Abstract

Ramba Field, located in the Corridor Block of southern Sumatra, produces oil from coral-rich packstones and wackestones in the Lower Miocene Baturaja Formation at a depth of about 800 meters. The field consists of the "A" and "B" oil pools which are separated by a prominent intrabank paleochannel. These pools cover an area of approximately 24 km². The oil column in the "A" pool is about 45 meters thick and lies beneath a gas cap up to 25 meters thick. The "B" pool has an oil column about 35 meters thick beneath a very thin (< 5 M) gas cap.

The best reservoir rocks contain common fragments of branching corals with associated molluscs, and benthonic foraminifera in a micrite matrix. A similar fauna occurs in the more shaly and tighter offreef limestones deposited in slightly deeper water flanking the reservoir rocks. Still deeper offreef facies in the channel downdip from the reservoir lack corals but contain common planktonic foraminifera, intraclasts, and glauconite. They provide poor reservoir potential even where they occur within the oil column.

Cores from one of the best wells in the field (Ramba-4), a flank well (Ramba-12), and two "channel" wells (Ramba-5 located south of the "A" pool and Ramba-13 located in the intrabank channel) provide a detailed look at the variety of lithofacies in and around the Ramba Field reservoir. These cores indicate that the best reservoir rocks were deposited toward the end of the Early Miocene transgression in shallow, sheltered conditions as mudbanks over basement peleohighs. Little of the coral is in growth position indicating extensive reworking of the bottom sediments by storms and currents.

Porosity is best developed in both clean (non-shaly) coral-rich limestones and some foraminiferal limestones. It averages 16% and 18% in the "A" and "B" pool reservoir rocks, respectively. Calculated reservoir permeabilities in the two pools are 3600 and 500 md. Initial reservoir pressure in the "A" pool, was 1260 psi with 37.6° API gravity oil. In the "B" pool these values were 1200 psi and 37° gravity oil. Reservoir drive mechanism in the "A" pool is gas cap expansion, solution gas, and gravity segregation; in the "B" pool it is solution gas with a poor partial water drive. Since its discovery in 1982 through June, 1992, Ramba Field has produced nearly 60 MMBO.