ISS ASSESSMENT OF THE INFLUENCE OF NONPORE SURFACE IN THE XPS ANALYSIS OF OIL-PRODUCING RESERVOIR ROCKS

The application of X-ray photoelectron spectroscopy (XPS) to oil-produ cing reservoir rocks is new and has shown that pore surface concentrat ions can be related to rock wettability. In the preparation of fresh f ractures of rocks, however, some nonpore surface corresponding to the connection regions in the rocks is created and exposed to XPS. To asse ss the potential influence of this nonpore surface in the XPS analysis of rocks here we use ion scattering spectroscopy (ISS), which has a r esolution comparable to the size of the pores, higher than that of XPS , with an ion gun of He+ at maximum focus. Sample charging effects are partially eliminated with a flood gun of low energy electrons. All th e ISS signals are identified by means of a formula which corrects any residual charging on the samples. Three rock samples are analyzed by X PS and ISS. The almost unchanged ISS spectra obtained at different poi nts of a given sample suggest that the nonpore surface created in the fracture process is negligibly small, indicating that XPS data, from a larger surface spot, represents the composition of true pore surfaces . The significant changes observed in ISS spectra from different sampl es indicate that ISS is sample specific. (C) 1997 Academic Press.