The Middle Miocene Miri Formation outcrops in Miri, Sarawak which represent possible surface analogue to the offshore Baram field, consist of meter-scale, multiple- stacked, fining-upward successions which predominantly comprise medium to fine- grained, trough cross-bedded sandstones. These rocks represent transgressive, tide- dominated deltaic or estuarine sedimentation. The Baram field is a small oil field situated in the north-eastern side of the Baram Delta Province, offshore Sarawak. The cored intervals of the Upper Cycle V (Late Miocene) of Baram field display reservoir successions dominated by thick swaley cross stratified (SCS) sandstones and other associated shallow marine, wave and storm- dominated facies. The vertical facies organisation of these successions suggest deposition during shoreface pro gradation associated with fall of relative sea level. Parasequence stacking patterns in the Upper Cycle V (Late Miocene) of Baram field suggest that deposition and build-up stratigraphy were controlled by superimposed short-term, medium-term and long-term sea level changes. Well log correlation of these stacking patterns reveal three scales of depositional cyclicity, the parasequence (-10 to -30 m thick), the parasequence set (-45 to -130 m thick) and the major cycles (-600 to -800 m thick). The parasequences and the parasequence sets represent the -20K precessionary and -lOOK eccentricity Milankovitch cycles, respectively. This interpretation places the lower boundary of the Upper Major Cycle (Upper Cycle V) to -6.3 Ma, which correlates with a major eustatic fall on the Haq et al. (1988) sea level curve. Modal analysis of the Baram sandstones reveal that mechanical compaction resulted in the reduction of about 30 to 60 % of the original intergranular porosity. Siderite, chlorite-smectite, chlorite and quartz overgrowth represent the principle authigenic phases. Siderite cementation occurred early, initially below sediment-water interface and continued to depths of -450 to -800 m. Chlorite-smectite, grain-coating and pore-filling chlorites, and authigenic quartz characterize advanced stage burial diagenesis, possibly at temperatures between 100 to 150°C. Other minor phases identified include hematite, kaolinite, framboidal pyrite, and localised carbonate and feldspar dissolution. The reservoir sandstones of Baram field record porosities and permeabilities ranging from 11.0-28.6% and 1.13-819 mD respectively. Primary depositional fabric strongly influenced the poro-perm characteristics of these sandstones. The very well- sorted SCS sandstones constituted the principal reservoir units (-20-26% porosity; -50.00-400 mD permeability). The heterogeneous sandstones also show good reservoir quality (10-25% porosity and 10.0-681 mD permeability). Overpressuring contributed to porosity preservation at depths. The presence of detrital mud clasts, clayey laminations, bioturbation and siderite cement commonly resulted in the reduction of reservoir quality.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:553106 |
| Date | January 1994 |
| Creators | Rahman, Abdul Hadi Abd |
| Publisher | University of Reading |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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