The Wolfcamp and Bone Spring formations of Lower Permian age in the Permian superbasin of west Texas and southeast New Mexico have contributed greatly to the US shale oil revolution in the recent decade. These formations were deposited in a rapidly subsiding basin at the southern margin of the North America craton during collisional tectonics in Late Paleozoic times. Here we analyze production data from nearly 4,800 horizontal wells drilled into Welfcamp and Bone Spring to constrain petroleum accumulations. The data indicate a bimodal distribution of oil and gas occurrence in Wolfcamp suggesting the migration of oil and gas from deeper (∼11,000 ft.) to shallower levels (4,000-6,000 ft.) in the same formation. Oil accumulation in Bone Spring is concentrated at the depths of 11,000-7,000 ft. with some intervals being more productive mimicking the alternating position of shaley and sandy layers in this formation. High gas-to-oil ratios are found at shallower levels for both formations. This study offers an application of reverse engineering to petroleum system analysis and supports the concept of intra-formational migration in the tight (shale) oil formations.