Application and Assessment of Open vs. Closed Kerogen Isolation Methods for Spectroscopic and Isotopic Characterization of Gas Shale Kerogens

The ever increasing interest in unconventional natural gas resources has spurred much research into the origin and mode of entrapment and expulsion of gas from the diverse resources. In the case of “gas shale”, questions remain unanswered on whether the organic matter serves only as a source of the gas or also as a reservoir and an entrapment site within the shale matrix. One aspect to consider is the role of kerogen, the insoluble organic matter from which the hydrocarbons are formed, with regard to its chemistry, its complex internal structure, and its rheology and petrophysical properties. A pre-requisite to any kerogen study is the isolation and separation of the kerogen in its purest and most intact form from the rock matrix that it is intimately imbedded in. Previous work showed that the isolation methods used can impact kerogen recovery, purity and elemental composition of the organic matter. Still to be investigated are the impact of the isolation process on the interpretation of the isotopic, spectroscopic and structural properties of the kerogen.

In this study, we will report on the effects of the isolation processes on isotopic, spectroscopic and structural properties of kerogens recovered in the closed-system vs. open-system isolation processes. We compare and contrast the properties of kerogen of four major gas shales including the Barnett, the Marcellus, the Haynesville and the Silurian gas shale of Poland. Though strictly not a gas shale, we will report our observations on the characteristics of the Monterey Shale to address the effects on sulfur-rich, Type II-S kerogens. The carbon, sulfur and hydrogen isotopic compositions, the organic elemental composition, the Solid-state 13C NMR spectroscopic properties, and the pyrolytic behavior of kerogen concentrates isolated by the closed-system conservative method will be compared to the corresponding properties of kerogens isolated by conventional open-system methods. The impact of the isolation processes on the organic microscopy, mineralogy, sulfur forms, sulfur isotopes and inorganic elemental composition of the 1st and 2nd transition metals of the native rocks vs those of the isolated kerogens will be described. Observed differences will be addressed in terms of degree of fractionation and its implications on interpretation of kerogen’s petrophysical properties, gas-retention and storage capacity.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California