Abstract: A downhole tool and method for determining properties of a formation. The method comprising irradiating the formation with a nuclear source; performing both a resistivity and a sigma measurement at each of different radial depths into the formation with a plurality of detectors; and inputting the measurements into a predetermined model for determining the properties of the formation.

Abstract: A logging tool capable of making different types of measurements is provided, and a plurality of measurements on a formation are obtained. Certain mineral properties of the formation are assumed and a mixed properties theorem for the formation is invoked. Upper and lower velocity limits for sonic waves traveling through the formation are determined using the above information. Biot's constant is also computed using the above information. LWD data on the formation is obtained and an effective stress is determined. A total stress is determined and, from that and the other information, a pore pressure is determined.

Abstract: A downhole tool and method for determining properties of a formation. The method comprising irradiating the formation with a nuclear source; performing both a resistivity and a sigma measurement at each of different radial depths into the formation with a plurality of detectors; and inputting the measurements into a predetermined model for determining the properties of the formation.

Abstract: A downhole tool and method for performing a base log sigma measurement of a formation. The tool comprising a nuclear source for irradiating the formation and a detector for obtaining a sigma measurement of the irradiated formation. The tool is mounted close to a drill bit arranged to drill a borehole into the formation and is capable of determining a base log sigma measurement of the formation while drilling into the formation.

Abstract: A method of determining the porosity and fluid saturation for a given volume of an underground formation through which a borehole is drilled, the method comprising: measuring, from within the borehole, parameters relating to characteristics of the underground formation; determining volumetric values for resistivity, hydrogen index, density and neutron capture cross-section for the given volume from the measured parameters, each volumetric value including a contribution from formation matrix, water, oil and gas; determining, from the volumetric values, the volumes of rock matrix, pore water, oil and gas, and the salinity of the pore water; and determining the porosity and fluid saturation of the volume of the formation from the determined volumes and salinity.

Abstract: A method of determining the porosity and fluid saturation for a given volume of an underground formation through which a borehole is drilled, the method comprising: measuring, from within the borehole, parameters relating to characteristics of the underground formation; determining volumetric values for resistivity, hydrogen index, density and neutron capture cross-section for the given volume from the measured parameters, each volumetric value including a contribution from formation matrix, water, oil and gas; determining, from the volumetric values, the volumes of rock matrix, pore water, oil and gas, and the salinity of the pore water; and determining the porosity and fluid saturation of the volume of the formation from the determined volumes and salinity.