Recognizing pitfalls of seismic structure and amplitude anomalies in Osprey field, Brunei

Abstract

The Osprey gas field was discovered in 1975 in the western offshore area of Brunei Darussalam. Two discovery wells penetrate the hanging wall of two separate growth faults and are associated with bright seismic amplitudes. Multiple thin reservoirs are separated by shales of 10 to a few hundred meters in thickness. In addition to seismic tuning of thin layers, there are many shallow gas clouds above these reservoirs, which cause significant and varying amount of push-down in seismic time data. Visualizing as seismic depth data reduces the amount of push-down but doesn’t remove it totally because, to date, the velocity model has been neither detailed nor accurate enough to fully constrain the seismic processing. Time-to-depth conversion includes significant uncertainties in the reservoir time interpretation, with only one well control per fault block. The shallow gas clouds also cause severe amplitude distortion at reservoir levels in the time and depth data, resulting in difficulties for discriminating gas distribution from stratigraphic variations or tuning effects. Furthermore, the shallow gas clouds cause false or enhanced rollover structures and artificial fault features adjacent to expressions of “real” features in the seismic data. Thus, estimates of reservoir spatial distribution and hydrocarbon volumes have large uncertainties. Recognizing these structure and amplitude pitfalls and handling them properly are important to improve reservoir understanding and volumetric estimations and to optimize development of the field. We have uncovered a variety of pitfalls and developed some mitigation methods. One method uses the seismic flat spot to correct the gas push-down effect, and the check-shot velocity was adjusted accordingly. This led to reduced uncertainty in the reservoir depth prognosis of the planned wells. Four wells were drilled in 2014, and the actual depth in all wells came out within only a 4-m difference, validating the correction method for shallow gas push-downs.