Abstract: Exploration for Structurally Controlled Areas of Enhanced Coalbed Gas Content: A Case History from Coastal British Columbia

D. Nikols, C. Cathyl-Bickford, G. Hoffman

High volatile bituminous coals of the Upper Cretaceous Nanaimo Group are extremely gassy. Gas yields from deep coal mines locally approach 140 metres per tonne of coal mined. These yields are 7 to 10 times the generative capacity of the coals. Structural and hydrodynamic traps with effective seals, have held the gas in the coal since it was generated during an Eocene thermal event.

Discovery of the gas was a by-product of early mine development. Significant gas flows were encountered in boreholes in two coalfields. In the Nanaimo coalfield, the dominant gas-bearing structures are northwest-trending folds associated with thrusts and strike-slip faults. Coals at Nanaimo are readily mapped on seismic profiles, owing to good acoustic contrast with their shaly roofs and floors. In the Comox coalfield, the dominant gas bearing structures are northwest-trending folds associated with listric extensional faults. Coals at Comox are difficult to map on seismic profiles owing to marginal acoustic contrast with nearby thick sandstones.

Between 1984 and 1986, 16 wildcat tests were drilled by two operators. Some of the wells encountered gassy coals but none were completed owing to financial constraints. Exploration in the Nanaimo Group has recently been given new impetus by construction of gas transmission lines on Vancouver Island, and another round of drilling is likely to occur in the near future.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994