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.