GEOCHEMISTRY OF OILS AND CONDENSATES, K-FIELD, OFFSHORE TAIWAN - A CASE-STUDY IN MIGRATION FRACTIONATION

The K Field, offshore Taiwan, produces mainly gas along with small amounts of condensates and paraffinic oils. Detailed geochemical analyses of 21 fluids have shown that they belong to a single family generated from mixed organic matter derived from microbial, gymnosperm and angiosperm sources deposited in a fluvio-deltaic environment. Maturity assessment using several molecular indicators shows that the liquid hydrocarbons have maturities equivalent to vitrinite reflectances of around 0.7-0.9%. The source and maturity evidence indicate the fluids were generated in situ from Late Oligocene-Miocene coals and shales associated with the reservoirs. Large variations are observed in the bulk properties of the condensates and paraffinic oils (e.g., API gravities from 32 to 58-degrees), certain molecular ''source'' indicators (e.g., Pr/Ph ratios from 5.1 to 9.2) and carbon isotopes (e.g., deltaC-13 values of saturated hydrocarbon fraction from - 28.9 to - 26.9 parts per thousand/PDB). These variations are ascribed to the process of evaporative fractionation in which a gas-condensate cap separates from its associated oil and migrates to a shallower reservoir. In the K Field, thermogenic gas resulting from volcanic activity during the Middle/Upper Miocene provided the necessary gas charge to the emplaced oils. This study provides the first documentation of the effect of evaporative fractionation on a wide range of saturated and aromatic biomarker and carbon isotope indicators commonly measured in crude oils and condensates. Recognition of the process of migration fractionation and understanding its effect on petroleum composition are critical because molecular and isotopic geochemical parameters are widely used in oil-to-oil and oil-to-source correlations as well as indicators of alteration, source type and thermal maturity.