Geochemical characteristics and implications of hydrocarbons in reservoir rocks of Junggar Basin, China

A practical sequential extraction approach has been applied to two core sam ples collected from oil zones of Triassic sandstones in Junggar Basin, nort hwest China. Five bitumen fractions were obtained from each sample using th is approach. These fractions are free oil components in inter granular pore s (bitumen 1), adsorbed oil components on grain surfaces and liberated by H Cl treatment (bitumen 2), oil-bearing fluid inclusions hosted by grains (bi tumen 3), and adsorbed oil components on clay mineral surfaces and liberate d by HCl and HCl:HF treatments (bitumens 4 and 5). Striking differences have been observed both in bulk compositions and in al iphatic biomarker distributions (e.g., terpanes and steranes) among the fiv e bitumen fractions. This phenomenon is ascribed to the indigenous composit ional variations and differential maturation behaviors between adsorbed oil components in a network of polar compounds (resins and asphaltenes) and fr ee oil components in even a same oil charge, in addition to fractional adso rption effects and compositional changes of the oil charges in reservoir ro cks during filling process. The molecular compositions and homogenization temperatures of oil-bearing f luid inclusions in these two samples indicate that trapping of these types of inclusions apparently ceased in sample A, while continued in sample B af ter the initial oil charge, suggesting that the relative content of polar c omponents in the initial oil charge compared to the later oil charges plays a dominant role controlling the trapping of oil-bearing fluid inclusions i n an oil saturated zone. Sequential extraction studies are important and useful for reservoir geoche mistry, especially in the cases where only limited samples of oil and reser voir rocks are available, and/or where compositional heterogeneity of free oils has disappeared in a reservoir [Karlsen, D.A., Nedkvitne, T., Larter, S.R., Bjorlykke, K., 1993. Hydrocarbon composition of authigenic inclusions : application to elucidation of petroleum reservoir filling history. Geochi m. Cosmochim. Acta 57, 3641-3659]. (C) 2000 Elsevier Science B.V. All right s reserved.