An Integrated Analysis of the Depositional Control, Sedimentology and Diagenesis of Cenozoic Carbonates from the Sarawak Basin, East Malaysia

Abstract

This study is focused on an integrated geological analysis of both onshore and offshore Cenozoic carbonates from the Sarawak Basin, NW Borneo. The principal aims of this thesis are to (1) investigate the regional factors that control the carbonate development, geometry and distribution, (2) determine sedimentological and high order sequence stratigraphic models, and (3) analyse the diagenesis of these isolated carbonates and better constrain their overall reservoir characteristics. The study is based on (1) four carbonate build-ups in the offshore Central Luconia Province (Middle-Late Miocene), and (2) four onshore carbonate units (Late Eocene-Early Miocene). The regional synthesis indicates that the sites for carbonate growth were mainly controlled by palaeo-basement tectonic structures that partially determined the buildups’ shape and morphology. Some carbonates developed on flat rift blocks show flat-top morphology, while others grew on tilted sub-blocks and basinal often display conical shapes. The tectonics also controlled subsidence, uplift, tilting, faulting and later influenced local relative sea-level fluctuations, which strongly affected facies variability, stratal geometry and carbonate demise. A good match between the stratigraphy and global sea-level curves suggests that eustacy was the main controlling mechanism for carbonate growth. Eustatic sea-level changes have also influenced the rate of growth, erosion, and buildup geometry, that ultimately impacted the reservoir property distribution. The sedimentological study reveals variability between the individual onshore build-ups, which display unique depositional patterns, facies and biota systems eventhough they are the same age and located only several kilometres apart. During the Late Eocene, the Lower Batu Gading Limestone was typified by massive nummulitic facies, that is occasionally interbedded with matrix-rich limestone once it moves away from the bank of ramp setting. The Upper Batu Gading Limestone (Early Miocene) sits on a disconformity and is composed of finely bedded and brecciated limestones. In contrast, the Suai Limestone is characterised by fining upward parasequences of larger foraminifera dominated by large Eulepidina spp, while the Subis Limestone is rich in massive/branching corals, benthic foraminifera and algae. The Bekenu Limestone is composed of finely laminated marls and shale rich in planktonics that represents an analogue for calci-turbidite carbonates as shown by strong seismic attributes offshore. These variations suggest that the isolated carbonates onshore northwest Sarawak grew with different depositional constraints, such as variations in light intensity, water depth, salinity and nutrient levels. In contrast, the offshore carbonates show very favourable conditions for carbonate growth, as indicated by much greater similarity in terms of facies, foram size, biota systems and sequences.The onshore carbonates have undergone less complex diagenesis than their offshore equivalent. The latter have been affected by ten major stages of calcite cementation and dolomitisation. Each stage shows large variations in elemental and isotopic compositions. Tm ice of inclusions indicates that they were trapped from evaporitic waters during burial or mixed marine/meteoric waters at relatively low temperature (<50oC). However, these cements completely obliterated the pores and made the reservoir quality onshore relatively poor. In contrast, the offshore carbonates have undergone more complex diagenetic pattern, including reversal in reservoir properties. The build-ups have been affected by at least nine stages of calcite, dolomite and dedolomite cementation together with three major dissolution and several minor alteration episodes. A little variation in elemental composition but significant changes in isotopic ratios of these cements suggests drastic changes in diagenetic palaeo-fluid systems over time. The presence of high temperature minerals and high Th (130oC) indicate the possible involvement of late stage corrosive fluids of hydrothermal origin that migrated upward along faults and was responsible for large porosity and permeability enhancement of the reservoirs. This study provides a new insight into the factors that controlled carbonate development and distribution, geometry and facies variability for prospect and reservoir characteristics. These findings can be used as an analogue to better understand the older and deeper carbonates within other parts of the Sarawak Basin, which will help future exploration and development strategies.