Diagenetic evolution of fault zones in Urgonian microporous carbonates , 1 impact on reservoir properties ( Provence – SE France )

6 Microporous carbonate rocks form important reservoirs with high a permeability variability depending 7 of sedimentary, structural and diagenetic factors. Carbonates are very sensitive to fluids-rock 8 interactions that trigger to secondary processes like cementation and dissolution leading to reservoir 9 properties modifications. As they can act as drains or barriers, fault zones influence the fluid flows in 10 the upper part of Earth crust and increase the fluid-rock interactions. The aim of this study is to identify 11 fault zone impact on fluid flows and reservoir properties during basin geodynamic history. The study 12 focuses on 2 fault zones of the Eastern part of La Fare Anticlinal (SE France) where Urgonian 13 microporous carbonates underwent polyphase tectonics and diagenesis. We took 122 samples along 4 14 transects cross-cutting two fault zones. Porosity values have been measured on 92 dry plugs. Diagenetic 15 properties of samples have been determined on 92 thin sections using Polarized Light Microscopy, 16 cathodoluminescence, red alizarin, SEM and isotopic measurements (δC and δO). Height calcite 17 cement stages and 2 micrite micro-fabrics have been identified. This study highlight that fault zones 18 acted as drain canalizing low temperature fluids at their onset, and induced fault zone cementation with 19 two cementation phases, what has strongly altered and modified local reservoir properties. 20


DIAGENETIC EVOLUTION OF THE FAULT ZONES 296
The chronological relations between cements can be established via cross-cutting relations and 297 inclusion principles. Indeed, the veins filled with C2 cement cross-cut C1a and C1b cements (Fig.   298 5B). Thus, C2 cementation post-dated C1 cement. C3 veins cross-cut the C2 veins, but are included 299 within FR1 clasts (Fig. 5B). Hence, C3 cement is prior to FR1 development but is subsequent to

305
fault core formation and are related to the fault nucleation. Replacive dolomite is found within FR1 306 matrix (Fig. 5E), therefore, it developed after FR1 formation. Finally, the C4 cement can be noticed 307 within FR2 matrix indicating that C4 cementation event post-dated FR2 formation. The fault rock 308 2 (FR2) developed during strike-slip reactivation of the studied faults. The combined superposition, 309 overlap, cross-cutting principles and isotopic signature of cements brought out the chronology 310 between phases, and revealed the paragenetic sequence (Fig. 7).

311
The Urgonian carbonates in La Fare anticline underwent 3 major diagenetic events, which impacted 312 the host rock and/or the fault zones. We discriminate among diagenetic events that occurred before 313 and during faulting.

338
The micrite re-crystallization strongly increased rock porosity due to enhanced microporosity (Fig.   339 9B1a). Resulting from this event, Urgonian carbonates formed a type III reservoir sensu Nelson    The Al enrichment of C2 could result from the erosion of Albian and Aptian deposits during the 391 Durancian uplift (Guendon and Parron, 1985;Triat, 1982).

392
As the fault zone grew, new fracture sets formed, leading to new phase of calcite cementation (C3) 393 in veins and intergranular porosity (Step 4 on Fig. 8). The δ 18 O isotopic values of C3 range from -10.40‰ to -6.73‰ with δ 13 C values between -2.09‰ and +1.22‰. As C3 cementation occurred during the Durancian uplift and denudation, it most probably did not cement in deep burial 396 conditions (maximum depth of 500 m; Fig. 9C4). The negative δ 13 C values tend corroborate the 397 hypothesis of cementation induced by meteoric fluids rather than marine ones. Hence, C3 would 398 correspond to a shallow burial/meteoric cementation phase. Due to this cementation, rocks in this 399 zone tightened with porosity down to <5%. The porosity did not change since this event (Fig. 9B5).  In a later stage, the fault core formed and the fault plane sensu-stricto developed, leading to FR1 406 breccia with a permeable matrix with quartz grains >100 µm in size (Step 5 on Fig. 8). These grains 407 either came from silica found inside C2 cement described above or from Aptian overlying rocks.

408
Silica crystals in C2 veins are scarce and smaller than 10 µm. Thus, quartz grains may rather come 409 from Aptian rocks like the ones found in C2 veins. The presence of Aptian quartz in the fault core 410 proves that the Castellas fault affected also Aptian rocks, which were later eroded during the 411 Durancian uplift. According to this, the fault activity occurred before total erosion of Aptian rocks.  values of C4 cement (from -9.2 to -6.1‰ for δ 18 O and from -5.01‰ to -1.0‰ for δ 13 C) highlight 460 the strong influence of meteoric fluids. This is coherent with the occurrence of karstic infilling due 461 to fluid circulations in vadose zone, with alternating dissolution and cementation (Swart, 2015).  Walsh et al., 1999Walsh et al., , 2003. Consequently, the fault complexity, the 472 fracture intensity and the fracture-strike range are increased (Kim et al., 2004;Sibson, 1996). This process in the studied area resulted in a well-connected fracture network that increased the permeability and favoured local fluid circulations. In transect 2, the increase of the local indicating an organic matter input (Swart, 2015). This implies fluids percolating soils, as results 499 from a near surface fluid circulation. We deduce that the D19 faults was lately reactivated after the 500 folding of the La Fare anticline. There is no such cementation with similar isotope values in the 501 fault zone, meaning that fluids and cements did not alter the fault zone diagenetic properties.

502
Eventually, the late exhumation of the Urgonian carbonate host rocks led to flows inducing 503 dissolution of MF3 grains in the host rock. This phase produced the moldic porosity and increased 504 the porosity/permeability (Step 8 on Fig. 9B and C). These fluids, however, did not affect fault 505 zones.

EVOLUTION OF FAULT ZONES RESERVOIR PROPERTIES 507
The host rock presents a monophasic evolution and switch from a type IV reservoir where matrix 508 provided storage and flow, to a type III reservoir where fractures behave as pathways towards fluid 509 flow but the production mainly comes from the matrix (Nelson 2001, Fig. 10A). The fault zones 510 present a more complex polyphasic evolution than the host rock. Indeed, their reservoir properties 511 evolved from a type IV reservoir corresponding to the host rock to a type I reservoir where fractures 512 provide both storage and flow pathways (Nelson 2001 , Fig. 10A). Both fault zones present slight differences. The Castellas fault zone was completely tight soon after C3 cementation.
Consequently, it did not fit to the Nelson reservoir type classification. However, after fault core 515 formation, the fault zone presents a high fault core permeability. In this study we propose a new 516 approach with a triangle diagram taking into account fault core permeability to remove the flaws 517 of this method (Fig. 10B). The percentage assigned to the fault core or to the matrix are qualitatively

528
matrix and 50% to the fault core during dilation band development (step 2 on Fig. 10B). Thereafter, 529 during the two fracture events permeability is mainly linked to fracturing (C2: 30% fault core, 70% 530 fractures; C3: 15% fault core, 15% matrix, 70% fractures; step 3, 4 on Fig. 10B). Then, after fault 531 core formation and during dolomitization event, permeability is solely provided by the fault core 532 (step 6, 7 on Fig. 10B). Lastly, after fault zone reactivation, the permeability is due to 20% to the 533 fault core and 80% to fractures (step 7c on Fig. 10B). The D19 fault zone permeability during its 534 development was related for 20% to the matrix, 20% to the fractures and 60% to the fault core (step 535 7a and 7b on Fig. 10B).  This regional study allows to draw broader rules for complex faults with polyphasic activity 557 affecting granular carbonates at shallow burial conditions (Fig. 9).