Challenges for geothermal exploration- fluviatile systems and new interpretation techniques

What are the main challenges of exploring geothermal reservoirs with low temperatures, so-called low-enthalpy systems? A typical situation in European countries, especially Germany, is regionally different in high information density due to former intensive exploration activities for hydrocarbons. Regionally there are concepts of more or less successful exploitation for geothermal projects based on special subsurface conditions, which allow high water production at high temperatures. The development of such regions where geothermal energy plays an essential role in the energy supply extends over decades. Uncertainties in reservoir characterization lead to high investment risk and a lack of economic viability. The increase in energy costs and the intensified search for climate-neutral energy has recently intensified the search for suitable reservoirs for geothermal use. In this context, the development times of geothermal projects must be shortened, and possible reservoirs must be systematically surveyed. Exploration plays one or even an essential role. Even if the state assumes costs and risks, concepts for exploration must be developed according to the current state of the art.

A closer look reveals that even in areas with a high information density, more than knowledge about the subsurface beyond the hydrocarbon reservoirs is needed for geothermal use. The exploration goal in geothermal energy so far has been mainly structural exploration. To better estimate the reservoir quality, understanding the depositional space and subdivision into different facies is necessary. These tasks are then the basis for estimating petrophysical parameters for reservoir characterization. Here, newer interpretation techniques help to perform such tasks quickly.

An example of an interpretation of a 3D seismic dataset from the Bavarian Molasse illustrates this. The Tertiary sediments of the Molasse basin are composed of marine sequences, redeposition of debris fans of the adjacent Alpine region and drainage systems of the basin. Due to regionally different uplift and subsidence of the basin, east-west directed trends exist in the respective depositional systems. In the region studied here, these changes are extreme. The goal was to find and map evidence of fluvial systems in the depositional settings. The seismic patterns are very heterogeneous, and there are only a few continuous reflectors to subdivide the study area. Indications of fluviatile deposits are not evident at first.

By calculating local inclinations and inversion techniques, a variety of so-called phantom horizons can be generated and then visualized with different seismic attributes. This way, fluviatile patterns could be recognized in a 300m thick part of the Aquitaine. The mapping of these patterns followed a further step by exploiting the corresponding signals' similarity and spatial connections. Machine learning can perform further facies discrimination but cannot recognize these fluviatile patterns.

The work showed that existing data, with appropriate processing and new interpretation methods, can be efficiently used to search new geothermal reservoirs systematically. The scientific challenge that industrial orders cannot solve is a more detailed analysis of the evolution of the sedimentation space. However, this is necessary for the transfer of results to neighbouring areas.