Bergquist, Christopher L.¹, Peter P Graham¹, Dennis H Johnston¹
(1) Devon Canada Corporation, Calgary, AB

ABSTRACT: Mackenzie Delta: Mobile Shale System and Exploration Implications

A shale diapir in recent 3D seismic provides evidence of a mobile shale system in the Beaufort Mackenzie Basin (BMB). The diapir is interpreted as having initiated as a shale swell in the hanging wall of large, linked growth and counter-regional faults within wrench-related pull-apart basins, then pinched off in the core of a large detachment fold that formed with the SW-NE compression that inverted the sedimentary succession.
Elsewhere mobile shale occurs as shale swells below “roller-type” growth faults or as vertical chimneys. Typically the mobile shale lacks seismic reflectivity, having a ‘wormy’ appearance, with upturned beds and gas ‘flags’ when associated with vertical chimneys along normal faults. At the surface are sea-bottom mounds and collapse calderas seen on sidescan sonar and features labeled as PLF’s (pingo-like features) on bathymetric charts.
Sands adjacent to mobile shales are highly overpressured where penetrated by wells, up to twice-normal hydrostatic pressure. The overpressures have implications for exploration in the BMB, which include shallow gas hazards and well design. More important are enhanced hydrocarbon columns where sealed by overpressures. The two largest Tertiary discoveries in the BMB - the 3TCF Taglu and 3.5TCFe Amauligak fields - also have the highest hydrocarbon columns and drill-proven overpressure seals. 3D structural mapping and AVO analyses of the Paktoa prospect flanking the Paktaq diapir indicate even larger hydrocarbon columns and recoverable resources may be present there.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.