Brown Shale Formation: Paleogene Lacustrine Source Rocks of Central Sumatra

Abstract

This chapter summarizes several earlier studies on the origin of oil in Central Sumatra basin, In- donesia, by Williams et al. (1985); Katz and Kelley (1987); Robinson (1987); Sundararaman et al. (1988); Katz and Mertani (1989); Longley et al. (1990); and Katz (1991) supplemented with unpublished data. The approximate ten billion barrels of oil in the Central Sumatra basin of Indonesia have been gen- erated from organic-rich shales of lacustrine origin of the Brown Shale Formation within the Paleo- gene Perna tang Group. During Paleogene time, large freshwater lakes developed within structurally controlled rift troughs. Geochemical analyses were used to identify the source beds, interpret the depositional environment of the source rocks, correlate oils to source rocks, and determine the maturation history of the source rocks. Systematic variations in organic type reflect depositional environments, and along with organic maturity largely determine the type of hydrocarbons generated. Asymmetry of the rift troughs is the primary factor governing the predominantly lateral migra- tion of hydrocarbons towards the gentle hinge margin. Minor vertical migration is related to fault and fracture systems that have diverted migration from its preferential lateral mode. Structure, combined with stratigraphy and aerial distribution of sealing shales, controls the entrapment of oil.