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Abstract: A High-Resolution, Multi-Previous HitSensorTop Gamma-Ray Tool

Pirmez, Carlos; David Goldberg and Aleksandr Meltser - Columbia University

Natural gamma-ray measurements provide one of the best means to investigate the cyclicity of sedimentary sequences, the variations in the amount and gross mineralogy of terrigenous sediments within a sequence, and it is the measurement of choice for correlating among different logging runs and between logging data and core measurements. The vertical resolution of gamma-ray logging tools is largely determined by: 1) logging speed; 2) vertical sampling rate; 3) detector dimensions; 4) logging and borehole environment (ship's heave, cable stretch, borehole diameter and fluid), and 5) characteristics of the gamma-ray source (strength, attenuation). We developed a new gamma-ray tool that will provide high vertical resolution gamma-ray measurements by addressing factors (1) to (4) above. The tool will employ an array of four small gamma-ray detectors together with a downhole accelerometer for precise determination of tool motion. The reduction in gamma-ray counts expected from decreased detector size will be offset by the increased number of detectors. The measurements from individual detectors will be stacked, taking into account oscillations in tool speed measured by the accelero-meter. Results from tool response modeling in thin-bedded sequences and under varying conditions of platform heave and logging environments will be compared with initial field results. The high vertical resolution data to be obtained by this tool will be ideal for detailed characterization of turbidite and other cyclic sequences over a wide range of sedimentation rates, and will enhance our ability to integrate core measurements with logging data.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil