Abstract

Stanvac's Raja field in South Sumatra was discovered in 1940 and currently has 68 wells in it. It produces oil mainly from the prolific Talang Akar Formation (Late Oligocene-Early Miocene). The formation is represented by a transgressive sand – shale sequence deposited under environmental conditions which changed with time from continental to nearshore. The Talang Akar reservoir rock distribution is laterally restricted and accurately mapping distribution is difficult but critical to efficient development of the field.

Environmental analysis has been used to aid in the understanding of Talang Akar sand distribution at Raja Field. Isopach maps of the varioius oil producing zones in the Talang Akar Formation consistently show two places where the section is thinner than in other parts of the area. Those thickness differences are more pronounced in the lower part of the formation. Apparently, those anomalies are caused by two old highs on the pre-Tertiary basement surface just below the Talang Akar sequence. The isopach maps also show that depositional strike during the Talang Akar deposition time was east-west. Consequently, beach complex sands were anticipated to have an east-west trend whereas channel sands would be expected to have a north-south trend.

Generally, the beach complex and channel sands can be identified on logs and mapped according to their respective trends. Data from new drilling in the Raja field helps refine the interpretation of the reservoir rock distribution and, in a broader sense, the geological picture of the area.