Detection and characterization of thin weak bedding planes in hydrocarbon-bearing laminated formations

A method of investigating a formation traversed by a borehole, the method comprising:

• exciting and detecting sonic shear wave waveforms and sonic compressional wave waveforms at a plurality of locations in the borehole using a sonic borehole tool that includes a sonic source and a plurality of sonic wave detectors;
• exciting and detecting pseudo-Rayleigh surface wave waveforms and ultrasonic compressional wave waveforms at the plurality of locations in the borehole using an ultrasonic borehole tool that includes an ultrasonic transmitter and at least one ultrasonic receiver in a pitch-catch mode;
• determining shear wave slowness at the plurality of locations in the borehole using the sonic shear wave waveforms;
• determining sonic compressional wave slowness at the plurality of locations in the borehole using the sonic compressional wave waveforms;
• determining pseudo-shear wave slowness at the plurality of locations in the borehole using the pseudo-Rayleigh wave waveforms;
• determining ultrasonic compressional wave slowness at the plurality of locations in the borehole using the ultrasonic compressional wave waveforms;
• at each of the plurality of locations, comparing the shear wave slowness with the pseudo-shear wave slowness;
• at each of the plurality of locations, comparing the sonic compressional wave slowness with the ultrasonic compressional wave slowness;
• and
• if (i) the shear wave slowness and the pseudo-shear wave slowness are different and (ii) the sonic compressional wave slowness and the ultrasonic compressional wave slowness agree at a particular location of the plurality of locations, identifying the particular location as a location of interfacial weakness in the formation.

A method of investigating a formation traversed by a borehole, the method comprising:

• exciting and detecting pseudo-Rayleigh surface wave waveforms at a plurality of locations in the borehole using an ultrasonic borehole tool that includes an ultrasonic transmitter and at least one ultrasonic receiver in a pitch-catch mode;
• imaging the borehole at the plurality of locations using a microresistivity borehole scanner tool that includes an array of electrodes;
• processing signals from the array of electrodes to generate an image of the borehole at the plurality of locations in the borehole;
• determining at each of the plurality of locations in the borehole whether at least two events are represented in the pseudo-Rayleigh wave waveforms;
• and
• if at least two events are not represented for a particular location of the plurality of locations, using the microresistivity image to identify whether the particular location presents a sharp contrast in resistivity relative to adjacent locations.