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.
Freshwater environment and high clay content are quite common in Indonesia. This introduces certain challenge in performing hydrocarbon identification and evaluation especially in already cased wells. In old producer wells, possible conditions such as fluid channeling behind casing and trapped hydrocarbon in annulus add more complexity in performing behind casing analysis to understand current reservoir condition. In order to increase the success in finding remaining hydrocarbon potential, PERTAMINA has deployed pulsed neutron logs (PNL) to accurately pinpoint the targeted interval for perforation. Since 2017, the PNL campaign has covered approximately 160 wells in PERTAMINA's development fields across Indonesia up until now. PNL service offers nuclear-based statistical measurement such as sigma, thermal neutron decay porosity (TPHI), and carbon-oxygen yield that allows simultaneous oil and gas saturation evaluation without any dependence on water salinity and other electrical properties of the formation and fluid. It also allows computation of elemental dry weight from elemental spectroscopy data which can be utilized to determine lithology to complement the standard open-hole logs dataset. The more advanced PNL tool raises the bar even further by offering new measurement of fast neutron capture cross section (FNXS) log which is useful to identify gas even in tight rock formation. The latest generation also features self-compensation algorithm resulting in more robust TPHI and sigma log under complex circumstances such as multi-casing/tubing. This paper showcases several prominent success stories of oil and gas findings identified from PNL interpretation in development wells. There are also several examples of elemental spectroscopy data utilization from PNL to prevent non-economical perforation by means of providing accurate lithology and porosity analysis as compared to previous result built from old and/or incomplete open-hole logs dataset. This PNL campaign has also given valuable insights of borehole and reservoir condition which might have been overlooked such as hydrocarbon in annulus, low pressure gas zone identification and batman's ear boundary effect. Low pressure gas zone may be qualitatively identified whenever TPHI from PNL is noticeably lower than neutron porosity measurement from the open-hole log. Batman's ear effect is usually observed when a body of sand is sandwiched between carbonaceous shales or coal layers resulting successive oil-water-oil saturation profile in one homogenous body of sand, shown as oil peaks at the bed boundaries similar with the appearance of batman's ear. As the sand gets thinner, these two oil peaks might merge into one solid body of high oil saturation which might not depict the true oil potential of the sand.
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