Tertiary uplift and inversion history in the North Celtic Sea Basin and its influence on source rock maturity

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Abstract
Tertiary uplift and basin inversion have been widely recognized in the area of the North Celtic Sea Basin offshore Ireland, but the magnitude and timing of the various events have not been well documented. An understanding of these events is important because recent basin history generally has a major influence on hydrocarbon source rock maturity levels. A multi-disciplinary approach was used to investigate the timing and nature of uplift in the North Celtic Sea Basin, involving seismic data interpretation, well log correlation, vitrinite reflectance measurement, apatite fission track analysis and a Cretaceous Chalk Group isopach and velocity study. Both regional uplift and compressive inversion can be interpreted to have occurred, based on missing geological section, compressive reactivation of earlier normal faults and basin-centred uplift. Two Tertiary erosion events are recognized: a regional uplift in the Palaeocene, and compressive inversion characterized by basin doming and fault reversal, probably associated with an Oligo-Miocene phase of the Alpine Orogeny. Separating these was a period of basinal deposition in the Eocene to Oligocene. A map of the magnitude of erosion generated as a result of the two events shows that the amount of uplift is quite variable. In the northeast part of the basin, where the later event was most severe, uplift was as much as 1100 m. Reconstruction of the basin geometry before inversion, together with thermal modelling at selected well locations, suggests that Upper Jurassic source rocks reached maturity for oil generation only in the central portion of the basin, whereas Lower Jurassic source rocks entered the gas window in much of the basin prior to Tertiary uplift.