ABSTRACT: Petroleum system of the Senoro-1 Discovery, East Sulawesi, Indonesia

The Senoro-1 well was drilled in March 1999, resulting in a sizeable gas and oil discovery. The primary objective was a seismically well-defined Miocene reef, which was expected to have good reservoir properties based on existing well data for similar carbonate buildups along trend. These carbonate build-ups comprise the North Tomori petroleum system, a complex assemblage of Upper Miocene reefs, recently charged by older Miocene source rocks after burial by Pliocene synorogenic deposits along the Tomori collision zone. The well successfully penetrated a porous reefal facies of the Mantawa formation, containing gross gas and oil columns of 656 ft and 33 ft, respectively. The gas consists of 89% methane, 2% carbon dioxide, hydrogen sulfide (estimated at about 1%) and wet hydrocarbon gases (C2-C5+). Carbon isotopic composition of the methane is -41 per mil (PDB), suggesting a mixed thermogenic and biogenic origin. The oil has biomarker and isotopic signatures correlative with calcareous marine source rocks of the underlying Miocene Matindok and Tomori formations. Oil staining was observed within the gas column, indicating that oil once filled the reservoir to a structural level much higher than the present gas-oil contact. Using geochemical signatures and observations of residual oil staining, a probable hydrocarbon charge history was reconstructed. This involved early stage biogenic gas charge, followed by migration of oil, and then late stage thermogenic gas flushing. The high quality reservoir facies is capped by a thick shaley sequence with sealing properties capable of supporting very large hydrocarbon accumulations. Fluid compositions bear witness to the various stages of hydrocarbon fill in a geological setting that clearly contains a very active petroleum system.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia