Organic Matter Characterization of Shale Rock by X-ray Photoelectron Spectroscopy: Adventitious Carbon Contamination and Radiation Damage

Abstract

A detailed characterization of the organic matter in sediments gives a key parameter for the correct evaluation ofthe petroleum generation potential of a source rock and its modeling in an oil system. To understand the features of the organic matter stored in source rocks, a wide variety of routine techniques are used. Those techniques are mostly destructive, timeconsuming,and are not necessarily suitable for all the shale rocks. Thus, new technologies are being explored. XPS is a solid state,nondestructive, and direct method. It can be used to directly probe the speciation of organic carbon in sedimentary materials byC1s spectra measurements. In this work, we demonstrated that a heterogeneous layer of adventitious carbon (AC) is always present on the rock. This layer significantly altered the features of the organic compound fingerprint, which is measured by XPS of C1s. To acquire a reliable organic matter composition of a rock from XPS spectra it is necessary to remove the AC layer by sputtering it with Ar+. Further, the sputtering damage of the organic matter appears after an hour of radiation, and rocks become contaminated again during storage in the UHV chamber. The radiation damage by X-ray is also relevant in the XPS measurementof a rock. This damage is associated with an increment of AC contamination on the rock surface. To minimize this side effect,C1s spectra have to be measured immediately after the sputtering conditioning step. Thus, we developed a method for XPS measurements taking into account the AC contamination and radiation damage. We consider that the proposed method for outcrop shales can be applied to a wide range of rock sampling types, such as cuttings, damaged crowns, and so forth.