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Home Studies West and East Natuna Basins

 West and East Natuna Basins

JMJ I-MAP GIS Product Suite

West and East Natuna Basins 01

GIS G&G Project and A3 Hard Copy Report


A Petroleum Systems Evaluation of the West and East Natuna Basins.

In the West and East Natuna Basins Study, all blocks (open and concession) have been evaluated and ranked in terms of hydrocarbon potential and risk and uncertainty. At the time of the study, of particular interest were the open blocks available for the Indonesian 2008 Bid Round (i.e. Cakalang, Baronang and Kerapu, in the West Natuna Basin; and Cucut and Dolphin, in the East Natuna Basin).

To date, only the West Natuna Basin is producing both oil and gas. Gas in substantial commercial quantities has been discovered in the East Natuna Basin. Discovered in 1970 by Italy's Agip, the East Natuna D-Alpha Field is the largest in SE Asia, with an estimated 46 trillion cubic feet (tcf) of recoverable reserves, but has been developed only recently due to problems with CO2 extraction. A number of markets are available for the gas, with most recent interest coming from Thailand’s PTT. Other likely countries interested in purchasing large volumes of natural gas from the Natuna D-Alpha Block are Malaysia and China.

East Natuna has been little explored over the last 15 years and, despite the problems encountered so far with high levels of CO2, we believe that this large basin rift system, which extends roughly N-S along the eastern flank of the Natuna Arch, has considerable hydrocarbon potential, both in terms of gas and oil. Numerous prospects and leads, including both structural and carbonate reefal buildups, have been identified in this and adjacent blocks, which are presented in our GIS project.

West and East Natuna Basins 02 West and East Natuna Basins 03

Both basins are characterised by NW dipping NE-SW trending half graben. The northern West Natuna Basin swings more N-S and is divided by NW-SE trending dextral wrench zones. The East Natuna Basin also contains N-S trending graben, such as that of the Komodo Graben, which appear to be dextral transtensional structures. Although containing similar structural frameworks, both basins contrast in terms of, for example, timing of events, depositional environments and heat flow. For instance:

  • Late Eocene basin initiation in West Natuna appears to pre-date that in East Natuna, apart from in the northeast of the basin, where half graben of similar ages are thought to exist.
  • The basins are extensional-transtensional half graben or graben which experienced significantly different post Late Oligocene development.
  • Within the West Natuna Basin, the “post rift” section may actually reflect a period of mild E-W Late Oligocene extension.
  • 40% to 100% of the original extension in the West Natuna Basin was removed by Late Oligocene to Late Miocene inversion. By contrast, no inversion occurred in the East Natuna Basin.
  • In the East Natuna Basin, active rifting ceased in the Early Miocene, with high subsidence rates in the Late Miocene and Plio/Pleistocene due to subduction in the North Borneo Trough.
  • Generally, the West Natuna Basin is an overfilled basin and the East Natuna Basin an underfilled basin.

Just these observations alone make it clear that a detailed tectonostratigraphic framework has to be established, based on seismic and well data, in order to understand the dynamic history of subsidence and uplift throughout both basin systems.

A Base Tertiary Map for the entire Natuna region, constrained by seismic, well, magnetics and gravity data, and public domain sources, including depth converted TWT maps and georeferenced interpreted seismic profiles, has been generated and is included in the GIS project. This, together with GIS-based qualitative mapping of gravity data, has provided us with the geometry of both basin rift systems, the location and continuity of important basement fabrics and major crustal discontinuities, and an indication of the composition of the underlying crust.

West and East Natuna Basins 04

This has provided information on the tectonic and structural framework of the region, allowing us to better understand, for example, the local distribution of inversion. The inversion in the West Natuna Basin has created a wide range of potential traps, largely dependent on the strike of the original half graben relative to the principal stress direction during inversion, but has also compromised trap integrity across many structures. In the East Natuna Basin structuring is limited in the east (in the area of the D-Alpha Field), but may be more prospective for traps within syn-rift graben to the west (e.g., Komodo Graben). In terms of heat flow, the resultant tectonic and structural framework has helped to explain the contrasting basin dynamic histories of the two basins and the difference in heat flow regime. The two contrasting domains (basins) are separated by the N-S trending Natuna Arch, a major tectonically-defined basement high.

The distribution of Upper Eocene to Mid Oligocene syn-rift lacustrine shales with oil prone Type I and Type I/III kerogens is poorly understood in both basins, reflecting differing tectonic regimes and structural responses. Across both basins, fluvial to paralic coals and coaly shales form potential source rocks within the Mid Oligocene to Upper Miocene.

The distribution and quality of both proven and potentialsource rocks in the Natuna basins (West and East Natuna) have been assessed as a means of determining the spatial and temporal distribution of source families and their accumulation environments, with respect to modern-day and regional analogues, such as the Kutei Basin (e.g., oil potential from fluvial coals).dd

A summary map of predicted hydrocarbon types – deep water lacustrine, fluvio-deltaic, and shallow water fluvio-lacustrine facies – and general observations have been made in relation to the geochemical characteristics of “Rift” and “Sag” phase source rocks and sourced oils in both basins.

Non-hydrocarbon gases, particularly CO2, have also been considered in terms of risk, particularly in the East Natuna Basin.

Numerous burial/maturation history models for wells and pseudowells have been analysed in detail across both basins to deduce the timings of generation and expulsion of different hydrocarbon phases. A total of twenty-two modelling points (12 wells and 10 pseudowells) have been chosen to provide adequate coverage across the two basins, to cover both their depocentres and flanks. Modelled wells utilised to verify the heat flow models, together with observed Vitrinite Reflectance data, are provided; in addition to all burial/maturation models for both wells and pseudowells, showing transformation ratios, timing of maximum hydrocarbon expulsion and the amount of predicted expelled hydrocarbons and residual potential. All the models are presented in full and the results have been summarised in terms of depth of maximum expulsion and timing.

It has been established from basin modelling that there is potentially considerable variance between levels of thermal maturation and temperature in relation to oil expulsion. The key component in these basins is time. The relationship between expulsion and apparent source rock maturity (i.e. Vitrinite Reflectance determination of the source rock) varies with time and temperature. The results of the modelling have been assessed with respect to key moments in each basin’s tectonic history, which relate directly to the formation of traps and the possible breaching of traps. Within the West Natuna Basin, Oligocene and Miocene depocentres are offset by inversion. This affects both the location of differing source facies, the potential maturity of the source intervals and timing of expulsion across the basin.

Expulsion windows for Top Oil (0.8% Ro), Top Gas (1.0% Ro) and Top Main Gas (1.2% Ro) have been generated by integrating the results of the burial/maturation history with the Base Tertiary Map. These windows, together with known hydrocarbon occurrences (producing and non-producing discoveries) and mapped prospects and leads, have been used to assess the hydrocarbon potential in different parts of the two basins, particularly, in less well explored areas away from the main depocentres, such as the KB and Komodo grabens.

In the West Natuna Basin, we recognise six proven plays and two potential plays; whereas, in the East Natuna Basin, two proven and three potential plays are identified. For each play, whether it is proven or potential, the essential play elements have been analysed and the results of this assessment have been synthesised under the following headings: Reservoir; Source; Seal; Trap; Migration; Timing; Key Issues; Example. The relationships of the essential play elements are shown through play cartoons.

The following key factors have been considered during the evaluation of exploration opportunities in both basins: Effects of inversion (preservation of early hydrocarbon charge; integrity of traps; hydrocarbon phase within traps; timing of charge vs. inversion is different for different parts of the basin and for different source rocks; early uplift may preserve poro/perm in older reservoirs); Correlation difficulties due to significant variations in age of unconformity and degree of exhumation and removal of section; Multiphase expulsion and gas flushing; Seal integrity; Lack of mature source; Long distance migration; Present day heat flow; Oil vs. gas prone source rocks; Multiple play targets.

All mapped interpretational elements and underlying datasets are contained within the GIS project. The A3 hard copy report contains 188 colour pages.