The quantitation and origin of C40+ n-alkanes in crude oils and source rocks

A gas chromatographic method using n-alkane standards (n-C-20, n-C-30, n-C-40, n-C-50, and n-C-60) and a deuteriated alkane internal standard (n-C24D50) is presented for the quantitative analysis of high molecular weight alkanes (> C-40; HMWAs). The samples were injected in pulsed splitless mode. Using this method, HMWAs ranging up to C-100 can be determined, and a GC response factor for n-C-60 of up to 83% that of n-C-30 was obtained. The HMWA distributions in three typical high wax oils and a source rock extract from the Nanyang Depression, China, were determined. The results show that the concentrations of HMWAs in the three crude oils range from 22.8 to 38.0 mg/g oil, accounting for about 16% of the total concentration of C22+ n-alkanes. The concentration of HMWAs in the source rock extract from the Dong-10 well is twice as large compared to the concentration in the crude oil in Dong-12 well, which may be associated with fractionation during migration from the source rock to the reservoir. The formation of C-40-C-60 n-alkanes may be associated with cutan/cutin sources within kerogen [Org Geochern 31 (2000) 589], and also with biochemical and geochemical processes under strongly reducing conditions. The n-C-21 to n-C-35 n-alkanes in typical high wax oils from the Nanyang Depression have a weak odd-over-even carbon number predominance, whereas the n-C-40 to n-C-60 n-alkanes show a strong odd-over-even carbon number predominance. This phenomenon is tentatively considered to be related to the occurrence of different hydrocarbon generation and preservation characteristics of n-alkanes with different carbon numbers. (C) 2003 Elsevier Science Ltd. All rights reserved.