D/H ratios and hydrogen exchangeability of type-II kerogens with increasing thermal maturity

Abstract

Stable isotope ratios of non-exchangeable hydrogen ( δD n) and of carbon were measured in type-II kerogens from two suites of Late Devonian to Early Mississippian black shale, one from the New Albany Shale (Illinois Basin) and the other from the Exshaw Formation (Alberta Basin). The largely marine-derived organic matter had similar original stable isotope ratios, but today the suites of kerogens express gradients in thermal maturity that have altered their chemical and isotopic compositions. In both suites, δD n values increase with maturation up to a vitrinite reflectance of R o 1.5%, then level out. Increasing δD n values suggest isotopic exchange of organic hydrogen with water-derived deuterium and/or preferential loss of 1H-enriched chemical moieties from kerogen during maturation. The resulting changes in δD n values are altering the original hydrogen isotopic paleoenvironmental signal in kerogen, albeit in a systematic fashion. The specific D/H response of each kerogen suite through maturation correlates with H/C elemental ratio and can therefore be corrected to yield paleoenvironmentally relevant information for a calibrated system. With increasing thermal maturity, the abundance of hydrogen in the kerogen that is isotopically exchangeable with water hydrogen (expressed as H ex, in % of total hydrogen) first decreases to reach a minimum at R o ∼ 0.8–1.1%, followed by a substantial increase at higher thermal maturity.