Logging as a business can be split into 2 subgroups namely Open Hole or Cased Hole. Open Hole logs are performed during the drilling phase of an oil well. The main goal of Open Hole logs are to delineate productive intervals (if any) and provide data to estimate volumes of oil in place. Open hole logs can also be used to determine formation strike and dip, improve seismic imaging, and provide depth accurate core and fluid samples of reservoir rock.The engineer has several types of logs at his disposal to make these kinds of determinations - what logs are actually run depends on what the customer wants and how much about the reservoir is actually known. Most decisions as to whether or not a well is productive is based on data from Open Hole Logs.
Cased hole logs generally occur in wells where the drilling process is finished and the well is in the completion or workover phase. These Logs are run to perform a specific service or answer a specific question. Cased Hole logs can also be used to measure reservoir deplection, production characteristics, or casing integrity (over time) which can all affect the life of an oil well and the design of future wells.
Most logging engineers are specialised into either Open Hole or Cased Hole. There are a few that do both on a regular basis(this would be me). The specialisation (or lack therof) is mostly due to market forces rather than any particular desire of the engineer (or company).