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Department of Geology & Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen AB9 2UE, UK
1 PGI Consulting Ltd, 173 Deanston Drive, Glasgow G41 3JZ, UK
The thermal stress (maturity) of Oxfordian to Kimmeridgian rocks in 13 Inner Moray Firth wells and at outcrop has been assessed using sterane and hopane isomer ratios. Significant depth profile displacements between the maturity trends for the Beatrice suite, other groups of Inner Moray Firth wells and a Viking Graben calibrant well are considered to be partly due to differences in uplift during Tertiary inversion and partly to varying palaeo-thermal gradients. Present day temperature plots are used to measure cooling from biomarker-derived maximum temperatures and hence to obtain approximate estimates of inversion.
Consideration of errors in cooling estimates allows the limits of uplift to be constrained. Relative uplift between Inner Moray Firth fault blocks is likely to be subject to fewer estimation errors than absolute uplift (inversion) with respect to the Viking Graben. 12/26-1, the most uplifted well on the Central Ridge, is estimated to have experienced between 400 and 700 m of uplift relative to 12/24-1, the least uplifted in the Smith Bank Graben, although for other wells relative uplift is not always related to present-day structural elevation. Inversion (absolute uplift) of the Smith Bank Graben is probably at least 150 m although the error in this estimate is large. Inversion of well sections in the Beatrice Field is thus estimated to have been around 400 m.
