Analysis of the Aux Vases (Mississippian) petroleum reservoirs of energy field, Williamson County, Illinois

Energy Field in Williamson County, Illinois, was discovered in 1968. Oil production is from the sandstone bar facies of the Aux Vases Formation (Valmeyeran) at a depth of about 2,400 feet. The trapping mechanism is stratigraphic; however, oil-to-water production ratios are affected by structural position. Twenty-three oil wells, 17 dry holes, and one injection well have been drilled in the field area. The field is under development. The economically important siliciclastic zone ranges from 6 to 35 feet thick and consists primarily of sandstones and shales. Two sets of distinct, sandstone bar complexes developed within this zone: one in the northern portion of the study area and one in the southern portion. Both bar complexes have lower and upper reservoirs that are up to 24 and 10 feet thick, respectively. The reservoir facies consists of fine and very fine grained sandstones cemented by clay, quartz overgrowths, and calcite. The sandstones may pinch out between wells, a distance of 660 feet, and grade laterally and vertically into encapsulating shales, calcareous shales, and calcareous sandstones. The upper sandstones also grade into limestone. The lower sandstones are marine tidal ridges or bars, apparently deposited during a marine regression, as interpreted from the evidence of multiple occurrence, morphology, petrography, and sedimentary structures visible in core. The upper sandstones are winnowed, crestal sandstone bars deposited during a marine transgression. These interpretations are based on repeated areal association of upper and lower bars, ichnofauna, associated shallow facies, and differences in petrology of the bars. The mechanism facilitating formation of the upper bars is believed to be accumulation of sediments on topographically high areas over the lower sandstones. Winnowing of these sediments by wave action produced the porosity and permeability. Over most of the field, the lower and upper sandstones are separated by a thin, impermeable interval of algal-bound, calcareous sandstones, shales, and biocalcarenitic quartzose sandstones interpreted, on the basis of features observed in core and areal distribution, to be intertidal mud flat deposits. The lower sandstone ridges occur at an apparently predictable interval of 3/4 mile, a characteristic that may prove useful in exploration for similar reservoirs. The primary drive mechanism at Energy Field is solution gas. The oil-water contact is -1 ,923 feet (subsea) in the southern bar complex and -1 ,974 feet (subsea) in the northern bar complex. These contacts occur in the lower sandstones; a separate oil-water contact in the upper sandstones has not been observed. The different oil-water contacts and initial pressure data show that the northern and southern complexes are not in communication. The original oil in place (OOIP) is calculated to be 4,004,000 barrels of oil (BO). A recovery efficiency of 20.9% of OOIP was calculated for the field; primary and secondary methods are projected to be 7.3% and 13.6%, respectively. Remaining mobile oil in place, recoverable through primary and selective secondary recovery efforts, is estimated at a minimum of 531 ,000 BO.