Diagenesis and Petrophysical Rock-Type Study of Oligocene Berai Carbonate Platform in “KX-1” Well, CentralKalimantan: Implications for Depositional and Reservoir Models

Abstract

The “K” Oligocene carbonate platform is located in Upper Kutai Basin, interpreted as an isolated rimmed platform atoll system within a non-reservoir lagoonal environment in the central platform interior. Previous wells were drilled in the eastern rim tested significant volumes of gas. However, the KX-1 well which was drilled to test the western rim of the atoll also discovered gas with a pressure gradient indicating a similar spill point to the eastern rim. Integrated core and wireline log study of the KX-1 exploration well reveals that “K” carbonate platform developed in a land-attached setting in a semi-enclosed embayment without raised rims, under oligophotic conditions, related to encroaching clastic influx points from the North and Southwest. Interlayered reservoir-quality grainstone and packstone units developed across the platform interior as sand shoals with significant lateral extents, suggesting pressure communication in the saddle area between the rimmed margins. Economic porosity is present with average of 5.5%, as secondary porosity generated by: (1) matrix-specific intercrystalline microfractures, and (2) microporosity via non-fabric selective burial leaching and dolomite replacement. Isotope signatures of uplift may relate to tectonically induced microfractures that are associated with burial leaching and dolomite replacement in the platform interior. The microfractures are a response to funneling and leaching fluid pathways
connected to the platform margin through a continuous permeability zone. These findings open up a new exploration and development paradigm by identifying new elements as target criteria in Oligocene carbonate platform : (1) Layered grainstone units with extensive lateral continuity in the
platform interior setting, (2) Structural overprints that can create combination traps, thus extending the play-type target from what was previously a strict “reefal build-up” poro-perm model.