Shale as reservoirs?
Posted by D Nathan Meehan June 23, 2010

Shale as reservoirs?
Shales are known to be the principal sources for conventional hydrocarbon plays. They also often function as reservoir seals because of their low permeabilities.  They can also be the reservoir and trap under certain circumstances. Most of the gas created in such reservoirs would be thermogenic in origin although some shales (e.g. the Antrim) have significant quantities of biogenic gas. Gas is stored in shales either as free gas in the pore spaces or adsorbed onto the organic material or surface walls in the shale.

Most coal bed methane (CBM) gas production is dominated by gas that is adsorbed in the micro-porosity of the coal with lesser amounts of free gas present in the natural fractures. Measures of the available gas in a CBM reservoir are generally made with an isotherm as illustrated in the above Figure. In this figure, a Langmuir isotherm is used to describe the gas storage content of the coal. The y-axis is the adsorbed gas content in scf/ton and the x-axis is presssure.
The Langmuir volume of 375 scf/ton in this graph represents the total theoretical volume of gas that can be adsorbed in the coal. The Langmuir pressure is that  (lower) pressure that must be achieved in order for half of the theoretical gas to be recovered. In this example, the initial reservoir pressure is 3210 psi at which the Langmuir isotherm match would have predicted a little over 318 scf/ton of adsorbed gas. In reality, the coal bed methane reservoir only has 281 scf/ton of adsorbed gas and will produce water until the reservoir pressure is reduced to 1700 psi, the critical desorption pressure. The isotherm can then be used to predict the volumes of adsorbed gas that are released until the abandonment pressure is reached.

While this approach is common for screening CBM projects, cores taken from shale gas projects often show relatively small volumes of adsorbed gas with exceptions such as the Antrim shale.

Gas produced from shale reservoirs may either be thermogenic or biogenic or a combination.

Thermogenic gas was formed when organic matter was compressed at  high temperatures (and pressures) for a  long time. Just as in oil formation, thermogenic methane is a cracking process transforming organic particles carried in the clastic materials which became the shales. The nature of the organic material and time and pressure dictate what is formed in thermogenic processes. Thermogenic gas can contain significant quantities of heavier hydrocarbons; however, thermogenic gas may also be nearly pure methane.

Biogenic methane can be formed by microorganisms that chemically break down organic matter. Biogenic methane is generally formed at shallow depths in anoxic environments. While most such methane escapes to the atmosphere, some can be trapped and buried at depth. Modern landfills can also form biogenic methane. Biogenic methane is essentially unrelated to the processes that form oil. Biogenic gas primarily contains methane with very few heavier hydrocarbons. A standard way to determine whether a gas is thermogenic or biogenic is available in gas geochemistry. Thermogenic gas has less 13C compared to the predominant 12C than do biogenic gases.

Next time: A quick look at shale geochemistry

5 responses | Add Yours


Rodolfo Galecio says:

This is really an illustrative article, using a Langmuir isothermal curve we could address the forecasting issue in a CBM reservoir at early stages of development, I would like to know what is the uncertainty related to this method? can we apply this curve in gas shale formations without any restriction?

D Nathan Meehan says:

This technique is well established with CBM reservoirs but cannot be applied independently of many other factors there. It should not be used “without restriction” in shale gas reservoirs! It is in fact quite difficult to quantify recoveries, drainage areas and the degree to which adsorbed gas plays a role in shale gas reservoirs.

I plan on covering shale gas reserve estimation methodologies in more detail.

Yousaf says:

It is very informative article. I would like to have “Shale Gas Reserve Estimation methodologies” from you asap.
Suppose if we don’t have such experiments performed in past on target shale gas reservoir, how we can or calculate Gas Constant then? Can you update? Thanks

D Nathan Meehan says:

We are building such a methodology. Most operators have relied on performance data (decline curve analysis of some type) and analogies. A few use reservoir simulation models with a large number of inherent difficulties. I will definitely be posting more articles in this area once I take a pause from the economics section.

Baker Hughes Blogs Administrator says:

In talking with a geochemist, we found that this is not a strong concern for North America shales, as almost all of the significant basins contain only thermogenic gas. The exceptions are the Antrim and the New Albany shales, which are actually combination themogenic and biogenic (with the biogenic gas being secondary gas generation). These two basins are also unique in other aspects. They contain and produce significant amounts of water (unlike other shales in US and Canada) and must be “dewatered” like CBM prior giving up their primarily “sorbed” gas. A number or reservoir engineers believe that all of the early gas produced from the North American basins is “free gas”, and we have yet to really see any of the sorbed gas actually being produced. Of course, the debate still continues.
A further bit of research yielded that Exoma Energy Limited had conducted an in-house study on the different types of shale gas. Quoting from a May 10, 2010, on line article:
“Exoma’s in-house study highlights the differences between the three major types of shale gas: ‘conventional’ thermogenic gas such as the Barnett Shale and the two types of biogenic gas which differ depending on whether they are the product of primary or secondary gas generation by anaerobic bacteria. Biogenic gas was previously not seriously considered by explorationists in Australia but reported comprises over 25% of all the known natural gas reserves in the world, not including the deep-sea hydrates.
The Po Valley Basin in northern Italy has over 28 TCF of proven biogenic methane reserves that were generated from Pliocene sediments.
A summary of this present study is included on the Company’s website, and indicates that the gas is likely to be of mostly primary biogenic origin, with the potential for secondary biogenic gas being developed down dip… It is also likely that a large portion of the shales could still be within the biogenic shale gas window.” (Note: did not find study on Exoma’s site today)
Author: Robert “Bobby” Kennedy E-mail:

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