Log-derived petrophysical parameters

Applying the well log interpretation procedure described in Chapter A.6 for potential reser-voirs highlighted in the literature (Signorelli et al., 2004 ; Baujard et al., 2007 ; PGG, 2011

; Bancal, 2013) has revealed that petrophysical parameters derived from HU-2 logs should be used with caution and only qualitatively. Indeed, the poor quality of log data has not provided accurate and consistent numbers, as revealed by calibration with available core data, even when considering upscaling factors. Nevertheless, porous and non-porous zones could be efficiently categorized with the GR log, and effective porosity curves calculated in Techlog c with RHOB, DT and NPHI logs. As NPHI log appeared shifted, and RHOB

3.3. Reservoir properties 73

Laminated Bioturbated Fissure horse tail Stylolite

textures particle size cement type

stratigraphy

Upper BajocianLower Bajocian Alternances sup. de calcaires et marnesAlternances sup. de calcaires et marnesCalcaires à entroquesEq. to calcaires à polypiersAlternance inf. de calcaires fins 1910.0

Figure 3.14: Details of the Bajocian stratigraphic and sequential subdivisions at the 3^rdorder (?) scale according to Piuz (2004). Porosity values from report, logs and measurements on logs are compared, as well as RHOB (bulk density) and GRHO (grain density) values (right margin). The red curve has been derived from DT (sonic) log, according to equations 2.11 and 2.13 (Chapter A.6.4).

74 Chapter 3. Multidisciplinary study of Humilly-2 well was only acquired in the Triassic interval, only the DT-derived effective porosity curve has been presented. Information on the pore network could also be inferred from the velocity deviation log (Anselmetti and Eberli, 1999) on specific sections.

Box-plots of gamma-ray values along the entire well have been carried out to compare the relative shale content per stratigraphic units (Figure 3.16). The gamma-ray log has been corrected for discrepancies linked to fractures or wash-out effects, which could interfere with the geological signal and has therefore not be taken into account. This plot shows that the KimmeridgianReef Complex contains the lowest clay-content and variations, and thus is the cleanest carbonate unit in the sedimentary sequence. At smaller scale, few intervals in the Cretaceous, Dogger and Muschelkalk also present similar mineralogical properties. However, these units are thinner than the Reef Complex and the have not been differentiated from their larger stratigraphic unit.

3.3.3 Carboniferous

In the Carboniferous unit, no fluorescence has been reported on core, but gas indications recorded during production tests 4 and 5 in the overlying Buntsandstein unit highlight the source rock potential of this interval rather than its reservoir quality, as porosity-permeability measurements clearly demonstrates (φ ≈0.6%,k ≈0.005mD).

3.3.4 Triassic

The reservoir properties measured in the Buntsandstein interval vary according to the clay content. Porosity is commonly < 5% (mainly consisting of interparticle micropores) and permeability ranges between 0.1 mD and 2.7 mD (Figure 3.13). Although direct fluores-cence along fissures and slight gas indications were recorded in this interval, no sufficient commercial flow rate was obtained.

The Muschelkalk-Lettenkohle interval shows poor matrix reservoir quality. Effective porosity(φ_e) curves have highlighted three intervals of interest in dolomites. However, poros-ity measured on plug ranges from 0.2% to 3% only, and permeabilporos-ity (K) is commonly lower than 0.1 mD, the highest K values being linked to induced micro-cracks. Although fluores-cence was detected on fissures, and oil and gas indications were reported in these dolomitic intervals, results of two formation tests (TF3 and TF6) were negative. Nevertheless, mi-crocracks formation highlights the brittle behaviour of this unit, which might constitute a fractured reservoir along fault corridors.

The Keuper is entirely composed of shales and evaporites such as halite, gypsum and anhydrite, and is therefore considered a seal.

3.3.5 Jurassic

In the Liassic, only weak oil and gas indications were recorded, as well as bitumen-rich levels in the upper argillaceous unit, which affect sonic and density logs.

3.3. Reservoir properties 75

LaminatedBioturbatedFissurehorse tailStylolite textures particle size cement type stratigraphy

Kimmeridgian Complexe récifalCalcaires de Tabalcon

1010.0

Figure 3.15: Details of the Kimmeridgian Reef Complex unit and Calcaires de Tabalcon unit stratigraphic and sequential subdivisions at the 3^rd order (?) scale according to Meyer (2000a). Similarly to Figure 3.14, porosity values from report, logs and measurement on logs are compared, as well as RHOB (bulk density) and GRHO (grain density) values. The red curve has been derived from DT (sonic) log, according to equations 2.11 and 2.13 (Chapter A.6.4).

76 Chapter 3. Multidisciplinary study of Humilly-2 well Carbonate units of the Dogger display a high degree of cementation and therefore have poor matrix reservoir quality (φ between 1% - 3%; K usually < 0.1 mD). However, mud losses and salt water inflow in other wells where tectonic perturbations are more pronounced testify to the possibility of having open fractures in these units (Savoie-106 & 107, Toillon-1, Faucigny-1, Chaleyriat-1, Chatillon-1D)(Régie Autonome des Pétroles, 1954a,b ; PREPA, 1958 ; ESSO REP, 1970, 1989, 1991 ; PGG, 2011). At surface and in the shallow subsurface, these units are also exploited for drinking water in the Bellegarde area and Jura Mountains (PGG, 2011).

In terms of reservoir, the Malm unit can be subdivided into three compartments. The upper one comprises the TithonianTidalites de Vouglans Fm, which presented few gas indi-cations in its lower part, but isa priori not considered as a reservoir, because it mainly shows tight, muddy carbonate facies in HU-2. The second unit comprises the Upper Kimmeridgian Calcaire de Tabalcon Fm and Etiollets Fm (commonly named Reef Complex unit) made of pure carbonates. As previously explained, the latter formation shows important lateral facies variations between lagoonal, perireefal and reefal environments mainly. The core recovered in this unit shows facies linked to the two reef-related environments only. Porosity measure-ments in the core reach up to 13%, and permeability ranges between 0.16 to 3.15 mD. The maximum permeability value (3.5 mD) comes from the well report, and new measurements show generally values close to 1 mD. Sonic log porosity values are usually below 10%, but still indicate a certain amount of porosity. The velocity deviation log in porous intervals shows negligible negative values, which could indicate microporosity occurrence (Anselmetti and Eberli, 1999). This assumption is in agreement with the extended microfacies analysis.

Test 1 was performed at the bottom of theReef Complex unit, in a lithology described in the well report as alternating sucrosic dolomite and limestone layers. 11.56 l/min of fresh water were recorded, testifying to a weak water circulation in this interval. On one hand, by anal-ogy with highly porous dolomitic intervals investigated in outcrops on a similar stratigraphic level, such fluid circulation might be linked to diagenetically enhanced matrix properties in these sucrosic dolomite layers. On the other hand, a fractured zone has been highlighted on logs above the tested interval and could also play a key role in enhancing the flow path and rate.

3.3.6 Cretaceous

According to the the final well report, theCalcaires urgoniens unit (Barremian) is a poten-tial reservoir with a sonic porosity reaching 10%. This unit is also karstified, and depending on the nature of sediment infill, karst pockets could potentially enhance the permeability through this interval. According to the report and deep resistivity log, the Barremian lime-stone likely bears fresh water.

3.3.7 Tertiary

The 7.5 m thick Eocene siderolithic sediments which overly the Mesozoic-Cenozoic uncon-formity shows quartz sandstones with good reservoir properties (sonic φ≈ 14%). This unit