Thursday, April 9, 2015

Gas in Seismic Reflections

The presence of gas in a reservoir often produces a detectable suite of responses in the seismic record; and it is obviously very important for the interpreter to be able to recognize these gas effects.

Acoustic-Impedance Effects


The way in which a reservoir responds to the presence of gas depends on the acoustic impedance of the gas-filled portion of the reservoir, the water-filled reservoir, and the cap rock; and the thickness of the gas-filled interval. If the gas column is thick enough and there is an acoustic-impedance contrast between the gas-/oil or the gas-/water filled portions of a reservoir, a reflection commonly called a flat spot will result.

As a rule of thumb, flat spots are likely to be found in porous sandstones or carbonates down to about 2.5 km. Below this depth the effect of gas on velocity is less marked and the chance of getting a good reflection from a gas contrast is reduced.

Flat spots will always have a positive reflection coefficients, appearing as a trough on seismic sections displayed with SEG normal polarity or a peak on reverse polarity sections. Although gas contacts are usually horizontal in depth, they do not always appear horizontally in time due to the push-down effect of the lower velocity in the gas interval.

Dim spots due to a reduced reflection coefficient of the top-reservoir reflector are more common with less porous or well-compacted sands and carbonate reservoirs. The carbonate would usually have strong positive reflection coefficient. Gas in the reservoir reduces the reflection coefficient, causing the top-reservoir reflector to lose amplitude and dim. Amplitude anomalies are sometimes accompanied by corresponding polarity changes. A polarity reversal of the top-reservoir reflector at the gas-oil or gas-water contact is a common feature of bright spots.

Velocity Effects

If the gas column is sufficiently thick, a push-down may be observed on underlying reflectors.

Diffractions are developed where there is a significant lateral contrast in acoustic impedance and are often seen at the edges of bright spots.

"Gas chimneys" or "gas cloud' -poor data zones above gas-bearing structure are quite common and can be very characteristics. Poor data zones are thought to be caused by scattering of seismic energy by escaped gas penetrating the cap rock above a gas reservoir. Gas leakage into the cap rock can occur through a variety of mechanisms (leakage along fault planes; fractures; or overpressure exceeding the mechanical strength of seal rocks.

Reference : Practical Seismic Interpretation, Michael E. Badley 

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