Why ,in the formation of petroleum products, it doesn't require the need of oxygen?
Absence of Oxygen is a very important for the formation of hydrocarbons. When oxygen is present, several things can happen. At the surface, when organic material is first laid down, the presence of oxygen means the presence of bacteria that can quickly consume the decaying material before it has a chance to be buried by sediment. This is why most petroleum deposits were once at the bottom of a sea or lake, often one with very low oxygen content, where sediment had time to accumulate before too much decay could occur in the presence of oxygen.
If oxygen is present, besides derailing the early stages of kerogen formation completely, it can also lead to the formation of acids and other molecules rather than strict hydrocarbon. These are usually detrimental to the formation of hydrocarbon and can even reverse formation that has already occurred.
Finally, levels of oxygen that are not high enough to prevent hydrocarbon formation can still be a problem Low levels of oxygen can lead to the buildup of toxic nitrogen oxide compounds as well as sulfuric and sulfurous acids. All these act as contaminants in petroleum, making it more expensive and difficult to refine.