Interest in shale resource plays has shifted from gas to gas condensate and oil because of economic pressures. From a geochemical perspective, this represents a shift in thermal maturity of the organic matter in the host rock. It is believed that the development of secondary nanoporosity in the organic matter is linked to the thermal maturity of the shale; therefore thermal maturity has an effect on both storage and permeability in plays dominated by organic porosity. Previous research on thermal maturity and organic porosity in gas shales shows a trend of porosity occurring in the gas window (> 1.2% Ro); however, to date, few investigations of microstructure in the oil window have been carried out.
To investigate the organic porosity in low thermal maturity shale samples, a combination of Focused Ion Beam (FIB) milling and Scanning Electron Microscopy (SEM) was utilized. This combination permits careful sample preparation that preserves the microstructure, minimizes artifacts, and provides the ability to image the nanoscale organic pores which are below the resolution of visible light. Initial FIB/SEM results on oil window shale samples suggest a completely different type of porosity development within organic matter in the oil window (~0.6-1.0% Ro) versus the gas window. Oil window porosity development in the organic matter is exhibits crack-like features that appear to be a product of hydraulic fracturing and shrinkage of organic matter. This is in stark contrast to the porosity observed in gas window shales.
The crack-like features observed in the organic matter of oil window shales have immediate implications for local expulsion of oil. Excess oil being "incompressible" must leave the generation site; this can be facilitated through pressure build up caused by the roughly 50% excess volume which leads to localized hydraulic fracturing.