Genetic mechanism of bicyclic sesquiterpanes in Upper Triassic source rock of Yinan 2 well from the Kuqa Depression of Tarim Basin, China

Bicyclic sesquiterpanes are widely distributed in terrigenous sedimentary organic matter and are serve as valuable biomarker in the field of organic geochemistry. However, their formation and evolution, especially in geological conditions, are still poorly understood. In the present study, a series of bicyclic sesquiterpanes, including C₁₅-C₁₆ rearranged and regular compounds with generally dominant isomers of 4,4,8,8,9-pentamethyl-trans-decalin, 8(β)H-drimane, and 8(β)H-homodrimane, have been detected in approximately 230-m-thick Upper Triassic source rocks from the Yinan 2 well in the Kuqa Depression of Tarim Basin, NW China. The ratios of the ∑C₁₅/∑C₁₆ drimanes, 8(β)H-drimane/8(β)H-homodrimane, and ∑C_{15 - 16} rearranged/∑C_{15 - 16} drimanes generally decrease from the bottom to the top of the source rocks. The decreasing of ∑C_{15 - 16} rearranged/∑C_{15 - 16} drimane follows the decrease of the illite content in the source rock, showing a strong correlation with R² = 0.82. The formation and evolution of bicyclic sesquiterpanes are restrained by multiple factors, including thermal maturity, depositional environment, parent input type, and clay minerals. Specifically, terrigenous higher plant input and the presence of clay minerals (such as illite) may facilitate the formation and rearrangement of bicyclic sesquiterpanes, respectively. For example, 8(β)H-drimane may undergo rearrangement through 1,3-methyl migration to form 4,4,8,8,9-pentamethyl-trans-decalin via clay mineral catalysis. These findings will significantly advance geochemical evaluations of the crude oils depleted in conventional biomarkers.