Integrated Approach to Stratigraphic Architecture and Reservoir Analysis of the Eutaw Formation: Eastern Gilbertown Field, Alabama

Abstract

Abstract: The Gilbertown Oil Field in Choctaw County, Alabama is the oldest commercial oil field in the state. Today the field is extremely mature and is at risk of abandonment. More than 85% of the oil produced in Gilbertown Field is from glauconitic sandstone of the Cretaceous-age Eutaw Formation. The Eutaw Formation comprises 7 major reservoir units dominated by glauconitic sandstone. This sandstone constitutes low-resistivity, low-contrast pay that is difficult to characterize using conventional geophysical log analysis. Highly conductive minerals such as glauconite and siderite, which make up as much as 30% of the reservoir rock, are the primary reason for the low resistivity pay zones in the Eutaw Formation. The primary goal of this study is to employ an integrated approach that utilizes wireline log analysis, core analysis, and thin section analysis to characterize the stratigraphic architecture and reservoir quality of the Eutaw Formation at Gilbertown Field. The results of this analysis could be crucial for guiding the future development of the oil field and for avoiding premature abandonment. The central hypothesis of this research is that the low resistivity signatures in wireline logs of the Eutaw Formation can obscure hydrocarbon-bearing zones, thus increasing the potential for bypassed pay. The eastern part of Gilbertown Oil Field has produced nearly 12 MMbbl of oil from the Eutaw Formation since initial development in 1945. An integrated method for volumetric analysis and pay evaluation in the eastern part of the field resulted in an estimated 82.4 MMbbl of original oil-in-place in the Eutaw Formation. To date, less than 15% of this oil has been recovered, and geological analysis indicates that the potential for improving recovery and revitalizing Gilbertown Field is high.