Source rock characteristics and hydrocarbon generation potential in Brunei-Muara district, Brunei Darussalam: a comparative case study from selected Miocene-Quaternary formations

Liyana Nadiah Osli,Md Aminul Islam,Mohamed Ragab Shalaby

doi:10.1007/s13202-021-01142-0

Liyana Nadiah Osli, Md Aminul Islam + Show 1 more

Open Access

https://doi.org/10.1007/s13202-021-01142-0

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Abstract

A comparative analysis on source rock properties has been carried out on the Miocene-Pliocene formations as well as the Quaternary terrace deposits using Rock–Eval pyrolysis results and organic petrography as well as some biomarkers results. Samples were obtained from outcrops of the Quaternary terrace deposits, Pliocene-aged Liang Formation together with the Miocene Miri and Setap Shale formations in Brunei-Muara district, with sample lithologies ranging from coal, coaly shale, shale and lignitic sand. High total organic carbon (TOC) and S2 values ranging from 41.8 to 62.4% and 7.40 mg HC/g rock to 122 mg HC/g rock, respectively, are identified in coals of the terrace deposit, Liang and Miri formations, making these as the best potential source rock due to the “good to excellent” generating potential. Meanwhile, a “fair to poor” potential is exhibited for the coaly shale, shale and lignitic sand samples as a result of their low TOC, HI and S2 values. The organic matter is composed of kerogen type III (gas prone) and type II-III (mixed oil and gas prone). Organic matter in all studied formations originate from a terrestrial-source, as proven by the abundance of huminite. Organic petrographical and biomarkers studies suggest that the coals and lignitic sand samples were deposited in a mangrove-type mire in a lower delta setting, under oxic and limnic to limnotelmatic conditions, except sample DD2-1, which is deposited in a less water-saturated environment. The samples display the presence of bi-modal and normal distribution of n-alkanes. For all of the samples, the dominating plant types in the palaeomire are of soft, herbaceous plants and this is supported by the low vegetation index and moderate Paq values. All the studied samples are thermally immature to early mature, as exhibited by the Tmax values that range from 300 to 437 °C and vitrinite reflectance readings of 0.22% to 0.46%.

Highlights

In Brunei Darussalam, good-quality source rocks exist only as centimeter-thick layers and lenses, without having a mappable thickness and lateral distribution (Scherer and Hitam 1992)
Preliminary classification of the samples based on lithology can be further improved with the use of ash content of the samples, which is a representative of mineral matter input of the samples (Diessel 1992)
The results show that MR1-1 and MR1-3 samples from terrace deposits have ash content of 11.3% and 90.8% respectively

Summary

Introduction

In Brunei Darussalam, good-quality source rocks exist only as centimeter-thick layers and lenses, without having a mappable thickness and lateral distribution (Scherer and Hitam 1992). These organic-rich layers are present in the Neogene sediments of Brunei, which are regularly studied on varying. Similar coal-bearing sediments are identified in other formations other than the Belait. Modified after (Wilford 1961). In the Brunei-Muara district, coals are identified in the Miri and Liang formations, as well as in terrace deposits. 1 and 2; Wilford 1961) The Miri Formation is lithologically similar with the Belait Formation and both of them are synchronous with each other, together with the Seria and Setap Shale formations (Figs. 1 and 2; Wilford 1961)

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