Conclusions on the Reef Complex reservoir

• Thermo-facies analysis in the SGMB (Sass and Götz, 2012 ; Homuth et al., 2015b) highlighted that dolomite units are particularly effective in terms of heat and flow con-duction. Dolomite appears more brittle than calcite, which improves its fracturability potential. Therefore, the high secondary porosity and fracture development enhance hydraulic conductivity, which in turn favours fracture enlargement by karstification.

Specific attention should then be paid on the understanding of equivalent facies occur-rence and its 3D distribution in order to predict its location within the GGB (Moeck et al., 2015).

• As mentioned above, facies analysis and measurements on rock samples, as well as results of production wells in the Malm reservoir of the SGMB highlighted overall tight, heterogeneous matrix reservoir properties. Thermal and flow behaviour is con-sequently thought to be dominated by fractures and karsts, whose expansion depends on facies (Homuth et al., 2014 ; Schulz and Thomas, 2012). According to the Reef Complex reservoir analysis showing comparable properties, such reservoir dynamic can be suitable for the GGB.

• In the GGB, advective heat transfer was identified along inverse and transpressive faults (Chelle-Michou et al., 2017), while it happens through normal faults in the SGMB (Moeck, 2014 ; Moeck et al., 2015). Inverse and transpressive faults might have originated from normal faults, created during Lower to Middle Jurassic extension phases (Gruber, in prep.). Many of them would have been later reactivated during the alpine compression, and only few would have preserved their original normal component (Clerc, in prep). Being in a more intense deformation zone of the Alpine orogeny, no or only rare extensive structural pattern seems to have persisted in the GGB, unlike the SGMB, where the flexured foreland wedge is widely extended.

The SGMB and the GGB show differences in terms of regional facies, diagenesis and struc-tural evolution, but specific smaller-scale patterns still present analogies. Comparisons with the extensively studied Malm geothermal reservoir of the SGMB have allowed to focus fur-ther research in the GGB, and have provided critical information for forthcoming exploration steps in an heterogeneous, fractured, and potentially karstified carbonate reservoir.

6.10 Conclusions on the Reef Complex reservoir

The present study of the Reef Complex unit, focussing on reservoir perspectives, includes investigations on sedimentological, structural and petrophysical properties and concepts. It has been carried out through the definition of an integrated rock typing workflow, which develops step-by-step different aspects of the reservoir assessment, which are later combined and successively superimposed. This approach consists of (1) considering independently the core, outcrop and log data, in order to preserve information provided at different scale, and better assess upscaling issues, (2) investigating the different factors which influence reservoir properties, (3) understanding which of these factors control the formation and distribution

186 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit of the pore(s) network(s), and (4) ensuring the possibility to integrate complementary data and analyses in any aspect developed, and at any step of the workflow. The latter condition has proved to be particularly important, with regard to the quality of the subsurface dataset, and planned future operations.

These investigations, integrating both surface and subsurface data of theReef Complex unit, provided the following new elements:

• Eight depositional rock types (DRT) have been highlighted, corresponding to micro-facies defined by Meyer (2000a), except DRT4 (mud mound/open lagoon), which has been newly defined according to the microfacies analysis. Additionally to microbial crusts associated with coral bioherms, microbial communities seem to build individual mud mounds, close to external, lagoonal setting, as suggested by the combination with Campbelliella-rich packstone facies. These microbial encrustment could play a key role in reef settlement while stiffening the uneven substrate.

• The entire Reef Complex unit was strongly affected by diagenetic processes, resulting in both poronecrosis and porogenesis. The distribution of diagenetic impacts is mainly controlled by original depositional facies.

• Resulting matrix pore types observed are micropores in automicrite mainly (micritic coating, microbial crusts), residual moulds, vugs and intercrystalline porosity in su-crosic dolomite.

• Two rock types defined according to diagenetically-affected DRT (DdRT) show the best reservoir properties, DdRT 4 and 5. The former is restricted to (patch) reef-related deposits, below small-scale sequence boundaries. The microporous network only appears effective according to MICP measurements, but analyses at a larger scale than plug should be performed to confirm this statement. The latter DdRT5 consists of porous sucrosic dolomite (intercrystalline mesopores), which also seem related to sequence boundaries. It is well developed at the base of the reef sequence (Calcaires de Tabalcon unit) and appears in patch reefs as well.

• Overall, the diagenetic trend responsible for porogenesis follows the depositional trend, largely affecting the more proximal western and northern part of the GGB, and de-creasing basinwards, to the ESE direction.

• Different fracture patterns have been observed in the Reef Complex unit. The dis-tribution and density of fractures are linked to intrinsic mechanical properties of the different DdRT. The early cementation, the mud content and the mineralogy, which strongly vary vertically and laterally, seem to be substantial parameters influencing the mechanical behaviour.

• Early cemented, reef-related facies and tight dolomite observed in outcrops present the most important, sub-seismic fracture networks, which has proved to be permeable in places according to well data. Complementary investigations should be performed in

6.10. Conclusions on the Reef Complex reservoir 187 order to quantify the contribution of these enhanced permeability features on reservoir transmissivity.

• Therefore, the depositional setting has revealed to be the main factor controlling the distribution of diagenetic impacts, which in turn constrain matrix reservoir properties and fracture development, according to analyses in cores and outcrop data.

• On log, different electrical rock types (ERT) have been defined, whose potential to pre-dict fractured intervals seems restricted to large faults only. When compared, porous ERT 3 and 7 correspond to DdRT 4 and 5 respectively. Upscaling to the log do-main is then conclusive for matrix reservoir properties, but a conceptual mechanical stratigraphy model has to be added to upscale properly the dual porosity-permeability network.

• The complex architecture of original depositional facies, the strong impact of diagenesis on matrix reservoir properties and the superposition of secondary, enhanced permeable features have resulted in a really heterogeneous reservoir, showing significant reservoir property anisotropy.

• The Malm geothermal reservoir of the South German Molasse Basin (SGMB) shows similarities in terms of matrix reservoir heterogeneity, property distribution and con-trolling factors. However, the reef sequence is thicker than the GGB one, and the basin exhibits a different structural configuration, which was established during the later stages of Alpine orogeny, being less tectonically constricted than the GGB. Frac-tures, and particularly the karst network have already proved to be of great influence on reservoir permeability.

Finally, this study has brought substantial insights on the sedimentology, petrophysical and mechanical property of theReef Complex heterogeneous reservoir, in support of geothermal exploration. It has also highlighted the lack of knowledge and data in specific fields, and has proposed complementary research avenues to be carried out in order to narrow the uncertainties associated with the nature of this reservoir unit in the subsurface. In particular, they should be considered to refine rock type definition and their relevance in predicting reservoir properties and quantify the influence of enhanced fracture permeability. These activities will therefore allow a proper static and dynamic modelling of this complex, dual porosity- permeability system reservoir.

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