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The L-Structure in the Natuna D-Alpha Block of the East Natuna Basin is located in the Natuna Sea in Indonesian waters, northwest of Kalimantan. The structure is a large structural stratigraphic hydrocarbon trap and is unique in areal extent and reservoir thickness among the various carbonate traps which are present in many southeast Asian basins. The porous reef complex (Terumbu formation) has a fairly homogeneous gas column, 5,200 feet thick with CO2 content averaging 72%. The purpose of this report is to summarize the exploration drilling problems in the reservoir relating to severe lost circulation occurrences and to explain the steps which have been taken to alleviate these problems.The thick Terumbu reservoir is a fractured, vuggy limestone. Furthermore, the long gas column in the reservoir presents a pore pressure profile which ranges from abnormally high in the top, down to a fresh water gradient in the bottom. The above factors compound the task of dealing with lost circulation and lead to additional drilling problems. Much progress has been made in reducing the severity of lost circulation and lessening its effects on drilling operations. Careful casing seat setection, improved drilling practices, strict control of drilling mud properties and use of lost circulation material in the mud while drilling have been major contributors to successful penetration of the entire reservoir.