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.