Abstract

The Soka oil field, a recent discovery in South Sumatra, is located in the central part of the Musi Platform, an area known for its gas production from the Baturaja Formation. The field lies on the southern rim of an elongate Pre-Tertiary basement high, the Bungur High, which trends NE-SW and transects the Musi Platform. The reservoir is composed of Early Miocene carbonates that are developed on a rugged basement terrain.

Conventional core and borehole image data acquired in delineation wells at Soka Field reveal that the early Miocene Baturaja Formation reservoir consists of a highly variable carbonate succession. Calibration of wireline log data with sedimentological facies interpretation allows subdivision of the carbonates into lithofacies on the basis of log response, bedding scale and contained sedimentary fabrics.

Within the Soka field, the Baturaja Formation lies unconformably upon a succession of highly fractured metavolcanics. Eight key lithofacies are identified at Soka Field. The lowermost sediments overlying the fractured metamorphic basement are comprised of mudstones and coarse basement-derived breccias with upward coarsening clast size trends, indicative of deposition by debris flow processes. A fan delta origin is interpreted for this unit. Subsequent carbonate deposition occurred within a variety of settings, including massive and vuggy reef facies, and also thinly bedded, often intraclastic, fine-grained facies, intercalated with glauconitic clays, and hence interpreted to be of deeper water origin.

The high resolution lithofacies characterization possible with borehole images has enabled identification of a number of upward shoaling lithofacies successions analogous to parasequences, providing a framework for inter-well correlation. Furthermore, the oriented fabric data evident on the high resolution images provide insight into slope and facies orientations.

The studies confirm that detailed lithologic subdivision within this type of carbonate succession can be achieved using borehole image data in the absence of cores. Also, detailed facies variations can be interpreted by complementing image data to information obtained from core data available from a limited number of wells, thereby enabling field-wide evaluation of reservoir quality.