Abstract A study group of the Alberta Society of Petroleum Geologists proposed the following classification of Lower Cretaceous strata in the Peace River area of west-central Alberta: Fort St. John group Shaftesbury formation Peace River formation Paddy sandstone member Cadotte sandstone member Harmon shale member Spirit River formation Notikewin sandstone member Falher member Wilrich member Bluesky formation Bullhead group The Shaftesbury formation is a dark marine shale. The Peace River formation contains fairly distinct units consisting of the Paddy continental sandstone containing some coaly beds, the Cadotte glauconitic marine sandstone, and the Harmon dark marine shale. The Spirit River formation is a variable succession of sandstones and shales grouped into the Notikewin sandstone, the Falher sandstones, shales, siltstones, and thin coals, some beds suggesting deltaic origin, and the Wilrich dark shales with interbedded sandstones. The Bluesky formation consists of glauconitic sandstone grading down to dark shale. The Bullhead group is a variable succession of conglomerates, sandstones, shales, and coals.

Abstract The Falher A Cycle is the youngest progradational, shoreline and nearshore clastic sequence in the Lower Cretaceous Falher Member of northwestern Alberta. In the Elmworth area of the “Deep basin,” this unit averages 25 m (82 ft) in thickness and contains vast amounts of gas reserves. The reservoir-quality shoreline sandstone and conglomerate form an east-to-west trending, 25- to 75-km wide (15.5- to 46.6-mi wide) belt. Gas is subnormally pressured and is trapped downdip (westward) from water. The eastern part of the “Deep basin” gas line, which marks the transition from downdip gas to updip water, trends northeast-to-southwest and cuts across the east-west oriented belt of shoreline sandstone and conglomerate. The position of this line is believed to have been controlled by a major distributary channel system, oriented in the same manner, which serves as an eastern updip seal. The northern part of the “Deep basin” gas line, which has an approximate east-west orientation, occurs at the facies transition between shoreline, reservoir-quality sandstone and conglomerate to the south, and nearshore and offshore, finer-grained and less permeable sandstone, siltstone and shale to the north. This transition acts as a northern updip seal. It is therefore concluded that the trapping mechanism of gas in the Falher A is facies controlled. Gas generated from coal in deeper parts of the basin migrates northeastward (updip) until it reaches the above-mentioned facies transitions where it is presently trapped. This trap is not completely tight, resulting in a slow but continuous eastward leakage of gas and its accumulation updip in conventional traps. This leakage is probably responsible for the subnormal pressure of the downdip gas.