Total organic carbon in the Bowland-Hodder Unit of the southern Widmerpool Gulf: a discussion

Abstract

This review of the article by Kenomore et al. (2017) on the total organic carbon (TOC) evaluation of the Bowland Shale Formation in the Widmerpool Gulf sub-basin (southern Pennine Basin, UK) reveals a number of flaws, rooted mostly in an inadequate appreciation of the local mid-Carboniferous stratigraphy. Kenomore et al. use the ΔLog R, the 'Passey' method after Passey et al. (1990), to evaluate the TOC content in two boreholes in the Widmerpool Gulf: Rempstone 1 and Old Dalby 1. We show here that Kenomore and co-authors used maturity data, published by Andrews (2013), from different formations to calibrate their TOC models of the Bowland Shale Formation (Late Mississippian-Early Pennsylvanian); the Morridge Formation in Rempstone 1 and the Widmerpool Formation in Old Dalby 1. We contest that this gives viable TOC estimates for the Bowland Shale Formation and that because of the location of the boreholes these TOC models are not representative over the whole of the Widmerpool Gulf. The pyrite content of the mudstones in the Widmerpool Gulf also surpasses the threshold where it becomes an influence on geophysical well logs. Aside from these stratigraphic and lithologic issues, some methodological flaws were not adequately resolved by Kenomore and co-authors. No lithological information is available for the Rock-Eval samples used for the maturity calibration, which because of the interbedded nature of the source formations, has implications for the modelling exercise. We recommend that more geochemical data from a larger array of boreholes covering a wider area, proximal and distal, of the basin are collected before any inferences on TOC are made. This is necessary in the complex Bowland Shale system where lithological changes occur on a centimetre scale and correlations between the different sub basins are not well understood.

Geochemical element mobilisation by interaction of Bowland shale with acidic fluids

“…The Roosecote 1 Borehole. A: chrono-and lithostratigraphy alongside the downhole gamma response [31,34] with indication of sample depths; B: Carboniferous palaeogeography of the Pennine Basin with location of the Roosecote 1 Borehole; C: Idealized cross-section throught the Millstone Grit and Bowland Hodder Unit as a key for the schematic colouring of A (following Hennissen and Gent [35] modification of Andrews [31]). Csrt.…”