RESEARCH INTO THE POSSIBILITY OF DIFFERENTIATING BETWEEN COMPACT AND KARST AFFECTED LIMESTONE IN THE SUBSURFACE BY SEISMIC TECHNIQUES
The joint project by the Belgian Geological Survey, Entec Energy Consultants Ltd and Prakla Seismos Gmbh, designed to develop a surface seismic technique to differentiate between tight and karst affected limestone at depth. The Lower Carboniferous limestone in North Belgium was chosen as test site because of its importance for low enthalpy geothermal energy. A seismic campaign (172 km) was carried out over the area. Initially an approach was designed based on the detection of elastic waves back scattered by the inhomogenous limestone. This approach was abandoned as initial seismic results and further geological study showed that the bulk properties of the karst affected limestone had to be emphasized rather than the scattering by larger karst features. The next approach was used on part of the original seismic campaign but also new seismic data obtained through the Geological Survey and Distrigas N.V. Indeed the initial campaign showed a completely unexpected new geological geometry in the paleozoic of the extreme north of Belgium; the newly discovered structure is of interest for the eventual presence of natural gas. Experimental processing was carried out on selected profiles and the most promising technique seemed to be the partial offset dependent stacking. A theoretical model was worked out to simulate the experimentally obtained amplitude changes with off-set in the Dinantian top reservoir. The approach was finally checked at the Merksplas-Beerse I well, where all necessary logs were available (full wave, sonic, density, VSP, and EVSP) and a seismic profile as well. All petrophysical parameters required as input in the model could reasonably well be derived from the log analysis and hence the theoretical response of the karst intervals in the well section could be simulated. The results fitted quite well the observed amplitude changes on the partial offset dependent stack sections produced from the seismic line through the well. Besides it was demonstrated that these changes would be absent in the absence of a karst interval. Also it was shown that the graphic representation of the amplitude changes with offset could lead to a three dimensional geometric visualization of the karst distribution. It is recommended to test the method in a new well.
Bibliographic Reference: EUR 9706 EN (1985) MF, 118 P., BFR 300, EUROFFICE, LUXEMBOURG, POB 1003 - NO BLOW-UP AVAILABLE!
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Record Number: 1989123039200 / Last updated on: 1987-01-01
Available languages: en