This article was contributed by Staffan Van Dyke
The objective of this article is to evaluate tight gas sandstones in relation to conventional reservoirs (sandstones/carbonates) as well as unconventional reservoirs (coalbed methane/shale gas), with reference to its constituent petroleum system parameters: source, trap, seal, reservoir properties (porosity and permeability), and time factors (timing of charge and migration). The article indicates significant differences between tight gas sandstones as compared to coalbed methane and/or gas shales. A thorough evaluation of the geological evidence studied for this article indicates that tight gas sandstones, as a reservoir, are closer to conventional type reservoirs than they are to unconventional type reservoirs, such as coalbed methane and/or gas shales.
Utilizing the framework described in this paper, tight gas sandstone reservoirs should then be considered as a sub-type category within the overall conventional reservoir definition, as the majority of its geological properties fall within this definition, and not that of an unconventional reservoir – note: the suggestions laid out in this article STRICTLY refer to the geological parameters of these reservoirs and NOT their engineering parameters (which are still very clearly considered “unconventional”).
Under this definition just laid out, the characterization of tight gas sandstones as an unconventional reservoir is simply inappropriate, as the geological setting / petroleum system / etc., as compared to coalbed methane and shale gas, are very different in their most basic geological constituents, making the comparison (and, hence, the argument that they are indeed very different from one another when viewed in a geological sense). Tight gas sandstones are simply reservoir rocks, whereas coal and shale are considered to be both the source and the reservoir rock.
Unconventional reservoirs are ones that cannot be produced at economic flow rates or they do not produce at economic volumes without the assistance from massive stimulation treatments, such as hydraulic fracturing (fracking) or other special recovery processes and technologies, such as steam injection (these are known as “secondary” and/or “tertiary” recovery techniques). Typically unconventional reservoirs have been described as: tight gas sands, coalbed methane, and gas shales (Holditch, 2003 and 2006). However it is an economic and reservoir engineering definition and does not take into account the geological processes behind the deposition of said deposits.
It is also important to understand that a conventional (sandstone/carbonate) reservoir with low natural pressure that depletes very quickly (in the order of weeks to months) that requires artificial hydrocarbon recovery techniques to maintain or increase its economic viability, is very nearly the exact definition of an unconventional reservoir, as the one given above. However, such reservoirs are still categorized as conventional in the geological sense.
On the other hand, since tight gas sandstones must be artificially stimulated (fracked) in order to produce its gas, it would only seem natural to place this reservoir criterion in the “unconventional reservoir” category.
Comparison of conventional and unconventional reservoirs

In the United States, the tight gas sandstone definition is applied to reservoirs with less than 0.1 mD of permeability (Meckel and Thomasson, 2008). Our investigation indicates that tight gas sandstones have significantly different characteristics in comparison to coal bed methane and shale gas. They are:
1. Tight gas sandstones act purely as a reservoir, whereas coalbeds and shales act not only as their own source rocks, but as well as their own reservoirs;
2. Shanley et al (2004) found that the low permeability reservoirs in the Greater Green River Basin of southwest Wyoming were not part of a continuous type gas accumulation but were low permeability rocks in conventional structural, stratigraphic, and/or combination traps. Earlier, Berry (1959) and Hill et al (1961), proposed that in the San Juan Basin, the gas within the sandstone reservoir was localized in a potentiometric sink associated with down-dip flow of water. In other words, it is a hydrodynamic type trap, thus, much more like the conventional trap settings found in conventional reservoirs;
3. Gas migrates into tight sandstones from the nearby source rock and the charged gas may be housed within the reservoir due to high capillary pressure conditions by virtue of low porosity and permeability, and up-dip presence of water due to regional or local hydrodynamic conditions, whereas in coal and shale gas, it is adsorbed into the matrix of organic matter (Bustin and others, 2009);
4. Many conventional reservoirs are porous and permeable but do not have enough primary energy to support hydrocarbon production unaided at an economic level, but are still categorized as conventional reservoirs. According to the unconventional reservoir definition given above, this quality should then define these reservoirs as unconventional, primarily because enhanced recovery techniques are required for them to be economically producible. Similarly, tight gas sandstone reservoirs need enhanced recovery techniques like fracturing, flooding, and
acidization to make them economically viable. However, instead of categorizing these low primary-energy conventional reservoirs as unconventional, it is the authors’ opinion that they should remain classified as conventional reservoirs, and that tight gas sandstones should be classified as a sub-type within the overall conventional reservoir petroleum system;
5. The only correlatable property of tight sandstones to coal and shale is their low porosity and permeability similarity, unlike the higher porosities and permeabilities typically seen in conventional sandstone / carbonate reservoirs. The geological aspects discussed above suggest that tight gas sandstone as a reservoir is closer to conventional reservoirs (sandstone / carbonates) than to coalbed methane and shale gas reservoirs. Table 1 summarizes the petroleum system and other parameters with respect to tight gas sandstones, coalbed methane, shale gas, and conventional reservoirs to elucidate the similarities between these reservoir types.
Conclusion
Evaluation of the above geological aspects suggests that tight gas sandstones, as a reservoir, are closer to conventional type reservoirs than to unconventional type reservoirs, like coalbed methane and shale gas. It is clear that tight gas sandstones act simply as a reservoir, whereas coal and shale act as a source rock as well as a reservoir for the gas. Tight sandstones may become a hydrocarbon reservoir only when a potential source rock is available within the basin, or a nearby region, capable of charging the reservoir. Utilizing the framework described in this paper, tight gas sandstone reservoirs should be considered as a sub-type conventional reservoir, as the majority of its geological and petroleum system parameters fall within this definition, and not that of an unconventional reservoir.