Formation and distribution of ethanodiamondoids in deeply buried marine oil from the Tarim Basin, China.

• Thermally stable ethanodiamondoids discovered in deeply buried marine oils. • Formation process of ethanodiamondoids investigated by thermal simulation. • Ethanodiamondoid-derived proxies established for oil maturity and cracking. • Abundance and isomerization of ethanodiamondoids changes during thermal evolution. • Phase type determined using concentrations of diamondoids and ethanodiamondoids. Diamondoids are often enriched in highly mature oil due to their high resistance to thermal stress. Ethanodiamondoids have similarly high thermal stability. We have detected ethanodiamondoids from 75 oil samples from deep marine carbonate strata in the Tarim Basin, China. Based on thermal simulation experiments on source rock and oil samples, we propose that ethanodiamondoids are gradually created with increasing thermal evolution and oil cracking. Their formation is likely due to free radical reactions of diamondoids with ethane or ethene formed from the cracking of alkanes. A proxy is proposed for the rapid determination of the degree of oil cracking and the petroleum phase based on the relationship between types and concentrations of ethanodiamondoids and diamondoids. [ABSTRACT FROM AUTHOR]