Target‐oriented AVO inversion of data from Valhall and Hod fields

Landrø, Martin; Buland, A.; D‘Angelo, R. M.

doi:10.1190/1.1437171

Cited by 5 publications

(2 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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“…The large range in porosity of chalk combined with the strong link between porosity and stiffness is reflected in an overall correlation between porosity and seismic impedance. This forms the basis for the use of seismic methods for predicting porosity within the chalk fields and for finding stratigraphic traps for hydrocarbons, including post‐stack inversion and amplitude versus offset techniques (Landroe et al ., 1995; D'Angelo et al ., 1997; Anderson, 1999). The difference in elasticity between water and hydrocarbons is a limiting factor for these seismic methods.…”

Section: Introductionmentioning

confidence: 99%

Relation of acoustic impedance to stiffness and porosity of Maastrichtian chalk in Dan field, Danish North Sea

Meireles

Welch

Fabricius

2022

Geophysical Prospecting

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In seismic data analysis, acoustic impedance may be used for predicting the porosity of a sedimentary layer with known lithology, where the influence on impedance from pore fluid is known. This method typically assumes that the stiffness of the rock frame is solely a function of porosity and that there is no variation in stiffness between rocks with the same composition and porosity. However, contact cementation can cause an increase in rock frame stiffness with no significant reduction in porosity. This means that rocks with similar porosities and varying degrees of cementation would have significantly different elastic moduli; thus, attempting to derive porosity from seismic impedance alone would incur errors. The primary goal of this study is to assess how significant this error is for porosity prediction. The secondary goal is to calculate the dry rock frame stiffness across the field and evaluate to what extent the time of hydrocarbon placement affects the lithification of the rock. To fulfill these objectives, we studied petrophysical logs and core data from the Dan field and derived seismic impedance, porosity and stiffness expressed in terms of Biot's coefficient for 14 wells across the field. Trends defined between acoustic impedance, porosity, and Biot's coefficient in the Maastrichtian unit of the Dan field showed that, for a given porosity and water saturation in the oil zone, the chalk located at the crest of the reservoir is softer than the chalk located at the flanks of the field. We also found that chalk is stiffer in the down-faulted north-western part of the field for a given porosity and water saturation in the oil zone. In the water zone, no difference was found. We speculate that contact cementation downflank in the northwest block caused an increase in the chalk stiffness without reducing the porosity, while earlier hydrocarbon filling of the south-eastern block and the crest of the structure delayed contact cementation between the chalk particles. This difference in contact cementation between rocks of the same porosity causes porosity prediction from acoustic impedance to vary by up to six porosity units.

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Effects of attenuation and scattering on AVO measurements

Adriansyah

McMechan

1998

GEOPHYSICS

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The trends and variance in amplitude variation with offset (AVO) observations caused by near‐surface structure, attenuation, and scattering are numerically synthesized by pseudospectral viscoelastic 2-D modeling. Near‐surface structure produces amplitude focusing and defocusing that significantly distort AVO observations in offset windows at a scale comparable to that of the lateral variations in the structure. Attenuation and scattering decrease absolute amplitudes at all offsets. Scattering and wave interference increase the variance associated with AVO measurements. Depending on the relative influence of intrinsic attenuation, apparent attenuation associated with scattering, and geometrical focusing, a normalized AVO response can increase or decrease with offset (relative to that for the associated elastic, nonscattering, 1-D solution), and so mimic the behavior predicted as a function of contrasts in density, velocity, porosity or Poisson’s ratio. If only relative (normalized) amplitudes are available, it is difficult to distinguish between effects of parameters whose main contributions are to absolute amplitude; for example, a trend of decreasing amplitude (relative to that for an elastic flat‐layered model) produced by intrinsic attenuation may be counteracted by focusing/scattering or anisotropic effects over wide aperture ranges. Diagnostic information on AVO effects of scattering and attenuation is lost when the noise level is sufficiently high. Interpretations of AVO observations based on homogeneous layered elastic models must therefore be used with caution as they are, in general, nonunique. Lateral variations in AVO parameters are the key to detecting hydrocarbons, so lateral changes in AVO produced by lateral changes in the overburden properties have potential for being misinterpreted, especially if the recording aperture is small.

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Target‐oriented AVO inversion of data from Valhall and Hod fields

Cited by 5 publications

References 0 publications

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

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

Relation of acoustic impedance to stiffness and porosity of Maastrichtian chalk in Dan field, Danish North Sea

Effects of attenuation and scattering on AVO measurements

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