Abstract

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. However, well placement close to faults is a significant risk factor due to expected mineralization of the fault and nearby shales.

A wide azimuth 3D survey was acquired to highlight areas exhibiting seismic velocity anisotropy for detection of open fractures. Sweet spots would be areas of anomalously low Poisson’s ratio (PR), away from faults, with high velocity anisotropy. This study documents the results of an integrated workflow of data processing, pre-stack seismic inversion and Ant Tracking to successfully characterize faults and fractures and to identify sweet spots in the subject gas shale.

During data processing, azimuthal anisotropy analysis was done to determine the dominant direction of Vfast and Vslow. In general, there is good agreement between the azimuthal seismic data processing fractogram and Ant Tracking results. Corrections for HTI medium were applied prior to VTI anisotropic Kirchhoff prestack time migration (KPSTM). Two full azimuth 3D seismic volumes were created: a prestack volume as input for inversion and a stacked volume as input for Ant Tracking.

ISIS simultaneous inversion was done on prestack data to invert for acoustic impedance (AI) and Poisson’s ratio (PR). A blind test of the inverted attributes is very encouraging. Areas of low PR away from faults are thought to be promising hydrocarbon leads or prospects.

Ant Tracking reduces the risk of drilling near faults by providing a high resolution image of fractures and faults beyond what can be interpreted from conventional seismic data.

The resulting integration of Poisson’s ratio, anisotropy, and Ant Tracking results provide effective delineation of areas with superior porosity/charge, areas with open fractures, and areas with faulting, outlining likely sweet spots as well as areas better avoided in drilling.

Integration of azimuthal anisotropic data processing with pre-stack seismic inversion and Ant Tracking provides a superior tool to explore for gas in gas shale.