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Our research on the Pyrenees revealed a phenomenon that is generally overlooked in the methodology for exploring mountain ranges and their foothills. This phenomenon — pre-orogenic capital — could be a crucial driver in making new discoveries.

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Sylvain Calassou

R&D

The Orogen Project to Rethink Mountain Range Formation

Mountainous regions have considerable oil potential, representing 15 to 18% of yet-to-find oil globally, yet they are among the least explored areas on the planet. These compressive systems, where plate tectonics resulted in a mountain-building event (also known as an orogeny), pose a number of problems that make exploration particularly risky. These include accessibility, poor quality seismic imaging and geological complexity.

Enhancing our geological concepts and developing new ideas about mountain belts are key to reducing these risks and guiding exploration to targets that are better predicted, understood and controlled. This is the challenge behind the Orogen research project, launched in 2015 in partnership with the French Geological Survey (BRGM) and the French National Center for Scientific Research (CNRS). The project’s objective is to qualify the processes involved before, during and after the formation of a mountain range, from the surface to the mantle, and to evaluate their impact on sedimentary basins, as well as changes in the circulation of related fluids, including hydrocarbons.

Using the Pyrenees as an Exceptional Geological Laboratory

The Orogen project focuses on the Pyrenees mountain range, from the Gulf of Lion on the Mediterranean edge to the Cantabrian Mountains on the Atlantic. A century of exploration has revealed mixed oil potential across the entire range.

This full-scale laboratory offers one of the world’s richest seismic and well data sets and features a remarkable variety of geodynamic phenomena, with, from west to east:

  • A passive continental margin inherited from the Cretaceous opening of the Bay of Biscay.
  • A subduction zone formed by oceanic-continental convergence from the Late Cretaceous to the Early Miocene (Cantabrian Mountains in Spain).
  • A collision zone where the Iberian and Eurasian plates came together, forming the Pyrenees from the Paleocene to the Eocene.
  • An extension zone related to the opening of the western Mediterranean in the Oligo-Miocene.

In other words, it gives us access to 1.5 Wilson Cycles, the scientific model that describes the movement of the continents on the Earth’s surface over geological time.

The Four Phases of the Wilson Cycle

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1. Rifting: A process in which the continental crust is extended and thinned.

2. Drifting: Continued extension that leads to the formation of new seafloor crust.

3. Subduction: The first phase in convergence, where the oceanic plate sinks under the continental plate.

4. Collision: Second phase in convergence that occurs when all the extension capital created during divergence (rifting and drifting) is subducted, leading to the formation of a mountain range.

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The geology of the Pyrenees is also unusual in that the passive margin inherited from rifting was partially preserved during subduction and collision. The different deformation states between the Bay of Biscay and the Gulf of Lion have allowed us to make a detailed study of the area’s architecture and evolution during compression and the post-orogenic phase. In particular, thanks to the presence of preserved Cretaceous basins accessible at the outcrop north of the Axial Zone, we were able to acquire a unique data set for this type of domain, notably concerning the significant swings in heat exchange during the period of extension. Some of the basins underwent extreme stretching that placed them directly on the lithospheric mantle as a result of crustal thinning.

This exceptional configuration allowed us to quantify physical, thermal and chemical processes over time in relation to the phases of deformation (extension followed by compression) and correlate those trends with changes in fluids of all types, including hydrocarbons.

Pre-Orogenic Capital: Looking at Mountain Range Exploration through a New Lens

While the formation of the Pyrenees range generated, enhanced or preserved structures in which hydrocarbons could accumulate, it did not create the petroleum system, as the source rock and hydrocarbon maturation pre-dated compression. In this case, and in most Alpine-type orogenic belts, the issue of pre-orogenic inheritance and the preservation of pre-orogenic capital is of key importance for exploration. This capital is determined during rifting (maximum heat), when the source rock matures and begins to migrate.

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If, as we believe, the formation of the mountain belt simply destroyed or displaced the pre-existing oil potential, the goal is to figure out where to look for any extensive pre-orogenic domains that may have been preserved. Using the Pyrenees as an example, we have developed a techno-sedimentary genetic map that allows us to picture petroleum systems in their original context and get an idea of their architecture and evolution as mountain ranges were formed. The key indicator of pre-orogenic capital preservation is what we call “divergence maturity,” a new concept that quantifies the amount of the extension generated by rifting and drifting that will be taken up by convergence so that collision and orogeny takes place. Collision is defined as the moment when the system has fully taken up the preceding extension. It is the moment when two continental lithospheres come into contact and the system creates landforms. Landform erosion and destruction is at maximum occurrence. The more mature the margin (from an aborted rift to a global ocean), the longer the time span before the moment of collision for any given convergence.

To test this new concept, we worked on a region with significant proven potential and on a mountain range where the consensus opinion was that any pre-orogenic capital had been completely destroyed by the range’s formation. Our research revealed that an extensive margin not taken up by convergence and linked to a petroleum system outside conventional exploration domains was partially preserved.

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