Provenance of Oligocene to Early Miocene Sediments in Sarawak, NW Borneo

Offshore Sarawak is an area rich in hydrocarbon resources, produced from the underlying Cenozoic sediments. Eight stratigraphic cycles (Cycles I to VIII) were introduced to correlate offshore sequences which were biostratigraphically tied using planktonic foraminifera and nannofossils. The oldest sequence, Cycle I, is interpreted to have been deposited in the Oligocene to Early Miocene. On land, the equivalent is the Nyalau Formation, but the correlation between the onshore and offshore areas is still uncertain. There have been no provenance studies utilising heavy minerals and detrital zircon geochronology in offshore Sarawak before this study. Six exploration wells from locations crossing from land to deep water were selected based on the oldest penetrated stratigraphic units, sand thickness, borehole wireline logs and seismic data. Samples of drill cuttings and several cores were processed for detailed analysis which included light minerals, heavy minerals, and detrital zircon U-Pb geochronology using the LA-ICP-MS technique. The sandstones were optically examined for framework analysis (QFL) and assessed for composition and textural maturity. Heavy mineral identification was carried out using three different methods: the Raman Spectroscopy (RS), Energy Dispersive Spectrometer using a Scanning Electron Microscope (EDSSEM), and conventional optical analysis. RS-identified heavy minerals were carefully checked and further validated optically and chemically using the EDS. The heavy mineral assemblages were then plotted to study the vertical and spatial variations in provenance; index ratios were used as indicators due to the insufficient number of grains. Concentrates of detrital zircons were picked and mounted for cathodoluminescence (CL) imaging to identify internal structures for laser spot position before U-Pb dating using the LA-ICP MS and to interpret their origin (e.g. magmatism). The study shows that the sandstones are quartz-rich, compositionally mature and texturally sub mature to immature. Poor presence of feldspar group minerals in the samples could have resulted from intense weathering due to the tropical climate in the region since the Late Cretaceous. Heavy minerals are dominated by the ultrastable and stable group minerals (zircon, rutile, tourmaline, apatite, garnet, cr-spinel, and siderite), and maturity index plots have revealed that their presence was not affected by post-burial diagenesis processes. CL images of detrital zircons internal structure (e.g. oscillatory, homogeneous) suggest they have magmatic origins. An age-provenance signature technique based on Phanerozoic population (top three most abundant age) coupled with the relative proportion of Phanerozoic and Precambrian grains (nPh/nPc), were used to characterise and correlate Cycle I and Cycle II between wells, and also applied to onshore zircon dataset from previous studies (e.g. the Nyalau and Belaga Formations). Cycle I has a pT>K>J imprint that is consistently exhibited by all six studied wells, as well as the Nyalau Formation with nPh/nPc = 0.9–2.1. It is very distinctively different from the overlying Cycle II, especially in the onshore and shallow water area. Cycle II has two types of imprint, Type 1 is K>pT>J (nPh/nPc = 1.9–6.7), and two variances for Type 2, Subtype 2A is pT>J>K (Bukoh-1), and Subtype 2B is pT>K>S (Bako-1) with nPh/nPc in the range of 0.7–1.3. The above unique characteristics have added a new important feature to the Cycle I and Cycle II definition that is useful for stratigraphic review and correlation of wells with poor biostratigraphic data, as applied in Teres-6 and Tenggiri Marine-1. The zircon imprints from the study also have revealed two important sediment source areas for Cycle I and Cycle II clastics, the Malay Peninsula and Borneo. The former is characterised by Permo-Triassic, Paleozoic, and Precambrian populations. Borneo materials are composed mainly of reworked materials sourced from the Kuching-Belaga Range. Cycle I in the onshore (Selungun-1 and Teres-6), shallow water (South Parsons-1 and Tenggiri Marine-1), and Bako-1 are interpreted to have been sourced from Borneo via a connected drainage system; except for Bukoh-1 that could be sourced from the west (Malay Peninsula) and north (southern Vietnam). The Borneo remained the important sediment source for the onshore and shallow water wells during the Cycle II deposition, but Bako-1 started to receive sediments from the west; Bukoh-1 remained unchanged.