FACIES MODEL AND STRUCTURAL FRAMEWORK OF THE MIRI FORMATION (MIDDLE – LATE MIOCENE),SARAWAK, MALAYSIA

Miri is the birthplace of Malaysian petroleum industry. It was discovered in 1910 and was totally abandoned in 1972 after producing about 80 million barrels of oil. However, geologically Miri remains one of the most interesting and challenging place to be resolved stratigraphically and structurally. The primary objective of the present study is to provide additional interpretation on the stratigraphy and structural geology of the Miri Formation in the Miri Field based on the new information gathered from new outcrops in the area. Eighteen outcrops were examined in detail on structural geology. Among these eighteen outcrops, five outcrops were chosen also for detail sedimentology and stratigraphy studies.

Based on lithology, sedimentary structures, bedding geometry and traces fossil, the sediments of the Miri Formation were grouped into fourteen lithofacies. These facies were grouped into two main facies associations which are: (i) tide-dominated estuary; and (ii) wave-and-storm dominated facies associations. The tide-dominated estuary system of the Miri Formation are includes variety of sub environments: estuary mouth or tidal channel and sand bars (characterized by trough cross-stratified sandstone with mud drapes facies), estuary channel or upper flow regime of sand flat (characterized by parallel stratified sandstone with mud-laminas facies), mixed-tidal flat (characterized by wavy and flaser bedded sandstone facies), and mud-tidal flat (characterized by rhythmic stratified sandstone-mudstone and lenticular bedding facies). The wave-and-storm dominated varied from lower to middle shoreface (characterized by hummocky cross-stratified sandstone and rhythmic parallel stratified sandstone and laminated siltstone facies), upper shoreface (characterized by swaley cross-stratified sandstone), lower shoreface (interbedded to bioturbated sandstone and siltstone facies), and offshore transitional (characterized by bioturbated sandstone and mudstone interbedding with parallel to hummocky cross-stratified sandstone facies).
Results for the structural geology analysis indicate that the development of the Miri structure is interpreted into two separate periods of deformation: (i) an early period of compression, indicated by the asymmetric anticline and the reverse fault; and (ii) a later period of extension, indicated by a set of normal faults. The early period of compression related to a NW / SE compressive stress field gave rise to anticlinal feature trending in NE-SW direction. Warping and uplifting of the Miri anticline accompanied the period of compression caused brittle deformation and rupture manifested by the reverse fault.

Later period of extension may have resulted from local to regional outer-arc extension. It was indicated by most of conjugate normal faults in Miri Formation has NE-SW strike orientation or parallel to the anticline axis of the Canada Hill. Rotational movements on the competent sediment of the Miri Formation also happen due to the local space created during the extension periods, thus it is possible to explain the existence of a very big contrast of thick sequence of vertically dipping section with a sub-horizontal or gently dipping sequence situated side-by-side at the Miri Hospital Road 1 outcrop. The effect of thrusting followed by the rotational movement of the sediments is the reason for the drastic change of the sequence within a very short contact zone without any significant brecciation.

The tectonic system of the Miri Formation was probably the result of compressional phase occurred during the early Late Miocene while the deposition of deltaic sediment of the Miri Formation started since the Middle Miocene times. The later stage of deformation was dominated by extensional phase and probably happened shortly before the depositional loading of the West Baram Delta ends, during Late Miocene to Pliocene times. This resulted in the developments of normal faults and the associated growth fault.