(1) Pemex Exploracion y Produccion / Colorado School of Mines, Villahermosa, Tabasco, Mexico
(2) Colorado School of Mines, Golden, CO
ABSTRACT: Influence of Collapse Zones on Multicomponent 4D Seismic Data at Vacuum Field, New Mexico
Phase VII of the Reservoir Characterization Project at Colorado School of Mines involved a 4D (time-lapse), multicomponent seismic study composed of the repeated acquisition of a 3D, 3C seismic survey during a CO2 injection program at Vacuum Field, New Mexico. As part of this project, an integrated analysis of multicomponent seismic data, well logging, soil and hydrologic studies, was conducted to provide an understanding of the nature of shallow heterogeneities, i.e. a chain of three small playa-lakes, and their effects on multicomponent seismic signal. Geologically, a salt/anhydrite section, extending from 1000-3000 feet below the surface, overlays the carbonate-prone section that hosts the reservoir rocks.
The integrated analysis characterized the three playa-lakes into two groups, the southwestern playa-lakebed composed of unconsolidated sediments, and the other two with thinner and more compacted composition. Evidence shows that the chain of playa-lake depressions is an aligned pattern of collapse events produced by evaporitic dissolution at depth. The dissolution was caused by long-term ground-water infiltration, under structural control.
Seismically, the water-table is the boundary for the P-wave near-surface layer but not for the shear-waves which are mostly attenuated and distorted by these heterogeneities. The thinning effect produced by the dissolution, introduces lateral velocity variations affecting the image beneath the evaporitic zone. Shear-wave anisotropy analysis revealed that the effects caused by playa-lakes on seismic signal are static, therefore, the low rate of dissolution can not be considered as a "dynamic effect" from the time-lapse interpretation point of view.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado