polymetamorphism in the schirmacher hills granulites east antarctica implications for tectonothermal reworking of an isobarically cooled deep continental crust abstract the precambrian basement of the schirmacher hills queen maud land east antarctica has evolved through multiple episodes of deformation and metamorphism the rocks have suffered at least five phases of deformation the imprint of the early deformation d 1 is preserved in some mafic isolated enclaves the second and the third deformations d 2 and d 3 are the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages the later deformations d 4 and d 5 produced two sets of open upright folds detailed mineralogical textural mineral chemical studies and geothermobarometry on khondalite leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint m 1 is broadly coeval with d 1 only in mafic granulite enclaves within enderbitic gneiss and took place at ca 10 kbar 900 c the mafic magma parental to the enclaves probably crystallized at 11 2 kbar following post peak near isobaric cooling the mafic granulites were transported to shallower levels by the enderbitic magma m 2 recorded in all the lithologies occurred at ca 8 kbar 800 850 c and synchronous with d 2 post peak m 2 evolution of the rocks was characterized by a pressure temperature drop of 2 kbar and 200 c respectively and textures indicative of both cooling and decompression are preserved in different rocks the relative timing of the two however cannot be worked out m 3 synchronous with d 3 took place at 6 kbar 600 650 c and evolved hydrous fluid flux correlation with available structural and geochronological data shows that both m 1 and m 2 could be of grenvillian event m 3 could well be pan african age

source rock palaeoenvironments and controls on the distribution of dibenzothiophenes in lacustrine crude oils bohai bay basin eastern china abstract the applicability of sulphur aromatic parameters based on ratios of dibenzothiophene dbt and its alkylated homologues as indicators of oil maturity has been investigated in 27 oils from the damintun and zhuanhua depressions bohai bay basin the oils were characterised by bulk composition and environmentally sensitive aliphatic biomarkers and were derived from a wide range of lacustrine source depositional environments ranging from highly reducing hypersaline sulphur rich oils to terrestrial input dominated freshwater very high wax oils with maturities ranging from immature to probable mid mature dibenzothiophene based maturity parameters are not well correlated with sterane isomerization ratios and immature oils in particular could not be discriminated by these aromatic maturity indicators in contrast dbt based parameters are strongly correlated with pristane to phytane and ts tm ratios high methyldibenzothiophene ratios mdr are associated with fresh water source deposition whilst low mdrs are found in oils of hypersaline source affinity unusually high relative concentrations of 2 3 mdbt were observed in some of the immature oils inferred to be sourced by lacustrine organic laminites the data provide further evidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils and that source depositional environments can exert a dominant influence when their range of variation is large absence of dbt in some biodegraded oils is tentatively attributed to its aqueous solubility during associated water washing

diagenesis provenance and reservoir quality of triassic tagi sandstones from ourhoud field berkine ghadames basin algeria the triassic tagi trias argilo greseux inferieur fluvial sandstones are the main oil reservoirs in the berkine basin algeria nonetheless their provenance and diagenesis and their impact on reservoir quality are virtually unknown samples from the ourhoud field representing the lower middle and upper tagi subunits were studied using a combination of petrographic mineralogical and geochemical techniques the lower tagi sandstones have an average framework composition of q98 3f0 6r1 1 and 95 of the quartz grains are monocrystalline by contrast the middle upper tagi sandstones have an average framework composition of q88 3f9 8r1 9 and 79 7 of the quartz grains are monocrystalline the lower tagi quartz arenites derived from paleozoic siliciclastic rocks whereas the middle upper tagi subarkoses originated mainly from metamorphic terrains this change in provenance is a potential criterion for correlation within the tagi also this change has contributed to the significantly different diagenetic paths followed by the lower tagi quartz arenites and the middle upper tagi subarkoses grain coating illitic clays are abundant in the lower tagi where they exert a critical control on reservoir quality these clays are interpreted as pedogenic and or infiltrated in origin and to have had in part smectitic precursors shallow burial fe dolomite cementation was favored in the downthrown block of the field bounding fault where it contributed to the poor reservoir quality magnesite siderite cements are multiphase the earliest generation is composed of fe rich magnesite that precipitated during shallow burial from hypersaline fluids with high mg ca ratios probably refluxed residual brines associated with the liassic evaporites later magnesite siderite generations precipitated during deeper burial from waters with progressively higher fe mg ratios authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite dickitization was followed by late stage illitization related to the dissolution of detrital and authigenic k feldspar quartz the most abundant cement was mainly sourced by the pressure or clay induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality overall quartz cement is more abundant in the lower tagi than in the middle upper tagi and this increase correlates with a decrease in average porosity within the lower tagi quartz cement abundance is stratigraphically very variable which is in part related to facies controlled variations in grain coating clay resulting in major vertical variations in reservoir quality anhydrite and barite cements postdate quartz overgrowth the sulfate necessary for their formation was likely sourced by deep subsurface dissolution of late triassic liassic evaporites

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

crustal thickening of the early cretaceous paleo pacific gondwana margin abstract the cause of 7 to 25 km of early cretaceous thickening of parts of the new zealand gondwana margin has been argued to be due to tectonic or magmatic processes new insights into this event have been gained from examination of the grebe mylonite zone an extensive amphibolite facies early cretaceous ductile shear zone that separates volcanic arc rocks of the outboard median batholith from metasedimentary and intrusive rocks of the western province inboard median batholith at lake manapouri in southwest new zealand ar ar and u pb geochronology shows that here exhumation of the outboard median batholith began 128 ma syn kinematic magmatism was taking place at 121 ma and measurable displacement had ceased by 116 ma although the mylonite zone is currently subvertical a combination of thermochronological cooling profiles lineation orientations and back rotation of tilted tertiary unconformities indicate that it most likely formed as an oblique sinistral reverse fault that dipped 60 e and therefore displaced the outboard median batholith over the margin of the western province however the high original angle of this and other compressional shear zones in analogous structural positions the apparent absence of a regional thrust or nappe sheet s and the subtly different ages of thickening for different areas indicate that tectonic loading can have played only one part in the crustal thickening event another factor was the contemporaneous emplacement of 30 000 km 3 of mafic to felsic magma which was likely accommodated by vertical depression of the middle and lower crust tectonic and magmatic processes leading to crustal thickening were therefore not mutually exclusive

infiltrated clays in fluvial jurassic sandstones of reconcavo basin northeastern brazil abstract fluvial sandstones and conglomerates of the sergi formation a jurassic unit of reconcavo basin northeastern brazil contain significant amounts of interstitial detrital clays which are interpreted as the product of early mechanical infiltration developed in an arid semi arid environment within such an environment the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium concentrating the clays in the upper phreatic zone near the sources of influent seepage petrographic textures considered to be typical of mechanically infiltrated mi clays are herein presented using examples from the sergi formation these textures include 1 ridges and bridges 2 geopetal fabrics 3 loose aggregates 4 anisopachous coatings of tangentially accreted lamellae cutans 5 massive aggregates and 6 shrinkage patterns developed during diagenesis in addition their detrital aspect as revealed by sem observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays in most hydrocarbon reservoirs in the sergi formation the clays described in this paper are among the main controls on porosity and permeability distribution commonly zones with maximum clay concentration are the main internal permeability barriers within the sergi formation reservoirs

modification of meander migration by bank failures meander migration and planform evolution depend on the resistance to erosion of the floodplain materials to date research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally this paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat the impact of stream bank failures on meander migration is conceptualized in our rvr meander model through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures simulation periods smaller than the time to cutoff are considered such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics cantilever failure continuously affects meander migration because it is primarily controlled by the fluvial erosion at the bank toe hence it impacts migration rates and meander shapes through the horizontal and vertical distribution of erodibility of floodplain materials planar failures are more episodic however in floodplain areas characterized by less cohesive materials they can affect meander evolution in a sustained way and produce preferential migration patterns model results show that besides the hydrodynamics bed morphology and horizontal floodplain heterogeneity floodplain stratigraphy can significantly affect meander evolution both in terms of migration rates and planform shapes specifically downstream meander migration can either increase or decrease with respect to the case of a homogeneous floodplain lateral migration generally decreases as result of bank protection due to slump blocks and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends with possible achievement of downstream bend skewness under certain conditions

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

stylolites as open fluid conduits abstract stylolites have hitherto been regarded as a direct result of dissolution in response to stresses applied during compaction zones of cementation and dolomitization and in some rocks associated internal sediments indicate that they were pathways to fluid flow yet clay residues and impermeable zones suggest that they are now less able to transmit fluids it is proposed that some stylolites represent former open fluid pathways developed in an over pressured system and that the familiar morphologies were only imposed during compaction as formation pore water pressures declined

the diagenetic destruction of porosity in plutoniclastic petrofacies the miocene diligencia and eocene maniobra formations orocopia mountains southern california usa

note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones oil emplacement retarded the rate of quartz cementation in the brae formation deep water sandstone reservoirs of the miller and kingfisher fields united kingdom north sea thus preserving porosity despite the rocks being buried to depths of 4 km and 120 degreesc quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time 15 m y the earlier the hydrocarbon fill the greater is the degree of porosity preservation failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg potentially leading in turn to poor decisions about the need for and placement of downflank water injectors during exploration the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

stability analysis of semicohesive streambanks with concepts coupling field and laboratory investigations to quantify the onset of fluvial erosion and mass failure abstractthe overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion the nature of this study is twofold first field and experimental analyses are conducted to generate data for channel cross section properties soil index properties and mechanical and erosional strengths at two sites in a representative midsize midwestern stream in southeastern iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure second the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established one dimensional channel evolution model namely the conservational channel evolution and pollutant transport system concepts version 2 0 langendoen and alonso 2008 to estimate the factor of safety for mass failure fsm and fluvial erosion fsf and simulate the bank retreat as a result of eithe

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

clay microporosity in reservoir sandstones an application of quantitative electron microscopy in petrophysical evaluation clay mineral microporosity in sandstones is measured using computer assisted image analysis of back scattered electron micrographs of petrographic sections diagenetic kaolinite has a variety of textures with microporosity values ranging from 15 to 61 diagenetic chlorite has a generally uniform grain coating texture and microporosity of about 50 fibrous illitic clays are difficult to characterize by the same method an average value of 63 microporosity was recorded but analysis of stereo pair micrographs from scanning electron microscopy analyses reveals that illite commonly has microporosity of approximately 90 clay microporosity data are used to calculate effective pore volumes and volumes of clay bound water for clay minerals in sandstones converting from wei ht percent clay to volume percent clay is important microporosity data are valuable input to vshale evaluation where water saturation is associated with clay mineral type texture and volume

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

anomalous porosity and permeability preservation in deeply buried tertiary and mesozoic sandstones in the cusiana field llanos foothills colombia reply abstract porosity in deeply buried 14000 ft 4200 m sandstones of the lower tertiary mirador formation in the cusiana field llanos foothills colombia is less than 10 porosity units p u however permeability is as much as 1000 md which is believed to be due to the simple quartzose detrital grain composition and large grain size the low porosity has resulted from pervasive quartz cement 10 15 fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated at a temperature of around 100 c during deep burial surprisingly thick 400 ft 120 m sandstones of the underlying upper cretaceous guadalupe group in the cusiana field have porosity as high as 20 p u these sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented quartz cement is not abundant and occurs as rims of microcrystalline quartz on quartz and phosphate grains in contrast interbedded quartz arenites within the guadalupe group are heavily cemented by quartz cement and have porosity similar to the overlying mirador formation the major control on porosity in these sandstones appears to be the presence of quartz cement the quartz cement is controlled by grain composition the reasons for the lack of quartz cement in the lithic sandstones of the guadalupe group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility the presence of high porosity sweet spots in deeply buried sandstones may be linked to the occurrence of phosphate beds a primary depositional control

numerical analysis of river channel processes with bank erosion this paper presents a numerical analysis of river channel processes with bank erosion the model can be used for investigating both bed deformation and bankline shifting in 2d plan form the basic equations are used in a moving boundary fitted coordinate system and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes the model was applied to examine the morphological behavior of experimental channels temporal changes in the plan form in a meandering channel can be classified into two patterns meander developing and meander straightening comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions on the basis of the numerical findings the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring the model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel

closed system burial diagenesis in reservoir sandstones examples from the garn formation at haltenbanken area offshore mid norway abstract the middle jurassic garn formation of the haltenbanken area has been studied using mineralogical and geochemical data from 21 wells ranging in burial depths from 2 0 to 4 1 km relative to seafloor rsf k feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth the content of pore filling authigenic illite increases sharply and the content of k feldspar and kaolinite decreases in garn sandstones presently at depths greater than 3 6 3 7 km rsf 120 130 c the depletion in k feldspar below 3 7 km rsf is not accompanied by lower potassium values in the bulk chemical composition wt k2o this suggests that the potassium released during k feldspar dissolution is retained in the sandstones and is precipitated as illite the variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy the shallower sandstones 1 3 have a k feldspar kaolinite ratio greater than one the deeply buried 3 7 km rsf sandstones with similar potassium contents contain excess k feldspar and most of the kaolinite is illitized however deeply buried sandstones containing an average of 0 38 wt k2o k al molar ratio 1 4 contain a significant amount of kaolinite but negligible k feldspar this suggests that the k feldspar kaolinite ratio before the onset of illitization was less than one and hence that the kaolinite illite reaction has been restricted by an insufficient supply of potassium absence of k feldspar this illustrates how illitization of kaolinite depends upon k feldspar as a local source of potassium prediction of illitization in sandstones therefore must be based on integration of models for provenance facies and early diagenesis in addition to burial and thermal history the formation of pore filling authigenic illite in these sandstones is an important influence on the total reservoir quality

revitalization of a mature oil bearing basin by a paradigm shift in the exploration concept a case history of bohai bay offshore china abstract the bozhong depression in bohai bay was previously ranked as an unfavorable exploration region due to concerns about reservoir trapping integrity and top seal risks in the mid 1900s a few explorationists began to view the petroleum accumulation conditions more positively they argued that rapid neogene subsidence could have generated a new suite of source potential and that recent continued fault movements may have provided the needed conduits for upward hydrocarbon migration they predicted the existence of lacustrine facies that could supply the needed lateral and top seals for the intensively faulted traps these radical thoughts were gradually formulated into a new concept for the neogene targets this paradigm shift in exploration concepts has since 1995 led to the discovery of nine 9 major oil fields with a combined in place reserves more than double those discovered in the previous thirty 30 years some of these neogene reservoirs appear to be at a dynamic balance between hydrocarbon escape and charge it is speculated that the required driving energy for the charge may have come from earthquake activities

salinity variations in north sea formation waters implications for large scale fluid movements abstract sodium chloride is the major salt dissolved in formation waters even at a considerable distance away from any evaporites chloride anions are not consumed to any significant extent by diagenetic reactions outside the salt deposits chloride ions are therefore preserved in the pore waters and the distribution of pore water salinities in sedimentary basins may help to constrain pore water flow and the transport of dissolved species of silica and carbonate minerals in the north sea basin and haltenbanken mid norway salinity gradients can be inferred from formation water samples measurements of pore water salinity from wireline logs are less accurate than formation water samples but have the advantage that a much larger database can be obtained data from 150 wells in the north sea and haltenbanken show that the salinities measured from well logs range from 20000 to 300000 ppm in the southern part of the norwegian north sea and haltenbanken there is a clear trend towards higher salinities with greater burial depth there is a pronounced salinity increase in the 500 m of section closest to the evaporites in the northern north sea where underlying evaporites are not known there is only a slight increase in the measured salinities with depth there are however no measurements of high salinity pore water 100000 ppm at depths shallower than 2200 m this pattern of salinity distribution in the pore water precludes large scale convection of the pore water during diagenesis it also indicates limited compaction driven flow log derived salinity data from the north sea suggest that the pore waters in upper jurassic and tertiary reservoirs are generally more saline than those of middle and lower jurassic reservoirs at the same depth the constraints on pore water flow that can be deduced from salinity variations puts severe limitations on the mechanisms of transport of dissolved solids in sedimentary basins and thus also on aspects of diagenetic models

quartz ribbons in high grade granite gneiss modifications of dynamically formed quartz c axis preferred orientation by oriented grain growth quartz ribbons form a well defined ls fabric in granitic gneisses sheared and metamorphosed in the upper amphibolite facies boundaries of quartz grains are smooth and sub perpendicular to ribbon walls suggesting post deformational growth whereas c axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c axes of relatively short grains exhibit across girdle pattern similar to that of the whole sample longer grains reproduce elements of the pattern of the short grains suggesting that oriented grain growth occurred grain boundary mobility which clearly ended in a post deformational static regime probably began during dynamic recrystallization owing to space limitations imposed by the feldspathic matrix grain growth in the ribbons ceased before the initial dynamic fabric was erased in summary this study shows that the c axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains reflecting an oriented grain growth which concluded in a static episode although possibly initiated in the dynamic regime

a simplified 2d model for meander migration with physically based bank evolution abstract the rate of migration calculated by numerical models of river meandering is commonly based on a method that relates the rate of migration to near bank excess velocity multiplied by a dimensionless coefficient notwithstanding its simplicity since the early 1980s this method has provided important insight into the long term evolution of meander planforms through theoretical exercises its use in practice has not been as successful because the complexity of the physical processes responsible for bank retreat the heterogeneity in floodplain soils and the presence of vegetation make the calibration of the dimensionless coefficient rather challenging this paper presents a new approach that calculates rates of meander migration using physically based streambank erosion formulations the university of illinois rvr meander model which simulates meandering river flow and bed morphodynamics is integrated with algorithms for streambank erosion of the us department of agriculture channel evolution computer model concepts the performance of the proposed approach is compared to that of the more simple classic method through the application to several test cases for idealized and natural planform geometry the advantages and limitations of the approach are discussed focusing on simulated planform pattern the impact of soil spatial heterogeneity the relative importance of the different processes controlling bank erosion hydraulic erosion cantilever and planar failure the requirements for obtaining stable migration patterns centerline filtering and interpolation of bank physical properties and the capability of predicting the planform evolution of natural rivers over engineering time scales i e 50 to 100 years the applications show that the improved physically based method of bank retreat is required to capture the complex long term migration patterns of natural channels which cannot be merely predicted from hydrodynamics only

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

formation of abnormally high porosity permeability in deltaic sandstones oligocene bozhong depression offshore bohai bay basin china abstract the porosity enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies however the mechanisms responsible for the formation of abnormally high porosity permeability remain unclear the oligocene dongying formation in the bozhong depression offshore bohai bay basin preserves abnormally high porosity i e 36 4 average or 42 1 median higher than background values this work selects samples with abnormally high porosity ahp samples with normal porosity np and samples with abnormally low porosity alp and compares their differences in physical properties porosities permeabilities and thin section porosities diagenetic minerals pore structures compaction and cementation detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity an effective export of dissolution by products al3 and si4 from ahp to np and alp leads to 2 01 average or 2 11 median newly created porosity in ahp which is still significantly lower than the porosity difference between ahp and np in comparison compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling ahp formation the content and distribution pattern of early carbonate cements compared to the sedimentary factors grain size and rigid grains have significant influences on compaction reduction only samples with moderate contents 0 5 3 0 of uniformly point distributed early carbonate cements and with effective export of dissolution by products can form ahp three formation models for alp and two formation models for np are also proposed in summary the evolution of porosity permeability is controlled by early carbonate cements and induced compaction reduction major causes and by feldspar dissolution and redistribution of by products minor causes this work provides new constraints on the formation of ahp and heterogeneity and is therefore important for sandstone reservoir quality prediction

geometric models of porosity reduction by ductile grain compaction and cementation pore volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains we represent the compaction behavior of grains with a purely geometric model which uses the cooperative rearrangement algorithm to produce dense random packings of partly interpenetrating spheres we varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains the predicted relationship between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with previously published experimental data in the literature the radius of the rigid core of the ductile grains is an effective way to represent different kinds of ductile material ranging from brittle rigid radius 0 9 to extremely ductile rigid radius 0 7 we simulated quartz cementation in our compacted rock by adding isopachous cement cement thickness was reduced on the smaller grains and increased on the larger grains to account for presumed export of pressure dissolved material from finer grained regions and the import of material into coarser grained regions these simulations yield descriptions of pore scale geometry resulting from processes common in sandstones modeled pore geometry provides insight into transport properties of such rocks for example the models predict to within a factor of five the permeability of samples of tight gas sandstones having little intragranular porosity

enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the triassic skagerrak central graben north sea uk current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation however the importance of overpressure developed during initial shallow burial in maintaining high primary porosity during subsequent burial has generally not been appreciated where pore fluid pressures are high and the vertical effective stress is low the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic the deeply buried fluvial sandstone reservoirs of the triassic skagerrak formation in the central graben north sea show anomalously high porosities at depths greater than 3500 metres below sea floor mbsf pore pressures can exceed 80 mpa in the upper part of the skagerrak formation at depths of 4000 5000 mbsf where temperatures are above 140 c the skagerrak reservoirs commonly have high primary porosities of up to 35 little macroquartz cement and variable amounts of diagenetic chlorite grain coats this research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the skagerrak formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

parameters influencing porosity in sandstones a model for sandstone porosity prediction thirteen parameters were reviewed for their influence on porosity in sandstone core material during compaction in basins of average geothermal gradients the first order parameters are age time of burial detrital quartz content maximum depth of burial and sorting overpressure may slow the compaction process depending on onset of overpressure during burial approximately 2 primary porosity may be preserved for every 1 000 psi 6 9 mpa above hydrostatic pressure depth related temperature and median grain size were found to be only second order parameters on the basis of 428 cases function coefficients were calculated for the relationship between porosity and the first order parameters this function permits porosity prediction with a high degree of accuracy th equation is valid for sandstones older than 3 ma with little or no cement no leaching depth of burial in excess of 500 m and little or no shear stresses secondary porosity is excluded and estimations will therefore be conservative where leaching occurred due to interrelationship of compaction and cementation the porosity reduction by cement is usually only a small fraction of the total reduction the function can therefore also be applied to little cemented sandstones

relationships between stylolites and cementation in sandstone reservoirs examples from the north sea u k and east greenland abstract the reservoir potential of hydrocarbon sandstone reservoirs may be significantly reduced by compartmentation as a result of the development of stylolites a petrographic and fluid inclusion microthermometric study was performed on sandstones containing abundant stylolites from the buchan galley and scott fields in the outer moray firth offshore scotland and from a palaeo oil bearing sequence in east greenland the main objective of this study was to further constrain the temperatures and burial depths at which stylolitization occurs in sandstone reservoirs the sandstones containing abundant stylolites are also characterized by their highly cemented nature numerous occurrences of quartz overgrowths clearly truncated by sutured stylolites are evident in all of the samples fluid inclusion microthermometry reveals that quartz cementation which is interpreted to be coeval with stylolitization occurred at minimum temperatures of between 86 and 136 c basin modelling of the scott and galley fields indicates that quartz cementation and stylolite development formed at depths greater than 2 5 km which were attained during rapid tertiary burial the occurrence of hydrocarbon fluid inclusions within healed microfractures orientated at high angles to the stylolites suggests that these microfractures provided pathways for hydrocarbon migration in the highly cemented low permeability zones associated with highly stylolitized sandstones

secondary current effects on cohesive river bank erosion 1 the emphasis of this research was placed on the development of a methodology that allows 1 an adequate representation of the distribution of the near bank shear stress s when secondary currents are present and 2 estimation of the critical erosional strength cr and other sediment erodibility parameters for fluvial erosion these two factors combine to form the overarching objective of this investigation which was carried out in union flat creek a stream with pronounced cross sectional irregularities located within the loess region of the palouse basin of washington state united states the study has shown that the presence of the secondary currents increases the magnitude of the depth averaged sidewall shear stress at least by a factor of 2 0 it was also found that the ratio of the maximum to the depth averaged sidewall shear stress is greater than 5 these findings suggest therefore that use of the may be a good approximation of the fluid shear stress magnitude for simple channel geometries but not for natural channels characterized by complex geometries the measured critical erosional strength cr 4 16 pa was comparable with the findings of other laboratory and field studies that also focused on palouse sediments

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

sandstone reservoir quality prediction the state of the art joanna ajdukiewicz joined exxon production research company in 1980 she was reservoir quality assessment and prediction team lead there from 1991 to 1995 and at imperial oil research centre in calgary from 1995 to 1997 subsequently she has worked a variety of exploration company assignments in the north sea gulf of mexico and middle east her current interests are in predicting the distribution of early diagenetic controls on deep reservoir quality rob lander develops diagenetic models for geocosm llc he obtained his ph d in geology from the university of illinois in 1991 was a research geologist at exxon production research from 1991 to 1993 and worked for rogaland research and geologica as from 1993 to 2000 he is also a research fellow at the bureau of economic geology to guess is cheap to guess wrongly is expensive chinese proverb reservoir quality predictive models will be a useful element of risk analysis until remote sensing tools are invented that accurately measure effective porosity and permeability ahead of the bit this issue of the aapg bulletin highlights recent advances in a new generation of reservoir quality models that have successfully predicted porosity and permeability in diverse siliclastic reservoirs under many different burial conditions most previous attempts at predrill reservoir quality prediction have relied on empirical correlations or on first principle geochemical simulations that incorporate laboratory derived input parameters wood and byrnes 1994 the new reservoir quality models differ from previous approaches in that although incorporating theory inspired algorithms they include terms with values that are explicitly designed to be calibrated by and tested against data sets of high quality petrographic analyses that are linked to thermal and effective stress histories petrographic observations therefore provide essential constraints in these models on the types timing and rates of key geologic processes affecting sandstone pore systems this approach avoids the pitfalls inherent

fluid flow in sedimentary basins abstract the pore waters in sedimentary basins are ultimately derived from sea water meteoric water mineral bound water or from the underlying basement geochemically these waters initially have different signatures however the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids meteoric water is driven downwards into sedimentary basins by the gravitational potential which approximately corresponds to the elevation of the ground water table above the sea or lake level it is difficult to model such flow however because the permeability on a large scale is almost impossible to represent realistically the continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin but this is again difficult to predict pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes the average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor the salinity of formation water provides important constraints on fluid flow in sedimentary basins in the north sea basin and the gulf coast basin these types of data indicate that vertical mixing by compaction driven flow and convection is limited compaction driven flow obeys darcy s law but there are complex interdependencies between pressure permeability and compaction given the low compressibility of water the flow which can result from pressure release alone is very limited except on a local scale along permeable faults and fractures the main flow of pore water during burial is driven by compaction which is a slow process even when overpressure is released abruptly with a resulting increase in the net effective stress in the case of high permeability faults the flow into the fault from low permeability sedimentary rocks at depth may be rate limiting the displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies diagenesis and the tectonic history of the basin extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals if basement fractures are overlain by a thick sequence of soft sediments the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover and conductive heat transport will dominate flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well cemented sedimentary rocks concentrated precipitation of dissolved components will occur near the surface where the rate of cooling is highest also sediment hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences

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

glacial interglacial climatic variations at the schirmacher oasis east antarctica the first report from environmental magnetism article i nfo we discuss in this paper the first detailed environmental magnetic record of glacial interglacial climatic varia tions in the schirmacher oasis east antarctica we determined environmental magnetic properties and inter parametric ratios lf fd arm s irm arm sirm arm lf arm fd s irm lf s ratio and hirm for sediment samples of a core from the sandy lake accelerator mass spectrometer ams 14 c dates were obtained on the organic matter from bulk sediment samples the sediment core spans the past 42 5 cal ka b p the magnetic minerals are mainly detrital and catchment derived as there is no evidence for the presence of authigenic greigite bacterialmagnetite ordiageneticdissolution the lastglacialperiodischaracterizedbya high concentra tion of ferrimagnetic minerals such as titanomagnetite high values of lf sirm etc and coarse magnetic grain size low arm sirm and arm lf values and high s ratio values deglaciation in the schirmacher oasis began around 21 cal kab p as suggested by the low magnetic mineral concentration the holocene period ischaracter ized by relatively warm climatic events as seen in the low values of magnetic susceptibility which is primarily contributed by fine grained titanomagnetite resulting from pedogenesis high fd v alues several of the rela

impact of antarctic climate during the late quaternary records from zub lake sedimentary archives from schirmacher hills east antarctica abstract the antarctic continental margin is marked with ice free areas which are host to numerous freshwater lakes these lacustrine systems are rich in sedimentary deposits which archive in them the regional and general climatic variations these lakes respond to the seasonal variations in climate over glacial interglacial timescales and can be inferred from the sedimentary proxies in this study a 79 cm long radiocarbon dated sediment core retrieved from a peri glacial lake is analysed for elementary corg norg isotopic 13com 15nom and particle size sand silt clay the radiocarbon dated sections 0 65 cm extends up to 43 kyr bp the time series of sedimentary organic matter om proxies corg 3 5 3 c natomic ratio 11 3 and 13com 14 4 indicate that the om in this lake sedimentary record is an admixture of terrestrial and lacustrine biomass distinctly higher lower values during the holocene lgm suggests presence of terrestrial and aquatic aquatic biomass indicating ice free ice cover and warm cold holocene glacial conditions which would result in an increased decreased lake productivity and fluvial wind input of sand and clay silt higher sand content 30 24 and 15 kyr bp silt content 24 kyr bp corg and norg 24 kyr bp within the last glacial stage lgs indicates intermittent warming period in coherence with the antarctic isotope maximum aim the transition in values corg norg c n ratio 13com sand content starting at 16 6 kyr bp closely following antarctic deglaciation to reach holocene optimum values at 11 3 kyr bp documents the influence of antarctic climate on regional areas

petroleum migration and accumulation in the bozhong sub basin bohai bay basin china significance of preferential petroleum migration pathways ppmp for the formation of large oilfields in lacustrine fault basins abstract this paper discusses migration styles and primary controls of petroleum migration pathways in heterogeneous carrier beds in bozhong sub basin bohai bay basin modeling of secondary petroleum migration pathways has been conducted using a simple three dimensional model which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces the modeling results have accurately predicted the petroleum occurrences and have been supported by petroleum geochemical studies most commercial petroleum accumulations are on the predicted preferential petroleum migration pathways ppmp formed by focusing of numerous small petroleum streams close to the kitchens and most large fields petroleum reserves greater than 1 10 8 ton have more than one preferential petroleum migration pathway to convey petroleum to the traps the focusing of petroleum originated from a large area of the generative kitchens into restricted channels is essential for the formation of large oilfields the strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds in the lacustrine fault basins are determined primarily by the morphology of the sealing surfaces at regional scales

quartz plastic segregation and ribbon development in high grade striped gneisses quartz microstructures and c axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high grade alem paraiba shear zone in southeastern brazil are documented cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons these ribbons are separated by feldspar rich domains devoid of quartz the stage at which individual stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode which is reflected by an abrupt increase in the smoothness of the ribbon boundaries this change is interpreted to represent a strain softening kink in the stress strain time path progressive ribboning is accompanied by strengthening of the c axis fabric z maximum indicative of continued plastic flow by basal glide operation of basal glide at these high temperature conditions 680 700 c is interpreted to be a consequence of relatively dry deformation conditions a model is then proposed for development of straight quartz ribbons in high grade striped gneisses where scattered quartz grains are continuously stretched and segregated by crystal plastic processes the small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons

diagenesis of a tight gas sand reservoir upper cretaceous mesaverde group piceance basin colorado abstract the upper cretaceous mesaverde group in the piceance basin western colorado contains thick sections of low porosity low permeability sandstones that are reservoirs for large accumulations of hydrocarbon gas the mesaverde sandstones are lithic arkoses and feldspathic litharenites containing quartz plagioclase variable k feldspar chert and volcanic rock fragments important diagenetic processes that have affected the sandstones include compaction dissolution of feldspar and or alteration to clay precipitation of carbonate and quartz cements precipitation of illite and alteration of detrital clays mixed layer illite smectite porosity is relatively constant a decrease in primary porosity downward is approximately balanced by an increase in secondary porosity with significant microporosity in authigenic and diagenetic clays k feldspar is almost completely absent below 5500 feet 1675 m fibrous illite is relatively abundant above this depth and variable in abundance below the k ar ages of the clays increase with depth from 40 m y at the top of the sampled section to 55 m y at the base indicating illitization of a precursor smectite at approximately the 100 c isotherm samples with abundant fibrous illite have relatively smaller age values mass balance calculations indicate that dissolution of k feldspar illitization of smectite and precipitation of fibrous illite were coupled these reactions suggest the transport of dissolved potassium upward hundreds of feet 100 300 m in the section possibly associated with water driven vertically by the migration of gas

impact of depositional facies on the distribution of diagenetic alterations in the devonian shoreface sandstone reservoirs southern ghadamis basin libya abstract the middle devonian shoreface quartz arenites present day burial depths 2833 2786 m are important oil and gas reservoirs in the ghadamis basin western libya this integrated petrographic and geochemical study aims to unravel the impact of depositional facies on distribution of diagenetic alterations and consequently related reservoir quality and heterogeneity of the sandstones eogenetic alterations include the formation of kaolinite pseudomatrix and pyrite the mesogenetic alterations include cementation by quartz overgrowths fe dolomite ankerite and illite transformation of kaolinite to dickite illitization of smectite intergranular quartz dissolution and stylolitization and albitization of feldspar the higher energy of deposition of the coarser grained upper shoreface sandstones combined with less extensive chemical compaction and smaller amounts of quartz overgrowths account for their better primary reservoir quality compared to the finer grained middle lower shoreface sandstones the formation of kaolin in the upper and middle shoreface sandstones is attributed to a greater flux of meteoric water more abundant quartz overgrowths in the middle and lower shoreface is attributed to a greater extent of stylolitization which was promoted by more abundant illitic clays this study demonstrated that linking the distribution of diagenetic alterations to depositional facies of shoreface sandstones leads to a better understanding of the impact of these alterations on the spatial and temporal variation in quality and heterogeneity of the reservoirs

factors in stability of relocated channels lateral and vertical stability are assessed for 100 unlined channels in different parts of the united states that were relocated by highway agencies 10 years or more before assessment median length of natural channel cut off by relocation is 50 times natural channel width or about 2 100 ft 650 m of all the relocated channels banks are assessed as stable at 78 percent and unstable at 22 percent of the shorter relocated channels less than 50 channel widths banks are assessed as stable at 84 percent and no significant degradation was observed after an initial period of adjustment the relocated channels tend to have about the same stability as that of the prior natural channel increase in channel slope by relocation is not clearly related to instability and stable channels of similar drainage area range widely in slope factors commonly associated with instability are 1 bends in channel 2 erodibility of bed bank materials 3 floods of large recurrence interval and 4 instability of natural channel before relocation

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

paleoclimatic controls on sedimentation diagenesis and reservoir quality lessons from miocene carbonates miocene carbonates worldwide were deposited in a wide range of paleogeographic and tectonic settings and form important reservoirs in several regions the reservoir quality of these carbonates appears to have been primarily controlled by the prevailing paleoclimate which exerted a major control on both the depositional patterns and diagenetic pathways two end members are discussed here 1 humid oceanic tropical subtropical settings and 2 arid land locked temperate subtropical settings in humid oceanic tropical subtropical settings e g miocene of southeast asia a warming trend paralleled by generally rising sea level allowed thick coral reefs and skeletal banks to develop most of which are seismically resolvable these carbonates typically represent several stages or cycles of development largely third order separated by discontinuities in platform growth with episodic subaerial exposure development of economic reservoirs is mainly related to relative sea level falls and associated meteoric diagenesis trends and patterns of reservoir distribution can be predicted within a sequence stratigraphic framework basal transgressive carbonates are mostly tight because of their relatively fine grained textures intense compaction and isolation from meteoric water influence best reservoir quality is commonly developed beneath subaerial unconformities in highstand buildups where effects of meteoric water leaching and karstification are most intense in contrast in arid land locked subtropical temperate settings unfavorable environmental conditions caused by periodically increased salinity and probably relatively low temperature restricted the full growth of carbonate buildups as a result relatively thin and narrow fringing coral reefs with small poorly developed lagoons in rhodalgal ramps are the most common facies type in the upper miocene of the mediterranean and the middle miocene of the gulf of suez and red sea whereas the lower miocene of the middle east is characterized by low energy shallow shelfal and lagoonal carbonates under arid climatic conditions recharge of fresh groundwaters is limited thus minimizing meteoric dissolution and cementation during subaerial exposure instead sea level fall and lowstand ommonly resulted in extensive evaporitic conditions hypersaline brines originated within evaporitic basins or lagoons were the main cause of the pervasive dolomitization leaching of metastable skeletal grains and generation of moldic vuggy and intercrystalline porosity often associated with these arid land locked carbonates is the widespread anhydrite cement that precipitated over a wide range of burial depths porosity distribution in these carbonates thus depends not only on the original depositional fabrics but more importantly on the degree to which the porosity occluding anhydrite cement was dissolved or the low permeability carbonates were fractured these miocene examples illustrate the contrasting diagenetic patterns and reservoir styles controlled by paleoclimatic and paleoceanographic trends and demonstrate the importance of the often overlooked hypersaline diagenesis in arid land locked subtropical temperate settings the development of exploration and production strategies for carbonates that were deposited in different climatic settings should acknowledge and use these fundamentally different styles of diagenesis and reservoir characteristics

mass transport in sandstones around dissolving plagioclase grains the dissolution of plagioclase and precipitation of kaolinite is a common late diagenetic process in sandstones the common occurrence on a thin section scale of patchy kaolinite adjacent to a dissolving plagioclase grain is consistent with diffusional transport of dissolved constituents combined with precipitation on a favorable nucleation site the absence in thin section view of a diffusionally related halo of kaolinite around a dissolving grain can be explained by a lack of a halo of favorable nucleation sites numerical modeling results are presented for the transport of aqueous al by diffusion coupled with fluid flow in sandstones at 100 c within a few months the al in the anorthite component of plagioclase grains can be removed by diffusion during diagenesis fluid velocities needed to overcome diffusion to prevent precipitation on the upstream side of a dissolving grain depend on the unknown minimum supersaturation needed for precipitation these velocities are less than those predicted by the peclet number but are of the order of metres per year at a distance of a few tenths of a millimetre from the dissolving grain the nonlinear inverse relation of these velocities to the distance from the dissolving grain implies that mass transport to precipitation sites is dominated by fluid flow at distances greater than 1 cm

petrography fluid inclusion isotope and trace element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones feldspar dissolution quartz cementation and clay cementation are significant diagenetic processes in deep water fan feldspar rich sandstones in the shengtuo area dongying sag east china the timing and origin of these water rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy fluid inclusion microthermometry oxygen isotope microanalysis and trace element microanalysis three distinct separate episodes of quartz overgrowths referred to as q1 q2 and q3 were distinguished by cathodoluminescence microscopy the q1 quartz identified in all porous sandstones from 2500 to 3600 m 8200 to 11 800 ft was formed at approximately 100 c 115 c 212 f 239 f before oil filled the reservoirs the q2 quartz was mainly precipitated at 115 c 130 c 239 f 275 f accompanying or slightly postdating the main phase of oil filling and was identified in samples from 2800 to 3600 m 9200 to 11 800 ft the q3 quartz was only identified in sandstones buried deeper than 3500 m 11 400 ft and was likely precipitated in the quaternary when temperature exceeded 130 c 135 c 266 f 275 f secondary ion mass spectrometry oxygen isotope microanalyses yielded 18ovsmow vienna standard mean ocean water values ranging from 21 42 to 24 35 for q1 quartz from 22 03 to 24 99 for q2 quartz and from 21 72 to 22 91 for q3 quartz a mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones positive 18o water values 0 5 to 4 5 vsmow of quartz forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water the variations of 18o cements and trace element compositions from q1 quartz to q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly further 18o water values of the pore water increased by approximately 2 vsmow after oil filling meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution the detected co2 and hydrocarbons in fluid inclusions in the quartz cements the existence of co2 in hydrocarbon rich natural gas and the pyrobitumen in the feldspar hosted pores suggest that acids derived from organic matter kerogen in source rocks and hydrocarbons in reservoirs probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones the ongoing development of q2 and q3 quartz suggests that quartz precipitation did not cease after oil filling further diagenetic reactions likely have proceeded from water rock interactions to hydrocarbon water rock interactions

mixing in flow perturbations a model for development of mantled porphyroclasts in mylonites abstract the presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry domains of closed and open flow lines are bounded by a separatrix that surrounds the porphyroclast the concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites the final shape of the recrystallized mantle depends on separatrix shape but also on the degree of intersection of the mantle and the separatrix and therefore on relative mantle width this model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample the occurrence of porphyroclasts that lack wings at high strain values and the presence of symmetric winged mantled porphyroclasts in ductile shear zones

diagenetic modeling to assess the relative timing of quartz cementation and brittle grain processes during compaction this study describes porosity reduction by brittle deformation and the application of touchstone sandstone diagenesis modeling software to assess the relative timing and interactions between grain fracturing and cement formation during burial compaction two examples from a previous study of compactional fracturing are used the oligocene frio formation gulf of mexico basin and the cambrian mount simon formation illinois basin united states grain fracturing during compaction creates intragranular fracture surfaces that are favorable sites for quartz nucleation compared to external grain surfaces that may bear coatings that inhibit the nucleation and growth of quartz cement thus the progress of brittle fracture processes during diagenesis affects quartz cementation in turn modeling of the quartz cementation process can serve to place fracturing into its proper context in burial history in the mount simon formation the extent of brittle deformation of quartz grains correlates with reconstructed effective stress at the onset of quartz cementation for frio formation samples however the extent of brittle deformation does not correlate well with reconstructed effective stress obtained using a one dimensional basin model that uses compaction disequilibrium as the dominant mechanism for overpressure generation judging from the observed degree of grain fracturing significant fluid overpressures in the frio may not have developed at the shallow depths indicated by our basin models the degree of compactional fracturing in sandstones constitutes observable evidence that can be used to decipher the complexities of pressure history

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults

high temperature mylonitization of quartzofeldspathic gneisses example from the schirmacher hills east antarctica abstract in the schirmacher hills most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism the microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth the variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain producing protomylonites orthomylonites and ultramylonites however a large part of the microstructural variation has resulted from syn to post tectonic recrystallization and grain growth of constituent minerals both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration the present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary the ratio varies between 1 5 and 2 presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of k feldspar suggest diffusion assisted dislocation creep these mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons the development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization the growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length at the ultramylonite stage the fine grained matrix of quartz and feldspar mostly accommodates the bulk strain

making diagenesis obey thermodynamics and kinetics the case of quartz cementation in sandstones from offshore mid norway calculation of the quantity and distribution of quartz cement as a function of time and temperature depth in quartzose sandstones is performed using a coupled dissolution diffusional transport precipitation model this model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and or clay grains at stylolite interfaces within the sandstones and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible integration of the coupled mass transfer transport equations over geologically relevant time frames is performed using the quasi stationary state approximation results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data are in close agreement with both the overall porosity and the distribution of quartz cement in the middle jurassic garn formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data when quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0 008 cm 1 to 0 34 cm 1 in either case these reactive surface areas are 2 to 4 orders of magnitude lower than that computed using geometric models

meander migration the observation method river meanders migrate over time and the consequences of this migration can create a problem for bridges and embankments near the river this is why it is important to predict the lateral extent of

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction there are several known causes of anomalously high porosity in sandstones one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement however there has been no mechanistic understanding of why or how microquartz grows or why it maintains high porosity in sandstones here we have used high resolution scanning electron microscopy electron backscattered diffraction and transmission electron microscopy to study the microquartz cemented late cretaceous heidelberg formation germany we have revealed that a nanofilm of amorphous silica 50 100 nm and a layer of chalcedony are between the detrital grain and microquartz cement the amorphous silica insulates the detrital quartz grains and prevents syntaxial growth while microquartz adopts the orientation of the underlying chalcedony with its fast growth c axis parallel to the grain surface thus preventing growth into the pore now that we know what controls microquartz growth and why it preserves porosity it can be used to help identify rank and appraise deeply buried petroleum accumulations

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

deltaic sedimentation in the devonian of western libya abstract middle and upper devonian deposits from the aouinet ouenine formation in the southern ghadames basin of western libya provide a well exposed example of a deltaic complex containing both progradational and transgressive facies progradational facies comprise both laterally accreting and incised distributary channels overlying prodelta deposits also present is a progradational beach environment showing build up from an offshore shelf through nearshore shelf to shoreface and foreshore sub environments over lying these progradational facies are transgressive tidal flat washover fan foreshore and nearshore deposits the characteristics and interrelationships of the different facies are explained by two sedimentation models progradational facies existed contemporaneously during phases of active sediment supply whereas the transgressive facies existed contemporaneously during periods of diminished or absent detrital influx

estuarine clay mineral distribution modern analogue for ancient sandstone reservoir quality prediction the spatial distribution of clay minerals in sandstones which may both enhance or degrade reservoir quality is poorly understood to address this clay mineral distribution patterns and host sediment properties grain size sorting clay fraction abundance and bioturbation intensity have for the first time been determined and mapped at an unprecedentedly high resolution in a modern estuarine setting ravenglass estuary uk results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolinite although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary fluvial supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite chlorite and kaolinite chlorite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse grained inner estuary dunes and tidal bars illite is relatively most abundant as well as most crystalline and most fe mg rich in fine grained inner estuary and central basin mud and mixed flats kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally influenced environments clay mineral distribution patterns in the ravenglass estuary have been strongly influenced by sediment supply residence time and subsequently modified by hydrodynamic processes there is no relationship between macro faunal bioturbation intensity and the abundance of chlorite illite or kaolinite based on this modern analogue study outer estuarine sediments are likely to be heavily quartz cemented in deeply buried burial temperatures exceeding 80 to 100 c sandstone reservoirs due to a paucity of clay grade material 0 5 to form complete grain coats in contrast chlorite enriched tidal bars and dunes in the inner estuary with their well developed detrital clay coats are likely to have quartz cement inhibiting authigenic clay coats in deeply buried sandstones

volatile organic acids produced during kerogen maturation amounts composition and role in migration of oil abstract maturation dependent generation of organic compounds from north sea source rocks has been investigated by hydrous pyrolysis of powdered source rock samples the generation of water soluble organic acids is emphasised and their composition compared with oil field water samples similar compositions are found matrix adsorption effects are not found to be quantitatively important timing of acid generation relative to oil generation is discussed effects of realistic concentrations of a volatile organic acid in the water phase on interfacial tension is determined and effects on fluid flow of crude oil through water saturated sandstone is investigated using a long core flow rig

a fracture model for quartz ribbons in straight gneisses abstract polycrystalline composite quartz ribbons in straight gneisses of the grenville province are explained as dilatant quartz filled fractures formed parallel to a strong preexisting gneissosity the ribbons are thin 3 mm but long and continuous and have straight and truncating contacts with bordering grains of other minerals they are similar in morphology to polycrystalline ribbons interpreted in the literature as resulting from high strains in contrast little strain is required if the ribbons form as quartz filled fractures a monoclinic symmetry of the shapes of ribbons and inclusions can be explained by fracture interactions as they propagate the symmetry is consistent with a late extensional south side down sense of shearing which has been documented elsewhere in the grenville province

development of successive generations of shear zones in the precambrian basement of schirmacher hills east antarctica abstract in the precambrian basement of the schirmacher hills ductile shear zones have developed at several stages over a wide time span of tectonothermal history the earliest shear zones were formed under granulite facies conditions producing well foliated gneiss the majority of the ductile shear zones developed under amphibolite facies conditions the sheared rock is a mylonite characterized by drastic grain refinement of the constituent minerals the shear zones under amphibolite facies conditions are of four broad categories an early set subparallel to the gneissic foliation an early and a late cross cutting types shear zones which developed within massive rocks and lastly those which developed at the contacts of strong rheological contracts the last phase of deformation produced discrete shear fractures all the different phases of ductile shearing were broadly synchronous with emplacement of quartzofeldspathic materials pegmatite was emplaced even along some of the discrete shear fractures the magnitude of shear strain could be measured from the orientations of newly formed foliations and also from the rotation of early foliations

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

tectonothermal history recorded in mafic dykes and enclaves of gneissic basement in the schirmacher hills east antarctica abstract there are successive generations of mafic dykes and other mafic and ultramafic bodies within the gneissic rocks of the schirmacher hills in east antarctica three major groups of events can be recognised by comparison of their internal structures with those of their charnockitic or granitic host and by identification of their successive stages of transformation by metamorphism and migmatization these events are 1 granulite facies metamorphism m 1 syntectonic with d 1 2 a second phase of granulite facies metamorphism m 2 and associated charnockitization syntectonic with transposition of d 1 foliation by isoclinal folding during d 2 and 3 emplacements of granites and pegmatites associated with amphibolite facies metamorphism m 3 broadly syntectonic with two generations of isoclinal folding d 3 the oldest mafic and ultramafic rocks occur as isolated enclaves and preserve a discordant d 1 foliation marked by metamorphic minerals characteristic of the first granulite facies metamorphism mafic bodies that postdate m 1 occur as dykes that were emplaced either 1 syn m 2 2 syn m 3 or 3 post m 3 it is suggested that m 1 m 2 and m 3 took place towards the end of the neo meso and palaeoproterozoic respectively

mechanisms for oil depletion and enrichment on the shijiutuo uplift bohai bay basin china the shijiutuo uplift is a major uplift to the north of the bozhong depression the largest generative kitchen in the bozhong subbasin bohai bay basin although the n35 2 trap on this uplift contains a medium size oil accumulation and the q32 6 trap contains china s third largest offshore oil accumulation the q31 1 trap between the n35 2 and q32 6 traps with very similar evolution history was confirmed to be dry biomarker associations of crude oil and source rock samples were analyzed and three dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift multiple parameter oil source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source related oil classes almost all oils from the shijiutuo uplift are derived from the eocene shahejie formation whereas oils found between the shijiutuo uplift and the bozhong depression either are derived from or have important contributions from the oligocene dongying formation variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the shahejie and then dongying formations in the bozhong depression which is reasonably supported by petroleum migration pathway modeling oil charge from two oil prone source rock intervals and more importantly focusing of oil originating from a large area of the bozhong generative kitchen into the same trap accounted for oil enrichment and formation of china s third largest offshore oil field in the q32 6 structure the complexity and primary control of the sealing surface top surface of the carrier bed morphology on the positions of migration pathways caused the q31 1 trap to be shielded from migration of oil originating from the bozhong depression resulting in oil depletion in this trap shadows to petroleum migration may occur because of the three dimensional behavior of petroleum migration and two dimensional migration modeling may be misleading in predicting petroleum occurrences

formation of secondary porosity in sandstones by quartz framework grain dissolution an understanding of the nature and origin of porosity in sandstones is crucial to the evaluation of their potential as hydrocarbon reservoirs primary porosity is normally destroyed by cementation compaction and pressure solution during burial but it has recently been recognized that significant secondary porosity can develop at depth1 3 this discovery has been called the most significant advance in the study of clastic diagenesis in the past decade 4 several genetic types of secondary porosity have been identified these include porosity formed by 1 fracturing 2 shrinkage 3 dissolution of sedimentary grains and matrix 4 dissolution of authigenic cements and 5 dissolution of authigenic replacive minerals5 formation of secondary porosity by framework grain dissolution has been thought mainly to involve decomposition of feldspars and lithic grains6 7 we demonstrate here that significant secondary porosity can also develop by dissolution of quartz framework grains if the rocks are exposed to alkaline pore fluids during deep burial and or uplift

hydrocarbon habitat of the offshore bohai basin china the bohai basin is a major continental petroliferous basin located in eastern china of which the offshore part of the basin contains more than one third of the total petroleum resources in the basin although the basin has been explored for nearly 40 years the offshore part of the basin is just reaching a moderately mature stage of exploration and its petroleum geological features are gradually becoming known the subduction of the pacific plate to the eurasia plate resulted in the convective upwelling of the mantle in the bohai area and triggered the development of nne and ne striking extensional faults along the mesozoic structures in the late paleocene causing the formation of the rift basin during the oligocene the depocenters and subsidence centers of the greater bohai basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin since the oligocene especially during the neogene due to the indentation of the india block to the eurasia plate the right lateral strike slip movement of the tan lu fault was relatively intense in the offshore part of the basin these characteristics led to the differences between the offshore and onshore sectors in sedimentation structural features and petroleum geology during the deposition of the dongying formation the offshore sector became the depocenter and subsidence center of the basin resulting in the deposition of the lower dongying formation mudstone source rocks while these mudstones are immature in the onshore part of the basin in the offshore bohai most major discoveries have been made in the drape anticlines on the rises in the upper tertiary reservoirs while the lower tertiary reservoirs within the half grabens only serve as secondary reservoirs as the neogene depocenter of the basin the offshore part accommodated some fine grained shallow lacustrine clastics of the lower minghuazhen formation serving as important regional seals for the neogene reservoirs a set of shallow faults and a central faulted anticlinal belt in the shallow upper tertiary sequences were formed during the neogene in the offshore sector under the influence of right lateral strike slip faulting of the tan lu fault these shallow faults networked with deep extensional faults serving as hydrocarbon conduits the central faulted anticline belt along the tan lu fault system also provides significant traps

measuring sandstone compaction from modal analyses of thin sections how to do it and what the results mean abstract calculation of compactional porosity loss from modal analyses of thin sections is a useful technique for characterizing the diagenesis and porosity evolution of sandstone reservoirs to obtain meaningful results however it is essential to assume an appropriate value for the original porosity and to use point counting categories that clearly distinguish between grains and intergranular materials because of the latter requirement it is commonly not appropriate to make calculations of compactional porosity loss from modal analyses that were not performed for the specific objective of measuring intergranular volume suites of modal analyses from deeply buried sandstone units typically show wide variations in the relative importance of compaction versus quartz cementation this heter geneity must reflect subtle variations in depositional sand characteristics in particular the distribution of detrital intergranular clays that tend to promote both mechanical grain rearrangement and stylolitic grain dissolution as well as locally inhibiting quartz cement growth despite wide variation in the mechanism of porosity loss total helium porosity tends to be relatively constant at individual well locations suggesting a close interdependence between compaction and quartz cementation with total porosity loss dependent mainly upon thermal exposure either maximum temperature or time temperature integral

geologic risking guide for prospects and plays the outlined procedure ties both the absolute level and the relative high grading of assessed prospect chances of success to the regional chances of the surrounding oil and gas play prospect group these estimated chances strongly influence exploration economic analyses the average prospect chance is established from the group vs the individual risk aspects of the play then the specific prospects are assigned higher or lower chances according to relative merit maintaining the average level keeps prospect and play assessments entirely compatible group and individual risk factors are documented fully to help make geologically proper summations and to calculate the proper expected value for economic evaluation any dependencies between risk factors are analyzed carefu ly to avoid risking the same basic factor twice chances are consistently related to exceeding a practical uniform minimum field size for the play the chosen minimum may range from very small to very large depending on the focus of the assessor data availability and maturity of the area postdrilling checks against actual experience include determining if the sum of the predrill risked mean hydrocarbon volumes approximately equals the sum of the discoveries the basic approach also works for assessing and aggregating different possible reservoir zones in a single prospect the system can make fullest use of play maps in locating and quantifying the potentially most rewarding areas for exploration and can be used by independents major companies and governments systematic geologic risk assessments should help explorers find oil and gas more profitably

volcanism mineralization and metamorphism at the xitieshan pb zn deposit nw china insights from zircon geochronology and geochemistry abstract the footwall volcanic rocks of the ordovician tanjianshan group in the world class xitieshan pb zn deposit have experienced prolonged arc volcanism followed by strong metamorphism and deformation this has resulted in a complex thermal history and led to ambiguity in interpretation of zircon geochronological results an integrated study involving textural characterization cl imaging trace element analysis ti in zircon thermometry and la icpms u pb dating has provided tight constraints on the age and genesis of the zircon groups in the volcanic rocks the temperature of metamorphism and deformation indicated by metacryst minerals and micro structures in the volcanic rocks ranges from 550 to 650 c which partially overlaps with the lower temperature range of zircon crystallization 600 750 c calculated using the ti in zircon thermometer cathodoluminescence images and trace element compositions confirm a magmatic origin for the zircons which have also been variably altered by metamorphic fluids two ranges of u pb ages 475 470 ma and 460 450 ma have been obtained on typical magmatic zircons and are interpreted to represent pre mineralization arc volcanism in the xitieshan deposit a younger age group of 440 430 ma for the fluid modified zircons is considered to record post ore metamorphism during the north qadaim orogeny thus we propose that the original exhalative ores at the xitieshan pb zn deposit formed at 450 440 ma

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

depositional and compositional controls on sandstone diagenesis the tetori group middle jurassic early cretaceous central japan abstract the tetori group is a representative middle jurassic to lower cretaceous deposit in the northern central part of honshu japan the tetori group is divided into the kuzuryu itoshiro and akaiwa subgroups with decreasing age it consists mainly of a marine sequence in the lower part kuzuryu subgroup and non marine sequence in the upper part akaiwa subgroup with transitional sequence in the middle part itoshiro subgroup diagenesis of tetori group sandstones comprises compaction cementation and replacement early diagenetic events include mechanical compaction and pore filling cementation of calcite siderite dolomite and formation of kaolinite late diagenesis includes formation of quartz cement and albitization of feldspar and formation of illite chlorite calcite ankerite and pyrite among the diagenetic minerals carbonate forms the major cement in tetori group sandstones calcite of each subgroup is subdivided into two or three types based on cathodoluminescence tetori group sandstones of marine fresh water and mixed depositional environments are characterized by distinct chemical compositions of early diagenetic calcites late diagenesis seems to have been mainly influenced by detrital compositions of the sequence burial temperatures and partially hydrothermal processes especially late diagenetic carbonate and authigenic clay minerals seem to have formed mainly from ions released from clay mineral transformations in the adjacent shales based on the illite crystallinity ic the kuzuryu subgroup exhibits higher diagenetic grades than the overlying itoshiro and akaiwa subgroups indicating that the diagenesis of the tetori group was mainly affected by depth of burial

case histories in soil and rock erosion woodrow wilson bridge brazos river meander normandy cliffs and new orleans levees this lecture presents four case history examples of erosion processes because the topic of soil and rock erosion is relatively underdeveloped in geotechnical engineering an introduction precedes the case histories to describe some fundamental aspects of erosion erosion involves the soil or rock through its erodibility the water through its velocity and the geometry of the obstacle through its size and shape knowledge of these three components is needed for any erosion problem to be studied and solved a set of fundamental issues are addressed in the first part including an erodibility classification for soils and rocks an explanation of the stresses imposed by the water on the soil water or rock water interface and an explanation of how the geometry impacts the problem the woodrow wilson bridge case history outlines a new and less conservative method to compute the scour depth and gives examples of bridge scour calculations the brazos river meander case history outlines a new method to predict m

feldspar dissolution authigenic clays and quartz cements in open and closed sandstone geochemical systems during diagenesis typical examples from two sags in bohai bay basin east china feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars we examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality one of the reservoirs is the eocene fan delta sandstone buried 2 5 4 0 km 1 5 2 5 mi below sea level bsl in the gaoliu gl area of the nanpu sag and the other is the eocene subaqueous fan sandstone buried 1 5 4 5 km 1 2 8 mi bsl in the shengtuo st area of the dongying sag both sandstones consist mainly of lithic arkoses and feldspathic litharenites and have secondary porosity formed by dissolution of feldspars in the gl sandstones the absolute amounts of authigenic clays and quartz cements generally 125 c 257 f the low abundance of authigenic clays and quartz cements and low pore water salinity indicate that much of the and released from leached k feldspars were exported from the gl sandstone system and the extensive feldspar dissolution enhanced much porosity and permeability in contrast the st sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays kaolinite and illite and quartz cements with almost identical volume of secondary pores kaolinite dominates in the st sandstones at shallower depth 3 1 km 2 mi bsl where temperature exceeds 125 c 257 f the presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the st sandstone system the dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars secondary porosity in thin sections can be up to 3 but little porosity the diagenetic difference between the gl and the st sandstones can be interpreted by assessing pore water evolution in these two areas the current pore waters with low salinity and negative hydrogen isotopic compositions in the gl sandstone system indicate the significant impact of meteoric water whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the st sandstone system indicate little trace of meteoric water access of meteoric freshwater to the gl area probably occurred during the late oligocene to neogene through widely developed faults in the paleogene and neogene strata the low salinity water could have been responsible for flushing of solutes derived from feldspar dissolution as such diagenesis in the gl sandstones is considered to have occurred in an open geochemical system whereas with limited faults and high water salinity the st sandstones acted as a closed geochemical system where precipitation of kaolinite illite and quartz cements occurred following dissolution of feldspars

extreme ductility of feldspars from a mylonite parry sound canada abstract single feldspar crystals in mylonites from the grenville structural province of canada have accumulated extreme strains by ductile mechanisms the samples studied are from a deep crustal shear zone and were collected at parry sound ontario the mylonites were derived from granite and leucogabbro and the feldspar crystals originated in late syntectonic pegmatite dykes optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb leading to recovery si al ordering and na k interdiffusion evidence for the operation of these processes is respectively ubiquitous subgrains development and coarsening of tweed texture and transition of monoclinic to triclinic k feldspar and exsolution domain reorganization these processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms the effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars the unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation in concert with a deformation enhancing point defect chemistry possibly associated with incorporation into the crystals of a species of water