The Petroleum System Paradigm and the Biogenic Origin of Oil and Gas
Wallace G. Dow
Consultant, The Woodlands, Texas
The distribution and compositional variations of oil and gas in sedimentary basins can only be explained by its origin in thermally mature, organic-rich source rocks and occasional post-accumulation alteration in the reservoir. This conclusion, based on a very old idea, was confirmed after many decades of research, analysis with modern laboratory instruments and methods and rigorous application of the scientific method. The input of abiogenic hydrocarbons to the oil and gas found in sedimentary basins, if any, is insignificant.
Oil and gas in sedimentary basins is not distributed evenly, but occurs in distinct geographic and stratigraphic trends. The problem at Amoco Research in 1970 was to learn why this is true and whether we could learn how to accurately extend current and predict future productive trends prior to drilling. The Williston Basin was one of the first areas to be studied in detail. We collected 184 oil samples from 107 fields and all of the productive reservoir rocks in the basin and analyzed them with the best analytical techniques available at the time. The oils fell into three major chemically distinct groups (Figure 1) and several minor groups. In an effort to understand where these compositionally different oils came from, we analyzed core samples from all of the organic-rich, non-reservoir rocks in the basin with the same techniques used to study the oils. Each oil group matched to and was closely associated with a different organic-rich interval. The three major oil-rock pairs are isolated from each other by thick salt deposits (Figure 2) and the oils were unmixed and unaltered except beyond the salt depositional edges where inter-group vertical migration was possible. These three major oil-rock pairs were termed “oil systems” and each oil system contained more than enough organic-rich rock volume to account for their related reservoired oils, even at very low generation/accumulation efficiencies. These observations let to the hypothesis that each major oil group was generated in and expelled from a different, chemically distinct, organic-rich source rock. The minor oil groups appeared to be related to limited source rocks in thin, localized, or less organic-rich intervals. There was absolutely no association with basement faulting and it seemed impossible for oil or gas migrating from deep sources to penetrate the regional salt seals. Oil originating in a single source, either biogenic or abiogenic, should exhibit a more uniform composition and not have the profound compositional differences and undeniable organic affinities of the three major Williston Basin oil types.
Corroborating evidence for this hypothesis was provided by further examining the three source rock intervals throughout the Williston Basin. Organic matter in the Bakken Shale, for example, changed systematically in composition between the shallow and deep portions of the basin and contained oil-like organic extracts only below about 5,000 feet near the basin depocenter. Here, the Bakken Shale is overpressured, under compacted and exhibited increased electrical resisitivity on wireline logs, all apparently due to internal oil generation. Cretaceous organic-rich shales were found to be the source of a different type of oil in nearby basins but are shallow and thermally immature in the Williston Basin and have neither generated oil nor are associated with any oil or gas accumulations. These observations led to an additional hypothesis that oil [and gas] source rocks must be subjected to burial heat and pressure in order to convert at least a portion of the organic matter they contain into expellable and oil and gas.
Additional corroborating evidence was obtained from hydrous pyrolysis experiments on immature organic-rich shales that have not yet generated oil or gas. Heating these shales in the laboratory under pressure and in the presence of water formed oil that has properties nearly identical to their naturally generated crude oil counterparts. More advanced analytical techniques such as gc/ms and gc/irms, developed since the initial work was done in 1970, have been used by many researchers to analyze Williston Basin oils and source rocks in considerably more detail, but this new data has changed none of the original conclusions. These techniques also provided additional detailed information on the types of organic matter in each parent source rock and their related oils and showed undeniable organic signatures. These data further corroborated the previous hypotheses and led to confirmation of the scientific theory that oil and gas are formed from thermally mature, organic-rich, sedimentary source rocks. This “biogenic theory” of oil and gas formation is the only theory consistent with all of the observations, analytical data, scientific facts, and experimental results relating to the natural process of oil and gas formation.
The presence of crude oil in fractured basement reservoirs such as those in the South Vietnam Cuu Long Basin is often cited as “proof” that these oils have an abiogenic origin. But detailed geochemical analysis with a host of modern analytical techniques on oils from both Miocene sandstone and fractured basement reservoirs show nearly identical characteristics and correlate well with solvent extracts from Cuu Long Basin Oligocene lacustrine shales. Furthermore, biomarkers in these oils are clearly related to fresh water algae, diatoms, and higher land plants (angiosperms) and it is difficult to imagine how such materials could have found their way into oils with an abiogenic origin. In the Cuu Long Basin, fractured and weathered granite and granodiorite reservoirs occur in up thrown fault blocks that are structurally higher than the immediately adjacent effective Oligocene lacustrine oil source rocks where they were generated. These observations demonstrate that the Cuu Long Basin Oligocene-Basement(!) Petroleum System is quite conventional and to invoke an abiogenic origin for these oils simply because they occur in basement reservoirs ignores abundant scientific evidence to the contrary.
This scientific method approach has been repeated in many sedimentary basins by many researchers, who always arrived at the same conclusions; pooled oil and gas in porous reservoirs can only be explained by it’s origin in thermally mature, organic-rich, sedimentary source rocks. This “biogenic theory” of oil and gas origin subsequently led to the “generative basin” concept and eventually to the “petroleum system” paradigm that is widely used with great success by the petroleum industry today. This paradigm integrates the data and ideas of geology, geophysics, petroleum engineering, mathematical modeling, and geochemistry into the conceptual framework within which most oil and gas exploration is carried out. No other scientific theory has taken the observations and experiments pertaining to the origin of oil and gas from the descriptive to the predictive stage and herein lays its value. Abiogenic hydrocarbons, primarily methane, are certainly present in some parts of the solar system, including planet Earth, but they have nothing whatever to do with the oil, gas, and coal that powers the world’s economy.
Figure 2. Schematic columnar section showing stratigraphic terminology. Vertical distribution of source rocks, reservoirs, and evaportie Seals in three Williston Basin petroleum systems. Modified from Dow, W.G., AAPG Bulletin, 1974, v.58, p. 1255.