The NW Palawan basin has undergone two major phases of structuring. The first involved rifting and separation of the basin from the southern coast of china, and is manifest by a series of half graben overlain by a post-rift "steer’s head" section which culminates in deposition of the Nido Limestone. The second was a compressional phase which occurred when the drifting NW Palawan micro-continent collided with the Philippine Arc. This effectively shortened and rotated the half graben in an anticlockwise sense and established a foreland basin configuration. Inverted and thrust-ramp anticlines developed basin wide, with the greatest relief and fault throw within the largest graben.
Detailed studies of seismic, well logs and core indicate that the depositional setting for the Nido Limestone was similar to that of the modern day Bahama Banks, i.e. development of shallow water reefal facies restricted to topographic highs, and peri-platform ooze elsewhere, with water depths not exceeding around 200 metres. This is different to the generally accepted model which discriminates between shallow and deep water (turbidite) settings. In the NW Palawan Basin, topographic elevation is associated with footwall blocks to the old rift basins. Retained porosity and permeability within the limestone is most likely to be associated with karsted reefal facies, highlighting the importance of distinguishing between paleo-structures associated with the underlying crustal configuration and structures related to inversion.
The Pagasa Formation, which followed the Nido Limestone, was deposited in the developing foreland style basin. This produced large scale lowstand progradation into deep water and the deposition of a series of basin floor and slope fans with associated thinner highstand deltaic sections.
In the southern part of the study area, the compression commenced within the latest depositional phase of the Nido Limestone. Small pinnacle reefs in the terminal stage of the Nido Limestone sit on top of small thrust ramp anticlines, and the Pagasa Formation downlaps onto the Nido upper surface. In the central area around the Matinloc and Galoc discoveries, a section of interbedded shales and sands lie conformably on top of the Nido Limestone. At Galoc, large rotated fault and slump blocks (which were originally interpreted as large carbonate mounds) lie on top of the clastic section.
Apart from a few large undrilled carbonate buildups (e.g. Coron North), the future for exploration in the basin may hinge around new plays. The most obvious of these is the lowstand basin floor and slope fan play in the Pagasa Formation. The new seismic shows many reflection strength anomalies, some of which appear to terminate downdip against structural culminations which may have acted as migration foci. In some cases there appears to be a co-incidence with the termination point and the position of oil water contacts. Other anomalies sit within local embayments and are interpreted as ponded turbidites. Some potential may also be present in clustered pinnacles reefs where single well productivity may be the control on whether they can be developed economically.
1. Beta4 Exploration and Production Consultants, Inc., Dripping Springs, Texas
2. Java Petroleum, London
3. Nido Petroleum Philippines Pty Ltd, West Perth
4. Schlumberger Oilfield Australia Pty. Ltd., Perth
We gratefully thank the joint venture partners for Service Contracts 14 and 42 for permission to publish this paper. The views expressed herein are those of the authors and are not necessarily accepted in full by all parties to the above Service Contracts.