A Comparative Source Rock Study of Two Proven Petroleum Systems: The Marine Madbi Formation of Yemen and the Terrestrial Nyalau Formation of Sarawak, Malaysia

Abstract

Two contrasting petroleum systems have been evaluated and compared. Marine shales of the Jurassic Madbi Formation in the Masila basin, Yemen (Fig.1a) and Tertiary coals and organic-rich sediments of the Nyalau Formation (offshore stratigraphic equivalent to the Cycle I & II of Balingian Province,) in Sarawak, Malaysia (Fig.1b) have been subjected to detailed organic petrological and organic geochemical studies. An assessment, based on organic facies characteristics, has been carried out on these sediments, in order to distinguish, characterise and evaluate source rocks deposited in marine versus paralic depositional sitting. The methods employed include evaluation of organic carbon content (TOC), biomarker distributions, pyrolysis-gas chromatography analysis and petrographic data. The maturity assessment of the samples analysed is mainly based on vitrinite reflectance (%Ro). The organic geochemical and organic petrological approach here has been able to clearly differentiate between marine and terrestrial depositional setting. Organic facies parameters such as Tm/Ts, Pr/Ph, Pristane/n-C17, Phytane/n- C18 and oleanane/C30 hopane ratios appear to reflect variation in depositional conditions and/or source input. Although there is a mixture of land-derived and<br>marine-derived organic matter in both sediments, the depositional conditions of these formations can be distinguished based on these organic facies parameters, whereby the Madbi shale samples were deposited in a reducing suboxic marine condition while the terrestrial Nyalau sediments in suboxic to oxic paralic condition of deposition (Fig.2) The Madbi shale samples possess vitrinite reflectance (% Ro) values ranging from 0.74-0.88% thus indicating an early mature to peak mature range, while the Nyalau sediments possess vitrinite reflectance values of 0.50-0.66% which suggest the samples are early mature for oil generation. 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 C32 hopanes (Fig.3). With regard to oil generation potential, good source rock potential is suggested by the high TOC values for Madbi shales and the organicrich sediments of the Nyalau Formation, as well as owing to their liptinite-rich nature (based on petrographic data) (Fig.4) and Py-GC dominated by n-alkane/alkene doublets (Fig.5). Within early to peak oil window maturity, the Madbi shale would be expected to be a better source rock for oil as indicated from its higher abundance of Type I and II kerogen compared to the Nyalau Formation which are dominated by Type II and III kerogen. Based on this study, good oil/gas generating potential is anticipated from the coals and carbargillite/coaly shales of the Nyalau Formation, owing to predominant n-alkane/alkene doublets and aromatic compounds (Fig.5).