Many of present-day producing naturally fractured reservoirs have been accidentally discovered when looking for some other type of reservoir. Some years ago McNaughton and Garb (1975) estimated that ultimate recovery from producing fractured reservoirs would surpass 40 billion stock tank barrels of oil (STBO). Today I firmly believe that this figure was very conservative. I am convinced that there are significant volumes of hydrocarbons that have been left behind pipe as undiscovered, or behind plugged and abandoned wells or because of vertical wells that have not intercepted vertical fractures.

Figure 1 1 shows the location of some important naturally fractured reservoirs. They are found all over the world, in all types of lithologies and throughout the geologic stratigraphic column. This is demonstrated in Chapter 5 dealing with Case Histories.

Figure 1 1: Location of some oil and gas fields producing from naturally fractured reservoirs. Some of the reservoirs are very prolific, some produce at more modest rates but still are economic, some are marginal, and some have tested at very low rates. Naturally fractured reservoirs are found all over the world in all types of lithologies and throughout the stratigraphic column all the way from the pre-Cambrian to the Miocene.