Seismic attribute extraction

Seismic attributes are extracted as a result of mathematical operations performed on a typical seismic trace. In map-based seismic attributes, an attribute can be precisely extracted at an interpreted seismic event, or can also be extracted through a defined time window. Often, this time window may be specified between 2 interpreted horizons, from one seismic horizon to a certain time value, or between two time/depth values.

In order to control the extraction process, two phantom horizons were created above and below the N-sand interval (Figure 25). The two phantom horizons delineate an envelope centered on an interpreted event of the N-Sand, defining the upper and lower boundaries for attributes extraction.

Figure 25. Vertical seismic section showing the two phantom horizons, in red color, centered on an interpreted blue event of the N-Sand. These two phantoms are used to define the upper and the lower boundaries for attributes extraction. (Troughs appear in hot color shades).

An a priori step for this procedure involves the accurate interpretation of the N- sand interval across the seismic survey. The classical approach of horizon tracking across inlines, cross-lines and closed loops is followed. Arbitrary traverses, normal to fault traces, are created, in order to track horizon interpretations across faulted surfaces (Figure 26). Subsequently, the autopicking tool is used to fill-in the gaps between interpreted inlines and cross-lines. The final product of autopicking is quality controlled and refined for accurate autopicking performance. Figure 27 shows a quality control map for the autopicking process, demonstrating the accurate autopicking result. Eighteen different attributes were extracted within an interval defined by 5ms above the upper phantom horizon and below the lower phantom horizon, shown in Figure 25. Finally, grid maps were created for each seismic attribute using a grid cell of 200x200ft, and the entire grid maps were quality controlled for any possible inadvertent errors. The extracted attribute maps successfully identified many anomalies regions that were also associated with previous drilling activities (Figure 28).

Table 3 shows a list of these attributes, a summary of their description, and their possible applications. The extracted seismic attribute maps are available in Appendix.

60 Arc Length Measure of the trace wiggle’s length within the analysis

window interval.

A measure of reflection heterogeneity which is used to quantify lateral changes in reflection patterns. Used as a stratigraphic sequence indicator.

Bandwidth Rating (Bias)

Statistical estimation of bandwidth of data within window. A measure of the frequency range in data, computed from the smoothed auto-correlation of the data.

A measure of reflection heterogeneity. It quantifies data similarity. Cyclic and shale sequences are presented by narrow bandwidth; However, widely varying lithologies more likely have a border bandwidth

Bandwidth Rating (Debias)

It is a statistical estimation of bandwidth of data within window. It is similar to Bandwidth Rating (Bias), but corrected for the bias effect of the smoother. This is usually the preferred measure of the bandwidth.

A measure of reflection heterogeneity. Generally more stable than Bandwidth Rating (Bias), unless window is small.

Half Energy It is the proportion of time required for the "energy" within a window to reach one-half of the total energy within the entire window. It shows the relative

distribution of energy in the window interval, where half of the energy is above, and half below.

A lithology and porosity indicator. A measure of reflection heterogeneity.

May be used to identify uneven reservoir features along the zone of interest. The attribute may indicate changes in lithology or porosity within a specified zone.

Mean Instantaneous Frequency

It is the mean Instantaneous Frequency within a window. Instantaneous Frequency is the time derivative of Instantaneous Phase, when an analytic seismic trace is decomposed into amplitude and phase components.

Often used to estimate seismic attenuation. A drop in frequency is associated with horizon occurring below oil and gas reservoirs. Also, it helps to measure cyclicity of geological in windowed intervals.

Mean Instantaneous Phase

It is the Mean of Instantaneous Phase component of an analytical seismic trace when decomposed into amplitude and phase components (Hilbert transform technique).

This measure can be use to track events of discontinuities such as pinchouts, angular

unconformities, and oil/gas or oil/water contacts along very limited windowed intervals.

Max Amplitude Maximum value of peak or trough Amplitude values within the analysis window.

A measure of direct hydrocarbon indicators. Used to identify amplitude anomalies of Bright spots due to changes in lithology or hydrocarbon accumulation.

Max Magnitude Maximum absolute value of Amplitude within the analysis window.

A measure of the strongest direct hydrocarbon indicators within the analysis window.

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window. indicator in a zone.

Mean Trough Amplitude

Average of the amplitudes of waveform troughs. Lithology and porosity indicator.

Min Amplitude Minimum value of Amplitude. A measure of the negative direct hydrocarbon indicators within the analysis window.

Number of Zero Crossing

The average number of zero crossings within the analysis window.

A measure of horizontal and vertical lithology changes. A high number of zero crossings indicate a great degree of vertical lithology complexity (i.e. a thick sequence of thin beds).

Polarity Ratio Ratio of number of positive samples to total number of samples.

A main lithology and porosity indicator. Used to detect lateral changes in thickness, and lithology.

RMS Amplitude A measure of reflectivity within a time window. The square root of the sum of time-domain energy (square of amplitude) within the window interval.

Indicates isolated or extreme amplitude anomalies. Used to track lithologic changes such as deltaic channel and gas sand, and used mainly as a direct hydrocarbon indicator.

Sum of Amplitudes Sum of Amplitude values within the window interval. A lithology and porosity indicator. A Large value may indicate a high net-to-sand ratio, or a high porosity interval.

Sum of Magnitudes Sum of the absolute Amplitude values within the analysis window.

A lithology and porosity indicator. Used to characterize sequences and indicate amplitude anomalies due to changes in lithology or hydrocarbon accumulation.

Sum of Negative Amplitudes

Sum of the negative Amplitude values within the analysis window.

A lithology and porosity indicator. A Large value may indicate a high net-to-sand ratio, or a high porosity interval.

Summarized from Sukmono, 2001; GeoFrame 4.0 documentation, 2002; Chen and Sidney, 1997.

** Gray shaded attribute indicate a main direct hydrocarbon indicator, whereas white shaded attributes indicate a main a lithology and porosity indicator.