--> --> Petroleum Source Rock Potential of Cenozoic Sandy Facies of NW Borneo: Deepwater Submarine Fan of Crocker Formation Versus Shallow Marine Deposits of Meligan Formation, Onshore Sabah, Malaysia

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Petroleum Source Rock Potential of Cenozoic Sandy Facies of NW Borneo: Deepwater Submarine Fan of Crocker Formation Versus Shallow Marine Deposits of Meligan Formation, Onshore Sabah, Malaysia

Abstract

Petroleum source rocks are commonly fine-grained sediments such as shales and calcareous mudstones. However, in the prolific petroleum basin of NW Borneo, the shaly facies are generally not organic-rich. On the other hand, some sandy facies such as the reservoir sandstones of the deepwater Kikeh Field offshore Sabah were shown to be organic-rich. In this study, organic petrological and geochemical analyses were performed to evaluate the oil-generating potential of sand-rich deepwater and shallow marine deposits from around Kota Kinabalu and Klias areas of western Sabah, Malaysia. The samples analysed are the submarine fan facies of the Crocker Formation (Eocene-Early Miocene) and the fluvio-deltaic to shallow marine deposits of the Meligan Formation (Middle-Upper Miocene). The Crocker Formation consists predominantly of alternating sandstone and mudstone layers characteristics of deepwater turbiditic deposits, whilst the Meligan Formation is dominantly massive cross-bedded sandstones interbedded with minor dark grey shale. The sandstones of Meligan Formation contain abundant coal fragments with common worm burrows. The Crocker Formation sediments are generally lower in organic richness compared to the Meligan Formation, although within the southern part of the study area abundant coal fragments do occur within the slump or debris flow mass transport deposits (MTD) of the Crocker Formation. Coaly terrigenous Type III and II kerogen are the most common oil-/gas-prone organic constituents in both these formations. In the MTD samples, oil-generative features were observed to be associated with perhydrous vitrinite and resinous constituents, and this is supported by reasonably high Hydrogen Index (HI) values in the range of 242 – 445 mgHC/gTOC. Oil-generative features were also microscopically observed in the Meligan sandstone facies which include occurrences of exsudatinite and oil-haze. This Meligan Formation is generally immature to early mature for oil generation as suggested by vitrinite reflectance values of about 0.5%Ro, although this value could have been suppressed due to impregnation of resinous constituents and/or early generated hydrocarbon/bitumen within the vitrinite matrix. On the other hand, the Crocker Formation shows variable thermal maturity in the range of 0.6 to 2.0% that correspond to early oil to gas generation stage.