I don't understand this point. The North Sea oil and gas producers will sell to France and Germany anyway, I don't see what being in the EU has to do with it. Does it offer some level of supply protection by treaty or something?
The EU is heavily reliant on gas and oil imports. The vast majority of these are from Russia, which is building a gas cartel. The EU’s energy dependency rate causes concern because of the shortfall between production and consumption; the EU’s increasing dependency on energy imports from non-member countries. In 2007, the EU was importing 82% of its oil and 57% of its gas.
Expelling Scotland, an EU oil and gas producer, means that dependency rate is worsened. A higher percentage of oil and gas needs to be imported. This strengthens Russia’s hand, even if Scotland doesn’t join the Russian gas cartel.
The point of the EU is that it is a single market. Barriers to trade, such as trade tariffs (import and export taxes), import quotas, and so on are eliminated, and so there is a free trade area for goods. The EU has a common external tariff; the same duties and tariffs etc apply to all imports into the EU, no matter which country is importing them. A similar agreement exists for the Customs Union of Belarus, Kazakhstan, and Russia.
If some of those goods are no longer inside the EU but outside it, then not only are the single market trading advantages lost, but the overall energy dependency balance is tipped further to the advantage of the Russian Customs Union. Even if Scotland doesn’t join the Russian Customs Union.
Germany is EU’s biggest net energy importer, but France, despite its large nuclear sector, is also high in the table of oil and gas importers. Although the euro crisis has put strain on the Franco-German relationship, they are still major drivers of the EU.
The EU energy security policy concentrates on how to produce more energy within the single market, and necessarily focuses on nuclear and renewable, since further gas and oil reserves cannot be manufactured. (Russia also has a strong hand in solid coal exports). Expelling an internal gas and oil supply will multiply the renewables or nuclear generation needed, and given the proportion, the effect would be greater than if the EU had a lower dependency rate. Since renewable are already struggling to achieve quotas, and nuclear is politically sensitive, this makes expelling an oil and gas supply even more critical.
In short, it will be a strong driver for Germany and France to want to keep Scotland in the EU.