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Types of Oil and Gas Accumulations – Conventional and Unconventional.

October 23, 2017

Conventional Oil and Gas –

Conventional oil and gas is crude oil or natural gas or sometimes both that are produced by a well drilled into a geologic formation in which the rock and fluid properties allow the oil and natural gas to flow or be pumped to the wellbore.  The principal rock properties in conventional oil and gas reservoirs are high porosity and permeability – the ability of the rock to accumulate oil and gas in pore space and the ability of the oil and gas to permeate the rock and flow to the well bore.  Many of the world’s conventional oil and gas reserves have been accessed already, leading producers to explore for and produce unconventional oil and gas resources.


Associated gas is produced in conjunction with crude oil production.  Associated gas is dissolved in the oil and produced as it is liberated during reservoir pressure decline, or is present as a gas cap above the oil.


Non-associated gas is gas in a reservoir that has only gas or possibly some natural gas liquids, but there is no oil.


Unconventional Oil and Gas – The following are the most common types of unconventional oil and gas accumulations.


Shale Oil and Gas – Shale is a sedimentary rock that has low permeability and is generally unable to be produced economically using traditional methods.  These reserves generally require the use of horizontal drilling techniques as well as hydraulic fracturing to be produced economically. 


Tight Gas – Tight gas is the term used to refer to low permeability reservoirs that produce mainly dry natural gas.  Many of the low permeability reservoirs that have been developed are sandstone, but gas is also produced from low permeability carbonates, shales, and coal seams.   Tight gas can generally not be produced economically unless the operator uses a horizontal wellbore, multilateral wellbores, or uses large hydraulic fracture treatments to stimulate production.


Coalbed Methane – Coalbed methane is produced from coal seams.  Coalbed methane is different from conventional gas reservoirs in that the methane is stored within the coal by a process called adsorption.  The methane lines the inside of pores within the coal, or may also be present within fractures in the coal.  Coalbed methane production is usually accompanied by the production of water in the coal which allows the methane to be produced.  Unlike conventional gas reserves, coalbed methane is almost entirely comprised of methane, with very little heavier hydrocarbons such as propane or butane.

These materials have been prepared solely for educational purposes to contribute to the understanding of oil and gas appraisal. These materials reflect only general concepts in the industry based on Colorado and may not apply to all circumstances.  It is understood that each case is fact ‐ specific, and that the appropriate solution in any case will vary.   These materials may not be relevant or apply to any particular situation.  While every attempt was made to ensure that these materials are accurate, errors or omissions may be contained therein, for which any liability is disclaimed.

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