Vertical Seismic Profile Experiments at the Békés-2 Well, Békés Basin

Abstract

Vertical seismic profile (VSP) experiments were conducted during 1986–1987 at the Békés-2 well site, where the well penetrated 2161 m of Mesozoic carbonate rocks beneath basin fill (TD = 5500 m). For these experiments a vertical-component, wall-locking geophone was used. Energy was generated by an explosive source for one near-offset and two far-offset VSP experiments, and by a Vibroseis source for a walk-away VSP experiment. These sets of VSP data were analyzed in conjunction with a seismic reflection profile recorded on the ground surface.

The aim of this work was to improve understanding of the structure of complexly deformed, buried Mesozoic sedimentary rocks at the well site. The geometry of deformed Mesozoic strata was modeled by means of ray-tracing techniques. The near-offset VSP data were used to identify prominent reflections below 3500 m in the Mesozoic section. These were interpreted as reflections from low-impedance layers within decollement folds. Although the far-offset and walk-away VSP data did not define the geometry of the deformed strata definitively, these data were useful in identifying truncations of strata. In contrast to the surface-recorded reflection data, the VSP data contained a lower signal-to-noise ratio for the deep part of the stratigraphic section. Because of complex deformation, individual Mesozoic stratum cannot be mapped as coherent seismic units over distances of more than a few kilometers in the vicinity of the Békés-2 well.