Impact of Oil-Prone Sedimentary Organic Matter Quality and Hydrocarbon Generation on Source Rock Porosity: Artificial Thermal Maturation Approach.

Abstract

This work investigates the relation between the molecular composition of the organic matter (OM), hydrocarbon generation, and porosity of artificially matured mudstones from the Kimmeridge Clay formation. Anhydrous thermal maturations, geochemical characterization (gas chromatography–mass spectrometry, gas chromatography with a thermal conductivity detector), scanning electron microscopy observations, and nitrogen adsorption measurements were carried out. The results were compared to the calculated OM porosity after maturation. Our results reveal that samples richer in phytoplanktonic OM generated more abundant oils enriched in alkanes and lighter aromatic hydrocarbons (i.e., biphenyls and naphthalenes) that are more able to fill the adjacent pores of the mineral matrix during maturation. Both the calculated and the observed OM porosity increases during maturation, but the measured rock pore volume shows a non-linear evolution related to the amount of gas generated and the ensuing ability of the rock to preserve the pores. The secondary cracking of highly oil-prone samples generated larger amounts of C1–C5 gases but lower pore volumes, less well preserved. OM composition and its ability to generate oil and gas seem therefore to affect the pore volume. These variations are nevertheless small compared to the effect of thermal maturity, which remains the major process controlling the evolution of porosity. During gas generation, high C2–C5 concentrations are generated compared to methane. The short thermal maturation duration of our experiments may have delayed the conversion of C2–C5 hydrocarbons to methane. In addition, slight differences in the concentrations of saturated and aromatic components and markedly different pore volumes and pore size distributions exist between artificially and naturally matured rocks. The conditions of artificial maturations may thus impact the thermal transformations of OM, emphasizing the necessity to investigate the role of the artificial maturation kinetics on OM and porosity.