Unconventional resources of oil and gas from a geologic perspective

For a clear knowledge of unconventional resources, the one firstly have to know the difference between both the conventional and the unconventional reservoirs. The conventional reservoirs are those in which wells can be drilled so that oil and natural gas can be produced at economic flow rates without large stimulation treatments or any special recovery process. On the other hand, the unconventional reservoir is the one that cannot be produced at economic flow rates or that does not produce economic volumes of oil and gas without the assistance of massive stimulation treatments or special recovery processes and technologies. With that rapid continuous decline in conventional oil and natural gas resources around the world that we witness, we became in a great need to exploit the unconventional resources to cover our shortages in energy.As the technology used in the petroleum industry is expanding and developing, introducing a new technology that exploits unconventional reservoirs has become a must to increase the world reserve by producing “unconventional” oil and natural gas resources that were previously out of the productivity scope. These unconventional reservoirs contain the future of our hydrocarbon supply.

Unconventional Resources Classification and Distribution

Classification of unconventional resources

The unconventional resources are classified into: shale gas, shale oil, tight gas, tight oil, coal seam gas/coal-bed methane and hydrates. Most of them will be covered in this article from a geologic perspective.

Distribution of the main unconventional resources

The distribution of the worldwide unconventional gas resources (after Rogner 1996 , taken from Kawata and Fujita 2001)
Region	Coalbed Methane	Shale Gas	Tight-Sand Gas	Total
	(Tcf)	(Tcf)	(Tcf)	(Tcf)
North America	3,017	3,840	1,371	8,228
Latin America	39	2,116	1,293	3,448
Western Europe	157	509	353	1,019
Central and Eastern Europe	118	39	78	235
Former Soviet Union	3,957	627	901	5,485
Middle East and North Africa	0	2,547	823	3,370
Sub-Saharan Africa	39	274	784	1,097
Centrally planned Asia and China	1,215	3,526	353	5,094
Pacific (Organization for Economic Cooperation and Development)	470	2,312	705	3,487
Other Asia Pacific	0	313	549	862
South Asia	39	0	196	235
World	9,051	16,103	7,406	32,560

Coal Bed Methane (CBM)

Formation of coal bed methane

In order to understand the formation of coal bed methane you have firstly to understand the formation of both coal and peat.

Formation of coal process Coal was formed from the remains of vegetation that grew about 400 million years ago. Therefore, it’s called fossil fuel. Formation of peat Peat is a soggy, dense material which is formed by accumulation of layers of sediments over the remains of dead plants and trees that sank to the bottom of the swampy areas, over long periods of time. The changes in the earth's surface caused deposits of sands, clay and other minerals to accumulate and bury the peat underneath. Then, sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water out from the peat. This depth associated with heat, gradually changed the material into coal. Scientists claim that 3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal. Formation of coal bed methane Biogenic methane is produced by anaerobic bacteria in the early stages of coalification.Thermogenic methane is mainly produced during coalification at temperatures ranging from 120 – 150 °C. However, some contrasting features exist between CBM reservoirs and conventional gas reservoirs. These features include: Gas CompositionGas produced from coal beds may be initially higher in methane content than the gas produced from conventional reservoirs. Methane is less adsorbed than ethane and other heavier saturated hydrocarbons. Consequently, they may not be as readily adsorbed at the onset of production.

The mechanism by which hydrocarbon gases are stored in the coal reservoir contrasts the mechanism of gas storage in conventional reservoirs. However, methane is held to the solid surface of coal by adsorption forces instead of occupying void spaces -as in the case of free gas- between sand grains (only 1-2%).

If we study the coal microspores we will see a clear illustration of an enormous surface area that the coal of one lb of has a surface area of 55 football fields, or 1 billion sq ft per ton of coal water production The water from coal's natural fractures must be removed first before methane can be desorbed. These 2 figures indicate that the large volume of water in the first 2 years of production decreases relatively rapid to small volumes for the remaining life of the well.

Gas Flow

Here we have an additional feature for coal; which is the mechanism of gas diffusion through the microspores of the coal matrix, we find here that the mass transportation depends on the methane concentration gradient across coal's microspores as a driving mechanism.

Distribution of coal bed methane

Distribution of coal bed methane is illustrated by the following table

Tight-Gas Reservoir

Definition of tight-gas reservoir

In the 1970s, the U.S. government decided that the definition of a tight gas reservoir is “the one in which the expected value of permeability to gas flow would be less than 0.1 md.” However, that definition had some political aspects which were related to the recovery produced form tight reservoirs. But if you use our main aspect which is the scientific one, a better definition could be revealed which states that a tight gas reservoir is “A reservoir that cannot be produced at economic flow rates nor recoverable economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores.”If we study consider that definition a rule, based on it we could say that there are no “typical” tight gas reservoirs, a tight gas reservoir could have various characteristics through depth, pressure, temperature, number of layers or even homogeneity.

Formation of tight-gas reservoir

What makes a tight reservoir?

There are a number of reasons that can make a reservoir tight.

But we can say that the effective permeability of a reservoir is the main reason for making a tight reservoir, after that being stated, we can then include some of the important parameters controlling the effective permeability, which are effective porosity, viscosity, fluid saturation and capillary pressure.

In addition to the factors related to the fluid nature, the rock parameters are equally important; yet those are controlled by depositional and post depositional environments the reservoir is subjected to.

Distribution of tight-gas reservoirs

Devonian

Jean Marie Member and related carbonates (NEBC)

Mississippian / Pennsylvanian / Permian

Mattson Formation (Liard Basin)

Stoddart Group (NEBC Foothills and Peace River Plains)

Triassic</span>

Montney ±turbidite play (Peace River Plains)

Doig ±shoreface/channel sands ±Groundbirch play (NEBC) Halfway ±NEBC Foothills, Peace River Plains</span>

Baldonnel / Pardonet ±(NEBC Foothills)>

Jurassic</span> Rock Creek (west-central Alberta) Nikanassin ±Buick Creek (NEBC, West-central Alberta) Kootenay (southwestern Alberta) Lower Cretaceous Cadomin / Basal Quartz (Alberta / B.C. western Plains and Foothills)

Bluesky / Gething (Peace River Plains, west-central Alberta) Falher / Notikewin (NEBC and adjacent Alberta)

Notikewin / Upper Mannville channels (west-central Alberta) Cadotte (west-central Alberta and adjacent B.C.) Viking ±(west-central Alberta)

Upper Cretaceous</span>

Dunvegan (west-central Alberta and adjacent B.C.)

Cardium ±Kakwa shoreface (west-central Alberta and adjacent B.C.) Belly River (west-central Alberta)

Shale Gas

Formation of shale-gas

Natural gas is not different from what you currently use to heat your home, cook with, or use to generate electricity, which is naturally trapped in its original source rock; the organic-rich shale that formed from the sedimentary deposition of mud, silt, clay, and organic matter on the floors of shallow seas..

Geological characteristics of shale-gas

Organic Material

They are rich in organic material (0.5% to 25%).

Thermal Maturity

Thermal Maturity is an indicator of how much pressure and temperature the rock has been subjected to.

The shale is usually more mature, has higher gas ratio and matured in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas.

Pore Space

The pore spaces here not the main core characteristics but the effective permeability in shale gas which ismuch less than 0.1 (md).

Distribution of shale-gas

Here is a map of major shale gas basis all over the world from the EIA report World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States.

Why is it hard to be studied?

Subsurface deposits have pressure and temperature specific conditions which is suitable for "methane hydrate".

So removing it from these conditions makes it unstable; as they are brought to the surface, the pressure is reduced and the temperature rises. This causes the ice to melt and the methane to escape so they can’t be drilled or cored for any studying matter

The most abundant unconventional natural gas source: Methane hydrates are considered to be the most abundant unconventional natural gas source, yet they are the most difficult to extract.

It is conservatively estimated to be 4,000 times the amount of natural gas consumed in the United States in 2010.

The following figure shows worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996. [[File:8.jpg|8.jpg]

Conclusion

This article illustrates the concept of unconventional resources and the different between them and the conventional ones, it briefly summarizes definitions, characteristics and distribution of main unconventional types, it also answers ;some interesting questions relating to some of that types.