What’s new in exploration

 Vol. 232 No. 10

The oil and gas industry is invited to explore Jurassic and Cretaceous deepwater reservoirs off Nova Scotia, in what the government hopes will be a succession of successful wildcats. Only one well, Shelburne G-29, has thus far tested the Jurassic to mid-Cretaceous deepwater succession on the western Scotian margin.

Although more than 120 wells have been drilled in the Scotian basin, they are concentrated in the productive Sable sub-basin, focused on rollover anticline plays. Several of the Sable Offshore Energy Project (SOEP) shallow-water fields-—Alma and North Triumph—are considered shelf margin delta complexes at the top of the Lower Cretaceous, with high in-place gas reserves (>500 Bcf) in excellent quality reservoirs with high flow rates. Regional specialists suspect that a continuation of this shelf margin delta play may exist in the deeper water of the Scotian slope.

The Canada-Nova Scotia Offshore Petroleum Board (C-NSOPB) is the independent joint agency of the Governments of Canada and Nova Scotia responsible for the regulation of petroleum activities in the Nova Scotia offshore area. The agency recently issued a call for bids on eight offshore blocks. The NS11-1 bid round includes four industry nominated and four non-industry nominated deepwater parcels on a largely unexplored part of the Scotian margin.

Scotian margin. According to a play fairway analysis (PFA) issued by the OETR Association earlier this year, there is strong evidence for Early Jurassic restricted marine oil-prone source rock along the southwestern Scotian margin beneath these parcels. The areas offered for lease are located above a major salt basin with numerous large untested salt-related structural traps (turtles, salt flank, folds, rotated blocks). This autochthonous salt basin underlies the three main provinces of the west Scotian shelf: LaHave Platform, Slope Detachment, and Allochthonous Salt and Minibasin.

There are two natural gas pipelines on the shelf operated by the Sable Offshore Energy Project, including a 26-in. pipeline with a capacity of 600 MMscfd and the EnCana Deep Panuke pipeline with a capacity of 300 MMscfd.

Prior drilling. C-NSOPB records show that few wells have been drilled near any of the NS11-1 parcels. Shubenacadie H-100 (which bottoms in Cenomanian strata on the slope) is located just inside Parcel 6. It was spudded by Shell in 1982 and was the first deepwater test off Nova Scotia. The primary target was interpreted as a Lower Tertiary turbidite fan and the secondary target was a Miocene bright spot believed to be a direct hydrocarbon indicator. The well did not encounter any hydrocarbons and the primary target was later determined to be an upper slope erosional remnant composed predominantly of chalks and marls.

The Albatross B-13 is in the northern part of Parcel 7 and reached late Middle Jurassic strata of the Abenaki formation at TD. The primary target was the Jurassic carbonate bank edge. Scattered porosity was observed throughout the Baccaro member with partial loss of circulation. Modest mud-gas peaks were recorded at two intervals but no hydrocarbon bearing zones were encountered.¹ The trap may have been breached by the prominent Early Eocene unconformity, which cuts down to the top of the carbonate bank.

The Shelburne G-29 well, which TDs in Upper Jurassic strata on the slope, is in the northeast corner of Parcel 8. The primary target, interpreted to be an Upper Cretaceous to Lower Tertiary submarine fan, was later determined to be an erosional remnant composed of chalks and marls, similar to what was found in the Shubenacadie H-100 well. The secondary target was interpreted as a salt-cored carbonate promontory draped by Middle Jurassic oolitic shelfal limestones.² A 6-m section of Jurassic carbonates (oolitic wackestone) was penetrated at the base of the well.

No wells have been drilled on parcels 1 through 5, but Torbrook C-15 (TDs in probable Miocene strata) is located 7.3 km north of Parcel 4. The primary targets in this well were Miocene submarine fans, which were found to be dominated by siltstone and claystone.

Potential traps. All eight NS11-1 parcels are located, at least in part, above the allochthonous salt and minibasin (ASM) province on the western Scotian Slope. Most potential hydrocarbon traps here are associated in some way with structures produced by or adjacent to allochthonous salt bodies (diapirs, walls or tongues). Multiple potential drilling targets are present in Cenozoic to Jurassic age strata that can generally be grouped into four structural categories: above salt diapirs; on salt flanks and below small salt overhangs; between salt bodies associated with welded turtles; and between salt bodies associated with contraction.

Turtle-back structures include strata mounded between salt diapirs, with a flat base and a rounded crest over a thickened sedimentary section which may or may not have a salt core. The turtle structure forms through structural inversion of a primary peripheral sink when salt is withdrawn from the margins into adjacent growing diapirs.

C-NSOPB is soliciting public comments on the NS11-1 round until Dec. 20, 2012, and bids must be received by Jan. 10, 2012. For more information, visit www.cnsopb.ns.ca.  

LITERATURE CITED
1 Kidston, A.G., Brown, D.E., Smith B.M. and Altheim, B., The Upper Jurassic Abenaki Formation Offshore Nova Scotia: A Seismic and Geologic perspective, Canada-Nova Scotia Offshore Petroleum Board, Halifax, 2005, p. 165
2 Kidston, A.G., Smith, B., Brown, D.E., Makrides, C. and Altheim, B., Nova Scotia deepwater offshore post-drill analysis – 1982-2004. Canada-Nova Scotia Offshore Petroleum Board, Halifax, Nova Scotia, 2007, p. 181.