Proterozoic Outcrop

Geological Field Study of Middle Proterozoic Platformal Carbonates for Hydrocarbon Reserve Potential, Alaska Highway Region, Northeastern British Columbia

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Summary:
Location: Blocks 94-K
Strata: Proterozoic (Outcrop)
Year of Study: 1995

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Introduction

This study evaluates the reservoir potential of Middle Proterzoic Bathtub Group strata exposed in the Tuchodi Anticlinorium. It focuses on the thick platformal carbonate successions of the Chischa and George Formations, which have a combined thickness of more than 1300 metres, and an intervening section of Tetsa Formation shales and siltstones about 300 metres thick.

The carbonates are essentially unmetamorphosed, and show remarkable textural preservation. They consist of cryptocrystalline to microcrystalline silty limestones, dolomitic limestones, and dolomites, entirely lacking matrix porosity. Some grainstone fabrics occur, but they are not a significant element in the study area.

Clastic strata, including arkosic and subarkosic siltstones to very fine sandstones, show evidence of carbonate dissolutions by acid meteoric water. They may also have experienced secondary porosity development through solution of feldspar by hydrothermal fluids. A major tensional open fracture system is pervasively developed and contains saddle dolomites, which suggests that it was a conduit for fluids during the Antler Orogeny. There should be potential for hydrothermal karst reservoirs to be developed within the carbonate successions, but direct evidence has not been documented.

Middle Proterozoic depositions took place in a setting characterized by extensional uplift of the crystalline basement. This occurred as the result of right-lateral movements on the Hay River Fault System, which separated the Nahanni magnetic high from the Fort Simpson high. Subarkosic clastic sediments were deposited on a glacio-fluvial outwash plain. Considerable aeolian transport occurred, particularly from the distal outwash onto the carbonate flats. Thus, these strata record the interplay between a shallow peritidal carbonate platform and regressive clastic outwash plain.

Analyzing the structural history of the area, we recognize that a regional detachment occurs above the Devonian. Therefore, wells located primarily on shallow (Laramide) structures may fail to test pre-Devonian structural closures.