Publications

The Eocene Tanjung Formation is a proven oil reservoir as well as a proven source interval primarily responsible for the Barito Basin petroleum system in the North Barito area of Kambitin, Tanjung, and surrounding fields. However, in the northwest end of the Barito Basin most of the wells which drilled through the Tanjung interval resulted in dry holes. This paper describes the reservoir characteristics of the Tanjung Formation in the Northwest Barito area, primarily from new outcrops in the Bintang Ninggi and Benangin areas with additional comparative subsurface data from exploration wells. The goal is to further investigate whether the reservoir or non-reservoir components of the petroleum system are responsible for the lack of exploration success in the Northwest Barito Basin. The Tanjung Formation has historically been divided into Lower and Upper members, the Lower Tanjung, which is more terrestrial, and the Upper Tanjung which is more transitional to shallow marine in character. The Lower Tanjung consists of three facies: alluvial fan, braided river, and meandering river. The alluvial fan facies is characterized by interbedded conglomeratic sandstone with coarse- to-medium grained sandstone. The fluvial facies is characterized by interbedded pebbly sandstone with coarse-to-medium grained sandstone and intercalation of shales, coal and paleosols. The Upper Tanjung consists of lower delta plain to delta front facies characterized by interbedded sandstone and shale with coal intercalation. The fluvial braided-to-meandering facies of the Lower Tanjung has better reservoir potential than the lower-delta-plain to delta-front facies of the Upper Tanjung. This is indicated by the results of surface observation and laboratory analysis showing that the fluvial braided to meandering facies has a sand-shale ratio of 73%, porosity up to 29% and permeability up to 16 mD with the predominance secondary porosity (dissolution). In the Upper Tanjung, the lower-delta-plain to delta-front facies has a sand-shale ratio of 52%, porosity 24% and permeability 19 mD with intergranular porosity. The differences in porosity types are due to the variable burial history of the Lower and Upper Tanjung facies. Maturity measurements and basin modeling indicate that the Tanjung Formation in the NW Barito basin is mature to over-mature indicating significant burial and uplift has occurred. As a result of the deep burial, over-maturity of the Eocene source rocks and secondary porosity development in the Tanjung reservoirs a new play concept has been formulated. Successful exploration efforts in the area could depend on the preservation of source rocks younger than Eocene, charging Eocene reservoir sections with enhanced secondary porosity.