GEOLOGICAL CONDITIONS OF DEPLETED GAS FIELDS IN THE TRANSYLVANIAN BASIN FOR CO2 STORAGE ASSESSMENT

Depleted gas reservoirs have attracted increasing interest as potential candidates for geological CO2 storage, particularly in mature gas provinces where subsurface conditions are well constrained by long-term exploration and production activities. The Transylvanian Basin represents such a setting, hosting numerous gas accumulations developed mainly within suprasalt systems and characterized by a long production history. This paper presents a screening-level geological assessment of depleted gas reservoirs in the Transylvanian Basin. It focuses on geological conditions relevant to containment in the context of potential CO2 storage.
The assessment is based on an integrated analysis of the basin-scale geological framework, including tectonic evolution, structural configuration, reservoir characteristics, and sealing conditions associated with the widespread Badenian salt. Particular emphasis is placed on the role of salt as a regional seal, the structural and stratigraphic compartmentalization of reservoirs, and the proven ability of these systems to retain gas over geological time. These aspects are considered essential for evaluating containment potential from a geological perspective. As a representative case example, the DepoMures underground gas storage facility, developed within a former gas reservoir, is discussed to illustrate how these geological conditions manifest at site scale and to highlight its relevance as an analogue for CO2 storage.
The results indicate that selected depleted gas reservoirs in the Transylvanian Basin exhibit several favourable geological features, including effective sealing conditions, well-defined structural traps, and documented gas retention, which support their consideration as potential candidates for CO2 storage. This study is intentionally limited to geological screening of containment-related factors and does not assess storage capacity, injectivity, pressure build-up/management, or geochemical and geomechanical behaviour. The results therefore provide a preliminary geological basis for selecting candidates for subsequent site-specific characterization and quantitative feasibility studies.