Recent exploration operations in the western part of Block-A, Sumatra, delineated exposures of Peutu Limestone (Lower Miocene) in the foothills of the Barisan Mountains These resistant carbonates cap a prominent ridge that extends N-S over a distance of 25 km. Primary rain forest covers the ridge, outcrop was accessible only by coordinating geological surveys with a seismic program active in the area in 1992-1993. Two stratigraphically equivalent lithofacies occur in the Peutu exposures: clean skeletal limestone and argillaceous limestone interbedded with shale. The skeletal limestone facies contains lithologies similar to subsurface platform carbonates that develop into reefal gas reservoirs at Arun, Alur Siwah, and the recent CO2-rich discovery at Kuala Langsa. The argillaceous limestone and shale is a basinal facies. A thick sequence (3000 m) of Baong Shale ( Middle to Upper Miocene) overlies the Peutu. Three lithofacies are recognizable in the Baong on outcrop and in the subsurface: a lower bathyal shale, rich in planktonic foraminifera, a middle unit consisting of interbedded shale and turbidite sandstones, and an upper shale unit that accumulated as prodelta and slope deposits associated with overlying sandstones of the Keutapang Formation. The Baong is overpressured in the subsurface and seals traps in underlying, lower pressured Peutu gas reservoirs. Geochemical analyses of outcrop samples indicate fair to good source rock quality, and the Baong may be significant as a source for gas/condensate accumulations in both the Peutu and the overlying Keutapang Formation.
Authors : Atang Barliana, Turcahyo Wahyudi & Mark Chamberlain