GEOCHEMICAL ASSESSMENT OF LATE PALEOGENE SYNRIFT SOURCE ROCKS IN THE SOUTH SUMATRA BASIN

Abstract

The potential of Late Paleogene synrift source rocks in generating hydrocarbons within the South
Sumatra basin has been evaluated using geochemical and pyrolytic techniques. The samples were collected from
the outcropping shales of different layers constituting the Upper Oligocene Talang Akar Formation. Results of
TOC analysis reveal minor quantities of organic carbons, indicating poor to fair potential source rocks. The S2/S3
ratios indicate two apparent kerogen types. The majority of the samples is type III kerogen; however, there
appears minor type IV kerogen, implying the oxydized organic matter and no expulsion. The S1+S2 measurement
of all the pyrolysed rocks, except one sample LP 12A, yields less than 1 mg HC/g rock, supporting the proposed
scenario of little potential to generate hydrocarbons. Evaluation of hydrogen and oxygen indices confirms a gas
prone source, but all the shales analyzed, disregarding one sample LP 18, provide a direct evidence that the rock
section is immature to early mature to expel oil. Important notes can be made that the early stage of organic
maturation might have occurred due to heating, as a result of deeper burial during Miocene time prior to the
subsequent uplift in response to the Plio-Pleistocene orogeny.