Comparison of hydrogeochemical logging of drilling fluid during coring with the results from geophysical logging and hydraulic testing Example of the Morte-Mérie scientific borehole, Ardèche-France, Deep Geology of France Programme

Abstract

A 980-m-deep well was cored on the Ardèche border of the Southeastern basin of France as part of the Deep Geology of France (GPF) programme. Hydrogeochemical logging was carried out during drilling, which involved the monitoring of physico-chemical parameters (pH, Eh, temperature and conductivity), and chemical parameters (concentrations of He, Rn, CO 2, CH 4, O 2 Ca, Cl and SiO 2) of the drilling fluid permanently circulating in the well. This logging programme was complemented by geophysical logging and two hydraulic tests. The combination of these measurements enabled identification of a transmissive interval due to fractures in the Jurassic carbonates, and of fluid inflow both at the base of the porous and slightly permeable Triassic sandstones and from an open fracture in the Permian conglomerates. These intervals are marked by changes in the drilling-fluid chemistry, such as an increase in chemical species content, or a drop in pH. The degree of modification depends on the natural permeability of the fractures and the salinity of the fluids. The porous and permeable intervals are also marked by He anomalies, which act as a tracer for these zones. Comparison between the geophysical and hydrogeochemical logs reveals that the latter provide information on the liquid phase, whether the fractures are productive or not, whereas the geophysical logs are more directly related to the solid phase.