Truncated Devonian and Fusselman fields and their relationship to Permian Basin reserves

The Permian System accounts for a majority of oil produced in the Permian Basin. However, the Devonian rocks and Silurian Fusselman rocks are excellent producing zones. These produce on both structure and truncation, but this paper deals with truncated aspects only. It appears that production in these fields is the result of an updip pinch-out of the formation along with lateral closure to trap the hydrocarbons. Lateral closure must be by faulting and/or reentrants along either or both sides of the pinch-out. When a prospect exhibits all these features, production should be assured. The truncated Fusselman along the Eastern shelf produces both from dolomite and limestone ranging in depth from 8,000 to 11,000 ft. It is believed that both the overlying Woodford Shale and underlying Sylvan Shale act as source rock and seal. When exploring for the truncated Devonian in southern Crane and northern Pecos Counties, Texas, a third requisite is required: tripolitic chert. This unique rock appears to be the result of subaerial erosion. The tripolitic chert is usually found in downthrown fault blocks or grabens which are present along the flank of the "Old Fort Stockton high." These faults appear to be adjustment features which formed as a result of movement along and around this positive area.