Brunei Deep Water Exploration: From Sea-Floor Images to Depositional Models in a Slope Turbidite Setting

Demyttenaere, Renat, Abdullah Ibrahim, and Jan-Pieter Tromp, Brunei Shell Petroleum, Seria, Brunei

Abstract

The Brunei deepwater, off the northwestern edge of the island of Borneo, is characterised by a steep slope from shelf to basin floor. Sea-bottom drops from 100 to 1200 m over a distance of 20 km. Shale ridges, related to deltaic toe-thrusts, pierce the seabed and are the major controls for turbidite sedimentation on the slope. Two recent discoveries, Meragi and Merpati, are in slope turbidites of late Miocene/Pliocene age.

Interpreting images of the present-day seabed, combined with slices through semblance data and other attribute maps from 3D seismic, helps in the understanding of the depositional model. The present-day slope is characterised by steep, locally meandering channel-levee complexes, controlled by existing seabottom topography. Highly sinuous channels show distinct levees. Straight channels are mainly erosive. Some of the channels abut against ridges and are deflected (losing energy in the process), resulting in turbidite deposition in mini-basins between the shale-ridges. Other channels erode the ridges but still deposit sediments on the leeward side. Examples of both have been tested in the Meragi and Merpati discoveries.

Also evident on the present-day slope is large scale slumping and gliding into major canyon systems. Steeply rotated blocks in deep erosional cuts deeper in the section are indicative of similar processes. Pockmarks on the seabed indicate active gas-charge, and shale ridges act as preferential pathways for the migration of hydrocarbons.