Abstract
The stable carbon and hydrogen isotopes of light crude oils from deep reservoirs of Cambrian-Ordovician strata in Tazhong area of Tarim Basin, NW China were investigated. The δ13C and δD values determined from the oil samples ranged from −32.94 to −29.53‰ and −121.36 to −87.78‰, respectively. A positive correlation exists between δ13Coil and δDoil when the δ13Coil < −31.15‰, while a negative correlation occurs at δ13Coil > −31.15‰. This basic geochemical data is quite instructive in that the δ13Coil value of approximately −31‰ may be an important threshold value to delimit the origin of the organic matter that formed the light crude oils.