Depositional environments

The vertical and lateral evolution of depositional environments and related facies of the Malm unit in the SGMB has been compared to the GGB, and few differences have been highlighted.

This unit is divided into six intervals according to lithological and sedimentological criteria, called Malm α, β, γ, δ, and ζ (Figure 6.32). Dirner and Steiner (2015), who performed facies modelling in Malm aquifers of the Munich area, provide a good overview of facies

178 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit

Figure 6.32: Stratigraphic framework of the South German Molasse Basin (SGMB) (Homuth et al., 2015a).

encountered in these different Malm intervals, and of their distribution across the SGMB (Figure 6.33). The first three intervals (α, β, and γ) correspond to tight bedded limestones and marls. Facies show mainly open-marine to basinal, muddy and marly sediments, in which reef buildups started to grow locally on palaeo-highs during the Middle Oxfordian. These build-ups are mainly characterized by siliceous sponges and microbial crusts. This unit might correspond to the Couches de Birmensdorf in the GGB, whose thickness is largely reduced comparatively. In the GGB, sedimentation evolved towards deeper environments during the Late Oxfordian-Early Kimmeridgian (Marnes d’Effingen to Couches à céphalopodes), whereas reef facies became progressively more developed in the SGMB.

The Middle Malm (Kimmeridgian) units δ and consist of a massive, dolomitized interval which is mainly made of oolithic platform sand associated with microbe-siliceous sponge mounds ("spongiolites"). These deposits form a significant belt of mixed bioclastic sand and biogenic constructions (Koch et al., 1994 ; Reinhold, 1998). Intra-basinal to lagoonal well-bedded limestones surround the reef facies, interfingering or onlapping onto the reefs (Gwinner, 1976 ; Pawellek, 2001 ; Homuth et al., 2015b). The top of the Malm unit is

6.9. The South German Molasse Basin: a geothermal reservoir analogue? 179

Figure 6.33: Model of the depositional environments for the Malm reservoir unit in the SGMB (Dirner and Steiner, 2015).

brecciated, and marks a regional sequence boundary at the end of the Kimmeridgian (Koch et al., 1994).

The Malm ζ unit is characterized by the large expansion of reefs following the transgressive trend at the beginning of the Tithonian. Coral fauna replaced gradually sponges at the end of the Jurassic period, which is marked by another large scale regression (Homuth et al., 2014, and references therein). Overall, the Upper Malm (Tithonian) ζ unit shows three main dif-ferent facies. The first two consist of reef buildups partially dolomitized and related peri-reef grainstones/rudstones rich in reef-builders debris (mainly sponge-algae-ooid association, and corals on top of the reef sequence)("Rifffazies", also called "Massenkalke"). Bioconstructions reach heights of 50 to 80 m and diameters up to 500 m (Meyer and Schmidt-Kaler, 1996).

The latter facies is made of bedded limestones intercalated with marl layers, which were de-posited in lagoonal and intra-basinal environments ("Bankfazies" or Lagunenfazies") (Meyer and Schmidt-Kaler, 1989 ; Wolfgramm et al., 2009) (Figure 6.34). Deeper-shelf, muddy to marly sediments known as Helvetic facies are finally found in the southern part, and define the southern boundary of the Malm aquifer in the SGMB (Villinger, 1988).

In comparison, the end-Jurassic regressive trend in the GGB is marked by the basinward progradation of patch reefs (to the ESE) during the Tithonian, while most proximal bio-constructions were progressively reworked. Reef-builder organisms were completely absent throughout the Tithonian, and theReef Complex unit was finally covered by more proximal, largely developed tidal flat sedimentation (Tidalites de Vouglans Fm), which comparatively

180 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit

Figure 6.34: Distribution of palaeoenvironments in the Malm reservoir of the SGMB (Wolf-gramm et al., 2009).

6.9. The South German Molasse Basin: a geothermal reservoir analogue? 181 appeared only during the Berriasian in the SGMB (Purbeck unit). Consequently, the GGB and SGMB reef sequences are slightly diachronic, and lasted longer in the latter basin.

Reef fauna is also slightly different between the two sequences, and bioconstructions are more largely developed in the SGMB than in the GGB. Whilst sponge reef precursor began to grow simultaneously during the Middle Oxfordian, their development was rapidly hampered in the GGB (higher subsidence rate leading to deeper platform setting?), while they thrived in the SGMB, forming a massive unit. Similarities between the GGB ad the SGMB appear more clearly on top of the reef unit, because microbe-sponge biotopes were colonized by coral fauna in the SGMB. This transition is usually characterized by aTubiphytes limestone layer, which shows alternating boundstone and packstone textures, and developed principally in high energy, shallow part of the reef front (Pomoni-Papaioannou et al., 1989). Thus, the latter interval is comparable to the Calcaires de Tabalcon unit, although Tubiphytes never formed boundstone in the GGB. Overall, in terms of facies and depositional environment, only the top of the reef sequence included in the Malmζunit can be considered as equivalent to theReef Complex unit of the GGB, although reduced thickness and expansion of reefs, as well as limited reef builder diversity in the latter testify to more precarious living conditions compared to the SGMB.

6.9.3 Reservoir properties

In the SGMB, tectonic structures and karstification control mainly the hydraulic conduc-tivity of Malm aquifers, whereas the development of karst and their characteristics depend on facies (Homuth et al., 2014 ; Schulz and Thomas, 2012) and on their position within the platform. Karstification is more effective in porous and permeable rocks, or along open frac-tures which enhance the hydraulic flow. Thus, in the SGMB, reservoirs are mainly located in reef units and/or dolomitized intervals, which hold higher intrinsic matrix porosity and per-meability than tight, well-bedded and platy limestones. Hence, δ, and ζ units contain the main producing aquifers (Böhm et al., 2013) (Figure 6.35). Fractures caused by differential burial between weak (platy limestones and marls) and more stiffen and brittle facies (bio-constructions) also promote fluid flow, and consequently dolomitization and karstification in reef bodies and their altered contours. Additionally, faults and fractures also favour fluid circulation in tight, well-bedded and platy limestones, and enable the connection between better reservoir bodies (Homuth et al., 2014).

Reef reservoirs are characterized by packstone/floatstone and grainstone facies which show the highest primary porosity related to reef framework. It is often overprinted by secondary, intercrystalline porosity which formed during multiple dolomitization and de-dolomitization events (Reinhold, 1998 ; Wolfgramm et al., 2011), enhancing reservoir properties. Dolomiti-zation can be total or partial. It formed mainly in residual primary porosity which survived the first cementation phase in reef buildups, and along permeable fractures and solution seams in tighter bedded and platy limestones as well. According to Reinhold (1998), it formed under shallow-burial conditions, with only slightly modified seawater in pores and pumped by differential compaction between stiff and muddy, softer facies. It is not dis-tributed regularly across the basin, especially in the Malm ζ unit. Resulting petrophysical

182 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit

Figure 6.35: Conceptual 2D cross-section of the Malm reservoir in the SGMB (Böhm et al., 2013).

properties are laterally and vertically very heterogeneous in this unit (table 6.4). Compar-atively, the massive δ and units are considered more homogeneous with respect to the distribution of productive facies, related petrophysical properties and hydraulic conductiv-ity. This could be explained by the fact that mound facies are more largely dolomitized, and dolomite occurrence has a substantial impact on thermal and permeability properties, be-cause secondary intercrystalline porosity have been preserved (Homuth et al., 2014). Overall, matrix porosity and permeability are usually quite low (φ< 15%; K from 0.001-10 mD), but can reach respectively 18% and up to 10 mD in reef, dolomitized facies.