Role of the mineral matrix during kerogen pyrolysis

Abstract

Much is known about the important interactions existing, during pyrolysis of rocks, between minerals and the hydrocarbon compounds coming from the cracking of organic matter (kerogens). Clay minerals have been shown to be the most active in “retaining” the heaviest hydrocarbon compounds and in promoting their coking as the temperature rises. Under pyrolysis conditions of the Rock-Eval type, this paper investigates the interactions between different pure clay minerals and the hydrocarbons coming from the cracking of an organic matter of marine origin (type II). For this, different methods were used to analyse the volatile compounds issuing from pyrolysis (hydrocarbons, hydrogen, CO 2, CO) and the carbon residues coming from the hydrocarbon compounds retained on the minerals. These methods included gas chromatography, mass spectrometry, Raman spectroscopy, electron microscopy and oxidation with temperature programming. The results show that: (i) The amount of hydrocarbons “adsorbed” increases with the specific surface area of the argillaceous minerals investigated. (ii) Montmorillonite of type Ca 2+ (Camp Berteaux montmorillonite) reveals “catalytic” activity which enhances the formation of light hydrocarbons, with a predominance of aromatics to the detriment of the heaviest adsorbed compounds. In this case, the carbon residue formed on the mineral locally reveals the appearance of “pre-graphitic” structures. (iii) The same is not true for other minerals, such as illite and attapulgite. Despite their high retention power, such minerals do not show any catalytic activity, and the carbon residues they form seem to have a less polycondensed structure (easier combustion of these residues, higher H/C ratio).