On the basis of an analysis of extensive data on the distribution of heteroelements and the phase state of hydrocarbon (HC) accumulations of the globe as a function of the temperature and pressure conditions of their location, the P-t parameters of substitution in the lower zone of the sedimentary cover with which accumulations of hydrocarbon gases (HCG) are replaced mainly by non-hydrocarbon gases (NHCG) have been found. This has been partly confirmed by physicochemical (thermodynamic) modelling of the process of conversion of depleted dispersed organic matter (DOM). It has been shown that from the ratio of heteroelements in natural caustobioliths it is possible to classify them according to the conditions of formation, and from the ratios of relict isoprenoidal alkanes to diagnose the mainly ＇＇marine＇＇ nature of crude oils enriched with sulphur.