RESERVOIR ARCHITECTURE OF GABUS AND UDANG FORMATIONS, WEST NATUNA BASIN, OFFSHORE INDONESIA

Albert W. Shultz¹ and L. Charles Ways^2
1 ConocoPhillips Upstream Technology, Houston, USA
² ConocoPhillips Indonesia Ltd., Jakarta, Indonesia

The Gabus and Udang Formations (late Oligocene - early Miocene) of the West Natuna Basin, Indonesia, contain significant clastic reservoirs for hydrocarbons in this rapidly developing region. Understanding of the reservoir architecture of these formations derives from conventional exploration and appraisal subsurface data, as well as from a limited amount of production history.

The place of the Gabus-Udang depositional system in the spectrum of lacustrine systems is not unequivocal. As a part of a transitional succession that evolved since Eocene time from isolated rift basins to widespread shallow marine conditions, deposition has been attributed to a fluvial, deltaic, and estuarine environment. Palynological and foraminiferal biostratigraphic analysis has documented both marine and lacustrine affinities. Sedimentological criteria seen in slabbed core include significant indicators of tidal processes, suggesting at least a partial marine connection. Very high quality seismic images of stratigraphic features, such as channels, valleys, and shorelines, have been obtained across some positive structures in the basin. This seismic data has allowed a good model of the depositional geometries to be applied in reservoir simulation. In other fields where seismic images are not well developed, depositional models for these sands have varied considerably. In order to construct credible geologic models for development planning and production forecasting, geoscientists have specifically confronted the difficulties of determining sand-body geometry, orientation and internal architecture.

As an example, the general stratigraphic nature of the Gabus and Udang Formations as revealed by wells in the North Belut field is dominated by a large number of more or less sandy layers separated by lacustrine mudstones. Upward-coarsening, upward-shallowing units of 5 to 25 m thickness suggest individual coastal regressive units of high frequency. These units are punctuated by subregional flooding surfaces and arranged in lower-frequency cycles of transgressive and regressive character. Whether fully lacustrine or restricted-marine in origin, these layers define the first-order architecture as highly layered.

Across much of the basin, internal details and lateral relations at a finer scale are hampered by limited seismic resolution and generally wide interwell spacings. Several alternative models can be proposed, differing mainly in whether sandbodies are elongate or equant in planview extent, laterally continuous or discontinuous at interwell distances, and if elongate whether oriented in the regional dip or strike direction. As a system, the reservoirs appear to be dominated by lacustrine deltas in which stratal architecture is constrained by baselevel cycles over a fairly narrow range in a tectonically quiescent basin.