The geology of onshore and offshore Sarawak shows a progressive deepening of basement from the Tinjar, Balingian, Penian High and Half-Grabens areas north and west towards the North and West Luconia blocks. The basement is characterised by folded and thrust faulted blocks of metasediment affinity and Cycle I and II sediments. This deepening basement is overlain by sedimentary basins and an extensive carbonate platform in the Central Luconia area. The many carbonate build-up structures (Figure 1) identified and drilled in Central Luconia play host to quality reservoirs containing hydrocarbons. Overlying the carbonate platform are prograding deltaic sequences that dominate in the Baram Delta and North Luconia blocks.
Carbonate and Sub-Carbonate plays
Much is understood on the variably complex porosity and geometries of the carbonate build-ups (example: Figure 2) from detailed seismic and well log data. However, what is less well understood is the geological make-up that contains the source rock material and potential for deeper plays. It is commonly accepted that sub-carbonate basins containing source rock material exist, but their shape and form remain unknown. Seismic coverage in the region is extensive but is generally unable to clearly image the deeper section where the carbonate sequences are complex.
Existing Air-FTG® data across the Sarawak Basin offers fresh insight to these previously hidden sections. Figure 3 shows a depth to Top Pre-Cycle Basement map extracted from a Depth Imaging FTG workflow that not only identifies variable depths to the dominantly metasediment basement, but also locates basins reaching 4km depth (cold colours). The FTG datasets also provide a range of other benefits including identification of key structural lineaments interpreted as faults predicting both transfer and normal faulting activity.
Carbonate Build-up structures in Luconia are often complex, usually suffering from deep weathering and containing high porosity sediment (Ramlan et al, 2013). Such features should give a negative response in an Air-FTG® survey, and indeed this can be seen in the existing data coverage in the area, as shown in Figure 4. As can be seen, the negative anomalies are dominantly associated with known carbonate structures.
New Plays at Top Basement
Recent activity in the Half Grabens and Tinjar areas involves targeting of shallow basement structures where they interact with Cycle I and II geology. Thrust folds and basins are known across Tinjar and anticipated to show up as a series of parallel positive and negative anomaly patterns. Figure 5 shows the Air-FTG® response as a series of positive anomalies (warm colours) tracking the anticlinal structures and negative anomalies locating Cycle I and II sediment filled troughs.
Plays from the Half-Grabens area have been recently successfully drilled with two oil accumulations from well NNG-1 (Jabber, 2015). The deeper of the two pools resides at Top Basement defined by a tilted fault block structure (Figure 6).
Exploring with Air-FTG®
Intuitive usage of Air-FTG® data with both currently existing and newly acquired seismic will facilitate direct 3D mapping not only the carbonate hosts to producing fields in Luconia, but also clearly locate their underlying source rock material and the migratory pathways that charge the reservoirs. Top Basement, overlying sedimentary units and fracture patterns and overlying carbonate complexity are all revealed with Air-FTG® technology. In addition, Air-FTG® is ideally suited to mapping the lateral extent of targeted anticlinal closures in Tinjar and tilted fault blocks in the Half-Grabens area.