Petroleum: To Be Or Not To Be Abiogenic
M. R. Mello1 and J.M. Moldowan1
1 High Resolution Technology & Petroleum. Av. Atlantica 1130, Copacabana, Rio de Janeiro, RJ, Brazil, 22021-000, E-mail: [email protected]
2 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 USA, and BIOMARKER TECHNOLOGY, 2501 Blucher Valley Road, Sebastopol, CA 95472 USA
Petroleum scientists have always searched for new trustful technologies to provide valuable evidences regarding the origin of hydrocarbons found in sedimentary basins and metamorphic areas around the globe. Present-day analysis of petroleum systems, when performed integrated with direct geochemistry, remote sense and high resolution geochemistry technology (HRGT), can provide irrefutable proof that 99.99999% of all the oil and gas accumulations found up to know in the planet earth have a biologic origin. The technologies can be so accurate and useful that they can predict pre-drilling insights regarding the quality and potential volumes of hydrocarbons to be found, including deep gas reservoirs, oil versus gas prone areas, degree of oil and gas cracking and of mixture of hydrocarbons derived from different sources, from different petroleum systems.
The recent application of those technologies, which consists of a complete characterization of the oil, gas and source rocks using Satellite images, piston core analysis, GC-MS, GC-MS-MS, Diamondoids, CSI-B and CSI-D methods, integrated with detailed geological and geophysical characterization, provide trustful and accurate approaches that allows the explorationists to play under a scenario of biological origin to obtain useful pre-drilling predictions in order to find oil and gas fields. The idea can be old but the practical use of this method is recent and represents an exploration breakthrough in petroleum technology.
The application of this new petroleum technology in petroliferous basins all around the world shows that oils can be attributed to organic-rich sedimentary rocks of specific geological age and depositional environments.
The abiogenic origin of petroleum thrived over a period in which scientific knowledge in biology, geology, and chemistry was in the dark ages. Mendeleyev, Kudryavtsev and Porfirev's abiotic evidences was well accepted at the beginning and in the mid-20th century because it offered an explanation for the presence of petroleum deposits in metamorphic rocks of the basement.
With the advances of analytical chemistry, around the fifties, geochemical evidence start to suggest, and latter proved that oils are related to biological precursors (Forsman and Hunt, 1958; Eglinton and Calvin, 1967 and Tissot, 1969). In the late seventies Albrecht, Seifert, Moldowan and Maxwell performed numbered studies that definitively proved the relationship between hydrocarbons and their putative biological precursor, burying the abiogenic hypothesis forever. Unfortunately, Thomas Gold convinced the Swedish Government and some neophyte people, in the nineties, that oil could be found everywhere in the planet, but more specific in an ancient meteorite crater, into fractured granite under the Siljan Ring, in Sweden. For this, two deep wells were drilled and millions of dollars were thrown in the deep earth granite (Gravberg-1 in 1986-1990 and Stenberg-1 in 1991-1992). In such adventure, no hydrocarbon was found and again the abiogenic hypothesis was put in its proper place (Kerr, 1990).
Today, the biogenic theory, which recognizes that all petroleum found in our planet is derived from biological precursors, is well proved and supported by laboratory experiments, in which petroleum composition is shown to reflect that of oil generated from kerogen by pyrolysis.
The application of high resolution biomarker technologies using GC-MS, GC-MS-MS, Diamondoids, CSIA-B and CSIA-D methods, integrated with detailed geological and paleontology cal characterization, provide scientific evidence that that oils can be attributed to organic-rich sedimentary rocks of specific geological age and depositional environments.
Oil samples related to sedimentary rocks of a certain depositional environment and geologic age show biomarkers derived from organisms that are known to have derived from biological precursor that evolved by that time (Figs 1 and 2). For example, oils that can be related to late Cretaceous and Tertiary source rocks generally show Oleanane, which derives from triterpane precursors in angiosperms that evolved and radiated in the Cretaceous and Tertiary, and/or they show the highly branched isoprenoid, which is synthesized by diatoms that evolved and radiated in about the same geologic time-span. Clear examples from major oil-producing basins are Venezuela, Nigeria and California (USA).
Tetracyclic terpanes, such as kaurane, beyerane and phyllocladane occur only in rocks and oils younger than the age of evolution of land plants, i.e., Silurian, because these are biogenic diterpenoid structures that are associated exclusively with hormone (gibberellins) synthesis required by all land plants. Such terrestrially dominated oils that show such compounds can be found widely in China, Southeast Asia, Australia, and in Venezuela. Oil samples that can be tied to early Paleozoic rocks, Cambrian-Devonian often show a unique n-alkane distribution with high odd/even predominance, terminating at n-C19 (Fig.1). This is the biochemical signature that has become identified and attributed to an early Paleozoic alga Gloeocapsamorpha Prisca, and can be found in certain oil habitats in basins of the Central USA, Australia, and Russia. It is also interesting to note that oils derived from marine and lacustrine source rock environments without higher plant influence lack terrestrial plant terpanes and oils derived from terrestrially dominated source rocks lack algally-derived C27 and C30 steranes. Another case, in point, is the presence of β-carotane derived from pigments in halophyllic bacteria that thrive in hypersaline environments, such as the Lagoa Feia source and derived oils of Brazil.
Another irrefutable proof of the biogenic origin of petroleum is the character of diamondoids in all petroleum liquids. One might expect an ultrastable hydrocarbon “non-biomarker” in oil, such as a diamondoid, to have an abiogenic origin. But, alas, it is not the case and it is proven by carbon isotopic composition. Diamonds are invariably formed from abiogenic carbon and, without argument, are abiogenic. They show carbon isotope ratios around 0 to 5 per mil indicating little, if any isotopic fractionation during their formation. However, the structurally related diamondoids in oil show high levels of isotopic fractionation in the range of -20 to -30 per mil, the same as most true biomarkers, indicating diamondoid derivation from enzymatic ally-created lipids with subsequent structural rearrangement during the process of source rock maturation and oil generation.
Application of diamondoid technology in the petroleum basins of the Gulf of Mexico has led to a uniquely detailed understanding of the oil and gas generating systems. Quantitative and isotopic analysis of diamondoid compounds in Mexican oils provide a depth of knowledge of the of the oil and gas biogenic origins, tying them to source ages and ranging from Oxfordian to Miocene in time, and from marine hypersaline to deltaic depositional environments, in which high amounts of higher plants were responsibly for the organic matter (Fig. 2).
K. E. Peters; C.M. Walters and J. M. Moldowan, 2005. The Biomarker Guide, 2nd Edition, parts 1 and 2, Cambridge University press, 1155p.
Figure 1. Fossil records of certain organisms suggest age-restricted biomarker concept. Age-related biomarker in oils has been developed using high resolution GC-MS-MS types of analyses, which are effective for differentiation of parts of the Phanerozoic source rock system and their depositional environments.