Controls on Reservoir Quality in a Depositional Framework of Formation 2, South Bongkot Area, North Malay Basin, Gulf of Thailand

Abstract

Greater Bongkot South (GBS) is part of Greater Bongkot Field (GBF), which is situated in the northern Malay Basin. The subsurface geology of GBF comprises a thick interval (6 km of Oligo-Miocene siliciclastic sediments) deposited in fluvial, delta plain to delta front environments of FM2. This study is focused on cored units in FM2 in the GBS area and to find that most potential reservoir sands in the Bongkot south area show significant evidence of tidal influence. Previous work argued that sands in whole Bongkot area accumulated in what were more fluvial-dominant settings. There is significant subsurface uncertainty in reservoir quality prediction due to variations in depositional environment in the GBS area. The outcome of this study is an improved understanding of factors controlling depositional environment, potential reservoir facies and reservoir quality. This was achieved by relogging relevant cores and integrating this new understanding with existing core data, core images, wireline logs, thin sections and other previous reports. In the FM2 sediments under study, the Tidal sand flat (TSF) facies contains the most significant and widespread grouping of potential reservoir sands. Internally in this grouping, porositypermeability is variable due to differing proportions of laminated clay versus structural and dispersed clay. When this unit is defined by its gamma signature, there is strong overlap with other significant facies sets, ranging from consistently excellent reservoir sands of Tidal channel (TC) facies, at the upper end of the poroperm value range, to less economically-significant thin sands in the tidal flat (TF) and lower shoreface (LSF) facies associations, which mostly occupy the lower end of the poroperm range. The FMI log is a valuable subsurface wireline tool that, given the nature of sediments in the GBS area, could be used to quantitatively differentiate laminated clay from structural and dispersed clay, work so far has shown clay content is mostly controlled by depositional environment. An FMI log would be much more reliable than a Gamma ray log when defining electrofacies in log suites run in the dominantly muddy FM2 environments present in the GBS area.