On 28 May 2006, PT Lapindo Brantas targeted gas in the Kujung Formation carbonates in the Brantas PSC area by drilling a borehole named the Banjar-Panji 1 exploration well. The drill string went into a thick clay seam (500–1,300 m deep), and then sands, shells, volcanic debris and into permeable carbonate rocks.[1] At 5:00 a.m. local time (UTC+8), the drill string went deeper to about 2,834 m (9,298 feet), after which water, steam and a small amount of gas erupted at a location about 200 m southwest of the well. Two further eruptions occurred on the second and the third of June about 800–1000 m northwest of the well, but these stopped on 5 June 2006. During these eruptions, hydrogen sulphide gas was released and local villages observed mud at hot temperature, around 60°C or 140°F.
From a model developed by a geologist, the drilling pipe penetrated the overpressured limestone, causing entrainment of mud by water. The influx of water to the well bore caused a hydrofracture, but the steam and water did not go through the borehole; they penetrated the surrounding overburden pressured strata. The pressure formed some fractures around the borehole to propagate to the surface 200 m away from the well. The most likely cause of these hydraulic fractures in the shallowest strata is by the unprotected drill string with a steel casing. Borehole protection by steel casing has been a common procedure in oil or gas exploration.
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