GEOCHEMISTRY AND ORIGIN OF CRUDE OILS AND CONDENSATES FROM THE CENTRAL PERSIAN GULF, OFFSHORE IRAN

Biomarker- and compound-specific carbon isotope analyses were used to compare oil samples recovered from Late Jurassic and Early to Middle Cretaceous reservoirs at South Pars and nearby fields in the Iranian portion of the Persian Gulf, and condensate samples associated with the super-giant gas accumulation in Permo-Triassic reservoirs at South Pars. The results indicate that all of the oil samples, including heavy oil from South Pars and oil from the Salman, Reshadat, Resalat and Balal fields, are genetically related. The most probable source rocks for these oils are Jurassic marine limestones or marls deposited under anoxic conditions. Based on the methyl phenanthrene index, source rock maturity was inferred to be equivalent to vitrinite reflectance values of about 0.8% Rc. The distribution and maturity pattern of the source rocks suggest migration from a depocentre located to the south, with inferred migration distances of up to 250 km. There is no genetic relationship between the heavy oil which has accumulated in Mesozoic reservoirs at South Pars and condensates which are associated with the super-giant gas accumulation in Permo-Triassic reservoirs there. Based on biomarker compositions, the condensates at South Pars appear to be derived from shaly marine or lacustrine source rocks deposited under dysoxic conditions. The delta C-13 values of short-chain n-alkanes and isoprenoids in condensate samples suggest a common source and an equal maturity for the source rocks. Pristane/n-C-17 versus phytane/n-C-18 characteristics are in agreement with published data for Silurian-sourced condensates. High thermal maturities equivalent to 1.7% Rc are also consistent with a Palaeozoic (Silurian) source rock.