OVERPRESSURE ANALYSIS AND TRAPPING MECHANISM AT FRACTURED BASEMENT IN THE SOUTH SUMATERA BASIN

The South Sumatra Basin (SSB) is a prolific hydrocarbon basin in Indonesia, where hydrocarbon is trapped in the pre-Tertiary basement reservoir and in Tertiary siliciclastic and carbonate reservoirs. The occurrence of hydrocarbon in the basement in SSB makes this basin as basement hydrocarbon producer in SE Asia, together with Chu Long Basin in Vietnam.

The basement reservoir could be categorized as non-conventional reservoir, where the tanks are composed of fractures without matrix porosity. The fractures were formed and intensified due to several deformation processes. For hydrocarbon to occur, the basement should be located at paleo-highs and the source rock should be onlapped into the basement, allowing hydrocarbon migration into the basement. The basement should also be overlain by thick shale sequence providing seal for hydrocarbon trapped in the basement.

We have analyzed 23 wells in SSB, and from basin modeling, the oil window is located at the depth of 1800 – 2000 m, while gas window at 2000 – 2800 m. The top of overpressure is located at the depth of 1500 m and top of hard overpressure at 2500 m. The top of overpressure is due to undercompaction, and at hard overpressure zone, there is a contribution from gas generation.

The depth to basement in the fracture basement fields in SSB is located at the average depth of 2000 m. All fields are gas field with Suban being the largest field with the reserve of 7 TCF. The pore pressure analysis shows that the sequence above the basements is in overpressure condition, and hydrostatic in the basement. This circumstance enhances hydrocarbon sealing capacity in the basement. The hard overpressure is located in the basinal depocenter, within the gas window. Since the depocenter is onlapped onto the basement, and the source rock is in hard overpressure condition, the gas migration into the basement is very efficient.

As a conclusion, several ingredients required for the occurrence of hydrocarbon in SSB are present, i.e.: 1) polyphase tectonism creating intensive fracture, 2) onlapped source rock onto the basement, 3) excellent sealing capacity due to overpressure above the basement, 4) hard-overpressure source rock providing effective hydrocarbon migration. These factors should be considered in finding similar basement fracture reservoirs in other areas in Indonesia’s basins.