FLUID RESERVOIR (QUESTIONABLE) BENEATH THE KTB DRILLBIT INDICATED BY SEISMIC SHEAR-WAVE OBSERVATIONS

Indications for the possible presence of a fluid accumulation below the drillbit of the KTB (German Continental Deep Drilling Project)-Hauptbohrung are provided by comparisons of seismic P-wave and S-wave reflection sections. A bright reflection with negative polarity at approx. 8 km depth is seen only on P-wave sections. It is absent on S-wave sections in contrast to deeper reflections. This observation can be explained by the following geological models or combinations of them: (1) brine accumulation within a strongly fractured rock reservoir, (2) liquid/gas interface within strongly fractured rocks, (3) rock composition with increased quartz content and corresponding low Poisson's ratio, and (4) varying symmetry system of seismic anisotropy due to rock foliation. Predictions concerning the nature and location of this reflector are expected to be verified by drilling and direct probing in spring-summer, 1993.