Pore network

The formation of the pore network in carbonate rocks is complex, and depends on pore types, which can be multiple, and on their inter-connectivity. It influences both the perme-ability and transmissivity, and is therefore critical to evaluate. The different pore types have been already presented above, whose development is related to the superposition of deposi-tional and diagenetic factors, and brittle deformation history. As mentioned, interrelation between these factors is not straightforward. Although causal relation between depositional environments and primary porosity exists, it can be easily modified by diagenetic and struc-tural/tectonic events. Their impact on the original sediments also relates to the duration of

6.5. Pore network 165

Figure 6.25: Mercury Injection Capillary Pressure (MICP) measurements in Humilly-2 samples for the entire sedimentary succession drilled. The blue dots representing the Reef Complex unit show the only potential matrix reservoir properties, which appear controlled by the microporous network.

such events. While the relative timing of diagenetic impact on the rock matrix have been framed through parageneses (Figure 6.11), the timing of fracturation is still under investi-gation (Clerc, in prep.). Nevertheless, observations on fractures discussed in the previous Chapter have demonstrated that such structures could potentially have an influence on the pore network, and especially on permeability. Therefore, a dual porosity-permeability ap-proach has been adopted to pinpoint at best the reservoir behaviour. On one hand, matrix properties have been detailed. On the other hand, the effect of fractures, when they poten-tially enhance permeability, has been superimposed, within the limits of current knowledge.

In samples where different matrix pore types have been recognized, the pore inter-connectivity in the rock matrix has been investigated through mercury injection capillary pressure (MICP) measurements (method developed in Chapter 2.2.8). In reef and peri-reef facies, where microporosity, residual intercrystalline (mouldic) porosity and vugs have been identified (Figure 6.12), only micro-pore throats (0.8 µm in average) contribute to the fluid flow path (Figure 6.25). This shows that larger, residual (mouldic) intercrystalline pores are not connected, or exclusively through surrounding micropores. The sample fraction investigated being reduced (about 1 cm³), the influence of vugs is not reflected in these mea-surements. However, because their distribution appears scattered in cores, and is usually focussed in recrystallised coral clasts/bioherms, they are thought to be rather disconnected.

In some cases, they could be connected through the microporous network, similarly to

resid-166 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit ual (mouldic) intercrystalline pores. Finally, intercrystalline pore throats in sucrosic dolomite intervals would likely be larger than those measured previously. No MICP measurements have been performed in such facies, but SEM images analysis corroborate this assumption (Figure 6.12).

Sonic log (DT), coupled with bulk density (RHOB) or neutron porosity (NPHI) logs can provide information on pore types, at greater scale than plug samples. The velocity deviation log method (Anselmetti and Eberli, 1999)(see Chapter??) has been applied in the only two wells with the necessary logs, i.e., Thônex-1 and Humilly-2 (Figure 6.26). In the former one, the deviation log has been calculated with RHOB and NPHI. Only the curve built with the RHOB has been presented, because it has provided more information, especially with respect to fractures. In the latter one, the NPHI log has been used exclusively, because the RHOB log was only acquired in the Triassic interval. In both wells, it is noteworthy that the deviation log in intervals considered as porous (light and dark blue intervals in ERT column, Figure 6.26) is close to zero (±500m/s), which corresponds to inter-particle/crystalline porosity, including microporosity and porous, sucrosic dolomite (Anselmetti and Eberli, 1999). This observation is in agreement with core and outcrop analyses, which have highlighted that microporosity is dominant in porous limestone samples, as intercrystalline porosity is in sucrosic dolomite samples. In Humilly-2, large negative deviations in theReef Complex and Tidalites de Vouglans units might be related to fractured intervals (Anselmetti and Eberli, 1993 ; Guadagno and Nunziata, 1993). On the contrary, the large fracture zones highlighted in the Tidalites de Vouglans unit in Thônex (red interval in ERT column, Figure 6.26) show positive deviations. These fractures apparently affected the borehole walls, as testified by oscillations of the caliper log and anomalous, low RHOB values. This artefact is probably responsible for these positive deviations.

In upscaling analyses performed in the core to log domain, the main effective pore network which developed in the calcitic matrix seems to be the microprosity. However, it cannot be excluded that a larger network connecting vugs and moulds would not be effective in places. This issue could not be addressed with MICP measurements, because the volume of rock considered is too small. Neither has the available log dataset allowed the differentiation of a larger matrix network, because the response to the deviation log for connected vugs and moulds would be likely similar to the microporous one. However, two different tools, looking at different scales, could provide critical information to complete this study. Firstly, 3D CT-scan modelling performed on core intervals could help distinguishing and visualizing different matrix pore networks, if present. Secondly, the nuclear magnetic resonance (NMR) log would also provide insights on movable fluids, and indirectly on the pore network (theory in Chapter A.6.7). Such devices were unfortunately not available when the studied wells were drilled, but should be considered for future well logging plans. To date, since no evidence of a larger matrix pore network has been proved, the pessimistic scenario, which only considers the microporous network as effective in calcitic rock matrix, has been adopted.

The development of karsts within the Reef Complex unit has also to be discussed, because intrinsic properties of this unit, intense fracturation in places and its current position on top of surrounding massifs are overall favourable conditions. Actually, large karst networks

6.5. Pore network 167

Thônex-1 Humilly-2

Electrofacies GR

ERT 1 ~high ~2 2.6 and less

2.7 2.6-2.7

~2.6

~2.7

~2.4-2.6

~2.7-2.8?

~52 marl

tight limestone

~porous limestone limestone Ф≈5%

marly, tight limestone Quartz sandstone potentially porous dolomite 47-49

~52 50-60 50-55 55-60 45-60?

<2 2-10

~5

≤2 variable

10-15 low

low low medium low to medium (0-60 GAPI)

~low with slight variations

fracture announced in reports or evident on logs ERT 2

ERT 3 ERT 4 ERT 5 ERT 6 ERT 7 ERT 8

Ф [%] RHOB [g/cm3] DT [μsec/ft] Main litho

Ti1

Kim5

Kim4 Figure 6.26: Velocity-deviation logs calculated in Thônex-1 and Humilly-2 (black curve on the right-hand side of each well sheet). See text for explanations.

168 Chapter 6. Rock typing of the Kimmeridgian - Tithonian Reef Complex unit have already been explored in the Jura Mountains and Mount Salève, but their extension throughout the deep subsurface have still to be demonstrated. Hence, in the pessimistic rock type scenario, karsts have not been taken into account.

In order to approximate better the effective pore network and related transmissivity of the Reef Complex unit, both fracture and matrix poroperm networks have to be superimposed.

The intercrystalline pore network in sucrosic dolomite could slightly enhance reservoir prop-erties of the rock matrix. It has been seen that porosity and permeability values in such intervals are slightly higher than in calcitic rock matrix. However, the distribution of both porous dolomite and calcite bodies is linked to reef-related depositional environments and the position of small-scale sequence boundaries, and is thus discontinuous within the Cal-caires récifaux unit. The sucrosic dolomite interval observed at the base of the reef sequence in the Calcaires de Tabalcon unit is more continuous. On the other hand, reservoir proper-ties are almost equivalent to that found in dolomitized patch reefs, i.e., of medium quality.

Considering these porous bodies together, the matrix pore network is unlikely to reach high permeability and transmissivity values required to fulfil medium to deep geothermal project expectations. Conductive fractures and fracture network could overcome this issue. Because fractures can be largely developed in this unit, a better reservoir potential is still expected.

Nevertheless, uncertainties linked to fractures and fracture networks specific attributes re-main challenging, while assessing their potential input on transmissivity at the reservoir scale (Correia et al., 2015). Further studies must therefore focus on that problematic, to integrate confidently and quantitatively this secondary -but necessary- poroperm network into rock types.