So, I made a trip to Canada, see post below, to study the McMurray Formation. The McMurray Formation hosts the single largest petroleum accumulation on the planet. However, the oil is more like tar and less like gasoline, making it a marginally commercial deposit.
Here I am at the base of the outcrop. The rocks I am walking on look and smell like fresh pavement. The sandstones are dark because they are stained by and full of oil.This is a view from the top of that same slope. As you can see it is beautiful. If this doesn't make you want to be a geologist at least a little bit you have no soul.
So what do I do? I make observations like, what is the grain size of the sandstone, what are the bedding attitudes, what is the oil saturation, is the evidence of paleontological activity (burrows, tracks, fossils)? From this we can tell what type of deposit it is (this deposit was a 50 m deep, meandering tidal channel). We can also determine which rock properties control where the hydrocarbons reside.