Cream separates to oil when heated
Generally if the cream separates, that means you've heated it too much -- either too hot, or too long.

Also, if you're making a cream-based pasta sauce, the standard method is to add the cream last to the other cooked ingredients in the sauce.  That way you're less likely to overcook the cream.  Most cream-based pasta sauces start with a base that has solids and oil, such as a white roux or a pesto, which further helps the cream hold together.