Numbers under the double wells

By Zozimusque Romanus, in Black Gold

When you drill under the double wells, you're not allowed to prospect first. The numbers of plumes will either be 2 or 5. But in the bottom right-hand corner of the number '2', there is a small black circle with a '3' inside. Does anyone have any idea what that is for? I can't find it anywhere.

Thanks!

Yes, you can find this in the rulebook at optional rules:

Deeper Double-rig Wells

When players drill at a double-rig well, they are
taking a big risk. For experienced gamers, this is
a calculated risk, and they must determine if such
a move is worth it. For inexperienced gamers who
feel compelled to build a derrick during every
single round, it can be crippling to discover two (or
more) double-rig wells with an oil value of “2.”
So, to lessen the blow of discovering multiple
shallow double-rig wells and to cater to a younger
audience, the Deeper Double-rig Wells optional
rule can be used. Under this rule, double-rig well
markers with an oil value of “2” are considered
to have an oil value of “3” instead. These well
markers show a circled “3” to act as a reminder
when using this optional rule. The wells with an oil
value of “5” remain unchanged.
Att
Alessandro