The present study integrates the laboratory data from sidewall cores (RCA, petrography, XRD and SEM) with sedimentological observations (cores, wireline and image logs). Compositionally the sandstones are fine to medium grained sublitharenites to litharenites, with lithic fragments of metamorphic, reworked sedimentary and volcanic rock types. The diagenetic events observed in the sandstones are compaction, dissolution of feldspar and lithics, cementation (pyrite, silica, calcite, siderite dolomite and ferroan carbonate), and clay formation (kaolinite and illite). The porosity type comprises of primary intergranular, secondary and microporosity. The environment show a gradual change from upper reaches of the lower coastal plain with fluvial dominant regime for Cycle I to lower reaches of the lower coastal plain with increasing marine influence in Cycle II. The reservoir quality is controlled by both petrofacies and grain size variations.
The ongoing drilling campaign in offshore Sarawak has resulted in the discovery of potential reservoirs from deeper Cycle I and II, previously considered tight and thus opening new frontiers for exploration. The study area lies within several geological provinces of Central Luconia, SW Luconia and West Balingian (Figure 1 and 2). The generalized stratigraphy, tectonics and paleogeographic evolution for the Cycle I and II are depicted in Figure 1 (Madon, 1999). In the subsurface the Cycle I and II sediments can be recognized using biostratigraphy, well logs and to a limited extent on seismic datasets; the concept of Cycle as originally defined by Ho Kiam Fui (1978) has been used for defining the cycle boundaries. Accordingly, the base of a cycle is taken for practical reasons at the base of the most transgressive interval. The electrolog signatures across the block show good lateral correlations and can be used with reasonable confidence (Figure 2).
The present study was undertaken to comprehensively evaluate the sedimentological controls for understanding the reservoir quality from eight exploratory wells obtained during the last few decades. The observations and datasets from limited conventional and sidewall cores form the basis of this study. Laboratory data (RCA, petrography, XRD and SEM), wireline and image logs were integrated to understand the petrofacies, diagenesis and depositional trends. The petrographic, mineralogical and textural data is limited especially for the deeper Cycle I sandstones. The analytical datasets have been standardized and plotted to make meaningful interpretations and comparisons. For understanding the sandstone compositions, the modal data obtained from petrographic examination was plotted using Folk (1980) techniques for classification of sandstones and lithic grains.
