Does grain size influence hydrocarbons generation and mesoporosity during artificial thermal maturation of an organic-rich mudstone?

Abstract

To assess how the grain size/rock fabric of laboratory-matured shale samples impact oil, gas production, and the formation of pores during laboratory thermal maturation, cores (4.5 mm*1 cm) of bulk rock, millimetric equidimensional rock fragments (3*3 mm), and fine grind powder (<200 μm) were obtained from an organic-rich mudstone (Kimmeridge Clay Fm. “KCF”, Yorkshire, UK) and artificially matured under anhydrous conditions. The composition of the organic matter (OM), porosity, and thermal maturity were characterized using nitrogen adsorption, Rock-Eval® VI pyrolysis, GC/TCD (Gas-Chromatography/Thermal Conductivity Detector), and GC/MS (Gas Chromatography/Mass Spectrometry). While only a few differences in geochemical composition and porosity exist between rock fragments and powders, a greater gap is observed with the cores. Probably due to lower surface contact between the organic components themselves and clay mineral surfaces (which can act as a catalyst of oil and gas generation) less intense OM thermal degradation processes were observed for the cores. The use of cores rather than powder and fragments results in: (i) the production of lower extractible OM concentrations; (ii) lower amounts of gas (C1–C5 and CO2); (iii) lower pore volumes and smaller pore sizes during maturation. This preliminary work highlights the importance of considering the fabric of the rock being artificially matured and shows that the use of cores with an intact rock fabric (e.g. cores), closer to natural conditions, could be more suitable when studying OM thermal degradation and porosity. In comparison with previous works, this also demonstrated that depending on the maturation system used (e.g. semi-confined, confined) the differences in the amount of hydrocarbons generated and porosity observed between powder and cores during maturation could differ significantly and find their origin in different processes.