Islip axis gas could support Central England power generation

Desmond H. Oswald
Canuk Exploration plc
Buckinghamshire, England

The Islip axis is part of a major structural feature in Central England that reaches its culmination about 4 miles north of Oxford (Fig. 1 [33325 bytes]). It crosses the culmination of the London platform at Twyford, 12 miles farther northeast (Fig. 2 [55259 bytes]).

During Mesozoic time this area lay between the relatively stable London platform in the east and the subsiding Worcester basin in the west. Paleozoic rocks, Cambrian to Devonian, underlie the Mesozoic rocks on the London platform, while to the west Permo-Triassic rocks rest on coal measures.

The subsurface structure and stratigraphy of the area have been established by many boreholes drilled in the early years of the century for coal and more recently by the British Gas Council exploring for a porous reservoir for gas storage.

The Paleozoics of the London platform come to their highest elevation in the area of Twyford, where they are overlain by Triassic (Fig. 3 [92455 bytes]). The earliest deposits were the basal Triassic Bromsgrove sandstone formation. The formation varies in thickness from 12.5-35 m, the variation in thickness probably reflecting in part the unevenness of the pre-Triassic floor and partly a regional thickening to the west into the basin. It pinches out against the London platform to the east where it was eroded in Rhaetic time. The formation generally consists of a poorly cemented, current bedded, pebbly and gritty sandstone with fining upward sequences of fine grained sandstone and silty mudstones.

The basal sandstone is succeeded by the Mercia Mudstone Group, finely laminated beds of silt alternating with beds of blocky mudstone, with nodules and veins of anhydrite and/or gypsum. Radioactive marker horizons provide evidence that the lower units of the formation thin to the east onto the London platform and also that deep erosion (?60 m+) took place. The eroded members are overlain by a thinly bedded mudstone, of probable estuarine origin, that becomes more sandy to the east. The group reaches a thickness of almost 150 m to the west.

The succeeding beds form the Penarth Group, which comprises the Westbury formation and the Cotham member, which are primarily mudstones with occasional partings of very fine sand and silt. A thin basal sand is sometimes present. The Cotham member is succeeded by the Langport member (White Lias), a porcellainous limestone that is generally taken to be the base of the Jurassic system.

To the east a marginal arenaceous facies, the Twyford beds, appears in the position of the Penarth Group. It comprises coarse sandstone, conglomeratic sandstones, and siltstones. Included are clasts of mudstone and micrite, possibly derived from the Mercia mudstone and penecontemporaneous fragments from the Penarth Group. Millet seed quartz grains and fragments of igneous and metamorphic rocks also are present. A Carboniferous foraminifer has also been recorded, indicating the widespread erosion that contributed to the deposition of the Twyford beds.

Liassic beds form the surface in most of the Twyford area. The history of early Liassic sedimentation is one of progressive overlap onto the London platform in four episodes, each of which was preceded by localized erosion at the edge of the basin resulting in a minor disconformity. The geophysical logs of the Gas Council boreholes provide excellent markers for demonstrating the thickness variations within the Lias.

The environment of deposition of the Triassic/Rhaetic succession suggests shallow marine and estuarine conditions. Pseudo-brecciated mudstones and evaporites in the Mercia Mudstone Group formed during periods of dessication. Marine bivalves indicate short lived marine incursions, but the main marine phase did not set in until deposition of the Langport member. Relative uplift of the London platform during Rhaetian times resulted in increased erosion and larger fragments of rock debris into the peripheral zone, as evidenced by the Twyford beds, which indicate rapid deposition with little reworking.

Petroleum potential Reservoir

The basal Triassic sand, the Bromsgrove sandstone member, is a very porous formation. It is laterally continuous with the Rhaetic basal sand, the Twyford beds, which pinch out against the London platform (Fig. 4 [53524 bytes]). The Twyford beds reach a thickness of almost 11 m in the Twyford 2 well and 13 m at Calvert West.

Cap rock

The Twyford beds are overlain by the Lower Lias, which is an impermeable shale more than 70 m thick in the Calvert West well. It forms an excellent cap rock throughout the area.

Structure

The Islip axis trends southwest/northeast crossing the culmination of the London platform on the Twyford area. The axis is defined at the surface by the presence of a series of Middle Jurassic inliers (Cornbrash and Great Oolite) within the Upper Jurassic Oxford clay. The Lower Paleozoics of the London platform reach a culmination, the Twyford high, in the Marsh Gibbon-Calvert-Twyford area, and the Twyford beds pinch out on the flank of this high, forming a perfect stratigraphic trap.

Source

Middle and upper coal measures up to nearly 1,000 m thick lie to the west of the Twyford high. Seven workable coal seams more than 0.6 m thick exist, and in all a total of 7.85 m of coal is present. The measures are overlain unconformably by the basal Triassic sandstone that forms the conduit for the passage of gas, generated in the basin, to the Twyford high.

Oil, gas occurrences

In the Twyford 1 well (Fig. 5 [53762 bytes]) gas at 900,000 cfd with a wellhead pressure of 138 psig was discovered in the Twyford beds. Gas was present in an interval of about 13.5 m in the Calvert West well; but the well drilled in 1911 did not measure the gas flow. A gas show was also present in the Marsh Gibbon well.

Economic potential

The volume of gas anticipated to be present in the Twyford area would probably not be sufficient for development as a gas field for input to the national gas grid.

However, small gas accumulations offer excellent potential for generation of electricity, which can be sold under recent changes in legislation direct to small industrial users. The gas in the Twyford area would be ideally suited to this type of development; gas analyses indicate methane values of over 80%. Helium is present up to 1.6%.

Canuk acquired an exploration license covering the Twyford area in the latest landward licensing round in the U.K. The company is currently seeking a competent operator to contribute financially to an exploration and development program in the area.

The Author