Offset-dependent seismic amplitudes from karst limestone in northern Belgium

In northern Belgium, karst limestones at the top of the Lower Carboniferous form a water-bearing reservoir at depths varying from 1000 to more than 2000 m. The temperature of the water and the hydrological characteristics make it an attractive reservoir for low enthalpy geothermal energy extraction. The depth range of the reservoir, its structure, and the sealing characteristics of the overlying rocks imply that it might form a trap for natural gas. The hypothesis that natural gas might he present in this Lower Carboniferous reservoir was recently confirmed by the discovery of a dissolved gas mixture, with methane, at the Merksplas-Beerse well. In the light of this varied economic potential of the reservoir, a preliminary seismic exploration programme was carried out by the Belgian Geological Survey. The reservoir properties, however, are rather heterogeneously distributed, as the pore space is made up of dissolution pockets and fractures. Therefore, a detailed analysis of the seismic reflection data was instigated in order to assess the potential of the seismic method for the detection of the karst distribution. This paper deals with the results of this analysis.