So, you’re strolling along the trails in Daley Ranch, enjoying a beautiful, mild sunny winter or spring day, when you come upon a puddle of water off the side of the trail. You notice that there’s an iridescent sheen on the surface of the water. “My gosh”, you think, “is that oil?” What starts running through your mind are scenarios like someone illicitly dumping used motor oil in the Ranch, or Jed Clampett shootin’ at some food, “and up from the ground came a bubblin’ crude...” Okay, maybe you’re not as mentally disturbed as I am, but you’re still wondering!!

Check out this page from the US Geological Survey for more info on the life of microbes in wet soils.

What you've observed isn't oil -- it's a biofilm produced by bacteria (Leptothrix discophora) that live in the soil. These bacteria have special requirements for life (which is why I mentioned above that you'd probably only see them during our Winter/Spring rainy season). When the soil is saturated, there's very little oxygen between the little grains of sand and pebbles in the soil; these are the anaerobic ("without air") conditions that Leptothrix needs to live. The soils of Daley Ranch contain a fair amount of iron (in the form of Fe(II) hydroxide). Leptothrix gets its energy from the oxidation of Fe(II) hydroxide to Fe2O3 (iron oxide -- rust!). This is why it's equally common (when you see the "oily" sheen on the surface of those puddles) that you'll also see orangey-red/rusty-looking material around the water. Leptothrix isn't just an iron-oxidizing bacteria; they can also oxidize manganese, which will lead to a black residue in and around the puddles. We don't see manganese oxidation so much, because manganese is a lot less common in the soil of Daley Ranch.
So, you're still not convinced that the oily-looking stuff isn't oil? Okay, pick up a small twig lying nearby, and gently and slowly stir through that iridescent film. A real hydrocarbon-based oil would swirl back together in the wake of the twig. A biofilm created by iron-oxidizing bacteria like Leptothrix will remain "broken" and separate.