Geochemical characteristics of inclusion oils from the Skarv field A segment and their implications for the oil charge and leakage history

Abstract

The oil-bearing fluid inclusions (FIs) in reservoir sandstones in the Skarv field in the Haltenbanken region offshore Norway recorded a complex oil charge and leakage history. Inclusion oils, hosted in quartz cements, and corresponding adsorbed residual oils on mineral surfaces from the Jurassic Garn and Tilje Formation (Fm) sandstones have been investigated using coupled gas chromatography-mass spectrometry, coupled gas chromatography-isotope ratio mass spectrometry and, as a new technique for FI oil analysis, Fourier transform-ion cyclotron resonance-mass spectrometry. The biomarker, stable carbon isotope and nitrogen, sulfur and oxygen-containing compound results suggest that FI oils from both the Garn and Tilje Fms and adsorbed (Ad) oil from the Tilje Fm were derived from the same source, whereas Ad oil from the Garn Fm shows a lower maturity level and differences in facies indicators. The similar geochemical signals for the FI oils from the Garn and Tilje Fms point to a first charge from the same source rock i.e. the late-Jurassic/earliest-Cretaceous Spekk Fm (Oxfordian to Ryazanian age). The fact that the Ad oil in the Tilje Fm is also similar to the respective FI oil indicates that the Tilje Fm was charged only once. In contrast, differences between the FI and Ad oils in the Garn Fm suggest that first charged oil has leaked and was replenished by a later charge sourced from a spatially different and less mature kitchen area of the Spekk Fm.