AVO Analysis Research Articles

Application of spectral decomposition as a direct hydrocarbon indicator within the challenging rock physics environment

A study was conducted to use low-frequency amplitudes obtained through spectral decomposition as a direct hydrocarbon indicator within the geologically and seismically challenging area. An offshore data set from the Gulf of Thailand with well-known geology and good well control for both water and hydrocarbon bearing zones were used in this study. The rocks physics reveals similar P-impedance for gas and water-wet sands. Moreover, organic shales have similar Poisson’s ratio as of gas saturated zones. This makes differentiation of fluids and lithologies through post-stack seismic inversion and AVO analysis difficult. Reservoir wedge modelling, spectral decomposition of the synthetic and field data and analyses were conducted to understand the spectral decomposition response to thickness variation and reservoir fluids. Spectral decomposition was also applied on partial angle stacks (near, mid and far). Gas sands show bright amplitudes at low frequencies (from 10 to 15 Hz). However, spectral decomposition of the synthetic seismogram of the organic shales indicates that it consists of only relatively higher frequencies (greater than 25 Hz). Therefore, low-frequency amplitudes of spectral decomposition may help to identify hydrocarbon saturated zones.

P-Wave Reflection Approximation of a Thin Bed and Its Application

“Thin-bed” reservoirs have become important targets of seismic exploration and exploitation. However, traditional amplitude versus offset/amplitude versus angle (AVO/AVA) technologies, for example, those based on Zoeppritz equations and their approximations for a single interface, are not sufficiently accurate for thin-bed stratigraphy. Analytic solutions of thin-bed reflectivity may become practical for thin-bed AVO analysis and inversion. Therefore, a linear analytic approximation of thin-bed P-wave reflectivity is developed under small-incidence and thin-bed assumptions. Numerical simulations show that the amplitude approximation errors are usually smaller than 10% for incidence angles less than 20 degrees, and the thin-bed thicknesses are less than one-tenth of the P-wave wavelength. Based on the least-squares approach, the inversion strategy is proposed using the approximate formula. A synthetic data test shows that the proposed inversion method can produce more accurate thin-bed properties than that based on the Zoeppritz equations, which reveals the potential of the inversion method based on the linear analytic approximate formula in the fine characterization of thin reservoirs.

Amplitude-Versus-Angle Analysis of Local Angle-Domain Common Image Gathers With Prestack Gaussian-Beam Migration of Seismic Data

The analysis of seismic reflection amplitude versus angle/amplitude versus offset (AVA/AVO) is an important tool for investigating the physical properties of subsurface rocks and/or reservoirs. We propose to generate local angle-domain common image gathers (ADCIGs) from the surface-recorded seismic data for AVA analysis via Gaussian-beam migration (GBM) using a deconvolution imaging condition. The traditional seismic reflection amplitude analysis is often based on the AVO response from common middle point (CMP) gathers. However, the AVO analysis based on the horizontally layered medium assumption can achieve success only in simple geological cases. Our approach provides an AVA response during the migration process that reveals true local physical properties in the image domain even if the image point is at a tilted interface or has a complicated overburden geological structure. We test our approach with a two-layer velocity model with a tilted interface, and with the benchmark Marmousi2 model that has a complicated geological structure. From these two numerical examples, we demonstrate that our method faithfully provides the AVA response that can be used to further interpret the rock/reservoir physical properties.

Symmetry between Theoretical and Physical Investigation of Water Contamination Using Amplitude Variation with Offset Analysis of Ground-Penetrating Radar Data

We evaluated the symmetry of theoretical and experimental analysis of water contamination such as non-aqueous phase liquid (NAPL) by using amplitude variations with offset analysis (AVO) of ground-penetrating radar (GPR) data. We used both theoretical and experimental approaches for AVO responses of GPR to small distributions of contamination. Theoretical modeling is a tool used to confirm the feasibility of geophysical surveys. Theoretical modeling of NAPL-contaminated sites containing wet sand—both with the water and light non-aqueous phase liquid—was applied by keeping in consideration the GPR AVO analysis in acquisition. Reflectivity was significantly altered with the changes in the contents of water and NAPL during modeling. The wet and dry sands introduced in our model changed two major phenomena: one, the wave pattern—implying a slight phase shift in the wave; and two, an amplitude jump with the dim reflection radar gram observed in the model. Experimental data were collected and analyzed; two observations were recorded during physical data analysis. First, relative permittivity confirmed the presence of NAPL in an experimental tank. Second, reflection patterns with jumps in amplitude and changes in polarity confirmed the theoretical investigation. Our results demonstrate that GPR AVO analysis can be as effective for detection of non-aqueous phase liquid (NAPLs) as it has been used to determine moisture contents in the past. The theoretical and experimental models were in symmetry, and both found a jump in reflection strength. The reflection pattern normally jumped with NAPL-intrusion. From the perspective of water contamination, this study emphasizes the need to take into account the impact of GPR AVO analyses along with the expert’s adaptive capacities.

Pre-stack gather optimization technology based on an improved bidimensional empirical mode decomposition method

This study proposes a pre-stack gather optimization technique based on the improved BEMD (bidimensional empirical mode decomposition) method with high stability and adaptability. An original gather signal was decomposed into different components with varying characteristics by the BEMD method, which combines morphological extremum point extraction and the bicubic spline interpolation method to obtain different-scale component signals. Then, wavelet threshold orthogonal filter was used to process the components, which correspond to noise, mostly in small-scale domain. Finally, correlation weighting reconstruction was performed on each component. The weight coefficient is determined according to the relationship between each component of the amplitude-preserving superposition gather and the original gather-preserving superposition gather. The testes on the data from Northeast Sichuan confirm that the method can optimize the pre-stack gather steadily and efficiently. The comparison of the AVO analysis results before and after optimization shows that the method is effective and feasible, which lays a good foundation for a prediction method based on pre-stack data.

SEISMIC EVENTS IN THE UPPER MIOCENE – PLIOCENE SEDIMENTARY SUCCESSION IN THE GULF OF İZMİR (WESTERN ANATOLIA): IMPLICATIONS FOR HYDROCARBON PROSPECTIVITY

An analysis of multi‐channel seismic reflection data integrating reflection tomography, pre‐stack depth migration, AVO analysis, seismic modelling and seismic attribute analysis was used to investigate the Miocene – Quaternary stratigraphy of the Gulf of İzmir, western Anatolia. In this area, the east‐west oriented Gediz graben intersects with the NE‐SW oriented Bakırçay Graben. A velocity‐depth model together with pre‐stack depth migration allowed two seismic stratigraphic units (SSU1 and SSU2) to be distinguished. These units can be correlated with the stratigraphic succession at the offshore Foça‐1 well and correspond to the Upper Miocene to Recent Bozköy, Ularca and Bayramiç Formations with a combined thickness of 1.75 km. The units rest on acoustic basement (SSU3) which has a basin‐and‐ridge morphology, and which corresponds to the Lower‐Middle Miocene Yuntdağ Volcanics. A number of lateral velocity variations were identified. In particular, a ~90 m wide and ~500 m long lenticular‐shaped low‐velocity zone with an interval velocity of 1.68 km/s was identified in the Quaternary Bayramiç Formation. The structure is bounded by negative reflections whose amplitude increases with offset at the top and by strong positive reflections whose amplitude increases with offset at the base, interpreted as possible bright and flat spots respectively. These amplitude events point to the presence of gas‐saturated sediments within the study area. The lenticular structure is bounded by strike‐slip faults on either side, and by a Miocene – Pliocene unconformity surface below and by shales of the Bayramiç Formations above. It is therefore interpreted as a possible structural – stratigraphic trap. The strike‐slip faults may allow the migration of hydrocarbons from source rocks located at greater depths. The presence of a low‐velocity zone above the lenticular structure reaching up to seafloor may indicate the upward leakage of hydrocarbons from the trap. These observations will contribute to future hydrocarbon exploration activities in the study area.

Integration of fluid replacement modeling and seismic AVO analysis towards simultaneous seismic inversion to delineate the distribution of Globigerina limestone gas reservoir

Integration of fluid replacement modelling and seismic AVO analysis towards simultaneous seismic inversion had been done to delineate the distribution of Globigerina limestone gas reservoir in Madura Strait, East Java Basin. This study was using backward modelling or well known as inversion modelling to finish the quantitative analysis. The purpose of the study is to map the distribution of Globigerina limestone gas reservoir so that in the end the final result will be used as a guidance to drill the next exploration well. It started with an optimized low frequency model building by integrating fluid substitution modelling and seismic AVO analysis. By integrating these two methods, the distorted top and bottom reservoir can be compensated, and the low frequency model will be optimized. The availability of the data includes 3D seismic data with acquisition bin size 18.75 m × 12.5 m, with two well logs. The cross plot result showed that the reservoir and non-reservoir lithology can be distinguished by Vp and Vs log. By analysing the acoustic impedance together with Vp/Vs ratio, the character of gas reservoir at surrounding F and H structures can also be delineated. The main cut off for acoustic impedance and Vp/Vs ratio were 5,500 gr/cm3*m/s and 2.0. The final optimizing result of low frequency model showed that the target G structure, which will be the next exploration target, showed an analogue response with its neighbour F and H structures. It is concluded that by integrating both two methods, fluid replacement modelling and AVO seismic analysis, the optimizing of low frequency model can be achieved and the simultaneous seismic inversion can successfully map other gas reservoir.

Frequency-dependent AVO analysis

A method for generalizing the conventional amplitude variation with offset model from an isolated interface to a scattering reservoir interval is presented. The advantage of this new method is that it can provide enhanced detection of subtle reservoir and pore fluid properties. First- and second-order expressions for the reflected compressional wave energy from a specified heterogenous interval are given. These expressions are applied to two problems of interest for reservoir description. One application is discriminating low versus higher saturations of hydrocarbons, and the other is detecting the extent of vertical stratification within a reservoir. The first-order expression is used for determining hydrocarbon saturations, and the second-order expression is used for detecting the magnitude of fine-scale layering within a reservoir. Synthetic models and field data examples are used in demonstrating the applicability of the proposed method.

Frequency-dependent AVO analysis using the scattering response of a layered reservoir

Sensitivity of reservoir properties to broadband seismic amplitudes can be weak, which makes interpretation ambiguous. Examples of challenging interpretation scenarios include distinguishing blocky reservoirs from fining sequences, low gas saturation from high gas saturation, and variable reservoir quality. Some of these rock and fluid changes might indicate stronger sensitivity to amplitudes over narrow frequency bands, which is a characteristic of frequency-dependent amplitude variation with offset (FAVO). We have developed a FAVO model for reservoir characterization, following a seismic scattering phenomenon through a set of isotropic elastic layers. The frequency dependency in our model comes from the time delays due to wave propagation within layers. The FAVO modeled response is a complex-valued amplitude varying with angle and frequency, and it is a function of the seismic velocities and thicknesses of individual layers, along with the conventional AVO response at all interfaces. Our FAVO seismic analysis consists of two main steps: (1) forward modeling using well logs to understand rock and fluid sensitivity to amplitudes to identify tuning frequencies with maximum amplitude excursions and (2) seismic analysis at tuning frequencies. With well-log models, we observed that the frequency-dependent tuning response is primarily dependent on the lithology stacking pattern of a reservoir; in the cases studied, the fluid and reservoir quality have secondary effects on the frequency dependence of the amplitudes. We evaluate synthetic models and field data from the Columbus Basin, Trinidad, to illustrate our frequency-dependent seismic analysis methods. For one of the sandstone reservoirs, a frequency-dependent attribute indicates better spatial resolution of the anomaly than a conventional amplitude extraction. FAVO attributes are complementary to conventional AVO attributes.

3C seismic data processing and interpretation: a case study from Carpathian Foredeep basin

Chałupki Dębniańskie seismic profile 2D–3C is located in Carpathian Foredeep basin, Poland, and is an object of interest for geologists and geophysicists due to the presence of gas-bearing layers. Multicomponent seismic plays a significant role in supporting reservoir analysis related to accumulations of crude oil and natural gas. The purpose of the research was the optimal processing workflow design, which integrated seismic images of three-component 2D seismic line (2D–3C seismic). A complete processing flow for vertical and both horizontal components was conducted to obtain stacks and prestack gathers with preserved amplitude relations (RAP processing). The main issue of the research was the interpretation of S-wave velocity, which was not provided by well log data. The obtained results increased the reliability of seismic interpretation within Chałupki Dębniańskie area. The research provided valuable information regarding amplitude anomalies and helped in the verification of the potential gas accumulations. Several reservoir analysis tools were tested, including seismic attributes and AVO analysis. Conducted research confirmed the existence of reservoir which is characterized by good reservoir parameters.

Verification of bright spots in the presence of thin beds by AVO and spectral analysis in Miocene sediments of Carpathian Foredeep

Twin structural traps that lie within the Miocene strata of the Carpathian Foredeep that are localized above Cierpisz and Mrowla-Bratkowice highs exhibit identical bright-spot seismic anomalies, but only those associated with the Cierpisz high are profitable gas reservoirs. Bright spots can be a result of weak gas or water saturation, but also seismic interference known as tuning effect. For these reasons, it is crucial to differentiate between seismic anomalies. In this article, we present the possibilities of verification of seismic anomalies that occur within the siliciclastic Miocene sediments of the Carpathian Foredeep with the application of AVO analysis and spectral decomposition. AVO methodology enabled to limit the number of anomalies that are present in the post-stack seismic data. These anomalies, however, may also be a result of tuning which is common for the heterolithic sequences in the Miocene sediments of the Carpathian Foredeep. For classification of anomalies in the view of the above, spectral decomposition based on the Basis Pursuit algorithm was applied. Spectral decomposition enabled to divide AVO anomalies in those that are the result of gas saturation and the tuning effect. Gas-saturated zones are characterized by higher spectral amplitudes of the lower frequency range, whereas tuning effect yields higher spectral amplitudes for the higher frequency content. This relation is visible for the data set and enables qualitative differentiation for the set of seismic anomalies.

Free Surface Multiple Removal Using 3D Surface Related Multiple Elimination Technique on 3D Seismic Data from Offshore Niger Delta

In addition to de-spiking, gun delay correction, correction for spherical spreading and earth’s absorption, gun and cable correction, zero-phasing, noise attenuation and correction for time shifts between sail lines due to changes in velocity of the water column, multiple elimination is the other important true relative amplitude processing routine desired to produce seismic gathers consistent with DHI and AVO analysis to de-risk the presence of hydrocarbon interpreted from seismic data. Multiple problems associated with seismic data acquired in the Niger Delta offshore are those due mainly to the free air-water interface and the seabed, and their presence constitute noise in the seismic dataset. In this study, we employed an approach in which we convolved all possible source and receiver peglegs for every multiple event that strikes the free surface irrespective of its path in the subsurface to model the multiple wave field in a 3D sense, in a partially processed pre-migration seismic data acquired in the Niger Delta deep offshore. The method of adaptive subtraction was then used to eliminate the modeled multiples from the dataset. Un-like other demultiple techniques such as tau-p deconvolution and radon, our multiple modeling and subtraction techniques do not require prior knowledge of the subsurface geology in terms of the velocity and reflectivity of the multiple wave field. The aim of the study was to improve the overall quality of the seismic data and signal-to-noise ratio, in addition to the DHI and AVO compliant gathers output from the process. The approach was successful and effective in removal of the free, air-water surface multiples from the dataset.

AVO analysis of a shallow reverse polarity in Sydney Basin

A vast shallow reverse polarity which is extended across the PEP11 block in offshore Sydney basin and the lack of any wells in the area has wondered the interpreters about the mechanisms which has caused such a seismic signature in the area. This study aims to characterize this reverse polarity using the AVO analysis and pre stack inversion techniques in a pilot study. Baleen 2004 2-D towed streamer data are the only recent seismic data acquired in PEP11 block. Shallow depth water has left significant amount of multiple energy in shot records. The presence of a very high velocity sequence of layers which terminate at the shore line cause a strong ray bending letting small amount of the energy penetrate to deeper parts. The reverse polarity reflector is part of a sequence of progradational sediments which are accommodated at the top of the high velocity layer.

Value-add and safe drilling from 4D AVO evaluation in the Enfield oil field, North-West Shelf, Australia

The Enfield oil field, on the Australia?s Greater North- West Shelf, started production in July 2006, and has already seen significant benefit from 4D seismic monitoring. The technique is particularly applicable in Enfield due to faulting and baffles within the reservoir, the need for multiple water injection wells, and the presence of a gas cap. Full-field monitor surveys in February 2007 and December 2008 have made major contributions to reservoir management and identification of infill drilling opportunities. This paper highlights the 2009 development drilling in the field?s Sliver Block, where 4D not only supported the business case for a new production and water injection well, but also provided important information to enhance safe and efficient drilling. 4D AVO analysis involved extraction of changes over various source-receiver offset ranges, driven by a rock property model in which nears and mids tend to show pressure changes, while fars and ultra-fars show saturation changes. A clear 4D AVO signal on the early monitor showed the drilling target area to be pressured up by a distant injector, contrary to pre-4D expectations. The later monitor confirmed this, and also showed the oil target intact ? with no significant water sweep. Using 4D, significant overpressure in the target was predicted. Accordingly, casing schemes for the wells were redesigned to enable safe reservoir entry; the distant water injector was shut in prior to drilling; and appropriate heavy brine was ordered for completion operations. Actual pressures fell within the predicted range, and a safe and successful drilling campaign was achieved.

AVO Didactic Analysis from Simplified Modeling of Seismic Data in Libra Oil Field of the Pre-Salt Region, Brazil

In this study, a certain zone of Libra oil field of the presalt region was selected to be analyzed for didactic purposes in order to simulate AVO responses. The region was modeled using seismic data, setting up a typical oil system with source rock (shale), reservoir rock (sandstone), and seal rock (salt). Thus, a set of parameters such as seismic wave velocity, density and Poisson’s ratio for the studied zone was proposed. The AVO analysis allowed the classification of reflectors and the distinction of the geology for sandstones saturated with both water and gas.

Влияние неупругости среды на AVO-анализ в сейсморазведке

Влияние неупругости среды на AVO-анализ в сейсморазведке

"Impact of AVO Analysis on Murree and Sakesar Formations in the Upper Indus Basin in Pakistan a Case Study"

Seismic data is extremely beneficial to envisage the subsurface lithologies. However, only seismic data is not sufficient to stipulate the gas reservoir. In this paper, by reviewing many signposts in the development of AVO methodology regarding the principles of AVO analysis, we applied AVO analysis to two formations (Murree and Sakesar). We have endeavored to cover the critical formulae and the most current tools in AVO analysis. After comparing multiple methods, we determined that the Aki and Richards equation is the best technique for the AVO analysis in this area because it provides the most accurate results. Because the Aki and Richards approximation is deviates at all angles, it will show the deviation when the angle is either small or large. Here deviation means that the curve plotted are matching to the Zoeppritz curve or not.

Estimation of Seismic Anisotropy Parameter and AVO Modeling of Field “G”

Conventional seismic model assumed that seismic wave propagates within subsurface rock at the same velocity in all direction. The rock assumed as medium isotropic. In fact, the earth has a complex structure. The seismic wave actualy does not propagate as ideal as within isotropic medium. Seismic anisotropy is defined as dependence of seismic velocity upon angle. The purpose of this study is to estimate the anisotropy parameters or Thomsen parameters of ε and δ for both reservoir and shale layer. The parameters have been estimated on the field “G”. It was done using well logs from 7 wells, including P-wave sonic log and the inclination angle of the wellbore. The velocity from the sonic log and the inclination angle were applied to a second order polynomial equation, wich includes the anisotropy parameters. The Matlab software was utilized to perform the calculations of anisotropy parameters, while Microsoft Excel was utilized to plotting velocity versus inclination angle. The result of anisotropy parameters at reservoir is -0.2 - 0.8 for ε and - 1.52 - 0.4 for δ. While the result at shale layer is -1.77 - 1.81 for ε and -1.31 - 1.44 for δ. The parameters were also applied in an AVO analysis for all wells to perform both the anisotropic and isotropic cases. The anisotropic was compared to the isotropic rocks by plotting reflection coefficient with incidence angle. Plotting the comparison shows that the difference between the isotropic and anisotropic case will begin to appear at small incidence angle and will be more noticeable at large incidence angle or at far offset.

Study of Converted-Wave Modeling: AVO Application for Shallow Gas Models

The application of converted-wave seismic method in hydrocarbon exploration has increased significantly. Since the conventional seismic ceases to provide an adequate result in complex geology area and it provides an ambiguous brightspot response. The main principle is that an incident P-wave produces reflected and converted P and SV wave when the downgoing P-wave impinges on an interface. Converted-wave seismic uses the multicomponent receiver that records both of vertical component and horizontal component. The vertical component is assumed to correspond to the compressional PP wave and the horizontal correspond to the PS converted-wave. In this research, a synthetic model with the shallow gas and the salt dome below are constructed. The purpose of this study is to analyze the brightspot due to the presence of shallow gas and its effect to the quality of PP and PS wave reflection below the gas zone. To achieve the goal, both vertical and horizontal seismic data processing are performed. In horizontal data processing, the best gamma function (Vp/Vs) value is estimated to produce the better and reliable image. The result shows that the brightspot response in conventional data doesn’t exist in converted-wave data and the imaging below the gas zone in converted-wave data is better than the conventional due to the attenuation and diffraction effect that caused by gas column. Processing is followed by AVO analysis to compare the AVO response of PP and PS data in characterizing gas reservoir. Both PP and PS AVO curve shows the consistency with synthetic AVO from well data. Gas reservoir is a class 1 AVO anomaly with positive intercept and negative gradient on PP data. However, PS AVO curve does not refer any anomaly. It is because S-wave is not sensitive to the existence of rock saturant.

Impact of frequency-dependent anisotropy on azimuthal P-wave reflections

Seismic anisotropy has widely been used for the characterization of fractures in a reservoir. Recent work has demonstrated the effect of seismic dispersion on producing a frequency-dependent reflection coefficient, which can be important in fracture characterization since large fractures often lead to frequency-dependent anisotropy. In this paper, we examine the impact of anisotropic dispersion on P-wave reflections based on an HTI sand model overlaid by VTI shale. Although VTI in the overburden does not lead to azimuthal anisotropy, its effect on angle dependence could significantly affect the azimuthal AVO responses at far offsets. We show a modest effect on the amplitude and large effect on the phase, the latter of which could even be mistaken for azimuthal velocity variations. We present a Bayesian inversion based on a forward modelling technique aimed at recovering water saturation, fracture density and fracture length of a HTI sand. Our results show potential of using seismic dispersion in azimuthal AVO analysis to discriminate large-scale fractures from micro-scale cracks.