AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

 

 

Mobile Shale in the MacKenzie Delta - The Paktoa Shale Diapir Complex

 

Gavin Elsley and Peter Graham

Devon Canada Corporation

 

 

The MacKenzie Delta is supplied with sediment from the second largest drainage basin in North America. It has been active since the earliest Tertiary and has experienced very rapid deposition of sands and organic rich shales, which has caused significant overpressuring. The effects of overpressure, combined with tectonic activity related to the Laramide Orogeny, and hydrocarbon generation have caused mobile shales to play an important role in the evolution of the western Beaufort-Mackenzie Basin.

Since oil and gas exploration began in the Beaufort-MacKenzie Basin (BMB) in the 1970s , there have been many mud volcanoes, pock marks and ‘Pingo like Features’ observed on the sea floor, as well as many interpreted shale diapirs identified on 2D reflection seismic profiles. A 3D seismic reflection survey acquired by Devon Canada in 2002 in the Paktoa area is the first in this basin to image a complete shale diapir complex. Paktoa is located in the Beaufort Sea about 100km North West of Tuktoyaktuk.

The Paktoa diapir is approximately 6km wide, up to 20km in length, and has about 4000m of present day vertical relief. It is characterized by having very little internal reflectivity, in contrast to the Taglu formation reflectors that terminate abruptly against its steeply inclined flanks (see figure 2). There are many crestal collapse features, as well as small offset faults sub-parallel to the flanks of the diapir, both of which likely formed as a result of de-watering and de-gassing of the mobile shale. The diapir was likely mainly intrusive, as there are no apparent mud volcanoes or mud flows resolvable in this seismic volume.

There are abundant seismic ‘bright spots’, which are known to correlate to shallow gas in this region. Some of these bright spots form a halo around the diapir in the immediately overlying sediments, while others are present above the crest of the diapir. The distribution of most of these shallow gas accumulations is controlled by shallow extensional faults related to movement of the diapir and fluid escape. These faults would have acted as conduits to fluid flow, but now appear to be sealing.

The source of the shale is interpreted to be the Late Cretaceous Boundary Creek Formation, which outcrops onshore in the Richardson Mountains to the South West as a dark, fissile, organic rich shale. It varies greatly in thickness throughout the western BMB, but reaches a maximum thickness of approximately 1500m in the Blow Trough immediately to the west of the Paktoa complex.

During the Eocene, Taglu deposition in the Paktoa area was influenced by extensional growth faulting, which set up a series of grabens, horst blocks and relay ramps. The greater influence of the Laramide orogeny during the Oligocene caused a large region of the BMB between the thrust front to the west and the rift margin to the East of the delta to be dominated by right lateral strike slip deformation and reactivation of many of the earlier extensional faults. In the Paktoa area (see figure 1) a releasing bend formed on one of these transtensional fault zones, which resulted in weakening of the overburden and movement up a fault plane of the Boundary Creek shale, which became the Paktoa diapir. The diapir initially moved vertically, but as it reached shallower depths and the fault tips out, it also began to move laterally along the pre-existing fault zone to the south east until it reached a restraining bend. This restraining bend formed the Paktoa/Tiggak complex, which are a pair of three way dip closures that terminate against the flanks of the diapir. Figure 1 shows the main structural elements of the Paktoa complex.

In January 2006 Devon Canada will commence drilling the first well in the Beaufort Sea in fifteen years. This well will test the Kugmallit and Taglu sands that overlay and flank the Paktoa diapir respectively.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1. Top Taglu Structure map showing the relationship between the present-day compressional ridge and extensional faulting to the strain ellipse for a right lateral pull-apart basin. The approximate positions of early extensional faults are also shown.

Figure 2. Seismic line through the Paktoa Shale diapir.