The Great Oolite Formation (Middle Jurassic) from the Upper Rhine Graben: sedimentological and mineralogical characterization from the Chalampé Well, and implications for geothermal reservoir

The Great Oolite Formation is a Middle Jurassic (upper Bajocian-lower Bathonian) carbonate Formation widespread across northern Europe, deposited in a shallow-marine epicontinental sea. This Formation is of great economic importance as a geo-resource, for example as a hydrocarbon reservoir in the UK and as a low-temperature geothermal reservoir in the Paris Basin. Comparable Jurassic oolitic carbonate Formations are also economically important worldwide (e.g.  the Arab Formation in the Persian Gulf as a hydrocarbon reservoir, and the Smackover Formation in the US as an aquifer reservoir).

In the Upper Rhine Graben (URG), the Great Oolite Formation has recently attracted new attention as a potential intermediate-depth geothermal reservoir. Despite its significance, geological knowledge of the Great Oolite Formation in the URG remains limited and is largely based on outdated mid-20th-century studies. Indeed, subsurface reservoir quality is poorly constrained from scattered data.

 An integrated analytical approach combining petrographic and facies analyses, with non-destructive micro–X-ray fluorescence (µ-XRF) mapping, were conducted on 64 core samples from the Chalampé well. This unique well represents a key reference well for the Great Oolite Formation in the URG, as it fully penetrates this Formation. The Great Oolite is only know at the surface from some limited outcrops, exposing incomplete sections of the Formation.

Petrographic analysis identified the diverse skeletal and non-skeletal components, from which five distinct carbonate textures (Mudstone, Wackestone, Packstone, Grainstone, and Rudstone) have been identified. Various post-depositional features, including micritization, pyritization, bioturbation, fracture, and stylolite, are documented. While the Great Oolite Formation from the URG was formerly presented as a monolithic unit, these preliminary results reveal a much heterogeneous Formation (i.e. lithology, textural fabrics, allochems content; vertical variations), implying lateral variability within the Formation. A first detailed sedimentological log of the Great Oolite Formation is now proposed for the URG. Comparison with surrounding Formations equivalent to the Great Oolite shows comparable vertical stratigraphic organisation and diversity of textures, suggesting a common regional depositional story.

Mineralogical composition maps derived from µ-XRF analysis are used to investigate the diagenetic story, related to the post-depositional processes and the geodynamic evolution of the URG.

So far, this ongoing work allows to list some main characteristics of the Great Oolite in the URG:

- a diversity of textures that reflects fluctuating depositional energy conditions, and controls variable primary porosity and permeability properties;

- fine grained intervals (mudstone texture) suggest the presence of potential intra-formational seals, which need to be investigated (thickness, continuity);

- a newly identified heterogeneity of the Formation, that suggests a possible layer cake model, and thus a more complex reservoir;

- a clear diagenetic impact on the primary porosity and permeability;

- the presence of fractures and stylolites which may modify fluid flow.