This relation seems to be interesting as a correlation, probably signaling a general lack of planning, but I don't see why one should lead to the other.

If anything there should be a relation between the country's external debt and the energy mix. The PIIGS are PIIGS because of their state budget deficit.. and it is not the state that consumes the oil. Moreover, Spain had a budget surplus in the last years, today's deficit is simply conjectural - a the oil-dependence is not.