Characterisation of carbon components and their isotopic composition in gas shales

Shales are an important reservoir of both organic and inorganic carbon that can be decoupled by incremental stepwise combustion without acid treatment. The study of shales from various depths of three cores of Haynesville-Bossier formation of Jurassic period indicates the presence of organic and inorganic carbon with comparable delta C-13 and nitrogen/carbon (N/C) ratios in all three. Both the isotopic and elemental ratios predominantly indicate a continental source but a minor N/C fraction corresponding to a marine/lacustrine source is also seen. This indicates either mixing from different sources and/or fractionation, perhaps due to preferential loss of nitrogen during organic matter degradation. Because some of the organic component is released at a high temperature of 1000-1200 degrees C although ungraphitised, it may be locked up in minerals like sulphides and K-feldspars, incorporated during diagenesis and hydrothermal alteration. Carbon locked in these minerals can be isolated from the carbon cycle for a longer period of time. Copyright (C) 2018 Elsevier Ltd. All rights reserved.