SEDIMENTOLOGY, DEPOSITIONAL ENVIRONMENT AND RESERVOIR QUALITY OF SANDAKAN FORMATION (ULU SIBUGA AND HARBOUR VIEW A OUTCROPS)

This study focuses on the sedimentological, depositional environment and reservoir
quality belongs to the Tertiary aged Sandakan Formation. This project was
conducted at the Harbour View A and Ulu Sibuga outcrops in Sandakan, Sabah.
Techniques involved in this study include facies analysis, permeability measurement,
gamma ray measurement, grain size analysis and petrographic analysis.
Four different lithofacies were identified at the outcrop. These are i)
sandstone with mud clast (F1), ii) hummocky cross stratification (F2), iii)highly
bioturbated (Skolithos) sandstone (F3), iv) thin parallel lamination with coal
fragment and carbonaceous material (F4), v) clean and massive sand (F5) and vi)
lenticular bedding (F6) . Permeability measurement gives the following reading: a)
F1 ( 41 mD to 184 mD), b) F2 (65mD to > 250mD), c) F3 ( 91mD to 199mD), d)
(200-250mD), e) F5 (44mD to > 250mD) and f) F6 (60mD to 80 mD). Gamma ray
count shows lowest reading in sandstone with the range of 3-7 API and highest
reading in mudstone with the range of 9-14 API. Based on the grain size analysis we
found that the grain size range from medium to very fine sand with the very fine
sand was found to be the dominant grain size. The grain sorting of the sediment is
poor to moderate.
The Sandakan Formation investigated in this study is interpreted to be
formed in wave and storm dominated environment. This formation suggests a
gradual overall shallowing upward environment which marked coarsening upward
sediment pattern. It also marked the change in depositional system from offshore to
lower shoreface. The thick mudstone succession in Ulu Sibuga may be succeeded by
lower shoreface deposit of hummocky cross stratification sandstone in Harbour View
outcrop that record the sedimentation in wave and storm dominated environment as
the relative sea level begin to fall.