Geochemical controls on shale microstructure

The uptick in hydrocarbon production from shale in the United States has generated interest in metrics of unconventional reservoir quality, like permeability. We use conventional gas sorption to characterize shale microstructure, which provides insight on the features that govern mass transport. The gas sorption data are analyzed to determine the surface area, A(S) (m(2)/g), and pore volume, V-P (cm(3)/g) of 30 samples from basins across North America. With this information, we quantify the effect of composition and thermal maturity on shale microstructure. In particular, we find that the specific surface area of the organic component evolves from similar to 50 m(2)/g total organic carbon (TOC) in immature shale to similar to 500 m(2)/g TOC for regions that produce dry gas. The increase in A(S) is accompanied by an increase in V-P and concomitant decline in average pore size (e.g., r(H) = 4V(P)/A(S)). We contend that the latter is due to the development of nanometer-sized pores in kerogen as it is converted to mobile hydrocarbon which is ultimately expelled. This hypothesis is supported by similar measurements on companion samples after bitumen extraction or combustion, which underscore the intimate spatial association of petroleum and kerogen. Coupled with information on accessibility to the porosity garnered by varying the particle size, these results establish a clear link between organic matter, thermal maturity, and reservoir quality in unconventional shale systems.