f Reservoir Properties and Depositional Environment of The Group M Cored Interval in Ledang Tengah, Block Pm309, Malay Basin

The early Oligocene Group M sandstone is relatively under explored in the Malay Basin but as exploration matures, the M group sandstones will be increasingly be targeted. The M group reservoirs in the Ledang-Anoa Field will be the oldest clastic reservoir in the Malay Basin to be developed. The Group M sequence forms part of the rift sequence and have been interpreted to be deposited within a continental setting in an alluvial-lacustrine environment. To determine the depositional environment, reservoir quality and identify the controlling factors in the reservoir quality variations, several cores were taken from the lower part of Group M, informally named as M40, M60 and M70 sandstones from the Ledang Tengah ST-1 well. From core descriptions and facies analysis work, supplemented by wireline log interpretations, the depositional system is interpreted to be a braid delta of McPherson et. al (1987). The braid delta system is characterised by progradation of a braided fluvial system into a lake. Depositional environments within this braid delta system composed of braided fluvial channels and in-channel bars, lacustrine mouthbars, palaeosols and open lacustrine muds. The almost complete absence of scour and fill structures (except for the base of the M70 core) that are common in many fluvial deposits is unusual and would suggest channel widening rather than channel incision as the main sedimentary response to fluctuating flow conditions. The sand bodies are thus likely to be aerially extensive and sheetform in geometry. Limited core and cutting samples have been analysed palynologically. In general, palynoligical recoveries were poor although samples were generally rich in organic debris. Terrigenous miospore assemblages were dominated by freshwater herbaceous swamps and cosmopolitan pollens. Mangrove taxa and savannah grassland pollens were rare. The high abundance of organis debris and highly degraded humic material indicated persistent soil erosion and deposition in a stagnant water, with brief periods of increased<br>freshwater run-offs marked by influx of freshwater algae and increase in miospores abundance and diversity. The absence of hinterland pollen species generally confirmed the reduced freshwater circulation and transport. Variations in reservoir quality in the cored section is controlled by primary factors, with sandstone grain size and clay content the dominant factors. However, the sandstones are generally texturally and compositionally immature. Although quartz and carbonate cements are present, the presence of authigenic kaolinite and to a lesser extent illite the results of secondary diagenesis, destroyed the primary permeability and rendered the quality of the sandstone reservoir poor. The best quality reservoir is associated with the coarsest grained and the more massive portions of the channel fill and delta mouthbar sandstones.