The Apco field of north-central Pecos County, Texas, discovered June 1, 1939, has produced, from 46 wells in an area of 1,840 acres, more than 2,646,000 barrels of 42.6° gravity oil to January 1, 1947. The field is on the northwestern flank of the Fort Stockton granite ridge. Oil is produced from a buried hogback of weathered Ellenburger dolomite (lower Ordovician) at depths varying from 4,300 to 4,600 feet. The Ellenburger dolomite rests on a basal sandstone which overlies the pre-Cambrian complex of igneous and metamorphic rocks. The reservoir consists of fracture and honeycomb types of porosity, the latter of which was probably developed by ground-water action during pre-Permian exposure and weathering. The reservoir trap is formed by overlapping and practically fl t-lying middle Permian shaly dolomite which envelopes the hogback. An area of pre-Cambrian granite, gabbro, schist, and gneiss, immediately below the Permian, is present on the south and east sides of production at elevations lower than the Ellenburger ridge. Insoluble-residue studies reveal a buried Ellenburger dip slope, held up by a resistant zone approximately 450 feet above the base of the Ellenburger. The restored Ellenburger structure dips from 600 to 1,000 feet per mile west and northwest. Shallow Permian structure, which is probably due to compaction, reflects the hogback as a small closure.