Seismic Reservoir Characterization of a Gas Shale Utilizing Azimuthal Data Processing, Pre-Stack Seismic Inversion and Ant Tracking
David Paddock, Christian Stolte, John Young, Lei Zhang, and Javaid Durrani
Prospective hydrocarbon-bearing zones in the subject gas shale are characterized by gas entrapped in the sediment matrix with some additional open-fracture component. This gas is economically recovered by horizontal drilling and fracturing. We show the results of an integrated workflow to get a good seismic image in the presence of anisotropy, find the sweet spots, and characterize fractures and reservoir risk.
Seismic azimuthal anisotropic analysis determines the dominant direction of the fast and slow seismic velocities indicative of maximum horizontal stress and / or fracture orientation and intensity. Corrections for HTI medium were applied prior to VTI anisotropic Kirchhoff pre stack time migration (KPSTM). Data processing created a prestack volume as input for inversion and a stacked volume as input for Ant Tracking.
Simultaneous inversion of the pre-stack data determined acoustic impedance (AI) and Poisson’s ratio (PR). A blind test of the inverted attributes is very encouraging. Sweet spots may be areas of anomalously low Poisson’s ratio (PR), away from faults, with high velocity anisotropy.
Ant Tracking reduces the risk of drilling near faults (a reservoir risk due to expected mineralization) by providing a high resolution image of fractures and faults beyond what can be interpreted from conventional seismic data. Faults were expected to be mineralized, with some mineralization extending out into the surrounding shales.
The resulting integration of Poisson's ratio, fractogram, and Ant Tracking provides effective delineation of areas with superior porosity and charge, areas with open fractures, and areas with faulting, outlining likely sweet spots as well as areas to be avoided in drilling.
Integration of anisotropic data processing with pre-stack seismic inversion and Ant Tracking provides a superior tool to explore for gas in gas shale.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas