Information Systems are evolving, integrating more and more mobile & heterogenous resources, in addition to those traditionally available on data centers and cloud platforms. Among the current trends leading to this new generation, called Pervasive Information Systems, we may cite the fog computing. This paradigm promotes an opportunistic use of devices on the edge of the network, by bringing computation nearest the user and the data production. Nevertheless, heterogeneity of fog environments complexify the resource management, and notably the task scheduling. In this paper, we tackle this issue by proposing a conceptual architecture supporting an opportunistic resource management of fog resources on PIS. The cornerstone of the solution is the use of context information for (i) guiding the tasks scheduling on heterogeneous & dynamic environment; and (ii) expressing the resources & services policies conditioning their use.