^PSPaleovalleys Revealed by Bedrock Topography and Drift Thickness Mapping Show Potential for Shallow Gas, Northwestern Alberta, Canada*

By

John G. Pawlowicz¹, Tami J. Nicoll¹, Mark M. Fenton¹, Jawwad Ahmad², Douglas R. Schmitt², Dean Rokosh¹, and Alain Plouffe³

Search and Discovery Article #10086 (2005)

Posted August 8, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.

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¹Alberta Geological Survey, Alberta Energy and Utilities Board, 4^th Floor, 4999 – 98 Avenue, Edmonton, AB T6B 2X3 ([email protected])

²Department of Physics, University of Alberta, Edmonton, AB

³Geological Survey of Canada, Natural Resources Canada, Ottawa ON

Abstract

The Alberta Geological Survey (AGS) and the Geological Survey of Canada (GSC) have been conducting bedrock topography and drift thickness mapping studies in northwestern Alberta as part of a four-year collaborative, multi-disciplinary project created under federal Northern Resource Development Program (NRD Project 4450) with additional support from the federal-provincial Targeted Geoscience Initiative (TGI-2). Additional Quaternary stratigraphic studies are being conducted in the Zama Lake area (NTS 84L) to characterise the drift for shallow gas potential through the acquisition of shallow high resolution seismic data collected by the University of Alberta and electrical resistivity tomography (ERT) data from a 10 km line over a deeply-buried paleovalley.

Natural gas has been produced in northwestern Alberta since the 1960s from Devonian, Mississippian and Cretaceous formations. Recently, economic quantities of shallow gas have been identified in glaciofluvial channels within the drift overlying the bedrock. Of importance to understanding these shallow gas reservoirs is the topography of bedrock surface and, in particular, the location of Quaternary paleovalleys deeply incised into the bedrock. Some of these buried valleys are infilled with more than 300 m of drift, consisting of interbeds of till, glaciolacustrine sediments, glaciofluvial sediments and preglacial fluvial sediments. Where these paleovalleys intersect the Cretaceous Bluesky Formation, gas may migrate upwards into the channel-fill sediments within the drift. Extensive layers of clay-rich till and/or glaciolacustrine clay deposited from multiple glaciations provide adequate seals to trap the gas.

Preliminary interpretation of shallow seismic reflection and refraction data from a 10 km line shows the presence of a buried valley to depths of over 300 m. Dipping reflectors and a thick region with low velocities overlying higher velocity rock indicate paleovalley incision through the entire Cretaceous sequence, including the Bluesky Formation, to the Devonian Wabamun Formation. Stacked channels and possible gas-bearing ‘bright spots’ within the drift appear to correspond to gas producing zones in nearby wells. Preliminary results from an electrical resistivity tomography (ERT) survey over the same 10 km line shows variations in electrical properties that also suggest a broad deep valley buried by more resistive drift sediments in contrast with the less resistive Cretaceous shale bedrock. A well defined zone of anomalously high resistivity suggests the presence of gas at depths of between 50 and 100 m. More detailed results of the geophysical surveys are presented in a second AAPG poster by Ahmad et al. entitled ‘Seismic Imaging of Quaternary Channels for Shallow Gas at Rainbow Lake, Northwest Alberta’ in the session ‘New Advances in Seismic Sequence Stratigraphy Analysis.’

Surface relief of NW Alberta and NE British Columbia.

Drift Thickness, Zama Lake area