SEDIMENTOLOGY, DEPOSITIONAL ENVIRONMENT AND RESERVOIR QUALITIES OF SANDAKAN FORMATION: TAMAN HARBOUR VIEW B AND ULU SIBUGA OUTCROP

Sandakan Formation (Late Miocene) is exposed across the Sandakan Peninsula in East Sabah and its estimated thickness is about 2300m (Khor et al, 2012). This study was carried out with the objective of characterizing the different sedimentary facies in recently exposed outcrop at Taman Harbour View B and Ulu Sibuga. Measurements and analysis such as permeability, gamma ray, thin section and grain size distribution were carried out. The dominant lithologies are sandstones and mudstones. Sedimentary features that is observed includes hummocky cross stratification, burrows, interbedding of sandstone and mudstone, flaser bedding and load casts; are documented in sedimentary log. Bioturbation features by Skolithos, Asterosoma and Monocraterion were also observed. Permeability and gamma ray measurement are corresponding to its respective lithology as in sandstones, the permeability value is high and gamma ray value is low while in mudstone, the permeability value is low and gamma ray value is high.
Five facies were identified from the outcrop namely (1) Hummocky cross stratified sandstone with coal lamination, (2) bioturbated sandstone, (3) sandstone-mudstone interbed, (4) amalgamated sandstone and (5) thick mudstone with sandstone pinch. Facies (1) is very fine grained with permeability range of 55 mD to 250 mD and gamma ray range of 3.87 API - 10.32 API. Facies (2) is fine sand with permeability range of 91 mD to 199 mD and gamma ray range of 6.45 API - 10.32 API while Facies (3) is fine sand and mud interbed with permeability range of 10 mD to 73 mD and gamma ray range of 9.03 API - 16.77 API. Facies (4) is fine sand with permeability value more than 250 mD and gamma ray range of 7.04 API to 9.03 API while Facies (5) is fine sand with permeability range of 7.87 mD to 48.17 mD and gamma ray range of 10.32 API to 16.77 API.
Based on these, the interpreted depositional environment of Sandakan Formation is said to be open marine shoreface to offshore shelf. Facies (4) and facies (2) have the best reservoir quality as they are deposited in more proximal area of upper and lower shoreface. Facies (1) and facies (3) have moderate reservoir quality as they were deposited in further offshore transition zone. Facies (5) are poor in reservoir quality as it was deposited in distal muddy shelf zone.