Abstract

The Kutai basin is one of the largest and most important hydrocarbon-producing basins of Indonesia. The Nilam structure is a 5 TCF mature gas asset with >1 BCF/day gas production. Reservoir engineering has identified a 180 BCF OGIP reservoir, called the G053B, within this thick prolific Middle Miocene deltaic sequence as an ideal horizontal well candidate.

The G053B interval is interpreted as an incised valley fill (IVF), deposited as a back-stepping sequence during a relative sea level rise, based on the integrated interpretation of all available core, well and 3D seismic data. An IVF interpretation, rather than a highstand systems tract distributary channel sandstone model, is based on the clear delineation of significant incision and corresponding basinward shift in facies, the development of coeval sediment-starved interfluves, and the abnormal aspect ratio of the valley system (3 km wide and 40 m thick). The recognition of an IVF is key to understanding both the internal and external architecture of the G053B reservoir, and explains the thick, multi-storey nature of these channel sands and the accordingly large gas reserves.

Multiple seismic attributes (RMS amplitude, half-energy, coherency, phase and isochrons) from the 3D seismic data delineate both the external morphology of the tank and offer glimpses of gross interval thickness, net gas pay and subtle structural elements, which influenced depositional trends. Forward modeling and AVO modeling have produced templates for recognizing incised valley fills and thick gas pay.

The recognition and mapping of an IVF provides a model for the classification and evaluation of the relatively small number of large tanks within the Nilam and Badak Fields that contain a majority of the field reserves, and will aid the siting of future infill and step-out wells.