The regional character of organic matter type and depositional conditions of Pendleian, Brigantian and Arnsbergian mudstones between the Craven Basin and the Widmerpool Gulf was compared through interpretation of biomarker and pyrolysis data from 201 samples recovered from 9 boreholes. The Carboniferous seaways have been determined to commonly host dysoxic conditions, enabling preservation of a mixture of marine and terrestrial organic matter types. Photic zone anoxia evidenced by aryl-isoprenoids was determined to be persistent during ‘marine’ conditions represented by marine band, high sea level and carbonate facies. Observation and correlation of diasteranes and Ts/Tm ratios within the samples and to other maturity parameters highlighted a significant clay mineral catalytic and/or hydrocarbon retention effect in the samples. This influenced both biomarkers as well as Tmax thermal maturity data reducing the reliability of such results in interpreting burial and ultimately reserve potential.