Confined carbonates – Regional scale hydraulic interaction or isolation?

Abstract This study examines the patterns of groundwater flow and salinity in a region of confined basement carbonate aquifer along with the region's unconfined adjacent part and siliciclastic confining strata. An understanding of regional-scale flow patterns in this setting may prompt a rethinking of the traditional view. According to that view confined carbonates are bounded and isolated by impermeable confining layers from their surroundings. A basin-scale analysis of the subsurface conditions promises better to accentuate otherwise unseen signs of hydraulic communication both horizontally and vertically between different parts of the flow domain. This study reveals that various flow regimes exist, in the area of the Paleogene Basin, Hungary. The pattern and intensity of these flow regimes depend on the elevation of basement carbonates and the structures, thickness, hydraulic conductivity and heterogeneity of the covering layers. Effects of gravity-driven regional groundwater flow were identified down to an elevation of −500 m asl including recharge and discharge areas. Hydraulic communication occurs both vertically and laterally in this zone but the direction and intensity of flow are influenced by aquitards or confining layers. Nevertheless, hydraulic boundaries (a colinear ridge in the north and a sink in the south) were recognized in the study area. This impedes horizontal hydraulic communication between the shallower unconfined-to confined carbonates in the west and the deeper confined carbonates in the east. Southeasterly through-flow can be observed below −500 m asl elevation which terminates in a regionally underpressured zone due to a regional aquitard in the zone of uplift. Both underpressured and overpressured blocks bounded by faults appear to influence vertical connections between siliciclastic confining layers and carbonates in the vicinity of significant strike-slip faults. The flow regimes thus recognized affect the subsurface salinity pattern, and hydrocarbon migration and as a result the planning of geothermal exploration. Consequently, a priori assumption of impermeability of confining layers and hydraulically isolated carbonate compartments below seems to be an oversimplification.