Petroleum and natural gas are formed by the breakdown of kerogen, which in turn is produced by the decomposition of organic matter. Although this process can give rise to conventional deposits, it can also take place within highly heterogeneous units such as shales. In this case, the kerogen is compressed within rocks a million times less permeable than in conventional hydrocarbon reservoirs. Temperature, pressure and maturation conditions are still largely unknown, which for now restricts the recovery of the gas they contain to the extraction technique known as hydraulic fracturing, or fracking.
In order to develop new, efficient yet environmentally friendly methods, scientists need new tools. Such tools should enable them to assess the amount and accessibility of resources and improve their understanding of adsorption1
and transport properties within gas shales. In fact, the researchers applied a hybrid method, combining experiments and simulations, in order to obtain molecular models of kerogens.
To collect data about the chemical composition, texture and density of kerogen, four samples were carefully selected. They differed in their origins and degree of maturation. The researchers studied their characteristics using a wide range of methods, including major facilities such as a synchrotron, in this case the European Synchrotron Radiation Facility in Grenoble, France, and a neutron source (at the Oak Ridge National Laboratory in the US).
The models obtained were then validated by comparing them with experimentally accessible kerogen properties. They provide a molecular view in a field where samples are generally macroscopic. This work should now help to unravel the microscopic structure of the disordered and heterogeneous materials that make up kerogens, adding one more piece to the arsenal of knowledge that may eventually make it possible to develop shale gas differently.
© M. Hubler (MIT) and J. Gelb (Carl Zeiss X-ray Microscopy)
X-ray microscopy (XRM) image of an untreated sample of gas shale, showing inclusions of pyrite, clay, organic matter and other minerals.
© Colin Bousige
Molecular model of a sample of the Marcellus kerogen studied (the organic phase constitutes the source of hydrocarbons in shale gas). Carbon, hydrogen and oxygen atoms are shown in grey, white and red respectively. The size of the image is 5 × 5 nm. Four samples with different maturities, that is, with different times and conditions of formation, were considered.
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1Adsorption is the adhesion of molecules to a solid surface.
Realistic molecular model of kerogene's nanostructure. Colin Bousige, Camelia Ghimbeu, Cathie Vix-Guterl, Andrew E. Pomerantz, Assiya Suleimenova, Gavin Vaughan, Gaston Garbarino, Mikhail Feygenson, Christoph Wildgruber, Franz-Josef Ulm, Roland J.-M. Pellenq, Benoît Coasne. Nature materials. 1 February 2016.