Ann: Additional Elevated Gas Shows in Primary Target, page-410

Sidetracks are drilled for a number of different reasons. Basically they fall into groups...planned and unplanned sidetracks.

Planned sidetracks are often drilled to economically access the maximum number of multiple targets off a single surface location.....you avoid the costs of rig moves and extra wellhead equipment. They can be drilled in order to cut a full hole core. When coring a single borehole you wouldn't know what you're coring until you actually core it but with a sidetrack core you've already evaluated the sequence in the original bore so have good control on where you start coring and how much you core. There are also other reasons for a planned sidetrack.

Unplanned sidetracks or remedial sidetracks are drilled to deal with unexpected scenarios and can include drill string or wireline tools left in the hole or bad hole conditions that prevent logging of the hole.

Re MAY's Zapato-1/ST wells.......they got their drill string stuck in the hole and ended up backing off the string and leaving a fish in the hole. They then cemented back and sidetracked the well. While this would classed as a remedial sidetrack the root reason is different from the reason for the Mukuyu-1 sidetrack.