred to have formed in three stages during the tertiary evolution of the adjacent north atlantic ocean the east greenland basaltic volcanism occurred over a period of 2 3 ma which was just prior to the probable date of onset of seafloor spreading in the early eocene shortly before magnetic anomaly 24b 52 ma the associated coastal flexure is interpreted as marking the edge of a zone of stretched and thinned continental crust beneath the inner denmark strait shelf and jan mayen microcontinent this represents a short lived rifting phase just prior to break up according to the evolutionary model the initial break up produced the margins south of the faeroe transform fault 68 5 n and north of the jan mayen fracture zone 72 n but in the intervening region the microcontinent remained attached to east greenland after break up the outer shelf of denmark strait formed from anomalously thick oceanic crust complementary to the iceland faeroe ridge the blosseville kyst margin 68 5 n to 70 n was initiated after anomaly 20 time 42 ma as the jan mayen microcontinent started to separate from greenland with spreading on both sides the new split occupied the line of the pre existing coastal dyke swarm and flexure the liverpool land margin 70 n to 72 n was also initiated at this time but the main separation of the northern part of the microcontinent only commenced after anomaly 7 time 26 ma when a single spreading axis was relocated west of the microcontinent it is suggested that the blosseville lavas subsided during deposition by loading and slight crustal stretching doming occurred near kangerdlugssuaq at the time of break up otherwise the topography of the lavas remained subdued until the relocation of the spreading axis at 26 ma after which the main regional uplift took place

glacially influenced deep marine sedimentation of the late precambrian gaskiers formation newfoundland canada the late proterozoic conception group exposed on the avalon peninsula in newfoundland canada is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites the gaskiers formation near the base massive and crudely stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine grained thin bedded turbidites these diamictites are interpreted as submarine debris flow deposits disrupted diamictites form strongly deformed units that contain large complexly folded rafts of mudstone and turbidite facies these diamictite units are interpreted as submarine slumps diamictites contain glacially striated and faceted clasts clasts and matrix are predominantly of volcanic provenance one outcrop shows interbedded volcanic agglomerate and diamictite and volcanic bombs can also be identified the interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below mall bay formation and above drook formation indicate a deep marine slope setting of diamictite deposition diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone the regional tectonic framework suggests the conception group accumulated in a deep southward opening ensialic rift basin with active but waning volcanic centres to the north the gaskiers formation may be representative of other late precambrian glacially influenced diamictite sequences that were deposited around the north atlantic region and in europe these deep marine diamictite sequences characterized by debris flows turbidites and slump deposits can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basinal settings

treitel ridge a unique inside corner hogback on the west flank of extinct aegir spreading ridge norway basin abstract detailed shipborne mapping sidescan sonar swath bathymetry seismic reflection 3 5 khz profiling gravity and magnetics of the extinct aegir spreading axis ca 55 25 ma in the norway basin revealed a highly unusual in a mor setting narrow 130 km long asymmetrical basement hogback treitel ridge tr on the southern west flank of the extinct rift close to its junction with the iceland faeroe aseismic ridge and the putative paleo iceland hotspot center the ridge seems to continue north another 130 km in the form of a wider more irregular basement rise for a combined length of ca 260 km the southern narrow part of tr approximately following or slightly younger than chron 18n ca 39 ma attains a basement relief of ca 1000 m in the south its summit descending gradually 1 7 100 towards the ne although an east flank conjugate appears generally lacking along both northern and southern parts of tr asymmetry of the southern portion steeper slope facing towards the extinct rift valley suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary the possibility of continental crust extending close to the southern end of tr may also be important because some of this crust may have been remelted at depth contributing magma to feed tr eruptions southern tr may record a brief 0 1 1 m y inside corner tectonic event probably a flexurally compensated normal faulted offset in effect a mega abyssal hill we suggest tr was probably not an expression of change in iceland hotspot activity or somehow related to nearby continental crust but rather represents the volcano tectonic response of the aegir accretion axis at the time it became the eastern edge of the new jan mayen micro plate to extinction of the mid labrador sea spreading center as proposed by nunns 1983 the annexation of the greenland plate by the north american plate forced a plate boundary reorganization east of greenland with northward propagation of the mid atlantic ridge rift along the greenland margin to form the jan mayen micro plate simultaneously two ca 107 trending transforms developed from the previously 146 trending faeroes fracture zone both tr formation and fracture zone trend changes may record aegir ridge response to this major plate kinematic reorganization because tr was formed during a short interval spreading along the mid labrador sea ridge may have ceased abruptly as well

the late ordovician diamictic formations from southwestern europe north gondwana glaciomarine deposits abstract this paper presents a review and a discussion of the clast bearing formations diamicts which occur in the uppermost part of the ordovician succession of numerous regions in southwestern europe lithology facies stratigraphic setting and age are discussed the strong similarities of these formations with contemporaneous glaciomarine deposits of the northern and northwestern margins of the african ice sheet and the late ordovician paleogeographic pattern based upon paleomagnetic data lead to the conclusion that the south european clast bearing formations are glaciomarine deposits supplied with clasts by floating icebergs released from the african ice sheet the proposed paleogeographic model confirms the close connection between southwestern europe and gondwana during early palaeozoic times

marine debris flows late precambrian tillites of the avalonian cadomian orogenic belt abstract correlative late precambrian strata of the avalonian cadomian orogenic cycle deposited after 670 and before 550 ma are now widely dispersed around the margins of the north atlantic these contain several prominent diamictite horizons long interpreted as tillites within thick turbidite successions associated with volcanic horizons the granville formation of the armorican massif in normandy france contains many pebby mud diamicite beds up to 20 m thick that have been interpreted as glaciogenic regionally the granville formation occurs within the lowest part of the upper brioverian supergroup which consists of several kilometres of turbidites and volcanogenic rocks at granville diamictites can be traced along striked into deformed and intermixed graded conglomerates and mudstones diamictites are identified as debris flows generated on a submarine slope by the emplacement of gravel turbidites on a mud substrate and their mixing during downslope slumping a glacial influence cannot be identified in north america the conception group of the avalon peninsula newfoundland canada contains nearly 4 km of turbidites with a conformable sequence of diamictites the gaskiers formation near its base massive and crudely stratified diamicite beds the diamictite successions of the granville and gaskiers formations provide significant data regarding facies types associations and the geometry of resedimented diamictite sequences in deep marine basins along active plate margins similar deposits at the same tectonic stratigraphic horizon occur in new england and africa this paper emphasizes the difficulty of identifying a direct glacial influence on diamictite sedimentation in such settings and the need for a sedimentological reappraisal of other supposed late precambrian tillites preserved in orogenic belts this is a necessary first step prior to modelling late precambrian paleoclimates

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

cementation of porous materials using calcite a technique of cementing porous materials artificially using calcite is presented the technique depends on flushing a mixture of chemical solutions through a porous medium leading to precipitation of calcite due to reaction of the solution ingredients the experimental procedures adopted to achieve the cementation are described the response of different soils to the cementation process is analysed the bond mechanism created by the calcite is illustrated using optical and electron micrographs the paper discusses the factors that appear to determine the response of geomaterials to calcite treatment it has been found that the strength of a calcite treated material increases with a intrinsic strength of the individual grains b density c decreasing particle size of the host grains d pre coating of grains with calcite and e roundness and non angularity of soil grains the cementation technique described in this paper has proved successful in reproducing the mechanical behaviour of natural

changes of shear moduli in carbonate rocks implications for gassmann applicability in laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions in particular these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation this change in shear modulus puts gassmann s assumption of a constant shear modulus into question and also explains why velocities predicted with the gassmann equation can be lower or higher than measured velocities

rock physics analysis and time lapse rock imaging of geochemical effects due to the injection of co2 into reservoir rocks the fundamental concept of time lapse seismic monitoring is that changes in physical parameters such as saturation pore fluid pressure temperature and stress affect rock and fluid properties which in turn alter the seismic velocity and density increasingly however time lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks this study springs from a series of laboratory experiments and high resolution images assessing the changes in microstructure transport and seismic properties of fluid saturated sandstones and carbonates injected with co2 results show that injecting co2 into a brine rock system induces chemo mechanical mechanisms that permanently change the rock frame injecting co2 into brine saturated sandstones induces salt precipitation primarily at grain contacts and within small pore throats in rocks with porosity lower than 10 salt precipitation reduces permeability and increases p and s wa

high rate flexure of the east greenland volcanic margin constraints from 40ar 39ar dating of basaltic dykes abstract in the north atlantic domain the se greenland volcanic margin developed in response to continental break up at 57 54 ma progressive tilting and intrusion of dykes reflect a major tectonic seaward dipping flexing of the continental crust we report eight new ages of tilted pre flexure and vertical post flexure dykes determined by 40ar 39ar incremental heating experiments despite strong excess argon on plagioclase measured 40ar 39ar plateau and mini plateau ages of dykes from detailed step heating experiments on mineral separates appear reliable and a systematic difference of ages is found between tilted and vertical dykes in both studied areas kap wandel 66 20 n and kap gustav holm 66 40 n tilted dykes show ages of 54 55 ma whereas vertical dykes are younger with ages of 51 ma therefore it appears that the crustal flexing of the volcanic passive margin is a short duration event

the effect of micrite content on the acoustic velocity of carbonate rocks abstractwe have demonstrated the effect of micrite content on the acoustic properties of well controlled microstructures that are created in the laboratory to closely mimic those of carbonate rocks in particular we examined the effect of micrite content on the acoustic velocity sensitivity of velocity to pressure and changes in velocity resulting from dissolution upon saturation with a reactive fluid we followed dunham s classification and fabricated the samples by mixing coarse sand sized and very fine micrite sized calcite grains in different ratios and then cold compressing the mixture the acoustic velocities were measured under benchtop conditions and as functions of confining pressure before and after the injection of a co2 aqueous solution the benchtop measurements indicated that the addition of micrite made the frame of the carbonate samples stiffer because the sensitivity of velocity to pressure decreases as the content of micrite increases we hypothesized a stiffer pore structure in

the e greenland rifted continental margin an examination of the coastal flexure the structure of the e greenland coastal flexure a major structure developed during continental break up is interpreted on the basis of field observations an earlier model which involves normal inland dipping antithetic faulting and seaward rotation of fault blocks is confirmed and amplified this model requires that regional dips are very much less than dips measured on bedding planes thus leading to considerably reduced thicknesses for the tertiary volcanic succession in e greenland detailed mapping confirms the validity of these thickness estimates and leads to the conclusion that the volcanic extrusion rate was similar to that of present day iceland thus obviating the need for special mechanisms of magma generation comparable structures occur in other regions of continental break up and voluminous basalt discharge and it is suggested that the flexure developed by collapse of the attenuated crust subsequent to the massive extrusion of plateau basalts

velocity porosity clay content systematics of poorly consolidated sandstones compressional velocities have been measured as a function of confining pressure for 14 west delta block louisiana sandstone cores the dry velocity data for these poorly consolidated sandstones at 10 mpa confining pressure plot on a linear velocity density trend with well consolidated sandstones but not loose sands the dry velocity data also show linear trends of decreasing velocity with increasing porosity along contours of constant clay content similar to those obtained at 40 mpa by tosaya and nur 1982 for saturated sandstones the spacing between clay contours indicates that a large portion of the velocity variation with clay content is due to microporosity which is not included in point counting within the clays the slope of the velocity porosity trends indicates that cementation increases with decreasing porosity

variations in amount and direction of seafloor spreading along the northeast atlantic ocean and resulting deformation of the continental margin of northwest europe the ne atlantic ocean opened progressively between greenland and nw europe during the cenozoic seafloor spreading occurred along three ridge systems the reykjanes ridge south of iceland the mohns ridge north of the jan mayen fracture zone jmfz and the aegir and kolbeinsey ridges between iceland and the jmfz at the same time compressional structures developed along the continental margin of nw europe we investigate how these compressional structures may have resulted from variations in the amount and direction of seafloor spreading along the ridge system assuming that greenland is rigid and stationary we have used a least squares method of palinspastic restoration to calculate differences in direction and rate of spreading along the reykjanes kolbeinsey aegir and mohns ridges the restoration generates relative rotations and displacements between the oceanic segments and predicts two main periods of left lateral strike slip along the main oceanic fracture zones 1 early eocene to late oligocene along the faeroe fracture zone and 2 late eocene to early oligocene and during the miocene along the jmfz such left lateral motion and relative rotation between the oceanic segments are compatible with the development of inversion structures on the faeroe rockall plateau and norwegian margin at those times and probably with the initiation of the fugloy ridge in the faeroe shetland basin during the eocene and oligocene the iceland mantle plume appears to have been in a position to generate differential seafloor spreading along the ne atlantic and resulting deformation of the european margin

episodic volcanism during break up of the north atlantic evidence from the east greenland plateau basalts abstract the lower tertiary upper paleocene lower eocene plateau basalts in the scoresby sund region of east greenland formed during three distinct magmatic episodes the activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence lower sequence the activity of the second episode was centred to the southeast offshore the present atlantic coast and this also produced a voluminous regional basalt sequence upper sequence the third episode was centred on the atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting the lavas are all tholeiitic basalts after an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height from tio2 rich feti basalts to tio2 poor basalts and ending with a reversal to tio2 rich basalts each of these cyclic patterns apparently resulted from a temporary rifting episode and much of the compositional variation found can be explained by magma development in open magma chambers the dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts and their tio2 p2o5 ratios indicate formation at a spreading ridge centre the pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region and the volcanism itself is closely related to this process

re os and 40ar 39ar ages of porphyry molybdenum deposits in the east greenland volcanic rifted margin precise re os dating of molybdenite from the malmbjerg and flammefjeld rift related porphyry molybdenum deposits indicates that they are among the youngest magmatic events 25 8 0 1 and 39 7 0 2 ma in their respective areas of the east greenland rifted continental margin an 40 ar 39 ar age for biotite from the malmbjerg granite stock 25 7 0 3 ma demonstrates that molybdenite mineralization is essentially concurrent with granite emplacement further the identical ages obtained by argon and re os geochronology suggest that for the youngest events in a region re os and argon based methods produce similar age results whereas this is generally not the case for older ore forming events that have been thermally overprinted in the latter case disturbance of argon systematics produces ages younger than those determined by re os both deposits postdate continental breakup and the associated flood volcanism episode at 56 to 54 ma we suggest that the malmbjerg and flammefjeld molybdenum deposits formed during later periods of lithospheric uplift which at least in the case of malmbjerg may have been related to plate tectonic reorganization of the northeast atlantic ocean basin this uplift may have caused decompression mantle melting and renewed generation of mafic magma that led to crustal anatexis and the establishment of major hydrothermal systems the suggestion that molybdenum mineralization occurred at times of late uplift of a volcanic rifted margin may be useful in prospecting

an ocean ridge type magma chamber at a passive volcanic continental margin the kap edvard holm layered gabbro complex east greenland the gabbros of the tertiary kap edvard holm layered serieshave a stratigraphic thickness of more than 5000 m earlier work has shown that the range in cumulus mineral compositions is restricted plagioclase an 81 an 51 olivine fo 85 fo 66 pyroxenes ca 43 mg 46 fe 11 to ca 43 mg 37 fe 20 field evidence of magma injections is common which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma a detailed study of a 600 m section 900 1500 m in the lower layered series reveals a period of crystallization when the magma chamber behaved as a closed system 900 1300 m the rocks formed during this periodare well laminated olivine gabbros 900 110 m which evolved to well laminated oxide gabbros 1100 1300 m compositional trends in the cumulusminerals are towards more evolved compositions plagioclase an 64 an 58 pyroxene mg from 80 to 76 with stratigraphic height from 1300 m to 1500 m granular olivine gabbros dominate with moreprimitive mineral compositions plagioclase an 67 an 76 pyroxene mg from 78 to 82 the transition olivine gabbro to oxide gabbro at 1100m is a consequence of fractional crystallization and it is shown how changes in activities of feo and fe 2 0 3 in the magma are reflected in the total iron content of plagioclases the transition from oxide gabbro to olivine gabbro at 1300 m results from replenishment by less evolved basaltic magma on the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the kap edvard holm layered series crystallized from a tholeiitic magma similar to morb melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill like bodies 0 2 2 0 m thick the melanogabbroic units consist of cr rich augite olivine plagioclase heteradcumulates and contain deformed mica crystals of pre emplacement origin these units crystallized from a wet mgo rich magma which was injected into the layered host gabbros after the formation of the cumulus pile but before the magma was completely solidified the kap edvard holm layered series has several parallels with the plutonic part of ophiolite sequences these include cumulus mineral assemblage compositions of the minerals and the restricted range in compositions with stratigraphic height field evidence of repeated replenishment of basaltic magma dyke swarms overlying the roof zone of the magma chamber and the existence of a late intrusive suite of wet mgo rich magma these parallels suggest that the processes involved in the formation of the kap edvard holm layered series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present day spreading ridges the kap edvard holm layered series is therefore believed to represent a shallow level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the north atlantic ocean

acoustic properties of ancient shallow marine carbonates effects of depositional environments and diagenetic processes middle jurassic paris basin france abstract examination of petrophysical properties acoustic velocity porosity permeability and density and petrographical characteristics texture facies composition and diagenesis of more than 250 core plugs from the middle jurassic carbonates of the eastern paris basin provides insights into the parameters controlling acoustic velocities in relatively low porosity carbonate rocks 20 the pore type observations reveal distinct acoustic velocities in samples with intergranular macropores and samples with micropores in subhedral micrite such that velocities in microporous mudstone wackestone lagoonal deposits are greater than in macroporous grainstone shoal samples at a given porosity range 15 20 the standard wyllie and raymer transforms fit very well with the linear regression between acoustic velocity and porosity from mudstone or lagoonal facies marls and fine grained deposits interpreted as lagoonal facies include statistically significant correlation r 0 9 between velocity and porosity however the data suggest that the wide scatter in velocity porosity relationship from grainstones are not the result of different sorting grain size pore type dolomite content or clay content instead early cementation greatly influences acoustic properties during diagenesis and are interpreted to account for the high variability of velocities over a given porosity range specifically at a given porosity acoustic velocities in compacted grainstone that did not undergo early cementation are higher than in early cemented grainstone petrographic observations suggest that early cementation limits mechanical compaction creating a heterogeneous medium from the earliest stages of diagenesis non touching grains preservation of intergranular macropores that are partially to totally filled by later blocky calcite cement the abundant interfaces between micritized ooids early cement fringes and blocky calcites in grainstones may induce significant wave attenuation as a result the standard time average equations fail to predict the effect of diagenetic features such as early cementation on sonic velocity conversely an absence of early cementation favors mechanical compaction grain to grain contact and suturing the result is a homogeneous micritized grain supported network that may facilitate wave propagation through demonstration of the key role of early cementation in the explanation of variability in acoustic properties the results of this study illustrate the complicated factors influencing velocity transforms in carbonates wyllie and raymer i e classical tools for predicting reservoir properties these insights on the interpretation of vp and the refinement of velocity porosity transforms in grainstone units may be broadly applicable to enhancing seismic based exploration in carbonate successions

laboratory measurements of the acoustic and transport properties of carbonate rocks and their link with the amount of microcrystalline matrix laboratory data and computer tomography ct scan image analysis of carbonate rocks were combined demonstrating a quantitative link between acoustic and transport properties the fraction of microcrystalline calcite micrite in the matrix and the grain to micrite matrix ratio samples from the monte acuto formation gargano italy a transgressive system tract tst whose microstructure ranges from basal pelagic mudstones to coarse calciturbidites were analyzed results indicate that high micrite rocks with low grain to micrite matrix ratios typical of low energy depositional settings are relatively stiffer and more impermeable in contrast lower amounts of micrite due to high energy depositional settings or its removal from intergranular macropores by progressive leaching of the matrix leads to high grain to micrite matrix ratios this process will increase porosity and reduce rock stiffness both of which reduce velocity decreasing micrite content correlates well with increasing macroporosity

palaeomagnetism of late precambrian cambrian volcanics and intrusives from the armorican massif france samples of the keratophyre tuffs of treguier 640 12 myr of the upper brioverian series and the microgranite dykes of loguivy 548 24 myr have been collected from the north coast of brittany france the samples were progressively demagnetized with af and predominantly thermal methods blocking temperatures coercivities and acquisition of isothermal remanent magnetization irm indicate that the keratophyre remanence is variably carried in magnetite hematite or occasionally in both after removal of a soft viscous component two stable palaeomagnetic directions were found one in magnetite the other in hematite the characteristic magnetite direction tb 300 580 c is a pre folding primary thermoremanent magnetization trm which after structural correction has a mean declination inclination of 217 9 30 4 the younger hematite component has a mean in situ direction of 320 4 20 3 the microgranite natural remanent magnetization nrm intensity distribution is distinctly bimodal forming high and low 1 6 a m 1 intensity groups upon demagnetization a single stable remanence in magnetite was removed between 300 and 580 c or 20 and 100 mt two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level low intensity samples have a mean direction of 201 9 2 3 and probably resulted from a late devonian early carboniferous local magnetization high intensity microgranites have a mean direction of 200 50 8 and this remanence is shown to be a primary trm palaeopoles obtained from the two keratophyre magnetizations magnetite 44 7 n 116 4 e hematite 39 5 n 233 6 e as well as the high intensity microgranite remanence 5 8 s 339 9 e show good agreement with similar age poles from the apparent polar wander path for the armorican massif comparison of these data to recent palaeomagnetic data from southern england and wales gives additional evidence to link that region with the armorica plate as proposed by van der voo and hagstrum et al

the jameson land basin east greenland a fission track study of the tectonic and thermal evolution in the cenozoic north atlantic spreading regime abstract the late mesozoic and cenozoic thermal history of the jameson land basin is constrained by new apatite and zircon fission track ft data of surface permian to jurassic sedimentary rocks the results show a general regional thermal evolution related to burial to temperatures close to and in excess of the maximum temperatures of the apatite annealing interval 125 c followed by cooling mainly due to cenozoic uplift and erosion faulting and differential movements in the basin generally occurred after cooling below the apatite partial annealing zone paz 75 125 c however in the northern part of the basin the data suggest a thicker sediment cover or localized heating related to an earlier fracture zone both apatite ft analysis and vitrinite reflectance values reveal a postmature signature for the studied rocks in the northeastern jameson land and premature to mature for the western central and southern jameson land rocks with respect to generation of hydrocarbons the chemical variations of apatite enhance the possibility of recognizing sample positions near maximum temperatures in the paz furthermore the pb zn mineralization pattern closely follows the tertiary maturity trend given by the ft data the type and distribution of mineralization suggest that it was influenced by the regional thermal evolution of the basin in the northeast domain circulating fluids may have overprinted the regional thermal record before ca 20 ma basaltic dyke and sill intrusions 55 45 ma locally caused resetting of apatite ft ages but generally the direct influence from upper crustal magmatic activity played only a minor role the thermal evolution in northeast jameson land is related to the late tectonic evolution of the northeast atlantic involving a change in ridge position at ca 25 ma which followed the passage of the proto icelandic mantle plume at 63 40 ma

regional hydrogeochemistry of the carbonate rock aquifer southern manitoba middle ordovician to middle devonian carbonates in the manitoba lowlands form an extensive aquifer system along the eastern edge of the williston basin the aquifer is divided into fresh and saline portions by a series of topographic lows defined by major river systems and lakes that create a north south trending hydraulic divide east of this divide ca mg hco3 fresh waters are derived by active modern day recharge focused in the sandilands and interlake regions stable isotope and geochemical data indicate that na cl saline waters west of the divide are a mixture of original basin brines and glacial melt water pushed into the basin during pleistocene glaciation high na cl and cl br ratios are consistent with significant salt dissolution by glacial melt water

palaeomagnetism of the ibero armorican arc and the hercynian orogeny in western europe van der voo1 has pointed out that further advances in the understanding of the hercynian orogeny require palaeomagnetic data from well studied orogenic areas we report here palaeomagnetic results of palaeozoic rocks taken in brittany portugal and spain in the ibero armorican arc iaa a large geological feature considered to be a key area for the hercynian orogeny in western europe they show evidence that the iaa formed in two stages extending results obtained in spain by ries et al 2 giving precise constraints for the former shape of the arc we can use pre hercynian palaeomagnetic directions found in spain and portugal to follow its evolution during palaeozoic times this megastructure linking brittany to iberia squeezed between gondwana and the northern continents during the carboniferous did break away from the former block during ordovician times

ultrasonic velocity porosity relationships for sandstone analogs made from fused glass beads using fused glass beads we have constructed a suite of clean sandstone analogs with porosities ranging from about 1 to 43 percent to test the applicability of various composite medium theories that model elastic properties we measured p and s wave velocities in dry and saturated cases for our synthetic sandstones and compared the observations to theoretical predictions of the hashin shtrikman bounds a differential effective medium approach and a self consistent theory known as the coherent potential approximation the self consistent theory fits the observed velocities in these sandstone analogs because it allows both grains and pores to remain connected over a wide range of porosities this behavior occurs because this theory treats grains and pores symmetrically without requiring a single background host material and it also allows the composite medium to become disconnected at a finite porosity in contrast the differential effective medium theory and the hashin shtrikman upper bound overest

variation in dynamic elastic shear modulus of sandstone upon fluid saturation and substitution experimental acoustic measurements on sandstone rocks at both sonic and ultrasonic frequencies show that fluid saturation can cause a noticeable change in both the dynamic bulk and shear elastic moduli of sandstones we observed that the change in dynamic shear modulus upon fluid saturation is highly dependent on the type of saturant its viscosity rock microstructure and applied pressures frequency dispersion has some influence on dynamic elastic moduli too but its effect is limited to the ultrasonic frequency ranges and above we propose that viscous coupling reduction in free surface energy and to a limited extent frequency dispersion due to both local and global flow are the main mechanisms responsible for the change in dynamic shear elastic modulus upon fluid saturation and substitution and we quantify influences

post breakup burial and exhumation of passive continental margins seven propositions to inform geodynamic models despite many years of study the processes involved in the post breakup development of passive margins remain poorly understood integration of apatite fission track analysis afta and stratigraph

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

factors controlling elastic properties in carbonate sediments and rocks carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks in particular cementation and dissolution processes continuously modify the pore structure to create or destroy porosity in extreme cases these modifications can completely change the mineralogy from aragonite calcite to dolomite or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock figure 1 all these modifications alter the elastic properties of the rock and therefore the sonic velocity the result is a dynamic relationship among diagenesis porosity pore type and sonic velocity the result is a wide range of sonic velocity in carbonates in which compressional wave velocity vp ranges from 1700 to 6600 m s and shear wave velocity vs from 600 to 3500 m s

on the universality of diagenetic trends for decades the main use of seismic data has been to delineate sedimentary bodies and tectonic features in the subsurface the mission of seeing inside the geologic body has been added recently mapping porosity lithology and other reservoir bulk properties inside the geologic body has become possible due to the recent dramatic improvement in seismic acquisition imaging and inversion quality as well as the accompanying progress of rock physics

tillites et tilloides du massif armoricain abstract a glacial origin had been suggested for the upper brioverian pebble mudstones of the armorican massif the lack of any true evidence of glacial abrasion on the pebbles the turbiditic sedimentary environment and the great amount of resedimentation figures would rather plead for a deposition by debris flow or mass flow the late ordovician diamictites which contain striated pebbles are inferred to be of glacio marine origin the new palaeomagnetic data are suggesting that the periglacial deposits of southeastern europa are located close to the late ordovician pole for gondwanaland

the equivalent pore aspect ratio as a tool for pore type prediction in carbonate reservoirs the equivalent pore aspect ratio provides a tool to detect pore types by combining compressional and shear wave velocities porosity bulk density and mineralogical composition of carbonate rocks the integration of laboratory measurements well logs and petrographic analysis of carbonate rocks from various depositional and diagenetic settings lower cretaceous presalt nonmarine carbonates from offshore brazil lower cretaceous shallow water platform carbonates from southeast france and upper cretaceous deep water carbonate from the gargano peninsula italy allowed the acoustic signature of a wide range of pore types to be quantified the equivalent pore aspect ratios are shown to be independent of pore volume and mineralogy they can therefore be regarded as relevant parameters for discriminating selected pore network architectures in carbonate reservoirs interparticle and intercrystalline microporosity and macroporosity may include a variety of distinct pore network structures and a wide range of acoustic signatures depending on particle shape nature of contacts packing and cementation intergranular macroporosity in a densely packed granular sediment secondary pseudofenestral etc the acoustic response of moldic pores appears to be mainly controlled by the shape of the dissolved allochems the present work reveals also that correct pore type detection from acoustic and porosity measurements is highly dependent on the pore type classification used a selection of petrographic and diagenetic features to be considered for the construction of a suitable pore type classification is proposed to improve the detection of the pore type effect on the acoustic properties of carbonate reservoirs at plug and well log scale

are some tillites impact related debris flow deposits abstracta number of ancient glacial deposits or tillites are currently interpreted as originating by subaqueous debris flow processes at glacial termini problems in identifying the glacial origin of such deposits can arise because some criteria commonly used for establishing glaciation especially outsize clasts interpreted as dropstones from icebergs can apparently be produced by nonglacial debris flows and debris falls could some of these debris flow diamictite deposits be related to large body impacts especially those that occurred at times other than established glacial periods debris flow ejecta of known impacts seem to be marked by mildly shocked clasts showing evidence of plastic behavior with brittle failure commonly resulting in multiple partially displaced fractures grading into crushed and brecciated clasts these deformation features entail brief periods of high confining pressures in accord with a hypervelocity impact origin i report here several debris flow diamictites currently

quantification of pore structure and its effect on sonic velocity and permeability in carbonates carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude traditional pore type classifications describe these pore structures but are inadequate for correlations to the rock s physical properties we introduce a digital image analysis dia method that produces quantitative pore space parameters which can be linked to physical properties in carbonates in particular sonic velocity and permeability the dia parameters derived from thin sections capture two dimensional pore size domsize roundness aspect ratio ar and pore network complexity poa comparing these dia parameters to porosity permeability and p wave velocity shows that in addition to porosity the combined effect of microporosity the pore network complexity and pore size of the macropores is most influential for the acoustic behavior combining these parameters with porosity improves the coefficient of determination r2 velocity estimates from 0 542 to 0 840 the analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small complicated pores estimates of permeability from porosity alone are very ineffective r2 0 143 but can be improved when pore geometry information poa r2 0 415 and domsize r2 0 383 are incorporated furthermore results from the correlation of dia parameters to acoustic data reveal that 1 intergrain and or intercrystalline and separate vug porosity cannot always be separated using sonic logs 2 p wave velocity is not solely controlled by the percentage of spherical porosity and 3 quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates

the batbjerg complex east greenland a unique ultrapotassic caledonian intrusion batbjerg is an ultrapotassic igneous complex of caledonian age 445 ma at the western extremity of kangerdlugssuaq fiord east greenland the part of the arcuate complex not covered by kangerdlugssuaq glacier extends 7 km with a 1000 m relief the dominant intrusive member is alkaline pyroxenite grading to jacupirangite and ijolite but containing leucite as well as nepheline dominant mineralogy is clinopyroxene magnetite nepheline and leucite no feldspar is present except as fingerprint intergrowths of nepheline with potassium feldspar intergrowths of kalsilite with potassium feldspar are thought to be derived from leucite by subsolidus breakdown leucite has not previously been reported from a plutonic intrusion and kalsilite only once other intrusive members are olivine gabbro dikes of syenite to granite and diatremes of appinitic character cutting the basement gneiss outside the alkaline pyroxenite a partial screen of limestone is the cauldron subsided remnant of a former paleozoic sediment

tillites diamictites and ballistic ejecta of large impacts the primary evidence for the temporal and spatial distribution of glaciation prior to the cenozoic is the occurrence of tillites these are diamictites poorly sorted mixtures of very coarse grained to fine grained sediments and associated deposits which have features considered diagnostic of glacial action the discovery that many diamictite bearing sequences were the products of debris flows and related mass flows with contemporaneous rainout of coarse sediment has led to the reinterpretation of many tillites as glaciomarine debris flow deposits fallout of ballistic ejecta from the impact of large asteroids or comets can also create large scale debris flows with simultaneous rainout of debris recent calculations of the predicted volume and distribution of impact generated diamictites suggest that they should be common in the geologic record moreover ballistic debris flow deposits can apparently exhibit a number of features considered characteristic of glacial deposits e g striated clasts er

the effect of chemical and physical processes on the acoustic properties of carbonate rocks carbonate rocks have major economic significance 60 of the world s oil reserves lie in carbonate reservoirs and the potential for additional gas reserves is huge however the relationship in carbonates between measured geophysical data and rock properties is complex due to the large variety of textures that arise during postdepositional diagenesis and to the chemical processes i e dissolution and replacement by newly formed phases that characterize carbonate forming minerals most experimental and theoretical rock physics research has focused on siliciclastic and shaly rocks however applying insights gained from clastics to carbonates is rarely straightforward and thus is questioned in the literature

phanerozoic equal area maps this paper outlines a method used to make paleocontinental maps and outlines its limitations 15 pairs of new lambert equal area paleocontinental maps are presented drawn at 40 ma intervals from the present day back to 560 ma early cambrian time

compressional velocity and porosity in sand clay mixtures laboratory measurements of porosity and compressional velocity were conducted on unconsolidated brine saturated clean ottawa sand pure kaolinite and their mixtures at various confining pressures a peak in p velocity versus clay content in unconsolidated sand clay mixtures at 40 percent clay by weight was found the peak in velocity is 20 30 percent higher than for either pure clay or clean sand a minimum in porosity versus clay content at 20 40 percent clay by weight is also observed such behavior is explained using a micro geometrical model for mixtures of sand and clay in which two classes of sediments are considered 1 sands and shaley sands in which clay is dispersed in the pore space of load bearing sand and thus reduces porosity and increases the elastic moduli of the pore filling material and 2 shales and sandy shales in which sand grains are dispersed in a clay matrix for these sediments the model reproduces the extrema in velocity and porosity and accounts for much of the scatter in