Depositional environment and diagenesis of early Miocene Nyalau Formation, Sarawak, Malaysia

Abstract

Diagenetic understandings tied to depositional sequences that developed over the past decades help to predict the reservoir quality in oil and gas exploration. Understanding the lithologic characteristics and diagenetic alterations is key to decipher the various complex controls on its porosity and permeability. This diagenetic heterogeneity is a product of a complex history of diagenetic modifications, however, there are not many studies addressing this problem. The results show that the exposed sections around Bintulu town comprise of massive sandstones intervals, laminated clays, and brackish-shales and lignite. Five main types of lithologies are identified in the stratigraphic sections: sandstones, shales, coal, siltstones, and mudstones. These rock types are further sub-divided into fifteen lithofacies based on sedimentary structures and other characteristics. Petrographic studies revealed that the sandstones are well-sorted quartz arenite, with fine to medium grained sand. The diagenetic processes recognized include compaction, dissolution and cementation. The early, middle, and late diagenetic stages are present in the lithostratigraphy. Bioturbation marks the start of post-depositional changes, and allowed rapid exchange of pore waters with overlying depositional water. Iron oxide occurred in early diagenesis rapidly after the sediments were buried. However, later grain dissolution provided the secondary porosity and permeability. Compaction rates were high in muddy sediments. Sandstone samples with high mud content have less diagenetic signatures. Later, quartz and calcite cement stabilizes the grains and decreases porosity generated by early dissolution. Middle stages of diagenesis involved the growth of chlorite perpendicular to quartz grains. The late diagenetic changes involved the growth of pyrite crystals associated with chemical composition of deep burial pore fluids. The cement types that have been observed in this formation include clay, calcite, quartz overgrowth and iron oxides whereas, the later phases destroyed the early generated porosity. The calcite cement in the early stage of diagenesis, though, resisted the compaction of sediments and aids to preserve the porosity. Dominant porosity is interparticle porosity that is present depositionally and later enhanced by secondary grain dissolution.