In the first few years of using logs measured while drilling (MWD), experience has revealed unexpected benefits for reservoir evaluation. MWD formation logs can deliver high quality measurements, often made before significant invasion or before the borehole has enlarged. The MWD induction resistivity log, the EWR*, is used to define reservoirs and accurately determine true formation resistivity. In fact, these measurements are often more accurate than conventional open hole wireline logs, as is proven with field examples. MWD users are combining "while drilling" logs with "after drilling" logs to observe hydrocarbon movement, or non-movement.

We discuss briefly the operating principles and characteristics of the EWR. Examples are shown which demonstrate the salient features of the logs. In particular, its thin bed resolution and good depth of investigation are obvious when compared with the conventional Deep Induction log. Field data will demonstrate the application of MWD logs for complementing or supplementing wireline logs, water saturation calculations, thin bed evaluation, and indication of hydrocarbon movement.

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