Abstract: Estimating Depth Conversion Uncertainty from conflicting data

FUGLESTAD, ODD, DAVE DRAPER, and ANDERS TORP

Generally the largest contribution to reserves uncertainty comes from uncertainty in the bulk rock volume. Even in a proven discovery with a known hydrocarbon-water contact, uncertainty in the rock volume can be significant. Discovery 34/11-J1 is a gas-condensate pool in the Brent Group reservoir in the Norwegian sector of the North Sea. The first well proved the presence of gas, and delineated the gas-water contact. The objective of the appraisal program is to reduce the uncertainty on gas-in-place.

One of the significant uncertainties is in the depth conversion, due to the low relief of the structure, and the presence of a low-velocity field over the crest. Two seismic datasets exist over the structure; the original exploration 3D survey, and a newer reprocessed cube. Interval velocities were extracted from these data, and analyzed using an in-house package, 3DEMIG, as well as ISATIS (Geovariances). Smoothing was performed on both datasets, to remove the effects of noise, and to eliminate any bulls-eyes and other features smaller than the effective spread-length of the data.

The two datasets gave significantly different results. Bulk rock volumes were computed for the discovery, using the two velocity fields derived in this study, plus other depth conversion methods. The largest and smallest bulk rock volumes differ by 20%. Depth conversion is therefore still a significant contributor to the reserves uncertainty, and the degree of uncertainty would not have been apparent from a single dataset.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria