Here, we discuss the use of Raman spectroscopy to characterize oil shale, particularly black shale, which consists of kerogen
(fossilized organic matter) trapped in sedimentary rock consisting of clays, calcites, silicates, and titanium dioxides among
other inorganic minerals. We show that Raman spectroscopy is useful for characterizing the varieties of kerogen structure
(chemical bonding and solid state), differentiating shales with different relative amounts of kerogen to inorganic mineral,
and differentiating polymorphs of naturally occurring inorganic oxides.
Although there has been a lot of attention given recently to oil shale, its discovery occurred many decades ago. In researching
this topic, I found a 1923 report in Chemical & Engineering News about the work of Ralph H. McKee of Columbia University, who advanced what were at that time "new arguments for the shale
oil recovery process" at a meeting of the American Chemical Society and the Providence Engineering Society (1). Quite remarkably,
the title of McKee's presentation was "Future Sources of Gasoline." He reported on his petrochemical studies of heating oil
shale to produce kerosene, gasoline, and various lubricating oils. More than 30 years later, one can read that "Northwestern
Colorado's oil shale deposits probably contain the greatest concentration of undeveloped energy to be found any place in the
world. So says Union Oil of California's president A.C. Rubel," from a 1957 issue of Chemical & Engineering News (2). The potential of and interest in oil shale as a source for fuel and other petrochemical products continues to this day
and attracts many to the Oil Shale Symposium held annually at the Colorado School of Mines in Golden, Colorado. In fact, I
attended the 32nd Annual Oil Shale Symposium in October 2012 and found it to be a very well organized, informative, and beneficial
conference with an opportunity to participate in a two-day field trip to the Green River Formation in northwestern Colorado
and southern Utah following the conference. I encourage readers who are interested in this subject and attending the 2013
Oil Shale Symposium to consult the Colorado School of Mines web site.
Let's begin with a clarification of the nomenclature used in this field. The term oil shale refers to the sedimentary rock from which oil is recovered through a high-temperature chemical (industrial retort) process.
The oil shale is porous rock in which organic matter called kerogen is trapped. As Jehlicka and colleagues stated, "Kerogen
is defined as dispersed organic matter from sedimentary and metasedimentary rocks or as the insoluble part of the organic
matter of rocks. The insolubility means here the insolubility in water, organic solvents, hydroxides and mineral acids (strong
oxygen containing acids excluded)" (3). I very much like the analogy that Professor Jeremy Boak of the Colorado School of
Mines uses when he compares the words oil shale to wine grapes. Of course, there is no wine in the grapes, but wine can be
generated from the grapes through chemical processes and fermentation, hence the distinction between oil shale and oil from
shale. Understanding and optimizing the chemical processes for producing fuel oil and other petrochemical products from sedimentary
shale permeated with kerogen (oil shale) remains a fertile area of chemical research.
Figure 1: Raman spectrum of black shale. The Raman bands at 1354 and 1603 cm-1are attributable to kerogen.