The creation of underground oil reservoirs is a complex biological and geological process, but with a little know how, the right ingredients, and some patience you can do it just like Earth!
It starts with phytoplankton and other microorganisms (that make up 80 percent of the organic mass in oceans) that die and sink to sea floors. Throughout most of the world, these tiny organisms get eaten and their calories cycle near the surface of the water, but in certain shallow seas the shower of organic matter settles undisturbed on the floor. Over 10 million years this process can create a layer of detritus one kilometer thick. As tectonic plates shift, seas get buried and the floor is pushed toward the earth's core. As it gets pushed, the pressure turns the sediment layers into hydrocarbon rich rock known as shale ( these hydrocarbons are molecule chains that repel water and come from the cells of the phytoplankton). The shales heat up as tectonic shifting pushes them toward the center of the earth. The heat cooks the hydrocarbon chains over millions of years, breaking them down into smaller units. They become lighter, turning from rock into a viscous substance, but they are still trapped in the shale, or "source rock". At this point, the substance is what we know to be petroleum.
It is ancient solar energy that has been collected, converted into carbon, and stored by Earth's marine plant life. These photosynthesizing microorganisms have been so prolific over the past 2.5 billion years that they have changed the composition of the atmosphere multiple times from high carbon or methane concentrations to more oxygen based atmospheres that allow other species to evolve. Large meteoric or geologic activity has spewed billions of tons of carbon into the atmosphere and the ocean's algae and plankton have used this atmospheric carbon and sunlight to multiply. They regulate the heat of Gaia by sequestering carbon, and further geologic activity buries it for millions of years. This carbon is not eliminated from the atmosphere permanently, it is released on a geologic time scale as shales get pushed to the surface. When humans pump it out of the ground and burn it really quickly (1 trillion barrels in 150 years), what do you think happens to the atmosphere?
At the point that the hydrocarbons become viscous petroleum, they are several miles below the surface. Humans would have never found out about this substance that has allowed us to temporarily stop deriving our energy from the sun if it stayed this far below our feet. Over 10's of millions of years, the "source rocks" of our reservoirs experienced intense geologic force pushing, shifting, and squeezing the rock. Under this pressure, the viscous hydrocarbons seeped upwards through tiny fractures or pore spaces. In many places there are more porous sandstones, the "reservoir rock" above the "source rock" shales where the oil can collect. Sometimes there is a layer of impermeable rock, the "cap rock", above the sandstones, capping the top of the oil seepage. And in even rarer sometimes, the cap rock is put under pressure and pushed upwards, creating a dome to trap the oil. This is the process and ingredients necessary to create a "worthwhile reservoir" ready for extraction.
One place that this precise mixture has occurred is in the Middle East, where the Arabian and Asian continents moved toward each other and eventually collided. Between them, as they were inching toward each other, was the Tethys Sea, a warm shallow sea teeming with marine life. For 100 million years the floor collected rich layers of organic sediments. The continental shifting made the sea recede and forced the sediments deep underground to experience a hot and pressurized geologic process. Layers of salt that remained from the disappeared sea folded into huge domes near the surface at the same time that the oil migrated upward from its source rock. This created the world's largest reservoirs. The new human species walked from Africa across the land bridge created by the continental collision and witnessed the transformed remains of the ancient sea slowly oozing on the ground.
It took awhile for humans to systematically exploit the energy in this ancient sunlight, but it has completely transformed our civilizations. For the majority of the time that our species has been around, we have only used current solar energy to power our lives. Civilization, generally thought of as arising with the advent of agriculture 10-12,000 years ago, has no doubt altered the biosphere and the atmosphere before oil, but the current magnitude and speed of change has no equal in the natural history of this planet. We have launched ourselves to the precarious peak of oil reserves and climate change and we do not know how to get down. Understanding the natural history of oil puts into perspective the scales of time at play in the creation and usage of this resource. Understanding also helps us build a connection between our species and the rest of Gaia. Oil has fueled our attitude that we are somehow disconnected and superior to the rest, but as you will see, oil actually links us to the ancestors of our current ecosystems. We are piped into the productivity of ancient marine plant life, the foundational species on which all complex life forms were built , and which derived their energy from the only known source: the sun.
For further reading, check out this educational website or read Crude or The Long Emergency or for an illustrated biological history of the earth read The Book of Life edited by Stephen Jay Gould (a fun reference book to own).
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