Estimation of Geothermal Reservoir Quality by Diagenesis Modelling of Gassum Formation Sandstones
Geocosm’s Touchstone system is proving to be useful for the exploration of geothermal energy resources for RQC consortium member GEUS (Geological Survey of Denmark and Greenland) as discussed in the presentation below at the 34th Nordic Geological Winter Meeting in Oslo, Norway, January, 2020.
Olivarius, M.1,*, Tremosa, J.2 , Mathiesen, A.1 , Weibel, R.1 , Dalgaard, M.T.1 , Vosgerau, H. 1 , Nielsen, L.H.1 , Whitehouse, M.J.3 , Bonnell, L.M.4 & Lander, R.H.4 1 GEUS, Geological Survey of Denmark and Greenland, Copenhagen, Denmark 2 BRGM, French Geological Survey, Orleans, France 3 Swedish Museum of Natural History, Stockholm, Sweden 4 Geocosm LLC, Durango, Colorado, USA * Email: mol@geus.dk
The effects of mechanical compaction and chemical diagenesis on the porosity and permeability of the Upper Triassic – Lower Jurassic Gassum Formation vary greatly across the Norwegian–Danish Basin due to variations in burial histories and initial mineralogical compositions. Hence, an integrated approach is necessary to estimate the reservoir quality and to establish where the Gassum Formation sandstones are suitable for geothermal energy exploitation prior to drilling campaigns. Diagenesis modelling of the sandstones is conducted for this purpose and the applicability of the modelling approaches in this setting is tested. Results of organic and mineralogical temperature constraints such as vitrinite reflectance, fluid inclusions in quartz cement, and oxygen isotopes in carbonate cement give comparable temperatures that are used to calibrate the basin modelling using PetroMod. Thermodynamic modelling of the mineralogical reactions by SURP provides important knowledge about the conditions for dissolution and precipitation of carbonate minerals, which result in low reservoir quality when pervasive carbonate cement is formed. The Mg availability in the pore water dictates where ankerite can precipitate, and the largest amounts of ankerite cement are able to form when both calcite and siderite are available for dissolution. Integration of petrographic observations, mineralogical compositions, reservoir properties, depositional environments, and burial histories in the calibration of compaction and diagenesis modelling by Touchstone produces results that are in good agreement with test samples and analog data. The calculated quartz cement volumes correlate acceptably with the measured values, showing the reliability of the modelled burial histories given that quartz precipitation is strongly dependent on the temperature evolution. The initial mineralogical composition and grain size distribution vary in relation to depositional environments and sediment source areas. Therefore, individual model scenarios need to consider each of these settings to obtain a detailed estimate of the reservoir properties. In this way, we were able to establish a robust model for estimation of geothermal reservoir quality of the Gassum Formation, which can be used to make pre -drill predictions in light of the burial history and depositional setting for sites of interest.