Heterogeneous exhumation in the Inner Moray Firth, UK North Sea: Constraints from new AFTA and seismic data

Journal of the Geological Society, Nov 2002 by Argent, J D, Stewart, S A, Green, P F, Underhill, J R

Quantitative thermal history interpretation of AFTA and VR data from these wells (following principles outlined byGreen et al. (2001a, 2001b, 2002)) is summarized in Table 2. This interpretation confirms the qualitative differences discussed above, and reveals further details of the thermal history of the sequences intersected in each well, as discussed in the following sections.

Well 12/23-1. Fission-track age and mean track length data were analysed for four samples at various depths within this well (Table 1). The AFTA results, taken as a whole, show that each sample has cooled from a higher temperature in the past. The evidence for this cooling event comes from the track length data in samples GC237-27, -28 and -29, which show a greater degree of reduction in track length than can be explained using the default thermal history (Table 2), and from fission-track age data in sample GC237-30, which show a greater degree of age reduction than can be explained by the default thermal history (Table 2 and Fig. 7).

Estimates of the magnitude of maximum palaeotemperatures and the time at which cooling from those palaeotemperatures began, derived from the AFTA data in each sample, are summarized in Table 2. Maximum palaeotemperatures increase downhole, from 65-85 deg C in sample GC237-27 to > 100 deg C in sample GC237-30. Assuming that the data from all the samples represent a common thermal event, comparison of the timing constraints from all four samples would be consistent with a single episode of cooling beginning sometime between 100 and 40 Ma, consistent with an early Tertiary event. AFTA data from a larger number of wells in the Inner Moray Firth, also analysed as part of this study (but not fully reported here), allow this timing to be refined further to the interval 60-55 Ma, showing that the sampled units began to cool from maximum postdepositional palaeotemperatures in the Early Tertiary (Fig. 9). All the samples illustrated in Fig. 9 broadly suggest an early Tertiary age for the onset of cooling within 95% confidence limits. However, by combining the data for all the samples a refined timing of 55-60 Ma for the onset of cooling is found via overlap of the sample cooling ranges. A single sample, from well 12/29-1, is excluded from this interpretation. The onset of cooling for this sample is later, sometime between 0 and 20 Ma, and is inconsistent with that derived from all other wells and outcrop locations. Whether these data are genuine, or are some kind of analytical artefact is not known at present. On this basis, we feel that the palacothermal effects recognized in well 12/29-1 are probably best interpreted in terms of the same Early Tertiary episode as recognized in the other samples. However, the possibility of a real difference in timing, with a later onset of cooling for the Banff High on which well 12/29-1 is located cannot be ruled out.

VR from downhole samples in well 12/23-1 are shown in Fig. 4. As summarized in Table 2, two VR values from the Barremian section and one from the Lower Permian section are higher than the values predicted by the respective default thermal history, suggesting that these units have been hotter in the past compared with their present-day temperatures. In contrast, VR values from the Upper Jurassic units in this well give anomalously low offset from the default thermal history compared with other samples from this well. This is probably due to the sapropel-rich character of the Upper Jurassic Kimmeridge Clay Formation, which is not considered to provide a reliable measurement of the true thermal maturity of this well. Maximum palaeotemperatures derived from the VR data in this well using the kinetic model of Burnham & Sweeney (1989) increase downhole from