Shale gas prospectivity is controlled by the amount and type of organic matter held in the shale, thermal maturity, burial history, micro-porosity and fracture spacing and orientation.
Factors influencing these properties relate to the depositional and post-depositional history of shale deposits. A wide range of skills and expertise are needed to address these topics:
The depositional environment of shales — marine versus non-marine, shallow marine versus deep marine, oxic versus anoxic — has a direct and primary influence on the type and amount of organic matter that they contain.
What 'makes' the organic matter in a shale and how much of it is present are related to the biological origins of the organic matter, and that varies with the original depositional setting.
Early Silurian shales in Poland, for example, which are prospective for shale gas, were deposited in a range of environments from shallow to deep marine. The types and proportions of organic matter present vary along this gradient, and so too does the prospectivity for shale gas. Determining the environment of deposition is therefore a first step towards mapping the spatial and temporal distribution of shale gas plays.
The BGS is able to offer expertise and advice in the following areas, all of which contribute to interpreting the environment of deposition of prospective gas shales.
Basin analysis, seismic processing and interpretation and sequence stratigraphy provide regional-scale overviews of depositional settings of prospective gas shales.
Sequence stratigraphic models for unconventional gas shales, aimed at investigating linkages between source rock quality and properties and sequence tracts or positions within parasequences, are being developed (e.g. Slatt and Rodriguez 2010, Hart 2011, Slatt and Abousleiman 2011, Lush 2011).
These are research areas in which the BGS has capability and to which it can contribute.
The BGS capability in biostratigraphy, the use of fossils to date and correlate rocks, helps to define the temporal and spatial extent of shale gas plays.
Shales from the Neoproterozoic onwards may contain organic-walled microfossils (or palynomorphs, which can include, depending on age, algae, acritarchs, chitinozoans, dinoflagellate cysts, spores, pollen) that are useful in biostratigraphy. These can be extracted from samples, along with other acid-insoluble particulate organic matter, in the BGS Palynology Laboratory.
Palynofacies analysis uses the abundance, composition and diversity of palynological assemblages to determine EoD.
Recent stable isotopes work, including work by the BGS (Stephenson et al., 2008), has demonstrated that δ13C (bulk organic matter) delineates marine and non-marine conditions because marine sedimentary organic matter (usually of algal origin) has a different δ13C value from that of terrestrial organic matter (mainly wood fragments and terrestrial palynomorphs).
Contact Ed Hough for further information or enquiries about BGS shale gas consultancy services