Geochemical exploration for oil and gas in northern South China Sea

Geochemical methods have been used to assist the petroleum exploration in sedimentary basins of the northern South China Sea_ A total of 1,560 stations have been sampled on 12 cruises for sea-surface air, near-bottom water, and seabed or near-surface sediments. Irregular grids or cross sections were deployed above target traps to identify geochemical anomalies. Known oiUgas fields and barren structures were sampled to provide anomaly models for comparison. About 50 chemical indices have been determined, including gaseous hydrocarbons (C l to Cs)' dissolved hydrocarbons, alternation carbonates (.1C), mercury, trace elements, ultraviolet and fluorescence intensities, as well as carbon isotopes in methane. These data were analyzed by robust statistical methods to ensure the reliability of anomaly delineation. Identified anomalies were interpreted based on geochemical theories and the comparison with known models. It is found that composite geochemical anomalies with various assemblages of indices and of halo or circularlhalf circular shapes appear above all of the oil/gas fields studied, in a clear contrast with the geochemical features of their adjacent barren traps. Geochemical methods provide important multivariate geochemical information for the assessment and prediction of hydrocarbon potential of traps identified by geophysical means.