Petroleum System

Source Rocks -The known source rock in the prospect area is the Green Point shale that has been credited for the oil produced by the Port au Port #1 discovery. This shale has TOCs of 8-10% where it is seen in outcrop. The shales are up to 50 metres in total thickness.

Fig. 2-5 - Green Point Formation Shale Source Rock. Photo Courtesy of Larry Hicks

Oil generation may have begun as early as the late Ordovician, but most date the start of oil generation to the Silurian. It may have continued to the mid-Carboniferous, probably in episodes during the end of the Taconian orogeny through to the Acadian orogeny, a period of some 100 million years.

Seal – The seal for the carbonate platform structural prospects are the shales and tight limestones of the Table Point Group. The tight Lourdes Limestone forms a regional seal for the updip, stratigraphic pinchout play of the carbonate platform and for the structural plays of the Humber Arm allochthon.

Fig. 2-6 - Stratigraphic Section of Anticosti Basin

It is probable that the reservoirs within the Humber Arm allochthon are also encased in shales of the platform slope facies and flysch shed into the foreland basin from the highlands. The earliest known oil generation for the basin post-dates the seal deposition in all cases.

Reservoir Development – Known reservoirs in the carbonate platform exist in the Aguathuna, Catoche and Watts Bight formations. (the St. George Group). Porosity values between 12% and 18% can be seen where primary porosity development is good. Karstification and fracturing enhance these values.

At the Port au Port #1 discovery well, karst porosity was well developed in the Aguathuna and Watts Bigh formations. Over 2200 barrels of drilling mud were lost to the Watts Bight Formation while drilling the well.

Onshore outcrops studies have demonstrated that dolomitization can be of the early, pre-burial type and of the later post-burial type induced by hydrothermal fluids. While post-burial porosity development can create reservoirs of good to excellent quality, early porosity development, in general, leads to better reservoir development and is enhanced by the later hydrothermal leeching.

Fig. 2-7 - Hydrothermal Dolomitization in St. George Group Strata at Table Point, NLPhoto Courtesy of Larry Hicks

Porosity development also results from karstification most likely the result of aerial or subtidal exposure of paleo-highs. The tilted fault blocks of the carbonate platform (figure 2-3) are good candidates for this model. The thrust sheet prospects (also figure 2.3) show evidence of erosion below their contact with the Lourdes Limestone and may also be candidates for karstification of the limestones found in the near slope facies.

Fig. 2-3 - Present Day Schematic of Western Newfoundland

Migration Pathways – The down-to-the southeast normal faults in the carbonate platform could place the source rock in the foreland basin directly against the potential reservoir sections in the upper part of the platform. Also, the regional, cross trend faults (transforms?) and fractures may act as conduits for fluid migration. Additionally, the potential source rock in the Black Cove shale (see fig. 2-6) may directly overlie potential reservoirs at the top of the platform providing both direct migration and a seal. The mapped prospects are within 10 – 20 km of the centre of the generative basin and are adjacent to or underlie the western edge of the generative basin. Thus, migration distances are short for the carbonate platform structural and stratigraphic prospects.

For the prospects involving the allochthon, the source material is either caught up in the thrust sheet or underlies it in a lower thrust sheet. Fracturing in these thrust sheets is highly likely and these would act as conduits for the migration of hydrocarbons.