Using 3D Seismic Imaging to Identify Areas of Increased Natural Fracturing in the

Basin-Centered, Tight-Gas Sand Play of the Piceance Basin

By

Douglas B. Ostby

Williams Production RMT Company, Denver, CO

Gas production in Parachute and Rulison Fields is primarily from very low permeability, lenticular fluvial sands of the Williams Fork Formation of the Mesaverde Group. Well EURs correlate poorly with net pay. Analysis of seismic data indicates that natural fracturing associated with faulting appears to be the primary control over well productivity.

A 47-square mile 3D seismic survey was acquired in 2001 over a portion of Parachute and Rulison Fields in the Piceance Basin, to provide a tool to identify, in advance of the drill bit, the more commercial areas associated with increased natural fracturing. The seismic survey was acquired with 1100 foot receiver line spacing and 1760 foot source line spacing. The group and source point intervals were each spaced 220 feet, yielding 110 foot by 110 foot subsurface bins. The source effort was mixed source, being a combination of vibroseis and shot holes drilled with conventional shothole rigs, buggy drills, and heliportable drill rigs.

The high quality 3D seismic data allows detailed mapping of faults, revealing a structural style characterized by transpressional strike-slip features. Azimuthally processing the data helps to isolate azmuthal variations in amplitude and velocity, providing predictions of fracture density and orientation through the volume, to better define areas of increased natural fracturing.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming