Geological Model of Shallow Marine Clastic Reservoirs in a Wrench-Faulted Province: St. Joseph Field, Offshore Sabah, Northwest Borneo
H. D. Johnson, J. W. Chapman, J. Ranggon
The St. Joseph field is situated along a major wrench-fault zone in offshore
Sabah (the Bunbury-St. Joseph-Bambazon "ridge") that divides the field into
several structural areas. The most prospective of these is the structurally
simple northwest flank (about 6 km long and 1 km wide) that dips uniformly to
the northwest (about 15°-20°) in a basinward direction away from the crestal
wrench-fault zone. The main hydrocarbon-bearing interval comprises a 1,350-ft
long oil column, which is contained within a highly heterogeneous sequence
of
late Miocene shallow marine sandstones and shales.
The main geologic uncertainties of the northwest flank concern lateral
variations in sand development, shale-layer continuity, and reservoir quality.
They have a major impact on reservoir recovery mechanisms, pressure-maintenance
schemes, and on field development strategy. Therefore, a reservoir geologic
model was developed that incorporates sedimentologic studies, well
-
log
facies
analysis
, reservoir mapping, and detailed structural interpretation (using a
full reservoir core and three-dimensional seismic data).
These studies demonstrate that depositional processes and tectonic setting had a major impact in controlling the thickness, quality, and distribution of the sandstone reservoirs. Features that had a particularly significant impact on hydrocarbon distribution, reservoir modeling and field development are: (1) a storm-dominated shelf-sand depositional system, (2) rapid vertical and lateral switches in sand supply, (3) a tectonically unstable, narrow (about 5-15 km wide) shelf, and (4) shelf-edge slumping (slump scars).
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.