New Carboniferous potential seen in Cumberland subbasin, Canada

Aug. 5, 2002
Considerable exploratory interest has come to focus on eastern Canada's Maritimes basin.

Considerable exploratory interest has come to focus on eastern Canada's Maritimes basin. This basin defines a large and complex tectonic depression of Late Paleozoic age, encompassing most of eastern New Brunswick, northern Nova Scotia, Prince Edward Island, and the offshore area that separates these provinces from Newfoundland (Fig. 1).

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Until recently, the entire onshore portion of the Maritimes basin, including some 40,000 sq km, had only one hydrocarbon-producing area-Stoney Creek field, discovered in 1909-and had attracted only sporadic, localized exploratory drilling.

Several factors have kindled new interest in the basin.

The completion in 1999 of the Maritimes & Northeast Pipeline, a major infrastructure highway to eastern Canada and the northeastern US, is one such factor. This pipeline has brought on-line a series of gas field discoveries in the shallow offshore near Sable Island, southeast of mainland Nova Scotia.

In addition, there are two new onshore gas discoveries in the Maritimes basin. One of these areas, opened in 1999, is 10 km south of Stoney Creek field and reportedly has two successful wells. The other is McCully field, roughly 45 km southwest of Stoney Creek, with six successful wells to date and estimated reserves on the order of 5-10 bcf/well.

Data from these new areas, coupled with re-evaluation of existing information, suggests a high level of prospectivity in other portions of the Maritimes basin as well. In particular, the Cumberland subbasin of Nova Scotia, a largely overlooked portion of the province, now appears to hold significant potential.

Tectonic setting

The Maritimes basin represents a collection of fault-bounded successor basins and intervening horsts that developed within an intracontinental rift system.

Recent studies suggest that these subbasins were created by a combination of transextensional and transpressional stresses generated within a major, regional strike-slip (transform?) deformational zone.1 Initial subsidence occurred in the Late Devonian, subsequent to the final phases of compression associated with the Acadian Orogeny.

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Most of the deeper subbasins, such as the Moncton, Cumberland, Windsor, and Magdalene depocenters, are bounded by faults with a combination of strike-slip and vertical offset. The complex pattern of horsts and grabens suggests significant lateral displacement of basement blocks (Fig. 2).

Subbasins are filled with up to 8 km of Late Devonian to Early Permian strata. Strata are dominated by terrigenous clastic sediments, deposited mainly under arid and semi-arid conditions in the Mississippian and semihumid conditions in the Pennsylvanian, with subordinate evaporite, marine, and volcanic material.

These strata indicate several major periods of deformation subsequent to the initial rifting event. Mid-Mississippian folding and faulting caused widespread uplift and erosion. Late Mississippian-Early Pennsylvanian compression affected all older deposits and was accompanied by salt movement. Late Carboniferous tectonism associated with the Allegheny Orogeny was mild, causing local strike-slip displacement.

Maximum burial was achieved in the Early Permian, followed by regional uplift and erosion that removed 1.5-2 km of Permian or younger sediments. Salt features, including salt anticlines and piercement structures, define important structural elements.

Stratigraphy

Formation names in the Maritimes stratigraphic succession have been generalized from the Cumberland subbasin, as nomenclature, thicknesses, and relationships all vary among the different subbasins (Fig. 3).

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The greatest amount of well data is concentrated in the Moncton subbasin, where Stoney Creek and the two new fields exist. Only four significant (deep) wells have been drilled in the Cumberland subbasin; none penetrated below Mid-Mississippian (Windsor Group).2

Basal strata are included within the Late Devonian Fountain Group, a succession of interbedded volcanic (rhyolite and basalt) and terrestrial clastic material from 300 m to 2,700 m thick, where present. These strata are overlain unconformably by conglomerates, sandstones, siltstones, limestones, and kerogenous mudstones of the Horton Group, indicative of alluvial fan, braided stream, and lacustrine settings. Fluvial and lacustrine-deltaic sandstones in this interval have good-to-excellent reservoir quality and form the major reservoirs at Stoney Creek and McCully fields.

Early-mid Mississippian deposits of the overlying Windsor Group mark the appearance of marine sediments. Windsor Group deposits, estimated to be up to 1,200 m thick,3 include multiple transgressive-regressive cycles consisting of evaporite (halite and anhydrite), carbonates, and fine-grained clastic material. Reef and bank facies are present in carbonate associated with the lowermost depositional cycle. Strata of this interval are mostly conformable above the Horton Group and are variably deformed, in part due to salt movement.

Overlying Late Mississippian-Early Permian sediments include mainly fluvial and floodplain mudstones, siltstones, fine-medium grained sandstones, and conglomerates of the Mabou, Pictou, and Cumberland Groups. Total thicknesses of these combined intervals range up to 5,000 m. Coals found in the Cumberland Group have been commercially mined in several areas.

Source rocks

Geochemical analysis of well and outcrop samples in the vicinity of Stoney Creek and McCully fields indicate petroleum source rocks exist in carbonates of the lowermost Windsor Group and in shale-dominated sections throughout the Horton Group.

Thick (100-150 m) kerogenous mudstones of the Albert formation (middle portion of Horton Group) are rich in Type I kerogen, primarily alginite, and are best considered oil shales.4 It has been estimated that an in situ reserve of 270 million bbl exists in these shales to a depth of 600 m in the Albert Mines deposit of the Moncton subbasin.5 Fine-grained carbonates and mudstones near the base of the Windsor Group are regionally observed to also be kerogen-rich and petroliferous.6

Both these intervals are interpreted to be present in the Cumberland subbasin. Regional relationships, structural history, and seismic mapping together suggest that the Horton Group is present at thicknesses of up to 2,000 m. Other than the Horton and Windsor Group shales, there exists an abundance of kerogen-bearing shale and mudstone intervals throughout the Mississippian and lower Pennsylvanian sections. Sample analysis suggests a dominance of Type-III, humic kerogen in these terrestrial rocks.

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Burial history modeling, based on samples from two wells in the Cumberland subbasin (Pacific Fox Harbour 1 and Sunoco Nappan 1A) indicates that the majority of this section (Horton, Windsor, Mabou Groups) lies within the zone of late mature oil generation and main gas generation (Fig. 4).

Fig. 4, which shows data for the Sunoco Nappan 1A, reveals that maximum burial was achieved in the Early Permian and that the source rock section of the lower Windsor and middle Horton Groups should have generated significant gas. Vitrinite reflectance values of >1.0 are reached near the base of the Mabou or within the upper Windsor Group. Horton Group source rocks should therefore be within the window of main gas generation throughout the Cumberland basin.

Reservoirs

The most prospective reservoirs in the Cumberland subbasin are interpreted to exist in fluvial and marginal lacustrine (strand and delta) sandstones of the middle Horton Group. These sands produce at Stoney Creek and McCully fields in a combination structural-stratigraphic trap, with seals provided by stratigraphically proximal shales and overlying evaporites of the Windsor Group.

The productive sandstones consist of fine-to-medium grained units 5-30 m thick interbedded with kerogenous mudstone and siltstone layers. As many as 30 separate sandstone units are productive in single wells at Stoney Creek. These units exhibit porosities of 12-20% and permeabilities of 10-80 md.

Detailed studies suggest that a significant portion of the porosity is due to dissolution of feldspar grains, carbonate material, and clays.7 Unstimulated gas flow rates of up to 18 MMcfd have been observed from these reservoirs in Stoney Creek field.

Subbasin structure

Newly revised interpretations of structure in the Cumberland basin, and the identification of quality exploratory leads, is largely based on reprocessed 2D seismic data.

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These data include a network of lines originally shot in the 1970s by Anschutz and in the 1980s by Chevron. Reprocessing has improved data quality considerably and has been especially important in the delineation of deep-seated faults and salt-related structures in the subbasin.

Fig. 5 shows that the Cumberland subbasin is bounded by faulted blocks of basement, the Cobequid Highlands inlier to the south and the Caledonia Highlands inlier to the northwest. It is separated from the Sackville subbasin to the north by a prominent buried uplift known as the Hastings horst.

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Surface mapping within the Cumberland subbasin has indicated two major anticlinal features, the Minudie and Claremont anticlines, formed by compression along several east-west trending wrench faults. These faults extend to basement and control the location of salt diapirs and tectonic thickening within Windsor Group evaporites. The Claremont fold, in particular, is associated with a deep-seated reverse fault, indicated on the colored time-structure map (Fig. 6).

The map also shows the locations of three wells, the Gulf et al. Hastings 1, the Pacific Fox Harbour 1, and the Anschutz Wallace Station 1.

The Hastings 1 was drilled directly into the crest of the Hastings horst, where basal sediments include the uppermost portion of the Mabou Group, the Windsor and Horton intervals having been eroded or never deposited.

The Pacific Fox Harbour well reached TD 3,004 m (9,853 ft) in the lower-middle Windsor. The Wallace Station well was drilled on the Claremont anticline, penetrated a thickened salt-evaporite section and reached total depth in red bed deposits of the lower Windsor Group.

None of these wells, therefore, penetrated the prospective Horton section.

A reprocessed, north-south seismic line (Fig. 7, location shown as red line on Fig. 6), reveals that the Claremont anticline is the shallow expression of a large, deep-seated fold forming a subsalt trap within the Horton Group. The anticline is associated with a north-dipping thrust fault that soles out within or along the base of the salt. The deeper fold in the Horton may be related to a basement reverse fault interpreted as the "Deep Thrust Fault" in Fig. 6.

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Fig. 7 shows thinning within the Cumberland Group along the crest of the structure that suggests the fold was initiated after Mabou deposition. Comparison with Fig. 4 above confirms that maximum burial and hydrocarbon migration occurred after this structure had already formed.

Major four-way closure on the subsalt structure is indicated by the depth-structure map contoured on the interpreted base of the Windsor Group (Fig. 8). This closure exists roughly 3 km east of the seismic line of Fig. 7 and culminates at a depth of 3,800 m (12,500 ft). The structure covers approximately 4,000 ha, compared with the 600 ha of Stoney Creek field in the Moncton subbasin.

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Stoney Creek has a hydrocarbon column of 335 m (1,100 ft). Assuming similar reservoir quality and water saturations, recoverable reserves for the structure shown in Fig. 8, with 30 m of pay, would be on the order of 650 bcf.

Overview

Recent gas discoveries in the Maritimes basin highlight the significant potential that exists in this province. In particular, large, undrilled structures exist in the Horton Group of the Cumberland subbasin, Nova Scotia.

Reprocessed seismic data, well information, geochemical analyses, and subsurface studies together support the identification of such subsalt structures as highly prospective in the southern portion of the province, at depths of 3,800-4,100 m (12,500-13,500 ft).

All the necessary ingredients for a major accumulation are present in this area, including documented source rocks (lower Windsor carbonates and Horton shales), effective seals (Horton shales, Windsor evaporites), good quality reservoir rock (marginal lacustrine sandstones), migration pathways (faults), and structure (subsalt four-way closure).

A well sited on the crest of the noted Claremont structure, targeted on the deeper closure in Horton Group strata, should have an excellent chance of encountering major gas reserves.

Acknowledgments

The authors thank geoscientists with the Nova Scotia Petroleum Directorate and the Nova Scotia Department of Natural Resources for important groundwork studies, without which this article could not have been written. In particular, we thank Jack McDonald and Paul Harvey of the Petroleum Directorate and Bob Ryan, Bob Boehner, and John Calder of the DNR.

References

  1. Ryan, R.J., and Boehner, R.C., "Geology of the Cumberland Basin, Cumberland, Colchester, and Pictou Counties," Nova Scotia Department of Natural Resources, Mines and Energy Branches, Memoir 10, 1994, 222 p.
  2. St. Peter, C., "Maritimes Basin Evolution: key geologic and seismic evidence from the Moncton Subbasin of New Brunswick," Atlantic Geology, Vol. 29, 1994, pp. 233-270.
  3. Boehner, R.C., "Salt and potash resources in Nova Scotia," Nova Scotia Department of Natural Resources, Mines and Energy Branches, Bull. 5, 1986, 342 p.
  4. Chowdhury, A.H., Fowler, M.G., and J.P.A. Noble, "Petroleum geochemistry and geology of the Albert Formation, Moncton Subbasin, New Brunswick, Canada," Bull. of Canadian Petroleum Geology, Vol. 39, No. 4, 1991, pp. 315-331.
  5. See review of related information in S.L. Foley, 1989, "Geology of the Stoney Creek Oil and Gas Field, and its implications regarding the tectonic evolution of the eastern Moncton Subbasin, New Brunswick," New Brunswick Department of Natural Resources and Energy, Minerals and Energy Division, Geoscience Report 89-1, 77 p.
  6. Mukhopadhyay, P.K., "Source rock potential and maturation of Paleozoic sediments (Devonian-Carboniferous) from onshore Nova Scotia," Nova Scotia Department of Natural Resources, Mines and Energy Branches, Open-File Report 91-012, 1991, 186 p.
  7. Chowdhury, A., and Noble, J.P.A., "Provenance, diagenesis, and geochemistry of the Albert Formation, eastern New Brunswick, "Geological Survey of Canada, Open File Report 1997, 1989, 129 p.

The authors

Scott L. Montgomery ([email protected]) is a Seattle petroleum consultant and author. He is lead author of the "E&P Notes" series in the AAPG Bulletin. He holds a BA in English from Knox College and an MS degree in geological sciences from Cornell University.

John Sinclair is the exploration manager for AMVEST Oil & Gas, a private E&P company in Charlottesville, Va. He has 11 years' experience in the petroleum industry, with expertise in coalbed methane and tight gas sands exploration and development. He holds a BS and MS in geology from the University of North Carolina at Chapel Hill.

William G. Shaw is founder and principal of W.G. Shaw & Associates Ltd., consulting geoscientists, in Antigonish, Nova Scotia. Before its founding in 1999, Shaw worked for multinational corporations, small private companies, and government departments in the petroleum, base and precious metals, and industrial minerals sectors.