To this point, we have discussed the elements required for the formation of a hydrocarbon reservoir. We have also discussed the timing and histories of the formation of the reservoir traps. We have also mentioned that all of the elements of the hydrocarbon reservoir must be in place prior to the formation and migration of the hydrocarbons. We are now able to discuss the actual formation of the hydrocarbons themselves.
Petroleum and natural gas are naturally occurring substances composed of hydrocarbon molecules (molecules made up of hydrogen and carbon atoms) and, possibly, non-organic contaminants, such as CO2, H2S, N2, and O2.
The most widely accepted theory of the origin of petroleum and natural gas is the Organic Origin Theory which states that these fluids are generated from the decay of prehistoric plants and animals under the influence of the excessive pressures and temperatures that exist in the earth’s subsurface. Research indicates that petroleum and natural gas originated from zooplankton (single-celled plants found drifting freely in fresh or brackish water) and algae. During their lifetimes, these organisms create energy from photosynthesis to carry out their life processes, and it is this energy that we use today (thus the term, “fossil fuels”). Contrary to the popular myth, petroleum and natural gas do not originate from decomposed dinosaurs.
Over the geologic time scale, these free-floating zooplankton and algae die, settle to the ocean-, sea-, lake‑, river‑, or swamp-bottom, and accumulate as sediment. This process occurs simultaneously with the geological processes (erosion, transportation, and deposition) acting on the sand, silt, and clay sediments that form sedimentary rocks. As the organic material and rock materials accumulate on the sea bottom, the materials at the top of the accumulation exert increased weight (pressure) on the materials at the bottom of the accumulation. As this depositional process continues and the accumulation becomes thicker, the impact of the earth’s geothermal gradient also begins to act on the organic materials. At elevated pressures and temperatures, the decaying organic materials are transformed into a dark waxy material called kerogen. Kerogen is an intermediary stage in the development of petroleum. The process of converting the original organic material into kerogen is called the Catagenesis Process. The formation of kerogen requires that the rock containing the original organic material be impermeable. This requirement is essential because as kerogens are formed, they must remain trapped within the pore-spaces of the rock and cannot be allowed to escape to other rock formations or to the environment. As we have already discussed, the rock formation in which the original organic materials are converted to kerogen, and eventually petroleum, are called source rocks for the hydrocarbons. Typically, fine-grained, clay-rich sedimentary rocks, such as shales, are the most common source rocks for hydrocarbon basins because they satisfy two of the requirements for kerogen generation; they (1) are sedimentary rocks and (2) contain adequately low permeabilities.
During the catagenesis process, the kerogen is “cooked” in the high-temperature environment, and the long-chain kerogen molecules are “cracked” into shorter-chained hydrocarbon molecules. It may take several million years for the deposition process to bury the original organic materials and the source rock to achieve the appropriate temperatures (> 250 - 300 ºF) for kerogen generation and another several million years to generate commercial quantities of oil and gas. There is a specific temperature range, referred to as the “oil window,” in which oil is formed. At temperatures below the oil window, the catagenesis process cannot occur; while at higher temperatures the “cooking” and “cracking” processes are stronger and very short-chain natural gas molecules are generated (thermogenic gas).
A second, less accepted theory for the origin of hydrocarbons is the Inorganic or Abiogenic Theory. In this theory, hydrogen and carbon from inorganic sources are fused at the elevated pressures and temperatures in the earth’s subsurface and are converted to hydrocarbons. While the presence of inorganically sourced hydrocarbons cannot be discounted, there is abundant evidence that the vast majority of crude oil and natural gas have an organic origin.