AVO Anomalies Research Articles

DHI scenarios in exploration: a personal view

DHI scenarios can usefully be divided into 1) ‘high grade DHI’ scenarios that have DHI characteristics consistent with the trap and indicative of a fluid contact and 2) ‘low-grade DHI ’ scenarios that have amplitude and AVO anomalies or low confidence fluid indications. This categorization is justified based on rock physics principles and historical data. Highgrade DHIs warrant a positive uplift to the chance of success of the prospect whereas low-grade DHIs generally do not. In late stage basin settings (such as the North Sea), with pressure to find prospects that pass internal risk and volumetric thresholds, it is easy to have a myopic focus on seismic amplitude, misunderstand the relative importance of geological and geophysical information and to exaggerate the significance of amplitude and AVO anomalies. Offsetting these tendencies requires an integration of prospect lookback/base rate information with bias mitigation strategies in the risking process.

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.

The Great Australian Bight – from AVO prospectivity screening to potentially drillable targets in one of the world's remaining untapped basins

The Ceduna Sub-basin in Australia’s southern margin offers an untapped opportunity for significant petroleum resource as part of the global exploration portfolio. Analogous to the prolific Niger delta in both size and structural style, this highly-extensional province contains up to 15 km of largely untested post-rift sediments including two widespread Late Cretaceous deltas linked to world-class oil-prone marine Cretaceous source rocks. Regional interpretation of legacy 2D seismic across the Bight Basin brings the sheer scale and structural complexity of this giant Cretaceous depocentre into perspective, but it is only through the detailed analysis of 8001 km2 of dual-sensor towed streamer 3D seismic that its true potential can be quantified. Rigorous phase and amplitude AVO QC of the pre-stack information, coupled with optimised velocity models fed into the depth migration sequence, have ensured amplitude fidelity and phase stability across all offset ranges. This has enabled a systematic and robust exploration workflow of AVO analysis and pre-stack inversion despite limited well data. Numerous dual-sensor case studies have nevertheless demonstrated these Relative Acoustic Impedance and Vp/Vs volumes to be reliably robust for prospect de-risking because of the extended low frequency bandwidth. Frontier screening supported by a partially-automated high-resolution stratigraphic framework has led to the identification of numerous prospects at multiple stratigraphic levels across the survey area. This includes isolation of laterally extensive and vertically amalgamated fan-like structures within the shallow Hammerhead delta using horizon-constrained high-definition spectral decomposition, and the extraction of potential AVO anomalies within the deeper structurally-controlled White Pointer sands draped across large gravity-driven listric growth faults.

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.

A case study in quantitative interpretation ambiguity, lambda-mu-rho, and rock-physics modeling in the Otway Basin, Australia

A rock-physics study and AVO modeling study have been completed to assist in the interpretation of seismic amplitude and AVO anomalies in the Shipwreck Trough of the offshore Otway Basin of southeastern Australia. Elastic log data, core data (both full and sidewall), and associated thin-section analysis of composition and texture were available on several wells, and these data are important in calibrating proposed rock-physics models that suggest that incorporating cement is critical to understanding anomalies in seismic and measured log data. Lithoprobability volumes based on conventional interpretation paradigms, such as low VP/VS values indicating gas presence, that do not incorporate an understanding of rock physics lead to biased interpretations. Ratios in particular can be misleading because there is ambiguity about whether an anomalous ratio is driven by the numerator or denominator. As a classic gas indicator, low VP/VS values are interpreted to be driven by a decrease in VP associated with gas replacing brine in a rock. Using Lamé impedance terms λρ and μρ, however, provides an alternative interpretation template that does not use ratios and can improve insight into rock properties. As in this recent case study, using lambda-mu-rho (LMR) can be an important tool when shear velocity has increased relative to compressional velocity, irrespective of any pore-fluid change. In the reservoir rocks of the Shipwreck Trough, low VP/VS in both gas-saturated and brine-filled sandstone is caused by quartz cement. This presents a substantial challenge to the use of a standard rock-physics template. In LMR space, however, low VP/VS data points clearly are characterized by high shear rigidity — an important point to recognize and incorporate into AVO interpretation workflows.

Rock physics and quantitative interpretation using Lambda-Mu-Rho in the Shipwreck Trough, Otway Basin

SUMMARY A rock physics study and AVO modelling study has been completed to assist in the interpretation of seismic amplitude and AVO anomalies in the Shipwreck Trough of the offshore Otway Basin of southeastern Australia. Elastic log data, core data (both full and sidewall) and associated thin section analysis of composition and texture were available on a number of wells; and these data are important in calibrating proposed rock physics models

Velocity analysis using similarity-weighted semblance

Weighted semblance can be used for improving the performance of traditional semblance for specific data sets. We have developed a novel approach for prestack velocity analysis using weighted semblance. The novelty came from a different weighting criteria in which the local similarity between each trace and a reference trace is used. On the one hand, low similarity corresponded to a noise point or a point indicating incorrect moveout, which should be given a small weight. On the other hand, high similarity corresponded to a point indicating correct moveout that should be given a high weight. Our approach could also be effectively used for analyzing AVO anomalies with increased resolution compared with AB semblance. Synthetic and field common-midpoint gathers demonstrated higher resolution using the approach we developed. Applications of the proposed method on a prestack data set further confirmed that the stacked data using the similarity-weighted semblance could obtain better energy-focused events, which indicated a more precise velocity picking.

Improved stratigraphic interpretation using broadband processing – Sergipe Basin, Brazil

Mike Saunders, Laurie Geiger, Dan Negri, Jaime A. Stein, T. Altay Sansal and John Springman demonstrate that de-ghosting and bandwidth extension using the Continuous Wavelet Transform (CWT) technique can produce broadband data that allows for improved identification and mapping of thinner potential hydrocarbon reservoirs. The Sergipe Basin originated in the Early Cretaceous and is a relatively mature hydrocarbon province on the northeast coast of Brazil, comprising 44,370 km² both onshore and offshore. The onshore portion of the basin (12,620 km²) is considered mature, with more than 2 billion bbls in place and 816 wells drilled since 1935. The offshore Sergipe Basin is gaining prominence as Petrobras has made several discoveries since 2010, including the 2010 Barra well (1-SES-158) and the subsequent Barra 1 appraisal well (3-SES-165). These wells targeted oil and gas charged Maastrichtian sandstones that display readily identifiable AVO anomalies on 2D seismic profiles. Approximately 16,000 km of long offset 2D seismic data was acquired in 2014 using a flat streamer towed at a depth of 15 m. The data was processed through pre-stack time migration using conventional techniques, and through prestack depth migration including conventional and broadband processing techniques. The broadband processing included proprietary pre-migration de-ghosting and bandwidth extension phases. The application of these two technologies resulted in seismic data with a vertical resolution as fine as 5 m to 10 m. Improvements in the resolution can be quantified by analysing the sequence stratigraphic interpretation which was done on both the conventionally processed data set and a broadband data set.

On seismic monitoring of $$\hbox {CO}_{2}$$ CO 2 leakage from geological storages and its primary detection

Geological storage of $$\hbox {CO}_{2}$$ in mature sedimentary basins and deep saline water aquifer requires special considerations on the potential leakage pathways. Early detection of $$\hbox {CO}_{2}$$ leakage from geological storage is one of the primary objectives of any carbon capture and storage technique. Thermodynamic properties of injected $$\hbox {CO}_{2}$$ and formation fluid, and change in in-situ conditions may act as driving agent to $$\hbox {CO}_{2}$$ leakage. Therefore, it is important to monitor the saturation of $$\hbox {CO}_{2}$$ and identify the phase of $$\hbox {CO}_{2}$$ during the injection, storage and after storage stages in order to monitor its possible migration and leakage risks. Seismic monitoring of sequestrated $$\hbox {CO}_{2}$$ in subsurface is in practice in which Gassmann fluid substitution model is used to estimate porous rock and pore fluid properties. The seismic properties of $$\hbox {CO}_{2}$$ contaminated fluids are predicted by Wood’s volume average formula. The analysis reveals that Wood’s approach is unable to account properly the properties of fluid mixture especially near bubble point pressure of $$\hbox {CO}_{2}$$ /water mixture, thus unable to monitor primary leakage of $$\hbox {CO}_{2}$$ . A modified approach based on thermodynamics is adopted for the detection of primary leakage of $$\hbox {CO}_{2}$$ . The analysis shows that for a minute amount of free $$\hbox {CO}_{2}$$ , the P-wave velocity is much lower than the values predicted by the Gassmann-Wood approach. The resulting normal reflectivity and AVO anomalies are much stronger than attained by conventional Gassmann-Wood approaches.

The Uncertainty Analysis of the Key Factors That Affect the AVO Attributes in Sandstone Reservoir

AbstractTraditional seismic AVO forward study usually uses constant parameters to construct rock physics models, and such parameters in real formations have uncertainties in an exploration area. This work employed laboratory measurements of cores from the target formation to simplify the rock physics model with relationships of reservoir sandstone porosities and P‐ and S‐wave impedances under a dry condition. To study influences of model parameter uncertainties, the probability density functions of key model parameters were introduced based on core measurements and well logging analysis. Using the Monte‐Carlo method and Gassmann fluid replacement technique, the AVO responses of models were obtained for saturation fluids as brine, oil and gas. The comparative analysis indicates that the velocity uncertainty of covering mud is the primary factor affecting the fluid cluster's distribution and deviation from water background trend on AVO intercept‐gradient crossplots, while the porosity uncertainty of sand is also significantly affecting but less than covering mud. Thus, the correct interpretation of AVO anomalies for real seismic data requires forward simulation on a probabilistic rock physics model to acquire the information on reservoir physical properties and fluids as much as possible.

Chapter 24 Application of AVO analysis to seismic records on the Laptev Sea Shelf

Abstract The Laptev Sea shelf basin was affected by phases of Late Mesozoic–Cenozoic rifting and has a thick sedimentary complex associated with linear grabens and horsts, creating trends favourable for hydrocarbon generation. Two groups of complexes are identified in the sedimentary cover: syn-rift between seismic horizon A and LS4 and post-rift between seismic horizon LS4 and seabed. The syn-rift complexes include strata of Late Cretaceous, Paleocene, Eocene and Oligocene–Middle Miocene ages. The post-rift complex includes Pliocene–Quaternary strata. Six seismostratigraphic complexes are identified and traced in the area, using wave field's seismostratigraphic analysis. The boundaries of these complexes are represented by surfaces of unconformities. The wave field of the sedimentary cover contains reflections from gas-bearing sandstones with types 1 and 3 of AVO anomalies. A shift of the anomalies' features from type 3 towards type 1 occurs with increasing depth of the potential deposits in the section.

Converted-wave elastic impedance and fluid identification factor in fractured reserviors

Identification and prediction of fracture and its developmental zone is meaningful to reservoir exploration and development. According to solid mechanics ,fracture strata is anisotropic media . Previous formulation for S-wave elastic impedance neglects seismic anisotropic. In order to incorporate anisotropic , we use an approximation of converted PS-wave reflection coefficients to deduce converted wave elastic impedance in weakly anisotropic media for the first time. Through numerical simulation, the effect of Thomsen parameter, incident angle and azimuthal angle on the formulation are described in detail. At the same time, we analyze the precision of new approximation formulation and compare the hydrocarbon detection effect of PP wave and converted wave elastic impedance. The future work is to select the newest nonlinear inversion algorithm and realize elastic impedance inversion in anisotropic media.On the basis of sensitive fluid identification factor in isotropic media,we construct the new fluid identification factor in weakly anisotropic media, which is composed of PP wave and converted wave elastic impedance. The advantage of new fluid identification factor is to allow investigating AVO anomalies with the effect of anisotropic parameters .Then ,we can analyze the difference of fluid identification factor with azimuthal angle and anisotropic parameters in 3D space .Adding Thomsen parameter to the 25 formation models gived by Castagna and Smith, we analyze the identification effect. Numerical simulation results show that the new fluid factor can identify the gas sand and brine sand effectively.

Modelagem avo: estudo de caso em um poço no Campo de Namorado

Este trabalho apresenta simulacao de substituicao de fluidos e estudo da modelagem AVO com a finalidade de caracterizar intervalos de arenito em um poco do Campo de Namorado. O estudo da simulacao da substituicao de fluido foi realizado com a substituicao do oleo in s/tu por salmoura. A modelagem AVO simulou atributos que foram interpretados com o auxilio da amplitude e da analise de graficos de dispersao cruzada, indicando anomalia AVO de Classe IV.

Extending the Exmouth Plateau post-Callovian fairway: WA-390-P phase I exploration

Discovery of the Jansz-Io field in 2000 established the presence of the Jurassic W. Spectabilis fairway in the deep-water Exmouth Plateau, North Carnarvon Basin. In conjunction with the regionally prolific Triassic Mungaroo gas play, this assisted in exciting record bid levels during the 2006 Gazettal of offsetting acreage, with the award of WA-390-P to Hess for a work commitment of 3,135 km2 3D seismic data and 16 exploration wells. In the first phase of exploration the 3D acquisition was completed and four wells positioned using 2D data were drilled in 2007. Three of these, Glencoe–1, Briseis–1 and Nimblefoot–1, were discoveries in the post-Callovian interval, with Briseis–1 encountering additional gas pay in the Triassic Mungaroo Formation. The W. Spectabilis sandstones encountered in Glencoe–1 were analogous to those encountered in the Jansz–1 discovery well, 75 km to the northeast. These are interpreted to have been deposited in a shallow marine environment with accommodation space controlled primarily by the relict topography of the Triassic fault blocks. In contrast, Nimblefoot–1 and Briseis–1 both encountered gas pay in deep-water sandstones of Berriasian age. These are interpreted to be delta-front turbidites sourced from the Barrow delta to the south, analogous to the Scarborough gas field, but in fault-bounded traps in which fragments of the hanging wall systems have been preserved in the degraded footwalls of major faults. Both play types are characterised by classic seismic amplitude and AVO anomalies.

Velocity analysis using AB semblance

ABSTRACTI derive and analyse an explicit formula for a generalized semblance attribute, which is suitable for the velocity analysis of prestack seismic gathers with distinct amplitude trends. While the conventional semblance can be interpreted as a squared correlation with a constant, the AB semblance is defined as a correlation with a trend. This measure is particularly attractive for analysing class II AVO anomalies and converted waves. Analytical derivations and numerical experiments show that the resolution of the AB semblance is approximately twice as low as that of the conventional semblance. However, this does not prevent it from being an effective attribute. I use synthetic and field data examples to demonstrate the improvements in velocity analysis from AB semblance.

Automatic compensation of AVO background drift

Anomalous AVO reflectors can be defined as those whose seismic amplitude varies with offset or incidence angle in a manner markedly different from the majority of nearby reflectors (known as the “background”). An increase or decrease of amplitude with offset is not anomalous if most other reflection amplitudes vary in similar proportion. Such would be the case if an overriding offset-gain function were applied to the gathers, either intentionally or inadvertently. AVO anomalies are of keen interest in exploration geophysics, since they are usually caused by abrupt changes in the Vp/Vs ratio across a reflection boundary, which are often associated with the presence of sandy reservoirs or hydrocarbons.

Reflections on elastic impedance

The story goes back to 1986 when I was first hooked on AVO after reading an SEG expanded abstract called “Seismic lithologic modeling of amplitude versus offset data” by Gelfand et al. of Western Geophysical. That in turn led me to Shuey's paper. I spent the next couple of years experimenting with AVO processing, analysis, and modeling. I concluded that any attribute with heavy dependency on the gradient term would be unreliable given the errors inherent in estimating the gradient (at least on 1980s vintage 2D data). Furthermore, the value of forward modeling gathers was limited because it was a step removed from the prime benefit provided by seismic—the image. I felt that producing constant angles stacks to image the AVO anomaly was the way to go.

Lithology and Fluid Prediction in Lightly Explored Basins

In this paper we present a methodology for performing lithology and fluid prediction in lightly explored basins. We use the deep water area of the Orange basin, offshore South Africa to illustrate the methodology. In the Orange basin, there are numerous wells drilled on the shelf, however, there have been no wells drilled in the deep water. Firstly, petrophysical analysis of the shelf wells is performed to determine end member properties of the sands and shales. From the analysis, rock property trends such as Vp versus Vs and Vp versus density are determined. In addition, the uncertainties associated with the trends are also calculated. A critical step in extrapolating from the shelf to the deep water is to use seismic derived interval velocities to improve the estimate of Vp as a function of depth. Using seismic interval velocities and well data, we derive expressions for Vp of the sands and the shales that are functions both of depth and seismic interval velocity. The rock property trends are used to perform stochastic AVO modelling for single interfaces. The AVO modelling gives an estimate of the average response of interfaces such as shale-to-brine sand and shale-to-gas sand interfaces, as well as a measure of the uncertainty of the estimate. Therefore, a range of AVO responses is provided. Lastly, the AVO modelling is compared with AVO anomalies observed on the seismic data.

THE THYLACINE AND GEOGRAPHE GAS DISCOVERIES, OFFSHORE EASTERN OTWAY BASIN

The Thylacine and Geographe gas fields were discovered in mid-2001 in the offshore Otway Basin, in permits T/30P and VIC/P43 respectively. Geographe is 55 km south of Port Campbell and Thylacine is a further 15 km offshore, in the depo-centre of the Shipwreck Trough, in water depths of 80 m to 100 m. The Thylacine–1 well intersected a 277 m gas column in Turonian to Santonian aged reservoirs. Geographe–1 intersected a 233 m gas column in a similar sedimentary section. Thylacine–2, 5.7 km west of Thylacine–1, confirmed the field extent, and flowed gas at 28 MMSCFD (0.79 Mm3/D). Critical to the discovery of these fields was the Investigator 3D seismic survey, which covered about 1,000 km2 of the central Shipwreck Trough. The pre-drill chance of success of both structures was high-graded as a result of excellent structural imaging and the conformance of amplitude and AVO anomalies to mapped closures. The interpretation of this survey and the subsequent drilling of the Thylacine and Geographe Fields have dramatically increased the understanding of the structure and stratigraphy of the offshore eastern Otway Basin particularly in relation to the Shipwreck Trough and the Sorell Fault Zone.Combined dry gas reserves at the proved and probable level stand at 0.85 TCF and condensate reserves at 10.7 MMBBL. The fields are undergoing integrated sub-surface, development and environmental studies with the aim of supplying the nearby southeastern Australian gas markets. The preferred development concept is a small jacket structure at Thylacine, followed by a subsea tie-in of the Geographe Field with onshore processing facilities near Port Campbell.

The MacCulloch Field, Block 15/24b, UK North Sea

Abstract The MacCulloch Field lies within Block 15/24b in the UK Central North Sea and is located on the northern flank of the Witch Ground Graben. It was discovered by Conoco well 15/24b-3 in 1990. MacCulloch Field is a four-way dip closure at Top Paleocene over a deeper Mesozoic structure. The reservoir consists of Upper Balmoral Sandstones containing 32-37° API oils derived from Kimmeridge Clay Formation shales and sealed by shales belonging to the Sele Formation. The field contains recoverable reserves of 60-90 MMBOE. Reservoir quality is generally very good, with an average porosity of 28% and core permeabilities (Kh) between 200 mD and 2D. AVO anomalies and a seismic flat spot are associated with oil filled reservoir and the oil-water contact in certain areas of the field.