Figure 1: Topographic map of the area of Montpellier and positionon of the main outcrop location. (b) 1/1000000 Geological Map of France with the positionon of the main outcrop locations. (c) Outcrop location (1) displayed on the regional 1/50000 geological map. (d) Outcrop location (2) on the regional 1/50000 geological map. Map source: hps://infoterre.brgm.fr/ 

In the unique geological context of the eastern Languedoc area (Figure 1 (a) and (b)), this excursion aims at highlighting how various types of natural fractures have generated within distinctive sedimentary formations deformed during complex, multi-phased, tectonic histories. The geological set-up covers roughly the boundary between the northeastern limit of the Pyrenean Mountain’s foreland and the south of the Hercynian Massif Central (Figure 1(b)). The contrasted origins and evolutions of the fractures encountered during the trip as well as the precious information that they can provide as analogues to the ones controlling the dynamics of subsurface reservoirs will be discussed. Insights about their presence, the possibility to detect them and characterise them on borehole data will also be thoroughly tackled by a series of practical activities.

Two main locations will be visited (maps of Figure 1).

Location 1

Outcrops of the first location (Lodève Permian Basin) provide an opportunity to observe fractures resulting of early (i.e., syn- to late diagenesys) deformation episodes (Figure 2).

Figure 2: Various types of natural fractures observed in the Permian-age pelitic formations of the Lodève Basin (Southern France). (a) Frontal surface view of a sinuous vein with coarse sparitic calcite mineralisation (b) and (c) Oblique and top map views of rectilinear veins with fibrous calcite cement. More information in De Joussineau et al., 2005.

The observations will focus on syn-rift deposits filling a 20Km-size half-graben structure of Stephanian to Thuringian age (Lopez et al., 2008) at the southern limit of the French Massif Central. The formation of interest is the Salagou Formation defined by Broutin et al., 1992 as a very fine-grained playa-type series locally intercalated with sandy to conglomeratic levels. Its thickness varies across the structure of the half-graben and reaches a maximum close to the southern fault of Les Aires (Figure 3). It has been significantly eroded at the top and is overlain unconformably by Triassic beds in the southern part of the Basin. 

Figure 3 : Simplified NS cross-secon highlighting the structure and stratigraphy of the Carboniferous to Permian-age Basin of Lodève (figure by Lopez et al., 2008).

In this basin, fractures initiated and developed at very early stages. They appeared as mineralised veins in incompletely consolidated silisiclastic sequence (alternations of sandstones and pelites of continental origin). These fractures can be sorted in different sets characterised by distinct orientations and by contrasted geometries. A chronology of the different fracturing stages proposed by De Joussineau et al., 2005, is available. It is based on a systematic analysis of intersection criteria and on the interpretation of specific geometrical observations within the context of the sedimentary basin’s evolution.  

Figure 4 : Stratigraphic profile of the Permian of the Lodève basin (Schneider et al., 2006) 

Fracture mineralisation is well observable in outcrop conditions and provides information about fracture kinematics and about syn-fracturing to post-fracturing fluid circulations during the structuring of the sedimentary basin. This outcrop provides a good opportunity to the participants to observe natural fractures at the scale of wellbore images, far beyond seismic resolution. It will generate discussions about key issues such as the detection, characterization, and importance of small-scale discontinuities in various reservoir, source-rock, or seal lithologies in subsurface conditions.  

Location 2 

The second location (Coulazou Valley) offers several remarkable outcrops in folded and fractured carbonate series of Jurassic age (Figure 5 Figure 6). 

Figure 5 : Geological setting of the Coulazou gully outcrops (Bazalgette et al., 2010): (1) Structural sketch showing the location of the Coulazou gully folded outcrops. These outcrops are situated in the “Montpellier Fold”, which is interpreted as the northernmost termination of the North-Pyrenean zone (Maauer, 1971). (2) Structural sketch of the Coulazou gully folded and faulted area. (3) Simplified secon of the Montpellier Fold area (modified from Gèze, 1979). 

These sedimentary series were deformed in the context of the “Pli de Montpellier” structure, which forms the northernmost termination of the nappes of the North Pyrenean Front (Bazalgette et al., 2010). The structural style of the area is characterised by a succession of well-observable East-West trending mesoscale folds associated to extremely well-exposed fracture sets (Figure 6). Excellent exposure conditions allow clear observations of both fold and fracture geometries. The role of certain types of fractures and fractured zones in influencing the geometrical and kinematic evolution of folds will be discussed at the light of analogue experimental models (Figure 6 (c), refer to Bazalgette and Pe't, 2007 for more details) as well as the importance of some key geological parameters which appear to be controlling the interactions between folds and fractures. The impact of fold-associated fracture patterns as observed in the outcrops of the Coulazou Valley on aquifer or reservoir-scale fluid circulations will be examined (depending on weather conditions). The key inputs of borehole data as a support to identify such structures in the subsurface and to characterise their potential control on reservoir dynamics be tackled as central part of the discussions. 

Figure 6: Mesoscale folds and associated fracture patterns in Jurassic-age limestones of the Coulazou Valley, near Montpellier (Southern France). (a) Rounded-shape ancline hinge with well distributed bed-controlled fractures (Bazalgette et al., 2010). (b) Chevron structure with dense, well-localised fractures distributed in the zone of maximum curvature (Bazalgette et al., 2010). (c) Analogue experimental model mimicking the geometries and fracture distributions observed in outcrop (a) (Bazalgette and Pet, 2007). (d) Detailed interpretative line-drawing highlighting fracture distributions observed in the chevron fold of outcrop (b) (Bazalgette et al., 2010). 

Field Trip Itinerary 

Date: Sunday, 21 September 2025

Starting Time: 7:00AM

End Time: 19:00PM

Montpellier City Center to the outcrop (Lodève Permian Basin) - 1-hour drive.  Will spend 2-3.5 hrs in this stop; Departure around  11:00.

Outcrop 1 (Lodève Permian Basin) to Outcrop 2 (Coulazou Valley) - 45-minute drive to southeast of Outcrop 1. Will reach this stop not later than 14:00 and stay for 3-4 hours. It includes 1-1.5 hrs walk on a rocky or gravel-cover path (relatively easy). 

Leave Outcrop 2 to Montpellier City Centre no later than 17:30  or 18:00 - 30-mins to 1 hr (20 KM) drive. Arrive back at Montpellier around 19:00. 

The complete itinerary from Montpellier City Centre and back to Montpellier City Centre, with the visit to the two main stops is available via the following link. 

Polygone Montpellier à Polygone Montpellier - Google Maps

The suggestion would be to start early morning from Montpellier City Center to avoid peak traffic and allow an optimal time for observations and discussions on the outcrops. The first outcrop (Lodève Permian Basin) is located roughly one hour from Montpellier. The road itinerary is available on the map of Figure 1.

Figure 7: Road itinerary from Montpellier City Centre to Outcrop 1 (Lodève Permian Basin). Driving time from MontpellierCity Centre is estimated to about 1hr depending on traffic conditions. Google Maps hyperlink here

Figure 8: Road itinerary from outcrop 1 to outcrop group 2 (Coulazou Valley). Driving time is estimated to about 45 minutes, depending on the selected road. Google Maps hyperlink here

Figure 9: Road itinerary from outcrop group 2 to Montpellier City Centre. Driving distance is short (about 20 Km) but driving time is estimated between 30 minutes to 1 hour depending on traffic conditions which can be heavy by the end of the working day. Google map hyperlink here. 

The distance back from outcrop group 2 to Montpellier City Centre is short (about 20 Km).Driving time can still reach about one hour depending on traffic conditions, especially by the end of the working day (17:00 – 18:00).A conservative estimation of the total driving time for the trip would be of about 3 hours to3.5 hours. A morning start around 7:00 AM would be recommended, which would allow a 2to 3 hours stop on the Permian Basin outcrop, with a departure around 11:00 AM. Lunch could be taken as sandwiches (most time-efficient solution) or alternatively in one of the many restaurants available at a short distance of the outcrops. The plan would be to reach outcrop group 2 in the Coulazou Valley not later than 14:00 (preferably 13:00 or 13:30). The visit of this location includes a 1 to 1.5 hours walk on a rocky or gravel-cover path (relatively easy). The plan would be to stay there for 3 to 4 hours in total and to leave no later than17:30 or 18:00, with an evening arrival back to Montpellier around 19:00.