Publications

The CD Carbonate of Ngimbang Formation in East Java basin was deposited in early Oligocene and divided into 5 reservoir targets for development. These reservoir targets were subdivided into different flow units, predominantly due to secondary porosity resulting from karst processes. Identifying these karst zones therefore became important for formation evaluation. Classic high resolution borehole image, combined with sonic, density and neutron logs, and NMR are the most commonly used tools to identify secondary porosity. However, due to horizontal well design selection, conventional wireline logging was no longer possible so LWD has become the primary data acquisition method. The LWD tools selection criteria have to meet both the geosteering and formation evaluation objectives by considering all the drilling challenges. Even though all these common measurements are currently available in LWD technology, the complexity of the well trajectories and drilling risks associated with the long horizontal length in this particular project has made this technology unavailable. Therefore, different technique is required to identify karst zone and to determine saturation along the expected deeply-invaded karst interval. A statistic-based correlation technique using principal component variance between advanced measurements from the multifunction LWD tool such as sigma, neutron capture spectroscopy and standard measurements such as gamma ray, neutron porosity, density, caliper, and resistivity invasion profile is used to show an indication of karst- or matrix-dominated porosity. The spectroscopy provides more accurate lithology volume and information on grain properties. The application and limitations of the simultaneous sigma–resistivity inversion technique to determine Archie’s parameter if the formation water salinity has been determined accurately were also evaluated and discussed particularly for the actual reservoir environment. Additionally, the combination of all these techniques with information from state-of-the-art reservoir mapping technology has provided a more complete picture of the reservoir characterization along the horizontal section.