Abstract

Bunyu Island covers an area of about 122 sq. kms. and is located in front of the Sesayap River estuary in N.E. Kalimantan. It produces oil and gas from Neogene sediments of the Tabul, Santul and Tarakan Formations.

Sedimentological as well as electrofacies studies reveal that the productive sand intervals are deltaic in origin.

During the Middle Miocene, deposition in Bunyu area was mainly developed in a prodelta-marine environment Meliat Formation) and in prodelta-delta front environment (Tabul Formation). In Late Miocene, high terrigenous influx from the west resulted in the deposition of Santul Formation under a delta front environment. The overlying Pliocene Tarakan Formation was predominanly deposited in a lower deltaic plain condition. Sedimentation in Bunyu area was terminated by upper deltaic and alluvial plain deposits which comprise the Pleistocene Bunyu Formation.

By subdividing the lithologic section into time intervals or depositional cycles using maker beds (mostly coal beds) the isopach and isolith map at individual cycles were constructed. These maps, supported by dipmeter trend analysis, have enabled prediction of the general direction of delta progradation. The study reveals that during Late Miocene to Pliocene an ancient delta prograded from southwest to northeast and branching at Bunyu Island towards the north-northeast and north.

In general, hydrocarbons are found in distributary mouth bar and tidal bar sands. These sands, if distributed between 500 to 2500 m depth usually have excellent reservoir characteristics and may contain oil, gas or gas and oil. If they are dispersed at a greater depth, their reservoir characteristics are inferior and usually gas bearing.