A petrological study of core samples from the Toolachee and Patchawarra formations in the Dullingari Field indicates that the sandstones are predominantly quartz rich and also contain small amounts of mica and metamorphic and sedimentary rock fragments. Patchawarra samples are generally finer-grained, contain more mica and also have lower porosities and permeabilities for equivalent grain size than the Toolachee samples. Diagenetic changes which have affected both formations include widespread quartz overgrowth cementation, siderite cementation and replacement, authigenic clay formation, and later quartz dissolution and microstylolite formation. A combination of these processes has reduced permeabilities to below 1 millidarcy in many samples. Dissolution of unstable grains and carbonate has resulted in some secondary porosity being developed in many samples, but high permeabilities recorded from some Toolachee samples result from incomplete quartz overgrowth cementation which has allowed the retention of some primary porosity. The lower porosities and permeabilities which are typical of the Patchawarra Formation are due to more abundant authigenic illite and microstylolite, which in turn relates to the higher mica content of the Patchawarra Formation compared to the Toolachee Formation. Sandstones from both formations are sensitive to hydrochloric acid due to the presence of siderite, and the presence of authigenic clays may cause fines migration problems. However, few problems with clay swelling are anticipated.