Conventional vs. Unconventional Reservoirs
The definition of conventional and unconventional oil and gas resources is closely associated with the ease, cost, and techniques used to develop a reservoir. Conventional oil and gas resources are generally viewed as being easy to develop due to the permeability of the reservoir rocks, having defined hydrocarbon pools, and using vertical wells. As technology and drilling techniques have progressed, previously uneconomical resources are now becoming the focus of new oil and gas exploration and development. These unconventional reservoirs usually have lower permeability and porosity, and can be the source rock for the hydrocarbons they contain. The oil and gas in unconventional reservoirs are usually distributed throughout the pore spaces of the reservoir rock, making it more difficult to extract the hydrocarbons. Techniques such as horizontal drilling and hydraulic fracturing are necessary to release the hydrocarbons from unconventional reservoirs. For more information on hydraulic fracturing, please see the Association of American State Geologists, the U.S. EPA, the Geological Society of America, or the Watershed Council.
Unconventional resources that have contributed to Wyoming’s oil and gas industry include shale gas, tight gas and oil sands, shale oil, and coalbed natural gas. The recent increase in Wyoming’s oil production can largely be attributed to the exploration and development of unconventional reservoirs in the Powder River Basin (Sussex, Shannon, Turner, Parkman, Frontier, etc.).
Conventional wells are drilled vertically from the surface straight down
to the pay zone. Horizontal drilling begins in a vertical position then turns
horizontal to access a larger pay zone. Directional drilling often involves
multiple wells from a single well pad. Artwork by James Rodgers.
Shale gas is natural gas locked in shale formations. In these reservoirs, the shale is both the source and the reservoir rock. An example of shale gas in Wyoming is the Hillard-Baxter play in the Greater Green River Basin.
Tight gas and oil reservoirs contain natural gas and oil trapped in the pores of siltstones and sandstones with very low permeability (<0.1 millidarcy) and very low porosity (<10%). The prolific Jonah, Pinedale, and Wamsutter fields are Wyoming’s largest tight gas reservoirs.
Shale oil is oil locked in shales and associated tight siltstones or carbonates – all of which have low permeability and porosity. Examples from Wyoming include the Niobrara Shale and Green River Formation.
*Please note that shale oil should not be confused with oil shale. Oil shales are shales that contain kerogen. Generally, hydrocarbons cannot be produced from oil shale using wells. Mining or in-situ heating processes are necessary to extract and convert the kerogen.
Coalbed natural gas (CBNG), commonly called coalbed methane, is natural gas stored in coal beds. Wyoming’s Powder River Basin has produced large volumes of CBNG over the past 15 years, although production is currently declining. More information on CBNG in Wyoming can be found in the Coalbed Natural Gas section.
In general, exploration geologists and engineers need to locate hydrocarbon reservoirs that will be productive and profitable enough to outweigh the high costs of drilling a well. A single horizontal well can cost from $9 to $25 million.