Abstract

Among the many different petroleum systems in Southeast Asia, three stand out in terms of ultimate yield and widespread occurrence: (1) rift-sag basins on continental crust, (2) Miocene platform carbonates, and (3) shallow to deep water prograding deltas, largely Middle to Late Miocene in age.

1. Tertiary rift-sag basins can be modelled using the "steer's horns" analogy. They are similar in cross section but vary in time of development.

Sumatra-West Java Sea basins originated by rifting in Late Eocene time but shifted to a sag phase in Late Oligocene. The principal source beds are middle Oligocene lacustrine shales of the late rift phase and the main reservoirs are wedge-base deltaic sandstones and grain carbonates in the early sag phase. Some are inversion structures and some are not. Mio-Pliocene wedge-top sag sandstones in inversion structures provide additional reservoirs.

Significant basin-forming normal faulting in the Sunda Rift Basins (especially the Malay Basin) took place in Oligocene time. The sag phase was Early Miocene and inversion was Middle to Upper Miocene, culminating at a regional 10.4 Ma unconformity. Source rocks are rift phase oil-prone lacustrine shales and gas-prone sag phase coaly shales and coals. The major fields are inversion structures.

2. Platform carbonates of Middle-Late Miocene age produce significant amounts of oil and gas in rift/sag basins and on microcontinents (Malampaya appears to be Early Miocene). Outstanding examples are Arun, Block 6 Vietnam reefs, Luconia Shoals and Salawati.

3. There are three outstanding delta sag systems with major hydrocarbons: Bangladesh, Kutei and Northwest Borneo.

The eastern Bengal Basin in Bangladesh is structured by north-trending Plio-Pleistocene wrench-fault anticlines superimposed on a thick, heterogeneous Tertiary sedimentary sequence. Underlying the thick upper Tertiary clastics derived from the uplifting Himalayan Range, the main producing section consists of clastics in an east-to-west distributary system.

A mirror image to the Bangladesh geometry is provided by the Kutei Basin, wherein a west-to-east distributary system has been deformed by long North-trending folds.

Traps are structural-stratigraphic, with multiple thin sandstone reservoirs draped over folds that are perpendicular to the sedimentary input direction. Basement is formed by Eocene down-to-basin normal faults into the opening Makassar Strait. Current exploration is focused on deepwater turbidite fields in a variety of structural styles.

The Northwest Borneo Basin has been built out over an early Tertiary accretionary complex. Shelfal fields exhibit a variety of structural styles, ranging from complexly faulted anticlines (Seria) to fairly simple open folds (SW Ampa) and seawardadvancing fault-controlled slivers of reservoirs (Champion). Source rocks are probably largely Middle Miocene shoreline-associated coals and coaly shales. In Sabah there are both simple, crestal-faulted anticlines (Samarang) and complex wrench-faulted tight anticlines (St Joseph). Tembungo is the oldest producing turbidite field, now enjoying enhanced late development activity.

Presented at: 2005 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2005