Onshore Kutai Basin Pore Pressure Synthesis and Hydrocarbon Accumulation

Abstract

Hydrocarbon accumulation in the Kutai Basin's deltaic system appears to be related to pore pressure profiles, with the largest accumulations occurring mostly between 60 - 140degC bottom hole temperature. This interval is the "Accumulation Zone," which extends from the hydrostatic pressure regime to the top of hard overpressure zone. The zone occurs due to the chemical cementation, which lithifies the rock grains and creates an inner sealing system that traps hydrocarbon effectively at the said temperature range. Below the Accumulation zone, the "Expulsion Zone” exists starting from the temperature of 140degC, which corresponds to Ro = 0.6 (Hydrocarbon Maturity), situated on hard overpressure and has very low mobility (< 1 mD/cP). A "Loss/depleted Zone" could occur well below the Expulsion Zone, with excessively hard overpressure and close to fracturing pressure. These conditions will make forming a slip fault/fracture easier, leading to drainage to reach a pressure-stress balance. Finding an ideal complete zone of such systems in the present state of the onshore part of the Kutai Basin is difficult due to existing erosional events after uplifting, except for Nilam Field. However, by reconstructing the erosional event, it is possible to comprehend the zone system and explain the relationship between pore pressure, temperature, and mobility to today's hydrocarbon accumulation, where higher erosion intensity tends to result in lower hydrocarbon accumulation.