Depositional rock types

The model for the depositional environments of the Reef Complex unit has been largely derived from Meyer (2000a) and presented in Chapter 4. Details on depositional environ-ments and organism associations have been provided by microfacies analyses and resulted in proposing an alternative interpretation of the Kimmeridgian Reef Complex depositional model (Figure 6.7). The Calcaires de Tabalcon unit shows mainly forereef deposits, where typical thrombolitic peloidal fabric testifies to ubiquitous microbial activity. This unit is overlain by patch reefs, which developed on inherited topographic highs in a vast carbon-ate platform and were influenced by wave and tidal currents constantly mobilizing particles (Meyer, 2000a). Under such conditions, small coral bioherms formed, associated with di-verse micro-encrusters which probably helped to stiffen the substrate before coral settlement (Kenter, 1990 ; Leinfelder et al., 1993 ; Schlager and Reijmer, 2009 ; Guido et al., 2016).

Microbial (mud) mounds also played a significant role in building patch reefs, and seem to have preferentially grown in quieter, protected sides of bioconstructions, close to exter-nal lagoon environment. Between patch reefs, inner lagoon environments likely developed in protected zones, but seem more extended in the proximal setting, west and northwest of the GGB, because no related facies has been observed in the subsurface. Oolithic bars formed in more turbulent environments, and are particularly well developed in the Calcaire

6.3. Depositional rock types 135 Identification of potential flow barriers

Training image

Fracture log signal Well Log Interpretation

Porogenesis/poronecrosis

Figure 6.6: Rock typing workflow.

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

Middle Jurassic Upper Jurassic Coral bioherm

Microbial-Campbelliella bioherm

Oolithic shoal Distal, sponge bioherm

Internal lagoon Bioclastic peri-reef sed.

Figure 6.7: Depositional model of the Reef Complex unit during the early stage of patch reef development (Kimmeridgian, Upper Malm).

de Landaize unit. The latter covers the reef sequence and is made of back-reef to beach deposits infilling inter-reef spaces and prograding onto the bioconstructions. Bernier (1984) separated this Calcaires de Landaize unit from the reef sequence, but this interpretation is still debated to date because of several evident affinities with the reef environment (Enay, personal communication). Stratigraphically upward, the Tidalites de Vouglans unit marks the regressive trend and the demise of reef organisms. Tidal deposits finally smoothed the topographic heterogeneities.

The platform morphology evolved after reef bioconstruction and prograded eastwards, fol-lowing the overall regressive trend. According to Meyer (2000a), the development of reef bioconstructions accentuated the inherited relief on an original ramp-type platform with a flat top morphology since the Middle Tithonian. At the late time of reef development, the slope between shallow and deeper environments became steeper. A real barrier formed (Mount Salève reefs), which allowed the formation of a large, muddy lagoon in the studied area.

According to the different microfacies observed and their main components, five depositional rock types (DRT) have been identified in the core material. Two others, absent in cores, have been recognized in outcrops and added in order to have a full representation of the diversity inside theReef Complex unit:

• DRT1 Reef front (Figure 6.8-B): thrombolithic pack/grainstone to bindstone formed by microbial crust; main components areTubiphytes morronensis (renamed Crescentiella (Senowbari-Daryan et al., 2008)), Terebella lapilloides, encrusting microproblematica

6.3. Depositional rock types 137 (Baccanella, Radiomura?), sponge spicules (rhaxes, monaxons and triaxons). Pres-ence of microbial activity is ubiquitous (crusts, peloids, large micritic coating). Minor components are echinoderms (urchins and crinoids), sponge and coral debris, bivalves, brachiopods, nodosarid foraminifera (Lenticulina sp. mainly). Rare dasycladacean al-gae, Bullopora, complex agglutinated and microgranular foraminifera have also been observed.

• DRT2 Reef (Figure 6.8-C): boundstone to grainstone with thrombolithic fabric in places

; main components are corals encrusted by microbial crust, Bacinella, Lithocodium aggregatum with its symbiotic foraminifera Troglotella incrustans, encrusting micro-probelmatica (Baccanella, Radiomura?), gastropods, echinoderms (urchins mainly), bivalves including rudists (Diceratidae), dasycladacean algae (Salpingoporella annu-lata and Clypeina jurassica mainly),Thaumatoporella parvovesiculifera, calcimicrobes (Cayeuxia mainly), diverse foraminifera such as Kurnubia palastiniensis and other complex agglutinated foraminifera, Trocholina sp., , Nautiloculina sp., Textularia-Verneuilinoides-Trochammina (T-V-T); all the bioclasts show large micritic coating, and peloids are also frequent, even forming thrombolithes; few brachiopods shells, mil-iolids and monocristalline foraminifera (Spirillina sp?) have been noticed.

• DRT3 Peri-reef (Figure 6.8-D): intrabioclastic rudstone to grainstone; main compo-nents are reworked debris of reef-builders (corals, calcareous sponges), other bioclasts found in the reef facies, large bivalves (Diceratidae) and gastropods. The stromato-poroidCladocoropsis mirabilis has been observed in this peri-reef facies only. Bacinella oncoids and large micritic coating are ubiquitous, sometimes complicating bioclast recognition.

• DRT4 Mud mound/open lagoon (Figure 6.8-E): Bacinella association bindstone and Campbelliella packstone; main components are Baccinella irregularis (crusts and on-coids), Lithocodium aggregatum with related foraminifera Troglotella incrustans, and thrombolitic fabric in places. This facies forms firm, microbially constructed mud mounds (auto-micrite, formed in-situ biologically), which are associated with detrital micrite (allo-micrite) packstone rich in peloids, dacycladacean algae, especially Camp-belliella sriata (Vaginella auct.), Thaumatoporella parvovesiculifera, gastropod and bivalve debris. All bioclasts show large micritic coating.

According to several authors, Bacinella association (Leinfelder, 1992) occurs in shal-low water and reefal settings, forming oncoids as well as bind crusts (Leinfelder, 1992 ; Leinfelder et al., 1993, 1994 ; Kaya and Altıner, 2015). In theReef Complex unit, this association is found in coral reefs as mentioned above, but also seems to build individ-ual mud mounds, close to the external, lagoonal setting, as suggested by the combi-nation with Campbelliella-rich packstone facies. Such interpretation is also supported by Malchus and Kuss (1988) ; Reolid et al. (2007), who attributed similar organism association to lagoonal environments. This DRT has been distinguished from the coral reef facies because of its higher micrite content (auto- and allo-micrite). However, it is

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

Figure 6.8: Illustration of DRTs microfacies: A) DRT0, tidal flat (Savoie-104, sample ER-155), B) DRT1, reef front. Red arrows indicate Tubiphytes morronensis, and yellow arrows point typical thrombolithic, "auto-micrite" (Savoie-107, sample ER-100); C)DRT2, coral boundstone (Humilly-2, sample ER-68); D) DRT3, peri-reef deposits (Savoie-108, sample ER-142); E) DRT4, ud mound/open lagoon. Green arrows indicate the algae Campbelliella striata, and the yellow arrow showsBaccinela irregularis (Humilly-2, sample HU-2-7 (photo Y. Makhloufi)); F) DRT5, internal lagoon deposits. Green arrows point Clypeina jurassica algae which are very abondant in this facies, showing mainly a "allo-micritic" matrix. The blue arrow indicate a geopetal fills in fenestrae (Savoie-104, sample ER-154). G) DRT6, restricted lagoon deposits. The pink arrow points a stylolite which acted as dolomitizing fluid conduit (Savoie-107, sample ER-103); H) DRT7, oolithic shoal (the Col de la Faucille outcrop, sample COM-1 (photo Y. Makhloufi)).

6.3. Depositional rock types 139

CTabLandaizeTidalitesComplexe récifal

DRT1: reef front DRT0: tidal flat

DRT2: coral reef

DRT3: peri-reef DRT4: mud mount/

open lagoon DRT5: lagoon:

DRT6: restricted lagoon DRT7: oolithic shoal

Figure 6.9: Conceptual 2D cross-section representing the DRTs and their schematic distri-bution envisaged for the Reef Complex unit (Kimmeridgian) in the GGB region.

still closely related to reef environment, and could play a complementary role for reef settlement while stiffening the uneven substrate.

• DRT5 Internal lagoon (Figure 6.8-F): dasycladacean and complex agglutinated foraminifera-rich wackestone to packstone; this facies has not been observed in the core material, but was described in several outcrops by Bernier (1984) and Meyer (2000a). Main components are diversified dasycladacean algae and large complex ag-glutinated foraminifera. The reader is referred to the cited works for further details on organisms and species.

Bernier (1984) and Meyer (2000a) usually found this facies in theTidalites de Vouglans unit overlying theReef Complex . However, its presence in the latter unit is presumed, because a large proportion of its components is found reworked in the peri-reef and oolithic facies. Therefore, internal lagoon likely developed in more protected and prox-imal zones. According to Meyer (2000a), this facies was more affected by meteoric dissolution and dolomitization, which indicates a shallower position in the water col-umn.

• DRT6 Restricted lagoon (Figure 6.8-G): chalk to platy, bituminous limestone; simi-larly to the internal lagoon deposits, these facies have not been observed in the core material. According to palaeoenvironmental reconstructions of Meyer (2000a), such a

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

Figure 6.10: Porosity-permeability cross plot displaying results from analysis on plug sam-ples and coloured according to the respective DRTs. The scattered distribution of the DRTs highlights that reservoir properties (porosity and permeability) of samples cannot be pre-dicted properly by defining the depositional environment only.

depositional environment was largely recognized in outcrops west of the studied area, and considered to be quasi absent in the GGB. However, because large uncertainties due to the lack of subsurface data in the studied area limits subsurface, inter-well correlations, this depositional environment has been still considered.

• DRT7 Oolithic shoal (Figure 6.8-H): oolithic grainstone; main components are ooids, whose nucleus is usually made of lagoonal foraminifera and dasycladacean debris. This facies shows well-sorted grains and sometimes cross-bed stratifications, which testify to moderate to high energy hydrodynamic conditions. It is particularly characteristic of the Calcaires de Landaize unit.

The distribution and relative spatial relationship between these DRTs has been summarized on a conceptual, simplified 2D cross-section (Figure 6.9). This Figure takes into account sequence stratigraphic trends, showing an overall regression and a basinward progradation of the reefal system. Small-scale sea-level variations have been considered through small facies indentation, because they can become important for the following rock typing steps.

At the time of sedimentation, primary porosity is driven by depositional texture, depending mainly on the presence/absence of micrite. It consists mainly of (1) micropores in micrite-dominated textures, either bindstone, pack or wackstone (DRT1, DRT4, DRT5 and DRT6), and (2) intra- and inter-particle pores in micrite-free textures, grain-supported or organically