Even though reflectometry using Global Navigation Satellite System's signals (GNSS-R) was first envisioned for mesoscale ocean altimetry, nowadays a number of different applications have been developed (ocean scatterometry, ice monitoring, soil moisture retrieval, etc.). Recently, imaging of the ocean surface from a GNSS-R spaceborne receiver has been proposed by treating the measured delay-Doppler Maps (DDM) as a blurred image of the surface's scattering coefficient in the delay-Doppler domain. Thus, by deconvolving the DDM with the GNSS code Woodward ambiguity function (WAF), and appropriate domain transform, an image of the surface's scattering coefficient distribution can be obtained. In this work this technique is applied to oil slick detection. A realistic scenario is simulated, and the performance of the GNSS-R imaging technique is evaluated on the retrieved image. Error below 10% in the retrieved scattering coefficient distribution is achieved (except for regions affected by deconvolution artifacts), and the image resolution of the order of 2 km is comparable to that of a synthetic aperture radar (SAR) system with equivalent specifications.