AVO Analysis Research Articles

Amalgamating LMR, EEI and AVO Analysis to Monitor the Characteristics of Pliocene Gas Reservoir: A Case Study Offshore, East Nile Delta, Egypt

Amalgamating LMR, EEI and AVO Analysis to Monitor the Characteristics of Pliocene Gas Reservoir: A Case Study Offshore, East Nile Delta, Egypt

Evaluation of Hydrocarbon Potential Using AVO Analysis in the FORMAT Field, Niger Delta Basin, Nigeria

Evaluation of Hydrocarbon Potential Using AVO Analysis in the FORMAT Field, Niger Delta Basin, Nigeria

Identification of challenging gas-bearing reservoir based on machine learning (ML) and computed conversion-based AVO analysis: a study from Jaisalmer Sub-basin, India

Amplitude variation with offset (AVO) analysis is an important tool for identifying natural gas-bearing reservoirs. The changes in seismic amplitudes based on the variation of density and velocity of the rock matrix are captured through the AVO analysis. The current work was performed in the Ghotaru region of the Jaisalmer Sub-basin area, where limited and discrete hydrocarbon discoveries were observed from the Lower Goru Formation during the earlier various exploration campaigns. The discrete nature of the reservoir lithofacies developed challenging scenarios for the successful exploratory campaign. The campaign encountered more difficulties because of limited advanced datasets, which affected the study to capture the extension of hydrocarbon-bearing reservoir lithofacies and its characterization towards a successful exploration campaign. This study shows the way to overcome these challenges using an existing conventional dataset. The study shows the possibility of AVO analysis using a post-stack seismic dataset. A unique conversion method from post-stack to pre-stack seismic is introduced in this study based on the uses of the integrated velocity model. An integrated, robust velocity model was developed with consideration of anisotropy factors. Introducing a machine learning-based algorithm in the petrophysical study, including the conventional approach, provides a robust validation of this work. Intercept (A) and gradient (B) are the basic outcome of AVO analysis. The well-based study and AVO analysis based on intercept (A) and gradient (B) complement each other for finding hydrocarbon-bearing reservoir rock. Cross-plots and AVO analysis show the reservoir's lithofacies extension and fluids. The study reveals the potential of natural gas retained in the Lower Goru Formation, which is composed of patchy sandstone. Two AVO classes (Class I and Class III) of gas-bearing sandstone have been identified in this study. This study presents a unique method for identifying natural gas reservoirs with limited old conventional data.

Wide azimuth seismic data processing technology and application: a case study of tight gas reservoirs in western China

As the difficulty of oil and gas field exploration and development increases both domestically and internationally, onshore exploration targets have gradually shifted from the shallow to the deep and from conventional oil and gas reservoirs to unconventional ones. Particularly in the exploration and development of unconventional oil and gas horizontal wells, there is an increasing demand for higher precision and quality of seismic data to better identify formation lithology, rock fractures, and improve the characterization of reservoirs, reservoir positioning, and connectivity. Wide-azimuth seismic exploration possesses significant technical advantages in addressing exploration challenges such as lithologic exploration, small fault imaging, and detailed characterization of oil and gas reservoirs. Wide azimuth seismic data reduces blind spots in seismic acquisition and improves the imaging accuracy of small faults. Notably, there exist distinct anisotropic characteristics in fault areas and fractured reservoirs. Wide azimuth seismic data is particularly advantageous for studying amplitude variation with variations in amplitude with offset (AVO), incident angle (AVA), or azimuth (AVAZ), as well as velocity with azimuth (VVA). These variations aid in identifying faults, fractures, and changes in formation lithology. As the focus of oil and gas exploration gradually shifts to complex lithological reservoirs and unconventional oil and gas reservoirs, narrow azimuth seismic exploration has been gradually replaced by wide azimuth exploration. However, as observation azimuth increases, challenges related to velocity variations with azimuth, azimuth-related traveltime differences, and azimuth-related anisotropy arise. Based on wide-azimuth seismic data from tight gas reservoirs in western China, this study conducted wide-azimuth anisotropic velocity analysis, OVT domain data regularization processing, OVT domain prestack time/depth migration, and horizontally transverse isotropy (HTI) azimuth anisotropy correction techniques. After applying specialized processing to the wide-azimuth seismic data, significant improvements were observed in the S/N and resolution of the target layer. The delineation of fractures related to hydrocarbon sources also became more distinct. These advancements not only provided high-quality results for high-fidelity, high-resolution imaging of tight gas reservoirs but also provided azimuth volume corresponding to fast and slow wave velocities for seismic data interpretation, facilitating velocity variation with azimuth (VVAZ) fracture detection and AVO analysis research.

Reservoir Characterization of the UM Field in the Niger Delta Using AVO Analysis

Reservoir Characterization of the UM Field in the Niger Delta Using AVO Analysis

ОТДЕЛЬНЫЕ ВОПРОСЫ ДИНАМИЧЕСКОЙ ИНВЕРСИИ ВОЛНОВОГО ПОЛЯ

Currently, reservoir parameter prediction methods based on the amplitude wave field inversion play an important role in solving geological problems in the joint interpretation of seismic and well data. From a formal point of view, well data are used to obtain additional conditions for solving inverse problems of seismic. But the formal description of solution is conditional, there is rather an adjustment of results obtained independently from seismic and well data. The paper considers the following issues: stability of the simultaneous inversion parameter estimation; calculation features of background model and coordination of background and seismic solution components; compliance with restrictions on the maximum sourcereceiver distances by kinematic muting and the range of reflection angles during AVO analysis. The AVO effect is analyzed, it displays itself in a change of the reflection coefficient sign and, correspondently, phase inversion of reflected wave.

Deep Neural Networks for Direct Hydrocarbon Detection in Prestack Seismic Data Based on AVO Analysis

Deep Neural Networks for Direct Hydrocarbon Detection in Prestack Seismic Data Based on AVO Analysis

ON SOME FEATURES OF ULTRASOUND REFLECTION WATER-SAMPLE IN AN INCLINED FALL (PHYSICAL MODELING)

In recent years, the analysis of the dependence of reflection coefficients on the magnitude of the angle of incidence of reflected waves has been successfully used in the practice of seismic research. AVO analysis is one of the methods of dynamic analysis that is used to estimate changes in the amplitude of reflected waves depending on the distance between the explosion points and the receivers. The AVO method is based on the analysis of the dependence of the reflection coefficients on the angle of incidence. In real conditions, this dependence can be determined, for example, by the roughness of the boundaries. This determines the relevance of studying the features of reflection coefficients on uneven boundaries on objects with well-controlled properties. The aim of the work is to determine the nature of the influence of different-scale roughness of seismic boundaries on the reflection coefficients of elastic waves. The work also used the technique of isolating standing waves to determine the wave velocity. As a result, graphs were obtained demonstrating the dependence of the reflection coefficients on the magnitude of the angle of incidence of reflected waves from a rough surface. Reflection coefficients were also obtained for the boundary of an isotropic medium in the direction of the isotropy plane and possible ways of applying the results were analyzed. Based on the data obtained, we can say that when the azimuth changes relative to the direction of the surface, the reflection coefficients change significantly only at the supercritical angles of incidence.

ON SOME FEATURES OF ULTRASOUND REFLECTION WATER-SAMPLE IN AN INCLINED FALL (PHYSICAL MODELING)

In recent years, the analysis of the dependence of reflection coefficients on the magnitude of the angle of incidence of reflected waves has been successfully used in the practice of seismic research. AVO analysis is one of the methods of dynamic analysis that is used to estimate changes in the amplitude of reflected waves depending on the distance between the explosion points and the receivers. The AVO method is based on the analysis of the dependence of the reflection coefficients on the angle of incidence. In real conditions, this dependence can be determined, for example, by the roughness of the boundaries. This determines the relevance of studying the features of reflection coefficients on uneven boundaries on objects with well-controlled properties. The aim of the work is to determine the nature of the influence of different-scale roughness of seismic boundaries on the reflection coefficients of elastic waves. The work also used the technique of isolating standing waves to determine the wave velocity. As a result, graphs were obtained demonstrating the dependence of the reflection coefficients on the magnitude of the angle of incidence of reflected waves from a rough surface. Reflection coefficients were also obtained for the boundary of an isotropic medium in the direction of the isotropy plane and possible ways of applying the results were analyzed. Based on the data obtained, we can say that when the azimuth changes relative to the direction of the surface, the reflection coefficients change significantly only at the supercritical angles of incidence.

Characterization of seismic anisotropy using azimuthal AVO analysis (AVAz) - An application case study in the deep and tight carbonate reservoirs from Potwar Basin onshore Pakistan

Reliable characterization of subsurface fractures information within the carbonate's resources plays a key role in reservoir description in terms of the storage and movement of fluid. Commonly, anisotropy information classifies heterogeneity in the rock fabric due to fractures, pores, and local stress variations. Amplitude Variation with Azimuth investigation examines azimuthal variations in the seismic amplitude information to provide useful information about fractures and anisotropy of a horizontally transverse isotropic (HTI).This paper presents modeling, analysis, and interpretation of Azimuthal amplitude variation with angles (AVAz) from 3D seismic and calibrate results with the wells data to extract fractures information in terms of fractures magnitude and orientations. We employed two most used data-driven alternate approaches 1) near-offset Rüger's equation and 2) azimuthal Fourier Coefficient (FC) for analyzing the AVAz responses in seismic data to estimate the fractures density and their orientations, and results are calibrated with the bore hole image data (FMI). These approaches proved practically convenient to quantify the azimuthal amplitude variations with two sets of AVAZ attributes. The results of AVAz modeling and analysis required good quality of input data. Before AVAZ modeling and attributes analysis, seismic data is pre-conditioned, and the data quality of the 3D seismic dataset is improved. Several qualitative and quantitative QC measures (e.g., comparison of angle stacks, AVAz analysis, cross-plots of correlation coefficients, and time shifts) are employed to ascertain the improvement of the health of the seismic data. Azimuthal AVA modeling helped to understand the azimuthal amplitude response due to anisotropy caused by fractures/stress. The predicted seismic response from the models is used to assess the sensitivity of AVAZ with the parameters of the anisotropic model. After the AVAZ modeling, the outcomes of the AVAZ attributes are generated using to 3D seismic and calibrated with the well data set to identify the anomaly of anisotropic gradient (Bani) and second Fourier coefficients (R2) by relating to fractures density or stress magnitude. These attributes helped to identify the amplitude variations with offset and azimuth in the deep inhomogeneous tight carbonate reservoir interval (Patala) of DS hydrocarbon field discovered in the north Potwar Basin, onshore Pakistan. High score of Bani or R2 observed at well location at the tight Paleocene carbonate (Patala formation) reservoirs exhibited the presence of fractures which is confirmed from the image logs (e.g., FMI).The results demonstrated that the studied formations exhibit significant heterogeneity due to the faults/fractures and stresses associated with the complex geological structure of the basin as well as variations in the rock fabric. Hence, accurate analysis of the fractures information from seismic data is quite useful not only in understanding the reservoir heterogeneity to optimize the production, but also in predicting the completion and development strategies that may be most effective.

Integrated AVO inversion and seismic attributes for tracing hydrocarbon accumulation in Kafr El-Sheikh Formation, South Batra field, Nile Delta, Egypt: A case study

Utilizing advanced seismic techniques for delineating hydrocarbon accumulations in HST (High Stand System Tract) turbidite channel of Kafr El-Sheikh (KS) formation (Pliocene Age) located in the western part of South Batra field (SBF), Nile Delta – Egypt is investigated. 3D seismic data and four well logs helped in studying the properties of the channel. Three wells encountered the central part of the channel and showed thicker reservoir thickness than the fourth well, which encountered the levee. The channel is characterized by high porosity (25–30%) and relatively low water saturation (27–50%). The structural map showed that the channel is dipping towards the NE–SW. AVO analysis, scaled Poisson's ratio, post-stack seismic inversion, and seismic attributes confirmed the presence of Class III gas sand. Variance attribute helped in imaging the channel and faults. Maps generated from spectral decomposition showed that the dominant frequency is around 15 hertz. Results of integrating different techniques were consistent and successfully helped in determining the geometry of the channel, discriminating between different lithologies and fluids, in addition to delineating the sweet spots and recommended locations for drilling.

Common Transmission Point (CTP) Gathers: A New Domain for Amplitude Variation with Offset

Analysis of amplitudes of transmitted waves (TAVO) is an extension of the conventional AVO analysis using amplitudes of reflected waves. In this study, we introduce the common transmission point (CTP) gather, which is a new domain that is convenient for TAVO analysis. A CTP gather is formed by binning traces that have the same transmission point across a layer interface. We use the proposed domain to invert the ratios Δαα, Δρρ, Δββ, and βα in a model consisting of a gas channel nestled within an oil reservoir. The TAVO equations are fitted to amplitudes calculated by Zoeppritz equations within CTPs inside and outside the channel. Within each CTP gather, we use all traces with incidence angles less than 90% of the critical angle (if any) as TAVO approximations break down beyond this point. The proposed CTP TAVO analysis method estimated Δαα, Δρρ, Δββ, and βα in the gas channel within 1% of their corresponding true values.

A new AVO fluid indicator using the fluid line: Theory and application

A number of seismic fluid indicators are derived from seismic amplitude variations with offset. The fluid indictors typically measure the anomalous behavior of the seismic response related to hydrocarbon's saturation. This anomalous behavior can be observed in different domains such as P-wave velocity vs. S-wave velocity, P-wave reflectivity vs. S-wave reflectivity, and intercept vs. gradient, among others. In this paper, we introduce a new AVO attribute for fluid detection. The attribute is expressed as a linear combination of AVO's intercept and gradient and measures the deviation caused by pore fluid from the background fluid line defined in the Intercept-Gradient domain. The attribute can be calibrated to local geology for better performance. We test the attribute on data collected from different basins. We compare the fluid detection ability of the attribute against existing fluid indicators commonly used in AVO analysis and interpretation. The new fluid indicator shows promising performance and success in fluid detection. Next, we apply the attribute on synthetic data calculated from well data, and seismic field data from North Western Australia. The target formation is Mid-Jurassic fluvial-deltaic sandstone and is comprised of three reservoir units interbedded in shale. The attribute could help detect the gas-saturated reservoirs and distinguish them from the overall background reflectivity.

Reservoir characterization analysis in glacial reservoirs

This research aims to characterize reservoir properties by applying rock physics and AVO analysis followed by pre-stack inversion. Two approaches are investigated: One approach addresses the case in which there are wells and seismic data, and the other addresses cases where only seismic data are available. The former approach is achieved by using well-log cross-plots for rock physics modeling to determine the feasibility and pay zone through gas fluid substitution followed by AVO analysis. Pre-stack inversion is then used to predict porosity and gas saturation. In the second approach, a synthetic seismogram is generated and compared to the observed seismic trace at the location of interest by forward modeling P-wave interval velocity and density. The best-matching P-wave velocity and density are subsequently used to generate synthetic well logs at the same location. Pre-stack inversion is then performed on these synthetic wells to predict porosity and gas saturation. Property prediction is performed by a feasibility study and pay zone calculation using rock physics modeling of the nearest well to the seismic block. Finally, the results of this case are validated using real wells. This new approach of reservoir characterization using synthetic wells is applied on reservoir channels and yielded a fairly good porosity prediction but a less accurate prediction of gas saturation.

Frequency-dependent Amplitude Versus Offset analysis of a Cenozoic mass-transport deposit on the Namibian slope

Abstract A partially tuned, ‘soft’ seismic reflection, overlying a regional limestone marker on the west African slope, is shown to represent a sequence, up to 30 m thick, within which various individual stratigraphic units are identified, some of which display characteristics of a mass-transport deposit (MTD). The seismic reflection is described in terms of its two-way time structure, reflection amplitude pattern, complex seismic attributes and AVO response. Frequency-dependent aspects of the AVO response, resulting from velocity attenuation and dispersion of seismic signals, are shown to indicate variations in pore-fluid gas concentration. These are exploited to assess lithological, petrophysical and pore-fluid characteristics of the various sub-units within the sequence. A relative acoustic impedance inversion of the seismic data is combined with a low-frequency model, constructed from acoustic velocity data, to approximate absolute acoustic impedance characteristics of the sequence and to model behaviour of the various sub-units across the area. These pseudo-absolute impedance data are combined with evidence from AVO analysis, to construct a 2D model of the MTD unit and to generate a synthetic seismic record, which confirms the interpretation of stratigraphic, petrophysical and lithological properties of the MTD complex.

A New Fluid Mobility Calculation Method Based on Frequency-Dependent AVO Inversion

Fluid mobility (i.e., permeability to viscosity ratio) is a key parameter that can evaluate the reservoir permeability and delineate the fluid characteristic in hydrocarbon-saturated reservoirs. Based on the asymptotic representation for the frequency-dependent reflections in the fluid-saturated pore-elastic media and frequency-dependent AVO inversion, we propose a novel method for estimating fluid mobility from poststack seismic data. First, we establish the relationship between fluid mobility and frequency-dependent AVO analysis. Then, the fluid mobility is estimated using the theory of frequency-dependent AVO inversion. Tests on synthetic data reveal that the fluid mobility shows excellent imageability for the fluid-saturated reservoirs and can accurately delineate the spatial distribution shape of the gas-saturated reservoir. The application of field data examples demonstrates that the fluid mobility calculated by the proposed method produces less background interferences caused by elastic layers compared with the conventional frequency-dependent fluid indicator. The frequency-dependent fluid mobility takes into account the dispersion features associated with hydrocarbon reservoirs, and it provides a new way to detect the location of hydrocarbon reservoirs and characterize their spatial distribution.

Research on prediction method of S-wave velocity based on deep learning

Aiming at the s-wave velocity prediction problem, based on the analysis of the advantages and disadvantages of the empirical formula method and the rock physics modeling method, combined with the s-wave velocity prediction principle, the deep learning method is introduced, and a deep learning-based logging s-wave velocity prediction method is proposed. This method uses a deep neural network algorithm to establish a nonlinear mapping relationship between reservoir parameters (acoustic time difference, density, neutron porosity, shale content, porosity) and s-wave velocity, and then applies it to the s-wave velocity prediction at the well point. Starting from the relationship between p-wave and s-wave velocity, the study explained the feasibility of applying deep learning technology to s-wave prediction and the principle of sample selection, and finally established a reliable s-wave prediction model. The model was applied to s-wave velocity prediction in different research areas, and the results show that the s-wave velocity prediction technology based on deep learning can effectively improve the accuracy and efficiency of s-wave velocity prediction, and has the characteristics of a wide range of applications. It can provide reliable s-wave data for pre-stack AVO analysis and pre-stack inversion, so it has high practical application value and certain promotion significance.

Frequency-dependent AVO analysis: A potential seismic attribute for thin-bed identification

Identifying thinly bedded reservoirs is important in exploration, appraisal, and development. The thin beds that we are referring to are of meter scale or less. Conventional seismic attributes are not able to resolve thin-bed effects. A new frequency-dependent amplitude variation with offset (FAVO) seismic attribute is developed for the prediction of thin beds. It is based on seismic wave scattering theory for vertically inhomogeneous media, in which the seismic response from a laterally homogeneous interval is represented by the Born series. The Born series characterizes the P-P and P-SV wave reflections, transmissions, and interbed multiples within the target interval with different order terms. The newly derived attribute corresponds to the second-order term of the Born series, in which P-S wave reflection and transmission energy is taken into consideration, and it is called the second-order FAVO gradient attribute. In a thinly bedded interval where sand/shale interfaces are abundant, the P-SV wave mode conversions are numerous. This leads to a strong second-order FAVO gradient attribute response compared to that of a blocky sand, silt, or shale interval under the assumption that there is no significant rock- and fluid-property differences between the intervals. Therefore, the new seismic attribute has the potential to be a thin-bed interval indicator. We have adopted a strategy to estimate the new attribute using conventional P-wave seismic data. The attribute is extracted from seismic data at the frequency range at which the second-order scattering effect is the most prominent. Synthetic and field data examples from offshore Trinidad are studied to demonstrate the second-order scattering effect and the potential usage of the second-order FAVO attribute for thin-bed reservoir characterization.

Identification of nonstructural type traps within the limits of northern side of Dnieper-Donets Depression according to the data of AVO analysis and seismic inversion

Nonstructural type traps in the sedimentary cover of the northern side of the Dnieper-Donets depression are poorly studied by seismic methods due to many factors among which are the following: complicated geological structure and not so high quality of given data of geological-geophysical studies of last years. Identification of lithologically screened gas-saturated object has been demonstrated based on the studies of elastic dynamic characteristics by the methods of AVO-analysis and elastic seismic inversion. Acoustic and elastic properties have been analyzed in the wells with cross-dipole real and synthesized acoustics. Gas-saturated intervals have been identified based on the ratios VP /VS and acoustic impedance. According to AVO-studies within the northern side of DDD in the stratum of productive horizons of the moscovian horizon a positive answer has been obtained to the question «if AVO analysis works correctly in general within the limits of the northern side of DDD deposits». Determining factors that influence on the result during the application of AVO-analysis at the most part of gas condensate field of the northern side of DDD are effective thicknesses, depths of occurrence, lithology, poro-sity and quality of given seismic data and data from geophysical surveys of wells. Taking into account minor effective thicknesses, small values of porosity, significant depths of occurrence of productive layers typical for the northern side of DDD, the majority of gas-saturated intervals are not at all identified in the wave field by dynamic characteristics of seismic signal. All the seismic anomalies analyzed had effective thicknesses from 6 to 20 m. Within the limits of deposits of the northern side of DDD, AVO-anomalies of the 2nd class are modeled. Along with positive experience of identification of dynamically cont-rasting objects in the wave field limitations to use methods of AVO-analysis and elastic seismic inversion are generalized for the northern side shown in particular and the whole Dnieper-Donets depression.

Seismic interferometry vs. static corrections in seismic data processing

The attenuation of distortion caused by the shallow layer generates reliable subsoil’s pictures, which static corrections attempt to do, under the assumption of surface consistency. However, such an assumption is frequently invalid, and nowadays, this problem is still unsolved. Besides, the first break handpicking is monotonous, many men hours consuming task and prone to human error, mainly when different persons process a project’s high data volume. This paper presents a new method to redatuming seismic data by transforming the data to the ray path domain, avoiding the consistent surface restriction. As it does not require first break picking, it becomes automatable and less prone to human error. The application of static correction and seismic interferometry to synthetic and real data allowed the comparison between both approaches’ performances. Besides, an AVO analysis of the processed lines allowed knowing how interferometry affects the reflection amplitudes. Seismic interferometry has two steps: transforming the gathers to the ray-consistent domain and constructing a time corrected function and its inverse filter. The convolution of the inverse filter with the gathers generates seismic records free of distortions. The trace mixing constructs the reference wavefield, which has more stable data with a well-being S/N ratio. Compared with static corrections, the interferometry supplies enhanced and reliable subsoil images, particularly in deep and steep reflectors. Additionally, after applying interferometry, AVO analysis of data indicated errors comparable to those of static corrections. On the other hand, seismic interferometry is a more resource-consuming method that requires a high-skill processor.