seismic pore space compressibility and gassmann s relation the pore space compressibility of a rock provides a robust model independent descriptor of porosity and pore fluid effects on effective moduli the pore space compressibility is also the direct physical link between the dry and fluid saturated moduli and is therefore the basis of gassmann s equation for fluid substitution for a fixed porosity an increase in pore space compressibility increases the sensitivity of the modulus to fluid substitution two simple techniques based on pore compressibility are presented for graphically applying gassmann s relation for fluid substitution in the first method the pore compressibility is simply reweighted with a factor that depends only on the ratio of fluid to mineral bulk modulus in the second technique the rock moduli are rescaled using the reuss average which again depends only on the fluid and mineral moduli

modelling elastic properties of impure chalk from south arne field north sea in impure chalk the elastic moduli are not only controlled by porosity but also by contact cementation resulting in relatively large moduli for a given porosity and by admixtures of clay and fine silica which results in relatively small moduli for a given porosity based on a concept of solids suspended in pore fluids as well as composing the rock frame we model p wave and s wave moduli of dry and wet plug samples by an effective medium hashin shtrikman model using chemical mineralogical and textural input for a given porosity the elastic moduli correspond to a part of the solid the iso frame value forming the frame of an upper hashin shtrikman bound whereas the remaining solid is modelled as suspended in the pore fluid the iso frame model is thus a measure of the pore stiffness or degree of cementation of the chalk the textural and mineralogical data may be assessed from logging data on spectral gamma radiation density sonic velocity and water saturation in a hydrocarbon zone whereas the iso frame value of a chalk may be assessed from the density and acoustic p wave logs alone the iso frame concept may thus be directly used in conventional log analysis and is a way of incorporating sonic logging data the rigs 1 and rigs 2 wells in the south arne field penetrate the chalk at the same depth but differ in porosity and in water saturation although almost the entire chalk interval has irreducible water saturation our model combined with petrographic data indicates that the difference in porosity is caused by a higher degree of pore filling cementation in rigs 1 petrographic data indicate that the difference in water saturation is caused by a higher content of smectite in the pores of rigs 1 in both wells we find submicron size diagenetic quartz

compressional and shear wave velocities as a function of confining stress in dry sandstones a laboratory study was carried out to investigate the influence of confining stress on compressional and shear wave velocities for a set of rock samples from gas producing sandstone reservoirs in the cooper basin south australia the suite of samples consists of 22 consolidated sublitharenites with helium porosity ranging from 2 6 to 16 6 we used a pulse echo technique to measure compressional and shear wave velocities on dry samples cylindrical 4 6 2 cm at room temperature and at elevated confining stress 60 mpa compressional and shear wave velocities in samples increase non linearly with confining stress a regression equation of the form v a be dp gives a good fit to the measured velocities with improved prediction of velocities at high confining stresses compared with equations suggested by other studies the predicted microcrack closure stresses of the samples show values ranging from 70 mpa to 95 mpa and insignificant correlation with porosity permeability or clay content there is a positive correlation between change in velocity with core porosity and permeability but this association is weak and diminishes with increasing confining stress experimental results show that pore geometry grain contact type and distribution and location of clay particles may be more significant than total porosity and clay content in describing the stress sensitivity of sandstones at in situ reservoir effective stress the stress dependence of cooper basin sandstones is very large compared with data from other studies the implication of our study for hydrocarbon exploration is that where the in situ reservoir effective stress is much less than the microcrack closure stress of the reservoir rocks the variation of reservoir effective stress could cause significant changes in velocity of the reservoir rocks the velocity changes induced by effective stress in highly stress sensitive rocks can be detected at sonic log and probably surface seismic frequencies

development of microporosity in a micritic limestone reservoir lower cretaceous middle east abstract abundant intercrystalline microporosity is developed within the matrix of micritic limestones over several hundreds of meters of section in the thamama group lower cretaceous at sajaa field eastern u a e sediments in the upper shuaiba formation uppermost tamama group include muddy rudist algal coralgal and orbitolina dominated lithofacies matrix is composed of euhedral to subhedral micro rhombic low mg calcite crystals with average diameters of about 5 m microporosity development was not related to leaching chalkification of lithified micrite during post depositional emergence of the shuaiba formation features diagnostic of emergence are lacking in core below the upper contact of the unit matrix microporosity is believed to have been an early developed property of micritic limestones trace element compositions of matrix reflect a diagenetic process that involved conversion of a calcitic lime mud into micro rhombic micrite and microspar using a closed system geochemical model the observed matrix oxygen isotope compositions 18 o of 4 to 5 pdb are compatible with dissolution of aptian lime mud 18 o of 2 pdb and reprecipitation of calcite in fluids of marine composition under conditions corresponding to burial depths of 550 800 m porosity of 30 50 at 40 50 c closed system conditions are consistent with textural constraints for limited or arrested cementation in the matrix chemical compaction was responsible for partial occlusion of matrix microporosity as microporosity is decreased in zones of intense stylolitization slight corrosion of matrix crystals and leaching of peloids and orbitolina postdated cementation and chemical compaction during deep burial

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

controls on sonic velocity in carbonates compressional and shear wave velocities v p and v s of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore fluid pressures the lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones the velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy porosity pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations

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

vp vs ratio versus differential stress and rock consolidation a comparison between rock models and time lapse avo data the compressional to shear wave velocity ratio vp vs is an important parameter in seismic amplitude versus offset avo analysis and this parameter plays a key role especially for lithology and fluid prediction methods the p wave velocity is a key parameter in traditional pressure prediction methods because overpressure often results in a velocity reduction however for avo based pore pressure prediction methods one expects that the vp vs ratio also is a key parameter the hertz mindlin geomechanical model predicts a constant vp vs ratio as the differential stress changes in a dry package of identical spheres ultrasonic core measurements show increased vp vs ratios as the differential stress decreases especially for unconsolidated wet sands thus one is likely to assume that the vp vs ratio is dependent on rock consolidation by combining the hertz mindlin model with the gassmann model we show how to obtain a sim ple rock physics framework including both the differential stress and

the relationship between acoustic properties and the petrographic character of carbonate rocks laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine and air saturated carbonate rocks with a wide range of facies porosities lithologies and rock fabrics indicate that porosity is the major factor influencing both p and s wave impedance and velocity primary lithology and secondary mineralogy have only a small influence on impedance and velocity combined use of p and s wave velocity data discriminates porosity changes from lithologic changes all other variables including pore fluid type and petrographic fabric have no significant influence on velocities laboratory measurements of p wave velocity under simulated in situ conditions reproduce well log velocity values reliably laboratory porosity velocity trends agree with the time average equation when the correct matrix velocities are used rock property results were used to interpret porosity lithology variations for an inverted seismic section from the williston basin where well

the petrophysical basis for shallow water flow prediction using multicomponent seismic data physical properties of shallow water flow swf sands differ from most reservoir and seal rocks studied for petroleum purposes these materials exist near the transition zone between rocks and sediments our investigations the subject of this article suggest that physical properties of swf sands are amenable to a prediction methodology that uses high resolution multicomponent seismic data swf sands are a primary hazard to deepwater drilling operations in the gulf of mexico they have the potential to result in well failures and significant discharges of subsurface fluids into the ocean however at present most operators make only a modest attempt to identify swf before drilling they drill through these hazards and mitigate any resulting damage at significant expense if one is encountered this is not rare according to a report from fugro geoservices approximately 70 of all deepwater wells have experienced swf a method for predrill delineation of sands that are close to failure and thus likely to exhibit swf would be advantageous in selecting optimal drilling locations and in developing cost effective well plans the current method of identifying swf involves predrill hazard studies utilizing conventional and high resolution seismic data to identify zones that might produce swf however the high incidence of swf mitigation events in deepwater wells shows that existing techniques do not provide adequate resolution or accuracy at present no robust seismic method exists for accurately identifying and characterizing swf operators do not have the option to avoid the hazard instead of mitigating the problem after it has started a new method is required that will permit operators to identify the hazard before the well is drilled using seismic data for accurate understanding of the physical properties and deformational behavior of swf sands is essential to characterization prediction and interpretation of these stratigraphic units to date very few core and

parameters controlling sonic velocities in a mixed carbonate siliciclastics permian shelf margin upper san andres formation last chance canyon new mexico we have measured the acoustic properties and mineralogic composition of 48 rock specimens from mixed carbonate siliciclastic outcrops of the permian upper san andres formation in last chance canyon new mexico the goals were 1 identify and model the parameters controlling the sonic velocities 2 assess the influence of postburial diagenesis on the acoustic velocities the variation in sonic velocity in the 0 to 25 porosity range is primarily controlled by porosity and secondly by the ratio of carbonate siliciclastic material linear multivariate fitting resulted in a velocity porosity carbonate content transform that accurately predicts sonic velocity at different effective stresses the slope of the velocity porosity transform steepens with increasing carbonate content which may be explained by the higher velocity of carbonate minerals another reason may be the property of carbonate minerals to form more perfect intercrystalline boundaries that improve the transmission properties of acoustic waves and are less sensitive to changes in effective stress the velocity ratio vp vs is an excellent tool to discriminate between predominantly calcitic lithologies ratio between 1 8 and 1 95 and predominantly dolomitic and quartz rich lithologies ratio between 1 65 and 1 8 gardner s experimental curve overestimates and the velocity porosity transforms by wyllie and raymer underestimate the observed sonic velocities probably because they do not account for variations in texture carbonate mineralogy and pore geometry petrographic observations show that postburial diagenesis is minor and does not seem to significantly affect porosity therefore the outcrop data set can be regarded as a proxy for the subsurface analog these findings underline the significantly more complex acoustic behavior in mixed carbonate siliciclastic sedimentary rocks than in pure siliciclastics where mineralogic composition explains most of the observed relationships between porosity and sonic velocity

velocities of compressional and shear waves in limestones carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties porosity and permeability mainly resulting from various diagenetic processes compaction dissolution precipitation cementation etc these complexities make prediction of reservoir characteristics e g porosity and permeability from their seismic properties very difficult to explore the relationship between the seismic petrophysical and geological properties ultrasonic compressional and shear wave velocity measurements were made under a simulated in situ condition of pressure 50 mpa hydrostatic effective pressure at frequencies of approximately 0 85 mhz and 0 7 mhz respectively using a pulse echo method the measurements were made both in vacuum dry and fully saturated conditions in oolitic limestones of the great oolite formation of southern england some of the rocks were fully saturated with oil the acoustic measurements were supplemented by porosity and permeability measurements petrological and pore geometry studies of resin impregnated polished thin sections x ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores it is shown that the compressional and shear wave velocities vp and vs respectively decrease with increasing porosity and that vp decreases approximately twice as fast as vs the systematic differences in pore structures e g the aspect ratio of the limestones produce large residuals in the velocity versus porosity relationship it is demonstrated that the velocity versus porosity relationship can be improved by removing the pore structure dependent variations from the residuals the introduction of water into the pore space decreases the shear moduli of the rocks by about 2 gpa suggesting that there exists a fluid matrix interaction at grain contacts which reduces the rigidity the predicted biot gassmann velocity values are greater than the measured velocity values due to the rock fluid interaction this is not accounted for in the biot gassmann velocity models and velocity dispersion due to a local flow mechanism the velocities predicted by the raymer and time average relationships overestimated the measured velocities even more than the biot model

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

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

conditions during syntectonic vein formation in the footwall of the absaroka thrust fault idaho wyoming utah fold and thrust belt abstract the twin creek limestone in the footwall of the absaroka thrust sheet contains three sets of bed normal syntectonic calcite veins vein formation occurred during cretaceous motion along the absaroka thrust fault as indicated by 1 crosscutting relationships among these vein sets 2 a previously dated solution cleavage and 3 calcite twin analysis fluid inclusions in the veins and overburden estimates constrain inclusion entrapment temperatures to be between 175 c and 328 c results from stable oxygen isotopes indicate that the host and vein fluid compositions were in near isotopic equilibrium applying both reasonable geothermal gradients and constraints on overburden temperature yields fluid pressures during vein precipitation that are near hydrostatic all data taken together suggest both that vein formation within the twin creek formation occurred in a relatively closed system and that the veins filled near hydrostatic fluid pressure because the veins fill precursory cracks vein filling might not reflect the maximum fluid pressure that existed during the complete vein forming process

predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics based on the recently developed petrophysical classification of siliciclastics which takes into account the amount of the volumetric clay content c and textural position of clay it is shown that acoustic velocities can be fairly accurate tools in predicting lithology porosity and ultimately transport properties of these rocks four major petrophysical groups of carbonate and organic poor siliciclastics are distinguished 1 clean arenites c 35 percent the compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients this allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average wyllie equation or its improved modification raymer equations both of which neglect textural factors

velocity porosity relationships for water saturated siliciclastic sediments seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments we have brought together an extensive suite of both new and published log and core based data we undertook a detailed statistical analysis of ocean drilling program data from amazon fan to examine variables affecting compressional velocity in shallow unconsolidated sediments we identify three dominant variables porosity shale fraction and consolidation history and present two empirically determined boundary curves one for normally consolidated sediments and a second for highly consolidated environments e g accretionary prisms these two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water filled pores as a function of porosity and clay content for the full range of observed porosities velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content at fractional porosities higher than about 0 4 fluid dominates the elastic properties and velocity exhibits a subtle dependence on porosity remarkably the amazon fan data show that clay content has no direct influence on velocity at high porosities both clay content and sorting do indirectly affect velocity through their control of porosity burial affects velocity not only by compaction related porosity decrease but also by pressure induced increase of intergrain coupling because of the sensitivity of velocity to consolidation history particularly at intermediate fractional porosities of about 0 30 0 40 no single velocity porosity relationship can apply to all high porosity sediments the two proposed relationships fit the majority of published and new data they are applicable however only for normally pressured in situ conditions and water filled pores

micro rhombic calcite and microporosity in limestones a geochemical study of the lower cretaceous thamama group u a e abstract euhedral micro rhombic calcite forms a microporosity network in limestones of the lower cretaceous thamama group u a e isotopic and trace element data indicate that micro rhomb formation was a two step process associated with diagenesis in meteoric waters the micro rhombic calcite and the micropores are both products of the second event sr fe mn and mg contents of the micro rhombs are all relatively low ranging from 108 to 310 ppm 34 to 308 ppm 3 to 32 ppm and 0 42 to 0 68 mole respectively the low sr and mg contents indicate extensive flushing of these constituents from the rock the low fe and mn contents are probably inherited from the original sediments and indicate the lack of any fe or mn in the pore fluids during micro rhomb formation the 13c of the micro rhombs 3 1 to 4 5 pdm are also primary unaltered by diagenesis the 13o of the micro rhombs 4 5 to 6 0 pdb reflect alteration by mateoric waters a large number of pore volumes and an open water buffered system would be necessary to flush sr and mg yet decreasing 18o with increasing porosity indicates that 18o values in non porous intervals are partially rock buffered two diagenetic events are thus implicated the first flushing sr and mg and lowering 18o in all rocks and the second event actually forming the micro rhombs and further altering 18o as a function of porosity the first diagenetic event was most likely mineralogical stabilization in an unconfined meteoric aquifer immediately after shuaiba deposition if mineralogical stabilization was completed in the first event then calcite to calcite recrystallization is implicated for the second event the second event may have occurred in the unconfined meteoric aquifer or it may have occurred in a confined meteoric aquifer during the early late cretaceous

saturation pore pressure and effective stress from sandstone acoustic properties 1 acoustic velocity and vp vs ratio show different responses in saturated and dry rocks under increasing differential pressures velocity is also shown to be sensitive to pore pressure not just differential pressure under identical low differential pressure conditions velocity is higher in sandstones when pore pressure is high during simulation of inflationary overpressure conditions vp vs ratios of sandstones with high fluid pressures are shown to lie between dry conditions and saturated conditions with constant and low pore pressure thus the velocity differential pressure relationship is not unique however the use of the measured biot effective stress coefficient results in a unique relationship it is postulated that higher velocity and different vp vs ratios are caused by loss of compliance in microfractures due to stiffening by increasing fluid pressure

controls on acoustic properties of upper jurassic siliciclastic rocks boulonnais northern france more than 200 plugs from outcrop and a nearby borehole in a carbonaceous siliciclastic interval of late jurassic kimmeridgian to tithonian age were quantitatively analyzed for texture mineralogy and acoustic properties our primary goal was to study the effect of clay fraction smaller than 8 m silt sand fraction larger than 8 m and carbonate on the acoustic properties the quantitative nature and volume of the data made it possible to observe four dimensional relationships in contoured ternary diagrams primary control on the acoustic properties is exerted by porosity but the trend of this relationship significantly deviates from popular velocity transforms the contribution of clay silt sand and carbonate particular material and cement explains the remaining variation in acoustic properties although no clear linear thresholds are defined a general trend is that clay and carbonate content have opposite and overlapping effects on acoustic properties the influence of clay content progressively increases with decreasing carbonate content and visa versa with increasing carbonate content the variation of acoustic velocity at a given porosity value increases to nearly twice of that in the clay dominated sediment traditional classification boundaries are present but strongly overprinted by this interplay between clay and carbonate this study may have important implications for porosity and lithofacies prediction from wireline logs in similar mixtures of sediment in addition the quantitative character of the textural and mineralogical data may provide a direct link from acoustic properties to the primary depositional system and sequence stratigraphy

pressure and porosity influences on vp vs ratio in unconsolidated sands elevated pore pressures commonly encountered in the shallow unconsolidated section of the sedimentary column present a significant hazard during the drilling and completion of offshore wells the porosity of the ocean floor sediments is high and a cover of low permeability clay can prevent the underlying sediments from draining even at very shallow depths below the seafloor as further deposition loads the sediment the entrapped fluids impede normal compaction by becoming pressurized the lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non cohesive sediments resulting in very weak shallow sands this weakness can result in washouts or fracturing in the shallow strata potentially leading to the loss of the well and of neighboring wells