The organic geochemistry and thermal maturity of the Pennine Carboniferous Basin, England
The Pennine Basin has undergone a complex geological history with major episodes of rifting, thermal subsidence and inversion, the pattern of which was largely controlled by the structure of the Caledonian basement. The resulting block and basin topography and differential subsidence led to major variations in sediment thickness throughout the basin. In the Dinantian, carbonate sediments were dominant changing to clastic dominated sediments in the Namurian and Westphalian. Thick accumulations of peat led to the formation of numerous coals, mainly in the Westphalian. There were periodic marine incursions, many of them correlatable over the whole of NW Europe. The boreholes were selected to give an approximate E-W traverse across the deepest part of the basin, with additional field and deep mine sample coverage. The samples encompass an age range from the base of the Permian down to topmost Dinantian. Vitrinite reflectance is the most consistently reliable indicator of maturity, giving the best trends with increasing depth for all the boreholes, bar Carlow 2, Knutsford 1 and Hayfield Farm. The aromatic maturity parameters proved to be the most useful of the geochemical parameters and of these MPI1 and % Rc were the most reliable, giving good depth related trends for all the boreholes bar Knutsford 1, Calow 2 and Hayfield Farm. A problem with MPI1 and % Rc is that the alkylphenanthrene parameters reverse at higher maturities, thought to be due to a change in the reactions involved. Consequent to this there is a change in the equations used to calculate % Rc which, for this study, applies only to the samples below the lower fault in Up Holland 1 and to the sill sample from Gainsborough 2. The aliphatics parameters are at or near their end point for most of the samples. For Gainsborough 2, the C29αalphaα 20S/20S+ 20R ratio attains equilibrium at around 1000m depth and thus can be used as a further constraint on the thermal history in the basin modelling.