Sandstones from the Helez Formation (Lower Cretaceous) are the main oil reservoir rocks of the Helez-Kokhav field in the southern coastal plain of Israel. These sandstones make up about 10 percent of the Helez which is 200 m thick; their reservoir characteristics relate to environment of deposition and to postdepositional changes. The sandstones were derived from disintegration and redeposition of older Nubian sandstones which were exposed as a result of post-Jurassic uplift. Volcanic activity took place at the time of the uplift, as a result of which some of the sandstones are tuffaceous. The sandstones are cemented by calcite and dolomite. Amount and mineralogy of cement, which are related to the depositional environment, are the main factors influencing porosity. Perme bility is related to the amount of cement only. In sandstones with intergranular porosity, permeability is dependent on the average grain size. Petrographic study leads to the recognition of three different depositional environments--offshore marine, tidal channel or lagoonal, and dune. The dune sandstones have the most favorable reservoir characteristics; average porosity is 24 percent and permeability values average 200 md. Sandstones deposited in tidal channels or lagoons have an average porosity of 16 percent and permeability values averaging 50 md. The sandstones deposited in an offshore marine environment have a very low porosity and cannot rate as reservoir rocks.