Porosity identification using amplitude variations with offset: Examples from South Sumatra

Chacko, Soman

doi:10.1190/1.1442737

Cited by 22 publications

(4 citation statements)

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“…Rock‐physics modelling indicates that post‐stack and amplitude‐versus‐angle (AVA) behaviour for a given lithology are controlled by porosity and the presence of hydrocarbons (Castagna, Batzle and Kan 1993). AVA analysis has traditionally been applied to clastic environments but has, within the last decade, been used as a successful porosity predictor in carbonates (Chacko 1989; Landrø, Buland and D'Angelo1995; Santoso et al 1995; D'Angelo, Brandal and Rørvik 1997; Li and Downton 2000). However, AVA analysis as a fluid prediction tool in carbonates has only been applied successfully in a limited number of cases (Megson 1992; Chiburis 1993; Adriansyah and McMechan 2001; Japsen et al 2004) and unique hydrocarbon signatures are only found in extraordinary conditions of very high porosity and gas saturation because the hydrocarbon effect is believed to overlap the changes caused by variations in porosity (Megson 1992).…”

Section: Introductionmentioning

confidence: 99%

Elastic behaviour of North Sea chalk: A well‐log study

Gommesen¹,

Fabricius²,

Mukerji

et al. 2007

Geophysical Prospecting

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We present two different elastic models for, respectively, cemented and uncemented North Sea chalk well‐log data. We find that low Biot coefficients correlate with anomalously low cementation factors from resistivity measurements at low porosity and we interpret this as an indication of cementation. In contrast, higher Biot coefficients and correspondingly higher cementation factors characterize uncemented chalk for the same (low) porosity. Accordingly, the Poisson's ratio–porosity relationship for cemented chalk is different from that of uncemented chalk. We have tested the application of the self‐consistent approximation, which here represents the unrelaxed scenario where the pore spaces of the rock are assumed to be isolated, and the Gassmann theory, which assumes that pore spaces are connected, as tools for predicting the effect of hydrocarbons from the elastic properties of brine‐saturated North Sea reservoir chalk. In the acoustic impedance–Poisson's ratio plane, we forecast variations in porosity and hydrocarbon saturation from their influence on the elastic behaviour of the chalk. The Gassmann model and the self‐consistent approximation give roughly similar predictions of the effect of fluid on acoustic impedance and Poisson's ratio, but we find that the high‐frequency self‐consistent approach gives a somewhat smaller predicted fluid‐saturation effect on Poisson's ratio than the low‐frequency Gassmann model. The Gassmann prediction for the near and potentially invaded zone corresponds more closely to logging data than the Gassmann prediction for the far, virgin zone. We thus conclude that the Gassmann approach predicts hydrocarbons accurately in chalk in the sonic‐frequency domain, but the fluid effects as recorded by the acoustic tool are significantly affected by invasion of mud filtrate. The amplitude‐versus‐angle (AVA) response for the general North Sea sequence of shale overlying chalk is predicted as a function of porosity and pore‐fill. The AVA response of both cemented and uncemented chalk generally shows a declining reflectivity coefficient versus offset and a decreasing normal‐incidence reflectivity with increasing porosity. However, for the uncemented model, a phase reversal will appear at a relatively lower porosity compared to the cemented model.

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Section: Introductionmentioning

confidence: 99%

Elastic behaviour of North Sea chalk: A well‐log study

Gommesen¹,

Fabricius²,

Mukerji

et al. 2007

Geophysical Prospecting

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“…For some time, single angle stack inversion was regarded as sufficient for carbonates due to the expectation that there was little AVO associated with carbonates. The basis of this is the supposed and reported invariance of the V P ∕V S with respect to changes in porosity (e.g., Chako, 1989) and the insensitivity with respect to pore fluids (e.g., Rafavich et al, 1984). Recently, there have been 1 Ikon Science, Kuala Lumpur, Malaysia.…”

Section: Introductionmentioning

confidence: 95%

Seismic inversion of a carbonate buildup: A case study

Sams¹,

Begg²,

Manapov³

2017

Interpretation

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The information within seismic data is band limited and angle limited. Together with the particular physics and geology of carbonate rocks, this imposes limitations on how accurately we can predict the presence of hydrocarbons in carbonates, map the top carbonate, and characterize the porosity distribution through seismic amplitude analysis. Using data for a carbonate reef from the Nam Con Son Basin, Vietnam, the expectations based on rock-physics analysis are that the presence of gas can be predicted only when the porosity at the top of the carbonate is extremely high (>35%), but that a fluid contact is unlikely to be observed in the background of significant porosity variations. Mapping the top of the carbonate (except when the top carbonate porosities are low) or a fluid contact requires accurate estimates of changes in V P ∕V S. The seismic data do not independently support such an accurate estimation of sharp changes in V P ∕V S. The standard approach of introducing lowfrequency models and applying rock-physics constraints during a simultaneous inversion does not resolve the problems: The results are heavily biased by the well control and the initial interpretation of the top carbonate and fluid contact. A facies-based inversion in which the elastic properties are restricted to values consistent with the facies predicted to be present removes the well bias, but it does not completely obviate the need for a reasonably accurate initial interpretation in terms of prior facies probability distributions. Prestack inversion improves the quality of the facies predictions compared with a poststack inversion.

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“…In the case of carbonate sulfur deposits, seismic interpretation is mainly focused on porosity changes at the top of the layer and within the carbonate interval. An increase in porosity results in a drop of the amplitude in the wave reflected from the top of the carbonate sediments, which in some cases may result in a dim spot or even a phase reversal (e.g., [9]). To seismically detect the sulfur ore we should investigate relationships between the amplitude of the reflected wave and porosity, as well as the percentage of sulfur within the cap rock.…”

Section: Introductionmentioning

confidence: 99%

Simultaneous Inversion of Shallow Seismic Data for Imaging of Sulfurized Carbonates

Cichostępski¹,

Dec²,

Kwietniak³

2019

Minerals

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In this article, we present a high-resolution shallow seismic surveying method for imaging the inner structure of the Miocene evaporitic formation, where sulfur ore occurs. The survey was completed in the northern part of the Carpathian Foredeep (SE Poland) where sulfur deposits occur up to a depth of ca. 260 m. In this region, the sulfur ore is strata-bound and exists within a carbonate interval of a thickness of approximately 28 m. The average sulfur content reaches up to 30%. Five seismic profiles were acquired with a total length of 2450 m. The acquisition was designed to obtain high-resolution, long offsets and a satisfactory signal-to-noise ratio. In the field, we used 48 channels and variable end-on roll-along spread that allowed us to record offsets of up to 375 m. Data processing was aimed at preserving relative amplitudes (known as RAP, relative amplitude preservation processing), an approach that is necessary for seismic inversion application. With the utilization of well log data and results of simultaneous inversion, we were able to calculate the elastic properties of the deposit to evaluate sulfur ore content and changes in lithology. The sulfur content is strongly dependent on the carbonate reservoir’s porosity. To evaluate porosity changes and associated sulfur content, a simultaneous inversion procedure was used. This is a pioneering approach in which we applied pre-stack inversion methods to shallow carbonate sediments.

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Porosity identification using amplitude variations with offset: Examples from South Sumatra

Cited by 22 publications

References 0 publications

Elastic behaviour of North Sea chalk: A well‐log study

Elastic behaviour of North Sea chalk: A well‐log study

Seismic inversion of a carbonate buildup: A case study

Simultaneous Inversion of Shallow Seismic Data for Imaging of Sulfurized Carbonates

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