Publications

Petrophysical analysis plays an important role in a reservoir study as it provides primary input data in volumetric calculation.The occurrence of shale in reservoir rocks can result in erroneous values of water saturation and porosity as calculated from well logs. While Archie’s equation describes the electrical conductivity behaviour of shale-free rocks. Indonesia, Waxman-Smits and Dual-Water equation is used for shaly sand. Waxman-Smits and Dual-Water equation account for the dual conductivity pathways formed by pore brine and clay mineral exchange cations in shaly sands. The determination of water saturation in shaly sand cannot be accurately achieved by use of classical water saturation models/equations. The Lakat Formation in the Central Sumatera Basin is one of the most productive reservoirs that is comprised of a shaly sand reservoir. The formation comprises polymic conglomerate, sandstones and is interbedded of claystones, siltstones and tuff. The previous conventional petrophysical analysis study in this area shows a very pessimistic result compared to test data. This study uses the wireline log and core data including cation exchange capacity available. By using both sequential and simultaneous petrophysical analysis to analyse the effect of clay content in the reservoir and by comparing the results, the aim of this study is to make a decision on which method is suitable for estimating the water saturation in this area. Sequential petrophysics shows more appropriate result than simultaneous method, even though sequential method just use single matrix and shale parameter value that can be used in the analysis. Unfortunately simultaneous analysis which can use variable mineral and matrix parameters give inappropriate result caused by less suitable XRD data which is used. Both shale volume and CEC dependent methods analysed in this study give a relatively optimistic result and so the reservoir test result such as drill stem test and production test act as the criteria which are to be used to select the most suitable result. Keyword: Petrophysics, Shaly Sand, Lakat Formation, Central Sumatra, Water Saturation