2 Key ingredients for petroleum accumulation

2.3 Seals

Above permeable reservoir rocks there must be an impermeable layer (known as a seal or cap rock) to stop migrating petroleum from rising further towards the surface of the Earth. Seals are fine-grained or crystalline, low-permeability rocks such as mudstone, anhydrite and salt. Rock salt is by far the most effective seal, because it is crystalline and therefore impermeable. Seals are also enhanced if they are ductile (ductile deformation prevents the formation of open fractures and joints), substantially thick and laterally continuous; little surprise then that the largest oil fields in the Middle East are sealed by evaporites (Argles, 2005) with these characteristics.

However, seals are rarely, if ever, perfect. Hydrocarbons can migrate through almost all rock types, but at different rates that depend upon any fracturing and microscale fluid flow, and whether liquids adhere to or are repelled by the surfaces of mineral grains. Many oil and gas fields have active surface seeps of petroleum overlying them that provide a direct indication as to their location. In marine settings seeps may be detected as bubbles of gas rising from the sea bed, or as an oily sheen on the water. On land, plant communities are stunted, surface layers of rock and soil may be altered, tarry residues may encrust the surface, and sometimes there may be active oil seeps. The first oil fields to be developed in the 19th century were located beneath such obvious features. It is thought that ignition (by lightning strikes) of petroleum escaping above the huge oilfields of Iran gave rise to the fire-worshipping Zoroastrian religion. Even odder, the Ancient Greek Oracle at Delphi is thought to have made her prognostications while hallucinating under the influence of escaping natural petroleum gas.