Abstract

The Peutu Formation crops out in foothills along the eastern margin of the Barisan Mountains. Limestone exposures vary from thin planktonic beds intercalated with shale to skeletal carbonates up to 75 m thick. These units dip eastward beneath the coastal plain, where limestone buildups reach thicknesses of 300-500 m and contain substantial gas accumulations. Reservoirs consists of vuggy porosity developed in foraminiferal grainstones to wackestones and coral boundstones. Associated platform facies are much thinner (50 m) and consist of tight limestone, sandstone, and shale.

A deep drilling program in Block-A recently confirmed the widespread distribution of gas-prone reservoirs in the Peutu Limestone. Exploration success in this play depends on:

1) Field size: Presence of mounds or reefal buildups is critical to finding accumulations large enough to develop. Buildups occur on preexisting structural uplifts, and isopachs of overlying units assist in both documenting paleostructure and in showing depositional topography.

2) Gas composition: Peutu reservoirs contain H₂S and CO₂. The relatively small concentrations of H₂S are manageable, but CO₂ content varies from 6.5% to 81.5%. This CO₂ derives from thermal decomposition of carbonates, and highest concentrations occur where the Peutu is deeply buried and unconformably overlying Tampur dolomite or pre-Tertiary basement.