What is origin of petroleum forms?

Petroleum is derived from the remains of living things which contains a material called kerogen. Before dead organic matter becomes petroleum with time, the kerogen matures into an assortment of hydrocarbon molecules of all sizes and weights. The lightest (small) hydrocarbon molecules waft away as natural gas, and the heavier ones make up an oily liquid. Petroleum source rocks are of terrestrial and marine origin. Terrestrial source rocks are deposited in lakes, delta and river basins having woody plant matter, algae etc. Marine source rocks contain dead planktons, algae, organic remains etc. In both the settings, the mixture is buried under conditions of no oxygen. The kerogen are classified as type I,II& III as per their origin and are capable of producing oil or gas or both. Under the anaerobic conditions, the kerogen is transformed into a flammable substance called bitumen by the action of heat and anaerobic microbes in the sediment and natural catalysts. Most of the bitumen is eventually cooked into tarry asphalt releasing hydrocarbon molecules (as well as water and carbon dioxide) out of the source rock as it heats. Heavy oils form first, then light oils. As temperatures rise to and above 100° C, source rocks produce more gas. Being lighter than rocks, petroleum tends to rise upward through fractures and the pores of coarse sandstone beds. A small fraction of that leakage, perhaps 2% is preserved in large pools having an impermeable cap / seal over it.