The Cromwell field occurs in a belt or zone of en echelon faulting as expressed in the surface sediments which are composed of sandstone and shale belonging to the Francis formation of Pennsylvanian age. The surface irregularities and faulting are believed to be a reflection of subsurface folding, and possibly subsurface faulting. The Cromwell sand (Lyons sand) of Pennsylvanian age has been the source of practically all of the oil. The "Wilcox" sand, which is one of the sand members of the Simpson formation of Ordovician age, has produced a small amount of oil. The Cromwell and the "Wilcox" sands are found at average depths of 3,400 feet and 4,150 feet, respectively. The migration of oil up the dip from the west has been intercepted by two subsurface structural f atures, namely, a fault with more than 214 feet of displacement and a dome with 70 feet of closure. Production bears a close relation to these structural features, and there would have been a closer relation had local sand conditions not interfered.