New insights into the Kerendan Carbonate Platform: an Oligocene Reservoir from the Upper Kutai Basin, Kalimantan

Abstract

The Oligocene carbonate play in the Upper Kutai Basin is under-explored with exploration historically focusing on the Miocene deltaic and turbidite plays. These carbonates, mainly consisting of the Berai Formation, outcrop along the southern margin of the Kutai Basin. In the subsurface the carbonates are seismically imaged developed over basement highs forming small, isolated and larger-scale platform areas. The Kerendan Field in the Bangkanai PSC is the only Oligocene carbonate play hydrocarbon accumulation in the basin, which comprises c. 100 km2 of combined structural/stratigraphic trap with two culminations. The western culmination was recently proved to be gas bearing through the successful West Kerendan-1 exploration well located some 11 km to the southwest of the initial Kerendan-1 discovery. The combination of development drilling in the Kerendan Field and the West Kerendan-1 exploration well have provided a wealth of new information, including samples, logs and test data. This together with a re-evaluation of the existing well core data and 2D seismic coverage has led to potential reinterpretation of the depositional model for the reservoir.It was previously characterized as an atoll with outer slope, rim and internal lagoonal facies (Saller and Vijaya, 2002), but an alternative interpretation is of a more open platform or bank with a partially developed rim. On this platform a complex association of platform, reef and carbonate sand facies were deposited in a range of shallow to deeper photic water depths. Deposition mainly occurred under low to moderate energy conditions within a semi enclosed marine embayment. Pressure and well test data support the presence of reservoir quality rocks across the entire Kerendan structure and a common gas-water contact. Where identified on regional 2D seismic coverage in the area of the Bangkanai and West Bangkanai PSCs, the Oligocene carbonate sequence appears to sit on NNE-SSW trending Paleogene horst blocks. Some bi-directional down-lapping features with parallel, mounded or chaotic internal seismic reflectors are observed, and are interpreted as carbonates with the potential for reservoir development by analogy with the Kerendan area. Several of these highs are undrilled and represent targets for future exploration efforts including additional 2D seismic acquisition.