Publications

An integrated regional geological, biostratigraphic and geochemical study of the Langsa Block was undertaken by J.O.B. Pertamina - TCR Langsa Ltd. in cooperation with P.T. Robertson Utama Indonesia. The principal aim of this study was to resolve stratigraphic and regional seismic correlation ambiguities in the Lower to lower-most Middle Miocene and to develop a geochemical model capable of explaining hydrocarbon charge in, and areas marginal to, the Langsa Block.The results of this study indicate that within the study area the Miocene consists of a series of depositional multicycles, termed B, C and D separated by phases of non-deposition and erosion resulting in unconformities. The termination of these multicycles appears to be related to regional tectonic phases.Each multicpcle is composed of several cycles, all representing a gradual rise and subsequent fall in sea-level. Within the B-multicycle, four such cycles were recognized while within the C-multicycle, five cycles are considered present. The D-multicycle is thought to be composed of several cycles, however only one could be positively identified. Although Amulticycle sediments have not been penetrated on the Langsa Block, and consequently no subdivision has been made, they are interpreted on regional seismic lines.The fluctuations in relative sea-level recognized are considered the combined result of global eustacy, subsidence rate and sediment supply and are consequently thought to be specific for this basin. Paleogeographic reconstructions formed the basis for interpretation of source rock distribution and type within the study area. Geochemical analysis of hydrocarbons suggests that two principal source rock types have generated hydrocarbons within the Langsa Block and surrounding area. These comprise a predominantly algal, probably lacustrine, source type (initial A-Multicycle) and a mixed marine algalkerrestrial source type (later A-Multicycle) . Younger source rocks (B and C-Multicycles) have also been identified but no oils in the study area have been typed to these.Timing of hydrocarbon generation and migration from A-Multicycle source rocks varies according to subsidence and thermal histories of the depocenters. It is likely that oil generation may have commenced at the beginning of the Miocene in the centers of. the deepest grabens and continues to the present in the margins o the grabens. Gas generation commenced significantly later (Late Miocene) in most basinal areas and is still continuing.The application of sequence stratigraphic concepts in the Langsa Block served as an important tool in the resolution and evaluation of the Miocene and pre- Miocene stratigraphy, petroleum geochemistry, hydrocarbon prospectivity and in the definition of additional play-prospect trends.