A new wire-line formation resistivity imaging instrument employing a ‘two-electrode’ measurement configuration has been developed for application in low-resistivity formations drilled with non-conductive (oil-based) mud. A detailed explanation of the instrument and measurement principles is provided. Modeled synthetic responses and field examples are shown.

The high-spatial resolution of a ‘two-electrode’ arrangement has been well documented for nearly 25 years. The lower formation resistivity limit in oil-based mud boreholes for prior devices often precluded realizing their benefits in many important offshore deep water plays, such as Malaysia, Gulf of Mexico and elsewhere. The new instrument employs multi-frequency impedance measurements to overcome this limitation.

The impedance measurements produce two image logs: a real-part impedance image that is calibrated to yield formation resistivity and an imaginary-part impedance image that conveys information about image quality. The instrument employs six individually-articulated pads each containing ten sensor electrodes that provide a 75% surface coverage in an 8½ inch borehole. Typical spatial-resolution is better than one-half-inch vertically and one-eighth inch azimuthally. Pad-to-pad depth off-set is less than 6 inches, assisting to maintain borehole coverage in the presence of common instrument rotation during logging.

Field examples from typical logging environments found in offshore Malaysia are presented. Detailed geologic features such as thin-beds, slumping, cross-bedding, faulting and fracturing in low resistivity depositional environments are clearly resolved.

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