Pertamina EP has recently drilled an exploration well in Akasia Maju Field located in the West Java Basin

Area, Indonesia. The well penetrated the main reservoir over clastics play of Lower – Upper Cibulakan Formation. Recognizing these reservoirs from standard logging data (Gamma Ray, Neutron-Density and Resistivity) were very challenging, as they had the characteristics of tight, low contrast, low resistivity, limited to no neutron density crossover, and in some cases, high GR. Consequently, these reservoirs were easily missed on the previous exploration activity.

Considering this situation, Pertamina EP has decided to change their wireline logging strategy, from the standard wireline logging suite to the combination of advanced wireline logging suite. This advanced combination was comprised of borehole resistivity image logs, nuclear magnetic resonance, and dielectric logs. These were mainly used to help identify potential reservoirs, optimize DST intervals, and distinguish between high water saturation and residual pay zones. In addition, the formation tester tool was run to measure formation pressures, obtain fluid samples, and determine fluid types and contacts.

This combination has successfully revealed the presence of hydrocarbon zones on the intervals that had no clear indication of hydrocarbon occurrence in both sandstone and limestone zones of the Cibulakan Formation. In these hydrocarbon zones, the textural analysis of resistivity image logs indicates that they were correlated either with the well sorted rock fraction (in sandstone) or with the secondary porosity development (in limestone). Meanwhile, the nuclear magnetic resonance reveals potential reservoir candidates. The potential hydrocarbon zones were indicated by the prominence of specific porosity bins. Particularly, porosity bin number 6 (100-300 ms) and porosity bin number 7 (300-1000 ms). This is not usually the case, as hydrocarbon association cannot be determined using porosity bin distribution alone. The formation tester on Upper Cibulakan formation revealed the medium mobility zone in which pressure and fluid analysis data were acquired using the conventional formation tester. However, in the Lower Cibulakan formation, the mobility data showed very low to low mobility zone. Hence, the advanced formation tester was used to obtain pressure data and perform conclusive fluid analysis.

Using this data combination, Pertamina EP managed to optimally select four DST intervals in Cibulakan Formation. The DST result was satisfactory, with the maximum oil rate reaching up to 1700 BOPD and the gas rate reaching up to 6.1 MMSCFD, positioning this well as a significant recent hydrocarbon discovery in Indonesia.

From this case study, it can be concluded that the use of advanced wireline logging suite yielded definitive results.

* Pertamina EP **Schlumberger

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