Organic geochemistry of Palaeozoic Source Rocks of the Irish Sea, UK

The East Irish Sea gas and oil fields (Triassic reservoir e.g. Morecambe, Lennox) are believed to be sourced from the underlying Carboniferous strata (e.g. Armstrong et al., 1997; Quirk et al. 1999 and references therein). This study undertook a systematic screening of Rock-Eval and vitrinite reflectance data extracted from released legacy well reports with the aim of providing a data based, regional overview of Carboniferous source rock intervals and their levels of maturity in the wider Irish Sea study area. The method and description of calculated parameters used are given in Vane et al. (2015). The Palaeozoic stratigraphy of the region is described in Wakefield et al. (2016b) and the regional petroleum systems analysis in Pharaoh et al. (2016). Released geochemical data from the Palaeozoic of the Irish Sea is sparse and a variety of stratigraphical units have been sampled, mostly from units other than the likely main source rock interval (Bowland Shale Formation and equivalents). Nine wells were evaluated: 110/02b-10, 110/07-2, 110/07b-6, 110/09a-2, 111/25-1A, 113/27-1, 113/27-2 and 113/27-3. Well 110/12a-1 was not assessed due to the absence of key maturity information. In these wells, the Pennine Lower Coal Measures, Millstone Grit Group and Bowland Shale Formation are mainly gas-prone strata of poor-fair generative potential remaining and mature to the gas window at the sampled intervals in Quadrants 110 and 113 (Figure 1). Within the limited well sample set examined, high Total Organic Carbon (TOC) coal intervals have the best generative potential remaining. The Cumbrian Coastal Group, Appleby Group and Carboniferous Limestone Supergroup present in two wells in Quadrant 111 are at oil to gas window maturity levels, but have low TOC and low residual hydrocarbon generative potential.