Diagenetic evolution of fault zones in Urgonian microporous

Microporous carbonate rocks form important reservoir with high a permeability variability 10 depending on sedimentary, structural and diagenetic factors. Carbonates are very sensitive to 11 fluid-rock interactions that lead to secondary processes like cementation and dissolution that 12 modify the reservoir properties. Focussing on fault-related diagenesis, the aim of this study is 13 to identify the fault zone impact on reservoir properties. It focuses on 2 fault zones east to La 14 Fare Anticlinal (SE France) which cross-cut Urgonian microporous carbonates. 122 collected 15 samples along four transects orthogonal to the fault zones were analysed. Porosity values have 16 been measured on 92 dry plugs. Diagenetic elements were determined on 92 thin sections using 17 Polarized Light Microscopy, cathodoluminescence, red alizarin, SEM and isotopic 18 measurements (δC and δO). Eight different calcite cementation stages and 2 micrite micro19 fabrics were identified. As a main result, this study highlight that the 2 fault zones acted as drain 20 canalizing low temperature fluids at their onset, and induced fault zone calcite cementation 21 during 2 subsequent phases which strongly altered and modified the local reservoir properties. 22


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-In the D19 fault zone, the lowest porosity values are in narrow zones around the faults 169 (less than 2m) and in the lens between F4 and F5. Though, this porosity decrease is not 170 homogeneous in fault zone and high values are found north of F1 and F3 (Fig. 3B).

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From thin sections impregnated with blue-epoxy resin, a porous rock-type with ϕ>10% mainly 172 in micritized grains as microporosity and moldic porosity ( Fig. 3C a), and a tight rock-type with 173 ϕ< 5% where the porosity is mostly linked to barren styloliths ( Fig. 3C b, c) are distinguished.

DIAGENETIC EVOLUTION OF THE FAULT ZONES 274
The chronological relations between cements can be established thanks to cross-cutting relation 275 and inclusion principles. Indeed, the veins filled with cement C2 cross-cut cements C1a and 276 C1b (Fig. 5B). Thus, C2 cementation postponed C1 cementation. The C3 veins cross-cut the 277 C2 veins, but are included within FR1 clasts (Fig. B). Hence, C3 cement is ante-FR1 brought out the chronology between phases, and revealed the paragenetic sequence (Fig. 7).
The Urgonian carbonates in La Fare anticlinal underwent 3 important diagenetic events, which impacted the host rock and/or the fault zones. We discriminate among diagenetic events that 288 occurred before and during faulting.  The next sub-phase of cementation C1a partly fills intergranular porosity. This non luminescent 302 cement with isotopic values ranging from -6.8‰ to -3.9‰ for δ 18 O and from -1.0‰ to +1.3‰

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The micrite re-crystallization strongly increased rock porosity due to enhanced microporosity 314 (Fig. 9B1b). Microporous limestones have a high matrix porosity but low to moderate matrix      (Kim et al., 2004;Long and Imber, 2011;Walsh et al., 1999Walsh et al., , 2003. 443 Consequently, the fault complexity, the fracture intensity and the fracture-strike range are 444 increased (Kim et al., 2004;Sibson, 1996). This process in the studied area resulted in a well- in addition, explains why the early C4 cementation has not been recorded in D19 fault zone.

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The C4 cementation in T2 reduced the porosity to less than 8% on a larger zone (>60m) than 455 in both others transects (T1 ≈30m, T3>40m). indicating an organic matter input (Swart, 2015). This implies soils, and thus results from a near 471 surface fluid circulation. We deduce that the D19 faults was lately reactivated after the folding

EVOLUTION OF FAULT ZONES RESERVOIR PROPERTIES 479
The host rock presents a monophasic evolution and switch from a type IV reservoir where 480 matrix provided storage and flow, to a type III reservoir where the fractures are pathways for 481 flow but the production comes from the matrix (Nelson 2001, Fig. 10A). The fault zones present 482 a more complex polyphase evolution than the host rock. Indeed, their reservoir properties 483 evolved from a type IV reservoir corresponding to the host rock to a type I reservoir where 484 fractures provide both storage and flow pathways (Nelson 2001 , Fig. 10A). Both fault zones 485 present slight differences. The Castellas fault zone was completely tight soon after C3 486 cementation. Consequently, it did not fit to the Nelson reservoir type classification. However, 487 after fault core formation, the fault zone presents a high fault core permeability. In this study 488 we propose a new approach with a triangle diagram taking into account fault core permeability 489 to remove the flaws of this method (Fig. 10B). Thus, for Castellas fault zone, the permeability 490 evolves from the host rock permeability (100% matrix; step 0 on Fig. 10B) to a permeability 491 due to 50% to the matrix and 50% to the fault core during dilation band development (step 2 on 492 Fig. 10B). Thereafter, during the 2 fracture events permeability is mainly link to fractures (C2: 493 30% FC, 70% fractures; C3: 15% FC, 15% matrix, 70% fractures; step 3, 4 on Fig. 10B). Then, 494 after fault core formation and during dolomitization event, permeability is solely located in the fault core (step 6, 7 on Fig. 10B). Lastly, after fault zone reactivation, the permeability is due fault core (step 7a and 7b on Fig. 10B).  In both cases, the cementation altered initial reservoir properties in the fault zone 509 vicinity, switching from type III to type I during the first stages of fault development.