Abstract: A two-stage mode of stable carbon isotopic profile of n-alkanes from crude oil in the Yingjisu sag of Tarim basin has been revealed, featuring a two-phase hydrocarbon accumulation process to date in this region. The first phase is related to the peak oil generation and accumulation of Lower Ordovician source rocks and exemplified by Well Longkou-1. The second phase is characterized by the highly matured condensate reservoirs exemplified by Well Yingnan-2. The distribution of light hydrocarbons in condensates shows that it is possibly derived from the high evolution stage of kerogen rather than from crude cracking.

Key words: Tarim basin, Yingjisu sag, carbon isotope, kerogen, cracking