Abstract

The reservoir interval studied forms a small part (20 to 25 m) of a thick succession of ncarshore and deltaic sediments that filled the Baram delta basin in northwest Borneo from early Miocene to Recent times. Cores recovered from the interval contain silty to sandy shale punctuated by sandstone units, which occur as either relatively thin (0.3 m), sharp-based, single, fining-upward units or as thicker amalgamations (0.3 to 2 m) of several, erosive-based, incomplete, fining-upward sequences. Available paleontological data, and the internal facies of the sandstones (parallel to low-angle laminae interpreted as hummocky/swaley cross-stratification), imply deposition in a wave-dominated shelf environment where the coarser sands were derived from the shoreface during storm events.

The vertical distribution of facies throughout the reservoir interval, in particular the upward increase in 1. sand percentage and sand bed thickness; 2. the preferential preservation of physical, as opposed to biological, sedimentary structures; and 3. the tendency for sand amalgamation, arc consistent with either regression and shoaling of the shelf or up-building of submarine bars. On account of close well control in the study area, individual sands and composite bed packages could be correlated and mapped. The resulting maps indicate that individual storm sands are laterally discontinuous and, where amalgamated, have the form of offshore bars that are approximately parallel to the coast.

Storm deposits have been increasingly recognized in recent years, but there is a lack of data on their three-dimensional geometry and architecture. This study indicates that a knowledge of that data is essential to the understanding of the depositional processes involved, as storm deposition may frequently be followed by extensive reworking by other shelf processes.