Abstract

The N.W. Palawan Basin is located offshore Palawan Island, Philippines, in water depths ranging from less than 100m to in excess of 1000m. Exploration of the basin commenced in the 1970's with the drilling of build-ups of the Nido Limestone Formation located in the shallow water Nido Shelf part of the basin. This initial exploration phase resulted in a number of small oil discoveries (e.g. Nido, Matinloc) which have since been produced. In contrast to the oil-prone Nido Shelf, exploration wells in deeper water to the west of the Nido Shelf (1982-1994) have discovered mostly gas, notably in the Malampaya/Camago accumulation which is estimated to contain 3.4 Tcf gas. With the exception of Malampaya/Camago, exploration results away from the Nido Shelf have been disappointing and have resulted in few potentially economic discoveries.

Few source rocks have been penetrated in the basin. In the absence of source rock samples, oil geochemistry has been used to deduce source rock types. Typing of oils in the N.W. Palawan Basin based on biomarkers, sulphur content and carbon isotopes has identified three separate oil families. The majority of hydrocarbons encountered appear to have been derived from a carbonate or marl source rock deposited in a marine environment. Ages of the source rocks are estimated to be Eocene to Lower Miocene. It is interpreted that source rocks have been deposited within restricted rift half grabens and/or isolated restricted intrashelf basins in the Post-Rift carbonate sequence which overlies the grabens.

Within the area of Malampaya/Camago charge is derived primarily from a deep half graben located between the accumulation and the Nido Shelf to the east. Estimates of thermal maturity suggest that source rocks in this graben are mature for both oil and gas. Geometry of seismic events in this graben suggests that migration from shallower (more oil-prone) intervals is predominantly towards the Nido Shelf to the east, whilst migration from deeper (more gas prone) intervals is mostly westwards towards Malampaya/Camago, providing a possible explanation for the oil vs. gas discovery pattern made in the area.