Reservoir potential of carbonate rocks in the Kutai Basin region, East Kalimantan, Indonesia

Abstract

Fifteen percent of the exploration wells drilled in the Kutai Basin region were targeted for stratigraphic play-types. Carbonate reservoirs comprise almost 70% of the objectives in these stratigraphic plays. There was need for a better understanding of the carbonate reservoir potential in the region. Accordingly, this study was carried out. The distribution, depositional environment as well as factors controlling the quality of carbonate reservoirs are reviewed and analyzed. Carbonate reservoirs in the study area can be found sparsely throughout the Kutai Basin. Carbonates range in age from Oligocene (Bebulu limestone) to Late Miocene (Dian limestone). The main constituents of these carbonate build-ups are platy-corals, encrusting red algae and larger benthonic foraminifera. Most of the carbonates were deposited in a shallow marine environment (inner to middle shelf) during rises in relative sea level. Highstand system tracts are characterized by well-developed carbonate facies-belts. The carbonate build-ups generally occur as isolated bedded mounds, from a few feet up to 1000 ft in thickness. The preservation of primary porosity is generally poor due to diagenetic processes during burial history, particularly the infilling of pores by non-ferroan calcite cement. The development of secondary porosity is limited, due to the retardation of subsurface fluid flow by non-permeable layers, and the absence of solution effects due to sub-aerial exposure and karstification. Preserved porosities are mainly present as vugs, best developed in coarse-grained shelf-margin facies, which may not have subsequently been completely filled by calcite cement. Early hydrocarbon migration may retard the diagenetic processes and preserve the primary carbonate porosity.