Publications

The Northeast Java Basin has made a major contribution to Indonesia’s oil and gas production since its earliest exploration in the late 19th century. Known as a mature Tertiary basin, the exploration and production in this basin has been primarily concentrated to Tertiary petroleum systems, in which known source rocks are particularly the Middle Eocene to Early Oligocene Ngimbang Formation. However, numerous geological, geophysical and geochemical studies have pointed out the possibility of other source rocks of Cretaceous age below the Ngimbang Formation. Previous seismic studies have confirmed the presence of sediments older than the Ngimbang Formation. The evidence of these sediments parallels to the tectonic reconstruction of the Northeast Java Basin which indicates the possibility of a Western Australia-derived microcontinent underlying the basin. The microcontinent drifted northward from Gondwanaland since the Triassic age to the southeastern margin of Sundaland where the future Northeast Java Basin is located. During the fragment’s movement, syn-drifting marine deposits of lower Cretaceous age were deposited on the fragment before the fragment was docked in its current place underneath the eastern margin of the Northeast Java Basin during Middle Cretaceous, where burial of future Cenozoic sediments along with tectonic activities further caused the Lower Cretaceous deposits to be mature. Those processes triggered the maturation of source rocks until they generated hydrocarbons which have migrated and currently have been trapped in geological structures of the Kangean area. This study reveals other evidence to support the presence of Mesozoic-aged Pre-Ngimbang source rocks based on a biomarker attained from aromatic fraction of three oil samples from Sepanjang Island, Kangean area namely triaromatic dinosteroid. Oils from Sepanjang Island-1, Sepanjang Island-3, and Sepanjang Island-4 well were sampled and their aromatic fraction was tested. The result shows a high number for the Triaromatic Dinosteroid Index for all samples which may highly correlate to the Cretaceous age, as also supported by previous studies mentioning the presence of lower than 20% Oleanane Index which is very common for samples from older sources than Cenozoic sediments. Based on this understanding, the exploration target of the basin may be shifted from Cenozoic sources to the recently discovered Mesozoic petroleum system of the Northeast Java Basin.