Molecular Characterization From the Earliest Days to the Present
Alfred Treib’s 1936 landmark study is credited with launching the beginning of molecular geochemistry. But, few geochemists today know how he was able to link the structure of geoporphyrins to chlorophyll. The discovery was not done in isolation but rested on nearly ~70 years of prior studies. A retrospection of our current analytical foundation can provide insight on how progress in petroleum geochemistry was made and how it will likely be continued in the future. People have used petroleum since Biblical times; however it is the most chemically complex mixture in nature and describing its composition has been an ongoing challenge. Studies using mainly distillation and wet chemistry conducted in the late 1800’s showed that petroleum was mostly composed of hydrocarbons with smaller amounts of NSO‐heteroatomic species. In 1928, the American Petroleum Institute launched Project 6, the first systematic study to identify specific components in petroleum. Over more than three decades, API Project 6 managed to isolate and identify only ~175 compounds, mostly in the gasoline‐range, using distillation, solvent extraction, and selective absorption. The API Project 6 approach became obsolete with the introduction of gas chromatography. Developed in the late 1940’s by James and Martin, Shell (Amsterdam) and BP (Sunbury) were at the forefront of embracing this new technology. It wasn’t long until GC was applied to isolate specific hydrocarbon biomarkers. The earliest GC detectors were based on thermal conductivity but flame ionization (1957) and mass spectrometer detectors (1959) quickly followed. Separation continued to improve as large‐bore packed columns gave way to small‐bore metal and hand‐drawn glass and then to fused‐silica capillary columns and new, more stable stationary phases. The introduction of commercial GC‐MS systems with computerized acquisition and data processing systems in the mid‐1970’s sparked two decades of rapid biomarker discovery that led to many of our current routine practices. With each subsequent advance in GC technology, from high‐temperature phases, on‐line pyrolysis, elemental specific detectors, and multi‐dimensional separation techniques, petroleum geochemists have been early and effective adopters. Liquid chromatography was invented in 1903 by the Russian botanist Michael Tswett to isolate plant pigments. His work was ignored and then 'reinvented' by Lederer in 1931 and soon after, liquid chromatography methods (e.g., paper, thin‐layer) became routine. For decades, LC was the primary means to separate petroleum into chemical fractions, first via open‐column then with medium and high pressure automated systems. Like GC, LC separation and detector technologies continued to improve and LC‐MS systems became practical. By the early 2000’s, techniques developed for natural products were being applied for the detection of bacteriohopanoids and archaeal lipids in geologic samples, leading to the development of key paleoclimate proxies. Mass spectroscopy is a key enabling technology for molecular characterization. First used in 1942 to characterize refinery products, MS was quickly embraced by geochemists. The discovery of carbon number preference of n‐alkanes by Evans et al. (1957) was based on MS analysis. Interfaced with GC, low resolution MS and MS/MS proved to be invaluable for identifying trace hydrocarbons. Today, the advent of ultrahigh resolution mass spectroscopy is ushering in a new era of molecular characterization allowing for the characterization of trace polar species at unprecedented numbers. The most advanced instruments can accurately measure the mass of ~50,000 ions that can be assigned unique molecular formulae within a single petroleum sample. As in the past, geochemists are at the forefront of investigating new ionization methods and applying GC‐ and LC‐ interfaced with ultrahigh resolution MS. We are currently able to identify several thousand individual compounds that comprise ~70 to 90% of a typical crude oil and assign molecular formulae to most of the remaining species. The ultimate goal of petroleomics, the complete molecular characterization of petroleum, remains a grand challenge. It may be an unobtainable goal as the complexity of petroleum is near infinite.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019