Hydraulic and hydrochemical properties of deep sedimentary reservoirs of the Upper Rhine Graben,Europe

Hydraulic and hydrochemical data from several hundred wells mostly drilled by the oil and gas industry within the four deep carbonate and siliciclastic reservoirs of the Upper Rhine Graben area in France and Germany have been compiled, examined, validated and analysed with the aim to characterize fluids and reservoir properties. Due to enhanced temperatures in the subsurface of the Upper Rhine Graben, this study on hydraulic and hydrochemical properties has been motivated by an increasing interest in deep hydrogeothermal energy projects in the Rhine rift valley. The four examined geothermal reservoir formations are characterized by high hydraulic conductivity reflecting the active tectonic setting of the rift valley and its fractured and karstified reservoirs. The hydraulic conductivity decreases only marginally with depth in each of the reservoirs, because the Upper Rhine Graben is a young tectonically active structure. The generally high hydraulic conductivity of the reservoir rocks permits cross‐formation advective flow of thermal water. Water composition data reflect the origin and hydrochemical evolution of deep water. Shallow water to 500 m depth is, in general, weakly mineralized. The chemical signature of the water is controlled by fluid–rock geochemical interactions. With increasing depth, the total of dissolved solids (TDS) increases. In all reservoirs, the fluids evolve to a NaCl‐dominated brine. The high salinity of the reservoirs is partly derived from dissolution of halite in evaporitic Triassic and Cenozoic formations, and partly from the fluids residing in the crystalline basement. Water of all four reservoirs is saturated with respect to calcite and other minerals including quartz and barite.     

Hydrochemical characterization of a coal mine plume detected by an airborne geophysical survey

A trial airborne electromagnetic (AEM) geophysical survey was carried out across a 13 × 9 km area of the northern Nottinghamshire (UK) coalfield. One of the objectives was to examine the influence of coal mines (collieries) and associated spoil heaps situated above the Triassic Sherwood Sandstone aquifer. The conductivity models obtained from the AEM survey revealed extensive zones of enhanced subsurface conductivity in the vicinity of all the collieries in the survey area. The present study provides information regarding subsequent investigations (ground geophysics and borehole) to confirm the AEM results and to investigate the geochemical nature of the conductive zone identified in the vicinity of one of the collieries.Following ground geophysical confirmation of the airborne results, three boreholes were drilled into one of the conductive zones to the east of a working colliery. Geophysical logs and analysis of pore fluid geochemistry demonstrate that there is a very strong correlation between the borehole induction logs (formation conductivity), total dissolved solids (TDS) and chloride in the pore waters. The three cored boreholes have confirmed that there is a groundwater plume with high concentrations of TDS, compared with background, moving east from the Thoresby Colliery on a hydraulic gradient of 0.023. Porewater geochemistry obtained from core has been compared with that of local aquifer waters and with samples obtained from the colliery. The results indicate that the plume chemistry could result from the mixing of a typical Sherwood Sandstone composition with colliery spoil leachate. The AEM conductivity models have been successfully correlated with ground geophysical models, borehole induction logs, TDS and chloride in the pore waters. By implication, the geophysical results obtained over a larger area indicate the wider extent of such impacts in relation to both former and current mining activities.     

Main and trace elements in KTB-VB fluid: composition and hints to its origin

The pilot hole (VB) of the German Continental Deep Drilling Program (KTB) was drilled to a depth of 4000 m, where large amounts of free fluids were met. The KTB‐VB 4000 m fluid can be related to either Mesozoic seawater or formation water from Permo‐Carboniferous sedimentary rocks of the Weiden embayment. During the Upper Cretaceous uplift of the Bohemian Massif both fluids could have passed organic‐rich Triassic to Carboniferous formations of the Weiden embayment before invading the uplifted and fractured basement rocks of Devonian amphibolites and metagabbros, where the chemical composition of the fluids was changed by albitization, adularization, and chloritization. Results of chemical mass balances for both sources are presented. In order to concentrate the formation water from the Weiden embayment significant amphibolitization has to be assumed. During a 1‐year pumping test the chemical composition of the 4000 m fluids remained constant. The accuracy of chemical analyses is critically reviewed. An improved preconcentration method of rare earth elements and yttrium in high‐Ca‐bearing saline fluids is described.     

Large‐scale chemical stratification of fluids in the crust: hydraulic and chemical data from the geothermal research site Urach,Germany

The Urach 3 research borehole in SW Germany has been drilled through a sedimentary cover sequence and reached gneisses of the Variscan crystalline basement at 1604 m below surface. An additional 2840 m has been drilled through fractured basement rocks. The borehole has been used for hydraulic tests in the context of a ‘hot dry rock’ (HDR) project. The sedimentary cover ranges from the Carboniferous to the Middle Jurassic (Dogger) in age and comprises mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic. Water composition data from 10 different depths include samples from all major lithological units. The total dissolved solids (TDS) increases from the surface to about 650 m where it reaches 4.1 g l^?1 in Triassic limestone. In lower Triassic sandstones, TDS increases very sharply to 28.5 g l^?1 and the water is saturated with pure CO₂ gas. With increasing depth, TDS does not change much in the clastic sediments of the Permian and Carboniferous. The crystalline basement is marked by a very sharp increase in TDS to 55.5 g l^?1 at about 1770 m depth. TDS increases within the basement to more than 78.5 g l^?1 at about 3500 m depth. The data suggest that there is limited vertical chemical communication over long periods of time. The CO₂ gas cap in the lower Triassic sandstones requires a gastight cover. The chemical stratification of the fluids relates to the permeability structure of the crust at the Urach site and fits well with hydraulic and thermal data from the site.