Interpretation of vitrinite reflectance profiles in sedimentary basins, onshore and offshore Ireland

Abstract

Abstract Vitrinite reflectance (VR) data ( R m %) have been compiled from 77 Irish offshore wells and 17 onshore boreholes. This database has facilitated the analysis of vitrinite reflectance v. depth relationships by both basin and stratigraphic interval. In general, VR gradients from the Carboniferous sections are defined by less scattered trends than those from Mesozoic and Cenozoic sections, reflecting the less complex vitrinite populations within Carboniferous coals and shales. A composite approach (display of profiles from a number of wells together) to the interpretation of vitrinite reflectance profiles has been utilized to characterize the thermal history and the prevalent heat transfer mechanisms within the various basins. Calculated peak palaeotemperatures from the wells are used to compute palaeogeothermal gradients and to estimate the magnitude of net exhumation at selected locations. Average palaeogeothermal gradients in the onshore Carboniferous basins range from less than 3°C km −1 at well IIP-2 in the Clare Basin to 119°C km −1 at well N998 in the Navan area of the Dublin Basin. Lateral variations in palaeogeothermal gradients recorded in the Carboniferous sections are consistent with a gravity-driven hydrothermal system discharging heated fluids, along fault systems, in a foreland platform area. In general, palaeogeothermal gradients are substantially higher in the Carboniferous sections (mean 60°C km −1 ) than in the Mesozoic or Cenozoic sections (mean 32°C km −1 ). Maturation levels in many of the Carboniferous sections are considered to be the consequence of burial, elevated heat flows and a regional advective system during late Carboniferous to early Permian times rather than Mesozoic or Cenozoic processes. Empirically derived methods of calculating peak palaeotemperature from VR are compared with kinetic models and, although differing in detail, within the resolution of this dataset are shown to produce similar trends. There is a considerable body of evidence to suggest that the extensional evolution of Ireland’s Late Palaeozoic to Cenozoic sedimentary basins has been punctuated by a multiphase inversion history. Regional stratigraphic evidence, combined with VR and apatite fission-track data, suggests at least two periods of pervasive exhumation occurred; one during Late Carboniferous-Late Permian time and another during Tertiary time. Both of these phases are characterized by a component of compressional inversion and the widespread occurrence of extrusive and intrusive igneous rocks. However, the contrasting thermal signature of these regional uplift events suggests that both the basin setting and the mechanism of regional exhumation exerted a fundamental control on processes that determined heat flow distribution within a basin. In terms of the hydrocarbon exploration of Ireland’s sedimentary basins the model presented here has important implications for the timing of maturation of Carboniferous source rocks in these basins. Where Carboniferous source rocks are present, they will make a significant contribution to the hydrocarbon budget only in those basins that experienced relatively low heat flow during Late Carboniferous-Early Permian time and where sufficient Mesozoic burial has occurred to subsequently expose the kerogen to higher temperatures. This observation is consistent with the presence of gas accumulations, which are postulated to have been derived from Carboniferous source rocks, in both the Slyne Basin and the Northwest Carboniferous Basin.