bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

a generalized biot gassmann model for the acoustic properties of shaley sandstones1 we obtain the wave velocities of clay bearing sandstones as a function of clay content porosity and frequency unlike previous theories based simply on slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids sand grains and clay particles and a fluid the theory which is consistent with the critical porosity concept uses three free parameters that determine the dependence of the dry rock moduli of the sand and clay matrices as a function of porosity and clay content testing of the model with laboratory data shows good agreement between predictions and measurements in addition to a rock physics model that can be useful for petrophysical interpretation of wave velocities obtained from well logs and surface seismic data the model provides the differential equation for computing synthetic seismograms in inhomogeneous media from the seismic to the ultrasonic frequency bands

the seismic response to overpressure a modelling study based on laboratory well and seismic data we investigate the seismic detectability of an overpressured reservoir in the north sea by computing synthetic seismograms for different pore pressure conditions the modelling procedure requires the construction of a geological model from seismic well and laboratory data seismic inversion and avo techniques are used to obtain the p wave velocity with higher reliability than conventional velocity analysis from laboratory experiments we obtain the wave velocities of the reservoir units versus confining and pore pressures laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions these measurements provide the basis for calibrating the pressure model overpressures are caused by different mechanisms we do not consider processes such as gas generation and diagenesis which imply changes in phase composition but focus on the effects of pure pore pressure variations the results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep north sea reservoirs

pseudo 3 d p wave refraction seismic monitoring of permafrost in steep unstable bedrock 1 degrading permafrost in steep rock walls can cause hazardous rock creep and rock slope failure spatial and temporal patterns of permafrost degradation that operate at the scale of instability are complex and poorly understood for the first time we used p wave seismic refraction tomography srt to monitor the degradation of permafrost in steep rock walls a 2 5 d survey with five 80 m long parallel transects was installed across an unstable steep ne sw facing crestline in the matter valley switzerland p wave velocity was calibrated in the laboratory for water saturated low porosity paragneiss samples between 20 c and 5 c and increases significantly along and perpendicular to the cleavage by 0 55 0 66 km s 10 13 and 2 4 2 7 km s 100 respectively when freezing seismic refraction is thus technically feasible to detect permafrost in low porosity rocks that constitute steep rock walls ray densities up to 100 and more delimit the boundary between unfrozen and frozen bedrock and facilitate accurate active layer positioning srt shows monthly august and september 2006 and annual active layer dynamics august 2006 and 2007 and reveals a contiguous permafrost body below the ne face with annual changes of active layer depth from 2 to 10 m large ice filled fractures lateral onfreezing of glacierets and a persistent snow cornice cause previously unreported permafrost patterns close to the surface and along the crestline which correspond to active seasonal rock displacements up to several mm a srt provides a geometrically highly resolved subsurface monitoring of active layer dynamics in steep permafrost rocks at the scale of instability

seismic and electrical properties of unconsolidated permafrost1 a model has been developed to relate the velocities of acoustic waves vp and vs in unconsolidated permafrost to the porosity and extent of freezing of the interstitial water the permafrost is idealized as an assemblage of spherical quartz grains embedded in a matrix composed of spherical inclusions of water in ice the wave scattering theory of kuster and toksoz is used to determine the effective elastic moduli and hence the acoustic velocities the model predicts vp and vs to be decreasing functions of both the porosity and the water to ice ratio the theory has been applied to laboratory measurements of vp and vs in 31 permafrost samples from the north american arctic although no direct measurements were made of the extent of freezing in these samples the data are consistent with the predictions of the model electrical resistivity measurements on the permafrost samples have demonstrated their essentially resistive behaviour the ratio of resistivity of permafrost in its frozen state to that in its unfrozen state has been related to the extent of freezing in the samples electromagnetic and seismic reflection surveys can be used together in areas of permafrost firstly an em survey to determine the extent of freezing and then the acoustic velocity model to predict the velocities in the permafrost the necessary transit time corrections can thus be made on seismic reflection records to compensate for the presence of permafrost

glacier bed characteristics of midtre lovenbreen svalbard from high resolution seismic and radar surveying we conducted a seismic and radar survey of the central part of midtre lovenbreen a small polythermal valley glacier in svalbard we determined the physical properties of the material beneath the glacier by measuring the reflection coefficient of the bed by comparing the energy of the primary and multiple reflections and deriving the acoustic impedance by making reasonable assumptions about the properties of the basal ice we determined the acoustic impedance of the bed material as 6 78 1 53 x 10 6 kg m 2 s 1 we interpret the material beneath the glacier to be permafrost with up to 50 ice and we speculate that the material may be frozen talus similar to a deposit observed directly by others beneath another svalbard glacier the implication for midtre lovenbreen is that the basal material beneath the present glacier is not able to support fast flow we conclude that midtre lovenbreen has most likely had limited capability for faster flow in the past with motion dominated by internal deformation midtre lovenbreen is used as a study glacier for the scientific community in svalbard and a large number of studies have been based there our results show that it cannot be used as an analogue for larger glaciers in svalbard having distinct basal boundary conditions

amplitude variations with offset of pressure seal reflections knowledge of pressure compartments is essential for identifying potential risks in advance of drilling and in understanding the mechanism of hydrocarbon trapping and migration these compartments are bounded by pressure seals which under certain conditions can be detected and characterized with seismic reflection data use of prestack seismic data requires the analysis of reflections generated at the pressure seals such as the top of a reservoir rather than the properties of the rock volume in this sense amplitude variations with offset avo techniques can be a good tool to obtain information from such pressure seals or pressure transition zones in this work i investigate the avo response of compartment seals by analyzing the plane wave reflection coefficients as a function of pore pressure above and below the seal incidence angle and seal thickness for the case investigated in this work berea sandstone with high gas saturation the avo effects are important at low effective pressures tha

analytic simulation of the elastic waves propagation in the neighborhood of fluid filled wells with monopole sources an analytic formulation to understand the scattering diffraction and attenuation of elastic waves at the neighborhood of fluid filled wells is presented an important and not widely exploited technique to carefully investigate the wave propagation in exploration wells is the logging of sonic waveforms fundamental decisions and production planning in petroleum reservoirs are made by interpretation of such recordings nowadays geophysicists and engineers face problems related to the acquisition and interpretation under complex conditions associated with conducting open hole measurements a crucial problem that directly affects the response of sonic logs is the eccentricity of the measuring tool with respect to the center of the borehole even with the employment of centralizers this simple variation dramatically changes the physical conditions on the wave propagation around the well recent works in the numerical field reported advanced studies in modeling and simulation of acoustic wave propagation around wells including complex heterogeneities and anisotropy however no analytical efforts have been made to formally understand the wireline sonic logging measurements acquired with borehole eccentered tools in this paper the graf s addition theorem was used to describe monopole sources in terms of solutions of the wave equation the formulation was developed from the three dimensional discrete wave number method in the frequency domain the cylindrical bessel functions of the third kind and order zero were re derived to obtain a simplified set of equations projected into a bi dimensional plane space for displacements and stresses this new and condensed analytic formulation allows the straightforward calculation of all converted modes and their visualization in the time domain via fourier synthesis the main aim was to obtain spectral surfaces of transfer functions and synthetic seismograms that might be useful to understand the wave motion produced by the eccentricity of the source and explain in detail the new arising borehole propagation modes finally time histories and amplitude spectra for relevant examples are presented and the validation of time traces using the spectral element method is reported

elasticity of partially saturated frozen sand seasonal thawing and freezing of near surface sediments significantly affect the interpretation of seismic reflection surveys and vertical seismic profiling as well as activity on the surface permafrost covers much of the earth s colder regions and is also subject to periodic thawing and freezing the significance of the regions with large seasonal temperature variations for exploration and engineering activities makes understanding the elasticity of frozen ground an important practical and scientific goal

the ridgeway gold copper deposit a high grade alkalic porphyry deposit in the lachlan fold belt new south wales australia a discussion sir wilson et al 2003 in their important paper on the ridgeway au cu deposit describe hydrothermal alteration about the ridgeway porphyry ore system however their findings are substantially more valuable if taken into account with the published works on regional burial metamorphism of the district smith 1968 1969 these works described burial metamorphism of the ordovician volcanic sequences including the hosts to the ridgeway deposit based on district and regional zonation of secondary mineral assemblages this discussion briefly presents this context and focuses on the key issue of discriminating the porphyry related hydrothermal alteration from the effects of burial metamorphism because porphyry related hydrothermal minerals and later metamorphic mineral assemblages can be similar their discrimination is crucial in mapping alteration styles as vectors in exploration for concealed ore

the effect of the extent of freezing on seismic velocities in unconsolidated permafrost we develop a model to relate velocities of seismic waves in unconsolidated permafrost is idealized as an assemblage of spherical quartz grains imbedded in a matrix composed of spherical water inclusions in ice the theory of kuster and toksoz based on wave scattering considerations is used to determine the effective elastic moduli and hence the wave speeds the hasin shtrikman theoretical bounds on the elastic moduli of heterogeneous materials and considerations establish the plausibility of the model the model predicts vp and vs to be decreasing functions of both the porosity and the water to ice ratio and the ratio vp vs to be an increasing function of these two parameters the theory is then applied to laboratory measurements of shear and compressional wave velocities in 23 permafrost samples from different sites in the beaufort sea mackenzie river valley and canadian arctic islands although no direct measurements were made of the extents of freezing in these samples the data are consistent w

seismic modelling study of a subglacial lake we characterize the seismic response of lake vostok an antarctic subglacial lake located at nearly 4 km depth below the ice sheet this study is relevant for the determination of the location and morphology of subglacial lakes the characterization requires the design of a methodology based on rock physics and numerical modelling of wave propagation the methodology involves rock physics models of the shallow layer firn the ice sheet and the lake sediments numerical simulation of synthetic seismograms ray tracing p transforms and ava analysis based on the theoretical reflection coefficients the modelled reflection seismograms show a set of straight events refractions through the firn and top ice layer and the two reflection events associated with the top and bottom of the lake theoretical ava analysis of these reflections indicates that at near offsets the pp wave anomaly is negative for the ice water interface and constant for the water sediment interface this behaviour is shown by ava analysis of the synthetic data set this study shows that subglacial lakes can be identified by using seismic methods moreover the methodology provides a tool for designing suitable seismic surveys

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

coal occurrence in the jurassic dhruma formation in saudi arabia inferences on its gas and surface mining potential abstract coal occurrence in central saudi arabia was previously noted in government internal reports but apparently was neither analyzed nor studied in detail in the present study a coal sample collected from a water well drilled in the az zilfi area of central saudi arabia was available for analysis to characterize the coal and define its potential to generate gas the sample was obtained from a silty sandy mudstone unit in the upper part of the middle jurassic dhruma formation at a depth of 70 m below the ground surface organic geochemical rock eval toc and organic petrographic analyses of the sample have confirmed the occurrence of low maturity coal in the dhruma formation in the az zilfi area which has potential for surface coal mining as well as for microbial gas generation regional geologic data highlights the potential for higher maturity coal occurrence at depths suitable for thermogenic gas generation deep and shallow well drilling programs would provide essential data to assess both the coal gas and surface mining potential

p wave seismic attenuation by slow wave diffusion effects of inhomogeneous rock properties recent research has established that the dominant p wave attenuation mechanism in reservoir rocks at seismic frequencies is because of wave induced fluid flow mesoscopic loss the p wave induces a fluid pressure difference at mesoscopic scale inhomogeneities larger than the pore size but smaller than the wavelength typically tens of centimeters and generates fluid flow and slow diffusion biot waves continuity of pore pressure is achieved by energy conversion to slow p waves which diffuse away from the interfaces in this context we consider a periodically stratified medium and investigate the amount of attenuation and velocity dispersion caused by different types of heterogeneities in the rock properties namely porosity grain and frame moduli permeability and fluid properties the most effective loss mechanisms result from porosity variations and partial saturation where one of the fluids is very stiff and the other is very compliant such as a highly permeable sandstone at shallow depths saturated with small amounts of gas around 10 saturation and water grain and frame moduli variations are the next cause of attenuation the relaxation peak moves towards low frequencies as the background permeability decreases and the viscosity and thickness of the layers increase the analysis indicates in which cases the seismic band is in the relaxed regime and therefore when the gassmann equation can yield a good approximation to the wave velocity

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

the ridgeway gold copper deposit a high grade alkalic porphyry deposit in the lachlan fold belt new south wales australia ridgeway is a high grade gold copper porphyry deposit 54 mt at 2 5 g t au and 0 77 cu related to an alkalic intrusive complex of monzonitic composition the deposit occurs within the cadia district of new south wales australia which consists of a cluster of four late ordovician gold copper porphyry deposits and two iron copper gold skarn deposits with a combined gold resource in excess of 574 metric tonnes t the deposits lie on a 7 km long northwest oriented corridor of alteration and mineralization transverse to the axis of the postulated volcanic arc alteration and mineralization at ridgeway are zoned around a vertically attenuated intrusive complex of monzodioritic to quartz monzonitic composition distinct styles of veining and alteration are related to different intrusive phases of the monzonite complex with the intensity of alteration and grade of mineralization decreasing from early to late mineral intrusions early mineral intrusions are associated with intense actinolite magnetite biotite calc potassic alteration and up to four stages of high grade quartz magnetite sulfide veining bornite is the most abundant sulfide formed during early stage alteration and correlates well with gold moderate to weak intensity orthoclase biotite plus or minus magnetite potassic alteration accompanies the inter and late mineral intrusions this alteration being associated with chalcopyrite and pyrite rich quartz orthoclase veins propylitic and sodic albite pyrite alteration assemblages are peripheral to and locally overprint the potassic alteration phyllic alteration is restricted to the margins of late stage faults the fluid inclusion assemblage comprises one and two salt bearing brine inclusions in addition to aqueous liquid vapor inclusions of low to moderate salinity no low density vapor rich inclusions are present indicating that the fluids from which the quartz veins precipitated did not enter the liquid vapor field of the h2o nacl system the brine inclusions undergo final homogenization to liquid via halite dissolution this phenomenon in addition to the absence of low density vapor inclusions suggests that the mineralizing fluids at ridgeway were nonboiling hypersaline brines that exsolved directly from the crystallizing magma the presence of mineralized aplitic vein dikes and comb quartz layering are interpreted to indicate that the early and transitional stages of mineralization at ridgeway formed at the transition between magmatic and hydrothermal conditions

noble gas tracing of groundwater coalbed methane interaction in the san juan basin usa the san juan basin natural gas field located in northwestern new mexico and southwestern colorado in the usa is a case type coalbed methane system groundwater is thought to play a key role in both biogenic methane generation and the co2 sequestration potential of coalbed systems we show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas we collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin and the underpressured low producing region as having a significant admix of thermogenic coal gas 3he 4he ratios increase from 0 0836ra at the basin margin to 0 318ra towards the center indicating a clear but small mantle he signature in all gases coherent fractionation of water derived 20ne 36ar and crustal 4he 40ar are explained by a simple rayleigh fractionation model of open system groundwater degassing low 20ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater associated gas by desorbed coalbed methane this rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well the quantified water volumes in both underpressured and overpressured zones range from 1 7 103 m3 to 4 2 105 m3 with no clear distinction between over and underpressured production zones these results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds there is no requirement of continuous groundwater flow for renewing the microbes or nutrient components we furthermore observe strong mass related isotopic fractionation of 20ne 22ne and 38ar 36ar isotopic ratios this can be explained by a noble gas concentration gradient in the groundwater during gas production which causes diffusive partial re equilibration of the noble gas isotopes it is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air derived noble gases can occur when such concentration gradients are established during gas production excess air derived xe and kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if xe and kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form ch4

mesozoic siliciclastic reservoirs and petroleum system in the rub al khali basin saudi arabia hydrocarbons have recently been discovered in upper triassic to middle jurassic siliciclastic reservoirs in the rub al khali basin in saudi arabia the reservoirs fill accommodation space created by triassic and early jurassic crustal scale basins on the order of 100 km 62 mi in wavelength and hundreds of meters in depth these basins are separated by highs that are interpreted as crustal scale epeirogenic folds lithologies include well sorted quartz arenites deposited in shallow marine shoreface and fluviodeltaic settings these sequences can be correlated across the basin to extensive escarpment outcrops south of riyadh and beyond saudi arabia into well documented equivalents elsewhere in the middle east and east africa the gross architecture of the interval is imaged on reflection seismic showing clinoformal geometries and onlap onto the triassic structured surface geochemistry of tested fluids indicates a type iii kerogen source the simplest interpretation is that the system is self sourcing hydrocarbons from interbedded coaly material that is observed in the wells and at outcrop reservoir pressures are anomalously low relative to the overlying carbonate reservoir systems these low pressures are interpreted to indicate lateral communication from the rub al khali basin westward to outcrop in contrast with the overlying carbonate fairways that are known to contain facies boundaries that trend across the regional dip onlapping geometries in the siliciclastic fairway combine with cretaceous and cenozoic compressional structures to create combined structural stratigraphic traps

gas generation and overpressure effects on seismic attributes drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones knowledge of pore pressure using seismic data as for instance from seismic while drilling techniques will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures we assume a simple basin evolution model with a constant sedimentation rate and a constant geothermal gradient oil gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow assuming a first order kinetic reaction with a reaction rate satisfying the arrhenius equation the oil gas conversion fraction is calculated balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations this excess pore pressure determines the effective pressure which in turn determines the skeleton bulk moduli if the generated gas goes into solution in the oil this effect does not greatly change the depth and oil gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure the seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas moreover the velocities and attenuation factors versus frequency are obtained by including rock frame fluid viscoelastic effects to match ultrasonic experimental velocities for the basin evolution model used here pore pressure is seismically visible when the effective pressure is less than about 15 mpa and the oil gas conversion is about 2 5 percent

velocity and resistivity changes during freeze thaw cycles in berea sandstone we report on simultaneous measurements of resistivity compressional vp and shear vs velocities made during the temperature cycling of berea sandstone while at elevated pressure temperatures were cycled through the freezing point of the saturating brine the responses of resistivity and velocities are in phase but are asymmetrical as the brine freezes and thaws limited freezing causes no discernible damage to the core free water is present below the nominal freezing point of the brine vp vs and resistivity change by 16 24 and 500 respectively over the temperature range of 4 c to 6 c at 6 9 mpa confining pressure

application of refraction seismics in alpine permafrost studies a review abstract refraction seismics is a common geophysical application to investigate permafrost in alpine studies this study reviews the physical basis of the use of refractions seismics and introduces the history of its application in alpine environments seismic properties are influenced by rock or soil properties and fluid or ice effects in laboratory measurements these influences are determined and linked to mechanical properties or used for calibration of field studies in field conditions supplementary environmental factors influence the seismic properties of landforms in this paper laboratory studies of rocks from different lithologies and with different properties as well as case studies on rock glaciers moraines talus slopes and debris covered slopes are collected and influencing factors quantitatively analyzed the data demonstrate how lithology porosity and anisotropy influence p wave velocity environmental factors result in a high variation of p wave velocities in landforms however p wave velocities enable the differentiation of active layer and permafrost in rock glaciers and moraines in talus slopes and debris covered slopes p wave velocity contrasts between active layer and permafrost layer might be insufficient and require the application of additional methods for a final differentiation

sonic to ultrasonic q of sandstones and limestones laboratory measurements at in situ pressures laboratory measurements of the attenuation and velocity dispersion of compressional and shear waves at appropriate frequencies pressures and temperatures can aid interpretation of seismic and well log surveys as well as indicate absorption mechanisms in rocks construction and calibration of resonant bar equipment was used to measure velocities and attenuations of standing shear and extensional waves in copper jacketed right cylinders of rocks formula in length formula in diameter in the sonic frequency range and at differential pressures up to formula we also measured ultrasonic velocities and attenuations of compressional and shear waves in formula diameter samples of the rocks at identical pressures extensional mode velocities determined from the resonant bar are systematically too low yielding unreliable poisson s ratios poisson s ratios determined from the ultrasonic data are frequency corrected and used to calculate thesonic frequency compressional wave velocities and attenuations from the shear and extensional mode data we calculate the bulk modulus loss the accuracies of attenuation data expressed as formula where q is the quality factor are formula for compressional and shear waves at ultrasonic frequency formula for shear waves and formula for compressional waves at sonic frequency example sonic frequency data show that the energy absorption in a limestone is small formula greater than 200 and stress independent and is primarily due to poroelasticity whereas that in the two sandstones is variable in magnitude formula ranges from less than 50 to greater than 300 at reservoir pressures and arises from a combination of poroelasticity and viscoelasticity a graph of compressional wave attenuation versus compressional wave velocity at reservoir pressures differentiates high permeability formula formula brine saturated sandstones from low permeability formula formula sandstones and shales

precambrian to jurassic rocks of arabian gulf and adjacent areas their facies depositional setting and hydrocarbon habitat the first sediments to onlap the metamorphosed precambrian arabian shield were infracambrian proterozoic to middle cambrian carbonates clastics and evaporites the oldest arabian reservoir rocks occur in the precambrian to lower paleozoic ara salt of the huqf group which forms the birba field of oman the middle cambrian sequence was followed by late cambrian through early permian marine sandstones and continental to littoral siltstones and variegated shales the first commercial oil discovered in the arabian gulf region occurs in fluvial sands of the ordovician to permian haima and haushi groups of the marmul field in south oman these strata are also productive in other fields and are sealed by unconformable contact with the al khlata formation or beneath shale of he albian nahr umr formation the deeply buried kerogen sediments of the huqf group to the southeast are believed to be the source rocks for these fields of south oman the late permian to triassic deposits of the arabian peninsula are mainly widespread carbonates and evaporites that were deposited during a period of relative tectonic stability their deposition on an epeiric shelf was punctuated by a series of transgressions and regressions significant gas reserves have been proven in deep wells in the arabian gulf these wells penetrate large deep structures in the permian khuff shelf carbonates these carbonates have developed secondary porosity and lie beneath interbedded shale and dolomites of the sudair or suwei formation the source of gas in the khuff is unknown but could lie in more deeply buried formations the large deep structures of the khuff are considered to be among the most attractive for gas potential in the region today the stable shelf depositional environment established during the permian and triassic continued through the mesozoic jurassic rock units of the arabian gulf formed a sedimentary megacycle of epeiric shelf carbonates these include relatively deep water mudstones and wackestones to shallow water grainstones and packstones with the greatest facies variations occurring in the post dhruma to arab d rocks some of the best and most extensive reservoirs of the area occur in the primary and secondary porosity of the upper jurassic arab formation and in the middle jurassic araej and dhruma formations of saudi arabia qatar and abu dhabi the sources for hydrocarbons in these jurassic sediments are thought to be in the dhruma and lower hanifa formation diyab equivalent of the gulf area n general the jubailah is a source in qatar and some of the offshore gulf the most common seals are the argillaceous carbonate at the top of the dhruma and the hith anhydrite the latter also marking the termination of jurassic deposition across the arabian platform the upper jurassic cycles of marine limestones and anhydrites of the arab and hith formations accumulated in response to changes of sea level caused either by regional epeirogenic movements or by eustasy

methanogenic pathways of coal bed gas in the powder river basin united states the geologic factor abstract coal bed gas of the tertiary fort union and wasatch formations in the powder river basin in wyoming and montana u s was interpreted as microbial in origin by previous studies based on limited data on the gas and water composition and isotopes associated with the coal beds to fully evaluate the microbial origin of the gas and mechanisms of methane generation additional data for 165 gas and water samples from 7 different coal bed methane bearing coal bed reservoirs were collected basinwide and correlated to the coal geology and stratigraphy the c1 c2 c3 ratio and vitrinite reflectance of coal and organic shale permitted differentiation between microbial gas and transitional thermogenic gas in the central part of the basin analyses of methane 13c and d carbon dioxide 13c and water d values indicate gas was generated primarily from microbial co2 reduction but with significant gas generated by microbial methyl type fermentation aceticlastic in some areas of the basin microbial co2 reduction occurs basinwide but is generally dominant in paleocene fort union formation coals in the central part of the basin whereas microbial methyl type fermentation is common along the northwest and east margins isotopically light methane 13c is distributed along the basin margins where d is also depleted indicating that both co2 reduction and methyl type fermentation pathways played major roles in gas generation but gas from the latter pathway overprinted gas from the former pathway more specifically along the northwest basin margin gas generation by methyl type fermentation may have been stimulated by late stage infiltration of groundwater recharge from clinker areas which flowed through highly fractured and faulted coal aquifers also groundwater recharge controlled a change in gas composition in the shallow eocene wasatch formation with the increase of nitrogen and decrease of methane composition of the coal bed gas other geologic factors such as burial thermal and maturation history lateral and vertical continuity and coalification of the coal beds also played a significant role in controlling methanogenic pathways and provided new perspectives on gas evolution and emplacement the early stage gas produced by co2 reduction has mixed with transitional thermogenic gas in the deeper central parts of the powder river basin to form old gas whereas along the basin margins the overprint of gas from methyl type fermentation represents new gas thus a clear understanding of these geologic factors is necessary to relate the microbiological biogeochemical and hydrological processes involved in the generation of coal bed gas

seismic and ultrasonic velocities in permafrost we calculate the compressional and shear wave velocities of permafrost as a function of unfrozen water content and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids solid and ice matrices and a liquid unfrozen water the compressional velocity for unconsolidated sediments obtained with this theory is close to the velocity computed with wood s model since biot s theory involves a wood averaging of the moduli of the single constituents moreover the model gives lower velocities than the well known slowness averaging theory wyllie s equation for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c computing the average bulk moduli by slowness averaging the ice and solid phases and wood averaging the intermediate moduli with the liquid phase yields a fairly good fit of the experimental data the proportion of unfrozen water and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity at the whole range of temperatures provided that the average pore radius and its standard deviation are known