Remember that this can include any work you've done as a contractor or as an employee in a refinery. It is ok if you have direct refinery experience, but it is also ok if you don't. Some employers would prefer to hire someone with little to no refinery experience so that any bad work habits aren't carried over from one company to the next. It is also sometimes easier to train someone who is new to the industry but has specific related experience. On the flip side, the employer might be looking for someone who has direct experience so that they don't have to start at square one acclimating you to the refinery environment. There is really no way to tell what the employer is looking for as far as experience, so don't second-guess that you have it all figured out. Just be honest and as specific as possible in your answer.