Amplitude Versus Offset Response Research Articles

AVO study on a known geothermal reservoir located in the fractured carbonate formations of the pre-Cenozoic basement, Northwest Hungary

Amplitude Versus Offset (AVO) analysis has been successfully applied in the hydrocarbon industry for more than three decades. This effective tool of the joint seismic and well-log data processing enables to predict and analyze fluid saturated porous geological formations. AVO methodology is based on the anomalous behavior of the pre-stack reflected amplitudes observed from fluid bearing rocks. However, the potential of AVO methodology is still unexploited in geothermal exploration, although the lithology and rock physical properties are very similar. In this study, we summarize the theoretical backgrounds and calculate synthetic AVO responses of a known geothermal reservoir located in the fractured carbonates of the Mesozoic basement. We demonstrate that the AVO response of a deep geothermal reservoir can be quite different from the amplitude response observed from a hydrocarbon bearing clastic formation. AVO attributes of the investigated geothermal reservoir are presented, and the potential of its detection by seismic amplitude data are discussed.

AVO Detuning Effect Analysis Based on Sparse Inversion

The wave field characteristics of thin reservoirs are extremely complex due to the tuning and interference between the top and bottom interfaces of the reservoirs, which leads to large uncertainty in thin layer AVO (Amplitude Versus Offset) analysis. In order to reduce the uncertainty of thin layer AVO analysis, we study the uncertainty dominant factors of the effect of thin layer on the AVO response characteristics from the aspects of theoretical derivation and forward simulation. Based on the research results, we use the AVO fitting forward method with offset and tuning utility as the joint inversion operator to establish an AVO detuning effect method, based on the sparse fitting inversion strategy, and study the objective function of the fitting inversion method. We optimize the sparsity constraints and the sparsity method to reduce the non-independence of multiparameter variables and seismic data, and the noise of inversion. Through the verification analysis of the model using actual data, the AVO detuning effect method studied in this paper has a correct and reasonable technical theory and obvious application effect.

Application of Far-Gather Seismic Attributes in Suppressing the Interference of Coal Beds in Reservoir Prediction

The sandstone reservoir of the Pinghu Formation in the Xihu Depression, East China Sea is characterized by great depth, small thickness, radical facies change and a widespread coal bed. It is difficult to describe the reservoir accurately using conventional reservoir prediction methods. In order to analyze the influence of coal-bearing strata on the prediction of the mid-low thickness sandstone reservoir, the seismic response of different sandstone–coal stratigraphic assemblages was simulated by seismic forward modeling. The modeling result indicates that the post-stack seismic response is dominated by coal bed, whereas the response of sandstone can hardly be recognized. In contrast, the difference between the pre-stack AVO (amplitude versus offset) response characteristics of coal seams and gas-bearing sandstones has been clarified based on the statistics pertaining to AVO characteristics of drilled wells. Therefore, we propose a method to reduce the interference of coal beds in sandstone reservoir prediction using far-gather seismic information. This method has significantly improved the accuracy of reservoir prediction and sand description in sand–coal coupled environments and has been applied successfully in the exploration of coal-rich strata in the Pingbei slope belt, Xihu Depression.

An integrated petrophysical-based wedge modeling and thin bed AVO analysis for improved reservoir characterization of Zhujiang Formation, Huizhou sub-basin, China: A case study

AbstractThe main reservoir in Huizhou sub-basin is Zhujiang Formation of early Miocene age. The petrophysical analysis shows that the Zhujiang Formation contains thin carbonate intervals, which have good hydrocarbon potential. However, the accurate interpretation of thin carbonate intervals is always challenging as conventional seismic interpretation techniques do not provide much success in such cases. In this study, well logs, three-layer forward amplitude versus offset (AVO) model and the wedge model are integrated to analyze the effect of tuning thickness on AVO responses. It is observed that zones having a thickness greater than or equal to 15 m can be delineated with seismic data having a dominant frequency of more than 45 Hz. The results are also successfully verified by analyzing AVO attributes, i.e., intercept and gradient. The study will be helpful to enhance the characterization of thin reservoir intervals and minimize the risk of exploration in the Huizhou sub-basin, China.

Seismic evidence of free gas migration through the gas hydrate stability zone (GHSZ) and active methane seep in Krishna-Godavari offshore basin

Abstract Seismic amplitude versus offset (AVO) analysis was performed on the multichannel seismic data in the vicinity of Site NGHP-01-10, where thick fracture-filled gas hydrate is reported, to understand the fluid migration in fracture dominated gas hydrate system. Amplitude preserved common image gathers (CIG) are used in extracting the seismic amplitude with offset for seafloor, bottom simulating reflectors (BSR), and different horizons within the gas hydrate stability zone (GHSZ). The extracted amplitudes are fitted with three-term Shuey's AVO approximation to estimate the intercept and gradient values. Based on the intercept and gradient values, two distinct classes of seismic AVO responses are observed; class IV AVO (decreasing negative amplitudes with increasing offsets) and class III AVO (increasing negative amplitudes with increasing offsets) for the BSR and other horizons above the BSR. The large negative intercept and gradient values which indicate class III AVO ranges from −0.2 to −1.8 for the inline seismic profile and from −0.01 to −0.3 for the crossline profile. The presence of class III AVO anomaly within the gas hydrate stability zone (GHSZ) provides evidence of existence of free gas which may be migrating along the fault system. Some of the active faults are exposed on the surface and show gas venting signatures such as gas-flares and pockmarks in the multibeam data.

Frequency-Dependent Amplitude Versus Offset Variations in Porous Rocks with Aligned Fractures

The theory of frequency-dependent amplitude versus offset (AVO) was developed for patchy-saturated model. In this work, we consider this theory in the case of an anisotropic medium based on a fractured-sandstone model. Thus, building on viscoelastic theory, we introduce a method for the computation of frequency-dependent AVO that is suitable for use in the case of an anisotropic medium. We use both analytical methods and numerical simulations to study P-P and P-S reflection coefficients, and results suggest that dispersion and anisotropy should not be neglected in AVO analysis. Indeed, for class I AVO reservoirs, the reflection magnitude of P-wave increases with frequency, while the responses of class II AVO reservoirs suggest that phase reversal occurs as frequency increases positively. In the case of class III AVO reservoirs, reflection magnitude decreases as frequency increases positively, while in the offset domain, the presence of anisotropy can distort or even reverse AVO responses. Thus, when compared to reflection coefficients for P-wave, reflection magnitude features of S-wave are more complex. The frequency-dependent AVO responses reported in this study provide insights for the interpretation of seismic anomalies in vertical transverse isotropy (VTI) dispersive reservoirs.

Using lame’s petrophysical parameters for fluid detection and lithology determination in parts of Niger delta

This study analyzed seismic and well log data in an attempt to interpret relationship between rock properties and acoustic impedance and to construct amplitude versus offset (AVO) models. The objective was to study sand properties and predict potential AVO classification. Logs of Briga 84 well provided compressional and shear velocities. AVO synthetic gather models were created using various gas/brine/oil substitutions. The Fluid Replacement Modeling predicted Class 3 AVO responses from gas sands. Log calibrated Lambda Mu Rho (LMR) inversion provided a quantitative extraction of rock properties to clearly determine lithology and fluids. Beyond the standard LMR cross-plotting that isolated gas sand clusters in the log, an improved separation of high porosity sands from shale was also achieved using λρ - µρ vs. AI. When applied to the calibrated AVO/LMR attributes inverted from 3D seismic, the λρ analysis permitted a better isolation of prospective hydrocarbon zones than standard AI inversion Thus, as an exploration tool for hydrocarbon accumulation, the Lambda Mu Rho (λµρ) technique is a good indicator of the presence of low impedance sands encased in shales which are good gas reservoirs. KEYWORDS : Impedance, synthetic gather, cross-plotting, porosity, inversion

Application of AVO analysis to gas hydrates identification in the northern slope of the South China Sea

Amplitude versus offset (AVO) analysis is a conventional seismic exploration technique in geophysical and lithological interpretation and has been widely used in onshore and offshore exploration. Its use in marine gas hydrate research, however, is still in initial stages. In this study, AVO analysis is applied to seismic profiles at drilling sites where hydrate samples have been recovered. The AVO responses of free gas, bottom simulating reflector (BSRs), and gas hydrates are discussed, and the AVO attributes in relation to gas hydrates are summarized. The results show that changes in intercept, gradient, fluid factor and Poisson's ratio clearly reflect: (i) location of free gas and the BSR, and (ii) spatial relations between blank zone, BSR, gas hydrate, and free gas.

Investigation of Fluid Properties and their Effect on Seismic Response: A Case Study of Fenchuganj Gas Field, Surma Basin, Bangladesh

The Fenchuganj Gas Field is one of the water-drive gas fields in the Surma Basin, Bangladesh. Due to gas production water saturation is increasing day by day. However, fluid properties of the reservoir at the present state in the gas depleted condition should be addressed with proper prediction. In this paper, we represent some modeling results which evidences that the pore fluids have a significant effect on the acoustic impedance and the Poisson’s ratio of the reservoir rock which is directly correlated with seismic amplitudes at constant pressure with Batzle-Wang model and Gassman-Boit models. Moreover, these models with varying pressure and water saturation conditions show the reasonable predicted fluid modulus against pressure. The reservoir modeling from irreducible water saturation conditions (90% gas saturation) to residual gas conditions (10% gas saturation) provides a pathway to calculate values at reservoir conditions from logging conditions. Fluid bulk density increases when water saturation increases with constant pressure and stay around constant when water saturation increases with pressure fall. But overall it increases through the production path that we assumed. Amplitude versus Offset (AVO) analysis also consistent with other studies models which show that seismic reflection of p-wave changes due to change of pressure and water saturation of the reservoir rock layers. The AVO response decreases with increases of water saturation of the gas zones. The evaluation of fluid properties enables seismic data to be used more powerfully. This study is also showing that both gas zones of the Fenchuganj are under gas sand category 3. We suggest that the modeling of fluid property in determining the usefulness of time lapse seismic, predicting AVO and amplitude response, and making production and reservoir engineering decisions and forecasting in the study field.

Reservoir Characterization by Investigating the Reservoir Fluid Properties and their Effect on Seismic Response of Fenchuganj Gas Field, Bangladesh

Fenchuganj Gas Field is located in the Surma Basin of Bangladesh and characterized by water-drive gas field. In the reservoir condition, water saturation increases as gas production rise. The fluid properties of the four individual gas zones of this reservoir at the present condition and at the gas depleted condition should be addressed with proper prediction. In this paper, we characterize the total reservoir with special emphasis on Upper Gas Zone and New Gas Zone I which are compared with other two gas zones (New Gas Zone III and New Gas Zone II) representing some modeling results (has done before by these authors) which evidences that the pore fluids have a significant effect on the acoustic impedance and the Poisson’s ratio of the reservoir rock which is directly correlated with seismic amplitudes at constant pressure with Batzle-Wang model and Gassman-Boit models. These models with varying pressure and water saturation conditions show the reasonable predicted fluid modulus against pressure for all four gas sands. The reservoir modeling from irreducible water saturation condition (90% gas saturation) to residual gas condition (10% gas saturation) provides a way to estimate values at reservoir conditions from logging conditions. Fluid bulk density increases when water saturation increases with constant pressure and stay around constant when water saturation increases with pressure drop. But overall it increases through the production path that we assumed. Amplitude versus Offset (AVO) analysis is also compared with other study models which show that seismic reflection of p-wave changes due to change of pressure and water saturation of the reservoir rock layers. This study is also showing that all four gas zones of the Fenchuganj are under gas sand category 3. We propose the modeling of fluid property in determining the convenience of time lapse seismic, predicting AVO and amplitude response, and forecasting in the study field and making production and reservoir engineering decisions.

Seismic studies of coal bed methane content in the west coal mining area of Qinshui Basin

The coal bed methane content (CBMC) in the west mining area of Jincheng coalfield, southeastern Qinshui Basin, is studied based on seismic data and well-logs together with laboratory measurements. The results show that the Shuey approximation has better adaptability according to the Zoeppritz equation result; the designed fold number for an ordinary seismic data is sufficient for post-stack data but insufficient for pre-stack data regarding the signal to noise ratio (SNR). Therefore a larger grid analysis was created in order to improve the SNR. The velocity field created by logging is better than that created by stack velocity in both accuracy and effectiveness. A reasonable distribution of the amplitude versus offset (AVO) attributes can be facilitated by taking the AVO response from logging as a standard for calibrating the amplitude distribution. Some AVO attributes have a close relationship with CBMC. The worst attribute is polarization magnitude, for which the correlation coefficient is 0.308; and the best attribute is the polarization product from intercept, of which the correlation coefficient is −0.8136. CBMC predicted by AVO attributes is better overall than that predicted by direct interpolation of CBMC; the validation error of the former is 14.47%, which is lower than that of the latter 23.30%. CBMC of this area ranges from 2.5m3/t to 22m3/t. Most CBMC in the syncline is over 10m3/t, but it is below 10m3/t in the anticline; on the whole, CBMC in the syncline is higher than that in anticline.

Analytical wavefront curvature correction to plane‐wave reflection coefficients for a weak‐contrast interface

ABSTRACTMost amplitude versus offset (AVO) analysis and inversion techniques are based on the Zoeppritz equations for plane‐wave reflection coefficients or their approximations. Real seismic surveys use localized sources that produce spherical waves, rather than plane waves. In the far‐field, the AVO response for a spherical wave reflected from a plane interface can be well approximated by a plane‐wave response. However this approximation breaks down in the vicinity of the critical angle. Conventional AVO analysis ignores this problem and always utilizes the plane‐wave response. This approach is sufficiently accurate as long as the angles of incidence are much smaller than the critical angle. Such moderate angles are more than sufficient for the standard estimation of the AVO intercept and gradient. However, when independent estimation of the formation density is required, it may be important to use large incidence angles close to the critical angle, where spherical wave effects become important. For the amplitude of a spherical wave reflected from a plane fluid‐fluid interface, an analytical approximation is known, which provides a correction to the plane‐wave reflection coefficients for all angles. For the amplitude of a spherical wave reflected from a solid/solid interface, we propose a formula that combines this analytical approximation with the linearized plane‐wave AVO equation. The proposed approximation shows reasonable agreement with numerical simulations for a range of frequencies. Using this solution, we constructed a two‐layer three‐parameter least‐squares inversion algorithm. Application of this algorithm to synthetic data for a single plane interface shows an improvement compared to the use of plane‐wave reflection coefficients.

Study on attribute characterization for reservoir dynamic monitoring by seismic

Study on characterizing reservoir parameters dynamic variations by time-lapse seismic attributes is the theoretical basis for effectively distinguishing reservoir parameters variations and conducting time-lapse seismic interpretation, and it is also a key step for time-lapse seismic application in real oil fields. Based on the rock physical model of unconsolidated sandstone, the different effects of oil saturation and effective pressure variations on seismic P-wave and S-wave velocities are calculated and analyzed. Using numerical simulation on decoupled wave equations, the responses of seismic amplitude with different offsets to reservoir oil saturation variations are analyzed, pre-stack time-lapse seismic attributes differences for oil saturation and effective pressure variations of P-P wave and P-S converted wave are calculated, and time-lapse seismic AVO (Amplitude Versus Offset) response rules of P-P wave and P-S converted wave to effective pressure and oil saturation variations are compared. The theoretical modeling study shows that it is feasible to distinguish different reservoir parameters dynamic variations by pre-stack time-lapse seismic information, including pre-stack time-lapse seismic attributes and AVO information, which has great potential in improving time-lapse seismic interpretation precision. It also shows that the time-lapse seismic response mechanism study on objective oil fields is especially important in establishing effective time-lapse seismic data process and interpretation scheme.

Shallow seismic AVO variations related to partial water saturation during a pumping test

High‐resolution shallow seismic reflection experiments were conducted during and after a pumping test of an agricultural irrigation well to image the cone of depression. Although variations in the reflection time from the top of the saturated zone were not observed, amplitude‐versus‐offset (AVO) analysis revealed changes in reflection amplitude responses that correlate temporally and spatially to expected changes to the partially saturated zone induced by the pumping and recovery of the aquifer. The AVO responses exhibit dependence on aquifer drawdown and recovery cycles and the distance from the pumping well. We propose that near‐surface soil heterogeneity and relatively rapid changes in the water table elevation during irrigation cycles caused a thickening of the partially saturated zone above the water table, which resulted in detectable changes in seismic reflection amplitudes. This study offers insights about the response of shallow seismic reflections to changes in subsurface water saturation and the potential application of seismic techniques to hydrogeophysical problems.

Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India

The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (MCSs). The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used wave-equation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone.

Temperature contrasts in the water column inferred from amplitude‐versus‐offset analysis of acoustic reflections

We show that seismic amplitude‐versus‐offset (AVO) analysis can be used to remotely quantify the temperature contrasts responsible for acoustic reflections in the ocean. We inferred the sound speed and density contrasts associated with two reflections in the Norwegian Sea by comparing their AVO response to that predicted by the Zoeppritz equations. Estimates of temperature contrasts were calculated from sound speed contrasts using Wilson's equation and compared with in situ temperature measurements from coincident expendable bathythermographs (XBTs). A strong reflection from Line 9 is best explained by a −6 m/s step in sound speed, corresponding to a temperature decrease of ∼1.5°C. A weaker reflection from Line 11 yielded a sound speed contrast of −1.2 m/s, corresponding to a temperature decrease of ∼0.3°C. The inferred temperature contrasts match those found in XBT data remarkably well. L1‐norm misfits between predicted and calculated values indicate that the acoustic impedance contrasts are principally controlled by sound speed contrasts, rather than density contrasts. AVO analysis offers an accurate and robust technique for estimating sound speed (and therefore temperature) contrasts in the water column.

Influence of porosity and pore fluid on acoustic properties of chalk: AVO response from oil, South Arne Field, North Sea

Amplitude versus offset (AVO) inversion provides direct evidence for the presence of light oil in high-porous chalk in the South Arne Field, North Sea. The elastic properties of the chalk were estimated at three scales by analysing core data, log-readings and AVO-inversion results. The velocity–porosity relation of the core data matches a modified upper Hashin–Shtrikman model for Ekofisk Field chalk and the model is extended to 45% porosity. A small clay content reduces porosity without affecting chalk stiffness and this content can be estimated from the water saturation, which is controlled by silicate content and particle sorting in the zone of irreducible water saturation. The model is, thus, scaled according to clay content estimated by the water saturation. Based on comparison with the model and measurements on core samples, it is found that the sonic log data represent chalk characterized by forced displacement of the oil by mud filtrate and, thus, a much higher water saturation than estimated from, for example, a shallow resistivity log. Forward modelling of the acoustic properties of the virgin zone results in a characteristic pattern of Poisson ratio versus depth. This pattern agrees with inverted seismic data, whereas it is not captured by conventional fluid substitution.

Application of AVO Analysis to Seismic Data for Detection of Gas below Methane Hydrate Stability Zone in Nankai Trough Area

Abstract. For the purpose of development of methane hydrate, occurring in the deep marine subsurface, as a resource, the most important issue is to understand the methane hydrate system (generation, migration and accumulation) as well as to delineate the methane hydrate reservoir properties. We have applied the Amplitude Versus Offset (AVO) analysis to the seismic data acquired in the Nankai Trough, offshore Japan, in order to confirm the occurrence of gas just below the methane hydrate‐bearing zone, assuming that gas will show a so‐called Class‐3 AVO response. Knowledge of the amount and occurrence of gas in the sediment below methane hydrate‐bearing zone is one of the keys to understand the methane hydrate system.We have utilized the qualitative analysis of AVO methodology to delineate how gas is located below the BSR, which is thought to be the reflection event from the interface between the methane hydrate‐bearing zone and the underlying gas‐bearing zone. In the region of MITI Nankai Trough Well PSW‐3, we observe two BSRs separated by 25 ms. After AVO modeling using well data, we applied AVO attribute analysis and attribute crossplot analysis to the seismic data. Finally we applied an offset‐amplitude analysis to CMP gather data at specific locations to confirm the results of AVO attribute analysis. The AVO analysis shows that there is very little gas located in the underlying sediment below methane hydrate‐bearing zone. This result supports the fact that we could not obtain any clear evidence of gas occurrence just below the methane hydrate‐bearing zone in the Nankai Trough well drilling.

Thin‐bed AVO: Compensating for the effects of NMO on reflectivity sequences

Estimating the amplitude versus offset (AVO) gradient for thin beds is problematic because of offset‐dependent tuning and NMO stretch. When AVO analysis is carried out before NMO correction, the nonparallel nature of the NMO hyperbolas results in differential interference conditions at each offset and complicates AVO interpretation. If AVO analysis is carried out after NMO correction, the data bandwidth is distorted and corrections must be made to recover the true AVO response. A correction for NMO stretch can be applied to Fourier spectra obtained after windowing the NMO‐corrected prestack data. This approach requires knowledge of the seismic wavelet but seems to be relatively insensitive to noise in the data or uncertainties in the wavelet estimation. The technique allows the interference conditions to be made independent of offset and the correct AVO gradient relative to the normal incidence amplitude to be recovered.

Analysis of multiazimuthal VSP data for anisotropy and AVO

Multioffset vertical seismic profile (VSP) experiments, commonly referred to as walkaways, enable anisotropy to be measured reliably in the field. The results can be fed into modeling programs to study the impact of anisotropy on velocity analysis, migration, and amplitude versus offset (AVO). Properly designed multioffset VSPs can also provide the target AVO response measured under optimum conditions, since the wavelet is recorded just above the reflectors of interest with minimal reflection point dispersal. In this paper, the multioffset VSP technique is extended to include multioffset azimuths, and a multiazimuthal multiple VSP data set acquired over a carbonate reservoir is analyzed for P-wave anisotropy and AVO. Direct arrival times down to the overlying shale and reflection times and amplitudes from the carbonate are analyzed. Data analysis involves a three‐term fit to account for nonhyperbolic moveout, dip, and azimuthal anisotropy. Results indicate that the overlying shale is transversely isotropic with a vertical axis of symmetry (VTI), while the carbonate shows 4–5% azimuthal anisotropy in traveltimes. The fast direction is consistent with the maximum horizontal stress orientation determined from break‐out logs and is also consistent with the strike of major faults. AVO analysis of the reflection from the top of the carbonate layer shows a critical angle reduction in the fast direction and maximum gradient in the slow direction. This agrees with modeling and indicates a greater amplitude sensitivity in the slow direction—the direction perpendicular to fracture strike. In principle, 3-D surveys should have wide azimuthal coverage to characterize fractured reservoirs. If this is not possible, it is important to have azimuthal line coverage in the minimum horizontal stress direction to optimize the use of AVO for fractured reservoir characterization. This direction can be obtained from multiazimuthal walkaways using the azimuthal P-wave analysis techniques presented.