Geological characterization of Hardegsen reservoirs

Abstract

Geothermal energy is strongly dependent on the geothermal gradient; this means that with increasing depth an increase in temperature is found. The targeted formation of our research is the Hardegsen which reaches depths of around three kilometers in the West Netherlands Basin. At three kilometers depth the temperature is around 90 C˚ which is interesting for geothermal exploitation. However, rock at increasing depth generally shows a decrease in reservoir properties.
This study investigates whether the reservoir properties of the Hardegsen formation at a depth of three kilometers are still interesting for geothermal exploitation. This is done by determining to what extent the Hardegsen has been influenced by depositional history, diagenetic processes and inversion.
Determining the reservoir properties is done by studying a data set that consists of core descriptions, core plug measurements, gamma ray logs, microscopy analysis, literature studies and a field study. From the core plug measurements a simple model was created describing how porosity and permeability of the Hardegsen behave with increasing depth.
The Hardegsen succession in the West Netherlands Basin consists mainly out of (cross-bedded) arkosic fine to medium grained sandstones intercalated by 0.2-1 meter thick laterally consistent shales and shaly/silty very fine sands. High minus-porosities (up to 45 %) are reported which could have played an important role in the preservation of the reservoir properties. Core plug measurements show that the Hardegsen has good prospects for a potential reservoir with porosities ranging from 10-20% and permeability’s ranging from 50-1000 mD.
The presence of the laterally consistent shale and sandy/silty very fine sand layers is heavily dependent on the location in the reservoir. This research shows absence of these layers in wells that are located closer to the basin margin. Since these layers decrease vertical flow drastically, further investigation of the extent of these layers is needed to give a better prediction of the quality of the reservoir as a whole.