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CONTENTS

Organic rich lacustrine mudstones are a major source of hydrocarbons in Western Indonesia and are widely understood to have been deposited in syn-rift settings. Syn-rift tectonic activity can have a major influence on the accumulation and preservation of lacustrine mudstones. Two field exposures of Brown Shale (Pematang Formation) lacustrine sequences in Western Sumatra illustrate aspects of syn-depositional tectonic influences on source rock deposition. At Karbindo in the Kiliran Sub-Basin a 200m thick well exposed syn-rift sequence illustrates the establishment of lacustrine conditions above a thick coal-palaeosol sequence. The sequence of floodplain to coal swamp to lake is a response to increasing rates of subsidence within the syn-rift setting. Relatively constant subsidence over a prolonged period of time allowed the deposition of a thick sequence of oil prone organic rich lacustrine mudstones. At Bukit Susah in the Sumai sub-Basin lacustrine mudstones are punctuated by pebbly sandstones and well defined palaeosols. Sedimentation is belived to be adversely affected by an accomodation zone where the Timpe lineament (a major E-W trending fault system) crosses the Sumai half graben. Five clearly definable exposure surfaces have been identified and are interpreted to reflect a strong tectonic control on deposition. Periodic uplift resulted in lake draining and exposure, with associated palaeosol development. As a result only 33% of the stratigraphic thickness comprises unmodified oil prone organic rich lacustrine mudstones. A knowledge of syndepositional tectonic activity and the depositional characteristics of lacustrine systems can greatly increase our understanding of the distribution and quality of lacustrine source rocks.