Abstract

The Tertiary sediments of the Central Sumatra Basin comprise different strata with source rocks deposited under lacustrine and marine conditions. The organic matter in the lacustrine Pematang Brown Shale is predominantly Lamalginite, whereas in the Petani and Telisa Formations organic matter is of mixed amorphous and structured vitrinite. The lacustrine Brown Shale is characterized by a narrow distribution of activation energies at 53-54 kcal/mole typical of Type I kerogen. The kerogens from Telisa and Petani formations have much wider distribution of activation energies typical of Type II-III kerogens. Because of this difference, the transformation of kerogen to hydrocarbons occurs over a narrow depth interval for the Brown Shale, and over a wider depth interval for the Type II-III kerogens. Type I kerogens (Brown Shale) of lacustrine ogigin show little variation in activation energy distributions, although quite variable in Hydrogen Index. These minor variations do not significantly affect the present day transformation ratios, due to the compensating effect of A and E values. In the case of Type II-III kerogens (Telisa and Petani Formations) there is an obvious relationship between activation energy distributions and vitrinite content. Kerogens with a larger percentage of vitrinite have higher activation energies.