oa Identifying Overpressuring Mechanisms in<br>Sedimentary Basins<br>from Wireline Logs

Accuracy in pore pressure prediction requires prior understanding of the formation of overpressure in the basin. This may be a challenging task as the generation of overpressure can be attributed to several geological processes, either increase in loading stress (compaction disequilibrium) or volume expansion, or combination of both. The most common cause is due to compaction disequilibrium, whereby the causal mechanism can be eadily verified if the porosity in the overpressured shales remains high (undercompacted). In many instances, the origins of overpressure are combination of several of these causes. In the Malay Basin variable pore pressure profiles are observed. Pore pressure profile is variable due to: (1) choice of methods (empirical or soil mechanics), (2) difficulties in selection of normal compaction trend, (3) multiple origins of overpressure, and (4) chemical ompaction effects. The variability of pressure profiles and log cross-plot trends found in the Malay Basin can also be observed in basins elsewhere in the world. Based on the pressure and wireline log analysis conducted for the Malay Basin, a flowchart has been developed to enable determination on overpressure origin in the basin. Overpressure resulting from disequilibrium compaction, fluid expansion, clay diagenesis, lateral transfer and chemical compaction can be recognized by a combination of prediction methods and well log signatures. Examples presented are taken from several overpressured basins around the world. The wider compaction trends observed in these basins are associated with variable sand and clay content, plus pure unloading signatures associated with fluid expansion mechanisms.