AVO Inversion Research Articles

Joint AVO inversion in the time and frequency domain with Bayesian interference

Amplitude variations with offset or incident angle (AVO/AVA) inversion are typically combined with statistical methods, such as Bayesian inference or deterministic inversion. We propose a joint elastic inversion method in the time and frequency domain based on Bayesian inversion theory to improve the resolution of the estimated P- and S-wave velocities and density. We initially construct the objective function using Bayesian inference by combining seismic data in the time and frequency domain. We use Cauchy and Gaussian probability distribution density functions to obtain the prior information for the model parameters and the likelihood function, respectively. We estimate the elastic parameters by solving the initial objective function with added model constraints to improve the inversion robustness. The results of the synthetic data suggest that the frequency spectra of the estimated parameters are wider than those obtained with conventional elastic inversion in the time domain. In addition, the proposed inversion approach offers stronger antinoising compared to the inversion approach in the frequency domain. Furthermore, results from synthetic examples with added Gaussian noise demonstrate the robustness of the proposed approach. From the real data, we infer that more model parameter details can be reproduced with the proposed joint elastic inversion.

Resolution enhancement of robust Bayesian pre-stack inversion in the frequency domain

AVO/AVA (amplitude variation with an offset or angle) inversion is one of the most practical and useful approaches to estimating model parameters. So far, publications on AVO inversion in the Fourier domain have been quite limited in view of its poor stability and sensitivity to noise compared with time-domain inversion. For the resolution and stability of AVO inversion in the Fourier domain, a novel robust Bayesian pre-stack AVO inversion based on the mixed domain formulation of stationary convolution is proposed which could solve the instability and achieve superior resolution. The Fourier operator will be integrated into the objective equation and it avoids the Fourier inverse transform in our inversion process. Furthermore, the background constraints of model parameters are taken into consideration to improve the stability and reliability of inversion which could compensate for the low-frequency components of seismic signals. Besides, the different frequency components of seismic signals can realize decoupling automatically. This will help us to solve the inverse problem by means of multi-component successive iterations and the convergence precision of the inverse problem could be improved. So, superior resolution compared with the conventional time-domain pre-stack inversion could be achieved easily. Synthetic tests illustrate that the proposed method could achieve high-resolution results with a high degree of agreement with the theoretical model and verify the quality of anti-noise. Finally, applications on a field data case demonstrate that the proposed method could obtain stable inversion results of elastic parameters from pre-stack seismic data in conformity with the real logging data.

AVO inversion in exploration — Key learnings from a Norwegian Sea prospect

AVO inversion of prestack seismic data, constrained by geologic knowledge and rock-physics modeling, is an essential technology that can greatly reduce interpretation risk during exploration. However, in spite of its great potentials, this technology suffers from a wide range of pitfalls and many uncertainties. In this study, we demonstrate the use of simultaneous AVO inversion in an area of the Norwegian Sea where several proven discoveries with strong AVO anomalies are present, all of which are structural traps. The inversion successfully delineated these discoveries, and a pure blind test of a prospect on a structural high correctly predicted the presence of hydrocarbons. The AVO inversion also gave support for a stratigraphic trap in a graben setting between the prolific structural highs. However, this prospect turned out to be a failure, as the observed AVO anomaly was false. Postdrill analysis showed that a thin, very hard, calcareous event right above the target had created a refraction that interfered with the target horizon below, creating a false AVO class II to III anomaly. The target interval was predominantly a very soft, thick, and immature organic-rich shale, which normally should show a class IV AVO anomaly. Moreover, the low-frequency model used in the inversion was derived from wells on structural highs combined with interval velocities and turned out to not be representative in the graben areas. The key learnings from this study are not to use angle ranges exceeding about 40° during AVO inversion in this area and, in general, to be careful extrapolating the low-frequency model away from well control, even when interval velocities are used, especially in areas with complex geology. More integration between geology and geophysics is necessary during all steps of the AVO inversion to avoid such pitfalls.

Introduction to this special section: AVO inversion

The goal of AVO inversion is to estimate subsurface rock properties using seismic P-P reflection data as input. This article serves as a nonmathematical introduction to the topic. The reader will not find any equations here. Instead, we will use words and diagrams to introduce AVO inversion at a conceptual level and then set the context for the articles that have been contributed to this special section on AVO inversion.

Prestack 3D and 4D seismic inversion for reservoir static and dynamic properties

We present a successful case study in which prestack 3D and 4D simultaneous AVO inversion is used in conjunction with rock-physics analysis to estimate saturation and pressure changes in a West Africa brownfield using time-lapse (4D) seismic as input. We show that, in 4D seismic, there can be many competing production effects that can be difficult to disentangle using traditional 4D interpretation methods, such as amplitude differences between the base and monitors. This begs the need for a more sophisticated approach to decouple these competing effects, such as the use of prestack simultaneous 3D and 4D inversions. Multiple substack seismic data are used to estimate a variety of 3D and 4D petroelastic attributes for mapping static and dynamic reservoir properties with the primary objective of influencing the continuous infill drilling and the overall reservoir management strategy. Facies-specific low-frequency models were used as priors for the 3D inversion, while velocity changes from time-lapse time shifts were used as priors for the 4D inversion. We also demonstrate the use of rock-physics templates coupled with a lithology-specific Gassmann fluid-substitution method to establish a nonlinear regression-based rock-physics model that obeys bound theory from classical rock physics and honors single and multimineral fluid-substitution theory. The resulting templates, when integrated with the prestack AVO-inversion technique, produce a set of attributes that accurately explain the time-lapse production effects observed on seismic.

AVO INVERSION WITH THE INVERSE OPERATOR ESTIMATION ALGORITHM

AbstractSeismic inversion is generally implemented with certain optimization algorithm. However, the inverse operator estimation algorithm proposed in this study is to perform the inversion of data matrix directly under the hypothesis that the inverse mapping exists in the empirically constrained subspaces. The key point of the proposed approach is to search those subspaces instead of searching for the solution indirectly as optimization algorithms do and it's more efficient. AVO/AVA (amplitude variation with offset or angle) inversion is widely utilized in exploration geophysics, and the inversion process is restricted by the quality of seismic data. L1 norm is applied in the construction of the kernel function of inversion by combining the constraint from initial models, which is helpful in enhancing the efficiency and stability of the inversion. Model and field data examples indicate that the proposed AVO inversion algorithm based on inverse operator estimation is more accurate and reliable.

Bayesian AVO inversion to rock properties using a local neighborhood in a spatial prior model

The spatial structure of the subsurface is an important factor when interpreting seismic data. The Bayesian methodology is a valuable tool for integrating these spatial relations in the inversion process as it merges the information together and assesses the uncertainty of the model. In the everyday use of the Bayesian methodology, however, the computational cost is a challenge. We describe a new approach that utilizes a local neighborhood to include the spatial constraints and assess the uncertainties in the inversion using fast and parallelizable computations. The approach is applicable for both discrete lithology-fluid prediction and estimation of rock properties, such as porosity and saturation.

Analysis and improvement for a linearized seafloor elastic parameter inversion method

AVO inversion is an effective seismic exploration method to predict elastic parameters. In this paper, we review and analyze the linearized AVO inversion method previously published for seafloor elastic parameters, and present a modification strategy. Before the linearized inversion is performed, a proper near-angle range in which the relationship between the reflection coefficient and sine-squared incidence angle is linear needs to be provided. However, the near-angle range is determined by the elastic parameters which are to be estimated by inversion. Therefore, only an approximated value of the near-angle range can be provided for the linearized inversion. Model tests show that a too large near-angle range may cause inversion fault, and a too small near-angle range may cause unreliable estimation. Further analysis shows that the estimation stability can be further improved even though the linearized inversion is performed under an exact near-angle range. To mitigate the strong dependence on the near-angle range, we use the seafloor elastic parameters estimated from the linearized method as the initial model for an unconstrained optimization method. Compared with the previously published method, the modified method is more robust to noisy data and shows less dependence on the near-angle range.

Least-squares migration — Data domain versus image domain using point spread functions

Conventional amplitude inversion assumes that the migrated image preserves relative-amplitude information. However, illumination effects caused by complex geologic settings, undersampled acquisition geometry, and limited recording aperture pose a challenge to even the most advanced imaging algorithms. In addition, standard depth-migration images can suffer from lack of resolution caused by wavelet stretch, attenuation, and suboptimal deghosting. Least-squares migration (LSM) can mitigate many of these problems and produce better resolved migration images suitable for AVO inversion. However, whether formulated in the data domain or the image domain, LSM is an inversion algorithm and is sensitive to inaccuracies in the source wavelet, velocity model, data preprocessing, and the propagator used. Practical considerations to mitigate these problems under nonideal conditions and cost-reduction strategies differ between the data- and image-domain formulations. The relative merits of each approach are evaluated by using example inversions for complex synthetic models, including free-surface ghost and attenuation effects. When a data-domain implementation of LSM is considered necessary, the image-domain implementation should be considered at the same time, especially when targeting localized reservoir targets under complex overburdens. Application of image-domain least-squares migration on a Gulf of Mexico field data set produces significant improvements in resolution and event continuity in the subsalt target region.

Is there a benefit to throwing the kitchen sink at geotechnical studies in an exploration phase?

In short, yes. This case study illustrates that the application of a thorough geotechnical workflow incorporating many new and advanced techniques can assist in exploration business case decision making. Is an exploration drilling decision made lightly? A workflow incorporating 3D seismic processing, AVO inversion and stratigraphic framework studies involving high-resolution biostratigraphic and chemostratigraphic analyses was used to assess the prospectivity of an exploration permit near giant gas fields in the offshore Northern Carnarvon Basin. The primary reservoir is the prolific Triassic Mungaroo Formation fluvio-deltaic sediments, and secondary reservoirs include mid-Jurassic marine sands. 3D seismic reprocessing xcombined a newly acquired broadband seismic dataset into a multi-survey multi-azimuth PSDM volume that conditioned data for input to an AVO inversion. New petrophysics and rock physics analysis and modelling on regional well data were then calibrated with the AVO inversion to statistically derive lithology and fluid prediction volumes. These data were used in conjunction with reservoir paleo-stratigraphy studies to derive a subsurface model for reservoir distribution and hydrocarbon prediction. A two-stage risking process was applied to each prospect that objectively applied risk based on the seismic amplitudes. This enabled a more accurate risked-volume assessment, combined with the ability to assess a prospect portfolio covering different plays. The resultant interpretation identified issues with interpretations made on vintage data that would not have been easily identified without undertaking these studies. The integration of these assessments resulted in an unfavourable exploration drilling business case and a decision not to renew the permit.

Unconventional resource evaluation and applied geophysics utilising LMR

Over the last decade the oil and gas industry has delivered conceptual and technical changes that have entirely changed the fundamentals of natural gas supply in North America. Underpinning the step change in natural gas reserves and market ready supplies has been the change in the perception of fine-grained, organic rich rocks (i.e. shales – although of course not all shales are organic rich). No longer are such rocks viewed only as source and seal candidates, but also as source rock reservoirs or shale gas plays. Although the geological continuity of shale gas plays have led to the production-line style operations seen across North America in mature unconventional plays, it is not factory-style efficiency improvements in isolation that allow the economic exploitation of shale gas. The large number of fit-for-purpose technologies, introduced by operators and service companies, has been critical in increasing production while keeping costs flat and/or reducing costs and time to production. 3D seismic data play a key role in unconventional developments as a unique look-ahead dataset. The role of seismic, however, has evolved to be far more than simply a tool for mapping major structures. For example, through AVO inversion we are able to make predictions regarding elastic properties of the formations of interest. The integration of AVO inversion data with engineering and rock physics data is providing new avenues of data exploitation. Seismic data are also being used to predict closure stress and stress anisotropy, which can be calibrated with data and analysis from hydraulic fracturing. Additionally, the integration of surface seismic data with microseismic provides a means of fine-tuning the estimation of stimulated rock volume.

Seismic Screening for Hydrocarbon Prospects using Rock-Physics Attributes

Rock-physics templates (RPT), in combination with seismic AVO inversion data, can be used to screen for hydrocarbon prospects during exploration. With the improved quality and increased use of elastic seismic inversion, there has recently been a paradigm change in prospect mapping in the oil industry, and quantitative interpretation has become a widely used jargon. Rock-physics models are essential in that they help in converting elastic parameters from inversion data to reservoir parameters. Rock physics models plays an important role in many of the stages of seismic AVO inversions; including the petrophysical log evaluations, well ties and wavelet estimation, the setting of parameter constraints during the inversion, and the interpretation of the inversion results in terms of reservoir properties. Rock-physics models can also be used to quality check and modify the low-frequency model used in seismic AVO inversion, and to assess the quality and uncertainties of inverted elastic parameters. In this presentation we will demonstrate the use of rock-physics models during the different stages of seismic inversion, and how these can improve our ability to reveal hydrocarbon-associated anomalies.

Pre-stack Simultaneous AVO Inversion and Lithology Prediction of Shale Gas Reservoir

Pre-stack Simultaneous AVO Inversion and Lithology Prediction of Shale Gas Reservoir

Seafloor elastic parameters estimation based on AVO inversion

Seafloor elastic parameters play an important role in many fields as diverse as marine construction, seabed resources exploration and seafloor acoustics. In order to estimate seafloor elastic parameters, we perform AVO inversion with seafloor reflected seismic data. As a particular reflection interface, the seafloor reflector does not support S-waves and the elastic parameters change dramatically across it. Conventional approximations to the Zoeppritz equations are not applicable for the seafloor situation. In this paper, we perform AVO inversion with the exact Zoeppritz equations through an unconstrained optimization method. Our synthetic study proves that the inversion method does not show strong dependence on the initial model for both unconsolidated and semi-consolidated seabed situations. The inversion uncertainty of the elastic parameters increases with the noise level, and decreases with the incidence angle range. Finally, we perform inversion of data from the South China Sea, and obtain satisfactory results, which are in good agreement with previous research.

Support Vector Machine (SVM) based prestack AVO inversion and its applications

AVO inversion can be used to estimate P-wave velocity, S-wave velocity, and density perturbations from reflection seismic data. The inversion of the density term, however, due to its little sensitivity to amplitudes and the paucity of large angle incident information, is usually difficult and unstable. The conventional method of linearized approximation is usually not accurate enough and tends to be affected by the background information, while the accurate method is more likely to be trapped in a local minimum and more computationally intensive. This paper delineates a novel method of AVO inversion based on Support Vector Machine (SVM). First, we describe the basic principle of SVM, and then we investigate an SVM procedure for the three-term AVO inversion problem. To demonstrate its performance, we compare it with the conventional Bayesian method. From the inversion results of both the synthetic and real data, we conclude that the algorithm of SVM leads to high-resolution P-wave velocity, S-wave velocity, and density perturbation, moreover, the resolution of the density term has large improvement, compared to the Bayesian method. They all demonstrate the feasibility and application of SVM on both synthetic and real data.

Sequence stratigraphy and reservoir potential in the Wenan inner-slope of the Baxian sag, Bohai Bay Basin, eastern China

The high-resolution sequences, depositional environments, and lithostratigraphic trapping styles of an Eocene lacustrine rift succession in the Wenan inner-slope of the Baxian sag in Jizhong depression of Bohai Bay Basin were studied using newly processed pre-stack seismic 3-D data, complemented by well logs and cores of new drilled well XL1. Four third-order sequences were identified in the study interval (the 3rd and 4th members of the Shahejie Formation). The lower two sequences mainly contain transgressive system tracts (TST) and highstand system tracts (HST), whereas the upper sequence only contains lowstand system tracts (LST). Seismic stratal slices demonstrate that the deltaic deposits were widely developed in the Wenan inner-slope. AVO inversion showed that the net sandstone of the study stratigraphic units decreased in thickness in the depositional-dip-oriented direction. A new potential reservoir model of the 4th member of the Shahejie Formation was built for the development of abundant undiscovered lithostratigraphic reserves and four major lithostratigraphic trapping styles were identified.

An Amplitude-normalized Pseudo Well-log Construction Method and its Application on AVO Inversion in a Well-absent Marine Area

AVO inversion is hard to be efficiently applied in unexploited fields due to the insufficiency of well information. For the sake of AVO inversion in a well-absent area, the most conventional method is to construct pseudo well-logs by defining seismic processing velocity as the P-velocity and computing S-velocity and density using empirical formulas, yet the resolution of the corresponding earth models and final inverted results could be extremely low, and a rough formula could destroy the inversion thoroughly. To overcome this problem, an amplitudenormalized pseudo well-log construction method that reconstructs pseudo well-logs in accordance with computed P-wave reflection amplitudes and nearby drilling data is proposed in this paper. It enhances the inversion resolution efficiently with respect to the real elastic parameter relationships, so that the corresponding AVO inversion results are reasonably improved. In summary, the proposed method is successfully applied in the AVO inversion of a well-absent marine area, and could be valuable in the early phase, particularly of the offshore hydrocarbon exploration.

Using eff ective refl ection coeffi cients for long-off set AVO inversion of PP-refl ections

Using eff ective refl ection coeffi cients for long-off set AVO inversion of PP-refl ections

Carbonate reservoirs dominated by secondary storage space: Key issues and technical strategy

Carbonate reservoirs of the Tarim Basin in China are buried deeply, with strong later diagenesis, and seldom are controlled by geologic facies. The effective storage spaces are mainly dissolution caves and fractures with few primary pores. Fluid distribution in the reservoir is extremely complicated. All these characteristics make it a world-class challenge for oil and gas exploration and production. Conventional poststack methods ignore the complexity of the reservoir medium with low accuracy of prediction results, by which the location of the dissolution cave is determined roughly. Only faults and fracture zones can be described to a certain degree instead of quantifying the fracture detection. Aiming at the key problems of the complex carbonate reservoirs in the Tarim Basin, a series of techniques has been developed to improve the successful rate of reservoir prediction, including the following aspects: the wide-azimuth and high-density data-acquisition technique to improve the quality of the original seismic data; a series of migration techniques including prestack reverse time migration; and amplitude-preserved Q migration to improve the quality of seismic gathers and the imaging precision with the matching precision of AVO gathers. The target reservoir data dominant frequency increased by more than 15% and by nearly 20 Hz. In addition, the effective DEM-Gassmann rock-physics model is proposed for velocity prediction, by which relative error of estimated shear-wave velocity reaches to less than 5%. Furthermore, an advanced inversion algorithm is introduced to improve the accuracy of extracted elastic information. By using new prestack inversion and frequency-dependent AVO inversion, the tie rate of the dissolution-cave reservoir and fluid prediction increased by more than 15%. Furthermore, the fracture-prediction tie rate can be enhanced by 20% through limited-azimuth anisotropic inversion, on the basis of which the quantitative identification of fracture fluid for actual seismic data can be realized.

Multi-disciplinary integration of seismic interpretation, AVO inversion and CSEM in the West Loppa exploration

Mehdi Paydayesh, Margaret Leathard, Federico Ceci and Ajai Kumar Sharma describe the integration of multiple geophysical methods for reducing uncertainty in identifying hydrocarbon potential during the predrill appraisal of a prospect in the West Loppa area of the Barents Sea.