INNOVATIVE APPROACH TO MAPPING SHALLOW NATURAL GAS IN QUATERNARY SEDIMENTS FOR SAFETY AND FOR PRODUCTION: NORTHWESTERN ALBERTA, CANADA

Natural gas has been produced in northwestern Alberta since the 1960s from Devonian, Mississippian and Cretaceous formations. More recently, in this region, gas has been discovered in glacial drift at depths as shallow as 30m. This shallow gas represents both a potential economic resource and a drilling safety concern. Unexpected near surface gas can produce “kicks” during drilling and in some cases blowouts, a few of which have already resulted in fire and loss of the drill rigs. The Alberta Geological Survey undertook, as a part of their surficial mapping program, to develop a method to map areas with increased potential for shallow gas.

The approach was to determine the stratigraphic setting and use this information to map the relevant geological units. Petrophysical logs from the few shallow gas wells suggest the gas reservoirs are glaciofluvial channels capped by till. Mapping of the bedrock topography, drift thickness and near surface bedrock stratigraphy began on the Zama Lake map sheet (NTS 84L); part of a project funded by the Alberta Geological Survey and the Geological Survey of Canada under the federal Northern Resource Development Program (NRD Project 4450) with additional support from the federal-provincial Targeted Geoscience Initiative (TGI-2). A 10 km test line was also set up to evaluate the applicability the shallow seismic and electrical resistivity tomography (ERT) data.

Of importance to understanding these shallow gas reservoirs is the topography of the 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, glaciofluvial and preglacial fluvial sediments. Where these paleovalleys intersect the Cretaceous Bluesky Formation, gas can 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. A combination of bedrock geology, drift thickness and bedrock topography mapping allowed the selection of areas with increased risk/potential for shallow gas. Work in adjacent areas will refine the predictive approaches.