Most analysts and journalists still draw a sharp distinction between the production of oil and gas from conventional fields and from unconventional resources such as shale. The reality is more complex.
In practice it is not easy to tell where conventional petroleum production ends and unconventional production begins. In assessing how quickly shale and other unconventional petroleum resources will be developed, location matters.
Large shale resources in countries which are already major conventional producers and exporters may not have much impact on oil and gas availability: the shale accumulations may end up "queued" behind a long line of conventional projects as the host nation restricts resource development to support prices.
By contrast, shale accumulations in countries which are net importers and/or have few conventional supplies of their own look set to be developed much more quickly.
Unconventional oil and gas resources are often located in the same sedimentary basins as conventional oil and gas fields. In many cases, the shale or tight rocks which are targeted by horizontal drilling and hydraulic fracturing were the original source for the oil and gas found in more conventional reservoirs.
Petroleum geologists talk about a "total petroleum system" (TPS) that includes all the essential elements (source, reservoir, seal and overburden rocks) and processes (generation, migration and accumulation) linked to a particular source of oil and gas.
A total petroleum system will normally include both discrete accumulations of oil and gas in conventional pools, reservoirs and fields, as well as more extensive "continuous-type accumulations" in shales and tight rocks ("The total petroleum system: the natural fluid network that constrains the assessment unit" U.S. Geological Survey, 2000).
For decades, North Dakota produced millions of barrels from wells drilled into conventional oil accumulations found in the Madison and Lodgepole formations. But the source of that oil was probably the Bakken and Three Forks formations, which are continuous-type accumulations, and are now being targeted directly by fracking firms.
The result is that much of the prospecting for shale oil and gas is occurring in areas that have already produced conventional oil and gas. Frackers have turned their attention to the Anadarko Basin beneath Oklahoma and Kansas, areas that have been producing substantial quantities of oil and gas for a century.
Argentina's enormous Vaca Muerta shale formation is thought to be the source for several conventional oil fields in the region. China's Sichuan basin, which is highly prospective for shale gas, has been the home of the