Abstract

North Robertson unit (NRU) produces from highly heterogeneous, shallow-marine platform carbonate reservoirs of the Permian (Leonardian) Clear Fork and lowermost Glorieta formations. Discovered in 1955, the productive area encompasses 5366 ac (2173 ha) along the northern border of the Central Basin platform (CBP) in the Permian basin of west Texas. Clear Fork/Glorieta production is widespread in this portion of the basin, with most fields, including NRU, now under waterflood. Primary production was on 40 ac (16 ha) spacing, with enhanced recovery dependent upon a combination of 20 ac (8 ha) infill drilling and waterflooding. Total recoveries in these fields have been typically below 25%, with many waterfloods performing below prediction due to a high level of lateral and vertical reservoir heterogeneity. Detailed geological and petrophysical analysis of new and existing cores from NRU, performed as part of a combined industry-U.S. Department of Energy (DOE)-sponsored project, has resulted in greatly improved reservoir characterization of the Clear Fork productive interval, ultimately allowing for the quantification and mapping of specific productive intervals within the 1000 ft+ (300 m+) gross reservoir interval. Relevant data have been used to identify optimal sites for strategic 10 ac (4 ha) infill development aimed at maximizing the efficiency and profitability of future oil-reserve growth within the unit. A total of 14 producing wells and 4 injector wells were drilled on the basis of the new geologic engineering model. Results of these wells were impressive and confirmed the value of targeted infill development in such a complex reservoir. Part I of this article summarizes field data and geologic information essential to the reservoir characterization effort in NRU. Part II of this article, to appear in the November issue of the Bulletin, will discuss petrophysical and engineering data, as well as the targeted infill drilling effort and its results.

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