Petroleum Formation
Petroleum formed chiefly from ancient, microscopic plants and bacteria that lived in the ocean and saltwater seas. When these microorganisms died and settled to the seafloor, they mixed with sand and silt to form organic-rich mud. As layers of sediment accumulated over this organic ooze, the mud was gradually heated and slowly compressed into shale or mudstone, chemically transforming the organic material into petroleum and natural gas.
Sometimes, the petroleum and natural gas would slowly fill the tiny holes within nearby porous rocks, which geologists call reservoir rocks. Because these porous rocks were usually filled with water, the liquid and gaseous hydrocarbons (which are less dense and lighter than water) migrated upward, through the earth’s crust, sometimes for long distances. A portion of these hydrocarbons would eventually encounter an impermeable (nonporous) layer of rock in an anticline, salt dome, fault trap, or stratigraphic trap. The impermeable rock would trap the hydrocarbons, creating a reservoir of petroleum and natural gas. Exploration geologists seek these underground formations because they often contain recoverable petroleum deposits. The fluids and gases caught in these geologic traps typically separate into three layers: water (highest density, bottom layer), petroleum (middle layer), and natural gas (low density, top layer).
Sometimes, the petroleum and natural gas would slowly fill the tiny holes within nearby porous rocks, which geologists call reservoir rocks. Because these porous rocks were usually filled with water, the liquid and gaseous hydrocarbons (which are less dense and lighter than water) migrated upward, through the earth’s crust, sometimes for long distances. A portion of these hydrocarbons would eventually encounter an impermeable (nonporous) layer of rock in an anticline, salt dome, fault trap, or stratigraphic trap. The impermeable rock would trap the hydrocarbons, creating a reservoir of petroleum and natural gas. Exploration geologists seek these underground formations because they often contain recoverable petroleum deposits. The fluids and gases caught in these geologic traps typically separate into three layers: water (highest density, bottom layer), petroleum (middle layer), and natural gas (low density, top layer).
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