Catalytic decarboxylations of fatty acids in immature oil source rock samples were examined in this study. The rock samples were obtained from seven oil fields in China. In order to clarify the effect of each mineral matter in the rock samples, both the Fe M?ssbauer effect and the X-ray diffraction (XRD) were used to determine the relative content of each mineral in the rock samples, and the catalytic activities of several minerals like clays, carbonates and pyrite were determined. The Fe M?ssbauer effect and the XRD studies show that clays are the main mineral components in the rock samples except for the samples from Biyang and Jianghan in which the main mineral component is ankerite. The other mineral components include calcite, plagioclase, quartz, feldspar, siderite, aragonite, pyrite, analcime, pyroxene and anhydrite. The studies of the catalytic decarboxylations of fatty acids suggest that carbonates and pyrite can make much greater contributions to the catalytic activities of the rock samples than clays. It is found that the overall catalytic activities of the rock samples are well related to the relative contents and the catalytic activities of clays, carbonates and pyrite in the rock samples.