The most important factor for petroleum becoming gas isn't a substance, it's temperature and pressure. Most reservoirs have varying degrees of both; the T&P dictates how much of each will form. Knowing the type of field tells you what you should expect, be it a natural reservoir (black gold), or peat bogs (oil sands).
Geologists often refer to the temperature range in which oil forms as an "oil window". Below the minimum temperature oil remains trapped in the form of kerogen. Above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level. The Athabasca Oil Sands are one example of this. – Petroleum
The oil in Texas, where 40% of America's crude comes from, is under a caprock, which traps a "three-layer cake with a layer of water below the oil layer and a layer of gas above it" (Wiki). This type of reservoir will contain much natural gas.
The oil sands of Canada, which has the largest mining operation by area in the entire world, extracts bitumen; aka: peat. To extract natural gas from oil sand it must be cracked, which costs money, as opposed to the freely available stuff under a capstone.
Peat forms in wetland conditions [terrestrial life, not aquatic], where flooding or stagnant water obstructs the flow of oxygen from the atmosphere, slowing the rate of decomposition.
A fossil fuel, petroleum is formed when large quantities of dead organisms, mostly zooplankton and algae [aquatic life], are buried underneath sedimentary rock and subjected to both intense heat and pressure.
TL;DR: the good stuff comes from poorly decomposed aquatic life. The harder stuff comes from poorly decomposed plant life.... Apparently the dinosaurs did not all die and turn into oil.
Apologies but, the most important source IMO would need to be determined by computing the amount of barrels produced, while knowing what type of field they came from. E.g., "Oil sands were the source of 62% of Alberta's total oil production and 47% of all oil produced in Canada." (Wiki) Now you just have to figure out how many barrels Canada produces, contrasted against the same information for every country or oil company throughout the entire world.