Thermal history and oil charge on the UK Atlantic margin

Abstract

Integrated apatite fission track analysis and vitrinite reflectance data show that well 204/19-1 in the West of Shetland region, UK Atlantic margin, has experienced only limited additional burial beyond present-day depths. Uplift and cooling to present-day levels probably occurred during late Cenozoic (Eocene to Miocene) basin inversion. Fluid inclusion data indicate that Paleocene-Eocene sandstones have experienced temperatures much higher than can be explained by burial alone. Temperatures up to 200 C indicate the passage of hot fluid through Cenozoic sandstones, which by-passed the pre-Cenozoic section in this and other wells. The hot fluid event must have been of very brief duration (up to 100 years) to show no record in the fission track and reflectance data, implying that the fluids migrated through fracture systems.Oil inclusions in the Cretaceous of well 204/19-1 have a chemistry that suggests derivation from a Kimmeridgian-aged source rock. They occur in cements that show no evidence for the hot fluid event and it is concluded that the cements pre-date the event. Oil inclusions in Cenozoic sandstones have a heavy, degraded character and were trapped at high temperature, suggesting that degradation was related to the hot fluid event. Present-day oils in the West of Shetland region are mixtures, which could reflect components from the two charges distinguished by the integrated thermal and geochemical histories. The inference of fracture-bound flow is consistent with existing models of overpressure development and hydrofracturing.

Late Palaeozoic hydrocarbon migration through the Clair field, West of Shetland, UK Atlantic margin

“…The principles involved in using AFTA to constrain thermal histories in sedimentary basins are described in detail elsewhere (Green et al, 1989a(Green et al, ,b, 2001(Green et al, , 2002(Green et al, , 2004Parnell et al, 1999Parnell et al, , 2005. In brief, optical microscope analysis of radiation damage features in detrital apatite grains separated from clastic units allows determination of the maximum temperature to which a sedimentary unit has been heated (up to a maximum of around 110-120°C, depending on apatite composition) and the time at which the sample began to cool from that palaeothermal maximum.…”

“…The variation of stratigraphic age with depth is also shown for each well, together with the variation in fission track age and mean track length predicted from the default thermal history scenario in each well. As explained in more detail elsewhere (Green et al, 2004;Parnell et al, 2005), this represents the situation in which all sedimentary units are currently at their maximum post-depositional temperatures, and forms the basis for extraction of information on the palaeo-thermal history from the AFTA and VR data.…”

“…Maximum palaeotemperatures attained by each sample and the time at which the sample began to cool from that palaeo-thermal maximum have been determined using procedures explained in detail elsewhere (Green et al, 2004;Parnell et al, 2005), with results summarised in Table 4. Two episodes of cooling can be resolved from the data in most samples from the Palaeozoic section.…”

Large-scale seismic thermal anomaly linked to hot fluid expulsion from a deep aquifer

“…However it is also possible to accumulate hot fluid at shallow levels in the crust, if the fluid is overpressured and associated with deviations from the normal geothermal gradient. Other examples of anomalously hot fluid at shallow levels in sedimentary basins are given by Rossi et al (2000) and Parnell et al (2005). Episodic fluid expulsion and anomalous heating are both typical for overpressured deep aquifers (Roberts and Nunn, 1996).…”

Evidence for kilometre-scale Neogene exhumation driven by compressional deformation in the Irish Sea basin system

“…The timing of these episodes of compressional deformation is very similar to that identified by this study. Along the UK Atlantic margin for example, compressional deformation at c. 16 Ma contributed to the development of a widespread intra-Miocene unconformity (Stoker et al 2005 (Parnell et al 2005). The correlation between the timing of the compressional deformation and exhumation of the UK Atlantic margin and the Irish Sea basin system (e.g.…”