However, exploration in basement in Sumatra remained minimal until 1991 when the first major basement gas discovery was made in the South Sumatra Basin.

Giant oil discoveries in naturally fractured and weathered basement in Venezuela in 1953, Libya in 1965 and Vietnam in 1975 (Koning, 2020) encouraged more exploration worldwide in basement. Most discoveries of oil and gas in basement were made вЂњby accidentвЂќ rather than by deliberately exploring the basement. This lack of interest in basement is due to a general perception of the industry worldwide that basement is typically вЂњtightвЂќ and did not warrant investigation. Repsol as operator has a 45% interest in the discovery along with partners Petronas with 45% and Mitsui with 10%.Дuring the many decades of exploration in Sumatra, little attention was placed on exploring for oil or gas in basement. The KBD-2X well was reported to have flowed gas at 45 MMcfpd. This was the largest hydrocarbon discovery in Indonesia in 18 years since the ExxonMobil Cepu oil discovery in 2001 in Central Java (Repsol, 2019, Wood Mackenzie, 2019). The well targeted the basement reservoirs that are highly gas productive in the nearby Corridor Block operated by ConocoPhillips. RepsolвЂ™s discovery well was the Kali Berau Dalam (KBD)-2X well, drilled on the Sakakemang Block, in the South Sumatra Basin and encountered at least 2 TCF of recoverable gas.

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