Electrical rock types

ever, occurrence of dolomite diminishes to the ESE, following the downdip depositional trend.

The Reef Complex unit was largely fractured, and different fracture patterns have been evi-denced (Figure 6.27c). Early diagenesis had a strong impact on rock brittleness, and seems to have controlled the distribution of heavily fractured units, such as DdfRT4, DdfRT5 and DdfRT6 especially (Figure 6.27d). In order to integrate the degree of fracture occurrence in rock type definition, a qualitative analysis on fracture density has been performed, con-sidering undamaged rock, meter-spaced near-vertical fractures, cm-scale fracture network and heavily fractured units. It has provided preliminary assessment to this dual-porosity-permeability reservoir. Further studies are still required to constrain better fracture at-tributes, which can strongly influence the potential of fracture conductivity.

Finally, the DdfRTs defined in core and outcrop samples, and distributed according to sedi-mentological investigations on the geometry of reservoir bodies (Figure 6.27d) have provided reliable petrophysical values for the rock matrix, and concepts at the reservoir scale. On one hand, they depict a pessimistic scheme of the Reef Complex reservoir quality. But on the other hand, they consider preferentially the most reliable analyses within the dataset available. In order to propagate rock types along wells and at the inter-well scale, respecting geological hazards, upscaling to the log domain is mandatory. This step is further developed in the next Chapter, following the rock typing workflow.

6.7 Electrical rock types

Electrical rock types (ERTs) are categories of log values (or ranges of log values) considering the different logs available, which correspond to different reservoir qualities. ERTs have been described first, independently from other rock types defined in core and outcrop samples (DRT, DdRT and DdfRT). In this study, because the log dataset is really heterogeneous both in terms of quality and logs available, "litho-porosity"-related logs have been used preferentially, i.e., GR, SP, RHOB, and DT mainly. In comparison, "fluid-related" logs such as resistivity logs were acquired with numerous different tools, whose signal correlation is not straightforward. Subsequently, the meaning of ERTs relies on the combination of lithological and porosity factors influencing log signals. Table 6.2 presents the different ERTs which have been distinguished in this study.

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

DdfRT2: tight, few fractures

DdfRT4: microporous (+residual moldic?), few fractures DdfRT1: tight, no reservoir

DdfRT3: tight, densely fractured

DdfRT6: intensively fractured dolomite DdfRT5: microporous, densely fractured

DdfRT7: large fault/fracture zone DdfRT8: intercrystalline porosity CTabLandaizeTidalitesComplexe récifalCTabLandaizeTidalitesComplexe récifal

DdRT3: vuggy porosity?

DdRT1: no reservoir DdR21: tight reservoir (PHI<5%)

DdRT4: microporosity (+residual moldic?) DdRT5: intercrystalline porosity CTabLandaizeTidalitesComplexe récifal

DRT1: reef front, packstone DRT0: tidal flat, wacke/packstone

DRT2: coral reef, boundstone

DRT3: peri-reef, grain/rudstone DRT4: mud mount/

open lagoon, pack/

bindstone

DRT5: lagoon, wackestone DRT6: restricted lagoon, mudstone

DRT7: oolithic shoal, grainstone

CTabLandaizeTidalitesComplexe récifal

1-5 meter-spaced fractures

large fracture/fault fractured “stiffen” Ca carbonate highly fractured dolomite

A B

C D

Figure 6.27: Summary of successive, rock types definition (A) Depositional Rock Types (DRT), (B) Depositional-diagenetic Rock Types (DdRT), C) conceptual mechanical stratig-raphy model for the Reef Complex and Tidalites de Vouglans units, (D) Depositional-diagenetic-fracturation Rock Types (DdfRT).

6.7. Electrical rock types 171

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Table 6.2: Values (range) attributed to each Electrical Rock Type (ERT).

Musiège-1

Thônex-1 Humilly-2 Savoie-107-autochtone Savoie-106 Savoie-107-allochtone Savoie-109 Savoie-108 Savoie-104

Kim 3 low to medium (0-60 GAPI)

~low with slight variations

fracture announced in reports or evident on logs ERT 2

Ф [%] RHOB [g/cm3] DT [μsec/ft] Main litho THX-1

Figure 6.28: Electrical Rock Types interpreted along each well crossing theReef Complex unit.

6.7. Electrical rock types 173 low to medium (0-60 GAPI)

~low with slight variations

fracture announced in reports or evident on logs ERT 2

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

Kim 4

Figure 6.29: Electrical Rock Types (ERT) represented on 2D cross-section, compared with real ERT defined along well. Even if real data show a higher variability of ERT succession, the concepts illustrated on the schematic cross-section are close to well data.

ERTs are generally defined to evaluate reservoir properties along a given stratigraphic in-terval. If correlations between ERT and rock types defined in core and outcrop samples are reliable, ERTs allow the upscaling of the space-limited core-rock types information vertically along wells. In the present study, ERTs have been first attributed along each well crossing the Reef Complex unit (Figure 6.28). Then, in order to see if these ERTs could be compared to DdfRTs, their potential distribution within the basin has been drawn on the conceptual 2D cross-section previously defined, based on core and outcrop information (Figure 6.29).

The real ERTs successions along wells have been superimposed to this schematic picture. It reveals that these ERTs are comparable and coherent with the DdfRTs and their conceptual distribution, even if actual well data are obviously more complex.

Although DdfRTs and ERTs are considered as correlatable, some information included in the definition of the former can not be clearly distinguished in the latter. Ranges of porosity have been calculated from DT, NPHI, and/or RHOB logs when available (see Chapter A.6.4 for methodology), highlighting porous, tight and non-reservoir intervals. As mentioned already in Chapter 6.5, pore types have been assessed according to the velocity-deviation method (Anselmetti and Eberli, 1999)(see Chapter ?? for theory), which shows results coherent with the matrix pore network observed in core samples, i.e., intercrystalline (micro)porosity

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

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Table 6.3: Values (range) attributed to each Petrophysical Rock Type (PRT).

mainly. Large fractures are also distinguishable on log signals, but other smaller-scale frac-ture patterns show no specific signafrac-ture. Except in dolomitic intervals, the distribution of these fracture patterns seems to follow the original DRT (see Chapter 6.4.3). Because mi-croporous, calcitic bodies can be observed on log (ERT3), as well as their relative position in the reef sequence, DdfRT4 and 5 can be distinguished. However, without information on the microfacies (evidence of reef-related environment), DdfRT2 and 3 show the same signal on logs, both corresponding to ERT2. Dolomite bodies can be separated from the calcitic ones, but their intrinsic reservoir properties (intercrystalline porosity in sucrosic dolomite versus tight, heavily fractured dolomite) cannot be distinguished on logs. These observations show that the current ERTs represent only partially the DdfRTs, and new, simplified petrophysical rock types (PRT) encompassing core, outcrop and log information have to be defined.