Broad Ion Beam–Scanning Electron Microscopy Characterization of Organic Porosity Evolution During Thermal Treatment of Bazhenov Shale Sample

Abstract

Organic matter-hosted pores are considered as the main type of porosity in organic-rich shales. At the same time, literature indicates the formation of pore space during pyrolysis of oil shales. However, controls, evolution, and types of organic porosity remain controversial. This study aims to experimentally investigate the evolution of organic pores in an organic-rich shale sample during thermal treatment. This paper reports the organic porosity evolution during an artificial maturation experiment of the Bazhenov Formation (BF) shale sample (West Siberian Petroleum Basin). The siliceous-argillaceous organic-rich shale immature nonporous rock sample was treated in an open system in the temperature range of 350–450°C with the step of 10°C. Organic porosity was characterized by the combination of broad ion beam (BIB) polishing and scanning electron microscopy (SEM). After each episode of treatment, the area of 1000×1000 μm was scanned with a resolution of 25 nm. The acquired mosaic SEM images were segmented by the neural network algorithm and quantitatively analyzed. We demonstrate direct experimental evidence that thermal maturation/thermal treatment influence on organic porosity development. Organic porosity evolution is shown within the individual organic matter (OM) particles throughout the experiment. Thermal treatment leads to the formation of two types of organic pores, which are shrinkage and spongy pores. The first shrinkage pores start to form after the evacuation of existing hydrocarbons; they are relatively large and might reach 7 µm. This type of pore dominates at the initial stages of treatment (350–390°C). Porosity at this stage does not exceed 1.4%. The second type is spongy pores, which are up to 3–5 μm in size and are potentially formed due to hydrocarbon generation from the kerogen. This type of porosity becomes major after 400°C. This is confirmed by the pore size distribution analysis. The porosity spikes up to 2.3% after 400°C and rises up to 2.9% after 450°C. Revealing of artificial organic porosity development during thermal treatment experiment shows the crucial importance of the thermal maturity level. The formation of pore space during the treatment is critical during the implementation of thermal enhanced oil recovery (EOR) technologies in shales for fluid flow, and a mandatory aspect that should be accounted during thermal EOR simulations.