The use of existing geological and structural maps, previous 2D seismic profiles, boreholes and correlation models between these data is sufficient to understand basin structure and thermal systems on a regional scale. However, this is not sufficient on a scale of a geothermal site to be sure of the hydraulic connectivity (or of the presence of a permeability barrier) between two boreholes 1.5 or 2 km apart. To ensure that there is enough hydraulic connectivity, it i s necessary to understand the controls on the network of fractures which affects the aquifer (fracture permeability) and the physical properties of the rock, namely the porosity and clay content in order to obtain a matrix permeability. The latest generation of broadband (six octaves) 3D seismic reflection will provide the following information: the similarity attribute will give an accurate structural map of the fault network at the seismic resolution and, in many cases, at a higher resolution than seismic; seismic velocity anisotropy analysis techniques will make it possible to visualize a 3D volume of information on the fracture network [Michel et al. (2013) Application of Azimuthal Seismic Inversion for Shale Gas Reservoir – Proceedings of the 11th SEGJ International Symposium, Yokohama]; acoustic impedance inversion or petrophysical inversion techniques will predict the porosity throughout the whole volume of the aquifer from a porosity log recorded in a pilot-hole. It allows a real 3D mapping of predicted porosity inside the aquifer much more reliably than from modelling alone. These seismic techniques were initially developed for petroleum exploration and development. They have rapidly progressed throughout the last decade, both in acquisition, processing and interpretation with new methodologies and high-performance softwares. They are efficient for modelling reservoirs to be produced. And, consequently, they can be used for geothermal applications as data to design dual deviated drillings with horizontal drains in carbonate and clastic reservoirs – not only for new projects, but also to revisit old ones to improve their performance or develop another reservoir. Broadband 3D seismic will secure the exploration of Triassic sandstones which stay an interesting prospect for deep geothermal projects. New prospects are proposed in the Paris Basin: Regional faults overlap the substratum. Inside faulted zones, hydrothermal circulations arriving by convection at the top of granitic basement could be geothermal objectives, as in the Alsace Upper Rhine Graben. A production pilot site is suggested to test superimposed aquifers and a regional fault and, at the same time, two different architectures of boreholes doublets: horizontal drains for aquifers and deviated wells for crossing a regional fault. The first site that will use this approach could be instrumented and used as an experiment with a small addition of measurements and sensors, thus becoming a showcase for geothermal energy in France. The objective of this experiment would be to determine the transit time, the heating time of the re-injected water and the circulation speed to define the optimal direction, spacing and length of drains, and also, to realize the thermal modelling of the site for different options.
This study is devoted to the lower series (Barremian-lower Albian) of Hole 549, just west of the continental shelf of northwestern Europe (Leg 80 of DSDP-IPOD). The lithological analysis is correlated with downhole logging, X-ray mineralogy, foraminiferal assemblages, and biological content. Biosedimentary environments and general pattern of distribution are interpreted and compared with Cretaceous exposures of northern Spain. Two successive paleogeographic phases are identified: a short transgressive episode giving a shallow environment (Barremian), and the opening (lower Albian) of a long period of pelagic sedimentation in a bathyal environment. They were separated by a gap in deposition, so that the record of intervening events is lost. The lower series begins with a record of marine transgression. The sporadic occurrences of oysters indicate a lowering of salinity. As the transgression became more extensive, the sediments became more calcareous, forming part of a carbonate platform. Trocholina species are associated with encrusting and other benthic foraminifers, bryozoans, corals, and algae. Subsequent deposition could not keep pace with subsidence, and the water deepened, as indicated by the presence of the planktonic foraminifer Hedbergella in association with small benthic foraminifers in a clayey-silty sediment interpreted as a quiet muddy seafloor. Clastic material is very mature, implying provenance in a very weathered area. In the lower part of the column kaolinite was the main mineral in the fraction smaller than 2 µm. In the carbonate sediments, kaolinite has been replaced by smectite. The second phase is characterized by planktonic foraminifers of the genus Hedbergella and by radiolarians, reflecting increased subsidence and bathyal conditions. The general trend of this evolution is chronologically in accordance with that known from the southern margin (Spain margin) of Biscay Bay.
Four holes at Sites 549 and 550 located along a transect across the northwestern European continental margin recovered calcareous carbonaceous sediments of mid-Cretaceous age. They are part of the more widespread occurrence of North Atlantic black shales. At both sites, alternating dark laminated calcareous siltstone and burrowed light-colored calcareous siltstone were recovered. At Site 549 the sequence is lower Albian to lower middle Albian and is 181 m thick. Terrigenous influx to the site was apparently relatively important because of its shallow paleodepth (500 m) and the proximity of an emerged land mass. At Site 550, which was the deepest site (2000 m paleodepth), biogenic calcareous sediments dominate the 91-m thick sequence of upper Albian to middle Cenomanian strata directly overlying oceanic basalt. A problem is raised by the cyclic character of the deposits. The cycles may have been climatically induced by consequent variations in turnover of bottom waters. Bottom currents seem also to have played a role in their formation. In spite of the similar aspect ç>f the sediments at both sites, the organic geochemistry data document differences between their environments of deposition. At Site 549 the low organic carbon content (< 1%) and the absence of marine organic matter in the dark facies both suggest that the bottom waters were moderately to well oxygenated. In contrast, at Site 550 the preservation of marine organic matter within the dark lithology implies a temporary restriction and oxygen depletion in the bottom waters during Cenomanian time. The coeval Cenomanian chalks deposited at the shallower Site 549 were well-oxygenated, pure white sediments. The narrow and complex geometry of the depression located in the oceanic trough, which opened during the Cenomanian, was probably favorable to restricting circulation of deep waters and also to the preservation of middle Cenomanian sediments, which have not been found very frequently in the deep North Atlantic.
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