Significance of sedimentary organic matter input for shale gas generation potential of Mississippian Mudstones, Widmerpool Gulf, UK

Carboniferous mudstones in central and northern England are shale gas prospects but the controls on the amount and composition of organic matter are not well understood, even though these parameters define the volumes of gas generated in fine-grained sediments. Organic matter in samples from basinal late Mississippian (Arnsbergian) mudstones in the Widmerpool Gulf was characterised by using semi-quantitative (n = 58) and quantitative palynofacies (n = 16) analyses, sporomorph counts and bulk rock geochemistry (total organic carbon, δ13C of bulk organic matter, Rock-Eval Pyrolysis). The results of this study suggest that most organic matter at this location was delivered to the sediment-water interface as aggregates of a granular translucent type of amorphous organic matter (AOMGr, mean 66.7±19.3%) via hemipelagic suspension settling. AOMGr represents fragments of algal material with subordinate inclusions of small plant fragments and pyrite framboids held together by microbial colonies. AOMBr (brown granular amorphous organic matter) is the second most abundant group (mean 15.6±8.5%) comprising similar microbial colonies that grew on suspended land plant-derived fragments in the water column. Palynofacies components representing clearly terrestrial organic matter are much less abundant and include gelified organic matter (G, mean 9.6±12.6%), black phytoclasts (PhBl mean 2.7±4.7%), brown phytoclasts (PhBr, mean 3.3±3.6%) and sporomorphs (mean 1.4±1.3%). Sediment delivery processes influence the balance between terrestrial organic matter and AOMGr. During low sea-level times, turbidity currents and debris flows delivered terrestrial organic matter (representing 12 to 40% of the palynofacies). Kerogen composition varies between Type II and III. In contrast, thin-bedded carbonate-bearing mudstones deposited during rising and high sea-level contain up to 95% AOMGr and these high abundances correspond to higher total organic carbon. Carbonate and AOMGr were generated by high bioproductivity in the water column. Type II (oil- and gas-prone) kerogens are dominant in these mudstones and therefore these intervals represent the best potential targets for thermogenic shale gas

An assessment of geochemical preparation methods prior to organic carbon concentration and carbon isotope ratio analyses of fine-grained sedimentary rocks

This study summarizes organic carbon isotope (δ13C) and total organic carbon (TOC) data from a series of tests undertaken to provide an appropriate methodology for pre-analysis treatment of mudstones from an Upper Carboniferous sedimentary succession, in order to develop a consistent preparation procedure. The main treatments involved removing both inorganic carbonate and hydrocarbons (which might be extraneous) before δ13C and TOC analysis. The results show that decarbonating using hydrochloric acid causes significant reduction in δ13C and total carbon (TC) of the bulk material due to the removal of inorganic carbonate. These changes are most pronounced where soluble calcium carbonate (rather than Ca-Mg-Fe carbonate) is present. Deoiled samples show only slightly higher mean δ13C where visible bitumen was extracted from the bulk sample. Moreover, the isotopic signatures of the extracts are closely correlated to those of their respective bulk samples, suggesting that small yields of hydrocarbons were generated in situ with no isotopic fractionation. In addition, further δ13C and TC analyses were performed on samples where mixing of oil-based drilling mud with brecciated core material had been undertaken. Brecciated mudstone material did not display distinct isotopic signals compared to the surrounding fine-grained material. Overall we show that the most accurate assessment of bulk organic carbon isotopes and concentration in these samples can be achieved through decarbonating the material prior to measurement via the ‘rinse method’. However, our results support recent findings that pre-analysis acid treatments can cause variable and unpredictable errors in δ13C and TOC values. We believe that, despite these uncertainties, the findings presented here can be applied to paleoenvironmental studies on organic matter contained within sedimentary rocks over a range of geological ages and compositions