ABSTRACT

A study of the Lower Cretaceous Bluesky Formation in west-central Alberta reveals the presence of anomalously thick Bluesky sediments in the Edson and Pine Creek field areas. Detailed core and well log examination indicates Bluesky sediments were deposited as an unconformity-bounded, estuarine, valley-fill sequence.

An unconformity at the base of the Bluesky Formation is interpreted to have formed during a fall in relative sea level following Gething deposition. During this lowstand, two northeast-trending valleys (Edson and Pine Creek field areas) incised into the underlying coastal plain and nearshore/shallow shelf sediments of the Gething Formation. During the subsequent rise in relative sea level, sediment backfilled within the valleys, and accumulated as transgressive estuarine deposits. The transgression reworked shoreface sandstones into shallow marine bars and shelf sandstones in inter-valley regions. A transgressive erosional surface and accompanying sandstone lag occurs at the

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top of the Bluesky Formation. Bluesky sediments are overlain by offshore shales of the Wilrich Formation.

Twenty-two lithofacies were identified from 54 cored wells within the study area (Twp. 51-58, Rge. 15W5-6th meridian). Nine depositional environments were interpreted from a combination of lithofacies assemblages. The valley-fill sequence comprises multiple, stacked, sand-filled estuarine channels, muddy (mixed sandstone/mudstone) estuarine point bars, sandstone shoals, and a wedge of open marine shales and siltstones. Inter-valley deposits include reworked shoreline sandstones, shallow marine bars and shelf sandstones.

The Edson and Pine Creek fields have cumulative oil and gas production to date of 1195 10³m³ (1.2 MMBO) and 7174 10⁶m³ (231 BCF), respectively. The primary reservoir units within the Bluesky Formation are thick (>30 m) estuarine channel sandstones with porosities that range from 9 to 16 per cent. The primary trapping mechanism of the Bluesky Formation within the valley-fill strata is a seaward pinchout of channel sandstones updip into tight marine mudstones, siltstones, and low permeability, muddy, estuarine point-bar deposits.

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