Of importance for petroleum chemistry and geochemistry, the problem of the composition of the "hump" usually observed in gas chromatographic analysis of different crude oils is considered. This hump also called the "unresolved complex mixture" of petroleum hydrocarbons, in its composition encompasses a considerable amount of the whole hydrocarbon fraction of crude oils, ranging within 30-50% or, in the case of naphthene-base oils, reaching 90-95%. Studies performed in recent years have shown that an important role in the hump hydrocarbons is played by the so-called T-branched C-15-C-30 hydrocarbons of the aliphatic and naphthenic series. The problem of their involvement in the preservation of petroleum hydrocarbons upon petroleum biodegradation is discussed.