History of the Petroleum System Concept

Leslie B. Magoon
Emeritus Scientist, U.S. Geological Survey, Menlo Park, CA

The petroleum system concept evolved over many years and is based on contributions by many investigators in geology, organic geochemistry, and computer technology. The oldest of these disciplines is geology, which dates to 1669 when Nicholaus Steno (1638-1686) published De Solido, which enumerated rules to interpret sedimentary rocks. Steno never mentioned time, which is so important in geology and evolution. The importance of time was recognized in 1788, when James Hutton (1726-1797) presented a paper to Royal Society of Edinburgh where he concluded that “….we find no vestige of a beginning, no prospect of an end.” Though Hutton developed the Theory of Uniformitarianism, his publications were overlooked until much later when he was recognized as the Father of Modern Geology. Sir Charles Lyell (1797-1875) gets credit for popularizing rock and time through his publication of the Principles of Geology in 12 updated editions. Lyell’s publication convinced Charles Darwin (1809-1882) that the Earth’s age was much more than 6000 years as interpreted by Archbishop James Ussher from the bible (Sunday at noon on October 23, 4004 B.C.), thus giving more time for evolution. Since the 1960’s, discoveries in geology, geophysics, and age dating have profoundly changed the way we look at Earth.

The second discipline required to make the petroleum system concept viable is organic geochemistry. In 1936, Alfred E. Treibs (1899-1983) correctly proposed the genetic link between the green pigment chlorophyll in living plants and many porphyrin pigments in crude oil, shale, and coal. He is generally acknowledged as the Father of Organic Geochemistry. During the 1960s, pioneers, such as Hunt, Philippi, Tissot, and Vassoyevich, published geochemical studies that clearly showed the origin of petroleum from fine-grained, organic-rich, sedimentary source rocks.

In 1966, the author was hired by Shell Oil Company to carry out a source rock study on the Santa Barbara Channel, California in preparation for an upcoming Federal offshore lease sale in 1968. Like other companies, this study was conducted because management was interested in improved exploration success. From 1966 to 1969, Wally Dow collected over 150 oil samples from the Williston Basin and he and Jack Williams at Amoco Research presented papers on the geochemistry of the oils at the AAPG Annual Meeting in Denver, Colorado in 1972. They introduced the concept of oil systems which depended on the ability to correlate crude oils to specific source rocks. I was present at their presentations and had just completed a similar study in the Big Snowy trough to the west of the Williston Basin.

Wally stayed in the petroleum industry and I went to work for the U.S. Geological Survey in 1974, where my primary effort was in domestic and worldwide oil and gas resource assessment. The original assessment procedure evolved from a primitive sediment volume approach to the more sophisticated play analysis concept. Unfortunately, geology and geochemistry were only included in a rudimentary manner. Most assessments relied on statistics, and in addition, there was no practical way to rank prospective areas. In 1982, the author began to develop what was to become the petroleum system concept and present a poster at the 1987 AAPG Annual Meeting in Los Angeles. We organized a very successful half-day session on the petroleum system for the AAPG Annual Meeting in Dallas, TX in 1991. Our effort culminated with publication in 1994 of AAPG Memoir 60, The petroleum system—From source to trap by myself and Wally. Fifteen years later, the petroleum system concept is widely used (Figure 1) and our biggest surprise is that this book is considered a “classic” that still sells at the original price, except at conventions.

The petroleum system is nature’s distribution system for hydrocarbon fluids, such as oil and gas. The deposition of sedimentary rock into a basin provides the setting into which this fluid system develops. Basin modeling numerically reconstructs this rock framework until the hydrocarbon fluid network forms which is then modeled as the petroleum system. Development of the petroleum system concept required refining and extending its vocabulary and establishing a series of graphic diagrams as a folio sheet (Figure 2). Generation-migration-accumulation, critical moment, and the pod of active source rock are important new terms needed for the visualization of the concept. The petroleum system map, cross section, and table, burial history chart and events chart are required to fully understand how the fluid system works in nature. The concept provides a new understanding the independent variables -- rock, fluid, time – needed to asses risk relative to petroleum prospects.

The petroleum system concept continues to grow as an exploration tool because of ongoing improvements in computing and 3-D visualization. Computers and information technology have been around for some time, but for many years their application was limited to experienced users. Early basin and petroleum system studies had to be carried out by dedicated specialists because almost everything had to be done by hand. Not so anymore; now all the drilled wells and seismic lines can be incorporated into the basin framework by way of computer graphics. Over the last decade, fast computers and jaw-dropping graphics have become accessible to almost everyone. Geochemical data on the source rock and hydrocarbon fluids can be readily incorporated into petroleum system models. Because of these advances, the plays and prospects that originate in the minds of the explorationist are now much more rapidly and reliably translated into discoveries.

Figure 1. Graph shows the number of papers and abstracts in the AAPG Bulletin (solid bar) and GeoRef (shaded bar) from 1985 that used the term petroleum system (left scale) compared to number of sold Memoir 60 (right scale).