Wan Hasiah Abdullah

Sedimentology and stratigraphic architecture of a Miocene retrogradational, tide-dominated delta system: Balingian Province, offshore Sarawak, Malaysia

Abstract The Balingian Province of NW Borneo is a major hydrocarbon-bearing region with production dominantly from Early Miocene (cycle II) coastal to lower coastal plain deposits. A regional sedimentological synthesis is outlined from the main productive interval of three offshore oilfields, which are representative of the depositional environments and stratigraphic architecture in cycle II. Four types of vertical facies successions are recognized: (1) fluvio-tidal channel; (2) tide-dominated delta front; (3) wave-dominated shoreface; and (4) barrier lagoon. The facies successions demonstrate that cycle II consists of a tide-dominated delta system that is partly analogous to the modern Rajang Delta and Lupar Embayment of southern Sarawak. Fluvio-tidal channel and tide-dominated delta successions represent periods of river-fed progradation. Wave-dominated shoreface and barrier lagoon facies successions represent depositional systems that developed during periods of transgression and/or delta lobe abandonment. Cycle II consists of several high-order sequences stacked together into two lower-order, c. 100–300 m thick fining-upwards, unconformity-bounded megasequences. This stratigraphic architecture reflects a combination of hinterland denudation and long-term eustatic sea-level rise. These trends were interrupted by frequent, higher-order relative sea-level falls, which were a result of movement along the West Balingian Line fault zone. Variations in the thickness of megasequences between the three oilfields reflect fluctuations in tectonically driven subsidence rates. This exerted a fundamental control on the reservoir architecture, zonation and correlation and provides a basis for comparing field behaviour.

Biomarkers and inorganic geochemical elements of Late Jurassic-Early Cretaceous limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq: implications for origin of organic matter and depositional environment conditions

Late Jurassic-Early Cretaceous limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq, were analysed based on a combined investigation of elemental composition and biomarker characterisation. These investigations were used to define the origin, type of organic matters and their relation to the environment conditions during deposition. The bulk geochemical characteristics indicated that the Late Jurassic-Early Cretaceous limestone sediments were deposited in a marine environment with calcium-rich seawater. Their biomarkers provide evidence for a major contribution by aquatic organic matter with a minor terrigenous organic matter contribution. This is confirmed by normal alkanes, hopanoids, steroids and related compounds of the saturated hydrocarbons. Moreover, the salinity stratification and relatively reducing bottom water conditions are evidenced in the Late Jurassic-Early Cretaceous limestone sediments, as identified from Sr/Br, V/Ni and Pr/Ph ratios. The occurrence of gammacerane biomarker in the analysed samples is also a strong indicator of reducing and salinity stratification during deposition. Therefore, a stratified water column with salinity and relatively anoxic bottom water conditions contribute to organic matter (OM) preservation.

Geochemical Characteristics of Crude Oils, their Asphaltene and Related organic Matter Source Inputs from Fula Oilfields

The oils were derived from alga organic matter, which was sourced from sediments deposited in a lacustrine environment under suboxic to relatively anoxic conditions and were generated from mature source rock with a wide range of maturity ranging from early to peak oil window. This is indicated by biomarker compositions based on saturated and aromatic fractions and bulk geochemical characteristics. Previous work by Makeen et al. (2013) on the Abu Gabra shales in the basin shows similar organic matter characteristics, palaeo-depositional environment and maturity, suggesting that the Fula oils are derived from the early Cretaceous Abu Gabra shales. This genetic relationship is also confirmed by the similarities in the structural moieties of the Fula oil asphaltenes and Abu Gabra shale kerogens (Makeen et al., 2015).

Organic Geochemistry, Burial History and HC Generation Modelling of Upper Cretaceous Sediments in the Chad (Bornu) Basin

The Upper Cretaceous Gongila and Fika Formation sediments, which are believed to be the major source rocks in the Chad (Bornu) Basin, were analysed using organic geochemistry and petrology. The total organic carbon (TOC) contents of the sediments range from 0.42 to 4.90%. The samples analysed have vitrinite reflectance in the range of 0.58 – 1.39 % Ro and pyrolysis Tmax in the range of 429 – 475 oC indicate that the Gongila and Fika sediments contain mature to late mature organic matter. Moderate oil-generating potential is anticipated from the sediments with fairly high hydrogen indices (150 – 250 mg HC/g TOC). This is supported by their Py-GC (S2) pyrograms with n-alkane/alkene doublets extending beyond n-C30. The sediments are dominated by Type II and Type III kerogen and are thus considered oil and gas prone (mainly gas). One-dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the Upper Cretaceous sediments in the Chad (Bornu) Basin based on the reconstruction of the burial/thermal maturity histories. This is to improve our understanding of the of hydrocarbon generation potential of the source rocks. Calibration of the model with measured vitrinite reflectance (Ro) and borehole temperature data reveals that the present-day heat flow in the Chad (Bornu) Basin varies from 55.0 mW/m2 to 60.0 mW/m2 and paleo-heat flow value at approximately 68 mW/m2. The source rocks of the Gongila and Fika Formation are presently at a stage of oil, condensates and gas generation with thermal maturity ranging from 0.58% to 1.39% Ro. The modelled burial history also suggest that maximum burial occurred in the late Miocene and that erosion might have been the cause of the thinning of the Tertiary sediments in the present time.