Thermal and seismic hints for chimney type cross-stratal fluid flow in onshore basins

Abstract

When modelling onshore sedimentary basins, modellers generally assume that semi-permeable layers (aquitards) greatly restrict vertical flow between aquifers. Aquitards are therefore considered as confining media and vertical flow is assumed to take place mainly within localised permeable faults, if any. In the offshore context, however, interpretation of seismic data frequently provides evidence of fluid flow between sedimentary layers via structurally disrupted formations (pervasive fractures) recognised as zones of reduced seismic amplitude and generically called “chimneys”. Here we show that chimneys are also present onshore, and that they crosscut confining layers. In the Anglo-Paris Basin, seismic data suggest 1 to 2 km wide zones of disrupted seismic signal spatially correlated to a hitherto unexplained major temperature anomaly of 20 °C. When included in geothermal models using a five-order increase in permeabilities with respect to confining layers, we find that fluid flows vertically through aquifers and confining layers, thereby explaining this major temperature anomaly. Despite the importance of their hydrodynamic and thermal impacts, chimneys – less obvious than faults – have been overlooked as fluid flow paths in many onshore sedimentary basins exploited for their resources. This indicates a clear need for better understanding of pervasive flow paths, especially as the resources and properties of basins (i.e. conventional and unconventional hydrocarbons, geothermal potential, CO2 storage, nuclear waste repository, drinking water, etc.) are increasingly being harnessed.