Unlocking New Potential From Weathered Basement Reservoir at South Sumatra Basin, Study Case Jabung Block and Pertamina Aset 2 Area

Abstract

Nowadays, the hydrocarbon potential of the reservoir basement is focused on the fracture zone. If there are no massive fault and fracture zone at the basement, it is believed that the basement reservoir has no hydrocarbon potential. Based on the evaluation and successful of the perforation program in the Jabung Block and Pertamina Asset 2 Area, a new understanding and opportunity was obtained in unlocking the potential in the weathered basement. Referring to this success, it is understood that a weathered basement reservoir can be a good reservoir even though it does not have an intensive natural fracture.

Analysis has been done using well log data and petrophysical evaluation for the characteristics of weathered basement zone. Also refer to successful fracturing program and production for this potential reservoir. The research area has a basement of granite and granodiorite. The weathered zone is characterized by high gamma ray and resistivity, which is as fresh basement zone, but the density value is lower than the fresh basement zone. In the previous method, the weathered basement reservoir was considered to be a basement part and did not have an intensive fracture. DST results from several wells obtained a small rate, than it was decided not to continue producing from basement and moved to the deltaic reservoir that has better reservoir property and more than 200 wells have been drilled to produce the deltaic zone. There are only a dozen wells that have log data up to a weathered basement. Based on the new concept, weathered basement can be separated from fresh basement. The weathered basement has better matrix property than fresh basement. In Jabung Block, it was decided to try hydraulic fracturing at this zone. The result of fracturing at INA-1 well in 2018 obtained gas rate of 3 MMSCFD, which is significantly increased compared with previous conventional perforation 0.6 MMSCFD gas rate. Through flow channel fracturing on oil well INA-2 and INA-3 in 2018 at the neighbor structure, gives oil rate 146 BOPD and 300 BOPD, respectively. Following those results, more detailed evaluation of electric logs, mud logs, and seismic attributes using AVO have been done to know hydrocarbon potential in weathered basement. In the Pertamina Asset 2 area, in one of the fields, there are 10 wells that penetrate the weathered basement zone. Perforation efforts have been made on 3 wells in 2004 & 2011 but only get a low flow rate of 0.3 - 1 BOPD.

A more detailed evaluation of the potential and completion methods include technology that could be implemented to produce a more massive weathered basement is still being carried out. It is believed that this reservoir potential also occurred at other structures in South Sumatera Basin and the other Basins in Indonesia.