This post is continued from my previous post on Oil Genesis on where oil comes from.........
*There are no big open caverns at oilwell depths. The oil is trapped in rocks with significant porosity, known as reservoir rocks. ("Significant" is more than 5%.) Roughly half of the world's oil comes from sandstone reservoirs. Almost all of the other half comes form limestone and dolomite. Limestones-which is calcium carbonate, CaCO3 -is formed mostly by marine plants and animals in ocean surface waters. Dolomite, a calcium-magnesium carbonate, CaMg(CO3)2, is still not fully understood, but it makes some of the best reservoir rocks.
*The pore spaces in the rock have to be connected to one another so that oil or gas can flow through the rock; the pore connectivity is called permeability. Gas bubbles in a solidified lava flow are examples of pores that are not connected to one another. Permeability depends on the square of the grain size. Doubling the diameter of the sand grains in a sandstone increases the permeability by a factor of four. Increasing the grain size by a factor of ten makes the permeability a million times larger.
*At least one rock layer between the oil reservoir and the surface has to make a leak-tight seal, called a cap rock. The first layer above a commercial reservoir rock is often mistakenly identified as the cap rock, although it usually is simply an oil saturated but noncommercial reservoir rock. Leak- proof cap rocks are special; very fine-grained mudstones and evaporite layers like halite (NaCI, table salt) and anhydrite (CaSO4).
From my first post on oil genesis until this post, there are 7 essential ingredients in crude oil. If one or more is missing, you will find zero oil.