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The Milk River Formation (Eagle Equivalent) in Alberta and Saskatchewan 1: General Production Characteristics, Lithology, and Facies

O’Connell, Shaun1 (1) Belfield Resources Inc, Calgary, AB

 

The giant gas reserves contained within the Santonian to Campanian Milk River Formation (Eagle Formation equivalent) constitute the largest Canadian gas fields ever discovered. Reserve estimates vary widely, but an initial in-place gas volume of 20TCF appears reasonable. The producing fields cover an area of some 35,000 km2, with over 30,000 producing wells. These fields have been producing gas for over 120 years, yet despite the maturity of the play trend there have been several recent major gas discoveries, and significant upward revisions of existing gas reserves.

 

Milk River reservoirs are characterized by their shallow depths, low permeability clay-rich sands, and immature, locally-generated biogenic gas. There is a large proportion of smectitic clay within reservoir sands and silts, which has given rise to unconventional production characteristics and log signatures. The clay component reduces permeability, decreases resistivity, increases ineffective porosity, and contains large volumes of bound water. This has resulted in: low production rates; difficulty in interpreting logs; difficulty in predicting gas and water production; and difficulty in estimating reserves.

 

The low permeability Milk River gas reservoirs are hosted within the Alderson Member of the Milk River Formation. The Alderson Member consists of very fine-grained sandstone, siltstone, and mudstone, and is up to 250m thick in Alberta and Saskatchewan. Alderson Member sands and silts were deposited in proximal offshore marine settings dominated by low-energy wave currents, generated by distal storm events. Depositional units are extensive and laterally gradational with no pronounced vertical cyclicity. These sediments were derived from a series of deltas that were present along regional shorelines to the south and the west.

 

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana