Shale-gas potential of the Mid-Carboniferous Bowland-Hodder Unit in the Cleveland Basin (Yorkshire), Central Britain

The shale-gas potential of mid-Carboniferous mudrocks in the Bowland-Hodder unit in the Cleveland Basin (Yorkshire, northern England) was investigated through the analysis of a cored section from the uppermost part of the unit in the Malton-4 well using a multidisciplinary approach. Black shales are interbedded with bioturbated and bedded sandstones, representing basinal-offshore to prodelta – delta-front lithofacies. The total organic carbon (TOC) content of the shales ranges from 0.37 to 2.45 wt %. Rock-Eval pyrolysis data indicate that the organic matter is mainly composed of Type III kerogen with an admixture of Type II kerogen. Tmax (436–454°C), 20S/(20S+20R) C29 sterane ratios, and vitrinite reflectance values indicate that organic matter is in the mid- to late- mature (oil) stage with respect to hydrocarbon generation. Sedimentological and geochemical redox proxies suggest that the black shales were deposited in periodically oxic-dysoxic and anoxic bottom waters with episodic oxic conditions, explaining the relatively low TOC values. The Rock-Eval parameters indicate that the analysed mudrocks have a limited shale-gas potential. However, burial and thermal history modelling, and VRr data from other wells in the region, indicate that where they are more deeply-buried, the Bowland-Hodder shales will be within the gas window with VRr > 1.1 % at depths in excess of 2000 m. Therefore although no direct evidence for a high shale-gas potential in the Cleveland Basin has been found, this cannot be precluded at greater depths especially if deeper horizons are more organic rich.