Abstract

Cores recovered from a 20 to 25 m thick reservoir interval in the Upper Miocene Miri Formation, Seria field of the Baram Delta Basin, Brunei, are composed predominantly of silty to sandy shales interspersed with numerous sandstone units. Subsurface correlation and isopach mapping in a small area of the field (2.5 km²) revealed the sands to be of two main types: 1. thin (0.1 to 1.5 m), fining-upward units with a patchy, sheet-like geometry and lateral extent of 100 to 1500 m; and 2. thicker (1.5 to 3 m), more extensive (up to at least 2 km) amalgamated sands. The latter most probably cover a wider area, as the boundaries could not be fully delineated in the studied region.

On the basis of lithofacies, depositional geometry and paleontological data, the thin sands are interpreted as the deposits of single storm events. In contrast, the amalgamated sands are interpreted as stacked storm sands of the lower-middle shoreface, either sheet-like sands, or coast-parallel lenticular bars. It is suggested that the bar-like geometry is a result of the modification of distal storm sands by semipermanent coastal currents.

Lithofacies trends and the paleontological analysis of the interval support an overall environmental interpretation of shelf regression and shoaling followed by transgression and deepening. Unlike classic models of shoreline regression, no foreshore or beach deposits have been identified in the sequence. Sequences of this type can be observed throughout the Miri Formation in the Seria field, often in a cyclic fashion. Analogies are drawn with similar, cyclic sequences in the Cretaceous Cardium and Viking Formations of the Rocky Mountain Foothills region of Canada.

Finally, the study demonstrates that the overall depositional architecture and three-dimensional arrangement of storm sands may be exceedingly complex and is often characterized by vertical and lateral offsetting of successive sand units.