Abstract We present a consistent synthesis of palaeothermal (apatite fission track analysis (AFTA) and vitrinite reflectance) data from UK Southern North Sea wells with the regional pattern of exhumation defined from sonic velocity data. Cenozoic exhumation across most of the region began in the Paleocene between 63 and 59 Ma. Amounts of removed section are around 1 km across the offshore platform, increasing to 2 km or more on the Sole Pit axis. Neogene exhumation within this area began between 22 and 15 Ma, and led to removal of up to 1 km of section. Along the eastern flank of the Sole Pit axis, sonic data define a pre-Chalk event, and AFTA data from these wells show that exhumation began between 120 and 93 Ma. This timing correlates with events defined from AFTA data in the Sorgenfrei–Tornquist Zone, further east, presumably reflecting a response to regional tectonic stresses. East of the Sole Pit axis, AFTA and sonic velocities suggest that Neogene exhumation dominates, while further east towards the central parts of the North Sea Mesozoic sediments appear to be at maximum burial today except for local effects related to salt movement. The multiple episodes of exhumation and burial defined here have important implications for exploration.

Abstract While several methods have been developed for assessing the magnitude of postdepositional heating of sedimentary rocks, apatite fission track analysis (AFTA ) can also define the time at which a sedimentary rock cooled from its maximum postdepositional temperature, up to ~110° C. This information is particularly important in hydrocarbon exploration, e.g., in defining the timing of hydrocarbon generation and identifying regions where the main phase of generation postdates formation of potential trapping structures. Based on analysis of naturally occurring radiation damage features (fission tracks) in detrital apatite grains, the foundation of the technique is a detailed understanding of the kinetics of fission-track “annealing,incorporating observations from both laboratory and geological field conditions and making explicit allowance for apatite composition (chlorine content), which exerts a crucial influence over fission-track annealing kinetics. The thermal response of fission tracks is dominated by the maximum postdepositional paleotemperature, and this fundamental aspect of the technique imposes strict limitations on the information that can be obtained. In particular, no information can be obtained on the thermal history prior to the onset of cooling from the maximum postdepositional temperature. However, one or possibly two additional episodes of heating and cooling can often be resolved following the paleo–thermal maximum. Integration of AFTA data with paleotemperature estimates from other methods, in particular vitrinite reflectance (VR), provides additional support for thermal history interpretations and can often help to refine solutions from AFTA. Most importantly, the combined use of AFTA and VR in a vertical sequence allows construction of profiles of paleotemperature with depth (or elevation), enabling quantitative determination of paleogeothermal gradient, which in turn allows identification of the mechanisms of heating and cooling. Heating due to deeper burial produces a linear paleotemperature profile with a similar gradient to the present temperature profile, whereas heating due primarily to increased basal heat flow will produce a profile with a higher gradient than the present temperature profile. Extrapolation of such profiles above the appropriate unconformity identified from AFTA to a suitable paleo–surface temperature allows determination of the magnitude of additional burial responsible for the observed heating. Estimations of additional burial in this way depend on assumptions concerning the lithology (i.e., thermal conductivity) of the eroded sequence and wherever possible should be combined with estimates of burial based on nonthermal processes, such as sonic velocity, in order to provide consistent constraints on the burial history. Nonlinear profiles are produced by contact heating around intrusive bodies and by passage of hot fluids within confined aquifer horizons. More complex situations that involve nonlinear profiles resulting from thick sequences with extreme thermal conductivities (e.g., coal or salt) can also be assessed by inspection of the variation of paleotemperature with depth. AFTA has been applied to hydrocarbon exploration in a wide variety of settings. Some of the most important outcomes of AFTA analysis, in terms of events that affect regional hydrocarbon prospectivity, are: (1) the recognition of regional kilometer-scale exhumation (implying earlier deeper burial), often in areas that have traditionally been considered stable; (2) definition of major Phanerozoic paleo–thermal events in Proterozoic basins; and (3) revelation of the importance of hot fluids in transporting heat in sedimentary basins. In basins with complex histories, for example, exhumed basins or those in which heat flow was higher in the past, AFTA can provide unique constraints on the timing of hydrocarbon generation, which can significantly reduce exploration risk in such areas.

Abstract Large tracts of the NW European continental shelf and Atlantic margin have experienced kilometre-scale exhumation during the Cenozoic, the timing and causes of which are debated. There is particular uncertainty about the exhumation history of the Irish Sea basin system, Western UK, which has been suggested to be a focal point of Cenozoic exhumation across the NW European continental shelf. Many studies have attributed the exhumation of this region to processes associated with the early Palaeogene initiation of the Iceland Plume, whilst the magnitude and causes of Neogene exhumation have attracted little attention. However, the sedimentary basins of the southern Irish Sea contain a mid–late Cenozoic sedimentary succession up to 1.5 km in thickness, the analysis of which should permit the contributions of Palaeogene and Neogene events to the Cenozoic exhumation of this region to be separated. In this paper, an analysis of the palaeothermal, mechanical and structural properties of the Cenozoic succession is presented with the aim of quantifying the timing and magnitude of Neogene exhumation, and identifying its ultimate causes. Synthesis of an extensive apatite fission-track analysis (AFTA), vitrinite reflectance (VR) and compaction (sonic velocity and density log-derived porosities) database shows that the preserved Cenozoic sediments in the southern Irish Sea were more deeply buried by up to 1.5 km of additional section prior to exhumation which began between 20 and 15 Ma. Maximum burial depths of the preserved sedimentary succession in the St George’s Channel Basin were reached during mid–late Cenozoic times meaning that no evidence for early Palaeogene exhumation is preserved whereas AFTA data from the Mochras borehole (onshore NW Wales) show that early Palaeogene cooling (i.e. exhumation) at this location was not significant. Seismic reflection data indicate that compressional shortening was the principal driving mechanism for the Neogene exhumation of the southern Irish Sea. Coeval Neogene shortening and exhumation is observed in several sedimentary basins around the British Isles, including those along the UK Atlantic margin. This suggests that the forces responsible for the deformation and exhumation of the margin may also be responsible for the generation of kilometre-scale exhumation in an intraplate sedimentary basin system located >1000 km from the most proximal plate boundary. The results presented here show that compressional deformation has made an important contribution to the Neogene exhumation of the NW European continental shelf.

Abstract In areas where significant unconformities are present, palaeotemperatures derived from apatite fission-track analysis (AFTA) and vitrinite reflectance (VR) data through a vertical rock section can be used to estimate palaeogeothermal gradients and (by extrapolation to an assumed palaeo-surface temperature) amounts of exhumation (palaeo-burial). AFTA also provides a direct estimate of the timing of exhumation. These parameters can be used to reconstruct more complete histories than those based purely on the preserved rock record. Precision and accuracy of these estimates are controlled by a range of theoretical and practical factors, perhaps the most important being the use of appropriate kinetic models. In extracting thermal history information from fission tracks in apatite, it is essential to use models that can describe variation in response between apatite grains within a sample. It is also important to recognize the limitations of the methods. AFTA and VR are dominated by maximum temperatures, preserving no information on events prior to a palaeo-thermal maximum. Recognition of this allows definition of key aspects of the history with greater precision. Results from NW Europe define a series of regionally synchronous palaeo-thermal episodes, with cooling beginning in Early Cretaceous, Early Tertiary and Late Tertiary times. Latest results show that Early Tertiary palaeo-thermal effects in NW England can be understood as being due to a combination of higher basal heat flow and deeper burial, and emphasize the importance of obtaining data from a vertical sequence of samples. Comparison with similar results from other parts of the world suggests that events at plate margins exert a key influence on the processes responsible for regional exhumation, as recognized through Mesozoic and Cenozoic times across NW Europe.

Abstract Thermal history reconstruction studies of four hydrocarbon exploration wells located in the Central Irish Sea Basin (CISB) reveal three major regional episodes of heating and cooling. Units throughout the pre-Quaternary section intersected in wells 42/12-1, 42/16-1 and 42/17-1 began to cool from their maximum post-depositional palaeotempera-tures in Early Cretaceous time, between 120 and 115 Ma. Cooling from subsequent palaeotemperature peaks began in Late Cretaceous–Early Tertiary (70–55 Ma) and Late Tertiary (25–0 Ma) time. Results from well 42/21-1 are dominated by the two more recent episodes, and show no evidence of the Early Cretaceous episode. This is thought to reflect a different structural setting of this well, within a North Celtic Sea–Cardigan Bay trend. Palaeotemperature profiles suggest that heating in each episode was due largely to deeper burial, with subsequent cooling caused mainly by uplift and erosion. A maximum of c. 3 km of additional Late Triassic to Early Cretaceous section is required to explain the observed Early Cretaceous palaeotemperatures. Appropriate values for the Late Cretaceous–Early Tertiary and Late Tertiary episodes are c. 2 km and c. 1 km, respectively. All of these cooling episodes correlate closely with similar episodes recognized from previous studies in surrounding regions, from onshore Ireland, Scotland, South Wales and northern, eastern, central and SW England, and each appears to be of truly regional extent. Exploration risk in the CISB generation can be significantly reduced through recognition of the major palaeothermal episodes that have affected the region, and the variation in the magnitude of their effects across the region. The challenge for future exploration in the region is to identify regions where the main phase of hydrocarbon generation post-dated structuring.