Elkton Reservoir of Edson Gas Field, Alberta

Abstract

ABSTRACT The Edson gas field of west-central Alberta comprises two separate reservoirs, a Cretaceous Bluesky pool, and a much larger Elkton pool in the Mississippian Turner Valley formation. This paper deals with the latter. The Elkton pool has an area of about 120,000 acres and reserves of gas in excess of 2 trillion cu. ft. Average net pay is 22.5 ft., average porosity 11.5 per cent, and depths to the pay zone vary from 8,600 to 10,400 ft. Condensates and sulphur are extracted from the gas stream at the Edson gas plant. The Elkton reservoir at Edson is one of several genetically similar entrapments in Alberta, including Brazeau River, Sundre, Harmattan-Elkton, Westward Ho, Carstairs, Crossfield, and East Calgary. These gas and/or oil pools are trapped in the extreme up-dip portions of tilted, erosionally truncated beds of the Elkton Member. Jurassic shale which directly overlies a mature eroded surface forms the cap-rock at Edson. The reservoir rock consists mainly of light grey, vuggy, very fine-crystalline dolostone. This commonly grades vertically into less porous, microcrystalline dolostone, which in turn grades into impermeable, argillaceous dolostone. Pelletoidal grains and fossil fragments comprise the carbonate grains still evident in the dolostones. Porosity consists of vugs formed by leaching of skeletal grains, notably crinoid fragments, intercrystalline void-space, and open fissures. The processes of leaching and dolomitization are believed to have affected the calcareous sediments soon after they were deposited. The excess magnesium ions required for dolomitization may have been derived from dense brines which formed on supratidal mud-flats (sebkhas), and migrated laterally and downwards via seepage refluxion.