Since 1936 the Penal oil field of Shell Trinidad, Ltd., situated in southern Trinidad, B.W.I., has been developed into a 6,500 b/d field producing both from a shallow pool and from deeper, Miocene Herrera sands at depths ranging from 4,000 to 10,000 feet. The Herrera has been proved productive from three superimposed tectonic levels, of which the intermediate one forms an overturned limb or an intricately built wedge. About 60 Herrera wells produce oil, condensate and/or gas; some of them came in with an initial production of more than 1,000 b/d, and five wells have reached a cumulative production of more than 1,000,000 barrels. The sands range in thickness from 50 to 300 feet in the overthrust, and are more than 800 feet thick in the second (intermediate) and third levels (underthrust). The distribution of oil, gas, and water is erratic and partly unexplained—a fact which, together with the complicated conditions of the tectonics and lithofacies, renders economic development of this interesting field somewhat speculative.