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Traditionally it has been assumed that Sihapas Formation facies are principally the product of the Duri-Bekasap Delta System which was constructed by a large river draining into the NE of the Basin from Sundaland, and that thicknesses and sand to shale ratios decline progressively to the SW towards the Barisan Mountains. Field work in Conoco Indonesias Mahato-Mandian Block shows that this concept must now be modified because a second and thicker, Barisan derived depocentre is present in the west of the Basin. This is related to the increasingly rapid uplift and erosion of basement rocks west of the Toru-Asik Wrench Fault (Ulu Air Fault) ahead of the magma which initiated the early MiocenE volcanic arc. Five units are recognised. The youngest, the Kanan Member, consists of marine glauconitic sandstones and defines the base of the transgressive Telisa Formation. The related Cubadak Member, which is only developed west of the Toru-Asik Wrench Fault, records a brief marine advance across the Barisan Mountains immediately prior to the onset of the main phase of volcanicity. The Sibung Member comprises fan and braided stream-type sediments associated with a sharp regression that heralded the beginning of volcanism. The undifferentiated Sihapas Formation is built up of three principal components, the fans of the Lubuk facies in the SW, the coal-bearing fluviatile Bukit Kalam facies on the crest of the Daludalu Uplift and the mainly fluviatile Pane facies in the NW. The Yang Asli Member accumulated in a rapidly subsiding half-graben, the Martona Trough, and was laid down in a predominantly tidally influenced environment. This localised depocentre, fed from the Lubuk facies fans, owes its setting to the juxtaposition of a narrow zone of unstable pre-Tertiary basement against the axis of Barisan uplift. Regionally the Kanan and Sibung correlate with the Bekasap Member of the Duri-Bekasap Delta System, the Yang Asli and undifferentiated Sihapas with the Bangko and Menggala Members.