THE UPPER CRETACEOUS MILK RIVER AND LEA PARK FORMATIONS IN SOUTHEASTERN ALBERTA

Abstract

ABSTRACT An estimated 141.5 billion m3 (5 trillion ft3) of recoverable gas are present in the subsurface of southeastern Alberta and southwestern Saskatchewan in the Upper Cretaceous Lea Park Formation. The gas is present in a sandy shale unit for which the name Alderson Member is proposed. The Alderson Member is of shallow-marine origin and is transitional between a sandy, nearshore, foreshore, shore and backshore facies (the Eagle and Milk River Formations) and a marine, offshore shale facies (the Lea Park Formation). The Milk River and Eagle Formations respectively outcrop in southern Alberta and northern and central Montana. The Lea Park shale is widely distributed in the subsurface of central Alberta and southeastern Saskatchewan. The Alderson Member is about 85 m (280 ft) thick and consists of thinly, lenticularly interbedded, bioturbated, silty shale and laminated, in places bioturbated, very fine grained sandstone. The two lithologies form a sedimentological unit which resembles the parallel laminated to burrowed sets present in Recent sublittoral sediments. Good porosity and permeability are restricted to the thin, laminated sandstone beds that form the lower part of the parallel laminated to burrowed sets, and to thin, lenticular sandstone beds, that are scattered throughout the lower part of the Alderson Member. In the area marginal to the depositional limit of the Virgelle Member of the Milk River Formation, the laminated sandstone beds in the upper part of the Alderson Member increase in number and thickness and comprise up to 30 or 40 per cent of the interval. However, the sandstone beds become progressively more bioturbated toward the top of the member and permeability decreases. Discontinuous zones of partly silicified siderite nodules, pebbles and bentonitic shale beds are present in the Alderson Member. The southeastern Alberta Milk River Gas Pool and two gas pools in Saskatchewan produce from the Alderson Member on the flanks and crest of the Bow Island Arch. The gas pools occur downdip from a freshwater aquifer (the Virgelle sandstone) which is connected with the present-day Milk River.