The Syn-Rift Sequences: Another Potential Play in Offshore Palawan

Joseph M. Foronda, Jaime A. Bacud, and Cielo F. Bastero

PNOC-Exploration Corp., Fort Bonifacio, Taguig City

The Upper Paleocene to Eocene "Syn-Rift" sequences in offshore Northwest Palawan consists mainly of marine deposits that filled rift depocenters. A mid-Oligocene unconformity marking the initiation of seafloor spreading in the South China Sea Basin separates the Syn-Rift from the widespread platform carbonates of the Nido Limestone. Buildups and folded fractured platform carbonates of the Nido Limestone, and the directly overlying turbidites of the lower Pagasa Formation have been the target of most exploration in NW Palawan. Very few play concepts have been developed for the Syn-Rift succession due to poor seismic data quality and the scarcity of wells that have penetrated it. In horsts where most wells have been drilled, most of the Syn-Rift successions, except for the topmost few decameters, have low sandstone:mudrock ratios, giving a perceived lack of reservoir quality sands.

The presence of turbidites in Syn-Rift-equivalent units in Central Palawan and new interpretation of 2D seismic data in the Syn-Rift successions predict the presence of turbidite reservoirs in slope and basinal paleoenvironments. Potential turbidite plays include basin floor fans and leveed channels. The basin floor fan may reach several km in one dimension and may be composed of more than one lobe. It is convex-up and shows bidirectional downlap. The leveed channel complex exhibits irregular and discontinuous reflections with occasional channel patterns <1 km wide.

One of the main risks in a basin floor fan play is the lack of a local condensed section above it to serve as seal. The main pitfall when exploring for leveed channels is drilling a sand-poor channel fill. The prospectivity of many of these deepwater plays is enhanced by its lateral proximity to half-graben kitchen areas and favorable burial history. Despite water depths from >1 to 2¹/₂ km, the Syn-Rift interval can become a potentially significant exploration target and warrants more detailed evaluation.