Palaeoceanographic controls on geochemical characteristics of organic-rich Exshaw mudrocks: role of enhanced primary production

Organic-rich source rocks have generally been attributed to enhanced preservation of organic matter under anoxic bottom waters. Here, geochemical analysis of kerogen and whole rock samples of organic-rich (lithofacies B1) and organic-lean (lithofacies B2) laminated mudrocks of the Devonian–Carboniferous Exshaw Formation, Alberta, highlight the importance of primary production in governing the quantity and quality of organic matter. Lower Si/Al, K/Al, Ti/Al and quartz/clay ratios in lithofacies B2, similar maceral types and the laminated fabric of the two lithofacies indicate that the quality and quantity of organic matter are not related to grain size, redox or organic matter source changes. High Total Organic Carbon (TOC) and Hydrogen Index (HI), low Oxidation Index (Ox.I. ratio of oxygen functional groups to aliphatic groups derived by FTIR), lighter δ15Ntot and heavier δ13Corg isotopes indicate that kerogen of lithofacies B1 accumulated during periods of high organic-carbon production and delivery of relatively fresh, labile, well-preserved organic matter to the sea floor. In contrast, low TOC, HI, high Ox.I., heavier δ15Ntot and lighter δ13Corg isotopes indicate low primary productivity and delivery, high recycling and poor preservation of organic matter during accumulation of lithofacies B2.