Hydrocarbon migration in the Porcupine Basin, offshore Ireland: evidence from fluid inclusion studies

A petrographic and microthermometric study of fluid inclusions in Jurassic and Cretaceous sandstones from the Porcupine Basin, offshore Ireland was integrated with innovative fluorescence lifetime measurements of hydrocarbon-bearing fluid inclusions to determine the compositions of the fluids associated with diagenesis and post-diagenetic fluid migration and the extent of hydrocarbon and aqueous fluid migration pathways. Petrographic analyses indicate that Jurassic strata were the main fluid migration pathways for hydrocarbon fluids and that hydrocarbon migration occurred relatively late in the diagenetic history of these sandstones. UV fluorescence and fluorescence lifetime measurements have recognized at least two chemically distinct hydrocarbon groups (Types 1a and 1b) with dissimilar lifetime-wavelength (τ-λ) profiles, consistent with at least two petroleum charges derived from different sources. Primary aqueous inclusions in authigenic cements show that cementation in Cretaceous sandstones occurred at relatively shallow levels at low temperatures (<50°C), while inclusions in authigenic cements in Jurassic sandstones were trapped at higher temperatures (70–120°C) and deeper levels. Aqueous fluid inclusions in intergranular trails indicate that post-cementation fluid migration occurred at high temperatures (up to 175°C). These high temperature fluid migrations are interpreted to be associated with plume-related activity during the opening of the North Atlantic.