As the sediments are buried pressure increases and temperature increases and the organic materials react to form oil (a complex mix of liquid hydrocarbons) and gas (a complex mix of gaseous hydrocarbons).
(the sediments; are buried; ) (pressure increases and temperature increases and the organic materials; to form; oil (a complex mix of liquid hydrocarbons) and gas)
the sediments pressure increases and temperature increases and the organic materials
are buried to form
oil (a complex mix of liquid hydrocarbons) and gas