Maturity and hydrocarbon generation in the Slyne Trough, northwest Ireland

Abstract

Abstract The Slyne Trough forms part of the Mesozoic basin system which stretches from the West Shetland Basin, north of Scotland, to the Porcupine Basin, west of Ireland. It comprises three asymmetric half-grabens containing a thick section of Permo-Triassic and Jurassic strata which are unconformably overlain by thin Cretaceous to Quaternary sediments. An extensive seismic database exists over the Slyne Trough but drilling to date has been limited to well 27/13-1, drilled in the Central Slyne Trough by Elf in 1982. This well penetrated a 2.5 km thick Middle/Upper Jurassic to Rhaetic section and encountered minor shows of oil in Bathonian-Bajocian sandstones, and of oil and gas in Lower Jurassic shales. The well has a present-day geothermal gradient of 27°C/km with a corrected bottom hole temperature of 74°C. It penetrated rich oil-prone source rocks in two intervals in the Lower Jurassic section. These are regionally correlatable with the Toarcian Portree Shale Formation and the Sinemurian Pabba Shale on the Isle of Skye. Geochemical studies of the shows in the Middle Jurassic sandstones indicate them to be biodegraded and water-washed. The strong similarity between their gas chromatograms and biomarkers and those of the Lower Jurassic source rocks suggests generation from more mature Lower Jurassic strata in the deeper parts of the Central Slyne Trough. The 27/13-1 well section, where the late Tertiary to Quaternary sediments rest unconformably on the Middle/Upper Jurassic, suggests that the Slyne Trough was affected by substantial uplift and erosion in the Late Jurassic to Late Tertiary. Apatite fission track data were used to study the timing and amount of this uplift and a maximum of 1.6 km uplift and erosion occurring between 100 and 30 Ma (Albian-Oligocene), with 80 to 60 Ma (Campanian-Danian) being inferred as the most likely timing. However, it is probable that the data represent the unresolved record of tectonic events in the Mid-Cretaceous and in the Late Tertiary. Maturity of the 27/13-1 well section was studied using vitrinite reflectance and biomarkers. Vitrinite reflectance ranges from 0.43 to 3.87% R 0 in the well, the very high values being associated with a thin sill intruded into the basal Jurassic section. The vitrinite reflectance/depth trend is consistent with the apatite fission track data, and places the palaeo-oil window (0.7% R 0 ) at 2600 m sub-sea. The Portree and Pabba Shale Formations’ source rock intervals have suppressed vitrinite reflectance values, having attained early maturity on structure. Hopane, sterane and triaromatic sterane biomarker data on bitumen extracts confirm the early maturity, with the Pabba Shale Formation being slightly more mature. Extrapolation of the vitrinite reflectance/depth trend indicates maximum uplift of 1.9 km. Biomarker data suggest lesser relative uplift of 0.7–1.06 km, which is comparable to the 0.7–1.05 km range determined from shale sonic log interval velocities. Geothermal modelling, incorporating the results of the geochemical and apatite fission track studies on well 27/13-1, suggests two periods of hydrocarbon generation in the Central Slyne Trough. The first occurred in the later Jurassic and was associated with rifting and rapid burial. Regional uplift and erosion in the Early Cretaceous ended this first phase of generation. Renewed burial took place in the Late Cretaceous to Early Tertiary, when the Jurassic source rocks attained their maximum burial. Subsequent phase of uplift and inversion in the Tertiary lifted the source rocks out of the hydrocarbon generation window. This event may have breached the structure tested by well 27/13-1, resulting in biodegradation and water-washing of the reservoired oils.