--> Geochemistry and Microstructure Characterization of Kerogen in Chinese Cambrian Shale: A Combined Experimental and Molecular Simulation Study

AAPG ACE 2018

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Geochemistry and Microstructure Characterization of Kerogen in Chinese Cambrian Shale: A Combined Experimental and Molecular Simulation Study

Abstract

Cambrian marine shale in Sichuan Basin is one of the most important target zones for shale gas exploration and exploitation in China. However, few studies have been performed to explore the geochemistry and microstructure of kerogen in Chinese Cambrian shale, which is a key control factor of gas storage and migration.

In this work, a combination of Fourier transform infrared spectroscopy (FTIR), solid-state C13 nuclear magnetic resonance (C13 NMR) and X-ray photoelectron spectroscopy (XPS) techniques were adopted to characterize the carbon chemical and structural features and the organic oxygen, nitrogen and sulfur species in kerogen from the black siliceous shale of the Chinese Niutitang Formation. These direct characterization results were utilized to develop a representative structural model using the molecular dynamics simulation. The microscopic structural properties of the generated kerogen model were further investigated.

C13 NMR and XPS results show that the carbon skeleton is composed of aromatic carbon, aliphatic carbon and carbonyl carbon. Aromatic carbon is the main component of kerogen skeleton, while aliphatic carbon and carbonyl carbon are mainly served as connections of aromatic units. Besides, aromatic units in the kerogen skeleton are highly condensed, and the inter-unit linkages are mainly short methylene and methylidyne groups. FTIR and XPS results indicate that majority of oxygen exists as ether forms. XPS results show that aromatic sulfur is the main form of organic sulfur, and nitrogen mainly exists as pyrrolic and pyridinic forms. Reasonable consistencies are observed on chemical compositions, physical density and porosity between simulated results and experimental data, which validates our kerogen model. Pore size distribution of the kerogen model shows a monopeak type, with the pore width corresponding to the peak site at ~1.6 Å. In addition to accessible micropores, the kerogen model contains a large amount of ineffective ultra-micropores, which contribute a lot to the high porosity and specific surface area. Moreover, the porous network of our kerogen model is highly connected with few dead pores.

This work gains deep insights into the geochemistry and microstructure of kerogen in Chinese Cambrian shale, and the generated kerogen model can serve as a starting point for further theoretical investigations on gas adsorption and transport mechanisms, representative of the kerogen in Cambrian shale at molecular scale.