Evaluation of shales of the Belaga Formation in Central Sarawak, Malaysia, using the Spectral Gamma Ray method

Abstract

The Belaga Formation is a deep marine turbidite interpreted in relation to a submarine fan and therefore a prospect worth evaluating. The scope of this paper will focus on the shales from the thick sedimentary sequence of the Belaga Formation (Upper Cretaceous to Eocene) consisting of five (5) members in Central Sarawak, Malaysia, of which attention is given to the depositional environment and clay minerals as factors to consider as predictive tools for organic matter richness and gas storage capacity using the spectral gamma ray method using Thorium (Th), Potassium (K) and Uranium (U) as indices for inferences. A mineralogical chart of Th/K (ppm/%) indicates that the dominating clay minerals of the shales are mixed-layer types. The Th/U as a redox indicator is used to assert that the shales are of anoxic conditions of deep-marine environments. Despite the relatively high insoluble Th values (9.93-20.13) ppm, the presence of U in substantial amounts, which only occurs in reducing conditions where it is preserved as a lower insoluble valence (U 4+) explains for the low Th/U values ranging between (1.86-4.57) ppm/ppm of the fifty-eight analyzed shale samples. The overall Th/U value of the evaluated shales remain less than 7, where Th/U < 7 is suggestive of marine sediments, grey and green shales, whereas Th/U < 2 is indicative of marine black shales of reducing conditions. The 30Ma interval of sediment deposition of the Belaga Formation Eocene recorded varied climatic fluctuations influential to clay minerals contained in the shales and ascribe the paleoweathering state of the shales, which from the standard imitative ternary plot of normalized K-U-Th of shales from Northern America address the shales of the Belaga Formation as of deep weathering residuum.