Paleozoic gas potential in the Weald Basin of southern England

Gas has been found in Mesozoic reservoirs in the Weald Basin, particularly along the northern margin. Most of the gas is dry, with a high methane content and often associated nitrogen. Isotopic evidence indicates that the gas is from a thermogenically mature marine source. Although there is evidence of some shallow, biogenic gas, only the lowermost Lias is projected to have reached the thermogenic gas window before Tertiary uplift. Estimated maturities from isotopic data from the main gas accumulations indicate significantly greater levels than those projected for Liassic shales: thus, the gas is thought to have originated from Paleozoic rocks. Data on the distribution of Paleozoic rocks subcropping the Variscan unconformity is limited. However, available data suggest that their distribution owes more to Acadian erosion than to Variscan. It is thought that the Upper Devonian and Lower Carboniferous transgressed over a thick, folded Tremadocian shale sequence in the west, and over folded Silurian and Lower–Middle Devonian rocks in the central Weald. There is some evidence for the presence of isolated late Carboniferous or early Permian clastics but no significant coals have been encountered to date. Regional source studies suggest that the only Paleozoic rocks with potential are post-Acadian-aged Devonian shales.