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Unconventional resources of oil and gas from a geologic perspective

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Overview

For a clear knowing of the unconventional resources, you first have to know the difference between both the Conventional and the Unconventional Reservoirs.

The conventional reservoirs are those which well can be drilled through, 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 one that cannot be produced at economic flow rates or that does not produce economic volumes of oil and gas without assistance from massive stimulation treatments or special recovery processes and technologies.

Think of the difference between a sponge and a piece of clay, it’s easy to squeeze water out of a saturated sponge, that’s a conventional oil and natural gas reservoir; squeezing water out of saturated clay is harder - that's an unconventional one.

Oil and natural gas are the main sources of energy around the world which are being exhausted decade after decade, that have resulted a decline in these conventional resources.

As technologies tackling the petroleum industry are always expanding and developing, introducing a new technology that tackles unconventional reservoirs became a must to increase the world reserve by producing “unconventional” oil and natural gas resources that were previously impossible to do.

And these unconventional reservoirs contain the future of our hydrocarbon supply which is the unconventionalresources.

Unconventional resources Classification and distribution

The classification of unconventional resources

The unconventional resources are classified into: shale gas, shale oil, tight gas, tight oil, coal seam gas/coalbed methane and hydrates, most of them will be tackled through the article from a geologic perspective.

The distribution of unconventional main resources

 Here we have a table showing the distribution of worldwide unconventional-gas resources (after Rogner 1996 , taken from Kawata and Fujita 2001)

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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 about 400 million years ago from the remains of vegetations that grew at that time, so it’s called a fossil fuel.

Formation of peat

Peat is a soggy, dense material which is formed by accumulation of layers 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, clays and other minerals to accumulate, burying the peat underneath

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 to coal. Scientists claim that, from 3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal.

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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.Contrasting features between CBM and Conventional Gas Reservoirs

1-Gas Composition

Gas 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 desorbed at first.

2-Adsorption

The mechanism by which hydrocarbon gases are stored in the coal reservoir contrasts with the mechanism of the gas storage in conventional reservoirs.

Methane is held to the solid surface of coal by adsorption forces instead of occupying void spaces -as a free gas- between sand grains (only 1-2%).

The adsorption mechanism creates the paradox of high gas storage in a reservoir rock of porosity less than 2.5%.

A clear illustration of the enormous surface area in the micropores of the coal is that 1 lb of coal has a surface area of 55 football fields, or 1 billion sq ft per ton of coal. 

3-Water Production

In the early production life of a well, before methane can be desorbed, the water from natural fractures in the coal must be removed.

The large volumes of water in the first year or two of production, decrease thereafter to relatively small volumes for the remaining life of the well.

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4-Rock Physical Properties

Conventional oil and gas formations are inorganic. Organic formations contain CBM; these formations may contain about 10–30% inorganic ash.

5-Gas Flow

For coals, an additional mechanism of gas diffusion through the micropores of the coal matrix is involved, where the mass transport depends upon a methane concentration gradient across the micropores as a driving force.

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Distribution of coal bed methane

Distribution of coal bed methane is illustrated by the following table

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Tight-Gas Reservoir

Tight-gas reservoir definition

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.

After that a better definition has been revealed which states as follows, 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.”

Based on this definition, there are no “typical” tight gas reservoirs. A tight gas reservoir can be deep or shallow, high-pressure or low-pressure, high-temperature or low-temperature, blanket or lenticular, homogeneous or naturally fractured, and can contain a single layer or multiple layers.

Tight-gas reservoir formation

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.

The important geologic parameters for reservoir conditions

The analysis of a tight gas reservoir should always begin with a thorough understanding of the geologic characteristics of the formation and the interaction between Quartz Cementation and Fracturing in Sandstones.

Interaction between Quartz Cementation and Fracturing in Sandstones

Quartz cementation and fractures are complexly interrelated. Quartz cementation influences fracture systems by affecting the rock mechanical properties at the time of fracture formation, which in turn, influences fracture aperture distributions and clustering.

Additionally, cementation affects flow properties of fracture networks by partially or completely occluding fracture pores, due to extensive cementation by authigenic clays. The matrix permeability of these sandstones is extremely low, on the order of microdarcies.

The important geologic parameters for a trend or basin are the structural and tectonic regime, the regional thermal gradients, and the regional pressure gradients.

On the other hand the important geologic parameters that should be studied for each stratigraphic unit are the depositional system, the genetic facies, the textural maturity, mineralogy, the diagenetic processes, cements, the reservoir dimensions, and the presence of natural fractures.

 

Tight Gas reservoirs distribution 

-Devonian

  • Jean Marie Member and related carbonates (NEBC)

-Mississippian / Pennsylvanian / Permian

  • Mattson Formation (Liard Basin)
  • Stoddart Group (NEBC Foothills and Peace River Plains)

-Triassic

  • Montney ±turbidite play (Peace River Plains)
  • Doig ±shoreface/channel sands ±Groundbirch play (NEBC) Halfway ±NEBC Foothills, Peace River Plains
  • Baldonnel / Pardonet ±(NEBC Foothills) 

-Jurassic

  • 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

  • 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 that has no difference 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.

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Geological characteristics of shale-gas

Shale Different Forms

Shale and mud rich rocks often differs in color and grain size as follows:

Shale that host economic quantities of gas and has common properties.

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.

Vitrinite reflectance (% Ro), where a value above approximately 1.0%–1.1% Ro indicates that the organic matter is sufficiently mature to generate gas.

Pore Space

Effective bulk permeability in shale gas is typically much less than 0.1 (md), although exceptions exist where the rock is naturally fractured (Antrim shale).

Here is a figure illustarating  shale gas compared to other types of gas deposits


 

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 .

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