Geochemical Characteristics and Families of the Crude Oils in the Yingmaili Oilfield, Tarim Basin

Abstract

Abstract:

The genetic type of the oil in the Yingmaili Oilfield in the Tarim Basin is rather complex. Multiple geochemical approaches, including GC and GC/MS analysis of hydrocarbons in a gasoline range, biomarkers and aromatics fractions as well as compound specific isotope analysis of nalkanes, are adopted to characterize and classify oils in the area. Two main genetic oil families have been recognized in the Yingmaili Oilfield, namely marine oil family (I) in the YM2 wellblock and terrestrial oil family (Ⅱ) in the YM7 well-block， which have typical characteristics of marine and terrestrial oils respectively. However, further investigations show that there are still some differences in either of the oil families. The oil family Ⅱ is divided into two subgroups: One is primarily normal black and heavy oils produced form the Palaeozoic and Mesozoic intervals (Ⅱa), the other (Ⅱb) mainly condensate oils from the Eogene. There are some disparities of compound specific carbon isotope of nalkanes and distinctive differences of the composition and relative distribution of aromatics in the oils. The Ⅱb oil is distinguished from Ⅱa by relatively high abundances of diphenyl and dibenzofurane series in aromatics, and significant advantages of various aromatics, such as naphthalene, phenanthrene and chrystane, each of which has a low molecular weight relative to that of the homologues with a high molecular weight. Furthermore, the Ⅱb oil is also characterized by a little heavier compound specific carbon isotope of nalkanes as compared with the Ⅱa. All those features of Ⅱb are similar to those of the oils in the adjacent Yangtake oilfield, suggesting that there must be a certain genetic relationship between them and that the former is likely to be mixedsource oils from the Mesozoic source rocks in the north Baicheng Depression of the Kuqu subbasin, which does not agree with the previous opinion about one genetic type of oil discovered there. Despite some similar biomarkers observed for the oil of family I in the YM2 wellblock, the distribution pattern of the compound specific carbon isotope of nalkanes in the oil of YM2 (O1) and YM201(O) wells in the block is impressively distinct, suggesting different sources and oil mixing probabilities. This study might be helpful for further source rock identification and hydrocarbons accumulation mechanisms unravelment as well as petroleum resource evaluation in the area.

Key words: Yingmaili Oilfield, biomarker, compound specific isotope, marine oil, terrestrial oil, mixed source oil

CLC Number:

TE122.1

LI Su-Mei, PANG Xiong-Qi, YANG Hai-Jun, XIAO Zhong-Yao, GU Qiao-Yuan. Geochemical Characteristics and Families of the Crude Oils in the Yingmaili Oilfield, Tarim Basin[J]. Geoscience, 2010, 24(4): 643-653.

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