Inter and intrafield hydrocarbon compositional variations in the Ula and the Gyda fields (Central Graben-North Sea)-implication for understanding the controls on hydrocarbon distribution within and between these fields

A suite of 34 oil samples from the Ula and 30 from the Gyda oil fields lying in the Central Graben of the Norwegian Continental Shelf, was analyzed using thin layer chromatography with flame ionization detection (TLC-FID), capillary gas chromatography with flame ionization detection (GC-FID) and gas chromatography-mass spectrometry (GC-MS). The results obtained were used to calculate several source and maturity based parameters. These parameters suggest that within the Ula Field, oils produced more recently have greater compositional homogeneity than those produced earlier. This has been attributed to a recent change in the crestal fault behavior. This change in fault behavior is probably production related. The Gyda Field oils are compositionally more homogeneous. However, oils from the Gyda South Culmination are compositionally different. This difference shows up in both maturity and source related parameters.