Lower Miocene Carbonate Mudbanks in the Ramba-Rawa Area, South Sumatra Basin, Indonesia

M. W. Longman, C. W. Zeliff, L. R. Beddoes, Jr.

Micrite-rich skeletal wackestones and packstones with common branching corals and benthonic foraminifera form the reservoirs in the Ramba and Rawa oil and gas fields. These limestones are part of the lower Miocene Batu Raja Formation, a widespread carbonate unit deposited over much of southern Sumatra and the Sunda Shelf on a relatively stable carbonate shelf.

Forty cores from the roughly 100 km² Ramba-Rawa area reveal that the reservoir rocks are part of a carbonate mudbank complex that formed on and around a basement paleohigh. The carbonates range in thickness from 12 m on the paleohigh to 65 m off the paleohigh and represent at least two periods of deposition separated by a time of subaerial exposure.

Micrite and fragmented fossil debris dominate the reservoir rocks. In-situ corals are rare and limited to a few small colonies. No evidence exists of a rigid reef framework or marine cementation. Instead, the sediments accumulated as micrite trapped by seagrass and/or non-calcareous algae in low-relief banks in a shallow marine environment. Deeper water clay-rich tight wackestones containing planktonic foraminifera and glauconite surround the carbonate mudbanks.

Porosity in the reservoirs is dominantly secondary and formed during dissolution and neomorphism of grains and matrix. Permeability is sufficient that the Ramba A and B pools produced 30 million barrels of oil during their first four years on production. Oil production in the Rawa area will begin in early 1988.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.