Exploration and Assessment Methods for Petrothermal Potentials in Crystalline Basements

Geothermal energy offers vast potential for sustainable heat and electricity production. Developing and exploiting this potential applying hydrothermal systems is often bound to permeable horizons or fault zone sections. Hydrothermal potential is therefore spatially constrained, and consumer and potential need to spatially overlap for economic exploitation. Petrothermal systems on the other hand, target tight reservoir or basement formations that are ubiquitously present. Therefore, petrothermal potential can be exploited in vicinity of heat or power consumer. Low natural permeability needs to be compensated by engineering processes e.g., reservoir stimulation and is therefore artificially enhanced. By such engineering processes, natural petrothermal potential can be exploited with e.g., enhanced (or engineered) geothermal systems (EGS). Stimulation can be performed hydraulically, chemically, or thermally but reservoir stimulation also increases initial investment. However, economic feasibility and success of petrothermal reservoir systems depend on a variety of chemical and physical properties. Before development, petrothermal potentials need to be evaluated carefully on both, a regional scale to identify high-potential areas but also on a reservoir scale to build detailed reservoir models. The presented assessment offers a scalable scheme to evaluate the natural or geogenic petrothermal potential based on physical input parameters such as petrophysical properties, reservoir volume and temperature. The scheme further allows to assess EGS potentials considering chemical constraints such as petrography or radiogenic heat production but also geological or structural circumstances such as reservoir geometry or depth or engineering factors such as the recovery factor. The geothermal potential is assessed and displayed on a parameterized 3D geological model. Geogenic potential is quantitatively evaluated based on heat in place calculations. Furthermore, in a qualitative potential assessment which is based on an analytic hierarchy process (AHP) for weighting input factors, EGS potentials are qualified. The assessment scheme is demonstrated on a regional scaled basement reservoir in the Hessian Mid-German Crystalline High. Potentials are evaluated on a total of eight modeling units, representing geological units of the Odenwald such as Frankenstein complex, Flasergranitoid zone, Tromm and Weschnitz intrusion but also Heidelberg granite. These units are traced beneath the sedimentary cover of the Upper Rhine Graben using geophysical survey data. Additionally, 3D gravimetric modeling helped to delineate three modeling units beneath the Buntsandstein cover of the Odenwald and adjacent NE striking Mid-German Crystalline High in the Spessart and Rhön region. Each modeling unit is studied extensively in a petrophysical property screening on outcrop analogues and if available completed with well samples. The dataset is round off with data gathered in a petrophysical and geochemical survey in Qinghai, China, investigating a comparable reservoir system as well as literature data if necessary.