A new pyrolysis technique using a diamond anvil cell: in situ visualization of kerogen transformation

Abstract

A pyrolysis technique has been developed to study hydrocarbon source rock potential by in situ visualization of kerogen transformation in a diamond anvil cell under a microscope (white and UV-light) using a CCD video monitoring system. The technique enables us to observe directly the processes and timing (or temperature) of kerogen transformation during pyrolysis in open or closed systems (either anhydrous or hydrous). It has been used to study a Green River kerogen and a lignite sample at heating rates similar to Rock-Eval pyrolysis under several pyrolysis conditions. The study has revealed: (1) the solid kerogen yields mobile oily liquids and solid residues and continues to evolve into gases. Most gases are generated later than oily liquids, probably from cracking of liquid. (2) The kerogen transformation is not significantly retarded in a closed system or at higher water pressure relative to an open system. (3) The neoformed oily liquid is immiscible with aqueous fluid at high pressure (8 kbar), but dissolves in the fluid at lower pressure (3 kbar). This confirms that the solubility of oily liquids in hydrothermal water decreases significantly with increasing pressure. The dissolved oily liquid can survive without cracking into gas at higher temperatures than the immiscible oily liquids. (4) An aqueous solution containing oxalic acid (which decomposes into CO 2 and H 2 at run conditions) dramatically retards the kerogen transformation. (5) Fluorescence of the total sample (kerogen with pyrolysate), which increases dramatically during transformation, may be used to follow quantitatively the rate of the reaction. (6) The preliminary results in an open, anhydrous system using a lignite sample with low hydrogen index (HI) show the decomposition of lignite into gases without the oily liquid precursor.