f Multicomponent VSP imaging of tight gas sands

Low porosity Bossier and Cotton Valley sands of the East Texas Basin, USA, have only a small acoustic impedance contrast with the encasing shales but a greater relative contrast in shearwave impedance. Thus multicomponent imaging may be more effective than P-wave imaging in exploring for these sand bodies.Vertical Seismic Profile data (VSP) acquired with both a near-offset and far-offset P-wave source clearly demonstrate the P-P reflectivity and P-S mode conversions within the Bossier section. While Bossier P-P reflectivity is low, it appears to be adequate for mapping thick sand bodies such as the York Sand, the main exploration target in this area. However P-P reflectivity is even lower for the overlying Cotton Valley Sands and is inadequate for imaging. In contrast, the far-offset VSP data demonstrate a high level of P-S mode conversion which is used to image this interval with high definition that is not provided by P-P reflectivity. This provides strong support for the use of P-S mode conversion imaging for seismic characterization of tight sand reservoirs. A P-P and P-S 3-D seismic survey was also acquired in conjunction with the VSP program. These data are shown in an accompanying poster. Near-offset shearwave VSP data acquired with a shearwave source show low signal/noise ratio and limited bandwidth for the downgoing waveform which we attribute to poor surface coupling or near-surface transmission effects. Such effects may also have a strong negative impact on multicomponent imaging of these sands using surface seismic techniques. We suggest that multicomponent 3-D imaging may provide a superior solution.