Hydrocarbon migration history, West of Shetland: Integrated fluid inclusion and fission track studies

In sandstone reservoir rocks in the vicinity of the Rona Ridge, West of Shetland, there is evidence for former episodes of hydrocarbon migration in addition to the episodes responsible for current accumulations. An integrated study of fluid inclusion microthermometry and apatite fission track analysis (AFTA) has helped to clarify the history of fluid migration in the region. The Upper Jurassic Rona Sandstone contains both hydrocarbon and aqueous fluid inclusions. Hydrocarbons occur in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths and authigenic feldspars. This indicates at least 2 phases of migration before the presently trapped oil entered the rock: fluorescence colours indicate that they are of distinct compositions. Associated aqueous inclusions yield minimum trapping temperatures which indicate that the earliest hydrocarbons were associated with a phase of hot fluid migration, and that cooler conditions pertained during later migration. AFTA data provide independent evidence for a passage of hot fluid early in the history of the rock, which may be related to Late Jurassic-Early Cretaceous rifting. AFTA data also suggest a second episode of heating and cooling in the Tertiary following uplift, which may have been responsible for a later stage of hydrocarbon generation and migration. The results reflect repeated expulsion of fluids from deep basinal areas onto adjacent topographic highs, throughout the late Mesozoic to Recent.