ESTIMATION OF RESERVOIR FRACTURING FROM MARINE VSP USING LOCAL SHEAR-WAVE CONVERSION

A marine VSP is designed to estimate the orientation and density of fr acturing within a gas-producing dolomite layer in the southern North S ea. The overburden anisotropy is firstly estimated by analysing shear waves converted at or just below the sea-bed, from airgun sources at f our fixed offset azimuths. Full-wave modelling helps confirm that the background has no more than 3% vertical birefringence, originating fro m TM anisotropy with a symmetry axis orientated perpendicular to the m aximum horizontal compressive stress of NW-SE. This finding concurs wi th current hypotheses regarding the background rock matrix in the uppe r crust. More detailed anisotropy estimates reveal two thin zones with possible polarization reversals and a stronger anisotropy. The seismi c anisotropy of the dolomite is then determined from the behaviour of locally converted shear waves, providing a direct link with the physic al properties of its fractures. It is possible to utilize this phenome non due to the large seismic velocity contrast between the dolomite an d the surrounding evaporites. Two walkaway VSPs at different azimuths, recorded on three-component receivers placed inside the target zone, provide the appropriate acquisition design to monitor this behaviour. Anisotropy in the dolomite generates a transverse component energy whi ch scales in proportion to the degree of anisotropy. The relative ampl itudes, for this component, between the different walkaway azimuths re late principally to the orientation of the anisotropy. Full-wave model ling confirms that a 50% vertical birefringence from TM anisotropy wit h a similar orientation to the overburden is required to simulate the field observations. This amount of anisotropy is not entirely unexpect ed for a fine-grained brittle dolomite with a potentially high fractur e intensity, particularly if the fractures contain fluid which renders them compliant to the shear-wave motion.