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In the context of circulation, a marker is a material such as a dye, sand, grain, etc., that can be recognized as the fluid is circulated. Useful for determining swept volume of the hole.
In the context of formations, a marker is an easily identified formation that is used to identify the start of a rock sequence. In the wellbore, a marker is a material placed in the circulating fluid that can be easily seen when the fluid returns to surface – helps identify the swept volume of the wellbore.