摘要：

Solid reservoir bitumens collected from Xujiahe(Upper Triassic,T3x),Feixianguan(Lower Triassic T1f) and Changxing Formation(Upper Permian,P2c) in Puguang and Yuanba gas fields and from Huanglong Formation(Middle Carboniferous,C2h) in western Hubei-eastern Chongqing region,the northeast of Sichuan basin,were systematically analyzed for bulk and molecular compositions,to investigate their genesis,character and origin.These bitumens are primarily characterized by poor solubility(ratios of chloroform extract over TOC<8% mostly),high reflectance(equivalent Ro>1.4%) and low H/C atomic ratios(<0.6),suggesting that they are derived from their original reservoired oil or migrated hydrocarbon by thermal degradation into gas,belonging to a pyrobitumen.Most bitumens in carbonate reservoir of Feixianguan and Changxing formations exhibit relatively high atomic ratios of S/C(mostly in 0.025 to 0.085),with δ34S values ranging mainly from 12‰ to 24‰,indicating a proportion of them possibly resulted from TSR.Unusual biomarker composition and distribution,such as bimodal distribution in n-alkanes,reduced Pr/Ph value,undifferentiated distribution in steranes and terpanes and reversal isomerization in sterane,are observed in these over-mature bitumens,and thus their conventional geochemical significances appear lost.However,the ratios of 2,6-/2,10-DMP,1,7-/1,9-DMP and 4-/1-MDBT in aromatic fraction are considered to be useful indicators for biological organic source and sedimentary environment of the source rocks from which the initial oils for the bitumens are derived.The aromatic ratios are relatively high in the bitumens of Xujiahe terrestrial reservoir,indicating a dominant terrigenous source input and oxic depositional environment for their source rocks.In contrast,they are significantly lower in the bitumens of Feixianguan,Changxing and Huanglong marine reaservoirs,suggesting an organic source predominated by aquatic organisms and a reducing condition.According to correlation between bitumens and source rocks by carbon isotopes and relative composition of dibenzothiophene series,the reservoir bitumens in Xujiahe Formation are believed to originate from adjacent source rock within this sequence,and those in Feixianguan and Changxing formations both from Permian source rock.Since Huanglong solid reservoir bitumen studied is isotopically heavier(δ13C values from-23.2‰ to-26.4‰),the original hydrocarbon could be primarily sourced from Mid-Lower Silurian Hanjiadian and Xiaoheba source rocks,rather than Longmaxi Formation.

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