Paleoweathering profile developed on homogenous sedimentary basement: an integrated approach from the CDB1 deep borehole (Rennes Basin, France).

摘要

Weathering profiles are mostly studied on their upper part (crust, saprolite) where leaching and concentration/precipitation of valuable element occur. For water resource and hydrocarbon purpose, the transition between saprolite and fresh basement is of utmost importance. Here is found the fissured layer, a highly fissured bedrock that it favorable for aquifer/reservoir properties. Such a weathering profile model is well known on igneous and metamorphic rocks, but curiously very poorly documented on sedimentary, non-metamorphic, basement. On behalf the CINERGY project, a 675m-long borehole (CDB1) was cored through the sedimentary infill (405 m), then the weathered basement (110 m) and finally the fresh bedrock (160 m). The basement is made of the "brioverian schists", which actually are epimetamorphic shales and fine grained sandstones, highly folded and cleaved. Here is presented an integrated (well-logging, mineralogy, petrography) study of the weathering profile that has been preserved under the sedimentary infill of the Rennes Basin. The well logging tools include Gamma-Ray, long and short resistivity, neutron porosity, gamma-gamma density, Pef, Full wave Sonic, BHTV and caliper. The cores were regularly sampled for total rock and clay mineralogy and some petrographic control were realized on specific facies and fracture fillings.