Low Resistivity Reservoir Identification of Gumai Formation Using Weighted Scoring Method in West Benakat, South Sumatra Basin, Indonesia

Abstract

Abstract The mature oil field of West Benakat is located in the South Sumatra Basin, one of the most hydrocarbon prolific Indonesian tertiary back-arc basins in Indonesia. Since its discovery in 1933, West Benakat Field has been operating and producing oil for nearly nine decades from Talang Akar Formation (TAF) as the main sand reservoir target with good porosity and permeability. Recently, Talang Akar reservoirs have been depleted through massive development programs, whereas more than 290 wells have been drilled for exploration, production, and injection wells. Decreasing oil production pushes the company's need for an alternative method that can increase oil production through subsurface evaluation in West Benakat Field to find the upside potential besides Talang Akar. One of the best alternative ways focused on the low resistivity pay zone from Gumai Formation (GUF) and Air Benakat Formation (ABF). The qualitative analysis through several well log data in Gumai Formation confirms the potential of low resistivity shaly sand reservoirs, GUF-1, GUF-2, and GUF-3, with neutron-density crossover. However, Talang Akar sand has an average resistivity value equally above 10 ohms. The low resistivity reservoir in Gumai turbidite deep marine sand could be occurred because of high shale content, fine grain sand, and the presence of laminated conductive clay minerals, such as glauconite. However, a low resistivity reservoir was initially considered unattractive because many major reservoirs which contain hydrocarbon would have a high resistivity value. Generally, the conventional petrophysical approach for low resistivity reservoirs in Gumai or Air Benakat Sand could lead to bias and pessimistic interpretation, yet in some cases, rocks with low resistivity have the potential to become hydrocarbon reservoirs. This study discusses how to identify and evaluate low resistivity pay zones and the success story of proving the Gumai hydrocarbon potential in West Benakat Field, South Sumatra.