f Determination of Fracture Parameters Using Local Shear-Wave Conversion

A fractured carbonate reservoir within the Upper Permian in the North-western part of the Southern Gas basin is imaged using a marine VSP. The specific objective is to determine details of likely fracturing within a 76m thick target layer at a subsea depth of 1106m. This layer is characterized by high seismic velocities, being encased in Zechstein evaporite sections with lower seismic velocities. The high seismic constrast generates a strong converted shear-wave from the incident compressional wave at moderate angles of incidence. The polarization properties of the resultant shear-waves are governed by the fracture-induced anisotropy of the Plattendolomit. Interference of the shear-wave with the direct compressional arrival creates a distinctive motion in the horizontal plane. This signature depends upon the density and orientation of the fractures within the Plattendolomit. Waveform modelling to match this behaviour indicates a high fracture intensity of about 50%, consistent with expected fracture intensity for this lithology. This technique may be of value for other future analyses of fractured reservoirs, as it is local and independent of the overburden.