alignment of near surface inclusions and appropriate crack geometries for geothermal hot dry rock experiments ubiquitous splitting of seismic shear waves indicates that most rocks in the upper half of the crust are pervaded by stress aligned fluid filled inclusions called eda cracks these inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures at depths where the overburden stress is sufficiently large typically below a few hundred metres this minimum stress is usually horizontal so that the eda cracks and hydraulic fractures are typically aligned vertically striking parallel or subparallel to the direction of maximum compression this is confirmed by the polarizations of the split shear waves along raypaths at depth in the crust at the free surface however the vertical stress is zero or approximately zero and cracks and hydraulic fractures at shallow depths in intact rock tend to be horizontal thus the directions of minimum stress and the orientations of hydraulic fractures are likely to swing through 90 near the surface of the earth since the behaviour of cracks and stress is often crucial to drilling operations the rotation of the crack and stress geometry near surface has important implications particularly for optimizing hydrocarbon production and geothermal reservoir management consequently evidence gained from experiments for example in hot dry rock geothermal heat extraction in inappropriate crack geometries at shallow depths may not be valid when applied to other crack and stress geometries at depth in hot rock

sources of aeromagnetic anomalies over cement oil field oklahoma simpson oil field alaska and the wyoming idaho utah thrust belt geochemical and rock magnetic studies undertaken to determine the cause of magnetic anomalies over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the wyoming idaho utah thrust belt have revealed different magnetic sources developed under different sedimentologic geochemical and structural settings at cement ferrimagnetic pyrrhotite fe7s8 typically intergrown with more abundant nonmagnetic pyrite fes2 formed as a result of hydrocarbon seepage sulfur isotopic data indicate that sulfur in the fe s minerals was probably derived from two different sources 1 isotopically heavy thermochemical h2s in petroleum and 2 isotopically light h2s generated by sulfate reducing bacteria that derived metabolic energy from leaking hydrocarbons or organic compounds derived from hydrocarbons although pyrrhotite may make a minor contribution to the reported total magnetic field anomalies at cement the measured anomalies are probably mostly caused by man mad

four dimensional seismology and true direct detection of hydrocarbons the petrophysical basis figure 1 published by m king hubbert in 1962 and again in 1969 shows the exploitation of fossil fuels in a historical perspective according to this graph about half of the fossil energy on earth is being used up in a period of about 200 years half of which is by now over

complex component analysis of shear wave splitting case studies summary complex component analysis has been applied to four shear wave data sets a four component single offset vsp in the lost hills kern county a multi offset single source two component vsp in the paris basin a four component reflection line in the lost hills and an in seam crosshole survey at german creek mine australia in all cases the polarization of the faster split shear wave or channel wave can be assessed from the colour coded record sections of the seismic attributes in particular the source independent coherent polarization on the colour sections of the complex components of in line and cross line sources allows the shear wave polarization angle to be determined without need for rotation of the instrument and source axes we conclude that complex component analysis can aid identification and estimation of shear wave splitting from seismic sections help stratigraphic interpretation and simplify the processing sequence of multicomponent reflection data in the presence of anisotropy

the range of effects of azimuthal isotropy and eda anisotropy in sedimentary basins summary combinations of bedding or lithology induced azimuthal isotropy with an axis of symmetry perpendicular to the bedding plane and crack induced extensivedilatancy anisotropy eda with a horizontal axis of symmetry are believed to be common in sedimentary basins and cause the widely observed phenomenon of shear wave splitting combinations of two such transversely isotropic forms of anisotropy with orthogonal axes of cylindrical symmetry lead to orthorhombic symmetry this has two major effects 1 the polarizations of the faster split shear waves may no longer be parallel to the strike of the cracks or fractures even for near vertical propagation and 2 such orthorhombic symmetry systems necessarily have a number of directions called shear wave point singularities where shear waves display disturbed or anomalous behaviour again possibly in near vertical directions unless these effects are correctly identified they could be interpreted mistakenly for the effects of structural irregularities or discontinuities in contrast recognition of the 3 d geometry of this behaviour places comparatively tight constraints on possible combinations of anisotropy in the rockmass in order to give some understanding of the geometry of these phenomena this paper presents 3 d patterns of the behaviour of shear wave splitting that have been computed for a range of combinations of crack and bedding induced anisotropy

pyritization processes and greigite formation in the advancing sulfidization front in the upper pleistocene sediments of the black sea pyritization in late pleistocene sediments of the black sea is driven by sulfide formed during anaerobic methane oxidation a sulfidization front is formed by the opposing gradients of sulfide and dissolved iron the sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content two processes of diffusion limited pyrite formation were identified the first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of fesn n 1 10 1 29 including greigite elemental sulfur and polysulfides formed from h2s by a reductive dissolution of fe iii containing minerals serve as intermediates to convert iron sulfides into pyrite in the second process a direct pyrite precipitation occurs through prolonged exposure of iron containing minerals to dissolved sulfide methane driven sulfate reduction at depth causes a progressive formation of pyrite with a 34s of up to 15 0 the s isotopic composition of fes2 evolves due to contributions of different sulfur pools formed at different times steady state model calculations for the advancement of the sulfidization front showed that the process started at the pleistocene holocene transition between 6360 and 11 600 yr bp our study highlights the importance of anaerobic methane oxidation in generating and maintaining s enriched layers in marine sediments and has paleoenvironmental implications

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

seismic versus sonic velocities a vertical seismic profiling study vertical seismic profiling vsp techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole the analysis of a vsp survey can also provide insight into seismic wave propagation especially when related to sonic measurements but vsp and sonic log velocities or traveltimes are often found to disagree recent field evidence of these differences suggests that the vsp traveltimes are delayed with respect to the integrated sonic times especially in the deep section 3000 ft by about 2 0 ms 1000 ft on the average the vsp has numerous applications in exploration geophysics such as calibrating the sonic log it is thus important to understand why the two measurements differ differences in the geometries source frequencies and instrumental errors of the two surveys are reviewed more detailed analysis of seismic wave propagation in the vsp shows that short path multiples and velocity dispersion can have a significant delaying effect on the seismic tr

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

fractured reservoir delineation using multicomponent seismic data the characteristic seismic response to an aligned fracture system is shear wave splitting where the polarizations time delays and amplitudes of the split shear waves are related to the orientation and intensity of the fracture system this offers the possibility of delineating fractured reservoirs and optimizing the development of the reservoirs using shear wave data however such applications require carefully controlled amplitude processing to recover properly and preserve the reflections from the target zone here an approach to this problem is suggested and is illustrated with field data the proposed amplitude processing sequence contains a combination of conventional and specific shear wave processing procedures assuming a four component recording the split shear waves can be simulated by an effective eigensystem and a linear transform technique can be used to separate the recorded vector wavefield into two principal scalar wavefields representing the fast and slow split shear waves conventional scalar processing methods designed for processing p waves including noise reduction and stacking procedures may be adapted to process the separated scalar wavefields an overburden operator is then derived from and applied to the post stacked scalar wavefields a four component seismic survey with three horizontal wells drilled nearby was selected to illustrate the processing sequence the field data show that vectormore wavefield decomposition and overburden correction are essential for recovering the reflection amplitude information in the target zone the variations in oil production in the three horizontal wells can be correlated with the variations in shear wave time delays and amplitudes and with the variations in the azimuth angle between the horizontal well and the shear wave polarization dim spots in amplitude variations can be correlated with local fracture swarms encountered by the horizontal wells less

a stress forecast earthquake with hindsight where migration of source earthquakes causes anomalies in shear wave polarisations abstract the stress aligned shear wave splitting observed throughout most of the crust is a three dimensional function of the effective elastic anisotropy of distributions of fluid saturated grain boundary cracks and preferentially oriented pores fluid saturated microcracks are highly compliant and crack geometry responds readily to changes in stress monitoring with shear wave splitting allows the accumulation of stress before earthquakes to be recognised and the time and magnitude of the impending earthquake to be stress forecast such increases in stress have been observed some 15 times with hindsight and one successful real time stress forecast here we report a further with hindsight stress forecast in northern iceland where increases of time delays implying stress accumulation are observed before a 4 9 earthquake at two seismic stations at epicentral distances of 50 km and 92 km there are however two anomalies changes in shear wave polarisations at the nearest station appear to be due to migration of shear wave source earthquakes in a uniform anisotropic symmetry into ray path directions with different anisotropic shear wave polarisations such directional changes in shear wave polarisations are occasionally observed above small earthquakes and may be misinterpreted as structural or temporal changes in anisotropy a further anomaly is that the station at 92 km displays variations in time delays but a station just 4 km farther away on the other side of a major fault appears to be in the stress shadow of the fault and does not display temporal variations despite having adequate source data

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

seismic vibrator control and the downgoing p wave while the need for phase compensation is well established the best method to measure the seismic vibrator output is not phase control of the force exerted by a seismic vibrator upon the earth s surface ground force is shown to be useful in producing consistent downhole p wave signatures experimental results are presented which compare the downhole correlation wavelets produced by phase locking to ground force reaction mass acceleration and baseplate acceleration as changes in vibrator type sweep bandwidth drive level and coupling medium are made the empirical results support earlier theoretical work which predicts with suitable assumptions that ground force and far field particle displacement are in phase except for a time delay

the potential of shear wave vsps for monitoring recovery a letter to management much of the ultimate success or failure of hydrocarbon recovery depends on the assessment usually implicit of the internal crack pore and stress geometry and pore fluids of the reservoir rock it also depends on the estimation again usually implicit of the changes to these quantities as the secondary and tertiary production or enhanced oil recovery eor proceeds at present we are able to recover only a small proportion of the oil in a reservoir amos nur wrote in tle september 1989 the most obvious way to significantly improve recovery is to obtain much better descriptions of the internal structure recent advances in seismology reported at the seg research workshop on recording and processing vector wave field data snowbird utah august 1989 have shown that many details of this internal structure may be monitored in situ by appropriate shear wave three component vsps

laboratory results for the features of body wave propagation in a transversely isotropic media much of the earth s crust appears to have some degree of elastic anisotropy crampin 1981 crampin and lovell 1991 helbig 1993 the phenomena of elastic wave propagation in anisotropic media are more complex than those in isotropic media shear wave propagation in an orthorhombic physical model is most complex when the direction of the wave is close to the neighborhood of the cusp on the group velocity surfaces brown et al 1991 the first identification of singularities in wave propagation through sedimentary basins occurred in the examination of shear wave splitting in multioffset vertical seismic profiles vsps at a borehole site in the paris basin bush and crampin 1991 where large variations in shear wave polarizations in propagation directions close to point singularities were observed computation of synthetic seismograms for layer sequences showed that the shear wave polarizations and amplitudes were irregular near point singularities crampin 1991

estimating the internal structure of reservoirs with shear wave vsps in the past 10 years vertical seismic profiles vsps have played an increasingly important role in identifying and delineating in situ layers such vsps have usually recorded p wave data three component shear wave trains carry three or four times more information about the source and structure of the rocks along the raypath than the equivalent p wave train so in principle shear wave vsps should be a particularly valuable technique for examining in situ layers

kara sea freshwater dispersion and export in the late 1990s 1 a regional coupled ice ocean model for the kara sea forced with boundary conditions from a large scale north atlantic arctic ocean model is used to study dispersion and export of freshwater from ob and yenisei rivers toward the arctic ocean and the laptev sea for the period 1996 2001 the years 1998 and 1999 were characterized by a strong positive sea level pressure slp anomaly in the siberian part of the arctic ocean owing to prevailing northeasterly winds the slp anomaly caused a blocking situation which suppressed the otherwise eastward freshwater export to the arctic ocean and laptev sea this reversal of the prevailing circulation scheme led to a dramatic freshening of the kara sea through accumulation of low saline river water in the central and western parts additionally the kara strait inflow from the barents sea which presents the main source for saline atlantic derived water was reduced and partly even reversed the suppressed freshwater export during winter 1998 1999 recovered in the following winter 1999 2000 when a significant pulse of low saline surface water left toward the laptev sea the variability of the river discharge plays a minor role for the investigated period because the interannual variability of runoff rates is generally too low to explain the observed hydrographic changes the results underline the importance of local shelf sea processes for the variability of the freshwater export from the arctic shelves to the central arctic ocean

suggestions for a consistent terminology for seismic anisotropy seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand i suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings it is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non specialist this not a manual of anisotropy and it is not intended for theoreticians it is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids

effects of the internal shear wave window comparison with anisotropy induced splitting in the absence of anisotropy the observed polarizations of shear waves should be those radiated from the source modified only by interaction with internal interfaces and free surface topography it has been suggested that shear wave polarizations may be systematically distorted by internal interfaces which could lead to difficulties in interpreting anisotropy induced shear wave splitting since the literature now reports many investigations of shear wave splitting possible complications caused by internal interfaces would be important and need to be assessed in this paper we investigate the polarization of shear waves in full wave synthetic seismograms incident from a point source at an isotropic to isotropic interface over a range of angles of incidence particle motion due to plane and curved wave fronts has different characteristics internal shear wave windows can be defined analogous to the shear wave window at the free surface within the innermost window the polarizations at subsurface geophones as in vertical seismic profiles are recorded with little distortion but outside this window the waveforms become increasingly distorted for larger angles of incidence this is similar to the behavior of shear waves with respect to the shear wave window at the free surface except that the behavior at internal interfaces is more complicated because of additional critical angles and additional windows we conclude that the interaction of shear waves with internal interfaces is only a serious difficulty when interpretating wide offset vertical seismic profiles wide angle reflections and cross hole data

p wave traveltime and polarization tomography of vsp data summary the presence of anisotropy requires that tomographic methods be generalized to account for anisotropy this generalization allows geological structure to be correctly imaged and allows the anisotropic parameters to be estimated use of isotropic inversion for imaging anisotropic structures gives systematic trends in the traveltime and polarization residuals however due to the limited directional coverage the traveltimes along may not be sufficient to study the anisotropic properties of the structure polarizations can provide independent information on the structure traveltime and polarization inversion are applied to synthetic examples simulating vsp experiments transverse isotropy and 1 d structure are assumed plots of traveltime and polarization residuals are an important tool to detect the anomalies due to the presence of anisotropy for receivers located in anisotropic layers polarization residuals display consistent anomalies of several degrees the synthetic examples show that even the simple 1 d problem is difficult when using direct arrivals only large a posteriori errors in anisotropic parameters are obtained by traveltime inversion in layers where available incidence angles are less than 45 resolution of the tomographic image of vsp data is greatly improved by a combination of traveltime and polarization information in order to obtain accurate inversion results the measurement error of polarization data should be kept to within a few degrees

recent hydrocarbon alteration sulfate reduction and formation of elemental sulfur and metal sulfides in salt dome cap rock abstract geochemical analysis of hydrocarbons and scanning electron microscopy sem provide insight to recent alteration of crude oil microbial sulfate reduction and mineral deposition in carbonate cap rock at damon mound brazoria county texas u s a previous work showed that intense aerobic biodegradation of hydrocarbons occurs in shallow cap rock where s 0 and metal sulfides are not abundant the present study focuses on some deeper carbonate cap rock characterized by more limited oxygen availability the c 1 c 4 hydrocarbons in deeper cap rock are a biodegraded residue of hydrocarbons derived from crude oil alteration of higher molecular weight crude oil components is best explained by the effects of water washing accompanied by some biodegradation sem of cavities in carbonate cap rock shows microbes in association with globular masses of solid crude oil and sulfate minerals late forming minerals in cavities include s 0 pyrrhotite pyrite sphalerite and aragonite the presence of s 0 and sulfide minerals indicates that microbial sulfate reduction is associated with limited hydrocarbon biodegradation in this deeper cap rock environment similar processes could explain the origin of some s 0 and metal sulfides at damon mound in the geologic past

inversion for subsurface anisotropy using estimates of shear wave splitting summary inversion of the parameters characterizing shear wave splitting polarizations time delays and differential attenuation provides a way of obtaining a good starting model for full wave anisotropic modelling and improves the convergence to a final waveform match this is necessary as the shear waves behave in a highly non linear fashion with respect to anisotropy here we discuss the extent to which polarization and time delay information estimated from split shear waveforms may supply details of this starting model 3 d patterns of these parameters for different anisotropic models are considered to investigate the potential of the inversion scheme guidelines are formed for the best arrangements of sources and geophones to provide optimum resolution of an anisotropic model the study indicates that in most cases different anisotropic models cannot be adequately distinguished using vertical seismic profiles vsps reflection profiles and cross hole surveys in which separate polarization or time delay measurements at one azimuth are made better results are obtained by combining polarization and time delays estimates preferably from more than one azimuth the type of model parameters which can be resolved depends crucially upon the geometry of the acquisition system and to a lesser extent on the background velocity structure individual model parameters are more difficult to extract but some details appear in principle to be resolvable for certain types of survey vsps are most suitable to monitor crack strike and density of parallel vertical aligned cracks and the dip of the fine layering contributing to layer induced anisotropy cross hole experiments are best for measuring the dip of the cracks the resolution of the crack content and aspect ratio are determined principally by the position of line singularities and either an offset vsp or an adapted cross hole survey may be adequate for resolving these properties if the line is shot at an appropriate azimuth with respect to the strike of the cracks the ratio of crack induced anisotropy to matrix anisotropy in sedimentary basins can be evaluated by measuring the positions of the point singularities in recordings of polarization and time delay patterns using a near offset vsp

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

algebraic processing techniques for estimating shear wave splitting in near offset vsp data theory1 a vector convolutional model for multicomponent data acquired in an anisotropic earth is used as a basis for developing algebraic solutions to interpret near offset vsp data this interpretation of the cumulative or interval medium response green s tensor for shear waves determines a polarization azimuth for the leading shear wave and the time delay between the fast and slow split waves the algebraic solutions effectively implement least squares eigenanalysis or singular value decomposition although the methodology for shear wave analysis is strictly relevant to a transmission response it can be adapted to surface data for a uniform anisotropic overburden the techniques perform well when calibrated and tested using synthetic seismograms from various anisotropic models noise tests demonstrate the sensitivity of the interval measurements to local interferences particularly if the shear waves are generated by one source although the algorithms are faster than numerical search routines this is not seen as their major advantage the solutions may have potential in near real time interpretation of shear wave data in well logging where they may be coded on a microchip to provide a direct stream of separated shear waves or polarization and birefringence information there may also be some benefit for large prestack multicomponent surface data sets where the solutions provide a direct transformation to the split shear wave components reducing the storage space for further processing

inverting shear wave polarizations for anisotropy using three component offset vsps synthetic seismograms summary a numerical algorithm is presented for interpreting shear wave polarization directions from offset vsp data as subsurface anisotropy the technique compares observed synthetic horizontal polarization angles with expected values for a range of physically plausible anisotropic models with different orientations the comparison is implemented in a database inversion scheme the anisotropic models are formed by combining the effective anisotropy of parallel aligned vertical cracks with matrix anisotropy due to thin layering or lithology different models are defined by varying the crack and matrix properties the technique is applied to synthetic offset vsp data for a multilayered anisotropic structure generated by multi azimuth sources analysis of the results indicates that polarization measurements from non zero offsets with at least two and preferably three different azimuths are required in order to reasonably constrain the final inversion solutions the parameters which are resolved depend upon the depth of the subsurface zone of interest crack strike is well resolved in the deeper layers for which directions of propagation are sub vertical whereas other model parameters such as aspect ratio and crack content can only be resolved for wider ranges of incidence angle the key to a successful inversion appears to lie in the selection of propagation paths through the relatively unique patterns of disruption created by singularities

remobilization and mineralization of selenium tellurium in metamorphosed red beds evidence from the munster basin ireland abstract selenium and tellurium are vital elements for the ongoing development of carbon free energy generation but are only recovered in small volumes as by products of base and precious metal processing security of supply of these commodities is of high importance as their demand is forecast to increase in the near future thus more research is required on the varying mineralization systems where their occurrence and behavior are poorly understood such as sedimentary basins and metasedimentary terranes the munster basin of southern ireland is a good study area for sedimentary se te in such environments as it has a regionally high soil selenium anomaly and is characterized by devonian terrestrial red beds which were metamorphosed during the hercynian variscan orogeny the area also contains ore grade biogenic sediment hosted copper and remobilized vein hosted copper sulfides which are prospective for chalcophile selenium and tellurium quartz vein hosted se te mineralization was identified across the munster basin accompanied by trace gold silver mercury accessory phases new sulfur isotope data reinforces the model of sulfur and selenium remobilized from a biogenic red bed source which reflects a wider association of selenium deposits with oxidizing environments the co occurrence of tellurium mineralization suggests that it too may be concentrated in oxidizing environments associated traces of gold add to a growing body of evidence that gold is also transported and precipitated in oxidizing conditions in continental basins

shaker tests on downhole seismic tools shaking a downhole seismic tool in situ is a powerful test procedure able to detect abnormal acoustical behavior it consists of feeding a swept voltage to a special geophone located inside the tool close to the triaxial measurement geophones the special geophone creates a swept force which excites the tool formation system the resulting motion of the tool is recorded through the triaxial geophones in this paper a simple model of the tool formation system excited by the shaker is given predictions from this model compare nicely with results from a field experiment conducted with a very compact seismic tool another field experiment conducted with a longer tool shows that tool intrinsic modes are clearly visible in the shaker data their frequencies and amplitudes depend upon the geometry of the contact between the tool and the formation a comparison between shaker data and impulsive shot data shows that moderate modal vibrations do not significantly deteriorate the quality of vsp data but that

remagnetization of the rush springs formation cement oklahoma implications for dating hydrocarbon migration and aeromagnetic exploration the permian rush springs formation above the cement anticline in oklahoma contains a late permian early triassic chemical remanent magnetization crm that is interpreted to reside in authigenic magnetite the crm is found in bleached carbonate cemented sandstones that were altered by hydrocarbons and contain authigenic magnetite the magnetite presumably precipitated in the late permian early triassic as a result of chemical conditions created by hydrocarbons or associated fluids that migrated from underlying reservoir units red sandstones around cement that were not altered by hydrocarbons contain a permian crm that resides in hematite the red and bleached sandstones have similar magnetization intensities and susceptibilities this raises questions about the use of aeromagnetic surveys in hydrocarbon exploration

interpreting data matrix asymmetry in near offset shear wave vsp data poor experimental control in shear wave vsps may contribute to unreliable estimates of shear wave splitting and possible misinterpretation of the medium anisotropy to avoid this the acquisition and processing of multicomponent shear wave data needs special care and attention measurement of asymmetry in the recorded data matrix using singular value decomposition svd provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear wave polarization azimuth three examples demonstrate how variations in shear wave polarization and acquisition inaccuracies affect the svd results in different ways in the first example analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected in the second example a known source re orientation and polarity reversal is detected by applying svd to near offset shear wave vsp data recorded in the romashkino fiel

degree of supersaturation and precipitation of organogenic dolomite this paper integrates the degree of carbonate saturation into a general model for the origin of dolomite in organic rich marine sediments degradation of organic matter by sulfate reducing bacteria promotes early dolomite precipitation by simultaneously increasing the carbonate alkalinity and lowering the sulfate ion concentration to near zero the interstitial water in organic rich sediments of several deep sea drilling project sites where dolomite is believed to be precipitating is up to 10 sup 3 times supersaturated with respect to near ideal dolomite and up to 10 times supersaturated with respect to calcite the highly supersaturated interstitial waters appear to overcome low temperature kinetic barriers to precipitate a nonideal fine grained calcian dolomite the degree of supersaturation with respect to dolomite decreases and the interstitial water approaches carbonate equilibrium with increasing burial depth at several of these sites removal of sulfate ion is important because sulfate ion may inhibit dolomite precipitation a calcareous sediment is not required but the presence of calcite or aragonite appears to greatly enhance dolomitization by providing an additional source of ca and carbonate ions

effects of point singularities on shear wave propagation in sedimentary basins summary in most directions of propagation in anisotropic solids seismic shear waves split in regular and predictable ways that in principle can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass in all anisotropic solids however there are directions of propagation known as shear wave singularities where the split shear waves have the same phase velocities for directions of propagation near the commonest type of singularity the point singularity the relationship between the phase and group velocities may undergo rapid variations for small changes in direction this results in shear waves along rays propagating at the grozq velocity behaving anomalously with irregular polarizations and amplitude changes as if they were propagating near cusps although the degree of anisotropy may be too small to cause conventional cusps on the group velocity wave surfaces the effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well established phenomenon of azimuthal isotropy transverse isotropy with a vertical axis of symmetry caused by horizontal lithology or by fine layering ptl anisotropy combining with the more recently recognized azimuthal anisotropy caused by distributions of near parallel near vertical fluid filled inclusions eda anisotropy this paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins if not identified correctly the effects of such point singularities could be mistakenly attributed to structural irregularities and if correctly identified the directions of such singularities can place tight constraints on possible combinations of ptl and eda anisotropy in sedimentary basins

relations among hydrocarbon reservoirs epigenetic sulfidization and rock magnetization examples from the south texas coastal plain this paper focuses on the association between concentrations of iron disulfide fes2 minerals in the shallow subsurface and underlying hydrocarbon accumulations such fes2 concentrations are the result of migration of either h2s or organic constituents from the underlying hydrocarbons the h2s from reservoirs is produced inorganically from sulfate in the reservoir rocks at high temperature 90 c and migrates to shallower beds to react inorganically with iron to form fes2 organic constituents from reservoirs in contrast provide nourishment for sulfate reducing bacteria in shallow relatively cool 90 c beds sandstone in the ray point uranium district in live oak county texas contains abundant fes2 which formed both from deep seated h2s and from h2s produced in the shallow subsurface by bacteria that utilized organic materials from depth deep petroleum reservoirs were physically connected to near surface 100 m beds containing epigenetic fes2 by the oakville fault epigenetic iron sulfide format

paris basin vsps case history establishing combinations of fine layer or lithologic anisotropy and crack anisotropy from modelling shear wavefields near point singularities summary this paper examines shear wave splitting in multi offset vsps at a borehole site in the paris basin and demonstrates that the sedimentary structure must have at least orthorhombic anisotropic symmetry it is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine layer or lithologic anisotropy and crack anisotropy transition zones close to shear wave singularities where there are rapid changes of shear wave polarizations and delays can account for the anomalous observed waveforms observations of large variations in shear wave polarizations in propagation directions close to point singularities are probably the first positive identification of the effects of singularities in sedimentary basins

magnetic forward models of cement oil field oklahoma based on rock magnetic geochemical and petrologic constraints magnetic forward models of the cement oil field oklahoma were generated to assess the possibility that ferrimagnetic pyrrhotite related to hydrocarbon seepage in the upper 1 km of permian strata contributes to aeromagnetic anomalies at cement six bodies having different magnetizations were constructed for the magnetic models based on geology and on petrologic and geochemical results supplemented by rock magnetic measurements of shallow core and outcrop samples the column of rock through which hydrocarbons have passed is divided into three sulfide zones on the basis of pyrrhotite content and the column is capped by a 30 m thick zone that contains ferric oxide minerals formed mainly from oxidized pyrite red beds unaffected by sulfidization as well as a zone of rock depleted in hematite but lacking sulfide surround sulfidic zones the synthetic magnetic profiles are controlled mainly by pyrrhotite bearing strata at depths of 200 500 m the magnetizations of these bodies are calculated from 1 pet

crack distributions which account for a given seismic anisotropy summary the effect of microfractures or small scale inclusions within earth structures may be measured seismically wavespeeds as a function of wave normal direction lead to effective or mean elastic parameters and the difference between these and the values for unfractured material is related to parameters for the size distribution and internal conditions of the cracks or inclusions thus it appears that measurement of the seismic wavespeeds for a sequence of different propagation directions can lead to information about fractures or inclusions within the rock even though the scale size of this microstructure may be very small compared with a wavelength however the crack parameters are continuous functions of orientation and the elastic parameters for the material provide values for only 21 independent quantities this is enough to describe the crack distribution in terms of spherical surface harmonics up to fourth order only the relationship between the effective elastic parameters and t he crack distribution is accurate to second order in the number density which limits the applications to materials where the contribution of microfractures is less than 10 per cent the relationships referred to above show that inherent material anisotropy is equivalent to a distribution of hypothetical cracks embedded in an isotropic material it then follows that the effect of microfractures in an anisotropic solid may be calculated by imposing both systems of cracks real and hypothetical on an isotropic matrix for which solutions are known

sulfidization and magnetization above hydrocarbon reservoirs geochemical and rock magnetic studies of strata over cement oil field anadarko basin oklahoma simpson oil field north slope basin alaska and the edwards deep gas trend south texas coastal plain document changes in original magnetizations caused by postdepositional iron sulfide minerals that are or may be related to hydrocarbon seepage at cement ferrimagnetic pyrrhotite fe sub 7 s sub 8 formed with pyrite and marcasite in permian red beds the fe s minerals contain isotopically heavy abiogenic sulfur derived from thermal degradation of petroleum and or isotopically light sulfur derived from sulfate reducing bacteria fed by leaking hydrocarbons at simpson ferrimagnetic greigite fe sub 3 s sub 4 dominates magnetizations in upper cretaceous nonmarine beds that contain biodegraded oil sulfur isotopic data are consistent with but do not prove a genetic link between the greigite delta sup 34 s gt 20 per mil and seepage in middle tertiary sandstones of southeast texas pyrite and marcasite formed when abiogenic h sub 2 s migrated upward from deep reservoirs or when h sub 2 s was produced at shallow depths by bacteria that utilized organic material dissolved in migrating water from depth the sulfide minerals replaced detrital magnetite to result in a systematic decrease in magnetic susceptibility toward faults that connect deep petroleum reservoirs tomore shallow sandstone the authors results show that abiologic and biologic mechanisms can generate magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage the magnetizations in such zones are diminished most commonly by replacement of detrital magnetic minerals with nonmagnetic sulfide minerals or are unchanged if such detrital minerals were originally absent less

behaviour of shear waves in rocks with two sets of parallel cracks summary the behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations both in earthquake and exploration seismology cracks in surface outcrops however frequently display multiple sets of parallel vertical intersecting cracks this paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks biplanar cracks to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks monoplanar cracks this study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal and unlikely to be easily recognized in the field we conclude that it is frequently not possible from analysis of surface observations of shear wave polarizations alone to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set the difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window however in areas with complex fracture and stress systems time delays may be much harder to estimate than the polarization angles of the leading split shear waves and it may not be easy to distinguish from seismic data alone the difference betwekn parallel and multiplanar sets of vertical fractures

velocity determination from offset vertical seismic profiling data two methods of computing interval velocities from offset vertical seismic profiles are developed based on a horizontally layered earth model the first method is a simple iteration procedure by which layer velocities are sequentially computed for any given layer the take off angle for the ray arriving at the geophone at the bottom of the layer is varied until the velocity computed from travel time differs from the velocity obtained from snell s law by less than some specified value this is similar to the shooting method in ray tracing this method requires as many layers as geophone locations the second method is more general and the only requirements are that the number of layers be equal to or smaller than the number of geophone locations and that the rays arrive at the geophones from above this method is based on the iterative solution of a linearized problem obtained by expanding travel times and horizontal distances in taylor series as functions of velocities and p parameters up to first order in dv s and dp s perturbations in velocities and p parameters respectively and assuming initial estimates vo s and po s close to the optimum solution the resulting linear system is solved using a simplified version of the levenberg marquardt algorithm due to the simple structure of the ensuing matrix of derivatives for each iteration the problem reduces to the solution of an inverse problem for the velocities alone plus the solution of a vector equation for the p parameters this method is fast and convergence is attained even in the case of poor initial estimates

extensive dilatancy anisotropy relative information in vsps and reflection surveys1 yardley g and crampin s 1990 extensive dilatancy anisotropy relative information in vsps and reflection surveys geophysical prospecting 39 337 355 shear waves have complicated interactions with the free surface particularly in the presence of low velocity surface layers topographic irregularities and the expected near surface crack and stress anomalies consequently it has been suggested that shear waves should be recorded subsurface in vertical seismic profiles vsps in order to extract accurate information about the in situ crack and stress geometry contained in shear wave splitting this paper compares the information in synthetic shear waves in reflection gathers and vsps in order to assess the relative merits of the two techniques for investigating shear wave splitting synthetic seismograms demonstrate that in the presence of even very simple surface layers shearwaves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth in contrast shear waves recorded in vsps are relatively unaffected by surface layers and near surface stress and crack anomalies and the behaviour of shear wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones matrix rotations of multicomponen t multisource shear wave reflection data to extract the information contained in the split shear waves are found to be directly meaningful only in situations where crack orientations do not change with depth

diagenetic alteration of permian strata at oil fields of south central oklahoma usa abstract many oil fields in south central oklahoma including cement velma carter knox and eola have a chimney of altered rock within slightly deformed permian red beds the chimneys occur above and within the oil reservoirs where the chimneys crop out the altered rock is commonly well exposed the alteration consists chiefly of removal of haematite from sand grains cementation of sandstone by pyrite marcasite calcite and dolomite and at the cement field replacement of a bed of gypsum by calcite the total volume of these late diagenetic minerals is vast many hundreds of millions of tons the diagenetic alteration of red beds occurred when crude oil and abundant natural gas migrated vertically from major anticlines in pennsylvanian and older strata into the gently folded gypsum bearing permian red beds during cementation the rush springs sandstone in the cement field had a high porosity 40 50 which suggests that here alteration occurred before substantial burial in the red beds anaerobic bacteria mediated the oxidation of ch 4 and possibly alkanes within crude oil to co 2 and simultaneously reduction of so 4 4 anions to h 2 s the cementation resulted when carbonate species reacted with ca 2 and mg 2 and the bleaching resulted when sulphide species reacted with fe 3 and fe 2 the evidence for the bacterially mediated alteration is the anomalously low isotopic values for carbon and sulphur carbonate minerals have 13 c values as low as 39 2 0 and most values are less than 15 0 o sulphide minerals have 32 s values as low as 37 9 and most values are less than 8 5 0

arctic warming evolution and spreading of the 1990s warm event in the nordic seas and the arctic ocean 1 observations in the arctic ocean revealed changes in oceanic temperature salinity and ice cover of the 1990s as compared with earlier data with a numerical model we favorably reproduce the development and subsequent propagation of temperature anomalies in water of atlantic origin in the 1980s and 1990s these propagated into the arctic ocean via the barents sea and the fram strait two warm anomalies entered the arctic ocean through these passages while the first smaller anomaly only warmed up the western eurasian basin the second large anomaly spread far into the eastern eurasian basin and across the lomonossov ridge into the western arctic basins intensified boundary currents during the high nao state in the first half of the 1990s significantly influenced the amplitude and speed of propagation of the temperature anomalies inside the arctic ocean in contrast to the notion of a continuous warming process during the 1990s our model results suggest the warming of the atlantic layer in the arctic ocean occurred in the form of events the event with the largest anomalous heat input during the modeled period entered the arctic between 1989 and 1994 it is possible to trace back the additional heat input into the arctic to an increased volume inflow via the faroer scotland passage and reduced heat loss to the atmosphere in the early 1990s after a weaker warm inflow in the second half of the 1990s the most recent observations and the model results point to a recurring warm anomaly in the inflow from 1999 onward

carbonate replacements after sulfate evaporites in the middle miocene of egypt abstract massive zones of diagenetic carbonate which replace sulfate deposits are observed in the middle miocene evaporites in several outcrops along the egyptian coast of the gulf of suez these carbonates display marked similarities of morphology lithofacies and stable isotope composition to diagenetic carbonates described in some portions of messinian gessoso solfifera formation of sicily and in the castiles of the upper permian castile formation of the delaware basin the mechanism of mineral transformation is basically bacterial sulfate reduction because the low 13c values of these carbonates argue for the organic origin of carbon the 18o values fall i to two distinct groupings indicating that the carbonates were crystallized during two different diagenetic stages the early diagenetic stage occurred soon after deposition in pore waters of marine origin and at different levels of evaporation organic matter used by bacteria was produced in situ the late diagenetic stage occurred when the tectonic or halokinetic uplift of the sequence caused flushing of the sediments by groundwaters of the phreatic aquifer and hydrocarbon migration furthermore to explain the very low 18o values of these carbonates it is inferred that locally ambient temperatures reached more than 35 c due to the large quantities of energy released during the bacterial sulfate reduction

modelling orthorhombic anisotropic effects for reservoir fracture characterization of a naturally fractured tight carbonate reservoir onshore texas usa in this study we present a step by step theoretical modelling approach using established seismic wave propagation theories in anisotropic media to generate unique anisotropic reflection patterns observed from three dimensional pure mode pressure 3d pp full azimuth and full offset seismic reflection data acquired over a naturally fractured tight carbonate field onshore texas usa our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns from the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shear wave splitting and variation in p wave velocity with offset and azimuth amplitude variation with azimuth avaz analysis also confirmed multi crack sets induced anisotropy which is characteristic of orthorhombic symmetry evident as multiple bright and dim amplitude azimuth directions as well as complete reversal of bright amplitude to dim amplitude azimuth direction as the angle of incidence increases from near 15 to mid 30 offsets finally we fitted the generated p wave velocity into an ellipse to determine the intensity and orientation n26e of the open crack set as well as the direction of the minimum in situ stress axis n116e within the reservoir the derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir which was on production decline despite reservoir studies that indicate un depleted reserves

fluid rock interactions in the salt dome environment an introduction and review abstract the salt dome environment in the gulf of mexico coast u s a provides one of the most diverse and long lived records of fluid rock interactions in an active sedimentary basin dominated by saline formation waters the geochemical record most of which is isotopic appears to span a temperature range between earth surface temperatures and lower greenschist facies most mineral forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters papers in this issue of chemical geology document some of the current research on fluid rock interactions in the salt dome environment these papers highlight the diversity of opinion about salt dome processes offering clues for further research the present paper reviews some of the key and more current literature on salt dome geochemistry fluid convection around salt domes and fluid rock interactions within salt and within the salt dome environment

giant calcite cemented concretions dakota formation central kansas usa giant spheroidal concretions cannonball concretions some nearly 6 m in diameter in fluvial channel fill sandstones at two localities of the dakota sandstone formed by import of cement constituents at a burial depth of 1 km during cannonball concretion growth a self organizational process restricted concretions to a relatively few but widely spaced and locally evenly spaced sites other forms of calcite cements at these localities are cement patches in the form of intergrown grape size concretions grapestone and locally pervasive cement an early episode of invasion by thermogenically generated h2s which reacted with iron oxides on detrital grains generated scattered pyrite crystals and decimetre scale spheroidal pyrite concretions intergranular volumes igv in the concretions range from 36 to 27 the absence of a trend in igv and of carbon and oxygen isotope ratios from cannonball centres to margins indicates that these concretions did not cement progressively outwards from the centre rather the modern spheres represent the spatial extent of nucleation sites that were not otherwise organized within that volume carbon and oxygen isotope values for concretion calcites plot along a swath between depleted values of 18c of 36 and 18o of 13 and enriched values of 4 and 6 respectively four groups of calcites are evident on the basis of trace element content and suggest that the calcite precipitated across a range of oxidation conditions that do not correlate strongly with the isotopic compositions although fluvial overbank sandstones have some pedogenic calcite the channel sandstones have at most a trace of pedogenic calcite and carbonate rock fragments so that the bulk of cement components were imported to the sandstones carbon and calcium sources for calcite cement include marine limestone carbonate shells and anhydrite in addition to hco derived from oxidized methane most likely derived from beds underlying or laterally in communication with dakota sandstones hco in ascending formation waters released during compaction mixed with meteoric water whose temperature and composition varied with time to generate the 7 range in 18ocalcite values measured

epigenetic zoning in surface and near surface rocks resulting from seepage induced redox gradients velma oil field oklahoma abstract abstract surface and near surface permian sandstone has been drastically altered over the productive part of the structurally complex velma oil field as a consequence of petroleum microseepage buried permian sandstone along the northwest southeast trending anticline is cemented with abundant pyrite ferroan calcite and ferroan dolomite at the surface along the anticlinal crest iron disulfide is scarce isotopically anomalous carbonate cemented sandstone is overlain by sandstone that is massively impregnated by hematite cement permian sandstone is normally reddish brown throughout southern oklahoma but along the anticlinal flank it has been bleached yellow and white owing to iron loss some units contain abundant solid bitumen the mineralogy in the vertical section over the anticline follows the calculated stability relations for iron sulfide iron carbonate and iron oxide along a gradient from strongly reducing conditions at depth to oxidizing conditions at the surface reducing conditions were provided by seeping hydrocarbons from subsurface reservoirs of this multizone giant field production depths range from 120 to 2 975 m the principal evidence that these alterations are seepage induced is provided by reports of oil seeps in the early literature by zones of solid bitumen cements and by c13 pdb values for carbonate cements that range from 7 8 to 36 7 ppt

the amplitude and phase response of a seismic vibrator the amplitude and phase response of a simple model is compared with the performance of a real vibrator working in the field the field results show a characteristic phase response which confirms that the real drive force applied to the baseplate and its load impedance is faithfully represented by the acceleration of the reaction mass it follows that all the parameters necessary to calculate the load impedance and the true power dissipated in the earth can be measured at the output of the vibrator it also follows that the current method of baseplate phase compensation should be reconsidered

an integrated multi azimuth vsp study for fracture characterization in the vicinity of a well we present the analysis of a multi azimuth vertical seismic profiling data set that has been acquired in a tight gas field with the objective of characterizing fracture distributions using seismic anisotropy we investigate different measurements of anisotropy which are shear wave splitting p wave traveltime anisotropy and azimuthal amplitude variation with offset we find that for our field case shear wave splitting is the most robust measure of azimuthal anisotropy which is clearly observed over two distinct intervals in the target we compare the results of the vertical seismic profiling analysis with other borehole data from the same well cross dipole sonic and formation microlmager data from the reservoir section suggest that no open fractures intersect the well or are present within half a metre of the borehole wall furthermore a detailed dispersion analysis of the sonic scanner data provides no indication of stress induced seismic anisotropy along the logged borehole section we therefore explain the azimuthal anisotropy measured in the vertical seismic profiling data with a model that contains discrete fracture corridors which do not intersect the well itself but lie within the vertical seismic profiling investigation radius we show that such a model can reproduce some basic characteristics of azimuthal anisotropy observed in the vertical seismic profiling data the model is also consistent with well test data that suggest the presence of a fracture corridor away from the well with this study we demonstrate the necessity of integrating different data types that investigate different scales of rock volume and can provide complementary information for understanding the characteristics of fracture networks in the subsurface

iron sulfide minerals at cement oil field oklahoma implications for magnetic detection of oil fields aeromagnetic anomalies at cement oil field anadarko basin oklahoma have been attributed to authigenic magnetite the following characteristics of the magnetite however indicate that it is contamination introduced by drilling 1 occurrence as sharp angular blades and as spheres commonly with metallographic textures typical of industrial alloys and with associated steel and wustite feo 2 presence only in well cuttings and absence from core and quarry samples and 3 lack of association with detrital framework grains or with authigenic carbonate and sulfide minerals ferrimagnetic pyrrhotite occurs in well cuttings cores and quarry samples at cement and is a possible natural source of the magnetic anomalies pyrrhotite which is intergrown with more abundant fes 2 minerals formed as a result of hydrocarbon seepage pyrrhotite is confined to beds above oil and gas reservoirs these beds which lack detrital organic matter contain higher mineral sulfide and lower mineral sulfate sulfur 1 7 and 0 1 wt respectively than do correlative beds off the field 0 2 and 1 1 wt respectively in the field isotopic values of sulfide s show a systematic decrease upward through the permian section from positive values maximum 12 per mil at 610 760 m depth to negative values 1 to 11 per mil between 32 and 230 m 26 to 30 per mil at the surface geochemical results together with time temperature data derived from burial curves limit the major sources of the sulfide in the fe s minerals to two possibilities isotopically heavy sulfide was generated either inorganically at temperatures 90 c in beds beneath permian beds or by bacterial sulfate reduction at temperatures

the uranium potential of diagenetically altered sandstones of the permian rush springs formation cement district southwest oklahoma reconnaissance and detailed geologic radiometric and geochemical investigations one previously unreported uranium occurrence was discovered during outcrop reconnaissance areas of uranium favorability in the subsurface were identified by the examination of gamma ray well logs and the interpretation of stratigraphic cross sections derived from electric log data an aureole of diagenetic alteration which affects the permian sedimentary sequence overlying the pre permian cement anticline exposed and subsurface diagenetically altered sandstones of the rush springs formation within this aureole constitute a favorable environment for the occurrence of shallow joint controlled sandstone type uranium deposits nonchannel controlled peneconcordant deposits and classic roll type deposits three subareas have been identified as especially favorable exploration targets within the overall favorable area modified journal abstract

burial diagenesis and thermochemical sulfate reduction smackover formation southeastern mississippi salt basin the smackover formation in the southeastern mississippi salt basin has been buried from 3 03 to 6 06 km and subjected to temperatures of 100 to 200c pre hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite these phases formed at temperatures of less than 100c the oxygen isotopic composition of calcite ranges from minus 5 to minus 8 4 pdb and carbon isotopic composition ranges from 3 5 to 6 0 pdb the sulfur isotopic composition of late anhydrites ranges from 18 1 to 19 1 cdt post hydrocarbon migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur and precipitation of post bitumen calcite cement these diagenetic phases formed at temperatures above 150c and are present only in the deep part of the basin the oxygen isotopic compositions of calcites that replace anhydrite range from minus 3 0 to minus 4 0 pdb and carbon isotopic compositions range from minus 2 0 to 4 2 pdb the oxygen isotopic compositions of post bitumen calcites range from minus 3 4 to minus 5 6 pdb and carbon isotopic compositions range from minus 1 6 to minus 16 3 pdb two samples of elemental sulfur have sulfur isotopic compositions of 15 8 and 16 1 cdt the high temperature replacement of anhydrite by calcite more the presence of post bitumen calcite cement the sulfur isotopic composition of sulfates and elemental sulfur and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the mississippi salt basin at temperatures as low as 150c less

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

vertical seismic profiling vertical seismic profiling is one of the rapidly developing areas of geophysical technology for exploring in mature basins the measurement basically involves recording the total upgoing and downgoing seismic wave fields propagating through a stratigraphic section by means of geophones clamped to the wall of a drilled well

the effect of the aspect ratio on crack induced anisotropy1 media containing aligned cracks show anisotropy with respect to elastic wave propagation there are several models describing the wave propagation in cracked media most of them only valid for cracks with small aspect ratios one of these models crampin s model is compared with a model valid for all aspect ratios nishizawa s model the elastic constants and the group velocities are compared for both dry and liquid filled inclusions with aspect ratios ranging from 0 0001 flat cracks up to 1 spheres the difference between both models is small for small aspect ratios but becomes larger for increasing aspect ratios at a crack density of 0 05 both models give within an error of 5 the same results for aspect ratios up to 0 3 therefore crampin s model can be applied to a large range of cracked media even if the aspect ratio of the inclusions is not small the variation of the anisotropy as a function of the aspect ratio can be studied using thomsen s dimensionless parameters e and y they show how inclusions with large aspect ratios result in elliptical anisotropy