Abstract
Abstract Organic-rich shale contains significant amounts of gas held within fractures and micropores and adsorbed onto organic matter. In the USA shale gas extracted from regionally extensive units such as the Barnett Shale currently accounts for 6% of gas production and is likely to reach 30% by 2015. Shale gas prospectivity is controlled by the amount and type of organic matter held in the shale, its thermal maturity, burial history, microporosity and fracture spacing and orientation. Potential targets range in age from Cambrian to the late Jurassic, within the main UK organic-rich black shales: younger shales have been excluded because they have not reached the gas window, but they may possess a biogenic gas play. A geographic information system, showing the distribution of potential reservoir units, has been created combining information on hydrocarbon shows, thermal maturity, fracture orientation, gas composition, and isotope data to identify potentially prospective areas for shale gas. Some of these data are shown as graphs and maps, but crucial data is lacking because earlier exploration concentrated on conventional reservoirs. The prospects include Lower Palaeozoic shale basins on the Midland Microcraton (a high risk because no conventional gas has been proved in this play), Mississippian shales in the Pennine Basin (the best prospect associated with conventional fields and high maturity), Pennsylvanian shales in the Stainmore and Northumberland Basin system (high risk because no conventional gas discoveries exist) and Jurassic shales in Wessex and Weald basins (small conventional fields signify potential here).
Highlights
Organic-rich shale contains significant amounts of gas held within fractures and micro-pores and adsorbed onto organic matter
In the US, shale gas extracted from regionally extensive units such as the Barnett Shale account for ~6% of gas production
The Big Sandy shale gasfield in eastern Kentucky lies over the thickest part of the Devonian Brown Shale, which is bounded to the SE by the Pine Mountain thrust (Ray 1976) of the Appalachian (Variscan) fold belt
Summary
The Barnett Shale Formation (354 - 323 Ma) of the Fort Worth Basin is up to 300 m thick and underlies an area of ~13,000 km. A borehole drilled at Usk (east of Usk Anticline in Fig. 1) penetrated no potential source rocks in the Lower Palaeozoic, minor gas shows occurred in some sandstone units This well was drilled on a structural high missing a large section of Llandovery sequence where black shales might have formed in the depocentre beneath the Usk Anticline. Individual shale units that might represent potential source rocks include (Table 1) the Worston Shale Group, the Long Eaton Formation and the Widmerpool Formation The latter has a high TOC content in Duffield borehole (Fig. 67), but the gamma ray values are low (Aitkenhead 1977). TOC measurements in the Knott Coppy borehole are in the range 1-3%, making these shale units potential hydrocarbon source rocks, in this borehole TOC-rich shales do not always correspond to high gamma intervals
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