New perspectives on the source rock facies of the Andaman Trough, offshore North Sumatra based on the latest onshore oil sampling and analysis

Abstract

The Andaman sub-basin, located offshore Aceh Indonesia, is considered to be one of Indonesia’s most underexplored basins, despite its proximity to the giant gas and condensate fields of Arun, NSO A, NSO J and South Lhok Sukon, where in excess of 6 MMboe has been produced to date. The understanding of the Petroleum System in the offshore Andaman Trough, has historically been challenged by poor imaging of the basin architecture and limited penetration and retrieval of source rock intervals and hydrocarbon fluids for analysis. Mubadala Petroleum, as operator of the Andaman I PSC, conducted a geological field study to collect multiple oil samples from fourteen (14) onshore traditional wells across the Bireun and Aceh Timur Administrative District (Figure 1). Those samples were analyzed in laboratory for their physical properties and parameters derived from those analyzes where integrated to fully characterize oils produced in the onshore Aceh area and establish the organofacies and maturity of their source facies precursors. The results were then used as calibration for the analysis and subsurface modeling of the offshore petroleum system of the Andaman sub-basin. Previous authors have postulated that Late Oligocene to Early Miocene marine shales were the main source rocks for oil in the Andaman Trough. Oil samples collected onshore as part of this study however, were sourced by peak to late mature oil-prone lacustrine source rock facies, yielding high API (42.7 – 50.8°), low pour point, low sulphur, and low wax content fluids. Integration of these findings with the upgraded tectono-stratigraphic framework provided by the 2019 MC3D survey, reprocessed multi-vintage 2D, and reinterpretation of biostratigraphic analysis, has enabled the delineation of a postulated Paleogene lacustrine source rock facies in the Andaman Trough. This model does not preclude the potential of other source rock facies to be present and active within the Andaman Trough, including the gas-prone fluvial Eocene-Oligocene Parapat Formation, but it supports the possibility that oil may have been generated in the Andaman Trough.