Clastic Reservoir Description from Pre and Post-Stack Seismic Data

Keith Wrolstad and Frederic Lefeuvre

Clastic reservoir properties were estimated from pre and post-stack seismic data using different methods independently by TOTAL and UNOCAL. Sand versus shale and gas versus wet saturation were predicted using a post-stack, multi-attribute statistical method with 3D data. Sand thickness, sand porosity, and sand fluid saturation were estimated quantitatively on a 2D regional line crossing the 3D volume using pre-stack elastic synthetic and field seismic cross-correlation analysis. Both methods required calibration at one or more wells intersecting the reservoir. The two approaches were then compared to give more confidence in the estimated distribution of shale, sand porosity, fluid saturation, and sand thickness.

The results of the two approaches lead to very comparable images obtained without any prior geological interpretation of the seismic data and fit surprisingly well with the known geological model of the area. The reservoir structure is composed of shingled prograding bars toward the west with a NE/SW orientation linked to the main depositional input coming from a northeastern direction. Prior to any wavelet shaping which would force a synthetic tie, the correct properties in the sand reservoirs under investigation were automatically picked at Well 1. The lateral gas/water contacts were precisely resolved in the pre-stack inversion and agree approximately with the post-stack results in map view. Estimated porosity changes correspond to predicted attribute changes in map view. The 2-D i version was used to calibrate and increase confidence in the 3D post-stack inversion results which, in turn, should be used to better constrain the reservoir model for fluid flow simulations.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California