Abstract

One fifth of the United Kingdom's energy is derived from the gas fields of the southern North Sea. The area is divided into large fault blocks, which are laced with minor faults. Surface reflection seismic fault maps of the main producing horizon are distorted by salt rollers or pillows in the overlying high velocity Zechstein evaporites. We have developed an inexpensive meth-od of confirming the presence of minor faults in the relatively flat base of the Zechstein around a well that would simplify production planning.Deep well sonic logs indicate that seismic velocity drops abruptly beneath the base Zechstein and then rises gradually with depth. This profile is characteristic of a whispering gallery, or a buried, variable-index beam waveguide. If base Zechstein faults scatter energy efficiently into the waveguide, then the faults might be mapped by moving a seismic source over the surface and recording the mode converted arrivals at a geophone within the guide. To test this possibility, over 3000 shots were fired at 27-m intervals in thirteen lines spaced by nearly equal 500 m from a 4436-cu in. (0.075-cu m) marine air gun array into a 3 km deep triaxial VSP geophone, stationed beneath the Rotliegendes. Secondary arrivals were found to move out across the raw field geophone gathers in the manner expected of down-going compressional to both guided compressional and guided shear vertical mode conversions [P-P and P-SV] at the base of the Zechstein. Prestack migration yielded a map of mode conversion activity at the edges of known faults with throws of 30 + m over an area of 10 X 10 km 2 .We conclude that there is an extensive seismic waveguide beneath the Zechstein in the southern North Sea. Faults scatter down-going P waves efficiently into the guide; noise levels are low at the bottom of a borehole. Vertical reverberation follows a surface source as it moves away from the wellhead. Horizontal waveguiding reduces geometric spreading loss. These factors make it possible to map small reservoir faults with high-resolution at long ranges from an exploration well.

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