ABSTRACT

The Pegasus Ellenburger Field is located approximately 32 km (20 miles) south of the city of Midland in southern Midland and northern Upton counties, Texas, Mobil Oil is the operator. The Ellenburger is Lower Ordovician (Canadian) in age, and occurs at an average depth of 3960 m (13,000 feet). Approximately 14.3 million cubic metres (90 million barrels) of oil have been produced since its discovery in 1949. A volumetric estimate of original oil-in-place is 34.3 million cubic metres (216 million stock-tank barrels). Production is structurally controlled with contours on the top Ellenburger unconformity surface defining a fault-bounded, 3-dome anticline with a north-south axial trend.

The geological model developed in the early 1950's proposed fracture-controlled production with lateral and vertical communication throughout the reservoir. Porous, dolomite breccias spatially segregated by dense, non-brecciated dolomite were interpreted as being in communication via the fracture network. Three different secondary recovery programs have been predicated upon this model. This includes two abandoned, unsuccessful waterfloods, and an ongoing crestal gas injection.

An integrated reservoir engineering, operations engineering, and geological study was completed during a 3-year (1985-88) field re-evaluation. Four infill wells were drilled, two existing wellbores were deepened, and thirty-two re-entries and workovers were completed. Data obtained from these operations, plus 36 years of production and reservoir data, indicate stratified production in a complex paleocave or karsted reservoir system. Vertical communication is limited.

Primary storage capacity is located within an apparently extensive solution-collapse and cave-fill breccia interval, which occurs approximately 46 to 61 m (150-200 ft.) below the unconformity surface. Interclast porosity ranging from 3 to 20 per cent typifies this interval. Production from the overlying dense, non-brecciated Ellenburger cave roof is fracture controlled.

A relatively minor interval of porosity occurs approximately 30 m (100 ft.) below the prominent cave-fill horizon. Bottom-hole pressure data, well tests, and fluid analyses data confirm vertical isolation of these 3 productive intervals.

This multidisciplinary approach to reservoir description has provided a model for future field evaluation by integrating field performance data with geological interpretations. The revised reservoir model has identified considerable opportunity for incremental oil production by defining the distribution of remaining oil reserves. Modifications to the existing crestal gas injection program are being implemented to improve recovery efficiency.

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