Oil originally trapped in an oil reservoir, can, over millions of years, migrate laterally from one location to another, sometimes many tens of kilometers from the original reservoir. We may think of present-day reservoirs as static and unchanging, but even now processes forcing the migration of the oil are continuing. Oil and petroleum products can escape through horizontal and vertical fractures and fissures and ultimately discharge at the surface as vapors and liquids. Vapors most easily penetrate such fractures and as they escape, they leave a residue by product of concentrated oil or tar. This concentrated by product may be too viscous to migrate and will largely remain stationary. If the residue is not highly viscous, the product may migrate with facility along fractures and also be discharged at the surface. Most oil discharged to the surface becomes evapoconcentrated as volatile components escape into the atmosphere. This thickened oil is tar, the most famous occurrence being the La Brea tar pits, in Los Angeles.
Oil products can also become concentrated and solidify as an asphalt, called asphaltite or gilsonite. This product can be exposed at the surface and has an appearance like coal. Upon closer examination, however, asphaltite may appear as a soft, light density, glassy material, that spalls with concoidal fractures like obsidian. Gilsonite will burn, and can be refined into heavy oil and other useful petroleum products. The refining process, however, is noted for flaring methane as a by-product of the refining process. Gilsonite is typically found in oil shale.
Keep in mind that in many instances, oil at depth may be within a reservoir that is very hot. The high temperature is due primarily to the geothermal gradient. This heat can cause the volatilization of oil thereby driving the volatile products through fissures and along faults to escape at the surface. The now thickened oil can also force open these same fissures and planes along faults and be deposited there. In the case of gilsonite, the thickened petroleum can form sills between bedding planes as well as dikes along fissures and faults. Erosion can later expose these deposits at the land surface.