Abstract
Karst reservoirs form through the dissolution of carbonate rocks in response to acidic fluid circulation, creating significant volumes of underground storage space for natural resources. The heterogeneity of karst systems and their internal structures poses a challenge to the development of reliable hydrological models. The recent discovery of “ghost-rock karstification” processes suggests the existence of substantial and unexpected volumes of groundwater resources.This CIFRE thesis has two main objectives:1- To create a tool for analysing karst reservoirs. We have developed a digital tool that enables the acquisition, centralisation, visualisation, and archiving of relevant data for karst analysis. This “3D GIS” offers a comprehensive solution for analysis, interpretation, and communication regarding karst reservoirs.2- To contribute to understanding the nature, geometry, and organisation of karst reservoirs in a carbonate massif, using the example of southern Larzac (southern France).The construction of a 3D geological model of the Jurassic interval in southern Larzac enables the delineation of the envelope in which the karst reservoir has developed over multiple phases, spanning long periods (1–10 Ma). We demonstrate that the karst reservoir’s framework consists of alteration corridors formed through a “ghost-rock” process, followed by the partial or complete removal of residual alterites. These alteration corridors were identified and mapped at both surface and depth in southern Larzac, allowing us to characterise the geological reservoir’s structure and texture.We identify three successive stages of undissolved material evacuation, associated with known shifts in hydrodynamic conditions driven by the tectonic and morphological evolution of southern Larzac. By integrating hydrological connections determined through dye tracing operations with the geological reservoir structure, we propose drainage paths for past and present groundwater flows across southern Larzac. Findings indicate that the interiors of the alteration corridors exhibit varied facies depending on their evolution stage. The influence of these facies on water circulation dynamics and on the estimation of groundwater reserves is examined.The study of the Barrois limestones (in the Meuse and Haute-Marne regions) near the Cigéo site served as a "demonstrator" to: i) test and enhance the methodological tool (GIS), ii) replicate the mapping method for alteration corridors, and iii) validate conceptual models of the three-dimensional organisation of karst structures and their role in shaping subsurface drainage paths.This thesis provides: i) a new understanding of the structure and texture of a karst reservoir, and ii) a tool adapted for karst studies, which offers a pragmatic and coherent response to questions posed by communities, decision-makers, and industry stakeholders regarding the nature of the subsurface in karst areas.