Online Journal for E&P Geoscientists

Hakimi, Mohammed H.^*1; Abdullah, Wan H.¹; Shalaby, Mohamed R.¹
(1) Geology, University of Malaya, Kuala Lumpur, Malaysia.

Organic matter content, type and maturity as well as some petrographic characteristics of the Tertiary coals and rich-organic matter sediments of the Nyalau Formation exposed in the Sarawak were evaluated, and their depositional environment was interpreted. Geochemical (Rock-Eval pyrolysis, extract analysis and biomarker distributions) and petrological are the main methods used in this study.

Biomarker parameters such as Tm/Ts, Pr/Ph, pristane/n-C17, phytane/n-C18 and oleanane/C30 hopane ratios appear to reflect depositional conditions and/or source input. Although there are a mixture of land-derived and marine-derived organic matters in both sediments, the depositional conditions of these sediments can be distinguished based on these organic facies parameters, whereby the Nyalau sediments were deposited terrestrial environment under an oxic to suboxic paralic condition.

The Nyalau Formation samples have vitrinite reflectance in the range of 0.50-0.66%, which suggests the samples are early mature stage for hydrocarbon generation. Tmax values range from 430-445oC, which is in good agreement with vitrinite reflectance data. The level of thermal maturity attained by these samples is also reflected in their biomarker distributions as indicated by the approximate 60/40 ratio of the S to R isomerisation of the C31 and C 32 hopanes.

The Coals and rich-organic matter of the Nyalau Formation are typically dominated by vitrinite, with moderate amounts of Exinite and liptinite. With regard to oil generation potential, good source rock potential is suggested by the high TOC values, as well as owing to their liptinite-rich nature (based on petrographic data). This is supported by their high hydrogen indices of up to 370 mgHC/gTOC and Py-GC (S2) with n-alkane/alkene doublets extending beyond n-C30, and their being in the early oil-window range. Petrographically, the most significant evidence of the oil-generating potential of these coals is the generation of petroleum-like materials (exsudatinite) visible under the microscope. Exsudatinite is a secondary maceral, commonly considered to represent the very beginning of oil generation in coals, which is shown here to also have an important role to play in hydrocarbon expulsion.