Bow Island

Bow Island Formation, Southern Alberta

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Summary:
Location: T1-14, R20W4-30W4
Strata: Bow Island
Year of Study: 1991

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Introduction

A multidisciplinary (surface geology, subsurface geology, petrography, hydrodynamics, geophysics) study of the upper Bow Island Formation in southwestern Alberta documents controls on production, and outlines new prospect fairways.

The major Bow Island reservoirs of the study area are wave-formed lower to middle shoreface deposits. The best reservoir types, highly porous and permeable conglomerates, are found near drowned river mouths. These sediments reflect shoreline progradation associated with localized tectonic activity in southwestern Alberta and northwestern Montana and Idaho during a time of fluctuating sea level.

Excellent porosity and permeability are encountered in the upper Bow Island at a variety of depths. The main postdepositional influences on porosity development are compaction, quartz overgrowth precipitation, calcite cementation and the formation of authigenic kaolinite.

The study area is divisible into two main hydrodynamic entities: a northeastern area characterized by dynamic aquifers, and a Deep Basin area in the southwest. The northeast is characterized by downdip water flow associated with updip gas traps found at structural and stratigraphic changes, and light oil pools in stratigraphic or structural traps found at the regional gas-water interface (e.g. Keho Lake at Township 11, Range 22W4M). In contrast, all porosity is hydrocarbon-charged in the southwestern Deep Basin. The dominant hydrocarbon is gas, which displays an increase in liquids content with depth, but the deepest Bow Island production is oil at Claresholm (Township 13, Range 27W4M). As the Deep Basin regime is generally underpressured, there is a high probability of formation damage within prospective Bow Island units in wells which were drilled to deeper zones.

Geophysical resolution of productive Bow Island units has historically been difficult due to difficult surface conditions and a prior emphasis on deeper (i.e. Paleozoic) targets. Amplitude-versus-offset modelling suggests that near-trace data gives the best indication of the thickest, gas-charged zones; reprocessing an example of existing data affirms this hypothesis.

Several significant producing wells and areas are present, such as Keho Lake (6?8?11?22W4M; 3.7 BCF), Blood (7-8-22W4M; 5.72 BCF), Granum 7-25-11-26W4M; 1.40 BCF) and Claresholm (7-20-13-27W4M; 269,000 bbls). A large expanse of underexplored acreage between Granum and Blood is present, and several pools with reserves of 20 to 40 BCF probably remain to be discovered within this trend. Further west, additional oil potential is present along strike with the Czar Claresholm 7-20-13-27W4M well. In the deepest part of the basin, one may encounter normally pressured or super-charged lenticular channel fills which would be similar to highly productive Cretaceous zones found in west-central Alberta at depths greater than 2400 metres.