During March to May 1992, the Australian Bureau of Mineral Resources (BMR)and the Philippine Office of Energy Affairs (OEA) conducted a joint marineseismic and sniffer survey in four Philippine offshore sedimentary basins. Theproject was funded by the Australian International Development AssistanceBureau (AIDAB), and the survey was conducted using the Australiangovernment's geoscience research vessel Rig Seismic. A total of 2750 km of192-channel seismic data, plus geochemical sniffer, gravity, magnetic andbathymetric data was obtained jointly. This comprised 580 km in the NEPalawan Shelf, 730 km over the Cuyo Platform, 490 km in Tayabas Bay, and950 km in Ragay Gulf. Because of the extensive fishing activities at night time,particularly in Ragay Gulf, acquisition of seismic data after dark was abandonedin the latter part of the survey. During these hours the geochemical sniffer program was continued over identified anomalies, and an additional 900 km of sniffer, gravity and bathymetry data were recorded.
Although not a lot can be deduced before the seismic data are processed, theship-board seismic monitors show plenty of structuring, and deep depocentreswith up to 4 seconds of stratified section. Because the weather during thesurvey period was very calm and therefore conducive to the collecting of low-noise data, the quality of the processed sections is expected to be excellent.
Geochemical anomalies were recognised in the NE Palawan Shelf and in RagayGulf. These were particularly significant in Ragay Gulf and were related to: (1)previously identified prospects (ie R-1 and R-2 in the 0EA-World Bank Report,and the Alibijaban prospect identified by Far East Resources); (2) faults; and (3)deep diapir structures. They fall into two distinct anomaly types: type 1 is
characterised by high methane, ethane and propane with traces of butane andpentane, indicating dry to wet thermogenic gas; type two has high methane andtraces of C2+, indicating very dry gas.
In order to better understand the flux rate of hydrocarbon release and thecomponent fractionation and magnitude of the source of the seep, Ragay Gulfwas revisited after the seismic shooting was completed. Here, eightgeochemical vertical profiles were recorded and 28 gas samples collected forisotope analysis. In one example, hydrocarbons were concentrated under thethermocline at about 140 m water depth.
The newly acquired data will be processed in BMR, integrated with existing seismicand well data from previous industry sources, interpreted and analysed for petroleumpotential. The final analysis, together with basic data, will then be presented to theexploration industry, initially in the Philippines and Australia, and laterinternationally, in order to promote further exploration.
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