Overpressure and Compaction in the Peciko Field, Kutai Basin, Indonesia

The Peciko Field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral overpressure gradients. We have analysed overpressure and compaction in this field by using wireline log, pressure, temperature, and vitrinite reflectance data. The top of overpressure is located below 3 km burial depth, below the depth range for transformation of smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for mudrocks show unloading responses within the transition zone into hard overpressure below 3.5 km depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with the onset of gas generation. Moreover, mudrock density continues to increase with depth in the overpressured section. We conclude that gas generation and chemical compaction are responsible for overpressure generation, contradicting previous interpretations that disequilibrium compaction is the principal mechanism for generating overpressure in this area. The transition into hard overpressure may be identified from reversals on the sonic and resistivity log responses in the mudrocks.