Joint inversion of temperature, vitrinite reflectance and fission tracks in apatite with examples from the eastern North Sea area

Abstract

As sediment accumulation indicates basin subsidence, erosion often is understood as tectonic uplift, but the amplitude and timing may be difficult to determine because the sedimentary record is missing. Quantification of erosion therefore requires indirect evidence, for example thermal indicators such as temperature, vitrinite reflectance and fission tracks in apatite. However, as always, the types and quality of data and the choice of models are important to the results. For example, considering only the thermal evolution of the sedimentary section discards the thermal time constant of the lithosphere and essentially ignores the temporal continuity of the thermal structure. Furthermore, the types and density of thermal indicators deter-| 441 EAGE NIELSEN and BALLING

Joint inversion of temperature, vitrinite reflectance and fission tracks in apatite with examples from the eastern North Sea area—response to discussion

“…The observed temperature gradient from top to bottom is close to 30°C/km, and below the homogeneous chalk section, it is close to 40°C/km. The observations clearly demonstrate temperature profiles governed by thermal conduction, thermal conductivity variations, steady background heat flow and a transient Late Quaternary surface temperature (Fuchs et al, 2020; Nielsen & Balling, 2022). Also shown is the linear present‐day and Late Tertiary temperature profiles of Green (2002) and hence of Japsen et al (2007), which, with a temperature gradient of 24°C/km and surface temperatures of 4°C and 10°C, respectively, predict much too cold fission track samples in the past.…”

“…Our analysis suggests that general thermal models, which include the observed temperature gradient rather than ad hoc helping hypothesis, are a better approach in this form of analysis. Japsen and Green (2023) question the results of our thermal modelling of four wells in the eastern North Sea area presented in Nielsen and Balling (2022) and focuses on one particular conclusion that our modelling suggests no evidence for extensive Neogene erosion. This contradicts the conclusion from Japsen et al (2007) and a previous compaction study (Japsen, 1998) suggesting extensive erosion.…”

“…North Sea area-response to discussion Japsen and Green (2023) question the results of our thermal modelling of some wells that they have also studied (Japsen et al, 2007). Although both studies use the same palaeothermal indicators, the respective thermal analysis methods are very different.…”

“…Green (2002) and hence Japsen et al (2007) used the model of Sweeney and Burnham (1990), which for many years was a so-called industry standard. However, also industry standards must adjust in the face of data (Suggate, 1998), and we use Nielsen et al (2015), which is calibrated and tested on many wells, and which incidentally is quite similar to some of the latest easy family members (Burnham, 2019; figure 2 in Nielsen & Balling, 2022). In the immature to early mature range relevant to the present wells (%R o ≤ 0.73), the improved vitrinite reflectance models require higher temperatures to achieve a certain level of maturity.…”

“…shows the measured present-day borehole temperatures of Aars-1 and Farsø-1(Nielsen & Balling, 2022) and the depth of apatite fission track samples (circles) plotted on the temperature profiles. The observed temperature gradient from top to bottom is close to 30°C/km, and below the homogeneous chalk section, it is close to 40°C/km.…”