Résumé
Advanced downhole hydrogeophysical logging of Oman Drilling Project Phase 2 boreholes using conventional and novel downhole probes was conducted to obtain a continuous record of petrophysical, hydraulic and fluid properties shortly after drilling in March 2018 in collaboration with the Ministry of Regional Municipalities and Water Resources Sultanate of Oman. The Centre National De La Recherche Scientifique (CNRS) and the Universite de Montpellier/Geosciences Montpellier deployed slimline tools to measure petrophysical, geochemical and fluid properties: electrical resistivity, magnetic susceptibility, natural gamma, sonic velocities, borehole televiewer (optical and acoustic), spontaneous electrical potential, borehole fluid physico-chemical properties (pressure, temperature, electrical conductivity, pH, Eh, dissolved oxygen), and downhole flowmeters (impeller and heat-pulse). In addition, a complete oilfield logging suite, including High-resolution Lateralog Array (HRLA), DLL, DSI sonic, spectral gamma, density, neutron porosity, Pe (photo electric), FMI electrical images, and "Lithoscanner" was deployed by Schlumberger Oilfield Services in the crust-mantle boundary holes (CM1B and CM2A). The downhole wireline data provide a unique continuous subsurface record of petrophysical, geochemical and hydraulic properties on the mm-to m scale. Log analysis is ongoing but preliminary results of the dynamic hydraulic and physico-chemical properties show distinct permeability barriers in correlation with alkaline and hyperalkaline fluids, discrete fractures with groundwater flow and indication of present day fluid-rock interaction in the crust-mantle boundary as well as in the partially serpentinized peridotite boreholes. The borehole fluid physico-chemical properties correlate well with electrical resistivity, spontaneous potential and "Lithoscanner" logs. Overall, the downhole logging data can be used to map alkaline and hyperalkaline fluid types, fluid type boundaries, permeability zones, and zones of fluid-rock interaction as a function of depth and to correlate them with petrophysical properties of the different rock types as well as with zones of different subsurface microbial activity.