differences in atmospheric circulation between the development of weak and strong warm events in the southern oscillation abstract in this study temporal and spatial aspects of el nino warm event development are explored by comparing composite sequences of sea level pressure slp surface wind and sea surface temperature sst anomalies leading into strong and weak events el nino strength is found to be related to the magnitude and spatial extent of large scale slp anomalies that move in a low frequency mode in association with this it is also intricately linked to the amplitude and wavelength of the rossby waves in the southern midlatitudes the primary signature of the southern oscillation is a more pronounced standing wave of pressure anomalies between southeastern australia and the central south pacific leading into stronger events a strong reversal in the strength of the annual cycle between these two regions causes a stronger weaker slp gradient that drives southwesterly northwesterly wind stress forcing toward away from the western equatorial pacific in austral winter spring of year 0 1 thus pressur

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

application of theoretically derived rock physics relationships for clastic rocks to log data from the wilmington field ca relationships between porosity and shear modulus are applied to logs recorded in the miocene upper terminal formation of the wilmington field ca to predict porosity from shear wave velocity data recorded through casing using a dipole sonic logging tool a relationship based on a modified hashin shtrikman lower bound appropriate for the unconsolidated nature of the formation allows determination of porosity from the shear modulus the results are similar to those obtained using standard log analysis in sands standard logs predict higher porosities in smectite bearing wackes because of the inclusion in the standard analysis of bound water or micro porosity in the clays which is not included in the porosity value derived from the shear wave velocity

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

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

the effect of fluid composition on the behavior of well cemented quartz rich sandstone during faulting abstract previous studies o kane a onasch c m farver j 2007 the role of fluids in low temperature fault related deformation of quartz arenite journal of structural geology 29 819 836 cook j dunne w m onasch c m 2006 development of a dilatant damage zone along a thrust relay in a low porosity quartz arenite journal of structural geology 28 776 792 found that quartz arenite within two fault zones in the appalachian foreland thrust belt displays very different structural styles and histories despite deforming at similar pressures and temperatures during the alleghanian orogeny a comparison of the grain scale deformation and fluid histories using transmitted and cathodoluminescence microscopy and fluid inclusion microthermometry shows that fluid composition was a controlling factor for causing these differences the cove fault zone deformed in the presence of aqueous fluids first a cacl 2 brine and then an iron rich fluid the precipitation of quartz cement from the brine kept pace with brittle deformation maintaining overall rock cohesion in the fault zone the introduction of iron rich fluids caused a switch from precipitation to dissolution of quartz along with precipitation of goethite in a damage zone along a backthrust in the cave mountain anticline early deformation occurred in the presence of an aqueous fluid from which quartz was precipitated the latest deformation however occurred in the presence of a methane rich fluid which inhibited the precipitation of quartz cement producing porous breccias and open fractures despite deformation at 6 km depth fluid composition not only affected cementation in the fault zones but also the selection of grain scale deformation mechanisms therefore it is a controlling factor in determining the behavior and strength of these fault zones

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

sandstone porosity as a function of thermal maturity sandstone porosity decreases in the subsurface as a power function of thermal maturity phi a m sup b where phi is porosity and m is a measure of thermal maturity representing integrated time temperature history a and b are constants for a given sandstone of homogeneous properties but vary between data sets the commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power function relation when temperature increases linearly with depth the consideration of sandstone porosity in terms of time temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings the recognition of unusual porosity within a sandstone sequence and the prediction of porosity ahead of the drill and at times in the geologic past

compaction law for cretaceous sandstones of alaska s north slope lithic arenites and wackes of the national petroleum reserve in alaska contain a significant volume of ductile grains and have suffered substantial physical compaction with burial as determined by stratigraphic studies these rocks were buried to maximum depths of 500 to 4 400 m initially the grain fraction grain volume as a percent of total rock volume must have been near 59 but increased to over 90 at depth statistical analysis reveals that the grain fraction gf is primarily a function of burial depth in meters d percent ductile grains such as phyllite rock fragments p and matrix content m multiple regression suggests the following linear equation gf 90 0 23p 0 72m 0 0018d simple grain rearrangement operates to a depth of 550 800 m and accounts for a 25 increase in grain fraction at greater depths compaction is by plastic deformation however the increase in grain fraction appears to fall off considerably below 1 000 m and compaction due to plastic deformation increases grain fraction on average by only 7 early quartz cementation never progressed beyond a beginning stage and thus was ineffective at preventing compaction on the other hand clay matrix drastically impedes compaction grain fraction for lithic wackes with an abundant matrix never exceeds 83 the compaction law derived in this study may prove useful in further exploration for petroleum reservoirs for example lithic arenites with a low ductile content should retain an intergranular porosity of 6 to a burial depth of 2 000 m

diagenetic alterations and reservoir heterogeneity within the depositional facies a case study from distributary channel belt sandstone of upper triassic yanchang formation reservoirs ordos basin china abstract diagenesis is of decisive significance for the reservoir heterogeneity of most clastic reservoirs linking the distribution of diagenetic processes to the depositional facies and sequence stratigraphy has in recent years been discipline for predicting the distribution of diagenetic alterations and reservoir heterogeneity of clastic reservoirs this study constructs a model of distribution of diagenetic alterations and reservoir heterogeneity within the depositional facies by linking diagenesis to lithofacies sandstone architecture and porewater chemistry during burial this would help to promote better understanding of the distribution of reservoir quality evolution and the intense heterogeneity of reservoirs based on an analogue of deltaic distributary channel belt sandstone in upper triassic yanchang formation 83 sandstone plug samples were taken from 13 wells located along this channel belt an integration of scanning electron microscopy thin sections electron microprobe analyses rate controlled porosimetry rcp gas flow measurements of porosity and permeability and nuclear magnetic resonance nmr experiments together with published data were analysed for the distribution mineralogical and geochemical characteristics of detrital and diagenetic components and the distribution of reservoir quality within the distributary channel belt distribution of diagenetic alterations and reservoir heterogeneity within the distributary channel belt sandstones include i formation of high quality chlorite rims in the middle part of thick sandstones with coarser grain sizes and a lower content of ductile components resulted from the greater compaction resistance of these sandstones providing larger pore spaces for chlorite growth leading to formation of the intergranular pore wide sheet like throat and intergranular pore intragranular pore wide sheet like throat 15 k 1md in the middle part of thick sandstones ii formation of thinner chlorite rims in the middle part of thinner sandstones is associated with the intergranular pore intragranular pore narrow sheet like throat 9 18 o vpdb 23 4 to 16 6 13 c vpdb 4 0 to 2 3 favoured in the top and bottom of the channel sandstone which near the sandstone mudstone bouding surface destroying pore space

compaction creep of simulated anhydrite fault gouge by pressure solution theory v experiments and implications for fault sealing the sealing and healing behaviour of faults filled with anhydrite gouge by processes such as pressure solution is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite bearing sequences such as those present in the seismogenic zone beneath the apennines we have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge allowing for dissolution diffusion and precipitation control to estimate the time scale on which such sealing and healing effects occur we compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge tested under wet upper crustal conditions the results confirm earlier indications that compaction under these conditions likely occurs by diffusion controlled pressure solution applying our most rigorous model for diffusion controlled pressure solution constrained by the fit to the experimental data we infer that anhydrite fault sealing will occur in a few decades at most which is rapid compared with both co2 storage time scales and with the recurrence interval for seismicity in the apennines

chlorite meniscus cement implications for diagenetic mineral growth after oil emplacement the possible influence on diagenesis of oil migration into reservoir sandstones is important for understanding their development although many studies have addressed the question of continued growth of diagenetic minerals after oil emplacement there is little clear evidence of the timing of mineral growth relative to oil emplacement nor of the possible mechanisms of continued mineral growth this study presents an example of continued cementation by pore lining chlorite during and after the migration of oil into a palaeocene turbidite sandstone reservoir near the faroe islands chlorite developed in two different textures one texture occurs on framework grain surfaces adjacent to open pore space in contact with oil the other texture occurs close to framework grain contacts where contact with oil appears to have been prevented by residual pore water menisci the meniscus crystals are larger than those from open pore implying that diagenetic clay mineral growth has continued after oil emplacement

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

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

tomographic p wave velocity and vertical velocity gradient structure across the geothermal site gros schonebeck ne german basin relationship to lithology salt tectonics and thermal regime 1 seismic wide angle data were collected along a 40 km long profile centered at the geothermal research well grsk 3 90 in the northeast german basin tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model indicative of cenozoic to pre permian sediments wide angle reflections are modeled and interpreted as top zechstein and top pre permian changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2 3 km depth and localized salt structures are imaged suggesting a previously unknown salt pillow in the southern part of the seismic profile the zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows which is confirmed by sonic log data this decrease in velocity could be explained by the mobilization of less dense salt which moved and formed the salt pillows whereas the denser salt remained in place at the salt lows we interpret a narrow subvertical low velocity zone under the salt pillow at grsk 3 90 as a fault in the deep permian to pre permian this wnw ese trending fault influenced the location of the salt tectonics and led to the formation of a fault bounded graben in the rotliegend sandstones with optimal mechanical conditions for geothermal production thermal modeling showed that salt pillows are related to chimney effects a decrease in temperature and increasing velocity the assumed variations in salt lithology density and strain must thus be even higher to compensate for the temperature effect

porosity reduction in sandstone by quartz overgrowth a mathematical model for predicting porosity histories due to quartz overgrowth in quartz rich sandstone reservoirs has been developed the model neglects secondary dissolution porosity and other diagenetic processes such as compaction and precipitation of carbonate and clay minerals nevertheless the model is straightforward and easy to use to calculate porosity the calculated porosity corresponds to measured porosity of simple quartz cemented sandstones in 27 wells in north and south america the model is based on fluid flow through initially porous unlithified sands in which the fluid phase is saturated with si oh 4 and is always in equilibrium with quartz as the continuously circulating fluid migrates updip toward the basin edge it cools and quartz precipitates into the pore spaces causing a loss of porosity basinal fluid velocities may be calculated assuming 1 thermal convection and 2 hydrostatic pressure due to recharge at the edge of the basin porosity diagrams relating porosity to geothermal gradient burial rate and depth of burial are compared to thermal maturation models of source rocks fluid flow history and grain size distribution the rate of porosity reduction depends on the following variables in decreasing order of significance buria rate age initial porosity basin size dip angle fluid dynamics initial permeability and geothermal gradient

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

modeling porosity evolution and cementation of sandstones abstract recent models of quartz cementation of sandstones attribute the source of silica to dissolution of quartz at stylolites bjorkum 1993 walderhaug 1994 aase et al 1996 oelkers et al 1996 quartz dissolution is believed to be catalyzed by the presence of mica illitic clayswhich are not dissolved or destroyed during the quartz dissolution process bjorkum1993 a supersaturation of silica is created at the stylolites which leads to a diffusive transport of silica into the interstylolite regionwhere the silica precipitates as cement interstylolite cementation is observed to be smooth which has led to the conclusion that the precipitation step is the slowest step in the entire quartz cementation process this is because the silica supersaturationcreated by dissolution of quartz at the stylolites is almost uniform throughout the interstylolite region the supersaturation is estimated to be 1 above the silica equilibrium concentration aase et al 1996 oelkers et al 1996 this estimate is based on the increase in the silica equilibrium concentration needed to grow microcrystalline quartz grains of a size 1 m petrographic studies of deeply buried jurassic north sea sandstones show that sandstones with microcrystalline quartz coatings have well preserved porosities microcrystalline quartz coatings are able to inhibit quartz cementation because the supersaturation created by dissolution of quartz at the stylolites is insufficient to grow the microcrystalline quarts aase et al 1996 the idea that stylolites are the source for silica cement was already suggested by heald et al 1955

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

porosity reduction by pressure solution a theoretical model for quartz arenites pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones this paper presents a one dimensional model for a well sorted quartz arenite sequence undergoing diagenesis the sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains this solution and precipitation process reduces porosity and forces the migration of pore fluid permeability decreases with porosity hindering fluid migration and excess fluid pressures are generated distributions of porosity pore pressure and fluid flow rates are given for several sedimentation rates and thermal gradients complete reduction of the primary intergranular porosity was accomplished in all cases examined by a depth of 3 5 km

comparison of deeply buried paleoregolith profiles norwegian north sea with outcrops from southern sweden and georgia usa implications for petroleum exploration abstract for the first time on the norwegian continental shelf deeply buried paleoregolith profiles have been identified as part of the petroleum reservoirs in recently discovered oil fields on the utsira high norwegian north sea reservoir properties porosity and permeability in the granitic basement on the utsira high are mainly the result of physical and chemical alteration of the rock occurring in the near surface environment during sub aerial exposure of the high in the mesozoic evaluating the reservoir potential of altered basement rocks requires a different approach than in conventional petroleum exploration in this paper macroscopic mineralogical and micromorphological alteration features observed in two deeply buried paleoregolith profiles are compared with surface paleoregoliths from ivo klack sweden and georgia usa the paleoregolith profiles are subdivided into specific weathering facies altered coherent rock facies saprock facies and saprolite facies based on the rock fabric and mechanical strength the reservoir potential of each weathering facies is controlled by the type and degree of alteration in the altered coherent rock facies porosity and permeability is mainly controlled by joints and microfractures that developed prior to subaerial exposure of the granitic pluton in the saprock facies intensified chemical dissolution of plagioclase enhanced porosity and the development of mesofractures improved the connectivity between pores in the saprolite facies progressive dissolution of plagioclase creates porosity but the precipitation of clays within voids and mesofractures has a destructive effect on the overall reservoir properties the deeply buried paleoregolith profiles from the utsira high display comparable macroscopic mineralogical and micromorphological alteration features to what was observed in surface paleoregoliths from ivo klack and georgia outcrop studies may therefore be an important tool when evaluating the reservoir potential in subsurface paleoregoliths

velocity depth trends in mesozoic and cenozoic sediments from the norwegian shelf sonic velocity density and resistivity log data from 60 wells on the norwegian shelf have been used to investigate velocity depth trends in sedimentary rocks as a function of sediment composition porosity pore pressure burial history and compaction processes a first order linear velocity depth trend line has been estimated from published velocity data the data analyzed in this study however show significant variations from this trend line indicating that no general velocity depth function can be used when performing more accurate analyses like depth conversion of seismic data pore pressure prediction or basin modeling lower tertiary smectitic sediments from the northern north sea and haltenbanken are characterized by relatively low velocities compared to the overlying pliocene and pleistocene sediments causing a distinct velocity inversion a significant velocity increase at a burial depth corresponding to 70 100c was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales overpressured jurassic sediments from haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference the reduced velocities may be a direct response to lower effective stresses and thus reduced elastic compaction low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy the high velocity depth ratio of barents sea sediments after correcting for tertiary exhumation is explained by the burial history of the area the subsequent thermal exposure of the sediments over time and thus the amount of quartz cementation

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

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

rayleigh convection mass transport and change in porosity in layers of sandstone the critical rayleigh number for the onset of thermal convection in a quartz rich sandstone layer is discussed for different thermal boundary conditions the mass transfer of silica due to rayleigh convection is computed assuming that the solution is saturated with respect to quartz and that the silica concentration is a function of temperature only since quartz solubility increases with temperature silica is dissolved from the sandstones in the lower part of the layer where the fluid is being heated and precipitated in the upper part where it is cooled the change in porosity is found as function of the vertical coordinate valid for small values of time it is shown that for small supercritical rayleigh numbers the time for the porosity to change up to 10 of its initial value is of the order 10 m y the spatial variation in porosity should be a characteristic signature of rayleigh convection and should be easy to recognize if convection has taken place for significant geological time

a fluid inclusion study of quartz cemented sandstones from offshore mid norway possible evidence for continued quartz cementation during oil emplacement abstract jurassic sandstones from the oil zone in a well from the haltenbanken area offshore mid norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths homogenization temperatures for the inclusions range from 92 c up to the present formation temperature of approximately 118 c since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92 c poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths and most calcite cement must be precipitated at temperatures below 90 c although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105 c none of the examined hydrocarbon inclusions homogenized below 102 c and this suggests that oil started entering the reservoir at temperatures of around 100 c the burial history of the examined sandstones indicates that temperatures of 100 c were not reached before the late pliocene and that filling of the reservoir has taken place during the last few million years homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress and that emplacement of oil did not necessarily stop precipitation of quartz cement possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water

secondary porosity creation of enhanced porosities in the subsurface from the dissolution of carbonate cements as a result of cooling formation waters abstract theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral if the formation water has an initial ph below 6 9 then cooling will result in a water with a considerable potential to leach calcite the temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin where faults cease to act as permeable pathways the escaping formation water may be forced through an aquifer in a partly carbonate cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth

mechanical and chemical compaction model for sedimentary basin simulators abstract this article presents a sediment compaction model for sedimentary basin simulators the concepts previously used in sedimentary basin models are generalized and described in our model based on the formalism specific to rock and soil mechanics sediment compaction is described on a geological time scale by an elastoplastic model in which the elastic modulus and the strain hardening modulus increase when deformation increases the plastic limit is the maximum vertical effective stress reached by the sediment the rheology of the sediment is defined by a relationship that couples the porosity or volume of the sediment with the vertical effective stress assuming uniaxial deformation the model also incorporates a viscoplastic term in the compaction equation this component macroscopically considers viscous compaction phenomena such as pressure solution the viscosity coefficient is considered to be a function of the temperature some theoretical considerations allow us to conclude that the thermal dependency of the viscosity is given with an arrhenius law in which the activation energy ranges from 20 kj mole to 50 kj mole using viscosity coefficients extrapolated from previous laboratory experiments a sensitivity study shows significant effects of viscous deformation on the compaction of basins older than 1 ma in another study the viscosity coefficient is determined by matching the results of numerical simulations with laboratory and borehole data obtained from literature for chalk a constant viscosity coefficient of 2 5 gpa ma 8 1022 pa s has been determined assuming viscosity as a function of temperature with an activation energy of 40 kj mole chalk viscosity at 15 c is calibrated around 25 gpa ma simulations with different thermal gradients show that porosity is a function of the temperature furthermore simulations covering different lengths of time show that porosity is also a function of time

compaction and porosity evolution of pliocene sandstones ventura basin california sandstones of the pico and repetto formations were studied to quantify the processes involved in compaction and to determine the porosity evolution of the beds during burial the sandstones are mineralogically immature and except for local carbonate cemented beds have insufficient cement to have prevented compaction stratigraphic studies and graphic porosity depth plots suggest that the samples maximum burial depths were approximately 10 000 ft 3 048 m deeper than the present day depths of 850 13 000 ft 259 3 962 m compaction produced closer grain packing as measured by the contact index ci average number of contacts per grain and tight packing index tpi average number of long sutured and embayed contacts per grain for samples with less than 10 cement the ci depth gradient is 0 21 1 000 ft 0 69 1 000 m and tpi depth gradient is 0 27 1 000 ft 0 89 1 000 m assuming sands had 40 initial porosity the sandstones that are at present most deeply buried lost a total absolute porosity of 26 by all compactional processes which includes 14 by grain rearrangement and 6 each by pressure solution and ductile grain deformation porosity loss by nondeformational grain rearrangement was complete by a present day depth of 5 000 ft 1 524 m but in uncemented sandstones pressure solution plus associated grain rearrangement and ductile grain deformation continued to the maximum depths sampled compaction is most abrupt in sands with abundant micas and ductile rock fragments the ci gradient is 0 14 1 000 ft 0 46 1 000 m for samples with ess than 10 ductile grains but 0 22 1 000 ft 0 72 1 000 m for samples with more than 10 ductile grains compaction was almost entirely the result of overburden pressure and was not noticeably retarded in overpressured zones suggesting that overpressures developed after maximum burial was achieved for samples with less than 10 cement the ci is a better depth predictor than the tpi r2 0 58 vs 0 45 but the tpi is a better predictor of pre cement porosity than the ci r2 0 54 vs 0 31 secondary porosity averages 20 40 of the total thin section porosity secondary porosity is present at all depths but makes up the greatest percentage of total porosity between present depths of 6 000 and 9 500 ft 1 829 and 2 896 m except for grain fracturing secondary porosity formed only by dissolution of framework grains fractured grain porosity contributes up to 29 of total porosity 5 absolute porosity in some samples fractures formed during pleistocene tectonic episodes

relationship between reservoir diagenetic evolution and petroleum emplacement in the ula field north sea abstract the diagenetic and petroleum filling histories of the ula field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements this study shows that diagenesis continues actively after the arrival of petroleum in the sandstones although the reaction rates and petroleum saturation remain obscure microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110 c i e 2 5 km and that this cementation has continued to the present day synchronously with quartz cementation authigenic albite was formed as inter and intragranular cements large amounts of petroleum inclusions locally occur in the albite and quartz cements these inclusions were trapped in the cements as the main charge of petroleum arrived at the structure diffusion through thin water layers between the petroleum and the mineral surface would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite it is evident from analysis of the petroleum inclusions in early formed k feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth before the rapid subsidence in the last 25 my compared with petroleum in the free porosity of the ula formation the petroleum in the k feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature later trapped petroleum inclusions in quartz and albite have characteristics found both in k feldspar and in the ula formation dst oil and thus is likely to reflect the progressive change in the ula field petroleum charge which occurred during the time period of quartz diagenesis

influence of grain boundary structural evolution on pressure solution creep rates intergranular pressure solution is a well known rock deformation mechanism in wet regions of the upper crust and has been widely studied especially in the framework of compaction of granular materials such as reservoir sandstones and fault rocks several analytical models exist that describe compaction creep by stress induced mass transport and the parameters involved are relatively well constrained by laboratory experiments while these models are capable of predicting compaction behavior observed at relatively high porosities they often overestimate compaction rates at porosities below 20 by up to several orders of magnitude this suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models the implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions to address this problem we propose a new thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path here termed grain boundary evolution the resulting constitutive equations for single grain grain contacts are then combined and solved semianalytically the compaction rates predicted by the model are compared with those measured in high strain compaction experiments on wet granular halite a significant reduction in compaction rate is predicted when grain boundary evolution is considered which compares favorably with the experimental compaction data

diagenetic compaction experiments on simulated anhydrite fault gouge under static conditions faults that crosscut subsurface co2 storage systems offer potential leakage pathways especially if fault reactivation and dilation occur after reactivation however newly formed fault gouge is expected to gradually compact and seal as a function of time to estimate the time scale on which this occurs the processes that control compaction must be understood we performed uniaxial compaction experiments on simulated anhydrite fault gouge to investigate the deformation mechanisms that operate under postslip conditions in faulted anhydrite caprocks this involved constant stress 5 12 mpa and stress stepping experiments 5 7 5 10 mpa performed at 80 c under dry and wet conditions on fault gouge samples prepared from crushed natural anhydrite sieved into different grain size fractions in the range 20 500 m dry samples showed little to no compaction creep whereas wet samples i e flooded with presaturated caso4 solution showed rapid compaction our mechanical data and microstructural observations on wet samples suggest that for fine grain sizes 50 m and low stresses gouge compaction is controlled by diffusion controlled pressure solution with increasing grain size and stress fluid assisted subcritical microcracking becomes dominant pressurizing solution flooded samples with co2 15 mpa led to no significant effect on compaction rates in fine grained material but it decreased compaction rates in coarse samples since fine grain sizes are expected in reactivated faults we infer that pressure solution will dominate in anhydrite cap rocks with extrapolation of our lab data to reservoir conditions suggesting sealing time scales of a few decades

on changes in porosity and volume during burial of argillaceous sediments abstract the porosity and hence volume of argillaceous sediments is determined by 1 the magnitude of the effective stress acting within the sediment 2 the previous stress history of the sediment and 3 at shallow depths of burial by features such as the mineralogy and the nature of the depositional environment stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase it is found that all porosity effective stress curves converge at large stresses the change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress and thus is largely independent of the nature of the stress field acting on the basin compressional extensional etc because of the dependence of porosity on the mean effective stress no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated it is possible to contour the porosity effective stress diagram in terms of burial depths these data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies

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

influence of grain size and temperature on intergranular pressure solution quartz cementation and porosity in a quartzose sandstone abstract cathodoluminescence petrography and scanning electron microscopy sem reveal that grain size and temperature significantly influenced intergranular pressure solution quartz cementation and porosity evolution in the quartzose hartshorne sandstone middle pennsylvanian of the arkoma basin among samples from each locality a negative linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution in contrast a generally positive relationship exists between mean grain size and volume of quartz cement although considerable variability exists in this relationship a positive relationship also exists between mean grain size and porosity intergranular pressure solution directly reduced minus cement porosity by causing tighter pack ng of detrital grains it thereby controlled volumes of both cement and primary porosity for this reason little primary porosity is preserved in pressolved finer grained sandstones even though they contain small absolute volumes of quartz cement appreciably more primary porosity may be retained by unpressolved coarser grained sandstones even though they contain larger absolute volumes of quartz cement these relationships also vary with thermal maturity which increases eastward across the basin this thermal maturity trend is the result of elevated temperatures associated with mesozoic rifting and intrusive events in the mississippi embayment among samples of equal grain size more intergranular pressure solution less quartz cement and less porosity are evident in areas of higher thermal maturity as is the case with grain size the influence that temperature exerted on intergranular pressure solution is in turn reflected in volumes of quartz cement and porosity thus primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher in areas of higher thermal maturity more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity this suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones

generation of evenly spaced pressure solution seams during late diagenesis a kinetic theory stylolites and pressure solution seams can form during diagenesis of rocks subjected to stress through a feedback involving texture and pore fluid solute concentration this feedback destabilizes the state of uniform texture represented here only by porosity for simplicity a mathematical kinetic model that includes pressure solution diffusion and reprecipitation predicts the spontaneous generation even in initially uniform rocks of regions alternatingly in space more and less soluble a region that becomes more soluble also becomes simultaneously more porous the kinetic theory predicts that stylolites can form at roughly constant spacings this agrees with many field and petrographic observations bedding planes between beds that have initially different texture are shown by the theory to be the focus of a porosity instability that can lead to the formation of a stylolite and to the generation of more stylolites within each bed this prediction agrees with the observation that many stylolites have developed at bedding planes

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

kaolin coating of stylolites effect on quartz cementation and general implications for dissolution at mineral interfaces abstract the deeply buried 4 km 160 c quartz rich sandstones of the middle jurassic garn formation in well 6507 7 1 offshore mid norway typically contain 20 30 quartz cement and only 3 9 porosity the exception is a less than 1 5 m thick interval at the top of the formation where quartz cement volumes typically drop to 8 16 and porosities are in the range 7 19 the uppermost part of the garn formation is not isolated from the rest of the formation by any type of impermeable barrier the sandstones are water filled indicating that the change in cementation does not correspond to a hydrocarbon contact and there are no significant changes in grain size grain coating abundance grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1 5 m of the sandstones however in the samples from the less cemented interval illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete we propose that the cause of reduced quartz cementation in the top of the garn formation may be the coating of stylolites developed from primary illitic clay rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin rich rock adjacent to the stylolites is dissolved although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality our observations may help to unravel the mechanisms of quartz dissolution at stylolites specifically we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin quartz contacts similarly lack of dissolution at clean quartz quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica which may explain the extensive dissolution often seen at stylolites in limestones

a dual porosity model for evaluating petroleum resource potential in unconventional tight shale plays with application to utica shale quebec canada unconventional tight to shale reservoirs vary from tight sandstone siltstone to organic rich mudstone shale commonly with mixed lithologies in such reservoir systems matrix pores and organic pores with different origins and distinct physical and chemical properties co exist for hydrocarbon storage traditional resource assessment methods designed for conventional reservoirs cannot handle the two pore systems properly this study proposes a dual porosity model to respond to the need for a new method in assessing hydrocarbon resource potential in such reservoir systems the dual porosity model treats the two types of pores separately and derives the resource estimates from different sources of data thus better characterizing unconventional reservoirs with complicated pore systems the new method also has the flexibility of assessing resource potential for the entire spectrum of mixed lithologies ranging from a complete tight to a pure source rock organic rich shale mudstone reservoir the proposed method is illustrated through the assessment of the in place petroleum resource potential in the upper ordovician utica shale of southern quebec canada the results of the application suggest that the proposed approach effectively handles the two pore systems in tight shale reservoirs effectively and provides a useful tool for estimating resource potential in unconventional plays

compaction rock physics and rock properties of sandstones of the sto formation case study of five wells from the south western barents sea norway abstract five wells containing lower middle jurassic sandstones of sto formation from the hammerfest basin 7120 9 1 7121 7 1 the ringvassoy loppa fault complex 7119 12 1 7119 12 4 and the troms finnmark fault complex 7019 1 1 in the barents sea area are considered in this study the sto formation sandstones contain dominantly very fine to medium grained quartz arenites with occasional coarse grained sandstone layers feldspathic and quartz wackes are also present the effect of compaction and exhumation on reservoir properties porosity and permeability and seismic property p wave velocity of these sandstones have been investigated source of quartz cement has also been investigated forty polished thin sections embedded in blue epoxy were studied using optical microscopy scanning electron microscopy and cathodoluminiscence bulk mineralogy was also analysed using x ray diffraction the studied sandstones have experienced cenozoic exhumation ranging between 820 and 1050 m p wave velocity is higher porosities and permeabilities are lower in the western wells 7019 1 1 7119 12 1 and 7119 12 4 compared to the eastern wells 7120 9 1 and 7121 7 1 rock physics models and diagnostics show that the western wells are diagenetically more mature stiffer more compacted and more cemented than the eastern wells these trends are attributed largely to difference in burial history from the east to the west and less to textural variations quartz cement is the most important authigenic mineral in these sandstones quartz cement in the western well 7119 12 1 is predominantly derived from clay induced dissolution at macrostylolites whereas the eastern wells 7120 9 1 and 7121 7 1 are mostly sourced from clay induced dissolution at grain contacts or microstylolites while cementational porosity loss dominates in the western wells compactional porosity loss dominates in the eastern wells compaction can reduce porosities down to 26 and this might be the reason for better porosity preservation and reservoir quality in the eastern wells than in the western wells

an integrated model for transitional pressure solution in sandstones abstract a key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution however it is often not clear why some rocks of a given mineralogy depth and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not to address such difficulties which we believe arise from the natural non linear physico chemical processes a quantitative model for pressure solution applied to sandstones has been developed this model subsumes various factors 1 pressure temperature and burial rate 2 grain size shape and packing of the mineral grains 3 the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact pore fluid pressure salinity ph and temperature in this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential thus allowing the diffusive transfer of matter from one site to another the contact between two grains is divided into two parts the actual true contact between two grains is a site of water film diffusion and the inclusions inside the contact the free face inclusion contacts are sites of free face pressure solution on the pore surface site precipitation occurs it is shown that a kinetics transition from a reaction limited deformation to a diffusion limited deformation occurs in the upper crust a geometric transition occurs too due to the textural evolution of the rock with deformation the main goal of this paper is to show the dynamics of the shifting importance of actual and free face inclusion contacts using a unified model including both features the result is a predictive model for porosity variations with depth due to pressure solution in sandstones

overpressure preventing quartz cementation comment to stricker et al 2016 abstract there is increasing evidence that quartz cementation can be viewed as a process controlled by temperature and insensitive to effective stress this view of quartz cementation in sandstones is often referred to as the illite mica induced dissolution model imid which assumes quartz dissolution to occur along stylolites and clay laminae rather than at quartz quartz grain contacts in the present comment it is argued that the exceptional reservoir quality in the skagerrak formation of the heron cluster north sea is due to grain coatings and rapid cenozoic burial limiting the exposure to quartz cementation this line of reasoning implies overpressure has had neglectable porosity preserving effect in the heron cluster

diagenetic sequences of continuously deposited tight sandstones in various environments a case study from upper paleozoic sandstones in the linxing area eastern ordos basin china an integrated analysis of the petrographic characteristics and types and distribution of diagenetic alteration in the upper paleozoic benxi taiyuan shanxi and xiashihezi formations provides insights into the controlling factors on variations in porosity and permeability in tight sandstones 85 of the sandstone samples display porosity values 10 and 90 of the samples exhibit permeability 1 md diagenetic alteration includes mesogenetic compaction cementation by dolomite ankerite and quartz dissolution of feldspar and illitization of smectite eodiagenesis includes compaction development of smectite cementation by pore filling quartz and disordered kaolinite and precipitation of calcite and fe calcite chlorite and quartz preserve primary pores against damage whereas kaolinite illite smectite i s mixed layer and illite significantly diminish reservoir quality via permeability reduction chlorite and i s content decrease abruptly as depth increases whereas the kaolinite content remains elevated at depth because of the complete destruction of k feldspar the transformation from smectite to illite provides silica ions for the widely distributed quartz overgrowths as the depositional environment transformed from fluvial xiashihezi to deltaic shanxi and to epicontinental taiyuan and benxi the dissolution effect increased monotonically feldspar dissolution is dominant in the shanxi formation whereas the benxi and taiyuan formations commonly contain quartz dissolution pores the taiyuan formation has markedly higher porosities than in the overlying and underlying formations caused by strong dissolution and high silica content the decrease in porosity in the benxi formation results from the extensive formation of clay minerals caused by high frequency transgressions in a transitional environment

the effect of oil saturation on the mechanism of compaction in granular materials higher oil saturations lead to more grain fracturing and less pressure solution abstract whether addition of oil to sandstones slows stops or has no influence upon pressure solution and quartz cementation has long been disputed despite having major implications for petroleum exploration and appraisal strategies to elucidate the effect of addition of oil to compaction pressure solution and cementation processes this study utilizes an experimental approach simulating isochemical volumetric compaction using granular halite in the presence of variable brine oil mixtures each experiment at 500 kpa effective stress lasted 48 hours and involved repeated measurements of volumetric strain the lithified products of experiments were examined using sem techniques after 30 hours approximately 4 volumetric strain occurs when the brine oil ratio is 100 0 pure brine fractures are rare in samples from experiments undertaken with pure brine so that pressure solution and concomitant cementation must be the compaction mechanism when the brine oil ratio is decreased from 100 0 to 40 60 the volumetric strain remains about 4 but there are more fractures as quantified from sem image analysis these observations suggest that although pressure solution has occurred some of the volumetric strain is the result of fracturing implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine when the brine oil ratio is decreased further from 40 60 to 5 95 the volumetric strain increases up to 7 3 sem image analysis reveals that the strain is predominantly due to grain fracturing this change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses brine coated grain contacts undergo pressure solution but oil coated grain contacts experience increasing stress up to the point of failure as the surrounding grain pack compacts when the brine oil ratio is decreased to less than 5 95 volumetric strain decreased to about 1 with few induced fractures and little pressure solution compaction is negligible when the brine oil ratio is 0 100 only oil in the pores and fractures are negligible halite is more water soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz since even halite has its mechanism of volumetric strain affected when the brine oil ratio is less than 40 60 quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling this experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil

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

north sea chalk diagenesis cementation of chalks and healing of fractures abstract petrological and geochemical studies have been carried out on the chalk matrix and the fracture filling calcite from several oil fields in the great ekofisk area of the central graben norwegian north sea in order to understand the mechanism of chalk cementation and fracture healing a hypothesis involving an episode of ascending hot water from the underlying permian evaporites has been postulated by some workers to explain the healing of similar fractures in the chalk reservoirs of the norwegian and danish north sea and the danish sub basin however the results presented in this paper including cathodoluminescence microscopy major and trace element geochemistry c o and sr isotope composition and modelling of heating and compactional water flow rates suggest that the ascending hot water hypothesis does not satisfactorily explain the required water budget rate of flow and the source of the calcite cement heat flow simulations demonstrate that a fluid flow rate of 7 5 cm yr is required by the ascending hot water hypothesis this is much higher than the flow rates calculated for this and other sedimentary basins an alternative model is proposed which can account for the observed 18o values of the fracture filling calcite 12 7 to 8 5 pdb the chalk matrix 7 4 to 4 1 pdb and the formation water 4 smow in terms of closed to semi closed system diagenesis in this model calcite from the chalk matrix dissolves congruently and cement precipitates with an isotope ratio governed by the geothermal gradient similarity in the cathodoluminescence trace element composition and carbon isotope values between the fracture filling calcite and the chalk matrix also suggests that the cements in the fractures and the chalk matrix are similar and were probably derived locally by pressure solution processes

packing states of ideal reservoir sands insights from simulation of porosity reduction by grain rearrangement abstract the question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands ideal reservoir sands a numerical model rampage an acronym of random packing generator has been developed to address this long standing problem rampage represents a synthesis of various algorithms designed to simulate packing of equal sized spheres which have been used to represent ideal solids liquids and gases as well as natural porous media the results of rampage simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal sized spheres in the 2 d state space of porosity p versus mean coordination number n three end member packing states have been identified random loose packing rlp p 45 4 n 5 2 random close packing rcp p 36 3 n 7 0 and bridged random close packing bridged rcp p 39 5 n 5 2 unlike previously proposed models rampage can simulate the transition from rlp to any other point in the stability field the rlp state is fully consistent with wet packed porosities of synthetic sands with lognormal mass size distributions reported in the literature the much higher in situ porosity values reported for modern air packed sands are unlikely to be preserved at depth on geological time scales data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper 800 m of the sediment column corresponds to the evolution of rlp to rcp and is thus fully explained by non destructive grain rearrangement

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

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial