Changchun Pan

Molecular correlation of free oil and inclusion oil of reservoir rocks in the Junggar Basin, China

Free oils and inclusion oils (oil-bearing fluid inclusions) of 12 samples collected from the sandstone reservoir formations in the central, eastern and northern areas of the Junggar Basin, northwest China, were analyzed by GC and GC-MS. Analytical results indicate very similar biomarker distributions within each of the free oils and their associated inclusion oils in the two samples collected, respectively, from the northwestern and eastern border of the Junggar Basin. The free oil and inclusion oil of sample MD1-1, collected from the northwestern border, correlate well with the Permian source rock of the Fencheng Formation, while those of sample DN1-1 from the eastern border correlate with the Permian source rock of the Pingdiquan Formation. In contrast, in the central area, the free and inclusion oils vary significantly in most cases, which suggests variations of sources for oil charges during the filling process. These data, and the correlation between the free and inclusion oils, are consistent with the field and seismic data, which show that in areas where samples MD1-1 and DN1-1 are located, only one available source rock exists, while in the central area, multiple source rocks are present.

Geochemical characterization of free versus asphaltene-sorbed hydrocarbons in crude oils: implications for migration-related compositional fractionations

Based on the existing petroleum migration models that focus on the role of organic sorption, a laboratory study has been performed to simulate the migration effects on the various fractions of oil. In this study, one free oil fraction (defined as Bitumen 1, existing outside of or loosely attached to asphaltene), and three to four absorbed oil fractions (defined as Bitumens 2, 3, 4 and 5, more strongly attached to asphaltene macromolecules) that were obtained from three oil samples collected from the Bohai Bay Basin, East China, were analyzed in detail. Based on the laboratory partition of oil components between the solvent phase (oil) and the asphaltene phase, petroleum migrationrelated effect on saturate fraction was re-evaluated. Our observation indicates that the compositional variation from Bitumen 1 to Bitumen 4 or 5 includes the following: (1) The resin content increases while the saturate content and saturates/aromatics ratio decrease; (2) The relative abundance of short chain n-alkanes decreases while that of the long chain n-alkanes increases; (3) Pr/nC17, Ph/nC18 and Pr/Ph ratios stay virtually unchanged while the relative abundances of terpanes to n-alkanes and steranes to n-alkanes decrease; (4) The ratios of C21/(C21 þ C29) steranes and C23/(C23 þ C30) terpanes in the free oil fraction are similar to or even lower than those in the whole absorbed fraction, suggesting relatively stronger affinities of these smaller terpane and sterane molecules to asphaltenes in contrast to the relatively larger ones and (5) The isomerization parameters of C27 and C29 steranes are slightly higher in the free fraction than in the whole absorbed oil fraction. Large variations were observed in the distributions of steranes among the four bitumen fractions of the X52 oil, and were considered to be related to the heterogeneity of the source rocks, in addition to the partition of these compounds between the asphaltene phase and the solvent phase. q 2002 Elsevier Science Ltd. All rights reserved.

Sequential extraction and compositional analysis of oil-bearing fluid inclusions in reservoir rocks from Kuche Depression, Tarim Basin

The free oils, adsorbed oils and oil-bearing fluid inclusions have been extracted separately and analyzed by GC and GC-MS in reservoir rock samples collected from the Kuche Depression. The results demonstrate that the molecular compositions of oil-bearing fluid inclusions are significantly different from those of the free oils (the current oils). Compared with the current oil, the oil-bearing fluid inclusions are characterized by relatively high values of parameters Pr/nC17 and Ph/nC18, low values of Pr/Ph, hopanes/steranes, C30-diahopane/C30-hopane and Ts/Tm, low content of C29TS terpane and high maturities as indicated by C29 steranes 20S/(20R+20S). In addition, the oil-bearing fluid inclusions correlate very well with the oils in northern and central Tarim Basin, which were derived from Cambrian-Ordovician marine source rocks. The adsorbed oils appear to be an intermediate type between free oils and oil-bearing fluid inclusions. The above analytical data indicate that there are at least two oil-charging episodes for these reservoir rock samples. The early charging oils were derived from Cambrian-Ordovician marine source rocks, and the later charging oils, from Triassic-Jurassic terrestrial source rocks. The primary marine oils were overwhelmingly diluted by the following terrestrial oils.

Geochemical study and significance of sequential extraction in reservoir rocks of Turpan-Hamin Basin

Seven samples collected from the Turpan-Hamin Basin were treated by sequential extraction method. Analytical results indicate that the free oils, inclusion oils (oil-bearing fluid inclusions) and clay-adsorbed oils vary significantly. The geochemical characteristics of inclusion and clay-adsorbed oils correlate well with the lacustrine mudstones. The characteristics are as follows: (i) with higher Ph/nC18 ratio and lower Pr/Ph ratio; (ii) with higher values of C26 tricyclic/C24 tetracyclic terpane, C23 tricyclic terpane/(C23 tricyclic terpane+C3017α(H) hopane), C21/(C21+C29) sterane, Ts/Tm and C29Ts/C2917α(H) norhopane and lower value of C19/C21 tricyclic terpane; (iii) with relatively high content of C27 and C28 sterane and lower abundance of C29 sterane and diasterane. They indicate that the early charged oils are derived from the preJurassic lacustrine mudstones and the later charged oils are from the Jurassic coal-bearing strata or coals.