Publications

Exploration of fractured basement reservoirs has become a great subject of interest in oil and gas industries due to depleting hydrocarbon reserves in many conventional sources. In 2013, PetroChina Intl. Jabung Ltd. successfully drilled the Northeast Betara (NEB) Base-1 well located in South Sumatra Basin to 3000' MD / 2000' TVD through the basement, primarily targeting a fractured basement reservoir. Barefoot test showed that gas and condensate flowed from the fractured basement. Following the success of NEB Base-1 well drilling, NEB Base-2 well was drilled in 2015 at a greater depth than NEB Base-1 well to reach the basement. Interestingly, the barefoot test in NEB Base-2 well showed no indication of influx from the well. Therefore, further investigation on the fracture characteristics from both wells was conducted to better understand their effects on hydrocarbon accumulation. Mudlog and gamma ray log data suggested that both wells have similar granitic basement lithology. However, image log data showed resistivity contrast enhancement for NEB Base-1 well which was associated with open fractures. On the other hand, limited resistivity contrast was observed in NEB Base-2 well, suggesting that the fractures were closed or filled with minerals. The fracture orientation of NEB Base-1 well also indicated that the fracture azimuth is oriented towards ENE-WSW, which is in the direction of compressional tectonics. In contrast, the fractures observed in the NEB Base-2 well are oriented towards NNE-SSW orientation, which is in the direction of extensional tectonics. In addition, compressional sonic data from NEB Base-1 well showed more cycle skipping compared to NEB Base-2 well and NEB Base-1 resistivity log is lower than in NEB Base-2 well. The differences in fracture characteristics of both wells were attributed to the difference in their stratigraphic succession during the syn-rift phase of basin evolution. Palinspastic reconstruction in the NEB area suggested that the area of NEB Base-1 experienced inversion or up-lift in the post-rift phase. However, there is no indication of inversion after the syn-rift phase around the NEB Base-2 area. Hence, tectonic inversion played an important role in determining the location of hydrocarbon accumulations in the fractured basement. Keywords: Inversion Tectonic, Fractured Basement, Palinspatic Reconstruction