Abstract

The Permian Tannehill Sandstones represent a fluvial/deltaic meander belt that was part of an extensive depositional system which shifted laterally along the paleoslope of the Eastern shelf toward the slowly subsiding Midland Basin.

Within the producing interval, stratigraphic traps commonly are formed by structural closure due to differential compaction and updip pinchouts against clay plugs of former channel thalwegs. Porosities range from 20-30%, with permeabilities of 200-600 md. The Tannehill Sandstone is divided into an upper and lower member, separated by the Stockwether Limestone. An abnormally low resistivity value (2 ohm-meters) is observed in the oil-saturated portions of the sandstone. Core analyses show that this abnormal value does not result from high water saturation levels, but instead is probably due to the retention of water in clay lenses found within the sandstone bodies, and their highly conductive nature.

Ultimate recoverable oil in the Tannehill is estimated to average 400 barrels per acre-foot. The average initial production rate of the Tannehill wells in the study area is 61.8 barrels of oil per day, with an average specific gravity of 37.1°. The relatively shallow depth (2200-2700 feet) of the Tannehill and the low cost of drilling produce favorable exploration prospects.