estimating scattering attenuation from vugs or karsts cavities in carbonate rock such as vugs or karsts cause scattering attenuation in passing waves the degree of such attenuation is often difficult to quantify because the exact scale and structure of the cavities is unknown taking the exact vug structure from x ray computerized tomography scans of two carbonate cores we use 3d finite difference modeling to determine the p wave scattering attenuation in these cores at ultrasonic frequencies the predicted scattering attenuation is quite high with q of 15 and 8 near the source frequency of 250 khz in spite of the sharp contrast in medium properties between cavity and rock and the violation of the small perturbation assumption the computed scattering attenuation is roughly comparable to that predicted by various random medium scattering theories this suggests that if the volume fraction and typical size of vugs or karsts can be estimated then a rough estimate of frequency dependent scattering attenuation can be made

a model for seismic velocity and attenuation in petroleum source rocks petroleum source rock is modeled as a viscoelastic transversely isotropic medium composed of illite smectite and organic matter the wave velocities and attenuation of petroleum source rocks are obtained as a function of excess pore pressure initial kerogen content and water saturation the model generalizes a previous approach based on a pure elastic formulation of backus averaging and introduces the pressure effect and the presence of fluids oil and water the model allows the simulation of different maturation levels induced by pore pressure changes caused by the conversion of kerogen to oil the higher the oil saturation the higher the maturation level assuming that the source rock has a very low permeability the excess pore pressure can be calculated as a function of the conversion factor then the bulk modulus and density of the kerogen oil mixture are obtained with the kuster and toks oz model assuming that oil is the inclusion in a kerogen matrix finally backus averaging of this mixture with the illite smectite layers gives the complex stiffnesses of the transversely isotropic and anelastic medium computed p and s velocities and quality factors parallel to bedding are higher than those normal to bedding with attenuation anisotropy higher than stiffness anisotropy in particular for the north sea kimmeridge shale and at maximum anisotropy p and s parallel velocities are approximately 0 7 km s higher than the corresponding p and s normal velocities the maximum attenuation and stiffness anisotropies are obtained for 18 and 30 volumetric kerogen content respectively both velocities and quality factors decrease with increasing kerogen content at a given pore pressure the decrease in wave velocity is 2 km s for p waves and 1 km s for s waves when kerogen increases from zero to 100 moreover anisotropy increases and velocities decrease with increasing pore pressure i e with higher kerogento oil conversion finally the presence of water affects the normal bedding velocity i e higher water saturation implies lower velocities

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

inversion migration tomography seismic inversion broadly enough defined is equivalent to doing migration and reflection tomography simultaneously diffraction tomography and inversion work best when sources and receivers surround the region of interest as in medical imaging applications theoretical studies have shown that high vertical wavenumber velocity perturbations are resolved by inverting surface seismic reflection data but the low vertical wavenumbers must be obtained using a separate step such as velocity analysis or reflection tomography i propose that a nonlinear iterative inversion that updates a varying background velocity obtains all wavenumbers that are resolvable separately by migration and tomography the background velocity must contain reflectors to provide data on both upward and downward transmission paths through the earth and hence the low wavenumbers by considering the downward transmission paths to be between surface sources and buried image geophones and the upward transmission paths to be between surfa

reflection waveform inversion using local descent methods estimating attenuation and velocity over a gas sand deposit prestack seismic reflection data contain amplitudes traveltimes and moveout information waveform inversion of such data has the potential to estimate attenuation levels reflector depths and geometry and background velocities however when inverting reflection data strong nonlinearities can cause reflectors to be incorrectly imaged and can prevent background velocities from being updated to successfully recover background velocities previous authors have resorted to nonlinear global search inversion techniques we propose a two step inversion procedure using local descent methods in which we perform alternate inversions for the reflectors and the background velocities for our reflector inversion we exploit the efficiency of the back propagation method when inverting for a large parameter set for our background velocity inversion we use newton inverse methods during the background velocity inversions it is crucial to adaptively depth stretch the model to preserve the vertical traveltimes this reduces nonlinearity by largely decoupling the effects of the background velocities and reflectors on the data nonlinearity is further reduced by choosing to invert for slownesses and by inverting for a sparse parameter set which is partially defined using geological reflector picks applying our approach to shallow seismic data from the north sea collected over a gas sand deposit we demonstrate that the proposed method is able to estimate both the geometry and internal velocity of a significant velocity structure not present in the initial model over successive iterations the use of adaptive depth stretching corrects the pull down of the base of the gas sand vertical background velocity gradients are also resolved for an insignificant extra cost the acoustic attenuation parameter q is included in the inversion scheme the final attenuation tomogram contains realistic values of q for the expected lithologies and for the effect of partial fluid saturation associated with a shallow bright spot the attenuation image may also indicate the presence of fracturing

analysis of multiscale scattering and poroelastic attenuation in a real sedimentary rock sequence compressional waves in heterogeneous permeable media experience attenuation from both scattering and induced pore scale flow of the viscous saturating fluid for a real finely sampled sedimentary sequence consisting of 255 layers and covering 30 meters of depth elastic and poroelastic computer models are applied to investigate the relative importance of scattering and fluid flow attenuation the computer models incorporate the known porosity permeability and elastic properties of the sand shale sequence in a binary medium plane layered structure the modeled elastic scattering attenuation is well described by stochastic medium theory if two length scale statistics are applied to reflect the relative thickness of the shale layers when compared to the sand layers under the poroelastic biot squirt flow model fluid flow attenuation from the moderate permeability 10 14 m2 sands may be separated in the frequency domain from the attenuation due to the low permeability 5 10 17 m2 shale layers based on

scattering attenuation in randomly layered structures with finite lateral extent a hybrid q model since the pioneering work of o doherty and anstey 1971 much research has been devoted to understand the effect of stratigraphic filtering of seismic waves i e the problem of multiple scattering in 1d random structures e g burridge et al 1993 shapiro and hubral 1999 for many subsurface structures such as sedimentary basins the assumption of layering is reasonable as a first approximation however real geostructures do not show perfect layering but exhibit a finite lateral extent in their elastic properties this becomes particularly important when studying overburden effects in reflection seismology where amplitude information is used in subsequent data analysis for example malme et al 2003 showed that the amplitude variation with offset avo response for a vertical seismic profile vsp experiment over a north sea field is significantly distorted by nonlayered overburden inhomogeneities they demonstrated by seismic forward modeling that pointlike diffractors and large scale gas filled sand bodies can be responsible for strong amplitude fluctuations in this short note we study the transmission behavior of seismic primaries when the finite lateral extent of the inhomogeneities is accounted for physically speaking we intend to quantify the combined effects of scattering attenuation due to thin layering and random diffractions and refractions it is important to understand that our approach describes scattering attenuation of seismic primaries and not attenuation of the mean field ensemble averaged wavefield as presented in earlier works e g lerche 1986 we use the model of an anisotropic 3d random medium anisotropy in this context means that the randomly distributed inhomogeneities have three different characteristic length scales and are characterized by a spatially anisotropic correlation function we also assume that the medium is lossless so that no intrinsic wave attenuation occurs a sketch of such a structure is shown in figure 1 resembling a realization of a 2d

seismic study of the low permeability volume in southern france karst systems locating groundwater in deep seated karst aquifers is inherently difficult with seismic methods we studied the upper epikarst and the underneath low permeability volume lpv of several karst systems located in the southern quercy and larzac regions of france and found that refraction tomography was effective only in the epikarst and not in the lpv we evaluated a 3d case study using a combination of surface records and downhole receivers to overcome this limitation this 3d approach unveiled a set of elongated furrows at the base of the epikarst and identified heterogeneities deep inside the lpv that may represent high permeability preferred pathways for water inside the karst to achieve the same result when no borehole was available we studied seismic amplitudes of the wavefield recognizing that wave induced fluid flow in low permeability carbonates is a driving mechanism of seismic attenuation we developed a workflow describing the heterogeneity of the lpv with spectral attributes derived from surface consistent decomposition principles and we validated its effectiveness at benchmark locations we applied this workflow to the 3d study and found a low amplitude signal area at depth we interpreted this anomaly as a water saturated body perched above the aquifer

seismic identification of a karstified reservoir in the adriatic sea in this study an elastic prestack migration inversion m i scheme was applied to a 2 d seismic line collected in the adriatic sea the objective was to use m i images to describe a hydrocarbon reservoir consisting of karstified brine saturated limestones the m i technique offers the benefit of extracting maximum information from seismic reflection data to describe and in some cases characterize the reservoir target m i images are calibrated with well data and represent approximate changes in p and s wave impedances across the reservoir combining these two images yields a poisson s ratio image

scattering versus intrinsic attenuation in the vadose zone a vsp experiment abstractwe studied scattering versus intrinsic attenuation estimates in the vadose zone from a shallow vsp experiment conducted in the lawrence livermore national laboratory llnl facility using permanent downhole geophones and a vertical impact source we estimated effective attenuation of the downgoing transmitted p wave we compared theoretical scattering attenuation estimates and finite difference synthetics to the measured field q values 1 q being a measure of attenuation using a selected range of impedance profiles of variance typical for a sedimentary basin our estimates of qscattering are in the order of 20 85 given the short propagation pathlengths involved 30 m we show that attenuation due to lateral heterogeneity is not significant we analyzed additional distorting factors including near field presence local impedance effects and interference from reflections originating beneath receivers and found that they may significantly impact attenuation measurement in near surface studi

possible sources for ch4 rich fluid inclusions in olivine from troctolitic rocks of the bathtub intrusion duluth complex u s a abstract cumulus olivine from the babbitt deposit in the troctolitic bathtub intrusion duluth complex contains abundant ch4 rich fluid inclusions with common hydrous silicate and carbonate precipitates at their inclusion walls given the active rifting environment in which magma of the duluth complex 1 1 ga has intruded as repeated pulses fluid inclusions in olivine are traceable throughout an entire stratigraphic drill core section with comparable textural features that enable a classification of three generations 1 primary ch4 rich fluid inclusions oriented parallel to growth planes of olivine or arranged in clusters 2 intragranular fluid inclusion planes of ch4 rich fluid inclusions extending towards grain boundaries and 3 transgranular fluid inclusion planes of ch4 rich fluid inclusions closely associated with serpentinization the hydrous minerals and carbonates contained in fluid inclusions of generations 1 and 2 indicate that original constituents of the early homogenous magmatic fluid must have been h2o and co2 upon cooling below 700 c at fixed pfluid 2 kbar and fo2fluid at or below qfm conditions an increase of xch4 at the expense of co2 results in the ch4 rich chemistry of the fluid the formation of late secondary fluid inclusion planes of generation 3 is related to extensive magma country rock interaction these inclusions contain no solids and their composition reflects the reduction of co2 to ch4 during serpentinization in an upper stability limit of 620 c this study emphasizes fluid inclusions as powerful indicators in remodeling chemical changes of the magmatic fluid during crystallization of cumulus phases magma country rock interaction and subsequent alteration

effects of incomplete parameterization on full wavefield viscoelastic seismic data for petrophysical reservoir properties in inversion of viscoelastic full wavefield seismic data the choice of model parameterization influences the uncertainties and biases in estimating seismic and petrophysical parameters using an incomplete model parameterization results in solutions in which the effects of missing parameters are attributed erroneously to the parameters that are included incompleteness in this context means assuming the earth is elastic rather than viscoelastic the inclusion of compressional and shear wave quality factors qp and qs in inversion gives better estimates of reservoir properties than the less complete elastic model parameterization qp and qs are sensitive primarily to fluid types and saturations the parameter correlations are sensitive also to the model parameterization as noise increases in the viscoelastic input data the resolution of the estimated parameters decreases but the parameter correlations are relatively unaffected by modest noise levels

reflectivity method for dipping layers the reflectivity method used for the calculation of synthetic seismograms for a layered elastic earth is modified for the case where the stack of layers is inclined with respect to the free surface the application of appropriate coordinate transforms and a stationary phase approximation results in a solution that holds if shot point and receivers are not in the vicinity of the area of outcrop and if multiple reflections between free surface and layers are excluded the method is applied to a simple synthetic example which demonstrates that downdipping reflectors can drastically reduce the critical amplitudes in comparison to horizontal layering a further application of the method concerns refraction data from the area of the german deep continental drilling location oberpfalz where dipping layers revealed in the reflection section require their incorporation in the wide angle modelling

wave attenuation attributes as flow unit indicators in this article we present a model based interpretation algorithm to predict attenuation signatures associated with a reservoir s structure and its intrinsic properties scattering losses are caused by multiple reflections associated with the impedance contrast between different reservoir boundaries intrinsic losses are due to the fluids in the reservoir the rock fabric and the anisotropy due to vertical fractures because amplitude data are sensitive to these reservoir characteristics we calculate the wave attenuation response of the reservoir in the frequency range of surface seismic vsp crosswell seismic and borehole sonic a model of a plane wave traveling in a poroelastic multilayer earth medium allows us to include anisotropy the multilayer part of the model simulates elastic scattering the poroelastic part of the model simulates fluid flow effects in the reservoir and the anisotropic part of the model simulates reservoir fractures

compressional wave q estimation from full waveform sonic data there is significant evidence that the anelastic loss of seismic energy is linked to petrophysical properties such as porosity permeability and clay content thus reliable estimation of anelastic attenuation from seismic data can lead to improved methods for the prediction of petrophysical properties this paper is concerned with methods for the estimation of attenuation at sonic frequencies 5 30 khz from in situ data two independent methods have been developed and tested for estimating compressional wave attenuation from full waveform sonic data a well established technique the logarithm spectral ratio lsr method is compared with a new technique the instantaneous frequency if method the lsr method uses the whole spectrum of the seismic pulse whilst the if method uses a carefully estimated value of instantaneous frequency which is representative of the centre frequency of the pulse in the former case attenuation estimation is based on the relative variation of amplitudes at different frequencies whilst in the latter case it is based on the shift of the centre frequency of the pulse to lower values during anelastic wave propagation the if method does not assume frequency independence of q which is a necessary assumption for the lsr method and it provides a stable frequency log the peak instantaneous frequency pif log which may be used as an indicator for attenuation under certain limitations the development and implementation of the two methods is aimed at minimizing the effect of secondary arrivals such as leaky modes and involved a series of parameter tests testing of the two methods using full waveform sonic data of variable quality obtained from a gas bearing sandstone reservoir showed that the if method is in general more stable and suitable for full waveform sonic data compared with the lsr method this was evident especially in data sets with high background noise levels and wave interference effects for good quality data the two methods gave results that showed good agreement whilst comparison with other log types further increased confidence in the results obtained a significant decrease approximately 5 khz in the pif values was observed in the transition from an evaporite shale sequence to the gas bearing sandstone average q values of 54 and 51 were obtained using good quality data from a test region within the gas saturated sandstone reservoir using the lsr and if methods respectively

deciphering magmatic and metasomatic processes recorded by fluid inclusions and apatite within the cu ni pge sulfide mineralized bathtub intrusion of the duluth complex ne minnesota usa

thermodynamic constraints on the formation of condensed carbon from serpentinization fluids abstract recent studies have identified carbonaceous material in serpentinite bodies but whether these deposits have a biologic or abiotic origin remains uncertain in this work thermodynamic calculations were performed to examine the potential for abiotic condensed carbonaceous material to be produced from serpentinization derived fluids under hydrothermal conditions calculation of reaction pathways during serpentinization of olivine showed that fluid compositions should equilibrate with condensed carbonaceous material which controls the h 2 and co 2 activities fluids from laboratory serpentinization experiments and from the lost city and the rainbow hydrothermal field are shown to be consistent with this model the predictions indicate that carbonaceous material should be the dominant carbon product of co 2 reduction in these hydrothermal settings which would have significant implications for a number of processes including the deep earth carbon cycle by creating a pool of relatively immobile reduced carbon the extent of h 2 production in ultrabasic environments and mechanisms leading to abiotic reduced carbon compounds

seismic scattering in the upper crystalline crust based on evidence from sonic logs summary evidence from sonic logs indicates that velocity fluctuations in the upper crystalline crust are remarkably uniform this motivates a generic approach to classifying uppercrustal seismic heterogeneity and to studying implications for seismic wave propagation the resulting canonical model of upper crustal seismic structure is characterized by a spatially isotropic von karman autocovariance function with a z 100 m 0 15 and cj 300 m s small angle scattering theory is used to predict the transition from weak to strong scattering as well as phase fluctuations and scattering attenuation compared with exponential random media v 0 50 the high fractal dimension i e small values of v of upper crustal heterogeneity causes smaller phase fluctuations and transition from weak to strong scattering at lower frequencies and shorter path lengths acoustic finite difference modelling shows that seismic reflections from deterministic features surrounded by heterogeneities are severely degraded when they fall into the strong scattering regime conversely traveltime fluctuations of transmitted waves are found to be relatively insensitive to the transition from weak to strong scattering upper crustal scattering q is predicted to lie between 600 and 1500 which is one to two orders of magnitude higher than q values inferred from seismic data this suggests that seismic attenuation in the upper crystalline crust is dominated by anelastic effects rather than by scattering

localization and backscattering spectrum of seismic waves in stratified lithology using four sonic well logs from diverse geological environments we analyze the statistics of lithological layers relevant to seismic wave propagation the autocorrelation functions are found to be well approximated by exponentials with correlation lengths generally in the 1 5 to 3 m range we use localization theory to calculate the apparent attenuation caused by random scattering as a function of frequency this attenuation has a peak at 50 to 150 hz with a corresponding localization length of 1 6 to 4 8 km 1 to 3 mi and an apparent q of 120 to 450 at seismic frequencies 20 hz the attenuation is smaller with localization lengths in the 16 to 32 km 10 to 20 mi range and an apparent q of 300 to 700 these values of apparent q are consistent with observations of previous authors who used well log calculations using finite differences we compute synthetic traces for a 20 hz pulse with multiple backscattering from the lithology given by each of the well logs the power spectral density of the trace

secondary graphitization in mantle derived rocks the assemblage magnesite graphite ch4 has been identified by laser raman microprobe spectroscopy in fluid inclusions in olivine grains in the kao kimberlite and the duluth complex troctolite in both cases the assemblage essentially eggler s emog oxygen fugacity buffer is believed to arise from secondary reactions rather than from primary igneous processes the known stability field in p t f o2 space of the emog buffer coupled with some other petrologic constraints imposed by the host rocks indicates in both cases that the carbon bearing assemblage was precipitated at low pressures and temperatures 2 kbar 550 c at oxygen fugacities within about 1 log f o2 unit below the fayalite magnetite quartz fmq buffer the laser raman microprobe provides compositional and structural data on the coexisting phases including carbon that further constrain the stability field of the natural assemblages

integrative analysis of a lacustrine carbonate reservoir kambala field cabinda angola the presalt lacustrine rift basins of west africa are the continued focus of exploration and development in this prolific petroleum province kambala field produces from the barremian aptian lacustrine carbonates of the toca formation source rocks are the rich lacustrine shales of the bucomazi formation toca carbonates were deposited along major shorelines and around islands in large lake systems reservoir studies provide the necessary models and analogs to guide exploration and evaluation efforts the toca reservoirs in kambala field have been subdivided into two units the lower toca is characterized by limestones and cherty dolomites which range texturally from high energy ooid and oncoid grainstones and packstones to lower energy ostracod wackestones and marls the upper and lower toca are separated by a unit of middle bucomazi shale the upper toca is composed of thin beds of low energy facies and thick porous carbonate units which are pervasively dolomitized kambala field sits over a northwest southeast trending basement high the field is bounded by intense faulting on the west and dips gently to the northeast the faults on the eastern side are more widely spaced and have less throw than the ones on the west seismic amplitude maps on the upper toca show markedly highermore amplitudes on the western and southern edges of the field the high seismic amplitudes and intense faulting generally correlate with the zone of porous dolomite less

improving q estimates from seismic reflection data using well log based localized spectral correction most methods for deriving q from surface seismic data depend on the spectral content of the reflection the spectrum of the reflected wave may be affected by the presence of thin beds in the formation which makes q estimates less reliable we incorporate a method for correcting the reflected spectrum to remove local thin bed effects into the q versus offset qvo method for determining attenuation from seismic reflection data by dividing the observed spectrum by the local spectrum of the known reflectivity sequence from a nearby well log we obtain a spectrum more closely resembling that which would be produced by a single primary reflector this operation equivalent to deconvolution in the time domain is demonstrated to be successful using synthetic data as a test case we also apply the correction method to qvo with a real seismic line over a south florida site containing many thin sandstone and carbonate beds when corrected spectra are used there is significantly less variance in the estimated q values and fewer unphysical negative q values are obtained based on this method it appears that sediments at the florida site have a q near 33 that is roughly constant from 170 to 600 m depth over the length of the line

ancient graphite in the eoarchean quartz pyroxene rocks from akilia in southern west greenland i petrographic and spectroscopic characterization because all known eoarchean 3 65 ga volcano sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate to determine the biogenicity of carbon in graphite in such rocks carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon here we report the petrological and spectroscopic characteristics of carbonaceous material typically associated with individual apatite grains but also with various other minerals including calcite in a 3 83 ga granulite facies ferruginous quartz pyroxene unit qp rock from the island of akilia in southern west greenland in thin sections of the fine grained parts of akilia qp rock sample g91 26 mapped apatites were found to be associated with graphite in about 20 of the occurrences raman spectra of this carbonaceous material had strong g band and small d band absorptions indicative of crystalline graphite three apatite associated graphites were found to contain curled graphite structures identified by an anomalously intense second order d band or 2d band raman mode these structures are similar to graphite whiskers or cones documented to form at high temperatures raman spectra of apatite associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 c which are consistent with granulite facies metamorphic conditions three graphite targets extracted by focused ion beam fib methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and raman spectroscopy tem analyses of graphite in these fib sections showed a 0 0 0 2 interplanar spacing between 3 41 and 3 64 a for apatite associated graphite which is larger than the spacing of pure graphite 3 35 a and may be caused by the presence of non carbon heteroatoms in interlayer sites samples analyzed by synchrotron based scanning transmission x ray microscopy stxm also confirmed the presence of crystalline graphite but abundances of n and o heteroatoms were below detection limit for this method graphite in the akilia qp rock was also found to occur in complex polyphase mineral assemblages of hornblende calcite sulfides magnetite that point to high temperature precipitation from carbon bearing fluids these complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite facies metamorphic event at 2 7 ga several observations point to graphitization from high temperature fluid deposition for some of the akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite but ambiguities remain for the origin of this carbon

quantifying uncertainties in attenuation estimation at methane hydrate bearing zones using sonic waveform logs abstractapplication of seismic attenuation estimation using sonic waveform data is limited because the estimation methods have not yet been fully developed although the median frequency shift method is considered to be effective and robust compared to the conventional spectral ratio method we demonstrated that the median frequency shift methods strongly depend on reference data under lower signal to noise ratios we modified an existing median frequency shift method not to depend on arbitrarily choosing a reference value and to quantify the uncertainties in attenuation estimation furthermore we implied the optimum selection of receiver pairs used for more stable attenuation analysis our numerical experiments supported the advantage of the proposed method although our main findings by applying the proposed methods in methane hydrate bearing sediments are almost consistent with past field sonic logging measurements we find some differences in the magnitude of attenuation values compared to existing s

life in the hydrated suboceanic mantle in the roots of the ocean crust mantle derived rocks are progressively hydrated by hydrothermal circulation raman spectroscopic analyses of hydrated rocks sampled from the ocean floor reveal accumulations of organic matter which point to the hydration process as a possible energy source

lacustrine carbonate reservoirs from early cretaceous rift lakes of western gondwana pre salt coquinas of brazil and west africa abstract barremian aptian lower cretaceous pre salt lacustrine carbonates host large petroleum reservoirs in south atlantic rift basins yet remain poorly studied in part due to the scarcity of lacustrine carbonate reservoirs throughout the geological record this review provides an integrated discussion of the current state of knowledge on the coquina play having been deposited in extensive rift lakes during the early stages of the opening of the south atlantic thick successions of coquina are recorded from the santos to sergipe alagoas basins and the kwanza to gabon basins on the brazilian and west african continental margins respectively coquinas exhibit similarities in depositional processes across the south atlantic however have distinct diagenetic histories and stratigraphic stacking patterns having been deposited in lakes with different geologies climates and water chemistries west african coquinas exhibit depositional characteristics that are indicative of deposition on distally steepened ramps and rimmed platforms and accordingly facies are somewhat different to those in brazilian basins which were deposited on low angle ramps as petroleum exploration continues into ultra deep waters on the brazilian and west african margins accurate facies models will be important for the predictability of carbonate reservoir intervals to this end onshore outcrop in the sergipe alagoas basin has revealed that the molluscan faunas have marine origins suggesting that facies patterns may be more complex than previously considered it is hoped that this review will provide the impetus for further research on the coquinas especially given the presence of poorly studied easily accessible onshore outcrops

empirical estimation of viscoelastic seismic parameters from petrophysical properties of sandstone viscoelastic seismic parameters are expressions of underlying petrophysical properties theoretical and empirically derived petrophysical seismic relations exist but each is limited in the number and the range of values of the variables used to provide a more comprehensive empirical model we combined lab measurements from 18 published data sets and well log data for sandstone samples and determined least squares coefficients across them all the dependent variables are the seismic parameters of bulk density compressional and shear wave velocities vp and vs and compressional and shear wave quality factors qp and qs the independent variables are effective pressure porosity clay content water saturation permeability and frequency as the derived expressions are empirical correlations no causal relations should be inferred prediction of is based on volumetric mixing of the constituents for vp and vs predictions separate sets of coefficients are fitted for three water saturation condi

p and s wave attenuation logs from monopole sonic data in this paper a modification of an existing method for estimating relative p wave attenuation is proposed by generating synthetic waveforms without attenuation the variation of geometrical spreading related to changes in formation properties with depth can be accounted for with the modified method reliable p and s wave attenuation logs can be extracted from monopole array acoustic waveform log data synthetic tests show that the p and s wave attenuation values estimated from synthetic waveforms agree well with their respective model values in situ p and s wave attenuation profiles provide valuable information about reservoir rock properties field data processing results show that this method gives robust estimates of intrinsic attenuation the attenuation profiles calculated independently from each waveform of an eight receiver array are consistent with one another in fast formations where s wave velocity exceeds the borehole fluid velocity both p wave attenuation qp 1 and s wave attenuation