GEOCHEMICAL STUDY OF HYDROCARBON-PRONE SEDIMENTS OF LOWER JURASSIC AGE IN THE SLYNE BASIN, OFFSHORE IRELAND

Abstract

The Slyne Basin, a narrow Mesozoic basin located offshore Ireland, has been the subject of significant exploration with several proven commercial hydrocarbon discoveries. While the Corrib gas field within the Slyne Basin is sourced from Carboniferous Coal Measures, Lower Jurassic intervals represent a separately viable hydrocarbon source rock system of regional significance in the Irish offshore. In marine environments, and particularly in epeiric sea settings, the processes leading to deposition of these hydrocarbon source rocks are governed by complex interplay of local (different carbonate producers, transgressive–regressive cycles) and/or global (worldwide preservation of organic matter, variation of continental weathering, input of volcanogenic light CO2) mechanisms. Stable carbon isotope chemostratigraphy, RockEval pyrolysis and X-ray fluorescence (elemental geochemistry) have been applied to geochemically characterize samples from Lower Jurassic source rocks in two wells (18/25-1 and 27/13-1, Slyne Basin). To determine δ13C and δ18O, the selected samples were analysed by Continuous Flow–Isotope-Ratio Mass Spectrometry (for carbonates) and Elemental Analysis- Isotope-ratio Mass Spectrometry (for total organic carbon, TOC). Results from well 18/25-1, present a negative δ13C trend from 2610 m to 2664 m, while δ13C values from organic matter present less variation: a negative trend is observed from 2610 m to 2688 m followed by a generally positive trend from 2694 m to 2824 m - the negative excursion of the carbon isotope signature is thought to correspond, in part, to the Toarcian Oceanic Anoxic Event. The slight post-T-OAE negative carbon isotope excursion (CIE) recovery to more positive values and the continuation of a relatively high organic content is coeval with a regional organic matter preservation interval observed in several Northern-European locations. Total Organic Carbon content and Rock-Eval data show that Pliensbachian-aged source rock intervals are gas/oil prone (Type II/III) and Toarcian-Aalenian aged sediments are oil-prone (Type II). These new datasets and their interpretation add to the understanding of the palaeoenvironmental dynamics during the Early–Middle Jurassic in the Northern European domain and may offer new insights for hydrocarbon exploration offshore Ireland.