For more than 100 years, shale oil has referred to the product of pyrolysis of oil shale, whereas oil shale refers to organic rich (kerogen) rock that has never reached the oil window.
The oil shale industry, operating since the 1850s producing shale oil and electric power, never achieved the size of the petroleum industry – but this does not give the larger industry the right to expropriate the technical term.
Is oil shale a misnomer because the rock does not contain oil and is not shale? Wine grapes contain no wine, but are still wine grapes.
The connotative flexibility of English is why we have so much great poetry and so many lawyers. The language does not care whether oil shale describes a rock that yields liquid hydrocarbons upon being heated in a technologically complex process, or one that yields liquid hydrocarbons upon being fractured in a technologically complex process.
Is the rock properly termed a marl or marlstone – an assertion rarely accompanied by data? In industry we too easily refer to lithified sandstone bodies as sands, and marlstones as marls. Marl is a term for sediment, not sedimentary rock, and oil shale is lithified.
The Mahogany zone of the Green River Formation (GRF), one target in the richest, largest, most famous oil shale deposit, has low clay content. However, the Garden Gulch Member, being tested by American Shale Oil (AMSO) in Colorado, is clay-rich, laminated, and fine-grained – perfectly good shale. Other oil shale deposits contain substantial clay. The clay-poor oil shale of the GRF is rich in diagenetic feldspar and dawsonite (NaAl(OH)2CO3), resulting from clay mineral breakdown reactions in the saline lacustrine environment where it formed. The feldspathic mudstone and marlstone are unusual sedimentary rocks (see figure 43).
The more we look at shale reservoirs, the more complicated it gets (just like sandstone and carbonate reservoirs).
Referring to plays like the Bakken and Eagle Ford as shale oil plays, then correcting that to recognize the importance of dolomitic and silty rocks of the Middle Member of the Bakken, leaves us with widespread confusion.
I suggest the terms “oil-bearing shale” and “shale-hosted oil” for the rocks on one hand, and products or plays on the other (Allix et al., 2010). These terms clarify that the rock contains oil in accepted form for geologic terminology.
Each formation plotted in figure 1 has been called shale, although the data show many don’t fit classical concepts of shale (like Pettijohn’s 1975 average shale, highlighted in red) any better than the GRF.
Indeed, in “shale” plays the brittleness needed for hydraulic fracturing comes from low clay mineral content and high framework silicate (Barnett, Monterey) or carbonate (Eagle Ford, Duvernay) mineral content.
Potter, Maynard and DePetris (2005) suggested the term “shale” should be restricted to its original sense of laminated and fissile fine-grained sedimentary rocks (Hoosen, 1747), in line with the word origin from old German and old Norse words for “scale.” Shale, as well as traditional mudstone, marlstone and even carbonate mudstone all came from mud, so classing them under the generic name mudstone could be justified.
As for what we can do to define what we mean by shale, I believe a more consistent definition would require a thoughtful effort at consensus within the sedimentary and petroleum geology community. I present a few ideas here, but recognize the significant alteration such a process produces.
The public, press, financial and industry communities will continue to act like field geologists, and use informal terms. Shale has been an English word for longer than it has been a geologic term. Geologists have to live with that, because we borrow common terms from local languages, rather than creating Latin or Greek terms.
Thinking of the public arena as fieldwork allows us some informality in language.
It is increasingly important for us to engage the public, and in so doing, to consider words and terminology – and their definitions and interpretations – carefully. (References available at online Explorer).
Jeremy Boak, Ph.D., P.G., serves as the Chair, EMD’s Oil Shale Committee. He is the Director of the Center for Oil Shale Technology and Research (COSTAR) at the Colorado School of Mines, and served as the Chair of the 26th, 27th, 28th, and 29th Oil Shale Symposia, held at Mines in 2006-2009, and Co-Chair of the 30th Oil Shale Symposium (October 18-20, 2010).
The Energy Minerals Division (EMD), a division of AAPG, is dedicated to addressing the special concerns of energy resource geologists working with energy resources other than conventional oil and gas, providing a vehicle to keep abreast of the latest developments in the geosciences and associated technology. EMD works in concert with the DEG to serve energy resource and environmental geologists.