Tuesday, December 2, 2014

Flat Spots

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

Flat spots are perhaps the best indication of gas, athough other diagnoistic acoustic-impedance changes between the cap rock and gas-bearing reservoir affect the amplitude and polarity of the top-reservoir reflection.

Amplitude anomalies fall into two groups :
1. Anomalies of very high amplitude, commonly termed bright spots
2. Anomalies of very low amplitude, commonly termed dim spots

Bright spots are usually associated with porous sand. In a typical sand-claystone sequence, the claystone-to-sand reflection is ordinarily positive and of medium strength for water or oil saturated sands of poor porosity. The reflection becomes weak or nonexistent with higher porosity.

No comments:

Post a Comment