UK shale-gas resources

Abstract

In the USA gas has been produced from shale in commercial quantities for nearly two centuries. Production commenced in New York State in 1821, when wells were located by market and drilled with little appliance of science. Subsequently, production has been established in five shale-gas systems: the Ohio Shale (Devonian) of the Appalachian Basin, the Antrim Shale (Devonian) of the Michigan Basin, the New Albany Shale (Devonian) of the Illinois Basin, the Barnett Shale (Upper Carboniferous) of the Fort Worth Basin, and the Lewis Shale (Cretaceous) of the San Juan Basin. A recent study by the USGS concluded that total in place US shale-gas resources are in the order of 497–783 × 1012SCF with estimated recoverable resources of 31–76 SCF.Gas produced from organic-rich shales is of both biogenic and thermogenic origin, with matrix permeabilities of some 0.001D. For commercial production this is enhanced by fractures, natural or artificial. Shale gas has high calorific values (c.1200 btu), and is commonly wet, with over 10% ethane. After initial high-pressure ‘blows’ wellhead pressures stabilize at 300–500 psi with flow rates of between 50–100 cubic feet of gas per day and depletion rates of 10% pa. One shale-gas well may provide enough energy for a school, a hospital or a housing estate for decades. The economics of shale-gas production is too small to be of commercial interest to major energy companies, but is viable in the USA, where petroleum exploration and production may flourish on a cottage industry scale.Exploration for shale-gas requires a somewhat different mindset from that of conventional petroleum exploration, and a completely different technology, both in terms of petroleum genesis, seismic prospecting, well drilling and completion.Some 20 years ago it was suggested that, by analogy with the USA, the UK may possess considerable reserves of shale-gas. This was predicated on the assumption that shale-gas only resulted from the thermal maturation of organic-rich shales. Subsequently, it has been realized that shale-gas can be formed by methanogenic bacteria acting on organic-rich rocks, irrespective of age and thermal history, and especially as a result of post-glacial flushing of aquifers. This realization enhances British shale gas resources dramatically, making any fractured organic-rich shale prospective. Potential British shale-gas petroleum systems include the thermally overmature Caledonide fold belt, the Lower Carboniferous thermally mature basinal shales of northern England and the Midland Valley of Scotland. The Jurassic (Lias, Oxford and Kimmeridge) clays may have considerable potential for thermogenic and biogenic shale-gas. The leaner Lower Cretaceous (Wealden) and Eocene (London Clay) formations of southern England may have minor potential for biogenic shale-gas.