HERE ARE GEOLOGY, POTENTIAL OF ENGLAND'S WINDSOR ANTICLINE

Desmond H. Oswald
Canuk Exploration Ltd.
Buckinghamshire, U.K.

The Windsor anticline is the northernmost structure in the Weald basin. In southeastern England there are two intracratonic Mesozoic basins-Wessex and Weald. The distinction between them is formed by a structural arch resulting in slight differences in their stratigraphic successions.

They developed in Permo-Triassic time in an area of Variscan extensional tectonics and, in Mesozoic time, further extensional tectonics produced a series of grabens and horst block highs. Mid-Tertiary tectonics generated the Weald anticlinorium and complex folds in southern Wessex, some of which have been drilled.

The Wessex and Weald are the sites of the main producing basins in onshore Britain, the former containing Wytch Farm oilfield, producing from Triassic and Jurassic reservoirs with an estimated 350 million bbl of oil recoverable. A number of smaller oilfields produce from the Middle and Upper Jurassic of the Weald basin (Fig. 1).

STRUCTURAL HISTORY

Maximum fault-driven subsidence occurred in late Jurassic and early Cretaceous time.

An interplay between the global fall of sea level and gentle folding at the end of the Cretaceous resulted in a widespread pre-Tertiary unconformity. The basin was inverted in the mid-Tertiary Alpine orogeny resulting in major structural closures and redistribution of hydrocarbons from earlier formed traps into newly formed structures. Windsor anticline is one such structure.

Windsor anticline is a large surface structure exposing the Upper Cretaceous chalk formation within the surrounding Tertiaries of the London syncline. A seismic survey of the structure recognized the anticline as an inversion structure with a horst block typical of the structural style identified further south in the Weald basin.

Regional geological studies indicate that the Middle Jurassic Great Oolite formation, the main producing formation in the Weald basin, would be at subcrop in the Windsor area. The Lower Cretaceous (Aptian-Albian) Lower Greensand formation onlaps the eroded Jurassic to rest on the Devonian of the London platform (Fig. 2).

SOURCE

Source rocks of predominantly Type II kerogen were deposited in Lower and Upper Jurassic time.

The Lower Jurassic (Lower Lias) source rocks entered the oil window in the early Cretaceous and reached the peak of oil generation in mid-Cretaceous time in the southern part of the Weald basin. Reconstructed isopachs based on Sellwood et al. 1986 suggest that while the Lias may be fully mature in the central Weald basin, and possibly the Upper Jurassic Oxford clay, the Kimmeridge clay could not be a significant contributor to oil generation.

RESERVOIR, SEAL

The Lower Greensand is a highly porous and permeable aquifer in the London syncline and would form the main reservoir in the Windsor area, where it reaches a thickness of more than 130 ft in a water well drilled at Slough less than 3 miles north of the Windsor anticline.

The Lower Greensand is capped by the Gault clay, a shale formation some 260 ft thick, which forms a very effective caprock. It is overlain by the Upper Greensand, a chalky sandstone that passes up into the chalk formation, the outcropping formation on which Windsor Castle is built.

The seismic structure on the Base Cretaceous (Fig. 3) illustrates the horst produced by the inversion of an earlier graben. The closure could be picked as low as the 300 contour in fault blocks north of the Windsor fault, which would give a closure of more than 4,000 acres, but if taken overall at a conservative 270 contour the closure would exceed 2,000 acres.

Considering the highly porous and permeable character of the Lower Greensand, with a thickness of up to 130 ft, an accumulation of 100 million bbl recoverable would be a possibility. Additional reserves might be present within the Great Oolite formation and the underlying Paleozoic.

Depth to the top of Lower Greensand at the crest of the structure would be less than 1,000 ft.

CONCLUSIONS

It may seem surprising that such a large structure should have remained undrilled in a country where extensive exploration has taken place. The reason is that the structure underlies Windsor Castle, the home of Queen Elizabeth II.

The castle is one of the major tourist areas of the U.K., and the whole area is very environmentally sensitive. However, Canuk Exploration Ltd. acquired an exploratory license and approached the exploration with great regard to local conditions and has had no adverse comments to its geological and geophysical exploration.

It is planned to drill an exploratory well close to the crest of the structure and, if successful, to conduct the development program with horizontal drilling from a location more suited to such activity.

Canuk is seeking a competent operator to contribute financially to the drilling of an exploratory well on this prospect on a farmout basis.

ACKNOWLEDGMENTS

The author thanks Bruce Sellwood for helpful comments in the preparation of this article. The conclusions, however, are those of the author alone.

BIBLIOGRAPHY

Ebukanson, E.J., and Kinghorn, R.R.F., Oil and gas accumulations and their possible source rocks in southern England, J. Petrol. Geol., Vol. 9, 1986, pp. 413-428.

Hancock, F.R.P., and Mithen, D.P,, The geology of the Humbly Grove oilfield, Hampshire, U.K., in Brooks, J., and Glennie, K., eds., Petroleum Geology of North West Europe, Graham & Trotman, 1987, pp. 161-170.

McLimans, R.K., and Vldetich, P.E., Reservoir diagenesis and oil migration: Middle Jurassic Great Oolite limestone, Wealden basin, southern England, in Brooks, J., and Glennie, K., eds., Petroleum Geology of North West Europe, Graham & Trotman, 1987, pp. 119-128.

Penn, I.E., Chadwick, R.A., Holloway, S., Roberts, G., Pharoah, T.C., Allsop, J.M., Hulbert, A.G., and Burns, I.M., Principal features of the hydrocarbon prospectivity of the Wessex-Channel basin, U.K., in Brooks, J., and Glennie, K., eds., Petroleum Geology of North West Europe, Graham & Trotman, 1987, pp. 109-118.

Sellwood, B.W., Scott, J., Mikkleson, P., and Akroyd, P., Stratigraphy and sedimentology of the Great Oolite Group in the Humbly Grove oilfield, Hampshire, Mar. Pet. Geol., Vol. 2, 1985, pp. 4455.

Sellwood, B.W., Scott, J., and Lunn, G., Mesozoic basin evolution in southern England, Proc. Geol. Assoc., Vol. 97, 1986, pp. 259-289.