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.
Highlights
Hydrological regime is one of the main factors controlling temperature in the Earth’s continental crust[1,2]
Such faulted regions and related fractured areas are widely documented in reservoirs thanks to 3D seismic data acquisition for oil[7] onshore and offshore, and for geothermal[8] fields in the onshore context
Highlighting fluid flow paths onshore requires a field area where seismic, hydrodynamic and thermal data coverage is dense, as can be the case for fields exploited for hydrocarbons, water or geothermal resources
Summary
Hydrological regime is one of the main factors controlling temperature in the Earth’s continental crust[1,2]. Vertical flows through permeable structures can cause large thermal anomalies in comparison with regimes without fluid flow (i.e. purely conductive thermal regimes). Such anomalies can reach several tens of degrees Celsius, especially within permeable faulted regions[3,4,5,6] in sedimentary basins. A 20 °C temperature anomaly is observed within this formation at a depth of around 1,700 m NGF, extending over a few kilometres between the north and the south of Paris[13] (Fig. 1 and Supplementary Figs S1 and S2). No model has so far been able to explain this anomaly, whether it be a conductive model with heterogeneous geothermal flux at the bottom and radiogenic production[14,15] or an advective one with flow confined to the exploited aquifer[16]
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