Diagenesis modelling of Danish Upper Triassic - Lower Jurassic sandstones of the Gassum Formation: Estimation of geothermal reservoir quality. GEOTHERM - Geothermal energy from sedimentary reservoirs - Removing obstacles for large scale utilization. Innovation Fund Denmark: project 6154-00011B (Final report in WP3)

Diagenesis modelling of the geothermal reservoirs onshore Denmark was initiated because the effects of mechanical compaction and diagenesis on the porosity and permeability vary much across the area due to differential burial histories and mineralogical compositions. Hence, an integrated approach is necessary to estimate the reservoir quality, and the Gassum Formation is used to test the applicability of the modelling approaches. Results of organic and mineralogical temperature constrains such as vitrinite reflectance, fluid inclusions in quartz cement and oxygen isotopes in carbonate cement are used to calibrate the basin modelling using the PetroMod software, and comparable results are obtained for the different input parameters.

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 they form pervasive cementations. The Mg availability in the pore water is triggering for when ankerite can precipitate, and the largest amounts 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 reliable results as compared 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 since the quartz formation is strongly dependent on the temperature evolution.

The initial mineralogical composition and grain size varies in relation to depositional environments and source areas, so individual model scenarios take each of these settings into account to obtain a detailed estimate of the reservoir properties. In this way, a robust model for estimation of geothermal reservoir quality of the Gassum Formation has been establish, which can be used to make qualified pre-drill predictions on the basis of knowledge about the burial history and depositional setting. The establishment of corresponding models for the other geothermal reservoirs is recommended as well as using the existing seismic mapping to extend the estimated reservoir properties to obtain their spatial distribution by considering the temperature- and facies-dependence of the diagenetic processes.