Probing Multiscale Structure of Mineral and Nanoporous Kerogen Phase in Organic-Rich Source Rocks: Quantitative Comparison of Small-Angle X-ray and Neutron Scattering

Source rocks are expected to become increasingly important in the upcoming years for oil and gas production as well as for the storage of greenhouse gases. These rocks are bedded and heterogeneous media, composed of minerals, kerogen, and pore space. One of the most challenging issues is to better define the pore space, including pore size distribution, pore volume fraction, pore connectivity, and pore affinity for various fluids. The aim of this study is to achieve such relevant parameters using X-ray and neutron scattering technique complementarity. Rock thin blades cut normal to the bedding plane preserve sample integrity and allow their measurements in both techniques. Two-dimensional scattering patterns show anisotropy, due to preferential orientation of lamellar minerals, which allows us to assess an order parameter. This parameter is a valuable tool for mechanical and transport properties. A model based on a three-phase system (minerals, kerogen, and pores) is developed through the study of scattering length density of each component within both radiations. The model allows us to fit both X-ray and neutron 1D data using the same model parameters. This latter is then considered as selective. It allows us to extract the kerogen mass density, the kerogen pore size distribution and its associated volume fraction, and the chemical nature of kerogen pore content. This methodology has been applied to a series of five source rocks of increasing maturities from Barnett Shale Play and Montney-Doig formations. Mature samples show a kerogen density of similar to 1.6 g.cm(-3), a pore radius distribution centered on 0.5-0.7 nm accounting for a pore volume fraction of similar to 0.01-0.04. These kerogen nanopores are filled with light condensed hydrocarbons. However, the overmature kerogen exhibits a mass density of 1.74 g.cm(-3), an additional pore radius distribution centered around 3.5 nm with a pore volume fraction of 0.002, and an empty pore space. All these parameters are in agreement with Rock-Eval pyrolysis measurements and literature data on similar source rocks. These results pointed out that the three-phase model associated with X-ray and neutron complementarity could be applied to in situ studies.