Publications

The North Belut field is a major gas development in Block B, South Natuna Sea, Indonesia. The reservoir - Udang and Gabus sands - is a 1500 ft section of thin, stacked lacustrine and deltaic sands. Appraisal wells show significant variation in vertical and lateral reservoir development. Reservoir quantity, quality and connectivity are major uncertainties.To better understand the reservoir a number of focused studies of reservoir biostratigraphy (Morley et al 2007), core sedimentology and petrology, wireline log analysis (log shapes and petrophysics) and seismic stratigraphy were carried out and integrated.Some conclusions: Facies correlation to log motif is not always predictable. Biofacies and core sedimentology help facies analysis considerably. Deposition occurred within an increasingly distal lacustrine delta setting. Thin distributary channel and associated mouthbar sands dominate. Accommodation space was low. Older reservoirs were deposited in a freshwater lake which became entirely brackish lake by early reservoir time. Distributary mouth bar sand bodies are more amalgamated in the older reservoir (Gabus and early Udang) and become increasingly separated vertically and possibly also laterally up section (later Udang). Net sand to shale ratios are lower in the younger units demonstrating an over all lake transgression. Degree of sand aggregation - hence sand connectivity - will influence the number of development wells required. Intra-reservoir mapping at zone scale is possible, but sand body mapping using seismic so far is difficult. A few channel features can be seen in some horizon slices from the deeper reservoir, indicating axial transport of sediment towards the NE. However, seismic information on sand distribution in the upper reservoir is qualitative at this time. The relationship of porosity to permeability is, at best, moderate. Reservoir diagenesis is the main cause, both enhancing and reducing permeability without proportionate changes in porosity.