Abstract

Caltex Pacific Indonesia (CPI) uses formation pressure data to aid in prospect evaluation and generation in Central Sumatra. Over 3800 wireline formation pressure determinations have been compiled from more than 350 wells within its contract areas in Central Sumatra. Due to the large volume of fluids extracted since the inception of production some 43 years ago, most formation pressures are below normal hydrostatic. Analyses of these depleted pressure anomalies have allowed the definition and evaluation of structural and stratigraphic barriers to fluid flow.

Several major faults are observed to have significant pressure anomalies: the Sebanga, Daludalu and Sikladi Faults. Faults without pressure anomalies include the North Aman Border and Telinga Faults. In general, the sealing potential of faults can be related to fault type and trend (from greatest to least sealing potential): NW-trending reverse faults, N-trending strike-slip faults, and NE-trending normal faults. Stratigraphic barriers to fluid flow are observed both locally (interbedded marine shales, paleosols, laminations and cross-beds) and regionally (Bangko Formation and Telisa Formation).

Two major aquifer systems are observed: (1) the Petani Formation and (2) the Sihapas/ Pematang Groups. The Sihapas/Pematang aquifer is further subdivided into three sub-regional aquifers: (1) the Bekasap, Duri and Telisa Formations, (2) the Menggala and Pematang, Upper and Lower Red Bed Formations and (3) sandstones within the Pematang Brown Shale Formation. The Petani Formation system is generally at hydrostatic pressure and is well connected to surface exposures. The Sihapas/Pematang system are disconnected and isolated from the formation outcrops. The Bekasap/Duri/Telisa aquifer is depleted up to 600 psi and has large local pressure anomalies. The Menggala/Pematang aquifer is the most highly depleted aquifer (depleted 400–600 psi) and has remarkable pressure gradient uniformity. The Pematang Brown Shale Formation aquifer includes isolated sandstones with highly variable, normal to super-normal pressures.