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

A practical sequential extraction approach has been applied to two core samples collected from oil zones of Triassic sandstones in Junggar Basin, northwest China. Five bitumen fractions were obtained from each sample using this approach. These fractions are free oil components in inter granular pores (bitumen 1), adsorbed oil components on grain surfaces and liberated by HCl treatment (bitumen 2), oil-bearing fluid inclusions hosted by grains (bitumen 3), and adsorbed oil components on clay mineral surfaces and liberated by HCl and HCl:HF treatments (bitumens 4 and 5). ng differences have been observed both in bulk compositions and in aliphatic biomarker distributions (e.g., terpanes and steranes) among the five bitumen fractions. This phenomenon is ascribed to the indigenous compositional variations and differential maturation behaviors between adsorbed oil components in a network of polar compounds (resins and asphaltenes) and free oil components in even a same oil charge, in addition to fractional adsorption effects and compositional changes of the oil charges in reservoir rocks during filling process. lecular compositions and homogenization temperatures of oil-bearing fluid inclusions in these two samples indicate that trapping of these types of inclusions apparently ceased in sample A, while continued in sample B after the initial oil charge, suggesting that the relative content of polar components in the initial oil charge compared to the later oil charges plays a dominant role controlling the trapping of oil-bearing fluid inclusions in an oil saturated zone. tial extraction studies are important and useful for reservoir geochemistry, especially in the cases where only limited samples of oil and reservoir rocks are available, and/or where compositional heterogeneity of free oils has disappeared in a reservoir [Karlsen, D.A., Nedkvitne, T., Larter, S.R., Bjørlykke, K., 1993. Hydrocarbon composition of authigenic inclusions: application to elucidation of petroleum reservoir filling history. Geochim. Cosmochim. Acta 57, 3641–3659].