Oil-Bearing Reefs in Salwati Basin of Irian Jaya, Indonesia

Abstract

Recent exploration activity in the Salawati basin, along the northwestern tip of Irian Jaya, Indonesia, has resulted in significant oil discoveries. Reservoir and trap are provided by highly porous and permeable Miocene reef rocks, which have yielded oil at rates of up to 32,000 bbl per day from individual wells. The Salawati basin is a structural-stratigraphic basin containing more than 15,000 ft (4,600 m) of marine Tertiary sedimentary deposits. The basin is bounded on the north by the Sorong fault zone which separates the Australian continental plate from the Pacific oceanic plate. Basin development was initiated in the Miocene when local downwarping and increased water depths resulted in the deposition of a basinal limestone and shale sequence in the area. Toward the east and south margins of the basin these sediments are replaced by shallow-water carbonate rocks which form a well-defined shelf and associated shelf margin. The productive reef belt is just basinward of the shelf and shelf-margin area. The reefs, which reach heights in excess of 1,600 ft (490 m), range in areal extent from 2 to 48 sq mi (5 to 124 sq km). Porosities within the reefal carbonate rocks are as much as 43 percent and average 20 to 30 percent. Late Pliocene-Pleistocene normal faults which cut many of the reefs, combined with postreef structural tilt, have modified considerably the original configuration of the reefs and the associated oil accumulations. The knowledge gained from the current exploration program concerning reef morphology, porosity variations, tectonic history, and other factors which control the oil accumulations should expedite future exploration efforts in the basin.