The Structural-Stratigraphic Framework And Petroleum Systems Of The Sandakan Basin, Offshore East Sabah, Malaysia

Abstract

Decades of exploration activities in the Sandakan Basin offshore eastern Sabah, Malaysia, since the 1970s have yet to yield commercial hydrocarbon discoveries. Of the nineteen wells that have been drilled in the basin up to 2015, only five are classified as discoveries, all made between 1970 and 1995. There are essentially two main proven play types: (1) Early to Middle Miocene “Segama play”, in which the reservoir targets are the Tanjong Formation equivalents within the Segama Group, which were deposited as part of the synrift sequence. (2) Middle to Late Miocene “Sebahat play” in which the reservoirs belong to the Sebahat Formation, characterised by prograding deltaic shoreface and shelf sequences, advancing eastwards and southwards from an uplifting hinterland in central and northern Sabah. The best reservoir facies are shoreface sands, which have porosities greater than 20%, particularly at depths shallower than ~2000 m. Although the generative source rocks have not been penetrated, geochemical data indicate that they are present at depths greater than 3200 m. The source rocks are characterised by predominantly Type III and Types II/III organic matter, which are typical of deltaic settings. The data indicate that hydrocarbons were generated by source rocks with a maturity range of 0.7 – 0.8% vitrinite reflectance (Ro). The Sandakan Basin was affected by several compressional deformation events which are expressed as major erosional unconformities; most significantly, the Middle Miocene (“D2 event”, 13.0 Ma) and Late Miocene (“D3 event”, 8.6 Ma). The unconformities were the result of compression and faulting which, while being responsible for trap formation, may also pose significant risk to trap integrity and preservation. Modelling results indicate that hydrocarbon generation and migration took place during Late Miocene–Early Pliocene and continues today. The basin’s prospectivity, therefore, critically depends on the delicate interplay between the timing of trap formation and hydrocarbon migration. Understanding these processes requires detailed understanding of the structural evolution and petroleum systems of the basin.