Two quantities which can be calculated from conventional electric logs of shaly sands provide useful and reliable information on the reservoir rock and the fluid it contains. These quantities are the following. 1) Shaliness - a measure of the amount of disseminated clay material in the formation. This quantity measures the ease with which the rock gives up fluids and also makes an excellent mappable attribute for the construction of subsurface lithofacies maps. The agreement between this log-derived property and the results of cation exchange capacity measurements is good. 2) Saturation ratio - the ratio of mud filtrate saturation in the invaded zone to the interstitial water saturation in the non-invaded zone. The magnitude of this ratio is indicative of the amount of displaceable hydrocarbons in the formation. The values of shaliness and saturation ratio when viewed together are related to the performance of the formation under production tests. Definite ranges in the values of these quantities are associated with a) formations which produced hydrocarbons readily, b) formations which produced hydrocarbons in commercial quantities only when some form of artificial stimulation, such as sand fracturing, was applied, and c) formations which produced water along with the hydrocarbons. A single favorability criterion is developed which is a joint function of shaliness and saturation ratio. This criterion based on electric log-derived quantities is a numerical estimate of the production performance of a formation. Its use in exploration is demonstrated by maps of its variation in shaly sand reservoirs of several oil fields. It may have value for detecting from dry-hole data the proximity to good oil production.