--> Source Rock and Geochemistry of the Central Atlantic Margins: Geochemical Characterization of Lower Jurassic Organic-Rich Facies Offshore Ireland

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Source Rock and Geochemistry of the Central Atlantic Margins: Geochemical Characterization of Lower Jurassic Organic-Rich Facies Offshore Ireland

Abstract

The Slyne Basin, a narrow Mesozoic basin located offshore Ireland, has been the subject of significant exploration with proven commercial hydrocarbon discoveries. While the Corrib gas field within the Slyne Basin is sourced from Westphalian Coal Measures, Lower Jurassic intervals are viable hydrocarbon source rocks of regional significance in the Irish offshore. Stable carbon isotope chemostratigraphy is a tool with a high potential to characterize organic-rich facies. It can be applied to characterize and/or to trace changes in the various hydrocarbon reservoirs, from continental environments and atmosphere to oceans and organisms. In marine environments, and particularly in epeiric sea settings, these processes are governed by the 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. In this study, we have analysed 17 cuttings samples from the interval between 2610– 2824 m of the well 18/25-1 (Slyne Basin). This interval has been dated as Pliensbachian–Toarcian. The selected cuttings were analysed by Continuous Flow–Isotope-ratio Mass Spectrometry (carbonates) and Elemental Analysis- Isotope-ratio Mass Spectrometry (organic matter) to determine 13C/12C ratio and calculate δ13C. δ13C in carbonates varies from -5.16 to 2.22 ‰. A negative trend is observed from 2610 m to 2664 m. The determined δ13C tends to more positive values. The δ13C values determined in organic matter presents less variation: a negative trend is observed from 2610 m to 2688 m (-6.65 ‰) followed by a generally positive trend from 2694 m to 2824 m (+6.00 ‰). These δ13C range from -28.23 ‰ to -25.52 ‰. The obtained data were integrated with pre-existing Total Organic Carbon, pyrolysis Rock-Eval and Vitrinite Reflectance data, in order to evaluate the origin of organic matter and potential to produce hydrocarbon. The negative excursion of the carbon isotope signature is thought to correspond to the T-OAE. Ongoing investigation will discern the conditions that lead to the occurrence of this organic-rich facies offshore Ireland.