The prediction of porosity in the Permian (Zechstein 2) carbonate of eastern Netherlands using seismic data

Abstract

The depositional model of the Zechstein 2 carbonate section is of such a complex nature that it is not possible to predict high‐porosity zones solely from geologic data. We describe a two‐phase study into determination and prediction of porosities in the Zechstein 2 carbonate in the eastern Netherlands. Phase 1 was the basic feasibility study and consisted of two parts, namely, a model study followed by a well‐log study. In both parts, acoustic impedance logs were filtered to test the feasibility of resolving porosity changes within the frequency range of seismic data. Based on our comprehensive model of the environment of deposition of the Zechstein evaporites, a number of acoustic impedance logs were constructed, each log representing a particular environment. These logs then were filtered with a suite of band‐pass filters. It was our observation that the gross changes in porosity were easily recognized. The model study was followed by a study of acoustic impedance logs from actual downhole measurements taken over intervals with depositional environments similar to those represented in the model study. In spite of geologic variations, the changes in porosity were easily observable. Phase 2 involved the prediction and verification of porosity. An exploration well, planned to penetrate the Zechstein 2 carbonate, was connected to a previously drilled carbonate test by a good quality seismic section. In the previous well, porous limestone approximately 60 m in thickness was penetrated. From the combined well and seismic data, we predicted that a similar development of porosity could be expected in the new location. Subsequent drilling proved the presence of porous carbonate approximately 50 m in thickness developed in an environment of deposition similar to that encountered in the other well. This technique has allowed us to predict reservoir porosity and will be used to develop new structural/stratigraphic prospects.