Controlling Factors in Clastic Reservoir Diagenesis Offshore Bintulu – Sarawak Basin

Rock data from new well discoveries show that diagenetic factors such as pyrite, siderite, silica & ferroan-calcite/dolomite cementations, clay minerals (kaolinite, illite/smectite & chlorite) and compaction are the important controlling factors in reducing reservoir quality in Offshore Bintulu – Sarawak Basin. Pyrite and siderite are more abundant in shallow depths but their abundance decreases with the increasing depth (Fig.1). This phenomenon is because pyrite and siderite were formed as early diagenetic minerals in sandstone. The bulk rock volume of pyrite and siderite is small (<6%-10%). However these minerals reduce both pore space and pore throat size in the shallow section above 1500 mss. Quartz overgrowths are present from shallow to deep. Re-precipitation of quartz as overgrowths can occur at any depth but it is primarily controlled by pressure and temperature conditions. In the study area, the presence of quartz overgrowth(s) is more common in the deeper interval (Fig.2). In the deeper interval, reprecipitation of quartz as quartz overgrowths is a major agent of pore space and pore throat size reduction. Ferroan-dolomite starts to appear in the deeper over-pressured interval of the study area. The mineral is controlled by both the environment of deposition (the more proximal marine – the more abundant) and the pressure temperature conditions required to precipitating it. The bulk rock volume of ferroan dolomite is low (<6%) but it becomes increasingly important as an agent of the reduction of both pore space and pore throat size with depth (Fig.3). Within most of the study area sands, the proportion of clay is low (<20%). The predominant clay mineral of this fraction is illite+mica followed by kaolinite (Fig.4). The low abundance of pore bridging illite/smectite is evidence that the study area sands are at an early-mature stage of diagenesis. Clay minerals contribute to the reduction of both pore space and pore throat size. However because of its low abundance clay, whatever its form is not a major contribution to the reduction of pore space and pore throat size. Fig.5 shows that compaction is the most important controlling factor on the reservoir quality of the study area sands. At shallow depths, compaction is light. At deeper depths, compaction increasingly influences reservoir quality regardless of the facies. However the fine grain silty low energy facies exhibit a more rapid reduction in reservoir quality than the coarse high energy deposits. This paper describes the integration of core, petrography, XRD and mineralogy data from <br>SK309/SK311 new well discoveries in the study area. It attempts to contribute to an understanding of clastic reservoir diagenesis in an inverted coastal plain to shoreface depositional environments in Offshore Bintulu – Sarawak Basin.