Publications

Gas accumulations in the southern part of the North Sumatra Basin, from Rantau to Medan, occur in the Keutapang, Middle Baong Sandstone (MBS), and Belumai formations. All usually consist of gas associated with oil or condensate. Only Wampu Field, located in the Langkat-Medan area, produces gas with no associated liquid hydrocarbons.Twenty two gas samples were analysed from reservoirs in this area for composition and for carbon isotopes in the methane and ethane fractions: sixteen from the Keutapang Formation, five from the Middle Baong (MBS), and one from the Belumai. Cross plots of (1) methane isotopes vs. methane ratio (Sl3Cmethane vs C1/~Cn,),(2 ) isotopes of methane VS. ethane (S13Cmethane vs S13Cethane), and (3) methane isotopes VS. percentage ethane (S13Cmethane vs C2%) indicate that all samples are thermogenic gas formed during the catagenesis phase (oil associated gas).The gas samples can be classed into two groups based on type and distribution. The Rantau group in the northern part of the study area consists of gas associated with condensate, whereas the Aru-Langkat group to the south derives from non-associated sapropelic organic matter.Basin modelling was conducted to determine the source rocks for these two gas groups. According to Schoell, 1984, vitrinite reflectance indications of source rock maturity for gas generation are generally above 1.0 % Ro and in some cases more than 1.2 % Ro. Basin modeling indicates that black shales of the Bampo Formation have reached the gas generation phase and are possible sources for the gas accumulations. Hydrocarbon traps formed during the Plio-Pleistocene and may still be filling.