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

Indications for the possible presence of a fluid accumulation below th e drillbit of the KTB (German Continental Deep Drilling Project)-Haupt bohrung are provided by comparisons of seismic P-wave and S-wave refle ction 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 sec tions in contrast to deeper reflections. This observation can be expla ined by the following geological models or combinations of them: (1) b rine accumulation within a strongly fractured rock reservoir, (2) liqu id/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 foli ation. Predictions concerning the nature and location of this reflecto r are expected to be verified by drilling and direct probing in spring -summer, 1993.