CSEM Surveying For Geothermal Exploration In Deep Sedimentary Basins

Abstract

Sedimentary basins have significant potential for low to medium enthalpy, deep geothermal energy resources. These resources are generally assessed using standard seismic exploration techniques to resolve geological structures. However, the electrical resistivity parameter, which can be directly impacted by the presence of a geothermal reservoir, is rarely investigated in such context. Therefore, the development of alternative and complementary exploration techniques such as Electromagnetic (EM) techniques may have an important role in reducing the cost and uncertainty associated with geothermal resource assessment. While EM techniques have proven to be useful in geothermal exploration in high enthalpy areas in the last decades, only a handful of studies assessed their applicability in low enthalpy sedimentary basins. There, challenges include identifying which sub-surface features can cause changes in electrical resistivity as low enthalpy reservoirs are unlikely to exhibit the hydrothermally altered clay layer above the geothermal aquifer that is typical for high enthalpy reservoirs. Yet a principal challenge is likely to be the high level of industrialization in the areas of interest. Infrastructure such as train tracks and power cables can create a high level of background noise that can obfuscate the relevant signal. Within the frame of national and European research programs such as the FP-7 IMAGE project, we have therefore undertaken to tackle these challenges by developing EM techniques specifically tailored to low enthalpy sedimentary basins. Although we developed processing algorithms to enhance the signal to noise ratio of passive EM measurements such as Magneto-Telluric (MT) soundings, we focused mainly on active EM techniques and in particular on the Controlled- Source Electromagnetic technique (CSEM). Indeed, the use of EM transmitters allows acquiring high quality datasets despite the presence of powerful and unpredictable background noise. In this paper, we will present the results from our research and development for geothermal exploration in deep sedimentary basins and illustrate our workflow with CSEM surveys acquired in several different sedimentary basins in Europe. We will show that in areas with high level of industrialization, it is indeed the most effective technique to image deep resistivity variations. We will also touch upon the remaining resolution and interpretation challenges that remain when using resistivity measurements for geothermal project de-risking in sedimentary basins.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018