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Abstract: Anatomy of a Lacustrine Source - The Brown Shale of Central Sumatra, Indonesia

KATZ, BARRY J., WILLIAM C. DAWSON, Texaco Inc.; CARL ATALLAH, GUNARDI B. SULISTYO, PT Caltex Pacific Indonesia; L. MICHAEL DARNELL, EWA SZYMCZYK, and LOUIS W. ELROD, Houston Advanced Research Center


Over the past two decades sufficient geologic and geochemical data have been generated in Central Sumatra to permit the development of a lacustrine source rock depositional model based on the Brown Shale Formation of the Pematang Group. The exponential increase in data has revealed the complexity of lacustrine source rock depositional systems. Areal limits are placed on oil-prone source developments by variations in subsidence and proximity to clastic input. The oil-prone facies appear to develop within the more rapidly subsiding sub-basins and in more distal settings. Stratigraphically the oil-prone facies developed in the upper portion of the sequence. The stratigraphic control appears to be, at least in part, controlled by climatic and tectonic processes.

A number of geochemically diverse oil sub-families appears to have been generated. The geochemical character of the oils reflects variations in environmental factors (i.e., water depth, salinity, etc.) and the relative proportion of allochthonous organic matter. The diversity of these oils is far greater than observed from a single marine source rock. Each of these oil sub-families appears geographically restricted, with each subfamily appearing to be associated with a distinct sub-basin. The volumetric importance of each sub-family appears proportionate to the oil generating potential (i.e., source rock development) of the sub-basin.

Although a general depositional lacustrine source rock model can explain the majority of the basin's hydrocarbon reserves a more refined understanding is required to capture the smaller yet significant reserves within the basin where a well defined oil source rock does not exist.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah