Publications

The Arun and North Sumatra Offshore (NSO) gas fields are located onshore and offshore Sumatra. They both produce from Lower to Middle Miocene age carbonate build-ups on horst blocks formed during rifting in Palaeocene-Oligocene times. The carbonates are sealed by the regionally extensive Baong Formation (Middle-late Miocene). The Arun Field is characterised by abnormally high pore pressures (7105 psi (49 MPa) at 3048m which equates to Equivalent Mud Weights of 13.5 to 14.0 ppg). These elevated pressures can result in breaching of seals such as recently in the Porong-1 well, Java. Carbonate reservoirs are the targets of many drilling programs in Indonesia in such basins as Southern, Ardujuna and Jatibarang Basins. Understanding the pressure regimes in these carbonates is vital both for safe drilling and for reducing uncertainty in predicting actual reservoir pressures pre-drill. As there is no relationship between effective stress and porosity/velocity in carbonates, approaches based on changes in porosity using seismic velocity and/or log data will give false magnitudes of overpressure in these carbonate units. Therefore another approach is required, one based on understanding the mechanisms of pressure generation and build-up in a basin (a geological approach), coupled with shale-based prediction techniques in any clastic intervals above (and below) the carbonates. This paper presents evidence of a significant impact on the internal pressure regime of carbonates via pressures of any bounding clastics, both above and below the carbonates, i.e. carbonates themselves do not normally generate overpressure but have pressure transition zones that reflect the pressures above and below. Pressure re- distribution is possible, as porous carbonates such as grainstones/packstones may be sufficiently well * Ikon GeoPressure ** University of Durham plumbed to allow either pressure escape or lateral transfer of pressures. Thus, the abnormal pressure in the Arun carbonates is considered controlled by pressures in the surrounding Baong shales.