A better definition of petroleum will include both a density range and a viscosity range.

A simple suggestion is that if the hydrocarbon is a gas at room temperature and pressure, then its a gas - this includes methane, ethane and propane. It the hydrocarbon is a liquid, even an extremely viscous liquid such as natural pitch, bitumen or tar, than it can be lumped into the 'petroleum' catch-all category - any hydrocarbon containing 4 or more carbon atoms, in general. Propane straddles that line, as it is commonly sold as a mildly compressed liquid. Hydrocarbon being, of course, molecules containing only hydrogen and carbon.

By the way, here is the definition of petroleum from Wikipedia, which sometimes is a reliable source if used with caution, "Petroleum, also known as crude oil or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that consist of refined crude oil." I don't have access to the Encyclopedia Britannica in order to make a qualified comparison of definitions. 

We should stick with broad definitions and lump all of these old carbon-rich material names into several headings such as petroleum or coal, as we are not concerned with gases here in MinDat. And lets not even get started with peat, char, or other naturally occurring energy sources.