Amplitude Versus Offset Analysis Research Articles

Three-Dimensional Amplitude versus Offset Analysis for Gas Hydrate Identification at Woolsey Mound: Gulf of Mexico

The Gulf of Mexico Hydrates Research Consortium selected the Mississippi Canyon Lease Block 118 (MC118) as a multi-sensor, multi-discipline seafloor observatory for gas hydrate research with geochemical, geophysical, and biological methods. Woolsey Mound is a one-kilometer diameter hydrate complex where gas hydrates outcrop at the sea floor. The hydrate mound is connected to an underlying salt diapir through a network of shallow crestal faults. This research aims to identify the base of the hydrate stability zone without regionally extensive bottom simulating reflectors (BSRs). This study analyzes two collocated 3D seismic datasets collected four years apart. To identify the base of the hydrate stability zone in the absence of BSRs, shallow discontinuous bright spots were targeted. These bright spots may mark the base of the hydrate stability field in the study area. These bright spots are hypothesized to produce an amplitude versus offset (AVO) response due to the trapping of free gas beneath the gas hydrate. AVO analyses were conducted on pre-stacked 3D volume and decreasing amplitude values with an increasing offset, i.e., Class 4 AVO anomalies were observed. A comparison of a time-lapse analysis and the AVO analysis was conducted to investigate the changes in the strength of the AVO curve over time. The changes in the strength are correlated with the decrease in hydrate concentrations over time.

Second-Order Approximate Reflection Coefficient of Thin Interbeds with Vertical Fractures

The horizontal fractures in the strata will close in the compaction effect of overlying strata, while the vertical cracks are widely developed, which can be equivalent to HTI (transverse isotropy with a horizontal axis of symmetry) medium. When an S-wave propagates into HTI media, the shear wave will divide into two types of waves: a fast S-wave and slow S-wave. When the strata of HTI are thin and overlapping, called the thin interbeds model, the wave field exhibits complex primary reflections, converted waves, and multiples. We introduce a new second-order approximation of the total reflection coefficient, with the incidence angle lower than the critical angle in thin-interbed HTI media using a recursive algorithm. We verify the effectiveness of the second-order approximation by analyzing the energy of multiples. Comparing the second-order approximate solution that degenerates the HTI medium into isotropic and Kennett’s exact solution, we find that our solution has an accuracy of over 99.9% in any azimuth, with the incidence angle lower than the critical angle under P-wave incidence. However, our solution of the SP wave field is suitable for incidence azimuth angles between 0–75° and 120–180°, with the lowest accuracy occurring at an incidence angle of 25° and a relative error of 6.4%. The approximate solution in the SS wave field has the same applicable range as the SP wave, with the maximum error of 6.3% occurring at the incident angle of 1°. This new second-order approximate formula for the total reflection coefficient of thin interbeds composed of HTI helps us to understand the reflection characteristics of complex thin interbeds. It also lays a theoretical foundation for the development of AVO (Amplitude Versus Offset) analysis and inversion techniques for lithological and stratigraphic oil and gas reservoirs.

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.

Fluid Accumulation Zone by Seismic Attributes and Amplitude Versus Offset Analysis at Solfatara Volcano, Campi Flegrei, Italy

The imaging of volcanic systems is a challenging topic that attracts the scientific community’s attention. The characterization of structures and rock properties by means of seismic active methods is becoming fundamental for providing ultra-high-resolution images of the structures of interest. The Solfatara Volcano is a quiescent volcano in the Campi Flegrei resurgent nested caldera that is continuously under investigation and monitoring for its shallow activity, such as fumaroles. The purpose of this work is to characterize the fluid accumulation zone in the first 150 m depth in the middle of the crater, using several post-stack seismic attributes and Amplitude Versus Offset (AVO) analysis to characterize the contact between the CO2 and condensed water in the shallower accumulation zone. The two 400 m-long profiles to which we refer in this work have been acquired during the active Repeated InduCed Earthquakes and Noise experiment. The profiles were deployed along with the NNE-SSW and WNW-ESE directions across the whole surface of the crater including the main surface anomalies of the fumaroles, in the eastern area, and the mud-pool of Fangaia, located in the western area. The seismic pre-processing, pre-stack processing, and post-stack analysis previously applied on the NNE-SSW profile are here performed for the first time on the WNW-ESE profile, while partial-stack AVO analysis is performed for both profiles. The post-stack attributes including time gain, envelope, energy, and root mean square have been computed and extracted for determining the maximum and minimum values of amplitude zones on the migrated post-stack seismic profiles. Such anomalies are provided by complex and geometrical attributes embedding information on faults and chaotic zones. The AVO technique has also been used as a direct gas indicator to enhance fluid discrimination and identification. Finally, the analysis of the profile, seismic attributes, and near-surface structural interpretation related to the Solfatara Volcano has been incorporated into the proposed analysis. The multi-2D image depicts fluids trapped in the Solfatara Volcano at depths ranging from 10 to 50 m below the crater’s surface, as well as their migration paths up to 150 m deep: this evidenced contact between the fluids has been probably due to the solfataric alteration of the minerals, caused by the arising plume and the abovecondensed water which decreases the permeability of the rocks and forms an argillic phase working as cap-rock and trapping the gases. The application of the AVO analysis, coupled with the seismic attribute’s investigation, provides a very detailed multi-2D image of the shallower Solfatara Volcano, which outperforms in terms of accuracy the ones obtained with different tools in previous works, and that evidences the presence and the position of the liquid and the gases in the north-east area of the Solfatara Volcano.

Detection of hydrocarbon- saturated reservoirs in a challenging geological setting using AVO attributes: A case study from Poseidon field, Offshore Northwest region of Australia

Amplitude Versus Offset (AVO) methods relate seismic amplitude variations to subsurface pore fluid and lithology changes. Over the past decades, numerous AVO attributes have been extracted from seismic data and combined to detect seismic expressions associated with hydrocarbon-charged sediments. In this paper, we use an AVO attribute combination composed of gradient and scaled-Poisson reflectivity (SPR) to detect hydrocarbon expressions. Irrespective of sand-shale impedance contrast, the SPR and the gradient G produce negative anomalies for shale over gas saturated reservoir. We demonstrate that SPR-G product is a good alternative to the Intercept-Gradient product which works only for unconsolidated sands. The dataset used in this study is from Poseidon field, North Western Australia. The gas reservoirs belong to the Middle Jurassic-aged Plover Formation. The Plover Formation comprises sandstone, mudstone, and coal that deposited in a fluvial-deltaic environment. AVO analysis at a well location indicates the reservoirs belong to AVO Class II, which is characterized by small zero offset reflectivity and an anomalously large G. Due to the low impedance contrast between the reservoirs and the shale intervals, the I × G product failed to detect the fluid expressions of the reservoirs. The SPR × G product and SPR-G crossplot showed better fluid detection and lithology discrimination. Furthermore, the computation of the product SPR × G volume helped highlight the fluid response and spatial distribution of the reservoir units. New prospective undrilled areas could be identified using the SPR-G product.

Quantitative seismic characterization for gas hydrate- and free gas-bearing sediments in the Shenhu area, South China sea

During China's first gas hydrate drilling expedition, a gas hydrate-bearing layer (GHBL) of ≈25 m thickness was identified above a bottom-simulating reflection (BSR) at site SH2 in the Shenhu area, South China Sea. This study considers the GHBL and the underlying free gas-bearing layer (FGBL) as a whole and evaluates them using quantitative seismic characterization. The reprocessed seismic data, correlated to well log data, reveal the BSR to be accompanied by newly-identified reflections of positive polarity from the top of the GHBL and the base of the FGBL. Of note are phase reversals along individual reflections that cross the top of the GHBL and the base of the FGBL. Phase reversals at the top of the GHBL are a previously little known seismic feature, and seismic forward modeling shows them to provide a direct indicator for the presence of gas hydrate. Amplitude versus offset (AVO) analysis indicates that the two new reflections have characteristics distinct from those of the BSR. Gas hydrate and free gas saturations derived by AVO analysis and seismic impedance inversion, respectively, are consistent with values estimated from sonic velocities, chloride anomalies and pressure cores. The saturations calculated from acoustic impedance inversion show that both gas hydrate and free gas of high saturation are discontinuous, and that free gas has a clear stratified distribution. The distribution of gas hydrate is generally consistent with that of free gas below the BSR, indicating that gas hydrate near site SH2 is mainly controlled by free gas in the underlying FGBL. The quantitative interpretation presented in this study provides a new and reliable method for the characterization of gas hydrate-bearing sediments in areas with no wells, and for the pre-drilling assessment of free gas hazard.

Geological Characterization of the 3D Seismic Record within the Gas Bearing Upper Miocene Sediments in the Northern Part of the Bjelovar Subdepression—Application of Amplitude Versus Offset Analysis and Artificial Neural Network

As natural gas reserves are generally decreasing there is a need to successfully characterize potential research objects using geophysical data. Presented is a study of amplitude vs. offset, attribute and artificial neural network analysis on a research area of a small gas field with one well with commercial accumulations and two wells with only gas shows. The purpose of the research is to aid in future well planning and to distinguish the geophysical data in dry well areas with those from an economically viable well. The amplitude vs. offset analysis shows the lack of anomaly in the wells with only gas shows while the anomaly is present in the economically viable well. The artificial neural network analysis did not aid in the process of distinguishing the possible gas accumulation but it can point out the sedimentological and structural elements within the seismic volume.

Determining the petroleum potential of sandstone reservoir GX field based on AVO analysis

Amplitude Versus Offset (AVO) is an effectively used seismic method in O&G exploration and production. Many studies indicated that the combination of AVO and seismic inversion allows reducing risks in the identification of gas-bearing sandstone reservoirs. Based on the input data including Pre-stack Time Migration, Post-stack Depth Migration and well logging at well 2X, the paper presents the method and procedure of analyzing AVO properties at GX field, block 05-2, Nam Con Son basin. The paper showed the process of building synthetic seismograms with common depth points, sketching graphs to analyze the behavior of AVO properties using intercept A and gradient B values calculated from the well logging data at a defined well, and predicting AVO’s behavior when the lithological and fluid properties in the reservoir change. Therefore, the abnormal section of the reservoir is constructed to determine the lithological features of the reservoir as well as the fluid within the GX field. The interpreted results reveal that the research object is a gas-bearing sandstone reservoir with the boundary in the time domain of 2920ms - 2936ms (or depth 3582.9m - 3616.1m), which is the foundation for determining the reservoir's potential scale.

New AVO Attributes and Their Applications for Facies and Hydrocarbon Prediction: A Case Study from the Northern Malay Basin

Amplitude versus offset (AVO) analysis integration to well log analysis is considered one of the advanced techniques to improve the understanding of facies and fluid analysis. Generating AVO attributes are one solution to give an accurate result in facies and fluid characterization. This study is focused on a field of Northern Malay basin, which is associated with a fluvial-deltaic environment, where this system has high heterogeneity, whether it is vertically or horizontally. This research is aimed to demonstrate an application of the scale of quality factor of P-wave (SQp) and the scale of quality factor of S-wave (SQs) AVO attributes for facies and fluid types separation in field scale. These methods are supposed to be more sensitive to predict the hydrocarbons and give less ambiguity. SQp and SQs are the new AVO attributes, which derived from AVO analysis and created according to the intercept product (A) and gradient (B). These new attributes have also been compared to the common method, which is the Scaled Poisson’s Ratio attribute. By comparing with the Scaled Poisson’s Ratio attribute, SQp and SQs attributes are more accurate in determining facies and hydrocarbon. SQp and SQs AVO attributes are integrated with well log data and considered as the best technique to determine facies and fluid distribution. They are interpreted by using angle-stack seismic data based on amplitude contrast on interfaces. Well log data, e.g., density and sonic logs, are used to generate synthetic seismogram and well tie requirements. The volume of shale, volume of coal, porosity, and water saturation logs are used to identify facies and fluid in well log scale. This analysis includes AVO gradient analysis and AVO cross plot to identify the fluid class. Gassmann’s fluid substitution modeling is also generated in the well logs and AVO synthetics for in situ, pure brine, and pure gas cases. The application of the SQp and SQs attributes successfully interpreted facies and fluids distributions in the Northern Malay Basin.

Identification of shallow gas by seismic data and AVO processing: Example from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea

This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (~3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50–200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data. • We identify shallow gas based on seismic data from SW shelf of Ulleung Basin. • Seismic chimneys, gas flares, enhanced reflections are most common. • We compare acoustic anomalies of shallow gas with different resolutions. • We verify the presence of gas by AVO analysis.

Characterisation of submarine depression trails driven by upslope migrating cyclic steps: Insights from the Ceará Basin (Brazil)

Abstract Circular to elliptical topographic depressions, isolated or organized in trails, have been observed on the modern seabed in different contexts and water depths. Such features have been alternatively interpreted as pockmarks generated by fluid flow, as sediment waves generated by turbidity currents, or as a combination of both processes. In the latter case, the dip of the slope has been hypothesized to control the formation of trails of downslope migrating pockmarks. In this study, we use high-quality 3D seismic data from the offshore Ceara Basin (Equatorial Brazil) to examine vertically stacked and upslope-migrating trails of depressions visible at the seabed and in the subsurface. Seismic reflection terminations and stratal architecture indicate that these features are formed by cyclic steps generated by turbidity currents, while internal amplitude anomalies point to the presence of fluid migration. Amplitude Versus Offset analysis (AVO) performed on partial stacks shows that the investigated anomalies do not represent hydrocarbon indicators. Previous studies have suggested that the accumulation of permeable and porous sediments in the troughs of vertically stacked cyclic steps may create vertical pathways for fluid migration, and we propose that this may have facilitated the upward migration of saline pore water due to fluid buoyancy. The results of this study highlight the importance of gravity-driven processes in shaping the morphology of the Ceara Basin slope and show how non-hydrocarbon fluids may interact with vertically stacked cyclic steps.

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.

AVO analysis aids in differentiation between false and true amplitude responses: a case study of El Mansoura field, onshore Nile Delta, Egypt

The seismic amplitude versus offset (AVO) analysis has become a prominent in the direct hydrocarbon indicator in last decade, aimed to characterizing the fluid content or the lithology of a possible reservoir and reducing the exploration drilling risk. Our research discusses the impact of studying common depth point gathers on Near, Mid and Far-offsets, to verify the credibility of the amplitude response in the prospect evaluation, through analyzing a case study of two exploratory wells; one has already penetrated a gas-bearing sandstone reservoir and the second one is dry sand, but drilled in two different prospects, using the AVO analysis, to understand the reservoir configuration and its relation to the different amplitude response. The results show that the missing of the short-offset data is the reason of the false anomaly encountered in the dry sand, due to some urban surface obstacles during acquiring the seismic data in the field, especially the study area is located in El Mansoura city, which it is a highly cultivated terrain, with multiple channels and many large orchards on the edge of the river, and sugar cane and rice fields. Several lessons have been learned, which how to differentiate between the gas reservoirs and non-reservoirs, by understanding the relation between the Near and Far-offset traces, to reduce the amplitude anomalies to their right justification, where missing of Near-offset data led to a pseudo-amplitude anomaly. The results led to a high success of exploration ratio as the positives vastly outweigh the negatives.

Prospects delineation and evaluation using the CDP gathers characterization, onshore Nile Delta, Egypt

In the last decade, the seismic AVO analysis have become prominent in the direct hydrocarbons indicator (DHI), aimed at characterizing the fluid content or the lithology of a possible reservoir and reducing the exploration drilling risk. This research discuss the impact of studying the common depth point (CDP) gathers on Near, Mid and Far-offsets, to verify the credibility of the amplitude response in the prospect evaluation, through analyzing a case study of two exploratory wells; one has already penetrated a gas-bearing sandstone as a reservoir and the other one is dry-sand, but drilled in two different prospects, using the amplitude versus offset (AVO) analysis, to understand the reservoir configuration and its relation to the different amplitude response. The results show, that the missing of the short-offset data is the reason of the false anomaly encountered in the dry sand, due to some urban surface obstacles during acquiring the seismic data in the field especially, the study area is located in El Mansoura city, which it is a highly cultivated flat terrain, with multiple channels and many large orchards on the edge of the river, and sugar cane and rice fields. Several lessons have been learned, which how to differentiate between the gas reservoirs and non-reservoirs, by understanding the relation between the Near and Far-offset traces, to reduce the amplitude anomalies to their right justification, where missing of Near-offset data led to a pseudo-amplitude anomaly. The results led to a high success of exploration ratio as the positives vastly outweigh the negatives.

INTEGRATED AVO, ELASTIC SEISMIC INVERSION AND PETROPHYSICAL ANALYSIS FOR RESERVOIR CHARACTERIZATION: A CASE STUDY OF GAS FIELD, SOUTH SUMATERA BASIN

Integrated Amplitude Versus Offset ( AVO), elastic seismic inversion and petrophysical analysis have been successfully applied to estimate the elastic parameters of the reservoir for a case study of the gas field in south Sumatera basin. This paper aims to have better understanding the petrophysical properties of the reservoir. The petrophysical analysis was carried out by performing routine formation evaluation that includes calculation of shale volume, porosity, and water saturation of basic well log data. Sensitivity analysis was conducted to evaluate the sensitivity parameters of the log for changing in lithology, porosity, and fluid content in the reservoir. For completing the availability of elastic parameter from well log data, shear wave logs were derived from Castagna’s mudrock line relationship. Further, P-impedance, S-impedance, VpVs ratio, LambdaRho (λρ), MuRho (μρ) and density(ρ) were then calculated through a Lambda-Mu-Rho (LMR) transformation. Prior to performing AVO analysis and elastic seismic inversion, super gather technique was applied to improve the reliability of pre-stack seismic data. Elastic seismic inversion was carried out to extract the lateral elastic properties to capture lithology and fluid changes in the reservoir. In addition, AVO analysis of pre-stacked data was applied to identify hydrocarbon-bearing sandstone at target zone. The petrophysical analysis shows that porosity versus density crossplot is able to distinguish sand-shale based on 34% shale volume cutoff, while LMR crossplot is able to delineate hydrocarbon zone at water saturation value under 65%. The predicted lateral elastic parameter shows slightly higher value compare to overlying layer.

Time lapse (4D) and AVO analysis: A case study of Gullfaks field, Northern North Sea

A 4D seismic or time lapse survey has been used to investigate the amplitude versus offset (AVO) effects on seismic data in order to identify anomalies in the Gullfaks field for three different reservoir intervals namely the Tarbert, Cook and Statfjord reservoirs. Repeatability analysis has shown that the earlier seismic vintages are the most unreliable for amplitude anomaly analysis as normalised root-mean square (NRMS) values are greater than 50%. This is above the threshold of good and medium repeatability. Fluid substitution models show increases in both P-wave velocity and density for increasing water saturations with a maximum change of 7.33% in the P-wave velocity, and this is in line with predictions from previous work using the Biot - Gassman equations. AVO modelling for the top Tarbert Formation interface produced scenarios of increasing amplitudes with offset for the presence of hydrocarbons, which dim out with 100% brine saturation. This correlates to class III gas sands for different situations of varying Poisson’s ratio across an interface, which has been previously modelled. Two anomalies were identified with one being related to increasing pressure due to water injection correlating to poor permeability around injector well 34/10-B-33. The second anomaly is a case of potential unswept hydrocarbons that displayed a consistent bright spot throughout all of the seismic vintages (in-inlines and crosslines). AVO attribute analysis of this event produced a class II anomaly. However, when comparing near and far offset seismic data, dimming effect was observed producing contrasting evidence. The dimming offset is viewed to have been as a result of poor repeatability values at far offsets. The modelling of the fluid contents in the studied formations to conform to existing literatures justifies the efficacy of the method.

Key preprocessing technology and its application for CBM AVO analysis

The hazards of amplitude versus offset (AVO) technique mostly applied in seismic explorations for predicting coalbed methane (CBM) content mainly derive from multi-stage amplitude modifications during seismic signal preprocessing. The modifications are undoubtedly necessitated by improving high signal-to-noise ratio (SNR) and high resolution, and achieve high fidelity to some extent; and nevertheless lead to an unfavorable possibility to implement CBM AVO analysis. Similar to sandstone reservoir with gas, AVO analysis preprocessing for CBM reservoir strictly abides by a relative amplitude preserved (RAP) principle and particularly emphasizes on preserving the evanescent class IV AVO anomaly. As for those indispensable dealings with linear noise attenuations, near-surface variety compensations and time/depth spatial imaging, the key technologies adapted to CBM AVO preprocessing should use radial trace (RT) filter, refined surface-consistent amplitude compensation (SCAC), and RAP Kirchhoff prestack time migration (PSTM). The theoretical analysis and one real 3D example in this paper demonstrate that three key technologies compliant to RAP principle in CBM AVO preprocessing can preserve the class IV AVO anomaly and benefit an operation of CBM AVO analysis and an improvement of CBM evaluation.

Detection of gas-bearing intervals using S-transform and AVO analysis

In geophysical studies, attributes are the main tools for hydrocarbon identification and can be applied either pre- or post-stack. The amplitude versus offset (AVO), on the other hand, lies within the inversion methods applicable to pre-stack seismic data specifically tailored to recognize fluid content. A recent method useful for detecting hydrocarbon-bearing intervals in a reservoir is the spectral decomposition and is applied on post-stack data. For any frequency in a time–frequency setting, a single-frequency seismic section can be drawn and the low-frequency shadows related to the hydrocarbon accumulation can be differentiated. In this study, the spectral decomposition and the AVO analysis methods are applied on synthetic and real data, the latter being an example from a gaseous sandstone reservoir. The synthetic data based on the nearby well-logs and the fluid substituted synthetic well-logs were constructed. Results indicate the superiority of spectral decomposition method in detecting gas-bearing intervals (gas saturation above 80 %) in comparison to the AVO method. While the AVO method successfully differentiates the hydrocarbon and water-bearing intervals, its sensitivity in detecting an interval of 80 and 35 % gas is indifferent. We empirically know that the dominant frequency for detecting shadows of low-frequency features is between 10 and 20 Hz. In this study, such shadows are only observed below high gas saturation intervals by which the economical gas-bearing zones could be discriminated from the non-economic ones.

The effect of anisotropy on amplitude versus offset (AVO) synthetic modelling in Derby field southeastern Niger delta

The effect of anisotropy on AVO synthetic modelling in Niger delta Derby field has been attempted. The objective of the study is to determine if anisotropic synthetic model can improve amplitude versus offset (AVO) analysis of CDP gathers by comparing them with isotropic synthetic model over shale-gas sand boundaries. Well logs and common depth point (CDP) processed gathers were used for the analysis. The result of the present study revealed that reservoirs in Derby field are overlain by intrinsically anisotropic shale and as such, anisotropic synthetic modelling yields the best fit to CDP gathers than the isotropic synthetic model. However, in conventional processing for AVO analysis, isotropic rather than anisotropic earth models are usually assumed, which could possibly affect the outcome of an AVO-based analysis at large offsets. Therefore, the effect of anisotropy should be incorporated and properly accounted for using the anisotropic synthetic modelling in the processing and interpretation workflows for quantitative AVO analysis in Niger delta to optimise hydrocarbon recovery.

Application of AVO attribute inversion technology to gas hydrate identification in the Shenhu Area, South China Sea

AVO (Amplitude Versus Offset) is a seismic exploration technology applied to recognize lithology and detect oil and gas through analyzing the feature of amplitude variation versus offset. Gas hydrate and free gas can cause obvious AVO anomaly. To find geophysical evidence of gas hydrate and free gas in Shenhu Area, South China Sea, AVO attribute inversion method is applied. By using the method, the multiple seismic attribute profiles and AVO intercept versus gradient (I-G) cross plot are obtained. Bottom-simulating reflector (BSR) is observed beneath the seafloor, and the AVO abnormal responses reveal various seismic indicators of gas hydrate and free gas. The final AVO analysis results indicate the existence of gas hydrate and free gas in the upper and lower layers of BSR in the study area.