Upper Jurassic Geothermal Reservoirs of South Germany: Characterization and novel exploration strategy using an integrated sequence stratigraphic approach

Abstract

This study systematically investigates the geothermal reservoir properties of the approximately 500-600 m thick Upper Jurassic carbonates in the subsurface of the Molasse basin, where only specific intervals contribute to the geothermal aquifer. Therefore, this study integrates all relevant aspects of geothermal reservoir characterization in carbonates following an 1-D (core and well data) to 2-D (correlations) to 3-D (seismic interpretation and integration) approach. This hierarchical workflow, which is based on reservoir characterization workflows established by the hydrocarbon industry is then modified to understand the characteristics and distribution of geothermal reservoir types involving the following steps: (1) core-based facies analysis, (2) borehole image and well log analysis, (3) sequence-stratigraphic framework, (4) interpretation of depositional environments, (5) identification of potential reservoir types and (6) integration of dynamic data. The core-based facies analysis reveals twentytwo lithofacies types, three depositional sequences for the Upper Jurassic and one for the Upper Jurassic/Lower Cretaceous Purbeck Formation. Large parts of the interpreted depositional environment correspond to a relatively deep (below storm-wave base) deposetting. Towards the top of the succession, a change to a shallower, higher water energy environment is observed, with clear indications of exposure. A newly established multi-proxy workflow for borehole image interpretation in carbonates reveals novel opportunities to link the core-based observations with the borehole images and shows how to maximize the value of information of such logs. The vertical stacking of nine distinct borehole image facies types, as well as a semi-quantitative charting of stratal surfaces, provided new insights into the sequence-stratigraphic development and the characterization of geothermal reservoir types. The evaluation of the 3D seismic survey yielded a best practice approach to differentiate three dominating seismic facies types based on their reflectivity and to differentiate between potential reservoir facies and non-reservoir facies. The results of the 1-D and 2- D analysis were used to validate the interpretation on various scales and draw 4 conclusions, that can be applied directly to the early exploration stage of emerging geothermal projects in the Molasse basin. The local, regional and basin-wide sequence-stratigraphic correlation show thickness variations of the Upper Jurassic carbonates. The integration of paleotectonic aspects into the sequence-stratigraphic interpretation adds to the understanding of these thickness variations. Along with the thickness variations, a change of depositional environment and facies types were documented. All these factors affect the characteristics and the distribution of potential geothermal reservoir types. Ultimately, the integration of dynamic data allows a ranking of the geothermal reservoir types and a subdivision into “high-k dominated” and “matrix-dominated” geothermal reservoir types. It shows that the investigated Upper Jurassic geothermal field of the study area is a combination of structural and stratigraphic traps. The sequence stratigraphic context explains the distribution of the preferential fluid pathways and is a crucial element for a successful and sustainable geothermal development strategy in Upper Jurassic carbonates.