Chalk porosity and Tertiary uplift, Western Approaches Trough, SW UK and NW French continental shelves

Abstract

Porosities have been derived from sonic logs for the Cenomanian–Maastrichtian Chalk of 22 hydrocarbon exploration wells in the Western Approaches Trough. Although the burial depth of the Chalk studied ranges from 40–1500 m below sea-bed, porosities are all within the range 0–45%. The most deeply buried Chalk sequences exhibit close to normal porosity/depth relations for offshore chalks. Abnormally low porosities in the remainder of the wells result from burial at greater depth than that presently observed. Low porosities were preserved during Tertiary, Alpine-related uplift of the area. For a given well the magnitude of uplift not reversed by subsequent subsidence (apparent uplift) equals the displacement of its porosity/depth relation from the normal relation along the depth axis. Apparent uplift in the Western Approaches Trough ranges from 0–1.3 km. In the Western Approaches Trough, Chalk porosity data provide evidence of more widespread Tertiary uplift than do seismic reflection records. Alpine-related compression and uplift (fault-bounded basin inversion) is apparent only on seismic records in the southern part of the Trough (Brittany and South-West Channel Basins), whereas Chalk porosity data suggest regional uplift occurred throughout the Trough and adjacent highs. Chalk porosity data were also used to quantify Tertiary uplift in the Cleethorpes-l well of the East Midlands shelf. The resultant value of 1.1km is considerably greater than that previously derived from shale sonic velocities (approximately 0.3 km) and slightly less than that based on apatite fission track analysis (1.3–1.7 km).