Abstract

The Embla field, located in the greater Ekofisk area of the Norwegian part of the southern North Sea, has been producing oil and gas since the early 1990s. The wells on the crest are the best and most continuous producers, whereas the wells on the flanks are only good producers over limited periods after pressure buildup. The reason for the poorer production from the flanks is reduced permeability caused by the presence of bitumen, a nonmovable biodegraded oil that fills the pore space. This oil is shown to be of Paleozoic age and is suggested to have charged the Embla field at the end of the Triassic. The structure was uplifted and eroded during the Jurassic, and the Paleozoic oil accumulation became biodegraded on the oil-water contact. The crest of the structure is believed to have been protected from biodegradation until the seal was eroded, which resulted in the escape of the oil and the biodegradation of the residual oil. Subsidence during the Cretaceous, the formation of a new seal, and the Upper Jurassic source rock intervals reaching oil window maturity resulted in the structure being recharged. Partly contradicting maturity estimates based on various biomarker ratios suggest that the analyzed oils represent different mixing ratios between the two oil generations. The change in light oil composition observed between the drill-stem tests sampled approximately 20 yr ago and the presently produced oil suggests that the Embla field currently is receiving Jurassic-sourced oil.

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