Nanoporosity of graptolite periderm by atomic force microscopy (AFM): A case study from the Baltic Basin, Poland

To date, the pore system of graptolite periderm in shales with thermal maturity at the transition phase from the oil to the gas window has been poorly researched and understood. It was also not clear whether the graptolite periderm develops secondary pores. Therefore, porosity, pore characteristics, and surface roughness of the graptolite periderm (graptolite reflectance GR = 1.30–1.83 %; vitrinite reflectance equivalent VRE = 1.11–1.50 %) in the Silurian shales of the Baltic Basin (northern Poland) were investigated by atomic force microscopy (AFM). The size of the 200 × 200 nm measurement area was chosen to allow observation of pores with diameters ≥1.4 nm. The graptolite periderm shows a very low porosity (0.16–0.28 %), which increases with increasing thermal maturity and decreases the mean equivalent pore diameter and surface roughness. With the onset of the gas window (at GR ≥ 1.58 %; VRE ≥ 1.31 %), limited secondary porosity develops, and percentage of pores <5 nm in size significantly increases. Elliptical or roundish micropores appear within the granular fragments, and sparse pores <5 nm in size emerge within the non-granular fabric. The ability of the graptolite periderm to generate gaseous hydrocarbons at the beginning of the gas window is poor. Granular type is more generative compared to non-granular type. The ratio between the two types of fabric is not significant for the overall source and reservoir potential of the host formation at the maturity level considered in this study. However, it can be important at higher maturities. Changes in the pore system and roughness parameters align with the alteration of the chemistry and microstructure of graptolite periderm. The development of secondary porosity can depend on thermal degradation of the graptolite periderm during and after burial.