Global correlations using sequence stratigraphy and chronostratigraphy have been exten-sively discussed over the last decades (Vail et al., 1977 ; Haq et al., 1987 ; Vail et al., 1991
; Mitchum and Van Wagoner, 1991 ; Miall, 1992 ; de Graciansky et al., 1998 ; Posamentier and Allen, 1999). Christie-Blick et al. (2007) suggest that correlations at the basin scale can be more confidently considered, because a reduced study area compared to global assessment allows a better constraint on local sea-level driving-processes such as tectonism, subsidence or climatic variations. Based on this argument, the following paragraphs locate interpreted 2nd-order sequences in the regional setting of the northern Tethyan realm.
Assessing correctly smaller-scale depositional sequences in carbonate rocks implies ad-ditional parameters, because of the higher diversity of stacked sedimentary bodies com-pared to siliciclastic systems (Read, 1982, 1985 ; Pomar and Kendall, 2008 ; Reijmer et al., 2015). Intrabasinal conditions, physical and ecological accommodation, biotic associations and seafloor morphology control the carbonate factory productivity and therefore platform and geobody geometry through time (Pomar and Kendall, 2008 ; Pomar and Haq, 2016).
These parameters are not easy to assess, and require the combination of numerous analyses (sedimentological study comprising structure identification at the macro and micro scale, microfacies by conventional and more sophisticated microscopy; mineralogical, palaeonto-logical and geochemical studies (Tucker, 1981)), performed in 3D, along continuous sections
80 Chapter 3. Multidisciplinary study of Humilly-2 well detailed at the centimetre scale. In this work, the core study provided a large part of this essential information on several scattered intervals. Since the available core material has a limited vertical extension, it has been associated with two well-documented, high-resolution regional field studies, which focus on the Bajocian (Piuz, 2004) and Kimmeridgian Reef Complex (Meyer, 2000a) intervals. Thus, this approach has enabled us to identify in the cored intervals sequences at a smaller scale than the 2nd order, and better constrain the carbonate platform evolution of these two potential reservoir units.
The nine discontinuities identified along the HU-2 sedimentary succession correspond to four successive transgressive-regressive (T-R) episodes, related to 2nd order sequences. The first one extends over the entire Triassic period, and the MFS interpreted in the Upper Muschelkalk is in line with observations in the northern part of the Central Europe Basin (CEB)(Franz et al., 2013, 2015). Marine ingresses at the base of the Muschelkalk are also supported by the microfacies analysis and interpreted depositional environment (subsection 3.2.3).
The second T-R episode covers the Liassic up to the Aalenian discontinuity. In the re-gional context, a wide transgression is known to occur from the Late Triassic-Early Liassic, until the transgressive peak in the Toarcian dark shales (Ziegler, 1990 ; Guillocheau et al., 2000). It supports the interpretation of this MFS in the Schistes cartons unit. In the liter-ature, this transgressive event is related to Alpine Tethys rifting (Ligurian part) (Dumont, 1988 ; de Graciansky et al., 1998 ; Guillocheau et al., 2000 ; Beccaletto et al., 2011).
The following T-R episode starts in the Aalenian, and ends at the beginning of the Cretaceous (Berriasian) with a facies and fauna which indicate shallow marine to continental environments, as well as repeated subaerial exposures (Goldberg Fm)(Strasser and Davaud, 1982, 1983 ; Müller, 1986). In this sequence, the basal unit (Aalenian to mid-Bajocian) was divided in several 3rd(?) to 4th-order shallowing upward sequences by Piuz (2004), based on the abundance through time of three facies associations (Figure 3.14). According to Durlet and Thierry (2000) and Thiry-Bastien (2002), these sequences are related to tectonic, low-amplitude eustatic variations. Overall, it corresponds to a lowstand systems track, while transgression evidences are observed since the Late Bajocian only. The MFS corresponds to the condensed Callovian interval, which appears as stacked discontinuities, rather than a single unconformity (in this case called "disconformity", as overlying strata are parallel to the underlying ones). Biostratigraphic evaluation of the underlying reduced transgressive and condensed beds have highlighted several sedimentary hiatuses inbetween (Mangold, 1984).
This could be the consequences of tectonic activity triggering uplift of a topographical high at that time in the Haute-Chaîne area during Late Bathonian and Callovian times, while continuous sedimentation took place westward in the external Jura area (Mangold, 1984).
Above this remarkable interval, highstand conditions favoured the development of a large carbonate platform, showing an overall regressive trend from distal open marine environment to the development of shallow water build-ups, forced to prograde basinwards at the end of the Kimmeridgian-Early Tithonian. The latter were finally covered by tidal deposits at the end of the Jurassic period. At smaller scale, Meyer (2000a) recognised a 3rd-order T-R episode in theReef Complex and the trend from microbial to coral build-up observed in the
3.4. Sequence stratigraphy 81 core recovered in this unit could correspond to the end of the highstand and beginning of the regressive trend (Figure 3.15).
The last T-R sequence spans from the Berriasian till the Barremian. The MFS is related to the high glauconite content recorded in the Grand-Essert Formation, which formed in deeper environment compared to the surrounding units. This event can be correlated at larger scale with the Paris Basin (Rusciadelli, 1996). The SB represents the most obvious unconformity in the GGB sedimentary record. In HU-2, this discontinuity brings in contact Cretaceous shallow water carbonates (Calcaires urgoniens) with Tertiary siliciclastic sedi-ments (Siderolithic unit), along an irregular fractured and kartsified surface. This event is evident in outcrops, but also in cores, logs, and on seismic because of the important litho-logical contrast between the two superposed lithologies. This discontinuity is related to the convergence of African and Eurasian plates and to the Vosges-Black Forest mantle dome which formed during the Early Tertiary and resulted in an extended basement uplift trigger-ing erosion of a large part of the Cretaceous units (Trümpy, 1980 ; Karner and Watts, 1983).
Indeed, in few other wells and outcrops, where Aptian-Albian units and also scarce Upper Cretaceous sediments are still preserved, evidence of drowning of the Urgonian platform, even in several successive episodes, argue for the beginning of a subsequent 2nd-order sequence from the earliest Aptian (Pictet et al., 2016). Subsidence, eustatic sea-level variations and rapid changes in climatic conditions during the Cretaceous are thought to be responsible for this transgressive event (Föllmi, 2012). This is also true for shorter time-scale sequences linked to high-frequency sea-level drops, whose evidences are well preserved in the shallow carbonate sedimentary succession characterizing most of the Cretaceous units (Strasser and Hillgärtner, 1998).
3.4.3 Implications of sequence stratigraphy on reservoirs
In carbonate rocks, discontinuities linked to subaerial exposure are of great importance, be-cause they could lead to the formation of karsts in the underlying units, which can strongly influence reservoir qualities. Kastified surfaces are generally related to sequence boundaries, and because karsts are sometimes hardly discernible in the space-limited well material, in-terpretation of such discontinuities is fundamental. In the GGB sedimentary succession, the Early Berriasian and top Mesozoic discontinuities show evidences of direct continental influence on sedimentation. In outcrops, karsts were recognised in the uppermost Cretaceous units (Charollais and Lombard, 1966 ; Charollais et al., 2013b ; Mastrangelo et al., 2013 ; Godet et al., 2016), and are also likely present in the subsurface, as it was recognised on seismic (Signer and Gorin, 1995), and cuttings material. The second potential karstified unit corresponds to the Tithonian Tidalites de Vouglans Fm and Berriasian Goldberg For-mation, where the SB has been interpreted. These two formations consist of carbonates and although the upper one shows more diversified facies, several of them are similar to the underlying Tithonian formation. Thus, if karsts developed in this interval, the poor facies contrast between matrix and karst infill could challenge their identification, which has not been confirmed to date.
Sequence identification is also important because reservoir properties are often
con-82 Chapter 3. Multidisciplinary study of Humilly-2 well strained by the depositional environment, whose evolution in 3D controls the distribution of reservoir bodies. Assessment of reservoir properties in HU-2 has highlighted mainly low matrix reservoir property values, except in the Reef Complex unit, where microporosity (mainly) can reach more than 10 %. Preservation of micropores is likely related to the pre-cursor micritic fraction composition, which prevented dissolution-recrystallization processes under moderate diagenesis (Volery et al., 2009). The rigid framework constituted by reef building organisms and/or early cementation (e.g., isopach cement) might have prevented porous micritic parts from compaction during burial diagenesis (Volery et al., 2010b) (more details in Chapter 6). Thus, the pore network in this unit seems firstly related to depositional facies, i.e., occurrence of bioconstructions. According to field observations, spatial distribu-tion of build-ups might be correlated at large scale to the overall regressive trend prevailing at the end of the Jurassic, as bioconstructions prograde basinwards towards the southeast through time (Deville, 1988 ; Meyer, 2000a). No other sequence-related reservoir develop-ment has been identified in the GGB sedidevelop-mentary succession, because potential carbonate reservoirs seem mainly dominated by fractures.
3.5 Conclusions of Humilly-2 well study
The HU-2 well has been fully re-investigated in the light of the geothermal reservoir as-sessment effort (GEothermie 2020 program), because it provides invaluable information on the complete sedimentary succession in the Geneva Basin. Analysis of facies variations and quantification of reservoir properties on logs, cores and core samples have confirmed the occurrence of five potential geothermal reservoirs (Lower Cretaceous, Kimmeridgian, Bajocian, Muschelkalk, Buntsandstein). It has given new insights on sedimentological as-pects and reservoir quality of these five intervals and additional cored units (Carboniferous, Bathonian-Callovian and Oxfordian). Among the intervals investigated, the Kimmeridgian Reef Complex unit presents the most promising reservoir properties with more than 10 % porosity and permeability averaging 1 mD in bioconstructions and peri-reefal deposits.
2nd order sequence stratigraphic cycles have been highlighted along the well section, and smaller-scale cycles have also been identified in the Bajocian and Kimmeridgian units.
Based on detailed field studies, the core material has been re-interpreted and stratigraphically better constrained in these intervals. Two potentially karstified horizons have been pointed out according to major sequence boundaries: the Cretaceous units at the top Mesozoic, where karst occurrence has been confirmed by cutting description, seismic survey and field analogues. The Tithonian-BerriasianTidalites de Vouglans andGoldberg Fms, which should be further investigated for the presence of karsts probably show less lithological contrast than in the Cretaceous interval.
Overall, the re-interpretation of stratigraphic boundaries in a coherent framework, fa-cies description and interpretation of depositional environments and their evolution through time, coupled with assessment of petrophysical properties in the most promising geothermal reservoirs in the HU-2 well provides consistent knowledge of the entire sedimentary succes-sion for future geothermal exploration. This complete "vertical" study will serve to anchor
3.5. Conclusions of Humilly-2 well study 83 subsequent global well analysis in the basin (Chapter 4), and help to understand better the distribution of productive reservoir facies within the GGB.
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