Here's my thought, for what it's worth.
Petroleum is not what's left of the Dinosaurs it is a natural byproduct of the earth. Petroleum is produced via the rotation of the earth's molten core that finds its way out of the core and into the Mantle by centrifugal force where it works its way slowly towards the surface. The different depths of various oils fields is due to the age of the deposits ejection from the core and how far through the solid rock it has worked itself towards the surface.
The La Brae tar pits in California is just one of many places where the petroleum deposit has worked itself to the surface. The fact that the La Brae tar pit is a reservoir of prehistoric skeletons of both mammals and Dinosaurs indicates that the deposits predated the ancient beasts and was not derived from them.