O Original oil in place evaluation started when the exploration drilling foundoil. The early development step of “X” Reservoir is determining the originalvolume of oil in place using volumetric method and material balance method.The type of the “X” reservoir is undersaturated toward saturated condition.It caused by the declining pressure from higher than the bubble point pressuretowards under bubble point pressure as time went. Drive mechanism determinedby using the qualitative analysis and from the drive index, the drive mechanism of“X” Reservoir is water drive.Based on the calculation of original oil in place in this Final Assignmentby using the volumetric method is 11,909 MMSTB, while using material balancehavlena odeh method the original oil in place is 11,202 MMSTB and usingmaterial balance method is 10,773 MMSTB. The percentage difference betweenvolumetric method and havlena odeh method is 6% and the percentage differentbetween volumetric method and material balance method is 10%