Avoiding False Amplitude Anomalies by 3D Seismic Trace DetuningAshley Francis & Samuel Eckford

By contrast, detuning of seismic amplitudes is a relative correction and sodoes not require well calibration. If detuning is applied to the seismic tracedirectly, this removes the requirement for seismic interpretation first,potentially speeding up the analysis and giving greater flexibility. Figure 3shows a new method of 3D seismic detuning called DT-AMP™ whichperforms such trace detuning in situ on seismic traces.

The method can also compute the input data for tuning curve analysiswithout seismic picking, being able to compute automatically the necessarytuning data on time slices or in windows. This gives me capability to makethe detuning function vary both spatially and temporally, adapting togeophysical and bandwidth changes in the dataset.

Summary

Amplitude maps derived from 3D seismic interpretation are an importanttool for understanding geological changes in the subsurface, includingfacies or porosity distribution. Many new exploration and near-field drillingtargets are associated with an amplitude anomaly, often interpreted as adirect hydrocarbon indicator or DHI.

Many factors affect 3D seismic amplitude and the interpretation ofamplitude changes is not unique. However, in the interpretation of seismicamplitude it seems to be widely unappreciated that the biggest factorcontrolling seismic amplitude variation is not sought-after geologicalchanges or DHIs, but is in fact thickness variation.

Thickness related tuning is well understood in geophysics, but the basicprinciple of removing tuning effects from amplitude maps is not widelyapplied. Published methods of detuning apply only to maps extracted viaseismic picking. In contrast to this approach, this paper shows examplesfrom a new and novel technique which directly detunes the seismic tracesin situ. The advantages of direct detuning of 3D seismic volumes (oralternatively 2D seismic lines) include the ability to investigate the wholetrace amplitude response (not just a window or extraction), generation oftuning curves without seismic data picking, rapid updating and modificationof parameters and improved stratigraphic and quantitative interpretation.

Figure 2 is a plot of the amplitude of the reservoir response from Figure1(b) plotted as a function of TWT thickness. Note how the amplitudereaches a maximum around 13 ms and then falls off to about half the peakamplitude at around 35 ms. This doubling of the amplitude as a function ofthickness is the tuning effect.While gas modelled here would boost amplitudes by around 25%, tuningeffects can more than double the amplitudes.

Detuning Seismic Traces

Typical approaches to detuning aim to detune and estimate seismic net payin a single step (Connolly, 2007). The method is applied to grids ofamplitude and thickness extracted via seismic interpretation picks. Seismicnet pay is a method of estimating an apparent net:gross from the seismicamplitude and then multiplying by the thickness estimate to give a netthickness (or pay).

Methods based on picking a top and a base of an event on relativeimpedance seismic data are potentially laborious and time consuming. Thecalculation of net:gross (and hence seismic net pay) requires wellcalibration; this also limits the application of the technique. Anotherdrawback to map-based detuning methods is that tuning curves can beover-fitted to local geological events. Only by selecting a wider set of datacan an unbiased tuning curve representing frequency content andbandwidth be generated.

Figure 1(a) Wedge model of a brine saturated sand for the Kadanwari Field, Pakistan

Figure 3(a) Pre-stack coloured inversion of EEI-70 lithology attribute over the main reservoir, Sea Lion Field, North Falkland Islands Basin. Reservoir is strong purple/blue

interval, inserted log is Vshale. Note strong amplitudes downdip despite lower net:gross at well 14/10-4. Inset graph on the right shows time thickness (x axis)

versus amplitude (y axis) with the position of the two wells plotted. Amplitudes would indicate the downdip well has the higher net:gross.

Figure 3(b) Pre-stack coloured inversion of EEI-70 lithology attribute after in situ seismic trace detuning using DT-AMP. Amplitudes in main reservoir now relate to

lateral changes in Vshale as observed in well logs. Inset graph on the right shows the detuned profile for the thickness vs. amplitude crossplot. The amplitudes have been

normalised to the green line and the correct relative amplitudes are now apparent. The crossplot now correctly shows the updip well with higher net:gross.

Figure 1(b) Relative acoustic impedance response of brine saturated sand model in 1(a). The modelled reservoir sand appears as the green/yellow event, with the brightening (green) very clear in the centre of the plotFigure 2 Tuning curve for Kadanwri brine saturated sand showing amplitude variation as a function of TWT thickness of reservoir

Wedge Model of Tuning

Geophysicists show the effect of tuning using a constant property wedgemodel. Figure 1 is an example for a soft sand reservoir in the KadanwariField (Francis and Syed, 2001), Pakistan. Note that all the layers in thismodel have constant properties: only the thicknesses are changing,thinning from left to right.

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Ardmore Field, Central North Sea

The following example (Figure 4) shows an amplitude anomaly mapextracted from a single horizon pick at Rotliegendes reservoir level overthe Ardmore Field, located in UKCS blocks 30/24b & 30/29c (Francis &Hicks, 2006). The reservoir is oil bearing in Jurassic, Zechstein, Permianand Devonian intervals, although the field is currently abandoned. The leftpanel shows the amplitude anomalies before detuning.

The amplitude map after detuning is shown in the right panel of Figure 4.With detuning the field outline is much more sharply defined, and there isclear closure separation in the north-western corner of the field. Thedetuned bright amplitudes in red no longer conform to the polygons andthe reservoir found in the wells in the south of the field is now defined.

York and Greater York, Blocks 47/2 & 47/3, UKCS

The York Field and the Greater York area comprise UKCS southern gasbasin fields and potential near field targets. The area also includes theRough gas storage field. Gas is present in both Rotliegendes andCarboniferous reservoirs; the presence of gas is known to brightenseismic amplitudes. The reservoir intervals are separated by a non-reservoir interval, which varies in being able to be resolved on seismic.DT-AMP detuning was applied to the seismic and automated, entirely self-consistent attributes were extracted using the existing interpretation pick.

Shallow Gas – Hazard or Resource?

The final example presented is taken from Netherlands block F09. Here,shallow gas is present and may be either a drilling hazard, or a potentialresource. Amplitude analysis is frequently used for attempting to identifygas hazards, but of course tuning is still the predominant amplitude driver,so removal of tuning effects improves the reliability of amplitude analysis forgas hazards. Detuning gives greater confidence in finding the potential gasaccumulation (figure 7).

Figure 4 Amplitude anomalies over the Ardmore Field at Rotliegendes level. Before detuning left) and after detuning (right)

Figure 5 Relative AI section, Rotleigendes and Carboniferous reservoir intervals (green colours) below Zechstein (strong purple). Here the two intervals are

separately resolved.

Figure 6 Original amplitude map (left) and detuned amplitude map (right)

Acknowledgements

We are grateful to Rockhopper Exploration Ltd and Premier Oil PLC for permission to show images from the Sea Lion Field. We are grateful to Centrica PLC for permission to show images from the York and Greater York area. The Kadanwari example was originally released by LASMO and the Netherlands shallow gas example was worked up with assistance and guidance from EBN. The Ardmore data was originally provided by Acorn Oil & Gas Ltd.

For shallow gas as a resource, a common criterion is to compare theconformance of the DHI to the structure. This is usually empirical and adhoc, little more than overlaying structural contours on an amplitude anomalymap. A better approach is to measure the conformance of the closure tothe amplitude anomaly using a suitable statistic such as MatthewsCorrelation Coefficient or F1 score, based on the binary intersection criteriadescribed in the following table:

References

Connolly, P., 2007, A simple, robust algorithm for seismic net pay estimation, The Leading Edge, October 2007.

Francis, A.M. and Hicks, G.J, 2006, Porosity and Shale Volume Estimation for the Ardmore Field using Extended Elastic Impedance. 68th EAGE Conference, Vienna, Austria

Francis, A.M., and Syed, F.H., 2001, Application of Relative Acoustic Impedance Inversion to Constrain Extent of E Sand Reservoir on Kadanwari Field, SPE / PAPG Annual Technical Conference, 7-8 November 2001, Islamabad, Pakistan

Conclusions

The effect of tuning on seismic amplitudes is widely recognized as an important effect but is rarely corrected for. In particular, there is a lack of awareness that thickness related tuning is the predominant physical effect controlling amplitude and that it can double the apparent amplitude of a seismic reflection. Because of this, all seismic amplitude data should be detuned before use. Detuning seismic data removes the most significant risk factor from the interpretation and makes amplitudes more easily interpretable in terms of geological criteria. Considering all the examples shown in this paper, it is clear that detuning is highly applicable to both lithological and fluid driven amplitude effects.

Seismic amplitude detuning is a self-consistent calculation and does not require external calibration such as well control. The use of 3D seismic detuning extends current map based approaches to detuning of seismic traces in situ in 2D lines or 3D volumes. This facilitates before and after detuning comparisons directly on seismic sections, before picking or further analysis. Detuning directly on the traces is significantly faster as it avoids the picking phase in both tuning curve analysis and interpretation. It also creates additional attributes which enhance the amplitude interpretation and ensure self-consistency.

Figure 7 Original Relative impedance (top) and detuned relative impedance

(Bottom).Notice the local

blue/black amplitude bright is now completely

reversed in spatial distribution.

Closure Amplitude Closure Binary

Amplitude Binary Description Abbreviation

In Closure In Anomaly 1 1 True Positive TPNot in Closure In Anomaly 1 0 False Negative FN

In Closure Not in Anomaly 0 1 False Positive FPNot in Closure Not in Anomaly 0 0 True Negative TN

Conformance 51%Conformance 51% Conformance 60%

Figure 8 Structural closure map (left), extracted amplitude (middle left), amplitude conformance by spillpoint (centre right) and amplitude conformance after the statistical best fit contact is calculated , improving the amplitude conformance to structure (right).