Carbon, Oxygen and Strontium Isotope Composition of Calcite Veins in the Carboniferous to Permian Source Sequences of the Junggar Basin: Implications on Petroleum Fluid Migration

Key words: 
• carbon /
• oxygen and strontium isotopes /
• calcite vein /
• reservoir diagenesis /
• hydrocarbon migration /
• the Junggar Basin /

Abstract: The presentlyexplored oil and gas in the central and northwestern Junggar Basin are mainly derived from CarboniferousPermian sequences. In this paper, we try to understand the basic features of petroleum fluid based on analyses on carbon, oxygen and strontium isotope composition of typical calcite veins hosted in source sequences. δ13CPDB  of 25 typical calcites ranges from －21.5‰ ~ 5.2‰, and δ18OPDB(δ18OSMOW) locates between －8.1‰ and -22.3‰(22.6‰ ~ 6.9‰). On the other hand, 20 87Sr/86Sr data (0.703 896~0.706 423) are all below the average value of modern seawater (0.709 073). Combined these isotope data and their geologic background, it is suggested that the petroleum fluid in the basin is a mixture of deepsourced hot fluid and oilgaswater. It carried volcaniccharacters by dissolution on the volcano and clastic rocks during hydrocarbon generation and migration. This result indicates that the effect of deep sourced hot fluid must be given enough consideration in the future study of reservoir diagenesis and petroleum migration.