SEM and FIB-SEM investigations on potential gas shales in the Dniepr-Donets Basin (Ukraine): pore space evolution in organic matter during thermal maturation

Abstract

Porosity and permeability are essential parameters for reservoir rocks. Techniques developed for conventional reservoir rocks characterized by large pores, cannot be applied to study gas shales. Therefore, high resolution techniques are increasingly used to determine reservoir quality of shale gas plays. Within the frame of the recent study, Upper Visean black shales (“Rudov Beds”) from the Dniepr-Donets-Basin (DDB, Ukraine) were characterized by X-ray diffraction, conventional SEM imaging and FIB/BIB-SEM.According to SEM and FIB/BIB-SEM data, nanopores are not abundant in primary macerals (e.g., vitrinite) even in overmature rocks, whereas they develop within secondary organic matter (bitumen) formed mainly at gas window maturity. Frequently occurring sub-micrometre porosity, probably related to gas generation from bituminous organic matter, was detected within mudstones at a vitrinite reflectance > 2.0 % Rr. However, such pores have also been detected occasionally in solid bitumen at oil window maturity (0.9 % Rr). Authigenic nanoscale clay minerals and calcite occur within pyrobitumen at gas window maturity.Furthermore, Rudov Beds can be subdivided into mineralogical facies zones by SEM imaging and X-ray diffraction. A basin-centred, brittle siliceous facies is most likely caused by increased contribution from deeper water radiolaria and is separated from a marginal clayey and carbonate-rich facies.