Seismic, Centroids and Sealing Capacity

By

Saad T Saleh¹, Kenneth Williams¹, Kuochang Chen¹

(1) Knowledge Systems, Inc, Stafford, TX

 Amplitude and AVO anomalies are commonly identified as drilling targets. Sometimes when these prospects are penetrated, they are found to be wet. The hydrodynamic concept known as the Centroid effect may account for some of these exploration failures.

This lithology-dependent difference in pressure gradients has significant consequences. Wells drilled high on a structure may encounter overpressures far in excess of that in the surrounding shales, even in completely water-wet sections. The overpressures at the crest may approach the fracture gradient of the topseal. Formation water flows up-structure from the deep, overpressured, downdip part of the sand and escapes at the weakest point of the topseal. As the water moves from high temperatures and pressures to lower ones, any gas contained within it may exolve. This gas, at low concentrations (5% or less), may be responsible for some of the amplitude anomalies that have been drilled. Use of the Centroid concept may help in the evaluation of topseal and hydrocarbon risk for undrilled prospects. .

Amplitude and AVO analysis can sometimes indicate the presence of gas in a potential reservoir interval, but has difficulty in distinguishing between low (noncommercial) and high gas saturations. A method is shown to determine whether an amplitude anomaly of a given measured vertical height is capable of being a hydrocarbon column or whether it is more probable that it is wet.