An array of oil resources on the map, filtered to those that don't have derricks built on them at the start of the game.
Overview
This array lists map-placed oil resources, which you can build a derrick on if there isn't already one there.
Depending on baseType setting and map design, some of the listed oil resources will have map-placed derricks already built on them when the game starts.
It is generally better to locate free oil resources using enumFeature() and existing derricks using enumStruct() than to use the derricksPositions[] array. In fact, I can't think of any reliable use for derrickPositions[] array...
Values
The 'derrickPostiions' array contains a numerically-referenced list of Position objects that define the co-ordinates of oil resources.