Case-3: Case Study: Use of 3D Seismic Interpretation Techniques to Define Shallow Gas Sand Reservoirs

Susie Mastoris

The use of advanced three-dimensional (3-D) seismic interpretation techniques and data processing applications are largely attributed to the success of a significant gas discovery drilled in December 1985, offshore California.

The 3-D seismic survey was interpreted using an interactive workstation. The techniques provided by this system allowed the seismic data to be examined in detail by highlighting subtle features such as faulting and stratigraphic thickening, and horizons to be mapped quickly and accurately. Rotational 3-D perspective views, also generated on the workstation, were used to study the 3-D relationships between the mapped horizons. These views showed gas distorted time sags along the crestal part of the structure, resulting in misleading structural highs on the sides and back portions of the structure.

To restore the true structural picture for an accurate well location, an analysis of the changing velocity patterns was studied on the unprocessed seismic gathers. This enabled the gas zones, defined by the slower velocity trends, to be mapped and incorporated into a 3-D velocity field. This resulted in velocity-corrected depth maps with the crests of the mapped horizons restored to their true spatial positions. The well drilled within a few feet of the expected tops and tested two product zones.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.