T he research area is in the X Well which located in the North Sumatra Basin. There have been several studies conducted in the area, but there is still very little information related to the age of the Formations in the basin. A total of 128 sample are used for lithostratigraphy analysis, in which 33 samples were analyzed further for the Foraminifera biostratigraphy study from the X Well at a depth of 6090 – 9919 feet. From the result of the analysis, 38 species from 16 genera successfully identified, as well as 5 datum species, namely Globigerinoides sicanus, Orbulina suturalis, Globorotalia menardii, Globorotalia plesiotumida dan Globorotalia tumida tumida. From the lithology data, three (3) lithostratigraphic unit successfully identified, namely the calcareous claystone, very fine calcareous sandstone and fine calcareous sandstone unit which is estimated to be comparable to the Baong, Keutapang and Seurula Formation. From the result of the study it’s concluded that the lithostratigraphic units found in the study area have the age range of N7-N18 or late part of the Early Miocene to the late part of the Late Miocene.