The importance of fluid pressures and migration to the hydrocarbon prospectivity of the Faeroe–Shetland White Zone

Abstract

The hydrocarbon prospectivity of the Faeroe–Shetland White Zone, located in the area between the Shetland and Faeroe Islands, was assessed in a regional study that integrated seismic and well interpretations with detailed source-rock geochemistry and predictive basin modelling.The Faeroe Basin formed during a Barremian rifting event followed by subsidence during the Late Cretaceous. The Paleocene began with a period of thermal uplift of basement highs and rapid sedimentation which infilled the submarine topography formed during the Cretaceous, and produced marked overpressuring in the basin. Gradual subsidence continued through the Tertiary except for a significant mid Tertiary inversion event that formed several interesting structures in the basin.New thermal models of basins and a new pressure mechanism for inducing hydrofractures that allow vertical hydrocarbon migration from Jurassic source rocks through Cretaceous mudrocks to Tertiary reservoirs, which we call the ‘whoopee cushion effect’, provide the key controls on the hydrocarbon charge mechanism, timing and petroleum composition.The other crucial elements, source, reservoir, and traps which are present at several stratigraphic levels in the White Zone, are summarized in this paper.The interplay of overpressure, hydrocarbon generation and migration during a complex basin evolution makes the White Zone a highly prospective frontier petroleum province.

Thermal history and oil charge on the UK Atlantic margin

“…Palaeotemperature constraints from apatite fission track analysis, while not definitely requiring any palaeo-heating, are consistent with those from vitrinite reflectance. Miocene-Eocene unconformity after Iliffe et al (1999). Figure 4 allows definition of the values of palaeogeothermal gradient which are consistent with the data.…”

“…with the reconstructed burial history for this well shown by Iliffe et al (1999), with around 650 m of section deposited and removed on the Late Cenozoic unconformity in this well.…”

“…The stratigraphy is summarized by Iliffe et al (1999) and geological ages are attributed according to the time-scale of Harland et al (1990). Since first returns were obtained from a depth of around 500 m below seabed, the stratigraphy of the shallowest part of the well is uncertain.…”

“…Two aspects of hydrocarbon migration attract particular attention: the process by which hydrocarbons reach Paleocene sandstone reservoirs and the sequence of oil charge in a multiple charge history. Models for oil charge in the West of Shetland region include (1) temporary residence in Mesozoic reservoirs followed by remigration into the Paleocene ('motel' model of Lamers & Carmichael 1999); (2) overpressure build-up in Cretaceous sandbodies followed by hydraulic fracturing and leakage into the Paleocene (Iliffe et al 1999); and (3) direct migration into the Paleocene due to overpressure retardation of hydrocarbon generation (Carr & Scotchman 2003). Evidence from organic geochemistry (Scotchman et al 1998;Holmes et al 1999;Goodchild et al 1999), basin modelling (Jowitt et al 1999) and fluid inclusion petrography (Parnell et al 1998) indicates that more than one oil generation is present.…”

“…Accordingly, this study is based primarily on well 204/19-1, adjacent to the Foinaven Field. The extensive stratigraphic range of this well has made it the subject of detailed assessments of burial and thermal history Iliffe et al 1999). Fluid inclusion temperature measurements from the Cretaceous-Paleocene section are interpreted within a thermal history framework constrained by apatite fission track analysis and vitrinite reflectance data throughout the well, to further understanding of thermal history in the region.…”