Vaughan's (1905) zonation of the Carboniferous Limestone in the Bristol district was a at

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Vaughan’s (1905) zonation of the Carboniferous Limestone in the Bristol district was a pioneer biostratigraphical study, which because of its meticulous execution can be reinterpreted in a modern context. The replacement of his scheme with a chronostratigraphical one by George et al. 1976 had a similar revolutionary affect on British Dinantian stratigraphy. However, it is now time to revise the British Dinantian stages so that they more closely correspond to biostratigraphical events. Dinantian biostratigraphy still requires considerable refinement, but it has now achieved a diversity of techniques and resolution far beyond that which was available at the time of these earlier proposals. It is the most pragmatic and closest approximation to widespread chronostratigraphical correlation available. This paper discusses these and related issues and presents a review and correlation of current biozonations.

SUMMARY The Worston Shale Group of Earp et al. (1961) is made up of four formations: the Clitheroe Limestone, Hodder Mudstone, Hodderense Limestone and Pendleside Limestone. Within these formations fifteen members are defined: the Thornton Limestone, Coplow Limestone, Peach Quarry Limestone, Bellman Limestone, Limekiln Wood Limestone. Phynis Mudstone, Whitemore Limestone, Hetton Beck Limestone, Leagram Mudstone, Buckbanks Sandstone, Embsay Limestone, Rain Gill Limestone, Chaigley Limestone, Twiston Sandstone and Rad Brook Mudstone. A particular bed within the Hodder Mudstone Formation, the Dunbarella Bed, is also recognised. Four unconformities are recognised. The most widespread is at the base of the Hodder Mudstone Formation; the others, which are more restricted, occur at the base of the Embsay Limestone Member (and equivalents), at the base of the Twiston Sandstone Member and at the base of the Pendleside Limestone Formation. Diagnostic Viséan foraminifera and conodonts first enter within the basal part of the Hodder Mudstone Formation, suggesting that the Tournaisian-Viséan boundary lies much higher than previously supposed. All the Dinantian stage boundaries in the Arundian to Asbian interval have been re-correlated. The Craven Basin is thought to have been underlain by an asymmetric graben within the pre-Carboniferous basement and the depositional history of the basin to have resulted from a combination of both syndepositional tectonics and eustatic processes. Consequently, the basinal sediments portray a transition from carbonate shelf to slope (Gawthorpe 1987), involving a change during deposition of the Clitheroe Limestone and Hodder Mudstone formations from shallow-water carbonates, generated within the basin, to deep-water hemi-pelagic sediments with limestone turbidites fed from the surrounding carbonate shelves and platforms. Intermittent supply into the basin of terrigenous sand and reworked faunas is related to the emergence and erosion of the surrounding areas.

New data from three shale gas exploration wells in the Bowland Basin of NW England contribute to the understanding of the stratigraphy, tectonic history and unconventional hydrocarbon resource potential of Lower Carboniferous strata. Three main prospective shales dominate the identified unconventional reservoirs: the Upper Bowland and Lower Bowland shales and the Hodder Mudstone, which are recognized by their distinctive lithology, corresponding log signatures and key zonal ammonoids. With a combined thickness of over 5000 ft ( c. 1500 m), this sequence of shales is one of thickest known potential self-sourced, unconventional hydrocarbon resources. The strata are organic rich with total organic carbon (TOC) values of between 1 and 7%, with an average of 2.65%, and organic maturity that ranges from the upper oil window (pyrolysis T max c . 450°C) in the higher part of the section to dry gas (R o  = 2.4%; pyrolysis T max >470°C) in the Lower Bowland Shale. The sequence is strongly heterolithic, and up to 60% free gas is stored in thinly bedded carbonate and clastic silty turbidites. Adsorbed gas is concentrated in more organic-rich, hemipelagic shales which are distributed throughout the sequence. Near maximum burial temperatures of c. 130°C are inferred from vitrinite reflectance (R o ) and are consistent with fluid-inclusion microthermometry of carbonate-filled fractures. This indicates oil generation in the Late Carboniferous, prior to Variscan uplift. Renewed subsidence through the early Mesozoic resulted in increased maturity and gas generation. In the Bowland Shale the gas per unit volume of rock ranges from about 0.6 to 1.5 Bcf (billion cubic ft) per metre per square mile. The thick interval of gas-charged strata provides the opportunity to exploit these major hydrocarbon resources by using stacked multilateral wells from a common, strategically located and environmentally optimized surface pad.