Environmental Considerations Associated with Hydraulic Fracturing Operations
by James A. Jacobs, Stephen M. Testa
3Geology of Unconventional Resources
3.1 Introduction
Oil and natural gas are hydrocarbons that reside in the pore spaces between grains of rock. As previously discussed, oil and natural gas resources can be divided into two general groups: conventional and unconventional. Conventional oil and gas resources must fulfill certain fundamental geologic requirements. Levorsen (1956) provided one of the earliest definition of what we would term a conventional oil and gas resource as
A porous and permeable body of rock called the reservoir rock, which is overlain by an impervious rock, called the roof rock, contains oil and gas or both, and is deformed or obstructed in such a manner that they are trapped.
Under favorable geologic conditions, these hydrocarbon‐bearing rocks are referred to as reservoirs where hydrocarbons can flow freely from reservoir rocks to oil and gas wells. It is the production from these types of rocks that is traditionally referred to as “conventional” hydrocarbon reserves. Conventional resources include oil and natural gas and its condensates. They are essentially buoyancy‐driven hydrocarbon accumulations, with secondary migration and structural and/or stratigraphic closures.
Conversely, unconventional resources commonly refer to low permeability rocks, which include sandstones, siltstones, shales, and carbonates, and contain continuous gas accumulations. Situated in basin centers and transition zones, oil and gas accumulations in these rocks are controlled ...