Distributions of methylated naphthalenes in crude oils: indicators of maturity, biodegradation and mixing

Variations in the distributions of trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs) and pentamethylnaphthalenes (PMNs) were studied in a set of crude oils, which differ mainly in their level of maturity. Comparison with mixtures obtained after heating methylated naphthalenes with clay led to the proposal of three new parameters, one for each class of naphthalenes. Each parameter is based on the same principle, which is the increase with maturity of stable isomers relative to less stable ones. It appears that the relationships between these three parameters are linear for those oils where thermal stress alone has determined the distributions. These relations can be conveniently displayed in a ternary diagram. It is proposed that when an oil plots in the restricted area in the diagram that is defined by these linear relations, the so-called maturity centre, the value of any of the three parameters is an accurate indication of its thermal maturity. When an oil plots outside the maturity centre, factors like in-reservoir mixing, biodegradation or mixing with indigenous organic matter have distorted the distributions of methylated naphthalenes. It is thought that the distributions of methylated naphthalenes reflect the extent to which 1,2-methylshifts and methyl transfer reactions, mediated by clay catalysis, have taken place. It is proposed that the methyl groups involved in these reactions form a ‘methyl pool’ to which most compounds present will have access. Increasing temperature and availability of suitable catalysts can remove methyl groups from this pool through formation of very stable compounds such as methane, leading to a decrease in the degree of methylation of aromatic hydrocarbons.