The main Wellington Seam (No. 1 seam) occurs in the Early Campanian Northfield Member at the base of the Extension Formation (Nanaimo Group) and generally dips northeast to east and strikes northwest. The seam is underlain by sandstone and overlain by sandy shale, sandstone or conglomerate. Two sets of faults striking north-northeast and west-northwest are prominent in the area. The largest continuous area of the Wellington Seam is in the Wellington Field where the Wellington Mines (Wellington Mine No. 1 Slope, Wellington No. 9 Mine, Northfield Mine and Wellington Colliery Shafts No. 1 to No. 6 (092GSW048) are located. Refer to the Bebans mine (092GSW026) for an explanation on the Wellington Seam in the Nanaimo Coalfield.

The seam occasionally shows sharp rolls and thickness varies from 0.9 metres to 3.0 metres. The great variability in seam thickness results from minor folds, faults and shale bands in the roof, while the floor is fairly regular. The seam consists of very finely striated humic coal with a semi-bright appearance (dull clarain with few vitrain bands or stringers) of high volatile bituminous rank.

Three upper seams are present in the Wellington No. 1 area and include the Wellington No. 2 or Little Wellington, Wellington No. 3 and Wellington No. 4. These seams are normally less than 0.6 metres thick and occur at intervals of 10.7 metres, 18.3 metres and 22.9 metres respectively above the Wellington No. 1 seam. The Wellington No. 9 mine exploits the Little Wellington seam over some of the Wellington mine workings.

Production from the North Wellington mines is included with Wellington (092GSW048).