3 | Multidisciplinary study of Humilly-2 well: the subsurface Mesozoic
3.2 Sedimentology, mineralogy, microfacies and deposi- deposi-tional environmentsdeposi-tional environments
3.2.1 Review of the stratigraphy
The present definition of the stratigraphy in HU-2 well (figure 3.2 and 3.3) includes the com-parison and revision of several data sources. It aims at providing a consistent interpretation, according to the most recent and integrated knowledge of the GGB. On one hand, refer-ences including the entire well are (1) well drilling-related documents such as the final well report, reservoir measurement tables, the composite log and the geological well summary (S.N.P.A., 1969) (2) the report on seismic velocities (CGG, 1970) and seismic interpretation reports of the GG87 survey (Gorin, 1989) and the 9000 survey (?) (3) Wernli and Gorin (1996) published in Donzeau et al. (1997), who provided a re-interpretation of the stratig-raphy along the entire well, based on well logs, and lithological characteristics compared with regional outcrop analogues (without reviewing cuttings and cores). These authors also reviewed lithological contrasts defining specific seismic markers. On the other hand, detailed review of particular stratigraphic levels in the well have also been used and were provided by Meyer et al. (2000) for the Liassic, Piuz (2004) for the Aalenian-Bajocian, Meyer (2000a) for the Kimmeridgian, and Charollais et al. (2007, and references therein) for the Tertiary intervals.
3.2.2 Carboniferous
The well penetrated only 10 m of Carboniferous sequence. It is composed of alternating sandstone and shale layers, corresponding to large variations of gamma ray and density logs responses. The sandstone mineralogy indicates a primarily granitic composition, with a clay content commonly reaching up to 20% (figure 3.4). Kaolinite is the dominant clay mineral, whose primary origin is likely detritic. However, large crystal morphology observed indicates burial diagenetic overprint, which induced recrystallisation and autigenous over-growth. Moreover, illitisation recognised in few kaolinite crystals supports burial diagenesis under even higher thermal gradient conditions than the kaolinitisation itself.
A 3.70 m long core was retrieved from this unit (Annexe A.2). It consists of brownish, subangular and subarkosic sandstone. The grain sorting shows repeated small-size fining upward sequences which show cross-bedded stratifications, disturbed by bioturbations in some places (figure 3.5). Coal chips are commonly found, as well as palynological fossils, which are the only biostratigraphic record in this interval and indicate a Westphalian D to Lower Stephanian age according to the well report (S.N.P.A., 1969). A single steeply dipping (70◦) fracture with 1 cm normal offset was observed along the core, as well as several slickensides.
According to the palynological content previously investigated (S.N.P.A., 1969), and cor-relation with regional outcrops of similar age (Gorin and Jan du Chêne, 1972 ; Marro and Manigler, 1998 ; Capuzzo and Wetzel, 2004), this interval was thought to have been de-posited in a continental environment. However, recent investigations on the organic matter
56 Chapter 3. Multidisciplinary study of Humilly-2 well QuaternaOligoceneEoceneBarremianHauterivianValanginianBerriasianTithonianKimmeridgianOxfordianCall.Bath.UBajoL Bajo.Aal.Toar.Pliensb.Hett.-Sin.Raet.KeuperLettenk.Musch.Bunt.Carb.
Gravels & sands
Figure 3.2: Humilly-2 well summary sheet
3.2. Sedimentology, mineralogy, microfacies and depositional environments 57
?
BARREMIANHAUTERIVIANVALANG.BERRIASIANTITHONIAN KIMMERIDGIANOXFORDIANLOWERLOWERLOW.LOW.LOWER
MIDDLE.
UPPER JURASSIC (Malm)MIDDLE JURASSIC (Dogger)LOWER JURASSIC (LIASSIC)CRETACElower
EOC.OLIGO- CENE Calc. marneux de la Rivière Calcaires urgoniens auct.
Urgonien jaune auct.
"Complexe des Marnes d'Hauterive" auct.
"Complexe de la Pierre jaune de Neuchâtel" auct.
Calcaires roux auct.
Calcaires à Alectryonia rectangularis Marnes d'Arzier
Formation de la Chambotte inférieure Formation de Vions
Formation de Pierre-Châtel
Formation de Goldberg (Purbeckien auct.) Tidalites de Vouglans
Calcaires à entroques et niveaux à polypiers Calcaires siliceux
Calcaires gréso-micacé à Cancellophycus Argiles + marnes à oolites ferrugineuses?
Alternances micacées à bancs durs
«Schistes cartons»
Marnes noires à nodules et Tisoa Dalles échinodermiques Fe
Marnes à amalthées Marnes calcaires à bélemnites Calcaire argileux à cassure conchoïdale Calcaires gréseux à Chlamys
Regional stratigraphy 1:5000Depth 6BS25 GR
0 150
QuaternaryOligoceneEoceneBarremianHauterivianValanginianBerriasianTithonianKimmeridgianOxfordianCallovianBathonianUBajoL Bajo.AalenianToarcianPliensbachianHettangian-SinemurianRaethianKeuperLettenkohleMuschelkalkBuntsandsteinPermo-Carb.
Gravels & Eq. to calcaires à polypiers
Alternances inférieuresdes calcaires fins Calcaires gréso-micacés
à Cancellophycus Marnes à amalthées
& Marnes noires à nodules et Tisoa & Dalle échino. Fe Calcaires et marnes
calcaires Argiles à estherides
& Dolomies & Grès blonds
Sel, Dolomies
Figure 3.3: Stratigraphic correlations between a synthetic stratigraphic column of the GGB (left hand side), based on Charollais et al. (2013b) and Donzeau et al. (1997), and the reviewed stratigraphy in HU-2, based on (S.N.P.A., 1969), the report on seismic velocities (CGG, 1970), seismic interpretation reports of the GG87 survey (Gorin, 1989) and the 9000 survey (Gorin, 1992), Wernli and Gorin (1996, published in Donzeau et al. (1997)) and this study.
58 Chapter 3. Multidisciplinary study of Humilly-2 well provenance using biomarkers, would suggest a possible mixed continental marine environ-ment (Do Couto and Moscariello, 2016). Grain shape and size coupled with the sedienviron-mentary structures observed suggest a proximal deltaic depositional environment, close to swamps developing in adjacent fluvial or deltaic plain, favourable to peat and coal formation. The normal fault identified can be related to extensional phases of the crustal basement that prevailed during deposition of this interval (Ziegler and Stampfli, 2001).
3.2.3 Triassic
Buntsandstein
The Buntsandstein interval is 10 m thick in HU-2. The transition form Carboniferous to the Triassic Buntsandstein interval is sharp on gamma ray, bulk density and neutron porosity logs, because of the important lithological change. The clay content in sandstones decreases drastically upwards in this interval, and shale layers are absent. Mineralogical measure-ments also demonstrate that the sandstone composition evolves from a subarkose (quartz, K-feldspar, siderite/dolomite (Fe-rich), clay minerals (kaolinite and illite)) to a purer quartz arenite towards the top (figure 3.4 and 3.6).
The Buntsandstein interval is almost entirely cored (9.5 m, Appendix ??). It is mainly composed by greenish, sub-rounded, coarse quartz grains and the limit with the underly-ing Carboniferous unit is clearly revealed by the color contrast between the two lithologies (brownish to greenish). Fractures sealed by quartz-baryte-galena mineral association are nu-merous, and probably responsible for high positive deflections of the bulk density log which overprint the primary sandstone log signature along the Buntsandstein interval.
The clear contrast between Palaeozoic and Mesozoic units emphasizes modifications of en-vironmental conditions between the two eras. It also highlights the (angular?) unconformity observed in regional outcrops at this boundary. Lack of marine evidence in Buntsandstein deposits suggests that this unit is genetically related to a continental depositional setting.
Quartz-baryte-galena mineralization in fractures testifies to hydrothermal circulation, and might be related to the "late Triassic event" described by Guillocheau et al. (2000) for similar mineralization observed in Triassic units of the Paris Basin, coeval with volcanic activity in the Alps.
Muschelkalk-Lettenkohle
The Muschelkalk and Lettenkohle units are 131 m thick. The lithology mainly consists of dolomite and anhydrite, as shown by mineralogical measurements and high values of the bulk density log (figure 3.2 and 3.7). Few shale layers are also present in this evaporite interval. The base of the Muschelkalk is marked by a decrease in clastic sediment input which coincides with glauconite occurrence, while first evaporites appear (figure 3.4).
45 m of cores were recovered in the Muschelkalk and Lettenkohle units. Because litholo-gies are very similar, both units are considered together. They consist of grey dolomitized grainstone with whitish anhydrite nodules, alternating with dark laminated anhydrite
lay-3.2. Sedimentology, mineralogy, microfacies and depositional environments 59
Figure 3.4: QEMSCAN measurements represented by mineral associations (at the level shown in Figure 3.2), whose total occurrence is re-sampled at 100 % in columns b, c, d. (a) Main components (b) Carbonates (c) Clastics, including quartz and detrital silicates, except clay minerals (d) Clay minerals. (e) Glauconite content.
60 Chapter 3. Multidisciplinary study of Humilly-2 well
A
C D
B
500µm 2mm
2.5mm
500µm
Illite Kaolinite
K-Feldspar Muscovite Other carbonates
Pores Quartz
Figure 3.5: Petrography of Carboniferous sandstone from Humilly-2. (A) Thin section image in transmitted light of subangular subarkosic sandstone containing coal chips and affected by solution seams - 3039.3 m depth. (B) Same as the previous sample under polarized light.
(C) Zoomed-out view under transmitted light of the same sample showing occurrence of bio-turbations and cross-laminae stratifications highlighted by the grain sorting and underlined by compaction features (solution seams). (D) Mineral mapping by QEMSCAN analysis of subangular subarkosic sandstone from 3039.8 m depth.
3.2. Sedimentology, mineralogy, microfacies and depositional environments 61
A
C D
B
500µm 500µm
2mm 2mm
Dolomite (Fe/Mn) Heavy mineral Illite K-Feldspar
Pores Quartz 81.8%
8
0.8 0.9 0.7 4.9
Other minerals0.2
Figure 3.6: Petrography of Buntsandstein sandstone (A) Thin section image in transmitted light of subrounded quartz sandstone with argillaceous matrix. Note the few altered siderite grains (heavy mineral in D) - 3035 m depth. (B) Same as the previous sample under po-larized light. (C) Zoomed-out view under transmitted light of the same sample, showing stratifications highlighted by the grain sorting and oxidized grains. (D) Mineral mapping by QEMSCAN analysis of subrounded quartz sandstone with argillaceous matrix - 3037.4 m depth.
62 Chapter 3. Multidisciplinary study of Humilly-2 well
A
C D
B
500µm 500µm
2mm 2mm
Dolomite Evaporites
Quartz
Dolomite Evaporites
Quartz 59.2
% 40.1
0.2 0.3
95.8
% 3.2
0.2 0.6
Pores Other minerals 0.2 Pores
Other minerals0.2
Figure 3.7: Petrography of undifferentiated Muschelkalk and Lettenkohle units (A) Thin section image in transmitted light of entirely dolomitized packstone with anhydrite over-growth (white minerals) - 2966.1 m depth. Note the large Cayeuxia sp. clast on the right hand side of the picture. (B) Thin section image in transmitted light of anhydrite with few small dolomitic rhombs and thin argillaceous drapes - 2904.7 m depth. (C) Mineral mapping by QEMSCAN analysis, showing the composite dolomite-anhydrite facies - 2966.1 m depth. (D) Mineral mapping by QEMSCAN analysis, showing the anhydrite-dominated facies - 2927.8 m depth.
3.2. Sedimentology, mineralogy, microfacies and depositional environments 63
A
C D
B
500µm
2mm 1mm
200µm
%68 25.4 2.2 2.9 0.4 Calcite
Illite Quartz Albite Dolomite
Pyrite 0.2 Other minerals0.9
Figure 3.8: Petrography of the Rhaetian unit (A) Thin section image in transmitted light of sandy limestone rich in bivalve and brachiopod shells - 2529.3 m depth. (B) Same as the previous sample, thin section stained and image in transmitted light, showing that shells exhibit different Fe content according to their precursor composition and related diagenetic impact (transmitted light) (C) Thin section image in cathodoluminescent light distinguishing few dolomite rhombs (yellow colour) and quartz grains from calcitic matrix and bioclasts -2529.3 m depth. (D) Mineral mapping by QEMSCAN analysis of sandy limestone rich in shells - 2529.3 m depth.
ers. The latter dip between 15◦and 30◦, and small-scale folds and perturbations are common.
Thin cracks and sub-vertical fractures filled with whitish anhydrite develop along dolomitic intervals, as well as solution seams and stylolites. Their frequency increases at the top of the cored interval, where vertical stylolites and fissure networks are also observed. Two principal microfacies can be distinguished: entirely dolomitized peloid and ooid grainstone/packstone comprising anhydrite nodules showing several fluid inclusions, and peloidal laminated an-hydritic layers. In both microfacies, allochem identification is often not possible because of anhydrite nodules, pervasive dolomitization, or large micrite envelopes.
The sharp decrease in siliciclastic input and coeval glauconite occurrence, which are followed by the first evaporite appearance, supports the current interpretation of a ma-rine transgression. The two microfacies described (figure 3.7) suggest that sediments were deposited in high-energy shallow marine to sebkha environments, which also support this marine influence. However, the dominance of microbial activity over the poor organism diversification points to hostile environmental conditions, in which microbial organisms (cal-cimicrobes, bacteria) are better adapted.
64 Chapter 3. Multidisciplinary study of Humilly-2 well Keuper and Rhaetian
The Keuper and Rhaetian units have an overall thickness of 331 m. The former unit is made of evaporitic sequences which show alternating halite, anhydrite/gypsum, dolomite and shale layers. The bulk density log signal is consequently perturbed, and shows typically low values in halite (≈ 2g/cm3), and sonic values (∆t) close to 70µsec/f t. The overlying Rhaetian deposits are made of shale and sandy limestone, whose siliciclastic content can be substantial (up to 25 % of subangular quartz grains).
One 6 m long core was recovered in the upper part of the Rhaetian, showing sandy limestone rich in bivalve and brachiopod shells, as well as echinid plates (Figure 3.8). Alizarin red-S and K-ferricyanide staining highlights that recrystallised shells are made of Fe-rich calcite, similarly to the interparticle microcrystalline cement. Only few brachiopod fragments show a non-ferroan calcite composition that resisted to diagenesis, likely because of the precursor low-Mg calcite composition of the shell. Oxides are also constantly found in such facies.
Unlike the thick underlying evaporitic layer related to a restricted environment such as a sebkha or small confined shallow basins, the upper shell-rich Rhaetian unit shows evidences of a several marine ingresses. Proximity to a detrital source is marked by the high siliciclastic input and subangular shape of grains, but clear signs of carbonate platform production indicated by enrichment in bivalves, brachiopods and echinids testify that the depositional environment progressively deepened.
3.2.4 Liassic
The Liassic interval is divided into a lower carbonate unit and an upper clay-rich unit. Their transition corresponds to increased values of the gamma ray and sonic logs, in response to a large clastic input and decrease in carbonate production. Three cores were recovered in the Hettangian-Sinemurian (25.1 m), but no additional investigation has been carried out on these because this interval constitutes a seal rather than a potential reservoir for geothermal exploration, as demonstrated by previous studies (Rybach, 1992 ; Baujard et al., 2007 ; Chevalier et al., 2010 ; PGG, 2011 ; Do Couto and Moscariello, 2016).
3.2.5 Dogger
The Dogger interval is about 315 m thick. The base of the Aalenian is difficult to distinguish from the underlying Toarcian shales because of their similar facies and log signature, and of the lack of outcrops of this unit preventing from calibrating interpretations along the well. Therefore, the Liassic-Dogger boundary is arbitrarily set where a clear lithological contrast is observed between shales and quartz and mica-rich limestone which corresponds to the"Calcaires gréso-micacés à Cancellophycus" Formation (Fm) (Figure 3.3). The middle and upper part of the Dogger unit consists of carbonate platform deposits, in which several consecutive shallowing upward trends were identified in the Bajocian by Piuz (2004). A deepening of the depositional environment is marked by marly intervals intercalated in the
3.2. Sedimentology, mineralogy, microfacies and depositional environments 65
A
C D
B
2mm 200µm
500µm 500µm
87.2% 12.5
0.1 0.1 Calcite Quartz Pores
Other minerals Dolomite 0.1
Figure 3.9: Petrography of theCalcaires à entroques unit, Bajocian (A) Thin section image in transmitted light of well sorted bioclastic grainstone rich in peloids and crinoid fragments - 2001.2 m depth. (B) Same as the previous sample, thin section stained and image in trans-mitted light, showing clear variations in Fe content between bioclasts (non-ferroan calcite) and cement, which evolves from Fe-rich synthaxial growth to non-ferroan blocky cement (transmitted light) (C) Same as the previous sample, thin section image in cathodolumi-nescent light, showing that these variations are underlined by shades of yellow colour in calcite. Quartz grains are also distinct (dark blue-purple) (D) Mineral mapping by QEM-SCAN analysis, revealing that the siliciclastic (quartz) input reaches more than 10 % - 1997.3 m depth.
66 Chapter 3. Multidisciplinary study of Humilly-2 well
A
C D
B
500µm 2mm
2mm 2mm
Pores Calcite Quartz Dolomite (Fe/Mn) Illite
Pyrite 88.6%
5
0.4 2.9 1.7
0.3 Other minerals1.1
Figure 3.10: Petrography of the Calcaires terreux unit, Bathonian (A) Thin section image in transmitted light of bioclastic grainstone with bimodal sorting and typical heterozoan assemblage - 1860.8 m depth. (B) Same as the previous sample, thin section stained and image in transmitted light, showing contrast between large non-ferroan calcitic bioclasts and Fe-rich composition of cement and small peloids. (C) Same as the previous sample, thin section image in cathodoluminescent light, highlighting silicification of few brachiopod shells. (D) Mineral mapping by QEMSCAN analysis - 1860.8 m depth.
Bathonian, up to the remarkable condensed Callovian interval. A clastic input is observed along the entire Dogger unit, but its amount varies, and trends are well reflected in gamma ray, density and sonic logs.
Two 5.9 m and 9 m long cores were taken at the Upper-Middle Jurassic boundary (across the Oxfordian-Callovian-Bathonian) and in the Bajocian, respectively. In both Bajocian and Bathonian cores, microfacies analyses revealed mainly the occurrence of bioclastic grainstone with dominant heterozoan assemblages, and sparse detritic quartz grains (maximum 1%
of total allochem amount). Grain orientation and sorting highlight thin foresets, which are sometimes underlined by wispy seams. Diagenetic features include Fe-rich synthaxial overgrowth and subsequent drusy mosaic cement composed of Fe-rich calcite, except at the base of the Bajocian core where it shows zonation in composition (from Fe-rich synthaxial, drusy and blocky calcitic cement to non-ferroan drusy/blocky calcitic cement) (Figures 3.9 and 3.10). Silicification of few echinid plates and brachiopod shells is also observed (Figure 3.10), as well as subsequent interparticle silica cement, particularly in the Bajocian unit. Few fractures filled by blocky Fe-rich calcitic cement are also reported in the Bajocian, and wispy seems and few grain imbrications restricted to the Bathonian emphasize a slight compaction.
The facies observed in both cores corresponds to the microfacies AF3 described by Piuz
3.2. Sedimentology, mineralogy, microfacies and depositional environments 67
A
C D
B
2mm 1mm
500µm 1mm
Calcite Quartz
Pores Chamosite Chlorite
Biotite Dolomite (Fe/Mn) Illite
94.9% 2.2
0.7 0.6 0.3 0.2 0.1 0.1 Unclassified0.9
Figure 3.11: Petrography of theMarnes d’Effingen unit, Oxfordian (A) Thin section image in transmitted light of bioturbated wackestone with ammonite (on the top left corner) - 1852.4 m depth. (B) Same as the previous sample, thin section stained and image in transmitted light, showing mainly non-ferroan calcitic composition and dark marls (C) Same as the previous sample, thin section image in cathodoluminescent light, showing that ammonite wall is partly recrystallised into Fe-rich calcite and also underwent partial silicification. (D) Mineral mapping by QEMSCAN analysis - 1853.2 m depth.
(2004, 2008), who studied in detail the Bajocian unit in the area, including that in HU-2. This author attributed these deposits to periodic sea level lowstands (tectonic, eustatism/climate (Durlet and Thierry, 2000 ; Thiry-Bastien, 2002), which triggered major changes in envi-ronmental factors resulting in fauna turnover from photo- to heterozoan assemblages on the mid-Jurassic carbonate platform. Foresets are thus interpreted as current ripples on hydraulic dunes. The latter testify to higher energy sedimentation, whose deposits filled inter-reef zones and topographic lows. Fe-rich calcitic cement is commonly attributed to burial diagenesis (Flügel, 2004). In this case however, it represents early diagenetic cement such as syntaxial overgrowth and is therefore more likely associated with changing redox conditions during eo- to mesogenetic processes.
3.2.6 Malm
The Upper Jurassic (Malm) unit is 1009 m thick. Lithologies grade from marls to pure carbonates in the upper part, as demonstrated by trend of the gamma ray log and miner-alogical measurements (Figures 3.2 and 3.4). The presence of partially dolomitized intervals is also common, even a pure sucrosic dolomitic bed is reported in the Reef Complex unit
68 Chapter 3. Multidisciplinary study of Humilly-2 well (Etiollets Fm) and Calcaires de Tabalcon Fm (S.N.P.A., 1969). In the latter, fractures are also indicated on logs, and are related to mud loss and fresh water invasion in the borehole (Figure 3.2).
Two cores were taken in the Malm interval. The deeper one is 3.1 m long. It crosses the Malm-Dogger boundary and extends from the Callovian up to the base of the Oxfordian unit, likely in the Marnes d’Effingen Fm. It shows a dark grey, bioclastic, marly limestone with few darker marl intervals. Thalassinoides-type bioturbations and compaction give a nodular aspect to the core. The microfacies is consistent in this interval (Figure 3.11), and shows sparse wackestones with mainly crinoid plates, thin-shell bivalves, planktonic foraminifera, sponge spicules and ammonites. Alizarin red-S and K-ferrocyanid staining reveals that main particles are made of non-ferroan calcite, and only thin cemented cracks and recrystallized allochems are composed of Fe-rich calcite. Few are even partially to totally silicified.
The shallower core is 5.9 m long and was taken at the top of the Kimmeridgian Reef Com-plex unit, which corresponds to theEtiollets Fm. It is composed of white, massive, bioclastic limestone, with a 15 cm thick dolomitic interval at the base. The successive microfacies ob-served include at the base microbial boundstone, alternating with bioclastic grainstone to rudstone, whose bioclasts show large micrite envelopes that can prevent their identification.
Microbial constructions are then progressively replaced by coral framestone, still interca-lated with bioclastic grainstone to rudstone. Relative chronology of diagenetic events can be summarized as: (1) a large bioclast micritisation coeval with the dissolution and recrys-tallisation of aragonitic and high-Mg calcitic organism walls into low-Mg calcite, as well as the degradation of soft parts and other residual organic components; (2) fibrous isopach
Microbial constructions are then progressively replaced by coral framestone, still interca-lated with bioclastic grainstone to rudstone. Relative chronology of diagenetic events can be summarized as: (1) a large bioclast micritisation coeval with the dissolution and recrys-tallisation of aragonitic and high-Mg calcitic organism walls into low-Mg calcite, as well as the degradation of soft parts and other residual organic components; (2) fibrous isopach