Mapping the Exploration Potential of NW Borneo in Sabah and Sarawak

Written by TGS | Nov 3, 2021 5:00:00 AM

The Sabah regional MC3D dataset has become the ultimate explorer’s tool in paving the way to better understand the frontier areas of deepwater NW Borneo. Its success is prompting a similar approach for Sarawak.

Since the award of Malaysia’s first-ever MultiClient 3D project offshore Sabah, back in 2016, the seismic joint venture consortium consisting of TGS, and Schlumberger (WesternGeco) has amassed 47 000 sq. km of contiguous broadband 3D seismic data covering the Sabah Basin. This enormous footprint, which was acquired in multiple phases, is the ultimate explorer’s tool covering various geologic terrains including the proven inboard Fold and Thrust Belt, the Sabah Trough, and the lesser-known Dangerous Grounds.

A newly accepted stratigraphic sequence for the Dangerous Grounds and the Sabah Trough was established, based on this high-quality measured broadband seismic dataset. This has enabled detailed seismic mapping of the basin fill, including imaging below the regional unconformity (MMU), which has historically been a challenge. Furthermore, AVO analyses of the clastic plays of the Fold and Thrust Belt have shown numerous untested low Vp/Vs anomalies, this ratio of compressional wave velocity to shear wave velocity is a good tool in identifying fluid type. Offshore Sabah, Vp/Vs anomalies indicate potential upside in hydrocarbon volumes near discovered fields, which may facilitate tie-back to existing infrastructure and easy increase in resources for development.

The Sabah MC3D project was a pilot for a new approach to regional offshore exploration in Malaysia and its success has prompted Petronas’ Malaysia Petroleum Management (MPM) to open additional areas for regional Multiclient 3D. Inspired by what the JV had accomplished in Sabah, the Sarawak offshore basin was nominated, and a call for tender went out in 2019. The project was awarded in 2020 to the same JV consortium, led by TGS. After consultation with the energy industry, phase 1 of a prefunded Sarawak MC3D project will commence in Q4 2021.

Sabah Geologic Setting and Potential

The Sabah Basin, situated offshore northwest Borneo, is filled with marine Tertiary sediments, typically more than eight kilometers thick. It receives its major sediment input from the Baram Delta, a prolific hydrocarbon province extending from Brunei to northwest Sabah and from the abandoned Champion and Meligan deltas.

Geologic Setting: The Sabah MC3D footprint covers multiple geologic terranes.

Gravity loading and thin-skinned deformation have resulted in a Fold and Thrust Belt in the inboard area. This initiated near the shelf in the Mid Miocene and then propagated north-westward in the Pleistocene. The Fold and Thrust Belt, which hosts turbidite reservoirs within anticlinal structures, has been the major focus for exploration and production by oil companies and is the most successful play type in the basin to date. Recently a play-opening well was drilled in the Sabah Trough, which discovered hydrocarbons in the Miocene carbonate.

The 2018 Tepat well in Sabah Trough was a new play opener, proving the petroleum system concept for the Dangerous Grounds and Sabah Trough. A carbonate pinnacle reef sits atop a pre-rift potential source rock, with shallow gas anomalies above it.

Further outboard of the Fold and Thrust Belt and beyond the Sabah Trough, lies the NW Sabah Platform, also known as the Dangerous Grounds. It consists of continental fault blocks in an extensional tectonic setting from the Eocene-Oligocene period. These sediments were deposited as pre- and syn-rift packages during this extensional phase and potentially host source rocks that are mature at present-day, to charge the overlying mid-Miocene carbonate targets (note: these are on the same play level as the Tepat-1 discovery).

Many indications of a working petroleum system can be seen on the Sabah MC3D seismic data below.

Seismic images from the Dangerous Grounds indicate Carbonates showing a ‘soft’ response with underlying potential source rocks, coupled with shallow gas and pockmarks indicating a working petroleum system.

In the well-proven and producing fields of the Fold and Thrust Belt, we can see a very active present-day syn-kinematic system of reservoir deposition coupled with active tectonic deformation. Reservoir variability has been a risk for explorers, as is evident from the image shown below. The key here is to carefully map each of the channel and splay systems throughout the sequence and only a high-quality and high-resolution dataset such as that afforded by the broadband multisensor Sabah MC3D can allow such precise detailed mapping of individual beds.

An RMS amplitude extraction (of just below the seabed) draped onto the seabed horizon. Present-day systems show reservoir variability and ponding of turbidite sands in the lows. The present is key to the past and from this, we can envisage a similar depositional setting during the Mio-Pliocene period in the Fold and Thrust Belt.

High-end QI workflows, generating Vp/Vs volumes, and working in the pre-stack domain, improve the chance of success and de-risk prospects. Initial work, on a relative Vp/Vs inversion cube using broadband data acquired with multisensor technology, shows that the reservoirs trapped in the low side of the folds, as seen on the present-day seabed, present a low Vp/Vs response, similar to that of nearby discovered fields. This finding suggests that there is an upside to hydrocarbons already discovered on block, boosting the economic value of the block operator’s asset portfolio.

QI: Relative Vp/Vs volume. Low Vp/Vs anomalies in strat traps close to discovered fields can substantially increase the economic value of the asset.

Sarawak Challenges and Opportunities

One of the many learnings from the Sabah MC3D data – and there are still more new findings to follow – is the long-term contribution of such a basin-scale MultiClient 3D dataset. In addition to supporting better evaluation of assets and facilitating operators' assessment of primary and nearfield opportunities, it can add enormous value to the industry's understanding of any basin.

In late October 2021, the JV consortium started acquiring data on the first phase (covering approximately 6 500 sq. km) of the Sarawak MC3D, on the Luconia Platform and in the Tatau Basin. Like Sabah, this area consists of Tertiary clastic and carbonate targets, being part of the greater northwest Borneo Sundaland geological setting. The consortium is aiming to work together with the industry to identify prospective areas and build upon this initial phase of seismic acquisition, to provide exploration insight and, ultimately, deliver a basin-scale regional broadband 3D dataset, akin to what has been accomplished for Sabah.

Match between GeoStreamer acoustic impedance section and acoustic well logs filtered to the seismic bandwidth.

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