FACIES AND SANDSTONE CHARACTERISTICS OF THE KUDAT FORMATION, SABAH, MALAYSIA

The Kudat Formation, formed the major rock unit of Kudat Peninsula in the northern tip of Borneo Island, is made up of interbedded sandstone and mudstone with fossiliferous limestone lenses. Lithostratigraphically, it is divided into Tajau Member and Sikuati Member. The Tajau Member differs from the Sikuati Member by the thicker coarsening upward sequence and the presence of limestone lenses. Field mapping and petrographic analysis were utilized to study the facies and sandstone characteristics. The formation as a potential reservoir is largely dependent on the original sandstone composition, which was influenced by deposition in a shallow marine setting and local sourcing activity. The succession of the sequence is composed of a major shoreface deposits during the late Early Miocene. The measured sections are predominantly composed of interbedded sandstone and mudstone usually show sedimentary structures which are indicative of wave and tidal processes, such as swaley, hummocky, trough and planar cross stratifications. The sequence grades up-section into facies packages of increasing tidal energy and terminate with the deposition within the upper regime shoreline settings. The selected Tajau sandstones in this study are texturally and mineralogically mature quartz arenites with good sorting. However, the presence of unstable lithic grains and feldspars during diagenetic processes contributed to the reduction of porosity, giving irregular porosity, due to deformation by compaction, and susceptibility to chemical alteration of labile minerals. Reservoir quality was also influenced by the subsequent diagenesis of the sandstones, which was driven by increase overburden pressure and could be culminated by increase temperature during the proceeding depth. Lithic fragments, feldspars and authigenic cements were altered to form diagenetic mineral suites, which tend to occlude porosity; however, dissolution and chemical reactions of some of these grains also enhanced secondary porosity development. Most observed porosity in the sandstone sequence is secondary, developed from dissolution of both carbonate cement and unstable framework grains. Mean porosity suggested a weak decreasing trend with depth of burial. However, detailed examination of several localities revealed that porosity development was strongly influenced by local factors. Thus, sandstones of the Kudat Formation are generally fair reservoir characteristics, due to the presence of unstable framework grains, early carbonate cementation and authigenic/diagenetic mineral suites