In conventional oil and gas accumulations, shales comprise the source rock from which hydrocarbons are generated following burial. Through geological time, these hydrocarbons migrate from the source rock, through carrier beds and ultimately accumulate in porous reservoirs (typically sandstone or carbonate) in discrete traps. These traps are typically located in structural highs on the margins of the basin centres. In the case of unconventional hydrocarbon accumulations (such as shale gas), this perceived wisdom is turned on its head – with shales acting as both source and reservoir rock, and the extensive basin centres becoming the exploration targets. Shale gas formations are “unconventional” reservoirs – i.e., reservoirs of low “permeability.
Figure 1. Schematic Geology of Natural Gas Resources (source: U.S. Energy Information Administration)
In shale gas plays, biogenic or thermogenic gas is present as two components: either adsorbed onto kerogen or clay particles, or present as free gas in pore spaces and natural fractures. Shale is predominantly comprised of very fine-grained clay particles deposited in a thinly laminated texture, but shale gas production may also come from layers of re-deposited limestone or thin clastic beds within the gross shale sequence. The clay particles fall out of suspension and become interspersed with organic matter, which is measured as the rock’s total organic carbon content (TOC).
Table 1. Type of Shale Gas
|Organic Rich Black Shale||Silt-Laminated Shale or Hybrid||Highly Fractured Shale|
|· High TOC & high absorb gas
· Low Sw
· High Sg
· Gas storaged as free and absorbed
· Mature source rock
|· Gas storaged in silt or shale
· Low to moderate TOC
|· Low TOC & low absorbed gas
· High Sw
· Low Sg
· Gas storaged in fractures
· Fractures reservoir in shale is the host rock
Through deep burial these muddy strata are compacted, and the pore water is expelled, resulting in a low-permeability layered rock called ‘shale’, which describes the very fine-grained and laminar nature of the sediment, not the rock composition, which is layered. Each of these layers creates a barrier to fluid migration, and this stacked system, called ‘composite layering’ is an effective vertical seal.
Matrix permeabilities (the ability of fluids to pass through them) of typical shale are very low compared to conventional oil and gas reservoirs (1 mD in conventional reservoir sandstones) which means that, in shale, hydrocarbons are effectively trapped and unable to flow or be extracted under normal circumstances, and they are usually only able to migrate to conventional traps over geological time.
Figure 2. Permability Properties Comparison Between Unconventional vs Conventional Reservoir (source: Polish Geological Institute – National Research Institute)
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British Geological Survey for Department of Energy and Climate Change, London, UK.
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Petrophysical Properties Of Shale Rocks. Polish Geological Institute – National Research Institute http://infolupki.pgi.gov.pl/en/gas/petrophysical-properties-shale-rocks, accessed on 15th May, 2016.
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