relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

water films at grain grain contacts debye huckel osmotic model of stress salinity and mineralogy dependence abstract water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure solution during geological processes in the upper crust this mechanism assumes that matter is dissolved inside the contact between two grains the resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film the main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains in this paper we show that the electrically charged surface of a mineral can attract counter ions from the pore and by the related change of osmotic pressure keep water within the contact this is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore this lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film these notions are made more precise by combining the theory of the debye huckle double layer with equations of osmotic pressure the resulting d h o theory predicts the dependence of the water film thickness on stress across the contact composition of the pore fluid and the identity of the minerals involved

grain boundary contact effects during faulting of quartzite an sem ebsd analysis abstract during low temperature faulting of cambrian quartzite assynt nw scotland stress concentrations develop at grain contacts either at the onset of deformation prior to the establishment of a through going fault plane or within the damage zone remote from the main displacement segment such concentrations contribute to the development of intragranular microfractures cataclastic microstructures and fault rocks this contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis the complexity of this deformation is revealed by scanning electron microscope sem electron backscattered diffraction ebsd dauphine twinning is a widespread feature associated with grain contact stress concentration and forms distinctive ebsd microstructures automatic sem ebsd analysis reveals that whilst initial indentation causes dauphine twinning of many grains continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and or microcracking which overprint the dauphine twins these observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests which reveal that indentation causes an intensely deformed region to develop comprising a high density of microfractures and a submicron scale blocky microstructure that accommodates any plastic deformation deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low temperature ductile fracture the new microstructures assist diffusive mass transfer dmt processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length concomitantly associated microfracturing perturbs local thermodynamic equilibrium leading to enhanced dmt crack healing and cementation overgrowths together these processes form the aseismic creep and sealing components of fault zone development

evidence for a long lived accommodation transfer zone beneath the snake river plain a possible influence on neogene magmatism geochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the snake river plain yellowstone province srp y magmatism crustal flow metamorphism and extensional exhumation of core complexes north of the srp occurred mostly between 55 and 42 ma as compared to 42 25 ma south of the srp with final exhumation of the southern complexes occurring only during younger miocene 20 0 ma basin and range faulting these significant differences in the timing of events suggest that the now lava covered srp which is at a high angle to cordilleran trends may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south following previous suggestions we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab shallower slab angle to the south used to explain both the locus of late cretaceous paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the srp during the cenozoic the details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused miocene srp y hot spot magmatism

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

potassium leached zones at the contacts of the garn formation mid norwegian continental shelf evidence for the role of shale in sandstone diagenesis abstract

rupture dynamics and ground motion from potential earthquakes around taiyuan china abstract using the curved grid finite difference method we develop dynamic spontaneous rupture models of earthquakes on the jiaocheng fault jf near taiyuan the capital and largest city of shanxi province in north china we then model the wave propagation and strong ground motion generated by these scenario earthquakes a map of the seismic hazard distribution for a potential m 7 5 earthquake is created based on dynamic rupture and true 3d modeling the tectonic initial stress fields derived from the inversion of focal mechanisms of historical earthquakes a nonplanar fault and a rough surface are considered in the dynamic rupture simulation based on the geological structure of the taiyuan basin normal faulting with a dipping angle of 60 is implemented for the scenario earthquake simulations the largest uncertainty of a potential earthquake in the jf zone is the hypocenter four cases are used to nucleate the earthquake at different locations using these dynamic rupture sources for the jf we further simulate and analyze both the seismic wave generated by the scenario earthquake and the strong ground motion it is found that the low velocity media of the taiyuan basin redistribute the ground motion well the effects of the regional stress fields on the dynamic rupture and hazard distribution are investigated and discussed further moreover a scenario earthquake which can cause great damage to the city of taiyuan is modeled and analyzed

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

mechanical compaction of deeply buried sandstones of the north sea abstract sandstones experience mechanical compaction when the overburden load exceeds the compressive strength petrographic evidence is rarely sufficient to determine the timing of mechanical compaction it is often assumed from indirect evidence such as regional porosity depth trends that mechanical compaction is a process that occurs exclusively during shallow or intermediate burial mechanical compaction without grain fracturing may occur at depth following pervasive framework grain dissolution and or if anomalously high porosity has been preserved due to the presence of small amounts of cement we describe examples from the fulmar sandstone formation of the central graben north sea that experienced late stage mechanical compaction following sponge spicule dissolution and microcrystalline quartz cementation deep burial mechanical compaction involving grain crushing may occur if the rate of grain contact quartz dissolution and or quartz overgrowth development cannot compete with the rate of stress increase at grain contacts some rotliegendes sandstones of the southern north sea that have been buried to 4 5 km offer a good example where the suppression of chemical compaction due to the presence of grain coating clays resulted in pervasive grain fracturing mineral veins are frequently associated with sandstones that have experienced pervasive mechanical compaction during deep burial these may reflect the sudden development of overpressure resulting from the transfer of load to the fluid during collapse of the sandstone framework

cretaceous depositional systems in the norwegian sea heavy mineral constraints deep water cretaceous sandstones in the norwegian sea were deposited by a number of distinct sediment transport systems tapping different sediment source terrains three distinct sandstone types k1 k2 and k3 have been identified distinguished and mapped on the basis of a combination of heavy mineral parameters sandstone type k1 occurs on the trondelag platform halten terrace and nordland ridge but does not appear to be present in the deeper water voring basin sandstone type k1 was ultimately derived from the scandinavian landmass with detritus sourced from metasediments of the caledonian fold belt intrusives of the trans scandinavian igneous belt and to a smaller extent svecofennian basement k1 sandstones were deposited on an unstable slope by debris flows and slumps with minor reworking by bottom currents sandstone type k3 was derived from the western gneiss region farther south on the scandinavian landmass sandstone type k2 occurs in more basinal locations in the norwegian sea and was not supplied by the systems operating along the scandinavian margin because its mineralogy contrasts with that of k1 and k3 k2 mineralogy is not consistent with a source in the lofoten area or east greenland therefore k2 is believed to represent the deposits of a separate axial transport system fed by a source that lay in northeast greenland k2 zircon ages indicate involvement of early proterozoic approximately 2000 ma and archean basement together with metasediments of the caledonian fold belt previous sedimentological models for the cretaceous of the norwegian sea suggest that sand deposition occurred as slumps and debris flows along the continental slope resulting in discontinuous and unpredictable sandstone units unless they become amalgamated into thick reservoir sequences the mineralogical evidence indicates that this model can be applied only to sandstone types k1 and k3 by contrast sandstone type k2 represents sediment introduced from the conjugate margin of the basin and occurs up to 200 km from its detrital source region therefore k2 is likely to occur as more predictable sheetlike bodies on the basin floor

the effect of stylolite spacing on quartz cementation in the lower jurassic sto formation southern barents sea abstract the shallow marine lower jurassic quartz arenites of the sto formation in the southern barents sea comprise 1 intervals where dispersed detrital clay is absent and where the spacing between clay rich laminae that evolved into stylolites upon burial is exceptionally large up to several meters and 2 intervals where minor detrital clay matrix occurs clay laminae are very common and stylolite spacing is typically less than a centimeter point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large 4 11 versus 10 20 outside these intervals there is also a correlation between distance to nearest stylolite and volume of quartz cement samples located a centimeter or less from a stylolite contain 10 20 quartz overgrowths for distances of 3 20 cm quartz cement content is 4 10 and only 3 8 when the closest stylolite is more than 20 cm distant modeling of quartz cementation with the exemplartm diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size degree of grain coating or content of quartz grains i e the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites on the contrary the modeling suggests that the samples situated more than 20 cm from stylolites contain 5 8 less quartz cement than what would have been the case given a more normal stylolite abundance this study indicates that sandstones with exceptionally few clay rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories however such sandstones seem to be highly unusual on the norwegian continental shelf this suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation

detrital quartz and quartz cement in upper triassic reservoir sandstones of the sichuan basin characteristics and mechanisms of formation based on cathodoluminescence and electron backscatter diffraction analysis abstract the relationship between detrital quartz grains and silica cement is significant because cement is often a major cause of porosity reduction in oil and gas reservoirs thus degrading hydrocarbon reservoir quality on the other hand the attributes of grain surfaces may inhibit cementation thus preserving porosity in the reservoirs in this study we focus on the microstructures in quartz grains and secondary overgrowths quartz cement in the heavily cemented sandstones from uplifted upper triassic siliciclastic rocks in the southern sichuan basin china electron backscatter diffraction ebsd optical and cathodoluminescence cl imaging methods have been used to address the microstructural and crystallographic attributes of the detrital grains and their overgrowths this study confirms that 1 the overgrowth zones of quartz are clearly visible in optical and cl images 2 all silica overgrowths are perfectly crystalline quartz 3 quartz cements have a syntaxial relationship with the host detrital grain indicating crystallographic continuity between the detrital quartz and its overgrowth and 4 crystallographic maps confirm that dauphine twins commonly occur both in the detrital grains and in the quartz cement most dauphine twins are developed at grain grain contacts including the margins of detrital grains where they are in contact with quartz cement some twins are limited to a single grain whereas others cross grain cement boundaries based on these twin distributions we can distinguish inherited twins from twins that formed during after burial compaction this study documents the grain deformation structures induced by sedimentary compaction and these structures can be accurately illustrated by ebsd data

dauphine twinning as evidence for an impact origin of preferred orientation in quartzite an example from vredefort south africa large meteorite impacts cause great changes to geologic features on earth ranging from cratering and ejecta blankets to phase transformations in minerals in this report we describe the effect of a shock wave on the crystallographic orientation of quartz crystals in quartzite from the vredefort impact site in south africa preferred orientation of bulk quartzite samples was measured by time of flight neutron diffraction with a random distribution of c axes a weak but distinct difference between the orientation distribution of positive and negative rhombs was observed as illustrated with pole figures and inverse pole figures results for the natural vredefort sample are compared with deformation

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

habitat of hydrocarbons on the norwegian continental shelf eds a m spenceret al norwegian petroleum society graham and trotman 1986 isbn 0 86010 833 3 price 65 00

mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy 1 tectonic and metamorphic data for the central alps switzerland italy are used to discuss this classic example of a barrovian metamorphic terrain notably the evolution of its thermal structure in space and time available p t t data indicate variable contributions of advective and conductive heat transport during collision and subsequent cooling and exhumation some areas experienced a prolonged period of partial melting while other areas at the same time show but moderate heating the barrow type metamorphic field gradient observed in the final orogen is the result of two distinct tectonic processes with their related advective and conductive heat transport processes the two tectonic processes are 1 accretion of material within a subduction channel related to decompression and emplacement of high pressure units in the middle crust and 2 wedging and related nappe formation in the continental lower plate the second process postdates the first one wedging and underthrusting of continental lower plate material produces heat input into lower crustal levels and this process is responsible for predominantly conductive heat transport in the overlying units the interacting processes lead to different maximum temperatures at different times producing the final barrovian metamorphic field gradient the south experienced rapid cooling whereas the north shows moderate cooling rates this discrepancy principally reflects differences in the temperature distribution in the deeper crust prior to cooling differences in the local thermal gradient that prevailed before the cooling also determined the relationships between cooling rate and exhumation rate in the different areas citation berger a s m schmid m engi r bousquet and m wiederkehr 2011 mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy tectonics 30 tc1007 doi 10 1029 2009tc002622

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

timing of extension in the pioneer metamorphic core complex with implications for the spatial temporal pattern of cenozoic extension and exhumation in the northern u s cordillera 1 the pioneer core complex pcc in central idaho lies along a transition between early eocene and ca 40 ma core complexes to the north and south respectively thus the age of extensional development of the pcc is important in understanding the spatial temporal patterns of core complex development in the north american cordillera new results including structural observations and u pb zircon shrimp and icpms geochronology constrain the early extensional history of the footwall for the first time high temperature strain with a top wnw shear sense is pervasive throughout metamorphic rocks of the northwestern footwall an isoclinally folded dike yields a crystallization age of 48 47 ma whereas a crosscutting dike yielded an age of 46 ma metamorphic rocks are also intruded by the 50 48 ma pioneer intrusive suite pis a w dipping granodiorite sheet displaying a magmatic fabric northwest trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility indicating that during emplacement the pis was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks therefore wnw directed extension spanning this structural section occurred between 50 and 46 ma following emplacement of crosscutting 46 ma dikes deformation was partitioned into the wnw directed wildhorse detachment motion on the detachment occurred between 38 and 33 ma as documented by previous 40ar 39ar thermochronology it is not clear however whether extension was continuous through the interval between these two time periods although early eocene extension in the pcc was synchronous with extension in core complexes to the north rates of footwall exhumation in central idaho were much lower this southward slowing is compatible with n s differences in inferred subduction zone geometry kinematics and in the internal character of the orogenic wedge

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic carbonate sediments northwestern qaidam basin northwest china abstract petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata with reservoir quality being a key the formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated however the mechanism of mixed siliciclastic carbonate reservoirs has not been well studied in this paper we address this issue using a case study in the northwestern qaidam basin northwest china n1 e1 2 age 2500 4500 m dissolution pores and fractures dominate the reservoirs however only the fractures were the focus of previous studies thus here we investigate dissolution and its impacts on reservoir formation providing a complementary understanding of the reservoir formation in the basin using core observations and examining thin section it was discovered that dissolution was both random and followed bedding the dissolved components primarily included calcareous and gypsum minerals with fingerprints that are characteristic of burial dissolution further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation the fluids passed through faults and fractures dissolution pores were an important component of the reservoir providing approximately 60 of the porosity in addition segments of high porosity generally above 5 are associated with dissolution based on these observations a schematic model was established to explain the impacts of dissolution on reservoir formation specifically organic acidic formation fluids enter tectonic fractures resulting in dissolution as they pass through them this dissolution enhances the size of the pore space and the reservoir properties of the rocks eventually developing a fracture dissolution reservoir

mechanisms of compaction of quartz sand at diagenetic conditions the relative contribution of cracking grain rearrangement and pressure solution during experimental compaction of quartz sand at diagenetic conditions was determined through electron and optical microscopy and image analysis aggregates of st peter sand 255 60 m diameter grain size and porosity of approximately 34 were subjected to creep compaction at effective pressures of 15 34 5 70 and 105 mpa temperatures of 22 and 150 c nominally dry or water saturated pore fluid pressure of 12 5 mpa conditions and for times up to one year all aggregates displayed transient decelerating creep and volume strain rates as low as 2 10 10 s 1 were achieved the intensity of fracturing and degree of fragmentation increase with volume strain and have the same dependence on volume strain at all conditions tested indicating that impingement fracturing and grain rearrangement were the main mechanisms of compaction throughout the creep phase the increase in fracture density and decrease in acoustic emission rate at long times under wet conditions reflect an increase in the contribution of subcritical cracking no quantitative evidence of significant pressure solution was found even for long term creep at 150 c and water saturated conditions comparison of our findings to previous work suggests that pressure solution could become significant at temperatures or times somewhat greater than investigated here

evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using ebsd and fabric analyser techniques quartz crystallographic preferred orientations cpo from three distinct orthogneisses using both the electron back scatter diffraction ebsd and fabric analyser fa techniques reveal a clear trend from basal and rhomb slip for high p t conditions 670 20 degrees c 9 kbar rhomb and basal slip for medium p t 590 15 degrees c 6 kbar and a dominance of prism slip for lower p t conditions 570 degrees c 4 5 kbar the textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences locally quartz cpus are different within one thin section and in comparison to bulk orientation measurements both ebsd and the fabric analyser techniques are ideal to determine such textural heterogeneities while the ebsd is a powerful technique to determine the full cpo measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the fa is an accurate tool to determine cpus of c axis orientations in uniaxial materials in a cpo focussed study the fa is a cheap alternative to ebsd as the analysis of whole thin section can be accomplished in a very short time with minimal sample preparation with the fa it is possible to evaluate the cpus of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity

stylolite interfaces and surrounding matrix material nature and role of heterogeneities in roughness and microstructural development rough pressure solution interfaces like stylolites are one of the most evident features of localized slow deformation in rocks of the upper crust there is a general consensus that the development of these rough structures is a result of localized stress enhanced dissolution of material along a fluid filled interface but little is known on the initiation of this roughness the aim of this article is to reveal the role of heterogeneities initially present in the host rock on roughness initiation this should give insights on whether stylolite roughness is generated by a stress induced instability or by the presence of disorder in the material i e quenched noise we use a microstructural approach based on sem ebsd analysis combined with orientation contrast oc image analysis of stylolites in limestones we found that the stylolite roughness is induced by heterogeneities in the host rock clay particles and detrital quartz grains in our case in addition close to mature stylolite interfaces matrix modifications occur which can be attributed to the compaction along the stylolite the grain size decreases by 15 25 and a pre existing shape and lattice preferred orientation spo lpo are significantly modified in the vicinity of the stylolite the results presented here imply that localized pressure solution along stylolites is not necessarily restricted to the actual interface but influences the adjacent matrix the heterogeneity data might serve as a quantitative basis for elaborate numerical models of localized compaction

the effect of dauphine twinning on plastic strain in quartz we present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies t 300 400 c granitoid protomylonite from the arolla unit of the nw alps mechanical dauphine twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane z almost orthogonal to the compression direction z twin orientation twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb r nearly parallel to the compression direction r twin orientation in contrast we report the lack of twinning in two porphyroclasts already oriented with one of the r planes orthogonal to the compression direction one twinned porphyroclast has been investigated with more detail it shows the localization of much of the plastic strain into discrete r twins as a consequence of the higher amount of elastic strain energy stored by r twins in comparison to z twins the presence of dauphine twins induced a switch in the dominant active slip systems during plastic deformation from basal regions without twinning to and pervasively twinned regions dynamic recrystallization is localized along an r twin and occurred dominantly by progressive subgrain rotation with a local component of bulging recrystallization part of the recrystallized grains underwent rigid body rotation approximately about the bulk vorticity axis which accounts for the development of large misorientation angles the recrystallized grain size piezometer for quartz yields differential stress of 100 mpa the comparison of this palaeostress estimate with literature data suggests that mechanical dauphine twinning could have a potential use as palaeopiezometer in quartz bearing rocks

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

slump and debris flow dominated upper slope facies in the cretaceous of the norwegian and northern north seas 61 67 n implications for sand distribution a regional sedimentological study of cretaceous sequences in the mid norway region norwegian sea and in the agat region agat field area northern north sea reveals that these sequences were predominantly deposited in an upper continental slope environment by slumps and debris flows examination of nearly 500 m of core from 14 wells shows eight distinct lithofacies facies 1 contorted conglomerate and pebbly sandstone represents deposits of sandy slumps and debris flows possibly in a channel setting facies 2 contorted sandstone is the most widespread and is the product of sandy slumps and debris flows facies 3 contorted mudstone indicates deposition from muddy slumps and debris flows facies 4 rippled sandstone suggests bottom current reworking facies 5 gr ded sandstone represents turbidity current deposits and is very rare facies 6 laminated mudstone is a product of pelagic or hemipelagic deposition facies 7 cross bedded sandstone is indicative of tidal processes and facies 8 laminated sandstone represents delta front and shelf deposits these facies and their association suggest a shelf edge delta to upper slope environment of deposition existing core data document deltaic facies only in the mid norway region the proposed shelf edge delta and upper slope model has important implications for sand distribution 1 this model provides an alternative to the conventional submarine fan model previously applied to these sequences 2 although slump and debris flow emplaced sands are usually discontinuous and unpredictable highly amalgamated slump and debris flow sands may develop thick reservoirs 3 by using the eocene frigg formation as an analog it is predicted that externally mounded seismic facies in the study area may be composed of sandy slumps and debris flows

compaction microstructures in quartz grains and quartz cement in deeply buried reservoir sandstones using combined petrography and ebsd analysis abstract sandstone diagenesis involves porosity loss by mechanical and chemical compaction in this study we have examined relations of diagenesis and microstructures in quartz grains and quartz cement in sandstones from relatively deep burial depths offshore mid norway electron backscatter diffraction analysis ebsd has been combined with optical and cathodoluminescence petrography study of samples with different degrees of compaction and quartz cementation quartz cement is shown to be syntaxial to the nearest host domain grain or subgrain both for monogranular polygranular undeformed and deformed quartz the quartz cement growth may have been initiated at different subgrain surfaces and preserving pre depositional deformation structures ebsd inverse pole figure imaging shows dauphine twins to be common in all samples both in quartz grains and quartz cement the dauphine twins appear in grain grain contacts and in cement crystal boundaries and commonly crossing grain cement boundaries on basis of twin distributions we suggest that both inherited twins from the source area and twins formed by compaction induced grain boundary deformation are present a general problem in sedimentary rocks is to distinguish inherited from sedimentary compaction induced intra grain deformation structures this study shows that the role of ductile deformation mechanisms e g twinning in sediment compaction can be clarified by more comprehensive following up studies using ebsd analysis in combination with other techniques

quantification of brittle deformation in burial compaction frio and mount simon formation sandstones abstract brittle deformation together with grain rearrangement and ductile grain deformation is a key mechanism of compaction in sandstones quartz cementation by virtue of its impact on sandstone mechanical properties is expected to affect the compaction progress by these various mechanisms sandstone samples of different ages and compositions taken from two basins with contrasting burial histories are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction exponential trends of increasing deformation by microfracturing are observed in both the lithic rich frio formation from the gulf of mexico basin r2 0 81 and in the quartz rich mount simon formation from the illinois basin r2 0 78 the two formations contrast in terms of the observed rate of grain fracture increase with depth a larger number of quartz grains in the mount simon formation undergo fracturing at shallow burial 3 km a larger number of quartz grains are fractured in the frio sandstones cathodoluminescence cl imaging conducted on an sem shows that grain fracturing and subsequent cementation of fractures by quartz can occur before during and after normal quartz overgrowth precipitation although fracturing is generally concurrent with cementation the intensity of brittle deformation for a single fractured grain is independent of depth as illustrated by plotting the ratio of the number of particles in fractured grains versus depth r2 0 21 for frio and 0 03 for mount simon suggesting that once fractured grains do not tend to become the locus of subsequent fracturing apparent fracture apertures in the frio grains are slightly wider average 5 m than in mount simon grains average measurable aperture width 4 m quartz grains in the frio have a variety of fracture morphologies including wedge shaped apertures intense comminution at grain contacts and grains with exploded fabrics apparent fracture apertures in mount simon grains are thinner and transect grains as straight uniform traces differences in fracture intensity and pattern are attributed to the fact that cementation in the frio began at greater depth than cementation in the mount simon combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures here referred to as cf or volume of intragranular cement allows calculation of the amount that brittle deformation influences compaction in the frio formation 0 12 to 8 37 of porosity loss due to compaction can be attributed to brittle deformation whereas the values for the mount simon formation lie between 0 25 to 2 16 the larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing

the simulation of rupture dynamics from potential earthquakes around xiluodu reservoir dam china abstract the xiluodu reservoir dam located on the jinsha river is one of the largest dams in the world nearby faults are very active and two m 7 strong earthquakes have occurred around the dam simulating the rupture dynamics of potential earthquakes on the surrounding faults is very important to estimate the seismic hazard for the dam in this research we use the curved grid finite difference method cg fdm to simulate spontaneous ruptures on the major faults around the dam the main results are that the leibo middle and south faults lbmf and lbsf are more likely to have large destructive earthquakes than the mabian yanjin east fault mb yjef asperities on faults that have an impact on strong ground motion are formed under the combined influence of the regional background stress field and fault geometry spontaneous ruptures on the lbmf and lbsf are mainly strike slip with minor thrust components while those on the mb yjef are strike slip and thrust these results are consistent with the focal mechanisms of recorded small and moderate earthquakes in this area