Sorry for such a late response, I haven't noticed an email notification and took a long break from Factorio, let alone mod development.
The oil fields are grouped on a best-effort basis using an algorithm as the game lacks an API for ore patches. I don't remember quite clearly why the grouping doesn't work on oil fields, but I remember trying to solve it to no avail.
Overall I don't have much motivation in further updating this mod, it's better to switch to something that's actively maintained, like https://mods.factorio.com/mod/resourceMarker
