**7. 13C enrichment of oils reflects O2 growth in the atmosphere and display four orogenic cycles from the Cryogenian period of Neoproterozoic era to the Miocene epoch of Cenozoic era**

Having examined the extensive collection of oils (504 oil samples) differing in age and origin, Andrusevich et al. [35, 36] established that in the course of geological time, the oils and their components have been consistently enriched in 13C (Figure 7). Model's logic allows concluding that the most likely reason for this enrichment is the intensified photorespiration of photo‐ synthesizing organisms as a result of the increase of average oxygen concentration in the atmosphere. One should take into account that, despite of variations, in the course of geological time; an average atmospheric oxygen concentration steadily grew up. The 13C enrichment of oils reflects that at least four orogenic cycles took place, accompanied by an increase in average oxygen concentration in the atmosphere. Oils inherit the enrichment from organic matter. The existence of four orogenic cycles in Phanerozoic agrees with the other proofs (see the previous section), evidencing for four waves of oil generation in the Phanerozoic.

**Figure 7.** Change of the average carbon isotope composition (δ13C, ‰) for saturate fraction C15+ of crude oils. Vertical bars are standard deviations, which increase with decreasing age. Arrows indicate Cambrian–Ordovician, Triassic–Ju‐ rassic, and Paleogene– Neogene boundaries where 13C enrichment occurs [36].

One more interesting observation indicating the validity of the model stems from the analysis of the above data. Since Jurassic, a scatter in carbon isotopic composition of oils has increased essentially. The variety of land photosynthesis conditions resulted in a wide spectra of δ13C values.
