Source Rocks and Oils of Atlantic Ireland and Comparison to Newfoundland-Labrador, Canada

Abstract

A major review of source rocks and oils from offshore basins in Ireland was carried out as part of a comprehensive study to further understand the development and distribution of source rock facies and their resultant hydrocarbon products between basins, and across the conjugate margin with Canada. The study included well data from most Irish basins including the Rockall, Porcupine, Slyne, Erris, Goban Spur, Fastnet and Celtic Sea basins. The database from offshore Ireland includes source rock geochemical analyses from 153 wells (Rock Eval, TOC), TOC log analysis of source rocks intervals in 35 wells, and 38 oil samples and source rock extract analyses. The interpretation of these data indicates around 15 different source rock horizons exist in the Irish basins within Mesozoic and Early Tertiary sediments, in addition to the Carboniferous gas-prone source rock. Oils in the Rockall Basin demonstrate a close affinity to oils in the North Sea and West Shetland basins due to the presence of 28, 30 bisnorhopane sourced from the Upper Jurassic ‘Kimmeridge Clay Formation’, suggesting a possible link at this time. Shallow borehole geochemical data from the margins of the Rockall Basin provides further evidence for Jurassic source rocks in the basin and in the flanking perched basins (4/97-03, 8/97-03 and (possibly) 83/20-sb01). Additionally, the Jurassic is an important proven source rock in the Slyne Basin where the Lower Jurassic is in the oil window. The Celtic Sea basins demonstrate Jurassic and Lower Cretaceous source rocks ranging from marine to lacustrine organofacies resulting in two main families. The Lower to Upper Jurassic were identified as the source of the oils found in the Porcupine Basin and these oils also all display a typical character of gammacerane which suggests a long standing stratification of the water column within the basin. Three main families of oil are categorised for the Porcupine Basin on the basis of sterane/hopane, C26/C25 tricyclic terpane and C35/C34 homohopane. The Irish oil samples were then compared to 89 oil samples from 38 wells in the Flemish Pass and Jeanne D’Arc Basins of Newfoundland-Labrador to determine if similarities exist in terms of source rock character and oil type. We successfully demonstrate that various source rock facies across the conjugate margin can be related to one another and grouped into families, and this is illustrated by comparing 10 oil samples from Canada (South Tempest G-88, Lancaster G-78, Hibernia J-34, Mara M-54, Egret K-36, Bay du Nord C-78, Mizzen O-16, Ben Nevis 1-45, Whiterose N-22, Adolphus 2K-41) and 6 oils and source rock extracts from Ireland (43/13-1, 49/13-2, 35/8-1, 35/8-2, 26/28-1, 12/2-1). We propose several “Super Families” of oils, including the regionally significant Upper Jurassic, highlighting numerous geochemical similarities attributed to analogous source facies across these basins. This particular geochemical study is underpinned by possibly the most comprehensive oil and source rock dataset accumulated for the Canadian-Irish conjugate North Atlantic basins. From this it is now possible to directly compare the oils and also the parent source rock facies both within and between each basin. This is especially important in the under-explored portions of some basins (e.g. the southern Porcupine Basin) where the lack of well data might be seen as a constraining element to exploration. Despite this lack of data, the results of the study have positive implications for petroleum prospectivity in offshore basins within the study area.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018